2024年2月21日发(作者：)
Guide to Concrete Floor
and Slab ConstructionReported by ACI Committee 302
ACI
302.1R-15
ACI 302.1R-15Guide to Concrete Floor and Slab ConstructionReported by Committee 302
Joseph F. Neuber Jr., ChairPatrick J. Harrison, Vice ChairDennis C. AhalBryan M. BirdwellPeter A. CraigAllen FaceC. Rick FelderEdward B. FinkelBarry E. ForemanGreg K. FricksTerry J. FricksJerry A. HollandPhilip S. KopfSteve R. Lloyd, A. MacDonaldArthur W. McKinneyDonald M. McPheeScott C. MetzgerJeffrey S. MillerScott L. NiemitaloRussell E. Neudeck, SecretaryNigel K. ParkesWilliam S. PhelanTim H. RobinsonJohn W. RohrerPaul A. Rouis, IIIDomenick Thomas RutturaBruce A. SuprenantScott M. TarrConsulting MembersCarl Bimel*Michael A. ClarkWilliam C. PanareseBrian J. PashinaBoyd C. Ringo**DeceasedThe quality of a concrete floor or slab is highly dependent on
achieving a hard and durable surface that is flat, relatively free
of cracks, and at the proper grade and elevation. Properties of the
surface are determined by the mixture proportions and the quality
of the concreting and jointing operations. The timing of concreting
operations—especially finishing, jointing, and curing—is critical.
Failure to address this issue can contribute to undesirable char-acteristics in the wearing surface such as cracking, low resistance
to wear, dusting, scaling, high or low spots, poor drainage, and
increasing the potential for te floor slabs employing portland cement, regardless of
slump, will start to experience a reduction in volume as soon as
they are placed. This phenomenon will continue as long as any
water, heat, or both, is being released to the surroundings. More-over, because the drying and cooling rates at the top and bottom
of the slab are not the same, the shrinkage will vary throughout
the depth, causing the as-cast shape to be distorted and reduced
in guide contains recommendations for controlling random
cracking and edge curling caused by the concrete’s normal volume
change. Application of present technology permits only a reduc-tion in cracking and curling, not elimination. Even with the best
floor designs and proper construction, it is unrealistic to expect
completely crack- and curl-free floors. Consequently, every owner
should be advised by both the designer and contractor that it is
completely normal to expect some amount of cracking and curling
on every project, and that such an occurrence does not necessarily
reflect adversely on either the adequacy of the floor’s design or the
quality of its construction (Ytterberg 1987).This guide describes how to produce high-quality concrete slabs-on-ground and suspended floors for various classes of service.
It emphasizes such aspects of construction as site preparation,
concrete materials, concrete mixture proportions, concrete work-manship, joint construction, load transfer across joints, form strip-ping procedures, finishing methods, and curing. Flatness/levelness
requirements and measurements are outlined. A thorough precon-struction meeting is critical to facilitate communication among key
participants and to clearly establish expectations and procedures
that will be employed during construction to achieve the floor qual-ities required by the project specifications. Adequate supervision
and inspection are required for job operations, particularly those
of ds: admixture; aggregate; consolidation; contract documents;
curing; curling; deflection; durability; form; fracture; joint; mixture propor-tioning; placing; quality control; slab-on-ground; slabs; slump TSCHAPTER 1—INTRODUCTION, p. 31.1—Purpose, p. 31.2—Scope, p. 3CHAPTER 2—DEFINITIONS, p. 3ACI Committee Reports, Guides, and Commentaries are
intended for guidance in planning, designing, executing, and
inspecting construction. This document is intended for the use
of individuals who are competent to evaluate the significance
and limitations of its content and recommendations and who
will accept responsibility for the application of the informa-tion it contains. ACI disclaims any and all responsibility for
the stated principles. The Institute shall not be liable for any
loss or damage arising there nce to this document shall not be made in contract
documents. If items found in this document are desired by the
Architect/ Engineer to be a part of the contract documents,
they shall be restated in mandatory language for incorporation
by the Architect/Engineer.
CHAPTER 3—PREBID AND PRECONSTRUCTION
MEETINGS, p. 33.1—Prebid meeting, p. 33.2—Preconstruction meeting, p. 3ACI 302.1R-15 supersedes ACI 302.1R-04 and was adopted and published June ght © 2015, American Concrete rights reserved including rights of reproduction and use in any form or by any
means, including the making of copies by any photo process, or by electronic or
mechanical device, printed, written, or oral, or recording for sound or visual reproduc-tion or for use in any knowledge or retrieval system or device, unless permission in
writing is obtained from the copyright proprietors.
1
2 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)CHAPTER 5—DESIGN CONSIDERATIONS, p. 65.1—Scope, p. 65.2—Slabs-on-ground, p. 65.3—Suspended slabs, p. 115.4—Miscellaneous details, p. 13CHAPTER 6—SITE PREPARATION AND PLACING
ENVIRONMENT, p. 146.1—Soil-support system preparation, p. 146.2—Suspended slabs, p. 166.3—Bulkheads, p. 166.4—Setting screed guides, p. 166.5—Installation of auxiliary materials, p. 166.6—Concrete placement conditions, p. 16CHAPTER 7—ASSOCIATED MATERIALS, p. 177.1—Introduction, p. 177.2—Reinforcement, p. 177.3—Special-purpose aggregates, p. 187.4—Monomolecular films, p. 187.5—Curing materials, p. 187.6—Gloss-imparting waxes, p. 197.7—Liquid surface treatments, p. 197.8—Joint materials, p. 207.9—Volatile organic compounds (VOCs), p. 20CHAPTER 8—CONCRETE MATERIALS AND
MIXTURE PROPORTIONING, p. 208.1—Introduction, p. 208.2—Concrete, p. 208.3—Concrete properties, p. 208.4—Recommended concrete mixture, p. 218.5—Aggregates, p. 238.6—Portland cement, p. 248.7—Water, p. 258.8—Admixtures, p. 258.9—Concrete mixture analysis, p. 27CHAPTER 9—BATCHING, MIXING, AND
TRANSPORTING, p. 319.1—Batching, p. 319.2—Mixing, p. 329.3—Transporting, p. 32CHAPTER 10—PLACING, CONSOLIDATING, AND
FINISHING, p. 3310.1—Placing operations, p. 3310.2—Tools for spreading, consolidating, and finishing,
p. 34
CHAPTER 11—CURING, PROTECTION, AND JOINT
FILLING, p. 5711.1—Purpose of curing, p. 5711.2—Methods of curing, p. 5711.3—Curing at joints, p. 5811.4—Curing special concrete, p. 5811.5—Length of curing, p. 5911.6—Preventing plastic shrinkage cracking, p. 5911.7—Curing after grinding, p. 5911.8—Protection of slab during construction, p. 5911.9—Temperature drawdown in cold storage and freezer
rooms, p. 5911.10—Joint filling and sealing, p. 60CHAPTER 12—QUALITY CONTROL CHECKLIST,
p. 6012.1—Introduction, p. 6012.2—Partial list of important items to be observed, p. 60CHAPTER 13—CAUSES OF FLOOR AND SLAB
SURFACE IMPERFECTIONS, p. 6113.1—Introduction, p. 6113.2—Random cracking, p. 6213.3—Low wear resistance, p. 6513.4—Dusting, p. 6513.5—Scaling, p. 6613.6—Popouts, p. 6713.7—Blisters and delamination, p. 6813.8—Spalling, p. 6913.9—Discoloration, p. 7013.10—Low spots and poor drainage, p. 7113.11—Slab edge curling, p. 7113.12—Evaluation of slab surface imperfections, p. 73CHAPTER 14—REFERENCES, p. 73Authored documents, p. 75
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CHAPTER 4—CLASSES OF FLOORS, p. 44.1—Classification of floors, p. 44.2—Single-course monolithic floors: Classes 1, 2, 4, 5,
and 6, p. 44.3—Two-course floors: Classes 3, 7, and 8, p. 44.4—Class 9 floors, p. 64.5—Special finish floors, p. 610.3—Spreading, consolidating, and finishing operations,
p. 3710.4—Finishing Class 1, 2, and 3 floors, p. 4410.5—Finishing Class 4 and 5 floors, p. 4410.6—Finishing Class 6 floors and monolithic-surface
treatments for wear resistance, p. 4410.7—Finishing Class 7 floors, p. 4610.8—Finishing Class 8 floors (two-course unbonded), p.
4710.9—Finishing Class 9 floors, p. 4710.10—Toppings for precast floors, p. 4810.11—Finishing lightweight concrete, p. 4810.12—Nonslip floors, p. 5010.13—Decorative and nonslip treatments, p. 5010.14—Grinding as repair procedure, p. 5210.15—Floor flatness and levelness, p. 5210.16—Treatment when bleeding is a problem, p. 5610.17—Delays in cold-weather finishing, p. 57
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 3CHAPTER 1—INTRODUCTION1.1—PurposeThis guide presents information relative to the construction
of slab-on-ground and suspended-slab floors for industrial,
commercial, and institutional buildings. It is applicable to
the construction of normalweight and structural lightweight
concrete floors and slabs made with conventional portland
and blended cements. This guide identifies the various classes
of floors based on use, construction design details, necessary
site preparation, concrete type, and other related materials. In
general, characteristics of the concrete slab surface and joint
performance have a powerful impact on the serviceability
of floors and other slabs. Because the eventual success of a
concrete floor installation depends on the mixture proportions
and floor finishing techniques used, considerable attention is
given to critical aspects of achieving the desired finishes and
the required floor surface tolerances.1.2—ScopeThis guide emphasizes choosing and proportioning of
materials, design details, proper construction methods, and
workmanship. Slabs specifically intended for the containment
of liquids are beyond the scope of this guide. Whereas this
guide does provide a reasonable overview of concrete floor
construction, each project is unique and circumstances can
dictate departures from the recommendations given in this
guide. Contractors and suppliers should, therefore, thoroughly
review contract documents before bid preparation (Chapter 3).CHAPTER 2—DEFINITIONSACI provides a comprehensive list of definitions through
an online resource, “ACI Concrete Terminology,” /store/?ItemID=CT13.
Definitions provided herein complement that ential set time—difference in timing from initial
introduction of water to concrete mixture at batch plant to
initial power -shake—dry mixture of hydraulic cement and fine
aggregate (either mineral or metallic) that is distributed
evenly over the surface of concrete flatwork and worked into
the surface before time of final setting and then floated and
troweled to desired e optimization indicator—intersection of the
coarseness factor value and the workability factor on the
coarseness factor g—creation of troughs in the soil support system in
response to applied wheel —creation of lines or notches in the surface of a
concrete pumping—vertical displacement and rebound of the
soil support system in response to applied moving slump—magnitude of slump, measured in accor-dance with ASTM C143/C143M, which is directly attributed
to the amount of water in the concrete of finishability—time period available for
finishing operations after the concrete has been placed,
consolidated, and struck-off, and before final ility factor—percentage of combined aggregate
that passes the No. 8 (2.36 mm) R 3—PREBID AND PRECONSTRUCTION
MEETINGS3.1—Prebid meetingThe best forum for a thorough review of contract documents
before the bid preparation is a prebid meeting. This meeting
offers bidders an opportunity to ask questions and to clarify
their understanding of contract documents before submitting
their bids. A prebid meeting also provides the owner and the
owner’s slab designer an opportunity to clarify intent where
documents are unclear and to respond to last-minute questions
in a manner that provides bidders an opportunity to be equally
responsive to the contract documents.3.2—Preconstruction meetingSuccessful construction of slabs-on-ground or suspended
floors or slabs involves the coordinated efforts of many
subcontractors and material suppliers. The slab designer
should schedule a preconstruction meeting to establish and
coordinate procedures that will enable key participants to
produce the best possible product under the anticipated field
conditions. This meeting should be attended by responsible
representatives of organizations and material suppliers directly
involved with either the design or construction of floors.3.2.1 Agenda items—The preconstruction meeting should
confirm and document the responsibilities and anticipated
interaction of key participants involved in slab-on-ground or
suspended floor or slab construction. Following is a list of
agenda items appropriate for such a meeting, including ones
for which the contract documents should establish a clear
responsibility. The following list is not all-inclusive:a) Site preparationb) Grades for drainage, if anyc) Work associated with installation of auxiliary materials,
such as vapor barriers, vapor retarder/barriers, edge insu-lation, electrical conduit, mechanical sleeves, drains, and
embedded platesd) Class of floore) Floor thicknessf) Reinforcement, when requiredg) Construction tolerances: base (rough and fine grading),
forms, slab thickness, surface configuration, and floor flat-ness and levelness requirements (including how and when
measured)h) Joints and load-transfer mechanismi) Materials: cements, fine aggregate, coarse aggregate,
water, and admixtures (usually by reference to applicable
ASTM standards)j) Special aggregates, admixtures, or monolithic surface
treatments, where applicablek) Concrete specifications including:1) Compressive strength, flexural strength, or both2) Recommended cementitious material content, if
applicable3) Maximum size, grading, and type of coarse aggregate
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4 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)4) Grading and type of fine aggregate5) Combined aggregate grading6) Air content of concrete, if applicable7) Slump of concrete8) Water-cement ratio (w/c) or water-cementitious mate-rial ratio (w/cm)9) Preplacement soaking requirement for lightweight
aggregates10) Finishabilityl) Measuring, mixing, and placing procedures, which
is usually by reference to specifications or recommended
practicesm) Strike-off methodn) Recommended finishing methods and tools, where
requiredo) Coordination of floor finish requirements with those
required for floor coverings such as vinyl, ceramic tile, or
wood that are to be applied directly to the floorp) Curing procedures, length of curing, necessary protec-tion, and time before opening slabs for traffic (ACI 308R;
308.1)q) Testing and inspection requirements;r) Acceptance criteria and remedial measures to be used,
if requiredAdditional issues specific to suspended slab construction
are:a) Form tolerances and preplacement quality assurance
survey procedures for cast-in-place constructionb) Erection tolerances and preplacement quality assurance
survey procedures for composite slab constructionc) Form stripping procedures, if applicabled) Items listed in 5.3 that are appropriate to the structural
system(s) used for the project3.2.2 Quality assurance—Adequate provisions should be
made to ensure that the constructed product meets or exceeds
the requirements of the project documents. Toward this end,
quality control procedures should be established and main-tained throughout the entire construction quality of a completed concrete slab depends on the
skill of individuals who place, finish, and test the mate-rial. As an aid to ensuring a high-quality finished product,
the specifier or owner should consider requiring the use
of prequalified concrete contractors, concrete suppliers,
accredited testing laboratories, and concrete finishers who
have had their proficiency and experience evaluated through
an independent third-party certification program. ACI has
developed programs to train and certify concrete flatwork
finishers and concrete inspectors and testing R 4—CLASSES OF FLOORS4.1—Classification of floorsTable 4.1 classifies floors on the basis of intended use,
discusses special considerations, and suggests finishing
techniques for each class of floor. Intended use requirements
should be considered when selecting concrete properties,
and the step-by-step placing, consolidating, and finishing
procedures in Chapter 10 should be closely followed for
different classes and types of resistance and impact resistance should also be
considered. Currently, there are no standard criteria for eval-uating the wear resistance of a floor, making it impossible
to specify concrete quality in terms of ability to resist wear.
Wear resistance is directly related to the concrete mixture
proportions, aggregate types, finishing, surface treatments,
curing, and other construction techniques used.4.2—Single-course monolithic floors: Classes 1, 2,
4, 5, and 6Five classes of floors are constructed with monolithic
concrete; each involves some variation in joint detailing
and final finishing techniques. If abrasion from grit or other
materials is anticipated, a higher quality floor surface may be
required for satisfactory service (ASTM 1994). Under these
conditions, a special mineral or metallic aggregate mono-lithic surface treatment is recommended. For slabs exposed
to vehicular traffic, enhanced detailing, including positive
load transfer (typically dowels) and edge protection at all
joints, is recommended.4.3—Two-course floors: Classes 3, 7, and 84.3.1 Unbonded topping over base slab—The base courses
of Class 3 (unbonded topping) floors and Class 8 floors can
be either slabs-on-ground or suspended slabs, with the finish
coordinated with the type of topping. For Class 3 floors, the
concrete topping material is similar to the base slab concrete.
The top courses for Class 8 floors require a hard-steel trow-eling and usually have a higher compressive strength than
the base course. Class 8 floors can also make use of an
embedded cement-coated hard aggregate, a premixed (dry-shake) mineral aggregate, or metallic hardener for addition
to the 3 (unbonded topping) and Class 8 floors are used
when it is preferable not to bond the topping to the base
course. This allows the two courses to move independently,
or so that the top courses can be more easily replaced at a
later period. Two-course floors can be used when mechan-ical or electrical equipment requires special bases and when
their use permits more expeditious construction procedures.
Two-course unbonded floors can also be used to resurface
worn or damaged floors when contamination prevents
complete bond, or when it is desirable to avoid scarifying
and chipping the base course and the resultant higher floor
elevation is compatible with adjoining floors. Class 3 floors
are used primarily for commercial or nonindustrial applica-tions, whereas Class 8 floors are used primarily for industrial
ed toppings should have a minimum thickness
of 3 in. (75 mm) for foot-traffic, but a minimum thickness
of 4 in. (100 mm) is recommended if the surface is to be
subjected to vehicular topping slab should have a joint spacing closer than
a slab placed on ground of similar thickness to minimize
the increased curling or warping stresses when placed over
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GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 5Table 4.1—Classes of floors on the basis of intended use and the recommended final finish techniqueClass1. ExposedAnticipated traffic typeExposed surface—foot
trafficUseOffices, churches, multiunit
residential, decorativeSpecial considerationsUniform finish, nonslip aggregate in
specific areas, curingColored mineral aggregate, color pigment
or exposed aggregate, stamped or inlaid
patterns, artistic joint layout, curing,
surface treatment, maintenanceFlat and level slabs suitably dry for
applied coverings, curingFinal finishNormal steel-troweled finish,
nonslip finish where requiredBurnishing or polishing to
enhance sheen as required2. CoveredCovered surface—foot
traffic3. ToppingExposed or covered
surface—foot trafficOffices, churches,
commercial, multiunit
residential, institutional with
floor coveringsUnbonded or bonded topping
over base slab for commercial
or nonindustrial buildings
where construction type or
schedule dictatesLight steel-troweled finishBase slab—good uniform level surface
tolerance, curingUnbonded topping—bondbreaker on base
slab, minimum thickness 3 in.
(75 mm), reinforced, curingBase slab—troweled finish
under unbonded topping;
clean, textured surface under
bonded toppingTopping—for exposed
surface, normal steel-troweled finish; for covered
surface, light steel-troweled
finishNormal steel-troweled finish4. Institutional/commercialExposed or covered
surface—foot and light
vehicular trafficExposed surface—industrial
vehicular traffic such as
pneumatic wheels and
moderately soft solid wheelsExposed surface—heavy-duty industrial vehicular
traffic such as hard wheels
and heavy wheel loadsExposed surface—heavy-duty industrial vehicular
traffic such as hard wheels
and heavy wheel loadsInstitutional or commercial5. IndustrialIndustrial floors for
manufacturing, processing,
and warehousingIndustrial floors subject to
heavy traffic; can be subject
to impact loadsBonded two-course floors
subject to heavy traffic and
impactBonded topping—properly sized
aggregate, 3/4 in. (19 mm) minimum
thickness curingLevel and flat slab suitable for applied
coverings, nonslip aggregate for specific
areas, curing; coordinate joints with
applied coveringsGood uniform subgrade, joint layout, joint Hard steel-troweled finishload transfer, abrasion resistance, curing6. Heavy
industrialGood uniform subgrade, joint layout, joint Special metallic or mineral
load transfer required, abrasion resistance, aggregate surface hardener;
curingrepeated hard steel-trowelingBase slab—good uniform subgrade,
reinforcement, joint layout, level surface,
curingTopping—composed of well-graded
all-mineral or all-metallic aggregate.
Minimum thickness 3/4 in. (19 mm)Mineral or metallic aggregate surface
hardener applied to high-strength plain
topping to toughen, curingBondbreaker on base slab, minimum
thickness 4 in. (100 mm), abrasion
resistance, curingClean, textured base
slab surface suitable for
subsequent bonded topping.
Special power floats for
topping are optional, hard
steel-troweled finish7. Heavy
industrial
topping8. Commercial/As in Classes 4, 5, or 6industrial
Topping9. Critical
surface profileUnbonded topping—on
new or old floors where
construction sequence or
schedule dictatesExposed surface—superflat
Narrow-aisle, high-bayor critical surface tolerance warehouses; television
required; special materials-studios, ice rinks, or
handling vehicles or robotics gymnasiums (ACI 360R)requiring specific tolerancesAs in Classes 4, 5, or 6Varying concrete quality requirements. Strictly following techniques
Special application procedures and strict as indicated in 8.9attention to detail are recommended when
shake-on hardeners are used. FF 50 to FF
125, superflat floor, curinga rigid base slab due to the effects of drying from the top
surface c sheeting, roofing felt, or a bond-breaking compound
is often used to prevent bond to the base slab. Reinforce-ment, such as deformed bars, welded wire fabric, bar mats,
or fibers is recommended to be placed in the topping in suffi-cient quantities to reduce the width of shrinkage cracks and
to bridge existing cracks in the base slab. Concrete should be
proportioned to meet the requirements of Chapter 8.
Curling or warping will also be more probable due to the
effects of drying from the top surface only. Reinforcement
of unbonded topping slabs is recommended due to increased
curling stresses and potential bridging of existing cracks in
the base slab.4.3.2 Bonded topping over base slab—Class 3 (bonded
topping) and Class 7 floors use a topping bonded to the base
slab. Class 3 (bonded topping) floors are used primarily for
commercial or nonindustrial applications; Class 7 floors are
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6 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)and can be designed with continuous reinforcement, while
eliminating joints.4.5—Special finish floorsFloors with decorative finishes and those requiring skid resis-tance or electrical conductivity are discussed in Chapter exposed to mild acids, sulfates, or other chemi-cals require special preparation or protection. Refer to ACI
201.2R for reports on the means of increasing the resistance
of concrete to chemical attack. Where attack will be severe,
wear-resistant protection suitable for the exposure should
be used. Such environments and the methods of protecting
floors against them are discussed in ACI certain chemical and food processing plants, such as
slaughterhouses, exposed concrete floors are subject to slow
disintegration due to organic acids. In many instances, it is
preferable to protect the floor with other materials such as
acid-resistant brick, tile, or resinous mortars (ACI 515.2R).CHAPTER 5—DESIGN CONSIDERATIONS5.1—ScopeChapter 5 addresses the design of concrete floors as it
relates to their constructibility. Specific design requirements
for concrete floor construction are found in ACI 360R for
slabs-on-ground, ACI 223R for shrinkage-compensating
concrete floors, and ACI 421.1R and 421.2R for suspended
floors.5.2—Slabs-on-ground5.2.1 Required design elements—Following are the
minimum items that should be addressed in the construction
documents prepared by the designer (ACI 360R):a) Slab-on-ground design criteriab) Base and subbase materials, preparation requirements,
and vapor retarder/barrier, when requiredc) Concrete thicknessd) Concrete compressive strength, flexural strength, or
bothe) Concrete mixture proportion requirements, ultimate
drying shrinkage strain, or bothf) Joint locations and detailsg) Reinforcement (type, size, and location) when requiredh) Surface treatment, when requiredi) Surface finishj) Tolerances (base, subbase, slab thickness, and floor flat-ness and levelness)k) Concrete curingl) Joint filling material and installationm) Special embedmentsn) Testing requirementso) Preconstruction meeting, quality assurance, and quality
controlIf any of this information is not provided, the contractor
should request it from the slab-on-ground slab designer.5.2.2 Soil-support system—Because the performance of a
slab-on-ground depends on the integrity of the soil-support
system, specific attention should be given to site preparation
Fig. 4.3.2—Saw-cut contraction for heavy-duty industrial applications subject to heavy
traffic and impact. The base slabs can either be a conven-tional portland cement concrete mixture or shrinkage-compensating concrete. The surface of the base slab should
have a rough, open-pore finish and be free of any substances
that would interfere with bonding of the topping to the base
slab (Fig. 4.3.2).The topping installation can occur either the same day
before hardening of the base slab or deferred until after the
base slab has hardened. The topping for a Class 3 floor is a
concrete mixture similar to that used in Class 1 or 2 floors.
The topping for a Class 7 floor requires a multiple pass hard-steel-trowel finish and the top course usually has a higher
strength than the base course. A bonded topping can also
make use of an embedded hard aggregate or a premixed
(dry-shake) mineral aggregate or metallic hardener for addi-tion to the surface. Bonded concrete toppings should have
a minimum thickness of 3/4 in. (19 mm). Proprietary prod-ucts should be applied per manufacturers’ recommendations.
Joint spacing in the topping should be coordinated with
construction and contraction joint spacing in the base slab.
Saw-cut contraction joints should penetrate into the base
slab a minimum of 1 in. (25 mm).If the topping is placed on a base slab before the joints are
cut, joints in the topping should extend into the base slab
and depth should be appropriate for the total thickness of
the combined slab. If the topping is installed on a previously
placed slab where joints have activated, additional joints in
the topping are unnecessary as shrinkage relief cannot occur
between the slab joints in the bonded topping. When topping
slabs are placed on shrinkage-compensating concrete base
slabs, the joints in the base slab can only be reflected in the
bonded topping slab if the bonded topping slab is installed
shortly after the maximum expansion of the base slab occurs.
Maximum expansion usually occurs within 7 to 14 days.4.4—Class 9 floorsCertain materials-handling facilities, for example, high-bay, narrow-aisle warehouses, require extraordinarily level
and flat floors. The construction of such superflat floors
(Class 9) is discussed in Chapter 10. A superflat floor could
be constructed as a single-course floor, or as a two-course
floor with a topping. A bonded topping would be similar to
a Class 7 topping. An unbonded Class 9 topping is similar
to a Class 8 topping, typically greater than 4 in. (100 mm),
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GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 7requirements, including proof-rolling (6.1.1). In most cases,
proof-rolling results are much more indicative of the soil-support system’s ability to withstand loading than from the
results of in-place tests of moisture content or density. A thin
layer of graded, granular, compactible material is normally
used as fine grading material to better control concrete’s thick-ness and to minimize friction between the base material and
slab. For detailed information on soil-support systems, refer
to ACI 360R.5.2.3 Moisture protection—Proper moisture protection
is essential for any slab-on-ground where the floor will
be covered by moisture-sensitive flooring materials such
as vinyl; linoleum; wood; carpet; rubber; rubber-backed
carpet tile; impermeable floor coatings; adhesives; or where
moisture-sensitive equipment, products, or environments
exist, such as humidity-controlled or refrigerated rooms.
ACI 302.2R provides recommendations for the design and
construction of concrete slabs that will receive moisture-sensitive or pH-sensitive flooring materials or coatings for
both slabs-on-ground and suspended slabs.5.2.3.1 Vapor retarder permeance—A vapor retarder/barrier is a material that is intended to minimize the trans-mission of water vapor upward through the slab from sources
below. The performance requirements for plastic vapor
retarder/barrier materials in contact with soil or granular fill
under concrete slabs are listed in ASTM E1745. According
to ASTM E1745 a vapor retarder/barrier material is to have a
permeance level, also known as the water vapor transmission
rate, not exceeding 0.1 perms as determined by ASTM E96/E96M or ASTM F1249. However, most flooring installations
will benefit by using a material with a permeance level well
below 0.1 perms (0.0659 metric perms = 5.72 ng/s–1m–2Pa–1).The selection of a vapor retarder/barrier material and
its level of permeance should be made on the basis of the
protective requirements of the material being applied to the
floor surface or the environment being protected. Although
conventional 6, 8, and 10 mil (0.15, 0.20, and 0.25 mm)
polyethylene has been used in the past, this class of material
does not fully conform to the requirements of ASTM E1745
and should not be considered for use as below-slab mois-ture protection. Any plastic vapor retarder/barrier material
to be used below slabs should be in full compliance with the
minimum requirements of ASTM E1745 and the thickness
and permeance of the material be selected on the basis of
protective needs and durability during and after r, for a material to be considered a true barrier it
would need to have a permeance level of 0.0 perms when
tested in accordance with ASTM E96/E96M or ASTM
F1249. The industry has not established a permeance level
that serves as the dividing point between materials classed
as vapor barriers or vapor retarders. It is most likely that
when a dividing point between barrier and retarder is estab-lished it will be at 0.01 perms or less. The laps or seams for
a vapor retarder/barrier should be overlapped 6 in. (150 mm)
(ASTM E1643) or as instructed by the manufacturer. Joints
and penetrations should be sealed with the manufacturer’s
recommended adhesive, pressure-sensitive tape, or both.
5.2.3.2 Vapor retarder/barrier location—The decision
to locate the vapor retarder/barrier in direct contact with
the slab’s underside had long been debated. Experience
has shown, however, that the greatest level of protection
for floor coverings, coatings, or building environments is
provided when the vapor retarder/barrier is placed in direct
contact with the slab. Placing concrete in direct contact with
the vapor retarder/barrier eliminates the potential for water
from sources such as rain, saw-cutting, curing, cleaning, or
compaction to become trapped within the fill course. Wet
or saturated fill above the vapor retarder/barrier can signifi-cantly lengthen the time required for a slab to dry to a level
acceptable to the manufacturers of floor coverings, adhe-sives, and coatings. A fill layer sandwiched between the vapor
retarder/barrier and the concrete also serves as an avenue for
moisture to enter and travel freely beneath the slab, which
can lead to an increase in moisture within the slab once it
is covered. Moisture can enter the fill layer through voids,
tears, or punctures in the vapor retarder/g concrete in direct contact with the vapor retarder/barrier requires additional design and construction consider-ations if potential slab-related problems are to be avoided.
When compared with identical concrete cast on a draining
base, concrete placed in direct contact with a vapor retarder/barrier shows more settlement and exhibits significantly
larger length change in the first hour after casting, during
drying shrinkage, and when subject to environmental
change (Suprenant 1997). Joints that open wider than what
is normally anticipated are called dominant joints (Walker
and Holland 2007). Dominant joint behavior can be made
worse when the slab is placed in direct contact with a vapor
retarder/barrier that reduces friction from the base. Where
reinforcing steel is present, settlement cracking over the
steel is more likely because of increased settlement resulting
from a longer bleeding period. There is also increased poten-tial for a greater measure of slab te that does not lose excess water to the base does not
stiffen as rapidly as concrete that does. If rapid surface drying
conditions are present, the surface of concrete placed directly
on a vapor retarder/barrier has a tendency to dry and crust over
whereas the concrete below the top fraction of an inch (milli-meter) remains relatively less stiff or unhardened. When this
occurs, it may be necessary to begin machine operations on the
concrete surface before the concrete below the top surface is
sufficiently set. Under such conditions, a reduction in surface
flatness and some blistering or delamination can occur as air,
water, or both, become trapped below the finish proposed installation should be independently evalu-ated for moisture sensitivity of anticipated subsequent floor
finishes and the level of protection and material strength
they might need. When placing concrete in direct contact
with the vapor retarder/barrier, the potential effects of slab
curling, crusting, and cracking should be considered. Design
and construction measures should be implemented to offset
or to minimize these effects. The anticipated benefits and
risks associated with the specified location of the vapor
retarder/barrier should be reviewed (Fig. 5.2.3.2) with all
parties before construction (ACI 302.2R).
American Concrete Institute – Copyrighted © Material –
8 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)Fig. 5.2.3.2—Flow chart to determine when and where a vapor retarder/barrier should be used.5.2.4 Supporting reinforcement—Deformed reinforcing
steel or post-tensioning tendons should be supported and tied
together sufficiently to minimize movement during concrete
placing and finishing operations. Chairs with sand plates or
precast concrete bar supports are generally considered to be
the most effective method of providing the required support.
When precast concrete bar supports are used, they should be
at least 4 in. (100 mm) square at the base, have a compres-sive strength at least equal to the specified compressive
strength of the concrete being placed, and be thick enough
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 9to support reinforcing steel or post-tensioning tendons at the
proper elevation while maintaining minimum concrete cover
welded wire reinforcement is used, its larger flex-ibility dictates that the contractor pay close attention to
establishing and maintaining adequate support of the rein-forcement during the concrete placing operations (Neuber
2006). Welded wire reinforcement should not be placed on
the ground and pulled up after placement of the concrete,
nor should the mats be walked in after placing the support spacing is necessary to maintain welded
wire reinforcement at the proper elevation; supports should
be close enough such that welded wire reinforcement cannot
be forced out of location by construction foot traffic. Support
spacing can be increased when heavier gauge wires or a
double mat of small-gauge wires is used. Installation should
be in accordance with the Wire Reinforcing Institute’s
(2008) recommendations.5.2.5 Steel fibers—Steel fibers are used to provide rein-forced concrete slabs-on-ground with increased strain
strength, impact resistance, flexural toughness, fatigue
endurance, crack-width control, and tensile strength (ACI
544.4R). Finishing floors with steel fibers is similar to other
floors, except for the potential for steel fibers to become
exposed on the surface of the slab. Typically, however, occa-sional exposed fibers do not cause a problem. Section 7.2.4
discusses types of steel fibers and dosages in detail.5.2.6 Synthetic fibers—Synthetic fibers are used to rein-force concrete against plastic shrinkage and drying shrinkage
stresses. Section 7.2.3 discusses types of synthetic fibers and
dosages in detail.5.2.7 Post-tensioning reinforcement—The use of high-strength steel tendons as reinforcement instead of conven-tional mild steel temperature and shrinkage reinforcement
introduces relatively high compressive stress in the concrete
by means of post-tensioning. This compressive stress
provides a balance for the crack-producing tensile stresses
that develop as the concrete shrinks during the drying
process. Stage stressing or partial tensioning of the slab on
the day following placement can result in a significant reduc-tion of shrinkage cracks. Construction loads on the concrete
should be minimized until the slabs are fully stressed (Post-Tensioning Institute 1990, 1996). For guidelines on instal-lation details, contact a concrete floor specialty contractor
who is thoroughly experienced with this type of installation.5.2.8 Causes of cracking over reinforcement—Bar shad-owing and subsidence cracking directly over reinforce-ment is caused by inadequate consolidation of concrete,
inadequate concrete cover over the reinforcement, use of
large-diameter bars (Babaei and Fouladgar 1997; Dakhil et
al. 1975), higher temperature of reinforcing bars exposed
to direct sunlight, higher-than-required slump in concrete,
revibration of the concrete, inadequate control of evapora-tion rate before concrete curing begins, or a combination of
these items.5.2.9 Joint design—Joints are used in slab-on-ground
construction to limit the frequency and width of random
cracks caused by volume changes and to reduce the magni-
Fig. 5.2.9—Appropriate locations for of slab curling. Slab designs with an increased number
of joints can result in decreases in curling and individual
joint opening width, resulting in less overall maintenance.
The joint details and layout of joints should be provided by
the designer. When the joint layout and joint details are not
provided before project bid, the designer should provide a
detailed joint layout along with the joint details before the
slab preconstruction meeting or commencing stated in ACI 360R, every effort should be made to
isolate the slab from restraint that might be provided by
another element of the structure. Restraint from any source,
whether internal or external, will increase the potential for
random ion, contraction, and construction joints are commonly
used in concrete slabs-on-ground. Appropriate locations for
isolation joints and contraction joints are shown in Fig. 5.2.9.
With the designer’s approval, construction joint and contrac-tion joint details can be interchanged. Refer to ACI 360R for a
detailed discussion of joints. Joints in topping slabs should be
located directly over joints in the base slab.5.2.9.1 Isolation joints—Isolation joints should be used
wherever complete freedom of vertical and horizontal move-ment is required between the floor and adjoining building
elements. Isolation joints should be used at junctions with
walls that do not require lateral restraint from the slab,
columns, equipment foundations, footings, or other points
of restraint such as drains, manholes, sumps, and stairways.
An isolation joint may be composed of sheet material or a
preformed joint material separating two adjacent concrete
elements; one example is where a slab abuts a wall. Where
the isolation joint will restrain shrinkage, flexible closed-cell
foam plank should be used with a thickness that accommo-
American Concrete Institute – Copyrighted © Material –
10 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)dates the anticipated shrinkage movement. The joint mate-rial should extend the full depth of the slab or slightly below
its bottom to ensure complete separation. Where the joint
filler will be objectionably visible, or where there are wet
conditions or hygienic or dust-control requirements, the top
of the preformed filler can be removed and the joint caulked
with an elastomeric sealant. Three methods for producing a
relatively uniform depth of joint sealant depth are:1. Use a saw to score both sides of the preformed filler at
the depth to be removed. Insert the scored filler in the proper
location. After the concrete hardens, use a screwdriver or
similar tool and remove the top section.2. Cut a strip of wood equal to the desired depth of the
joint sealant. Nail the wood strip to the preformed filler
and install the assembly in the proper location. Remove the
wood strip after the concrete has hardened.3. Use a premolded joint filler with a removable top to ACI 223R for guidance on isolation joints for
slabs using shrinkage-compensating concrete.5.2.9.2 Construction joints—Construction joints are
placed in a slab to define the extent of the individual concrete
placements, generally in conformity with a predetermined
joint layout. If concrete placement is ever interrupted long
enough for the placed concrete to harden, a construction
joint should be used. If possible, construction joints should
be located 5 ft (1.5 m) or more from any other joint to which
they are areas not subjected to traffic, a butt joint is usually
adequate. In areas subjected to hard-wheeled traffic, heavy
loadings, or both, joints with dowels are recommended.
Keyed joints are not recommended where load transfer is
required because the two sides of the keyway lose contact
when the joint opens due to drying shrinkage.5.2.9.3 Contraction joints—Contraction joints are usually
located on column lines with intermediate joints located at
equal spaces between column lines, as shown in Fig. 5.2.9.
Factors considered when selecting spacing of contraction
joints are:a) Slab design method (ACI 360R)b) Slab thicknessc) Type, amount, and location of reinforcementd) Shrinkage potential of the concrete, including cement
type and quantity; aggregate type, size, gradation, quantity, and
quality; w/cm; type of admixtures; and concrete temperaturee) Base frictionf) Floor slab restraintsg) Layout of foundations, racks, pits, equipment pads,
trenches, and similar floor discontinuitiesh) Environmental factors such as temperature, wind, and
humidityAs previously indicated, establishing slab joint spacing,
thickness, and reinforcement requirements is the responsi-bility of the designer. Because the specified joint spacing
will be a principal factor dictating both the amount and
the character of random cracking to be experienced, joint
spacing should always be carefully surface curling at joints is a normal consequence
of volume change resulting from differential moisture loss
from concrete slab to the surrounding environment. This
distortion can lead to conflicts with respect to installation
of some floor coverings in the months after concrete place-ment. Current national standards for ceramic tile and wood
flooring, such as gymnasium floors, are two instances that
require the concrete slab surface to comply with stringent
surface tolerances that cannot be met under typical slab
curling behavior. The designer should correlate the slab
design with the requirements imposed by the floor covering
specification in the design documents. The potential for the
joints to telegraph through the flooring should be addressed
by the random cracking should always be expected, even
with sufficiently close joint spacing. It is reasonable to expect
random visible cracks to occur in 0 to 3 percent of the surface
area floor slab panels formed by saw-cutting, construction
joints, or a combination of both. If slab curl is of greater
concern than usual, joint spacing, mixture proportion, and
joint details should be carefully analyzed. Reinforcement will
not prevent cracking. If the reinforcement is properly sized
and located, cracks should remain tightly in either direction can be reduced or eliminated
by post-tensioning that introduces a net compressive force
in the slab after all tensioning losses. The number of joints
can also be reduced with the use of shrinkage-compensating
concrete; however, the recommendations of ACI 223R
should be carefully ction joints should be continuous, not staggered or
offset. The aspect ratio of slab panels that are unreinforced,
reinforced only for shrinkage and temperature, or made with
shrinkage-compensating concrete should be a maximum
of 1.5 to 1; however, a ratio of 1 to 1 is preferred. L- and
T-shaped panels should be avoided. Plastic or metal inserts
are not recommended for constructing or forming a contrac-tion joint in any exposed floor surface that will be subjected
to wheeled traffic.5.2.10 Saw-cutting joints—Contraction joints in indus-trial and commercial floors are usually formed by sawing a
continuous slot in the slab to create a weakened plane, below
which a crack will form. Further details on saw cutting of
joints are given in 10.3.12.5.2.11 Joint filling—Contraction and construction joints in
floor areas subject to the hard wheels of material-handling
vehicle traffic should be filled with a semi-rigid filler to
minimize wear and damage to joint edges. Construction
joints should be saw-cut 1 in. (25 mm) deep before filling.
Joints should be as narrow as possible to minimize damage
due to wheels loads while still being wide enough to be
properly wet conditions or hygienic requirements exist,
joints should be sealed with an elastomeric liquid sealant
or a preformed elastomeric device. If there is also indus-trial vehicular traffic in these areas, consideration should be
given to armoring the edge of the joint through alternative
means. Refer to 7.8 for a discussion of joint materials and
11.10 for installation of joint fillers.5.2.12 Load-transfer mechanisms—Use of load-transfer
devices at construction and contraction joints is recom-
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 11mended when positive load transfer is required, unless
a sufficient post-tensioning force is provided across the
joint to transfer the shear. Load-transfer devices force the
concrete sections on both sides of a joint to undergo approx-imately equal vertical displacements when subjected to a
load and help prevent damage to an exposed edge when the
joint is subjected to vehicles with hard wheels such as lift
trucks. Dowel baskets should be used to maintain alignment
of dowels when used in contraction joints, and alignment
devices should be used in construction ed reinforcing bars should not be continued across
contraction joints or construction joints because they restrain
joints from opening as the slab shrinks during drying, unless
the reinforcement is properly designed as an enhanced
aggregate interlock load-transfer mechanism (ACI 360R).Keyed joints are not recommended for load transfer in
slabs-on-ground where heavy-wheeled traffic load is antici-pated because they do not provide effective load transfer.
When the concrete shrinks, the keys and keyways do not
retain contact and do not share the load between panels;
this can eventually cause a breakdown of the concrete joint
edges. For long post-tensioned floor strips and floors using
shrinkage-compensating concrete with long joint spacing,
care should be taken to accommodate significant slab move-ments. In most instances, post-tensioned slab joints are asso-ciated with a jacking gap. The filling of jacking gaps should
be delayed as long as possible to accommodate shrinkage
and creep (Post-Tensioning Institute 1990, 2000). Where
significant slab movement is expected, steel plating of the
joint edges is recommended for strengthening the edges.5.3—Suspended slabs5.3.1 Required design elements—The following items
specifically impact the construction of suspended slabs and
should be included in the construction documents prepared
by the designer:a) Frame geometry (member size and spacing)b) Concrete reinforcement (type, size, location, and
method of support)c) Shear connectors, if requiredd) Construction joint location and detailse) Steel deck (type, depth, gauge, and installation require-ments), if requiredf) Shoring, if requiredg) Tolerances (forms, structural steel, reinforcement, and
concrete)5.3.2 Suspended slab types—In general, suspended floor
systems fall into four main categories:a) Cast-in-place suspended floorsb) Slabs with removable formsc) Slabs-on-composite and noncomposite steel deckingd) Topping slabs on precast concreteDesign requirements for cast-in-place concrete suspended
floor systems are covered by ACI 318 and ACI 421.1R. Refer
to these documents to obtain design parameters for various
cast-in-place systems. Slabs-on-steel decking and topping
slabs-on-precast-concrete are hybrid systems that involve
design requirements established by The Steel Deck Insti-
tute, The American Institute of Steel Construction, Precast/Prestressed Concrete Institute, and tolerances of ACI levelness of suspended slabs depends on the accuracy
of formwork and strike-off, but is further influenced, espe-cially in the case of slabs-on-steel decking, by the behavior
of the structural frame during and after completion of
construction. The contractor should recognize that each type
of structural frame behaves differently and plan slab designer should discuss with the owner if the
slab surface is to be placed as near level as possible with a
varying slab thickness, or if it is more important to have a
slab with a uniform thickness, and then design the slab and
framing system accordingly. When placing slabs-on-steel
decking, the contractor is cautioned that deflections of the
structural steel members can vary from those anticipated by
the designer. Achieving a level deflected surface can require
increasing the slab thickness more than 3/8 in. (9.5 mm) in
local areas. Concrete placement procedures, increasing slab
thickness to place level surfaces, and the basis for accep-tance of the levelness of a completed concrete floor surface
should be established and agreed upon by key parties before
beginning suspended floor construction (Tipping 1992). In
many cases, the deflection will not allow a level slab to be
placed.5.3.3 Slabs with removable forms—Cast-in-place concrete
construction can be either post-tensioned or convention-ally reinforced. Both of these systems are supported during
initial concrete placement and will deflect when supporting
shores are -tensioned systems are normally used when larger
spans are necessary or when the structural system is rela-tively shallow for the spans considered. Post-tensioned
systems use high-strength steel tendons that are tensioned
after the concrete hardens using a hydraulic jack designed
for that purpose. The magnitude of floor slab deflection after
supports are removed is less than that of comparable floors
reinforced with conventional deformed reinforcing steel. At
times, dead load deflection is entirely eliminated by the use
of magnitude of deflection in a conventionally reinforced
floor system depends on a number of variables such as span,
depth of structure, age at the time forms are stripped, concrete
strength, and amount of reinforcement. In locations where
the anticipated dead load deflection of a member is deemed
excessive by the designer, an initial camber, generally 1/2
in. (13 mm) or more, can be required. The amount of camber
is determined by the designer based on an assessment of the
loading conditions discussed. Ideally, the cambered floor
system will deflect down to a level position after removal of
the supporting shores.5.3.4 Slabs-on-carton forms—Slabs-on-carton forms are a
special application of slabs with removable forms (Tipping
and North 1998). These slabs are necessary when slabs at
ground level should remain independent of soil movement.
Slabs-on-carton forms are most commonly used when soils
at the building site are expansive clays subject to significant
movement as a result of moisture variation. They provide
a more economical construction solution than conventional
American Concrete Institute – Copyrighted © Material –
12 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)framing systems, which require a crawl space to remove
forms. The cardboard carton forms deteriorate in the months
following construction, eventually leaving the desired void
space below the slab and forcing the slab to span between
supporting foundation ence has shown that certain types of wet cardboard
carton forms can fail locally under the weight of concrete
and construction activities, resulting in a loss of part or all
of the desired void space in the vicinity of the form failure.
This failure can occur immediately or up to 30 or 45 minutes
after strike-off. The latter type of failure, in addition to
reducing desired void space, can result in a loss of local slab
levelness. Forms that have been damaged by rain should be
replaced or allowed to dry thoroughly, with their capacity
verified, before concrete placement.5.3.5 Slabs-on-steel-deck—Construction of slabs-on-steel-deck involves the use of a concrete slab and a supporting
platform consisting of structural steel and steel deck. The
structural steel can be shored or unshored at the time of
concrete placement. The steel deck serves as a stay-in-place form for the concrete slab. This construction can be
composite or supporting steel platform for slabs-on-steel-deck is
seldom level. Variation in elevations at which steel beams
connect to columns and the presence of camber in some floor
members combine to create variations in the initial eleva-tion of steel members. Regardless of the initial levelness of
the steel frame, unshored frames will deflect during concrete
placement. These factors make the use of a laser or similar
instrument impractical for the purpose of establishing a
uniform elevation for strike-off of the concrete surface of
a slab-on-steel-deck, unless the frame is preloaded to allow
deflection to take place before strike-off and slab thickness
is allowed to vary.5.3.6 Composite slabs-on-steel-deck—In composite
construction, the composite section, which consists of the
concrete slab and steel beams, will work together to support
any loads placed on the floor surface after the concrete
has hardened. Composite behavior is normally developed
through the use of shear connectors welded to the struc-tural steel beam. These shear connectors physically connect
the concrete slab to the beam and engage the concrete slab
within a few feet of the steel beam with a resulting load-carrying element that is configured much like a capital T.
The steel beam forms the stem of the T, and the floor slab
forms the crossbar. Construction joints that are parallel to
structural steel beams should be located far enough away
to eliminate their impact on composite behavior. Questions
about the location of construction joints should be referred
to the project designer (Ryan 1997).Unshored composite construction is the more common
method used by designers because it is less expensive than
shored construction. In unshored construction, the struc-tural steel beams are sometimes cambered slightly during
the fabrication process. This camber is intended to offset
the anticipated deflection of that member under the weight
of concrete. Ideally, after concrete has been placed and the
system has deflected, the resulting floor surface will be level
(Tipping 2002).Shored composite concrete slabs-on-steel-deck are similar
to slabs with removable forms; both are supported until the
concrete has been placed and reaches the required strength.
Structural steel floor framing members for shored composite
slabs-on-steel deck are usually lighter and have less camber
than those used for unshored construction with similar
column spacings and floor loadings. One major concern with
shored composite construction is the tendency for cracks
wider than 1/8 in. (3 mm) to form in the concrete slab when
the supporting shores are removed. These cracks do not
normally impair the structural capacity of the floor but can
become an aesthetic problem. The contractor is cautioned
that this issue and any measures taken by the designer to
avoid the formation of this type of crack should be addressed
to the satisfaction of key parties before beginning suspended
floor construction.5.3.7 Noncomposite slabs-on-steel-deck—In noncom-posite construction, the slab and supporting structural steel
work independently to support loads imposed after hard-ening of the concrete slab.5.3.8 Topping slabs-on-precast-concrete—A cast-in-place concrete topping on precast/prestressed concrete units
involves the use of precast elements as a combination form
and load-carrying element for the floor system. The cast-in-place portion of the system consists of a topping of some
specified thickness placed on top of the precast units. The
topping can be composite or noncomposite. In either case,
added deflection of precast units under the weight of the
topping slab is normally minor, so the finished surface will
tend to follow the surface topography established by the
supporting precast units. The camber in precast members,
if they are prestressed, can change with time as a result of
concrete creep. Depending on the length of time between
casting of precast units and erection, this potential variation
in camber of similar members can create significant chal-lenges for the contractor. Care should be taken scheduling
such operations to minimize the potential impact of these
variations. The potential for significant variations in the
preplacement camber of supporting precast members and
the relative lack of options associated with field changes
or modifications create a unique challenge to the contractor
when the desire is to produce a surface that is uniformly
level or uniformly sloped. It is imperative that preplace-ment discussions be held concerning anticipated deflec-tion, minimum required topping thickness, and allowable
increase in topping thickness beyond that required by the
drawings that may be necessary to achieve the desired
product. A preplacement survey of the erected precast is a
critical component of those discussions.5.3.9 Reinforcement—For cast-in-place concrete
suspended slabs, reinforcing steel location varies as dictated
by the construction documents. The reinforcement amount
as a minimum should meet the ACI 318 building code
requirements. However, for slabs that will have surfaces
exposed to view, the slab designer should discuss with the
owner any additional reinforcement that may be required to
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 13control the crack widths to meet the owner’s crack width
expectations. It should be noted that, for exposed slabs, the
minimum amount of steel required by the building code may
not be sufficient to meet the owner’s expectations for crack
widths (ACI 224R). For suspended slabs supporting vehic-ular traffic such as cars, trucks, and lift trucks, reinforce-ment amounts should be carefully considered to maintain
the crack width sufficiently tight to minimize crack spalling
due to this fatigue-loading condition (ACI 215R). Post-tensioning reinforcement, when used, is enclosed in a plastic
or metal sleeve, unbonded, and tensioned by a hydraulic jack
after the concrete reaches sufficient compressive strength.
Elongation and subsequent anchoring of the ends of post-tensioning tendons result in the transfer of compressive
force to the concrete (Post-Tensioning Institute 1990).5.3.10 For slabs-on-composite-steel-deck, the composite
steel deck provides the positive moment reinforcement for
static gravity loads and, if needed, negative moment rein-forcement can be used. Composite steel deck is not recom-mended as the only reinforcement for use in applications
where the floor is subjected to repeated vehicular traffic
such as lift trucks or similar heavy wheeled traffic (ANSI/SDI C-2011). Temperature and shrinkage reinforcement for
composite steel deck slabs can be provided by deformed
reinforcing steel, welded wire reinforcement, steel fibers,
macrosynthetic fibers, or a combination thereof (ANSI/SDI
C-2011). For noncomposite slabs, the reinforcement is the
same as for cast-in-place suspended slabs with the steel deck
acting as a stay-in-place form (ANSI/SDI NC-2010).5.3.11 Construction joints—The designer should provide
criteria for location of construction joints in suspended
slabs. The following is a general discussion of criteria that
can influence these decisions.5.3.11.1 Slabs on removable forms—Construction joints
can introduce weak vertical planes in an otherwise mono-lithic concrete member, so they should be located where
shear stresses are low. Under most gravity load conditions,
shear stresses in flexural members are low in the middle of
the span. ACI 318 requires that construction joints in floors
be located within the middle third of spans of slabs, beams,
and primary beams. Joints in girders should be offset a
minimum distance of two times the width of any intersecting
beams.5.3.11.2 Composite slabs-on-steel-deck—An important
consideration when deciding on the location of construc-tion joints in composite slabs-on-steel-deck is that the joint
location can influence deflection of the floor framing near
the joint. A composite member, which is the steel beam
and hardened concrete slab working together, is stiffer and,
therefore, deflects less than a noncomposite member, which
is the steel beam acting alone. Most composite slabs-on-steel-deck are placed on an unshored structural steel floor
frame. Often, structural steel members have initial camber
to offset anticipated noncomposite deflection resulting
from concrete placement. After hardening of the concrete,
however, the composite member deflects much less than a
comparable noncomposite beam or primary beam.
The following are general guidelines for deciding
on the location of construction joints in composite
slabs-on-steel-deck:a) Construction joints that parallel secondary structural
steel beams should be placed near the middle third of the
slab between beams.b) Construction joints that parallel primary structural steel
beams and cross secondary structural steel beams should be
placed near the primary beam. The primary structural steel
beam should be excluded from the initial placement. Place
the construction joint far enough away from the primary
beam to allow sufficient distance for development of the
primary beam flange width. Placing the construction joint
a distance of 4 ft (1.2 m) from the primary beam is usually
sufficient for this purpose. This construction joint location
allows nearly the full dead load from concrete placement
to be applied to secondary beams that are included in the
initial concrete placement. The primary beam should gener-ally be included in the second placement at the construction
joint. This will allow the primary beam to deflect completely
before concrete at the primary beam hardens; construction
joints that cross primary structural steel beams should be
placed near a support at one end of the primary beam. This
will allow the beam to deflect completely before concrete at
the beam hardens.5.3.11.3 Noncomposite slabs-on-steel-deck—The place-ment of construction joints in noncomposite slabs-on-steel-deck should follow the same general guidelines discussed
for slabs-on-removable-forms in 5.3.11.1.5.3.11.4 Topping slabs-on-precast-concrete—Construc-tion joints in topping slabs-on-precast-concrete should be
placed over joints in the supporting precast concrete.5.3.12 Cracks in slabs-on-steel-deck—Cracks often
develop in slabs-on-steel-deck. These cracks can result from
loadings, drying shrinkage, thermal contraction, or variations
in flexibility of the supporting structural steel and steel deck.
In a composite floor framing system, primary beams are the
stiffest elements and generally deflect less than secondary
beams. The most flexible part of the floor framing assembly
is the steel deck, which is often designed for strength rather
than for flexibility ion as a result of power floating and power trow-eling operations can produce cracking over the structural
steel beams during concrete finishing operations if the steel
deck is flexible. As the concrete cures and shrinks, these
cracks will open wide if not restrained by reinforcing steel—usually deformed or welded wire reinforcement—located
near the top surface of the slab. Steel and macrosynthetic
fibers have also been effective in controlling crack widths.5.4—Miscellaneous details5.4.1 Heating ducts—Heating ducts embedded in a
concrete slab can be metal, rigid plastic, or wax-impregnated
cardboard. Ducts with waterproof joints are recommended.
When metal ducts are used, calcium chloride should not be
used in the concrete.5.4.2 Edge insulation—Edge insulation for slabs-on-ground is desirable in most heated buildings. The insulation
American Concrete Institute – Copyrighted © Material –
14 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)should be in accordance with ANSI/ASHRAE/IES 90.1. It
should not absorb moisture; be resistant to fungus, rot, and
insect damage; and not be easily tion should preferably be placed vertically on the
inside of the foundation. It can also be placed in an L-shape
configuration adjacent to the inside of the foundation and
under the edge of the slab. If the L-shape configuration is
used, the installation should extend horizontally under the
slab a total distance of 24 in. (600 mm).5.4.3 Radiant heating: piped liquids—Slabs can be heated
by circulating heated liquids through embedded piping.
Ferrous, copper, or plastic pipe is generally used with not
less than 1 in. (25 mm) of concrete under the pipe and 2 to
3 in. (50 to 75 mm) of concrete cover over the pipe. The
slab is usually monolithic and the concrete placed around the
piping, which is fixed in place. Two-course slab construc-tion has also been used, wherein the pipe is laid, connected,
and pressure-tested for tightness on a hardened concrete
base course. Too often, however, the resulting cold joint is a
source of distress during the service ting concrete made with vermiculite or perlite
aggregate or cellular foam concrete can be used as a subfloor.
The piping should not rest directly on this or any other base
material. Supports for piping during concrete placement
should be inorganic and nonabsorbent; precast concrete bar
supports are preferred to random lengths of pipe for use as
supports and spacers. Wood, brick, or fragments of concrete
or concrete masonry should not be g of the slab, where possible, can simplify sloping of
the pipe. Reinforcement, such as welded wire reinforcement,
should be used in the concrete over the piping. Where pipe
passes through a contraction or construction joint, a provi-sion should be made for possible movement across the joint.
The piping should also be protected from possible corrosion
induced by chemicals entering the joint. The piping should
be pressure-tested before placing concrete by air pressure,
but not water pressure, and should be maintained in the pipe
during concrete placement operations. After concreting, the
slab should not be heated until concrete curing is complete.
The building owner should be warned to warm the slabs
gradually using lukewarm liquid in the system to prevent
cracking of the cold concrete.5.4.4 Radiant heating: electrical—In some electrical
radiant heating systems, insulated electrical cables are laid
singly in place within the concrete or fastened together on
transverse straps to form a mat. One system employs cable
fastened to galvanized wire sheets or hardware cloth. The
cables are embedded 1 to 3 in. (25 to 75 mm) below the
concrete surface, depending on their size and operating
temperature. In most systems, the wires, cables, or mats are
laid over a bottom course of unhardened concrete, and the
top course is placed immediately over this assemblage with
little lapse of time to avoid the creation of a horizontal cold
joint (ASHVE 1955).Calcium chloride should not be used where copper wiring
is embedded in the concrete; damage to insulation and
subsequent contact between the exposed wiring and rein-forcing steel will cause corrosion. If admixtures are used,
CHAPTER 6—SITE PREPARATION AND PLACING
ENVIRONMENT6.1—Soil-support system preparationThe soil-support system should be well drained and
provide adequate and uniform load-bearing support. The
ability of a slab to resist loads depends on the integrity of
both the slab and full soil-support system. As a result, it is
essential that the full soil-support system be tested or thor-oughly evaluated before the slab is placed (Ringo 1958).The in-place compaction and moisture content of the
subgrade, the subbase (if used), and the base should meet
the minimum requirements imposed by the specifications,
project-specific geotechnical report, or local building code.
The base should be free of frost before concrete placing
begins and able to support construction traffic such as
concrete trucks (Fig. 6.1).The base should normally be dry at the time of concreting.
If protection from the sun and wind cannot be provided, or if
the concrete is placed in hot, dry conditions, the base should
be lightly dampened with water in advance of concreting.
There should be no free-standing water on the base or muddy
or soft spots when the concrete is day before slab placement, testing should be
conducted to establish the moisture content of the stone base
and the subgrade material supporting the stone base. Exces-sive moisture below the concrete can aggravate differential
top-to-bottom drying of the slab, which enhances curling
potential. Keeping a record of the moisture content could
help explain excessive curling should it develop later.
American Concrete Institute – Copyrighted © Material –
their chloride contents should comply with the limits recom-mended by ACI 222R.5.4.5 Snow melting—Systems for melting snow and ice
can be used in loading platforms or floor areas subjected
to snow and ice. The concrete should be air-entrained for
freezing-and-thawing resistance. Concrete surfaces should
have a slope not less than 1/4 in./ft (20 mm/m) to prevent
puddles from collecting. Piping systems should contain a
suitable liquid heat-transfer medium that does not freeze at
the lowest temperature anticipated. Calcium chloride admix-tures should not be used with snow-melting systems. Expe-rience has shown that these snow-melting piping systems
demand high energy consumption while displaying a high
potential for failure and thermal cracking.5.4.6 Pipe and conduit—Water pipe and electrical conduit,
if embedded in the floor, should have at least 1-1/2 in. (38
mm) of concrete cover on both the top and bottom.5.4.7 Slab embedments in harsh environments—Care
should be exercised in using heating, snow-melting, water, or
electrical systems embedded in slabs exposed to harsh envi-ronments such as parking garages in northern climates and
marine structures. If not properly embedded, systems can
accelerate deterioration by increasing seepage of saltwater
through the slab or by forming electrical corrosion circuits
with reinforcing steel. If concrete deterioration occurs, the
continuity and effective functioning of embedded systems
are invariably disrupted.
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 15Fig. 6.1—Proof rolling by loaded concrete truck.6.1.1 Proof rolling—In addition to compaction and mois-ture field testing results, proof rolling can be one of the most
effective ways to determine if the full soil-support system
is adequate to provide a uniformly stable and adequate
bearing support during and after construction. If applicable,
this process should be implemented after completion of the
rough grading and should be repeated before the placement
of the slab (Fig. 6.1).Proof rolling, observed and evaluated by the designer, the
designer’s representative, or the owner’s geotechnical engi-neer, should be accomplished using a loaded tandem-axle
dump truck, a loaded concrete truck, roller, or equivalent. In
any case, multiple passes should be made using a preestab-lished grid rutting or pumping is evident at any time during prepa-ration of the subgrade, subbase, base rolling, or slab place-ment, corrective action should be taken. Full soil support has
been achieved if the rolled area is observed to be firm and
unyielding, with no depressions greater than 1/2 in. (13 mm).Rutting normally occurs when the surface of the base
or subbase is wet, greater than three percentage points
above optimum moisture content, and the underlying soils
or subgrade are firm. Pumping normally occurs when the
surface of the base or subbase is dry and the underlying soils
are wet. Any depression in the surface deeper than 1/2 in.
(13 mm) should be investigated and repaired. Repairs should
include, but not be limited to, raking smooth or consolidating
with suitable compaction equipment.6.1.2 Subgrade tolerance—Industry practice is to plan and
to execute grading operations so that the final soil elevation
is at the theoretical bottom of the slab-on-ground immedi-ately before commencing concreting operations. Variations
in grading equipment, subgrade material, and construction
methods will result in inevitable local departures from this
theoretical elevation. Studies have shown that these depar-tures commonly result in slabs that vary in thickness as
much as 1-1/2 in. (38 mm) from that shown on the contract
documents (Gustaferro 1989; Gustaferro and Tipping 2000).The designer should explicitly state in the project speci-fications whether the slab thickness is to be the minimum
acceptable or a thickness other than that which would
commonly be expected from industry practice. Further,
the issue of minimum allowable slab-on-ground thickness
should be addressed in the bid documents and in a precon-struction meeting to ensure that all parties are aware of the
designer’s necessary grading of the subgrade, often called rough
grading, is recommended. Grading should conform to a
tolerance of +0 in./–1-1/2 in. (+0 mm/–38 mm). Compli-ance should be confirmed before removal of excavation
equipment. A rod and level survey should be performed by
a surveyor. Measurements should be taken at 20 ft (6 m)
intervals in each of two perpendicular directions.6.1.3 Base tolerance—Base tolerances, often called fine
grading requirements, should conform to ACI 117. Compli-ance with these fine-grade values should be confirmed using
measurements of the fine-graded base within individual
floor sections or placements. A rod and level survey should
be performed; measurements should be taken at 20 ft (6 m)
intervals in each direction. The use of laser-guided equip-ment to fine-grade base materials can significantly reduce
concrete material overruns while minimizing differential
slab thicknesses that can restrain slab movement. After
grading, the material should be recompacted with an appro-priate-size roller.6.1.4 Base material—The use of the proper materials is
essential to achieve the tolerances recommended in 6.1.3
(Suprenant and Malisch 1999a). The base material should
be a compactible, easy-to-trim, granular fill that will remain
stable and support construction traffic. The tire of a loaded
concrete truck mixer should not penetrate the surface more
than 1/2 in. (13 mm) when driven across the base. The use
of so-called cushion sand or clean sand with uniform particle
size, such as concrete sand meeting requirements of ASTM
C33/C33M, will not be adequate. This type of sand will be
difficult, if not impossible, to compact and maintain until
concrete placement is completed.A clean, densely graded granular material with a balanced
fine content that produces a low-friction surface while mini-mizing any wicking of moisture generally provides the best
support system. These densely-graded crushed products
are commonly called crusher-run materials. The following
material properties have proven to be adequate:a) Any local state department of transportation approved
road base material with 100 percent passing the 1-1/2 in.
(38 mm) sieve, 15 percent to 55 percent passing the No. 4
(4.75 mm) sieve and less than 12 percent passing the No.
200 (75 μm) sieveb) Material that satisfies the requirements of ASTM D1241
with the modified allowance of less than 12 percent passing
the No. 200 (75 μm) sievec) Material passing the No. 200 (75 μm) sieve should be
clean granular fill with less than 3 percent clay or friable
particles6.1.5 Vapor barrier/retarder—If a vapor barrier/retarder is
required to reduce the impact of moisture transmission from
below the slab on moisture-sensitive floor finishes, adhe-sives, coatings, equipment, or environments, the decision
whether to locate the material in direct contact with the slab
American Concrete Institute – Copyrighted © Material –
16 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)or beneath a fill course should be made on a case-by-case
basis. Each proposed installation should be independently
evaluated as to the moisture-related sensitivity of subsequent
floor finishes, project conditions, schedule, and the potential
effects of slab curling and a fill course is used over the vapor barrier/retarder,
it should be a minimum of 4 in. (100 mm) of trimmable,
compactible, granular fill (not sand) material in accordance
with 6.1.4. Following compaction, the surface can be choked
off with a fine-grade material to reduce friction between the
base material and the a fill course is used, it should be protected from taking
on additional water from sources such as rain, curing,
cutting, or cleaning adjacent work. Wet fill courses have
been directly linked to a significant lengthening of the time
required for a slab to reach an acceptable level of dryness for
floor covering applications. If a vapor barrier/retarder is to
be placed over a rough granular fill, a thin layer of approxi-mately 1/2 in. (13 mm) of fine-graded material should be
rolled or compacted over the fill before installation of the
vapor barrier/retarder to reduce the possibility of puncture.
Vapor barrier/retarder should be overlapped 6 in. (150 mm),
or per manufacturer’s recommendations, at the joints and
carefully fitted around service openings. Refer to 5.2.3.1
for more information on vapor barrier/retarder for slabs-on-ground (Suprenant and Malisch 1998).6.2—Suspended slabsBefore concrete placement, bottom-of-slab elevation, the
elevation of reinforcing steel, and any embedments should
be confirmed. Forms that are too high result in reinforce-ment, which is supported on uniform-height supports, also
being too high. The result, at best, prevents the slab surface
from being installed at the proper elevation while main-taining proper cover over the reinforcement. At worst, the
supported reinforcement can be above the desired elevation
of the slab. Screed rails or guides should be set at elevations
that will accommodate initial movement of the forms during
concreting. Screed rails may also be set at elevations that
will offset anticipated downward deflection of the structure
following concrete placement.6.3—BulkheadsBulkheads can be wood or metal. They should be placed at
the proper elevation with stakes and necessary support required
to keep the bulkheads straight, vertically aligned, and stable
during the entire placing and finishing procedure. Keyways are
not recommended. If specified, however, small wood or metal
keys should be attached to the inside of the it is necessary to set bulkheads on insulation mate-rial, such as in cold storage or freezer rooms, extra atten-tion should be given to keeping the forms secure during the
placing and finishing process. The insulation material should
not be punctured by stakes or pins. It may be necessary to
place sand bags on top of form supports to ensure stability
during concrete ar or square forms can be used to isolate the columns.
Square forms should be rotated 45 degrees. Walls, foot-
ings, and other elements of the structure should be isolated
from the floors. Asphalt-impregnated sheet or other suitable
preformed compressible joint material should be used. These
joint materials should not be used as freestanding forms
at construction joints or column block-outs but, instead,
installed after the original forms have been removed. After
removal of forms around columns, preformed joint materials
should be placed at the joint to the level of the floor surface
and the intervening area concreted and finished. These
preformed joint materials can be placed at the proper eleva-tion to serve as screed guides during concreting operations.
Preformed joint material should be of the type specified
and conform to ASTM D994/D994M, D1751, or D1752,
depending on the conditions of its use.6.4—Setting screed guidesScreed guides can be 2 in. (50 mm) thick lumber, pieces
of pipe, T-bars, or rails, with tops set to the finished concrete
grade, provided the design elevation of the reinforcing steel
does not change. Each type should have a tight-radius edge.
If the wet-screed approach is used to establish concrete
grade, the finished floor elevation for a slab-on-ground
may be laid out by driving removable grade stakes into the
subgrade at predetermined intervals that are appropriate for
the width of placement strips being installed. The tops of
these stakes should be set to the required concrete grade.6.4.1 Establishing grades for adequate drainage on slab
surface—When positive drainage is desired, the forms
and screed guides should be set to provide for a minimum
slope of 1/4 in./ft (20 mm/m) to minimize ponding. Posi-tive drainage should always be provided for exterior slabs
and can be desirable for some interior slabs. An exception
to these positive drainage recommendations for exterior
slabs is the accessible landing area and route to the building,
where the maximum slope can be limited to 2 percent.6.5—Installation of auxiliary materials6.5.1 Edge insulation—Insulation should preferably be
placed vertically on the inside of the foundation. It can also
be placed in an inverted L-shape configuration adjacent to
the foundation and under the edge of the slab.6.5.2 Heating ducts—Metal, rigid plastic, or wax-impreg-nated cardboard ducts with watertight joints are recom-mended; they can be set on a sand-leveling bed and back-filled with sand to the underside of the slab. Take precaution
to ensure that the position of the ducts is not disturbed during
concreting and that they are adequately protected from
corrosion or deterioration. If the ducts to be used are not
waterproof, they should be completely encased in at least 2
in. (50 mm) of concrete to reduce the potential for moisture
infiltration.6.6—Concrete placement conditionsWhen slabs are placed on ground, there should be no more
than 30°F (17°C) or, ideally, 20°F (11°C) difference between
the base or the ambient temperature at the area being poured,
whichever is lower, and concrete at the time of placement.
For trowel-finished slabs, the lower base or ambient temper-
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 17atures should be increased, as opposed to lowering concrete
material temperatures to minimize delayed concrete set slab installations should be undertaken in a
controlled environment where possible. Protection from the
sun and wind is crucial to the placing and finishing process.
The roof of the structure should be waterproof, and the walls
should be in place. The site should provide easy access for
concrete trucks and other necessary materials and suppliers.
The site should have adequate light and ventilation. Temper-atures inside the building should be maintained above 50°F
(10°C) during placing, finishing, and curing the concrete. If
heaters are required, they should be vented to the outside
(Kauer and Freeman 1955). Open-flame heaters that might
cause carbonation of the concrete surface should not be used.
When installation procedures are carried out each day under
the same conditions, the resulting floors are significantly
superior to those floors installed under varying or poor envi-ronmental conditions. Also, refer to 11.5.1 and 11.5.2 for
cold- and hot-weather considerations, humidity levels of any enclosed area should be checked
and rechecked hourly during concrete placement until start of
curing. If the humidity in the area falls below 40 percent in a
heated environment, early concrete moisture loss can exacer-bate surface drying, increasing potential for crusting, crazing,
mortar flaking, and plastic shrinkage R 7—ASSOCIATED MATERIALS7.1—IntroductionMany diverse materials are associated with slab-on-ground
construction. Several of these materials are discussed in this
chapter. Materials relating to concrete mixture designs are
found in Chapter 8.7.2—Reinforcement7.2.1 Reinforcing steel, mats, or welded wire reinforce-ment—Deformed bars, bar mats, or welded wire reinforce-ment are usually required in suspended structural floors
as part of the structural design. They can also be specified
for slabs-on-ground. Deformed bars should conform to the
requirements of ASTM A615/A615M or A996/A996M.
Bar mats conforming to ASTM A184/A184M can also be
used. Welded wire reinforcement should conform to ASTM
A1064/A1064M. The use of widely-spaced deformed rein-forcing fabric conforming to ASTM A1064/A1064M will
typically permit easier placement. The reinforcement steel,
mats, or welded wire reinforcement should be securely
located in the position required by the designer or its effec-tiveness could be reduced.7.2.2 Post-tensioning—Post-tensioning can be used in
slabs-on-ground and suspended slabs to address specific
design requirements. Prestressing steel for use in floors and
slabs should conform to the requirements of ASTM A416/A416M. The post-tensioning tendons can be bonded or
unbonded. Unbonded tendons should meet or exceed speci-fications by the Post-Tensioning Institute (2000).7.2.3 Synthetic fibers—Synthetic fibers for use in concrete
floors increase the cohesiveness of concrete and should meet
the requirements outlined in ASTM C1116/C1116M. The
most widely used synthetic fibers are polypropylene and
nylon, although other types are available. Polypropylene
fibers are available in both fibrillated and monofilament
form; nylon fibers are only available in monofilament tic microfibers, defined as a fiber with an equiva-lent diameter less than 0.012 in. (0.3 mm), are added to the
concrete mixer in quantities generally less than 0.2 percent by
volume of concrete. They are generally used in floors and slabs
in quantities of from 0.75 to 3.0 lb/yd3 (0.45 to 1.80 kg/m3).
Synthetic microfibers have a tendency to reduce the formation
of plastic shrinkage and settlement cracks at the surface by
increasing the tensile strain capacity of the plastic concrete.
Synthetic microfibers should not be used to replace conven-tional temperature and shrinkage reinforcement because they
have little impact on the behavior of concrete after it tic macrofibers are added to concrete mixtures
in quantities generally at the rate of 0.2 to 1.0 percent by
volume of the concrete. For flooring applications, typical
dosages can range from 3.0 to 15 lb/yd3 (1.8 to 12 kg/m3).
Synthetic macrofibers can reduce plastic cracking and drying
shrinkage cracking when used in a low shrinkage mixture.7.2.4 Steel fibers—Steel fibers for use in floors and slabs
should conform to the requirements of ASTM A820/A820M.
Steel fibers made from wire, slit sheet, milled steel, and melt
extract are available and are normally deformed or bent to
improve bond to the hardened matrix. Steel fibers are added
to the concrete mixer in quantities ranging from 0.0625 to 1
percent by volume of the concrete (8 to 132 lb/yd3 [5 to 78
kg/m3]), although quantities from 0.25 to 0.50 percent by
volume of the concrete (34 to 68 lb/yd3 [20 to 40 kg/m3]) are
more common (ACI 360R; ACI 544.1R).Steel fibers are used in floors to minimize visible cracking
and increase shear strength, flexural fatigue endurance,
impact resistance, and post-cracking flexural toughness.
The increases in mechanical properties achieved depend
primarily on the type and amount of fiber used, and can allow
a reduction in floor thickness and an increase in contraction
joint spacing (Tatnall and Kuitenbrouwer 1992).7.2.5 Fiber characteristics—Crack reduction, material
properties, and mixture proportions are thoroughly discussed
by Balaguru and Shah (1992). Additional information is
available in ACI 544.1R, 544.2R, 544.3R, and 544.4R.
Placing, finishing, and saw-cutting characteristics of the
fibers should be evaluated and accepted by the owner in a
test slab placement. Higher aspect ratio (ratio of fiber length
to diameter) fibers generally perform better in lab testing but
may not efficiently produce a desirable finished product.7.2.6 Dowels and load-transfer devices—Dowels installed
for load transfer can be round, square, or plates. Various
square bars and plate dowels provide a means for horizontal
movement and a more efficient use of steel for load transfer.
Plates can be used to replace dowels in both construction and
contraction joints. All materials used for dowels and load-transfer devices should meet requirements of ASTM A36/A36M. The diameter or cross-sectional area, length, shape,
and the specific location of load transfer devices and the
method of support should be specified by the designer.
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18 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)7.3—Special-purpose aggregatesDecorative and nondecorative mineral aggregate and
metallic hardeners are used to improve the properties of
the slab surface. These materials, applied as dry shakes on
top of the concrete, are floated and troweled into the plastic
floor surface to improve the abrasion or impact resistance, to
achieve nonslip surfaces, or to obtain a decorative finish. In
this guide, the term “dry-shake” is applied to premixed mate-rials, which may be mineral aggregate, metallic, or colored.
The term “embedded” is a more generic term used where the
material can be furnished in either premixed or bulk form.
Trap rock and emery are two examples of materials that can
be furnished in bulk form. These bulk materials should be
blended with locally available portland cement and should
meet the requirements of ASTM C150/C150M or C1157/C1157M before being introduced to the concrete surface.7.3.1 Wear-resistant aggregates—Hard, wear-resistant
aggregates such as quartz, emery, and traprock, as well as
malleable metallic hardeners, are frequently used as surface
treatments (ASTM 1994). They are applied as dry shakes
and finished into the plastic surface of the floor to improve
its abrasion and wear resistance. The use of aggregate
susceptible to alkali-aggregate reaction should be allic surface hardeners should be used on floors
subjected to heavy frequent forklift or hard-wheeled traffic
(Table 4.1). Metallic hardeners in sufficient quantity should
be considered for use when heavy steel wheel or intense point
impact loading is anticipated. Chloride-bearing admixtures
should not be used in conjunction with a metallic floor l aggregate and metallic surface hardeners are
factory premixed with specially selected portland cement
and plasticizers. Some mineral aggregates can be supplied
in bulk and mixed with cement on site. These aggregates, in
properly graded sizes, can also be used in topping mixtures.7.3.2 Surface treatment for electrically conductive
floors—Concrete floors can be made electrically conduc-tive by using specially prepared metallic hardeners lik dry
shakes. Electrically conductive floors are also required to
be spark-resistant under abrasion or impact. For protection
against abrasion sparks, care should be taken in the choice
of aggregates. Because construction techniques for these
floors are rather specialized, specific recommendations of
the product manufacturer and designer should be followed
(Boone et al. 1958).The electrical resistance of such floors can be determined
by reference to the appropriate specification of the Naval
Facilities Engineering Command (NFEC 1984). A typical
test for spark resistance under abrasion or impact is given
in the aforementioned specification and NFPA 99. A factory
premixed metallic surface hardener containing a conduc-tive binder is commonly used for these floors. This hardener
is floated and troweled into the surface of freshly placed
concrete. Special conductive curing compounds should be
used to cure these floors. Conductive floors should not be
used in areas expected to be continuously moist.7.3.3 Slip-resistant aggregates—Slip-resistant aggre-gates should be hard and nonpolishing. Fine aggregates are
usually emery or a manufactured abrasive. The slip resis-
tance of some aggregates can be improved by replacing the
fines with those of a more slip-resistant aggregate.7.3.4 Decorative aggregates—Decorative aggregates
can be of many minerals and colors. They should consist
of sound, clean, nonreactive, and of consistent quality.
The most common are quartz, marble, granite, and some
ceramics. Rocks, shells, brass turnings or other brass pieces,
and ball bearings have also been used. Shapes resembling
spheres and cubes are preferable to flat or highly irregularly
shaped pieces, which can become dislodged easily. Use
aggregates of only one sieve size when possible.7.4—Monomolecular filmsEvaporation-retarding chemicals, or monomolecular
films, can be sprayed on the plastic concrete one or more
times during the finishing operation to reduce the potential
for plastic shrinkage cracking. When used in rapid drying
conditions, including high concrete or ambient temperatures,
low humidity, high winds, direct sunlight or work in heated
interiors during cold weather, these liquid films temporarily
seal the surface to reduce moisture loss. These products
should be used in strict accordance with the manufacturers’
directions. They are not finishing enhancement materials
intended to be worked into the concrete paste and should not
be used during the final troweling operations because they
discolor the concrete surface.7.5—Curing materialsACI 308R lists many coverings and membrane-forming
liquids that are acceptable for curing concrete floors. Char-acteristics of curing materials suitable for flatwork are
discussed herein. Refer to ACI 302.2R for guidance where
floors are to receive moisture-sensitive floor coverings.7.5.1 Curing covers—Both reusable and single-use covers
or blankets in various forms are available for curing concrete
slabs. Curing cover materials should conform to ASTM
C171. The material shall exhibit a water vapor transmis-sion rate of no more than 10 g/m2 in 24 hours when tested
according to ASTM E96/E96M using the water method in
the environment (test cabinet) specified in ASTM C156.
When water is used in the curing procedure, the difference
in temperature between the water and the concrete surface
should not exceed 20°F (11°C) to reduce the potential for
surface crazing and cracking.7.5.2 Reusable covers—Most reusable covers available
are composed of an inorganic nonwoven fiber backing and a
plastic coating. On large projects, multiple uses make these
blankets more cost-effective.7.5.3 Single-use covers—Single-use covers can be used
for slabs where appearance is important to the client. The
cellulose, polyester, polypropylene, or rayon fiber backing
absorbs and wicks water evenly across the slabs surface
producing a uniform moisture seal. The plastic coating
retains the moisture.7.5.4 Plastic film or waterproof paper—Polyethylene
plastic film or waterproof paper should meet the requirements
of ASTM C171. These products should not be used on colored
floors or where the client requires a consistent appearance.
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 197.5.5 Wet burlap or combination polyethylene/burlap
sheets—If kept continually moist, burlap is an effective
material for curing concrete surfaces. Old burlap from which
the sizing has disappeared or has been removed is easier to
wet than new burlap. Care should be taken to assure the
burlap used does not stain the concrete or come from sacks
that once contained sugar; sugar retards the hardening of
concrete and its presence could result in a soft surface. The
requirements for burlap are described in AASHTO M182.
White polyethylene-coated burlap is available; the polyeth-ylene is helpful in keeping the burlap moist longer, but it
makes rewetting more difficult. Refer to ASTM C171.7.5.6 Membrane-forming curing compounds—Liquid
membrane-forming curing compounds should meet the
provisions of ASTM C309, which describes the require-ments for both clear and pigmented types. White or gray
compounds are used for their good light reflection. Colored
curing compounds are available for colored concrete. Dissi-pating or strippable resin-based materials can be used on
slabs receiving applied finishes or subsequent liquid surface
treatments providing that all traces of the material are physi-cally removed from the concrete surface before application
of the treatment. ASTM C309 allows moisture loss of 1.8 oz/ft2 (0.55 kg/m2) in 72 hours at a curing compound coverage
of 200 ft2/gal. (5.0 m2/L) when applied in compliance with
ASTM C156. Special conductive curing compounds should
be used to cure electrically-conductive and spark-resistant
floors. Some curing compounds are not compatible with the
installation or application of future floor floors designed for high wear resistance and optimum
top surface strength development, it is desirable to use
curing compounds that offer high water retention. When a
mineral aggregate or metallic surface hardener is used, the
curing procedure and specific product used for curing should
be approved by the manufacturer of the hardener. A high-solids-type curing compound can limit maximum moisture
loss to 0.008 lb/ft2 (0.04 kg/m2) at a coverage rate of 300
ft2/gal. (7.50 m2/L), which is less than 50 percent of that
allowed by ASTM C309 and ACI stringent criteria can be appropriate for some
projects. Manufacturers’ written instructions should be
followed for both the number of coats and the coverage
rate needed to meet the appropriate ASTM or project
requirements. Periodic field testing to evaluate actual
performance is a mineral aggregate or metallic surface hardener is
used, the curing method should be compatible with recom-mendations of the hardener manufacturer.7.6—Gloss-imparting waxesConcrete waxes to impart gloss to concrete surfaces are
available from various manufacturers. Curing compounds
should meet or exceed the water-retention requirements of
ASTM C309.7.7—Liquid surface treatmentsFloor slabs can have relatively pervious and soft surfaces
that wear or dust rapidly. Though the life of such surfaces
can be short, it can be extended by using surface treatments
containing certain chemicals, including sodium silicate and
magnesium fluosilicate (Smith 1956; Bhatty and Greening
1978). When these compounds penetrate the floor surface,
they react chemically with calcium hydroxide, a product of
cement broad classification covers a diverse group of prod-ucts that are intended to penetrate the concrete and form a
hard, glassy substance, which is also known as calcium sili-cate, within the pores of the concrete. Effective use of the
products generally results in reduced dusting and improved
density of the concrete surface. Depth of penetration into
concrete by these products varies with the porosity of the
concrete surface and concrete moisture content at the time
of application. Also refer to Chapter 11 for the purpose,
methods, and length of ts in this group are not specifically formulated for
curing applications and do not meet the requirements of
either ASTM C309 or ASTM C1315 for liquid membrane-forming compounds. Whereas their use may offer some
desirable benefits when applied after curing, they should not
be applied on fresh concrete for the purpose of these surface treatments are to be applied to new
concrete floors, the floor should be moist cured for at least 7
days and allowed to air dry in accordance with the product
manufacturer’s recommendations before application. Liquid
membrane-forming curing compounds should generally be
removed before application of surface treatments because
they prevent penetration of the liquid. The lone exception
to this requirement would be when compatible curing and
sealing products from a single manufacturer are anic materials, including fly ash and silica fume,
are siliceous or siliceous and aluminous materials that
react with calcium hydroxide to form hydration products
similar to those produced by portland cement and water,
which, in turn, can contribute to the strength development
and a reduced permeability of the concrete. This is the same
calcium hydroxide needed to react with the liquid surface
treatment that eventually hardens at the floor slab surface.
Therefore, less calcium hydroxide may be available to the
floor treatment for concrete mixtures incorporating pozzo-lanic materials, slag cement, or a liquid surface treatment is applied to concrete
mixtures incorporating pozzolanic materials or slag cement,
the application should be delayed 28 days to ensure that the
strength of the concrete has developed adequately before the
application of the surface quantity of pozzolanic material should be kept to a
maximum of 15 percent by mass of portland cement. If the
percentage is not limited, calcium hydroxide may be depleted
below the level necessary for the proper performance of the
liquid surface treatment. Contact the manufacturer of the
surface treatment for limits specific to the product to be view of the aforementioned, a number of specific items
should be considered when liquid surface treatments are to be
used. Specific considerations include, but are not limited to:a) Anticipated application conditionsb) Timing of application for maximum benefit
American Concrete Institute – Copyrighted © Material –
20 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)c) Eventual appearance of the treated surfaced) Resistance of treated surface to wear, dusting, and tire
markse) Coefficient of friction of treated surfacef) Anticipated results if applied on a surface that has
become carbonatedLiquid surface treatments react with materials found
in cement paste but not aggregate; they are not capable of
providing abrasion resistance equal to that obtained by use
of an embedded aggregate-type hardener.7.8—Joint materialsCertain two-component semi-rigid epoxy resins and poly-ureas can be used to fill joints where the joint edges need
support to withstand the action of small, hard-wheeled
traffic. These are the only materials known to the committee
that can provide sufficient shoulder support to the edges of
the concrete and prevent joint breakdown. Two-component
fillers are desirable because their curing is independent
of job-site conditions. Such joint materials should be 100
percent solids and have a minimum Shore A hardness of 80
when measured in accordance with ASTM D2240. Refer to
11.10 for more details on joint filling and med elastomeric sealants are useful for some appli-cations. They should not be used where subjected to the
traffic of small, hard wheels. They can be quickly installed,
require no curing, and, if properly chosen, can maintain a
tight seal in joints that are subject to opening and closing.
Preformed asphalt-impregnated or plain fiber materials
or compressible foam are used in expansion and isolation
joints, depending on the anticipated movement.7.9—Volatile organic compounds (VOCs)Many users and some states require materials to meet
volatile organic compound (VOC) limits. Liquid materials
are of the greatest concern because they are often solvent-based. Certification of compliance with the applicable VOC
limits should be required before the products are used.
Many curing compounds that comply with limits on VOC
are water-based. They should not be permitted to freeze. In
many cases, they cannot be reconstituted after R 8—CONCRETE MATERIALS AND
MIXTURE PROPORTIONING8.1—IntroductionConcrete produced in accordance with ASTM C94/C94M varies and produces concrete with different setting
and finishing characteristics. These standards offer a wide
window of acceptance (Bimel 1993). The specific concrete
mixture, therefore, should be investigated before the prepa-ration of mixture proportions for floors and slabs. The signif-icant characteristics of concrete for slabs-on-ground include
workability at time of placement and strike-off, duration and
uniformity of set, finishability, shrinkage of the hardened
materials, and economies of production and delivery. Mate-rials and proportions should be selected accordingly.
8.2—ConcreteBecause minimizing shrinkage is of prime importance,
special attention should be given to selecting the best possible
concrete mixture proportions. When necessary to determine if
a proposed concrete mixture has other than normal shrinkage
(ACI 209R), the proposed concrete mixture should be
compared with the specified or a reference concrete mixture
using ASTM C157/C157M testing, modifying curing, and
drying of specimens similar to Caltrans Test 530 (Caltrans
1995). It is essential that the concrete used in these tests be
made with the same materials, including admixtures, that will
be used in the actual addition to meeting the specified compressive strength
based on standard laboratory samples, a concrete mixture
proportion for use in a floor slab should also meet the flex-ural-strength requirements and w/cm limits, if specified. The
portland cement content and the content of other cementi-tious products, if used, should be sufficient to permit satis-factory finishability under the anticipated field conditions.
The setting characteristics of the concrete should be predict-able. The concrete should not experience excessive retar-dation, differential set time, or surface crusting difficulties
under the conditions of temperature, wind, sun, and humidity
expected on the project. Some admixture-cement combina-tions can cause these difficulties, particularly when multiple
admixtures are used. Because there is not a generally recog-nized procedure for establishing these performance charac-teristics, placement of a test slab should be as indicated in
8.4.4. Floor concrete requirements differ from those of other
concrete used in the structure. Project requirements should
be reviewed thoroughly before mixture proportioning. If
possible, the concrete contractor should have the opportunity
to review the proposed mixture proportions and to prepare a
sample placement to verify the workability, finishability, and
setting time for the proposed usage.8.3—Concrete propertiesA concrete mixture for floor and slab construction should
incorporate an optimized combination of locally available
materials that will consistently produce plastic concrete
with the required workability and finishability properties
during placement. After the concrete mixture hardens, it will
be required to develop engineering properties that include
surface abrasion, wear resistance, impact resistance, flexural
strength to accommodate anticipated loads, and shrinkage
characteristics that minimize potential cracking from
restrained shrinkage and curling te with good placing and finishing characteristics
that also meets the required engineering properties can best
be achieved by developing an understanding of the concrete
mixture. Concrete mixtures are commonly defined by the
proportions of individual materials that do not identify the
qualities of the blended mixture delivered and placed on
the site. To produce the best results, all parties involved in
the design and construction of the slab should understand
the characteristics of the combined materials in the mixture
delivered to the project.
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 21
In most flatwork, the placeability of the concrete and
finishability of the surface are at least as important as the
abrasion resistance, durability, and strength. The former
qualities will have a significant effect on the integrity of the
top 1/16 or 1/8 in. (1.5 or 3 mm) of the concrete surface.
Unfortunately, placeability and finishability are not easily
measured. There is a tendency for specifiers to emphasize
more easily determined properties, such as slump, w/c, and
compressive parameters being equal, a given concrete’s shrinkage
potential will decrease as its paste content is decreased and its
paste quality is optimized. The quality of the concrete paste
is reflected in the total water content necessary to produce
a workable mixture while maintaining a reasonable w/cm
that minimizes the amount of cementitious materials. The
w/cm specified should not require increases in the cementi-tious material or chemical admixture content, exceeding
what is necessary to achieve proper workability, setting
time, and finishing requirements of the contractor. Accep-tance of a mixture based on compliance with w/cm alone
seldom produces desired results without first minimizing the
total water content, which generally can be accomplished
through adjustments in the combined aggregate size, unifor-mity of distribution, or material source. Therefore, use of the
minimum amount of water necessary to produce the required
slump and workability is steel-trowelled slab-on-ground concrete, a minimum
amount of water is required to produce a workable, finish-able mixture with predictable uniform setting characteristics.
Currently available water-reducing admixtures perform best
when they are mixed with concrete that has enough water to
produce a water-induced slump of 3 to 4 in. (50 to 75 mm)
if no admixture was added. If this water slump is not achiev-able without the admixture, setting time and finishability
can vary when the concrete is subjected to normal varia-tions of ambient temperature and time between batching
and discharge (Harrison 2004). The slump envelope results
in predictable setting times and the required sequence of
finishing particular cementitious materials, aggregates, and
admixtures used can significantly affect the strength, setting
characteristics, workability, finishability, and shrinkage of
the concrete at a given w/cm (Tremper and Spellman 1963;
Kosmatka et al. 2002b). Furthermore, the amount of water
required to produce a given slump depends on the maximum
size of coarse aggregate, the uniformity of the combined
aggregate gradation, particle shape and surface texture of
both fine and coarse aggregates, air content, admixtures
used, and the temperature and humidity at time of place-ment. Using larger maximum-size aggregate and improving
the overall aggregate gradation reduces the mixing-water
optimum quality and content of fine aggregate in
concrete for floors should be related to the slump of the
concrete and the abrasive exposure to which the floor will
be subjected. Concrete should be sufficiently plastic and
cohesive to avoid segregation and excessive bleeding. Less
fine aggregate should be used in concrete with low slump,
which is less than 1 in. (25 mm), because this concrete does
not normally bleed or segregate. Decreased fine aggregate
contents can improve resistance to abrasion if the concrete
exhibits little bleeding and us field experience or laboratory trial batches should
be used to establish the initial proportions of ingredients.
Test placements can then be used to optimize the mixture
proportions. The laboratory trial batches can be omitted
if concrete mixtures have been recently used successfully
under similar t records of gradations of fine and coarse aggre-gates from concrete mixtures should be retained to develop a
combined gradation analysis. This gradation analysis should
evaluate the amount of aggregate retained on each of the
following sieve sizes, which is a percent of total mass: 1-1/2,
1, 3/4, 1/2, and 3/8 in. (38.1, 25, 19, 12.5, and 9.5 mm); No.
4, 8, 16, 30, 50, 100, and 200 (4.75, 2.36, 1.18 mm, and 600,
300, 150, and 75 µm).Trial batch proportions should be in accordance with
ACI 211.1 or 211.2. Adjustments of fine aggregate content,
however, may be necessary to obtain the best workability
(Martin 1983). The amount of the total combined aggregate
passing the No. 8 (2.36 mm) sieve for a uniformly graded
mixture should be between 32 and 42 percent of the total
combined aggregate. This index is called the workability
factor and should be evaluated in relation to the coarseness
factor of the larger aggregate particles, as illustrated later in
this chapter (Shilstone 1990). Adjustments in the fine aggre-gate content directly influence the workability factor.8.4—Recommended concrete mixture8.4.1 Required compressive strength and slump—Two
approaches for selecting mixture proportions are discussed.
Regardless of the approach, the specified compressive
strength fc′ shown in Table 8.4.1a should be used for the
various classes of concrete designer should be consulted as to the strength to be
achieved by the concrete before subjecting the slab to early
construction loads. To obtain this strength quickly, it may
be necessary to use more cementitious materials than the
recommended amounts shown in Table 8.4.1b, or to propor-tion the concrete for a 28-day strength higher than that
shown in Table 8.4.1a. The designer should consider that
the increased concrete strengths achieved through higher
cementitious material or admixture contents may adversely
affect the shrinkage characteristics and stiffness, or modulus
of elasticity, of the concrete. Drying shrinkage potential and
stiffness properties can greatly influence slab curling stresses
and ultimate load capacity of the slab-on-ground (Walker
and Holland 1999). Compressive strength testing should be
used to monitor batched material slump indicated for each floor class shown in Table
8.4.1a is the recommended maximum at the point of place-ment to minimize segregation while providing adequate
workability of the concrete. A one-time job-site slump
adjustment should be permitted as outlined in ASTM C94/C94M. Validation of total water content should be conducted
American Concrete Institute – Copyrighted © Material –
22 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)Table 8.4.1a—Recommended strength and
maximum slump at point of placement for
concrete floorsFloor class*1, 2, and 34, 5, and 67 base7 bonded topping‡8 unbonded topping§9 superflat*†Refer to Table 4.1 for floor class m slump is assumed to be achieved using a Type A water-reducing
admixture.‡§The strength specified will depend on the severity of m aggregate size not greater than one-third the thickness of unbonded
8.4.1b—Recommended cementitious
material contents for concrete floorsNominal maximum-size
aggregate*, in. (mm)1-1/2 (37.5)1 (25)3/4 (19)1/2 (12.5)3/8 (9.5)*†Cementitious material content†, lb/yd3 (kg/m3)470 to 560 (280 to 330)520 to 610 (310 to 360)540 to 630 (320 to 375)590 to 680 (350 to 405)610 to 700 (360 to 415)For normalweight m 400 lb/yd3 (240 kg/m3) portland cement content recommended for trowel
finished slabs-on-ground. Refer to ACI 318 for minimum portland cement require-ments for structural ically at the point of concrete placement, concurrent
with other specified site testing.8.4.2 Required finishability—Concrete for floors should
have other desirable characteristics in addition to strength.
There should be sufficient mortar content to allow the
finisher to completely close the surface and to achieve the
required surface tolerance, hardness, and durability (Martin
1983). The mortar fraction and the volume percentage of all
materials in the mixture, including the cementitious mate-rials, aggregate, water, and air that pass the No. 8 (2.36 mm)
sieve, should be balanced between the desired properties of
both fresh and hardened concrete. During construction, suffi-cient mortar is desirable for pumping, placing, and finishing.
Excess mortar, however, can increase shrinkage charac-teristics. Typically, a mortar fraction of 55 to 57 percent is
sufficient for a 3/4 or 1 in. (19 or 25 mm) maximum-size
aggregate slab-on-ground concrete placed directly from the
concrete truck. Larger aggregates, improved uniform distri-bution of the combined aggregate particle sizes, or both, will
decrease the mortar content needed. Smaller 3/8 to 1/2 in.
(9.5 to 12.5 mm) maximum-size aggregates can increase the
mortar content by as much as 63 percent.8.4.3 Required durability—The procedures for producing
durable concrete outlined in ACI 201.2R apply to floors and
ements based only on durability may yield concrete
compressive strengths much higher than normally required
American Concrete Institute – Copyrighted © Material –
Specified compressive
strength fc′ on 28-day
tests, psi (MPa)3000 (21)3500 (24)3500 (24)5000 (35)4000 (28)4000 (28)Maximum slump at
placement†, in. (mm)5 (125)5 (125)5 (125)3 (75)3 (75)5 (125)for structural concerns. Concrete floors and slabs subjected
to moderate and severe exposures to freezing and thawing, as
defined in ACI 201.2R, should have a w/cm no greater than
0.50. Concrete subjected to deicing chemicals should have a
w/cm no greater than 0.45. Reinforced concrete exposed to
brackish water, seawater, deicing chemicals, or other aggres-sive materials should have a w/cm no greater than ned air is necessary in concrete subjected to deicing
chemicals or to freezing and thawing when moist. Recom-mended air contents for hardened concrete for various
exposure conditions, aggregate types, and maximum-size
aggregates are given in ACI 201.2R. Properly air-entrained
concrete should achieve a compressive strength of 4000 psi
(28 MPa) before being subjected to freezing and thawing in
a moist condition, or to deicing chemicals. The use of any
deicing chemicals is not recommended in the first year of
slab entrainment of the concrete can cause a significant
strength reduction (more so in concrete mixtures of 4000
psi [28 MPa] compressive strength or higher than in lower-strength mixtures). Experience has shown the strength reduc-tion in concrete mixtures with higher cement contents to be
approximately 3 to 5 percent per 1 percent increase in air
content, with a corresponding reduction in abrasion resistance.
Air-entrained concrete should not be hard-trowel finished.8.4.4 Concrete mixture—In addition to meeting struc-tural and drying-shrinkage requirements, concrete for floors
should provide adequate workability and setting characteris-tics necessary to obtain the required finish and floor surface
profile. Total water content can have a major impact on the
bleeding characteristics of the concrete and the potential for
shrinkage, so use of the lowest practical quantity of water in
the concrete mixture is recommended. The amount of water
needed to produce a workable mixture is generally deter-mined by the characteristics of the combined aggregate mate-rials used in the mixture and is not effectively controlled by
specifying w/cm. If w/cm is specified, a w/cm in the range of
0.47 to 0.55 is common for most interior floors of Classes 4
to 9. Floors that are required to be impermeable; are resistant
to freezing and thawing and deicing chemicals; or meet the
requirements of ACI 211.2, 223R, or 318 should conform to
more stringent concrete mixture should be accepted on the basis of
a satisfactory mixture design submittal and a successful test
slab placement if appropriate to the project. The submittal
should include a combined aggregate distribution analysis
derived from current certified reports of gradations of the
individual aggregates. The test placement should determine
if the proposed concrete mixture is capable of producing
a floor of acceptable finish and appearance, and meet the
project a history of finishing properties is unavailable for a
concrete mixture, a test slab should be placed under job
conditions to evaluate the workability, finishability, setting
time, slump loss, and appearance of the concrete proposed
for use. Materials, including all admixtures, equipment, and
personnel proposed for the project, should be used. The test
panel should use at least 16 yd3 (12 m3), be placed at the
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 23specified thickness, and become part of the finished project.
The concrete contractor should review the proposed mixture
proportions and placement of the test slab before the precon-struction meeting. If a pump or conveyor will be used for
the placement of concrete materials, the test slab should be
placed with the same equipment.8.4.5 Consistency and placeability—The maximum slump
recommended for each class of floor is given in Table 8.4.1a.
These slumps should produce concrete of sufficient work-ability to be properly consolidated in the work without exces-sive bleeding or segregation during placing and finishing.
Excessive bleeding and segregation can contribute to poor
performance in concrete floors. If the finished floor is to be
uniform in appearance and grade, successive batches placed
in the floor should have approximately the same slump and
setting characteristics. Workability of a concrete mixture is
not directly proportional to the slump. Properly proportioned
concrete with slumps less than that shown in Table 8.4.1a
can respond very well to vibration and other consolidation
procedures. Increased slump alone does not ensure satisfac-tory workability ended slump values in Table 8.4.1a are for
concrete made with both normalweight and structural light-weight aggregate and assume the use of a normal water-reducing admixture, if required. Slumps in excess of those
shown in the table, not to exceed 6 in. (150 mm), are accept-able when high-range or all-range (polycarboxylate) water-reducing admixtures are used. If structural lightweight-aggregate concrete is placed at slumps higher than that shown
in Table 8.4.1a, however, the coarse lightweight-aggregate
particles could rise to the surface and the concrete could
bleed excessively, especially in cases where the concrete
does not contain an adequate amount of entrained air.8.4.6 Nominal maximum size of coarse aggregate—The
nominal maximum aggregate sizes in Table 8.4.1a are for
normalweight aggregates. The largest practical-size aggre-gate should be used if economically available, and if it will
satisfy the requirements that maximum size not exceed three-fourths of the minimum clear spacing of reinforcing bars or
one-third of the section depth. Structural lightweight aggre-gates generally are not furnished in sizes larger than 3/4 or
1 in. (19 or 25 mm); however, some lightweight aggregates
provide maximum strength with relatively fine gradings.8.4.7 Air content—Air-entraining admixtures should
not be specified or used for concrete to be given a smooth,
dense, hard-troweled finish because blistering or delami-nation could occur. These troublesome finishing problems
can develop any time the total air content is in excess of 3
percent. This is particularly true when embedded hardeners
are applied.8.4.8 Required yield and concrete mixture adjustment to
correct yield—A concrete mixture should be proportioned
to yield a minimum 27 ft3/yd3 (1 m3/m3). The yield of the
mixture proposed by a testing agency or concrete supplier
is the total of the absolute volume of the mixture ingredi-ents plus the anticipated volume of total air that, typically,
is entrapped only. This proposed mixture should have been
tested in accordance with the requirements of ASTM C138/
C138M to determine its density or unit weight and yield,
and the weights of the ingredients subsequently adjusted
as necessary. The concrete mixture should be sampled and
tested one or more times at the job site during placement of
concrete floor for the purpose of confirming yield. These job
site samples should be obtained from a mixer truck chute at
the point of delivery, and the tests performed by a certified
field technician as required by ASTM C94/C94M. Concrete
samples should be obtained after any necessary job site
slump adjustment. The concrete supplier should adjust the
concrete mixture as necessary to produce the proper yield.
Adjustments in concrete mixture proportions should be made
in accordance with the recommendations of ACI 211.1.8.5—AggregatesAggregates should conform to ASTM C33/C33M or
ASTM C330/C330M. Although these ASTM standards set
requirements for source materials, they do not establish
combined gradations requirements for the aggregate used in
concrete floors. Compliance with the aggregate gradations
discussed herein should produce a desirable matrix while
reducing water demand of the concrete mixture and reducing
the amount of cement paste required to coat the aggre-gate (Shilstone 1990). ASTM C33/C33M limits coal and
lignite to no more than 0.5 percent in fine or coarse aggre-gate and limits low specific gravity chert to no more than
5 percent in coarse aggregate. Although the concrete used
may comply with this standard, some popouts are always
possible. Concrete containing as little as 0.2 percent or less
coal, lignite, or low-density deleterious material may not be
acceptable, as this quantity of those products can affect both
the overall durability and appearance of the finished floor.8.5.1 Fine aggregate grading—Although ASTM C33/C33M and C387/C387M are acceptable specifications, Table
8.5.1 contains preferred grading specifications for slabs.
The amount of material passing through the No. 50 and 100
(300 and 150 µm) sieves should be limited as indicated for
heavy-duty floor toppings for Class 7. When fine aggregates
contain minimum percentages of material passing the No.
50 and 100 (300 and 150 µm) sieves, however, the likeli-hood of excessive bleeding is increased and limitations on
water content of the mixture become increasingly important.
Natural sand is preferred to manufactured sand; the grada-tion indicated in Table 8.5.1 will minimize water demand.8.5.2 Coarse aggregate grading—The maximum size
of coarse aggregate should not exceed three-fourths the
minimum clear spacing of the reinforcing bars in structural
floors, or one-third the thickness of non-reinforced slabs. In
general, natural aggregate larger than 1-1/2 in. (38 mm) or
lightweight aggregate larger than 1 in. (25 mm) is not used.
Although the use of large aggregate is generally desired for
lower water demand and shrinkage reduction, it is important
to recognize the overall gradation of all the aggregate. When
aggregate sizes larger than 1 in. (25 mm) are used, the coarse
aggregate can be batched as two sizes to prevent segrega-tion. Drying shrinkage can be minimized by the use of the
largest practical overall combined aggregate particle size.
American Concrete Institute – Copyrighted © Material –
24 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)Table 8.5.1—Preferred grading of fine aggregates
for floorsSieve designationsNormal-weight
aggregate10085 to 10080 to 9050 to 7530 to 5010 to 202 to 5Percent passingHeavy-duty
Lightweight toppings,
aggregateClass 7 floors10010085 to 10095 to 100—65 to 8040 to 8045 to 6530 to 6525 to 4510 to 355 to 155 to 200 to 58.5.3 Aggregate quality—Compliance with ASTM C33/C33M and C330/C330M generally ensures aggregate of
adequate quality, except where chemical attack or abrasion
in Class 7 and 8 floors is severe. Refer to ACI 201.2R for a
more complete discussion of precautions under these condi-tions. The guidelines of ACI 201.2R and ASTM C33/C33M,
including its appendix, should be followed where there is
concern about the possibility of alkali-aggregate reaction.8.5.4 Combined aggregate grading—Similarly, distrib-uted volumes of combined aggregates, amounts retained on
each given sieve size or uniformly graded materials, typi-cally produces the highest plastic and hardened concrete
performance characteristics at the lowest paste contents.
These characteristics include increased strength for a given
cementitious content and reduced and consistent water
demand while maintaining good finish and placing char-acteristics. Often, a third aggregate is required to achieve a
satisfactory combined aggregate gradation (Shilstone 1990).
Mixture proportions should be adjusted whenever individual
aggregate grading varies during the course of the work.8.6—Portland cement8.6.1 Cements for slabs-on-ground—Concrete floors can
incorporate a variety of portland cements that meet ASTM
C150/C150M, C595/C595M, C845/C845M, and C1157/C1157M. Of the four cements used in floors and slabs
described in ASTM C150/C150M, Type I or I/II is the most
common one used when the special properties of another
type are not required. Type II is also for general use, espe-cially when moderate sulfate resistance or moderate heat
of hydration is desired. Type III, if available, is manufac-tured to produce high early-age strength. Type V is used
when high sulfate resistance is required. When the aggregate
to be used on the project is possibly susceptible to alkali-aggregate reaction, the maximum equivalent alkali limits of
ASTM C150/C150M should be specified if supplementary
cementitious materials demonstrated to control alkali-silica
reactivity, or alkali-silica reaction-inhibiting admixtures, are
unavailable. In some regions, there are sands that contain
reactive shale that quickly react with alkali in solution,
resulting in small popouts in the surface. These alkali-silica
reactive popouts can appear as shortly as 48 hours after slab
placement. The popouts can be reduced by using low-alkali
cement; however, finishing practices that allow the alkali to
American Concrete Institute – Copyrighted © Material –
Standard9.5 mm4.75 mm2.36 mm1.18 mm600 μm300 μm150 μmAlternative3/8 . 4No. 8No. 16No. 30No. 50No. 100concentrate at the surface, such as hard troweling at high
temperature, can cause the reaction to occur even when these
cements are used. In these conditions, the concrete surface
should be flushed with vigorously flowing water at a rate
of 1 to 2 gal. (3.8 to 7.6 L) per square foot (square meter)
(Landgren and Hadley 2002). Refer to the appendix of
ASTM C33/C33M for further information.8.6.2 Blended hydraulic cements—Blended hydraulic
cements are produced by intimately and uniformly blending
two or more types of fine materials such as portland cement,
slag cement, fly ash and other pozzolans, hydrated lime, and
preblended cement combinations of these are several recognized classes of blended cements
that conform to ASTM C595/C595M:a) Type IS portland blast-furnace slag cementb) Type IPc) Type I (PM) pozzolan-modified portland cementd) Type S slag cemente) Type I (SM) slag-modified portland cementTypes IP and IS, however, are normally unavailable for
use in general concrete construction. The manufacturers of
these cements should be contacted for information regarding
the specific product and the effect its use will have on setting
time, strength, water demand, and shrinkage of concrete
proposed for the project under anticipated field conditions.
Conformance to the requirements of ASTM C595/C595M
does not impose sufficient restrictions on the cement to be
used. If the 28-day design strength is achieved, but shrinkage
is excessive and retardation significant, the cement may
be unsuitable for the project. ASTM C1157/C1157M is a
performance specification that establishes requirements for
six types of cement mirroring the attributes of ASTM C150/C150M and ASTM C595/C595M cement types.8.6.3 Pozzolans—A number of natural materials, such
as metakaolin, diatomaceous earth, opaline cherts, clays,
shales, volcanic tuffs, and pumicites, are used as pozzolans.
ASTM C618 pozzolans also include fly ash, silica fume,
and slag. When these materials, except silica fume, are used
in concrete, the time of set may be extended. The concrete
color can be different from that produced if portland cement
is the only cementitious C618 fly ash, Class F or Class C, is frequently
added to concrete. Fly ash can affect the setting time and is
often helpful in hot weather by delaying set time or as an aid
in pumping concrete (ACI 232.2R). In floors and slabs, fly
ash is often substituted for portland cement in quantities up
to approximately 20 percent fly ash by mass of cementitious
materials. The addition of fly ash can also result in a water-reduction effect, as the specific surface of the fly ash is less
than that of cement of the same cool weather, the addition of fly ash without other
adjustments of the mixture will usually delay the setting and
finishing of the concrete and can be added to delay bleeding
and crusting. Measures that may be taken to compensate for
the slower rate of reaction include increasing the concrete
temperature, using an accelerator, or adjusting the w/c in
consideration of the increased workability of the concrete
resulting from fly ash use.
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 25Care should be taken in using fly ash; the classification
system found in ASTM C618 is not descriptive of the prop-erties of the fly ash as a pozzolanic material. If possible, the
fly ash should come from one source, and stricter limits than
are required by ASTM C618 should be placed on the vari-ability allowed in the physical and chemical requirements
of the fly ash. When using a chemical bond breaker on slabs
that will later be a casting bed for tilt-up wall construction,
review the manufacturers’ specifications. Some chemical
bond breakers are sensitive to certain percentages of flash in
the mixture and will not perform properly at normal recom-mended fume is used as a portland cement replacement
or as a cementitious addition when using an accelerator
to compensate for low temperatures. The amount of silica
fume in a mixture typically varies between 5 and 10 percent
by mass of the total cementitious material. Silica fume can
both decrease the permeability and increase the compres-sive strength. Special attention should be given to avoiding
plastic shrinkage cracking during placing and finishing by
using evaporation-retardant chemicals sprayed onto the
plastic concrete surface, or by using fog sprays in the air
above the concrete. Early and thorough curing of the slab is
essential to minimizing cracking.8.6.4 Expansive cements—Types K, M, and S are expan-sive cements meeting ASTM C845/C845M specifications
that are used in shrinkage-compensating concrete floors.
Refer to ACI 223R for specific details on shrinkage-compen-sating concrete floors. Shrinkage-compensating concrete can
also be made by adding an expansive component. When a
component is used, it is essential that the component manu-facturers work with the concrete producer and testing labo-ratory to determine the rate and level of expansion that can
be expected under anticipated job conditions.8.7—WaterA good guideline for mixing water is to use potable water.
Often, nonpotable water can be used. Water that is not
potable may be used if testing shows 7- and 28-day strengths
of 2 in. (50 mm) mortar cubes made with it are equal to at
least 90 percent of the strengths of cubes made from similar
mixtures using distilled water and tested in accordance
with ASTM C109/C109M and ACI 318. ACI 301 discusses
mixing water, as do others (Kosmatka et al. 2002a). Also
refer to ASTM C1602/C1602M.8.8—AdmixturesAdmixtures should be used when they will effect a specific
desired change in the properties of the freshly mixed or
hardened concrete. They should be used in accordance with
the instruction and principles given in ACI 212.3R and the
guidelines for chloride limits given in 8.8.3. If more than one
type of admixture is used in the same concrete, each should
be batched separately. A second admixture can significantly
affect the required dosage of both admixtures; preliminary
tests are recommended to ensure compatibility. Sample slabs
made under the anticipated job conditions of temperature and
humidity can also be used to help evaluate admixture perfor-
mance and allow necessary adjustments affecting workability,
finishability, and setting time before the start of the slab instal-lation. Some admixtures are not compatible with shrinkage-compensating concrete because they adversely affect expan-sion, bond to steel, and shrinkage (ACI 223R).8.8.1 Air-entraining admixtures—Concrete for use in
areas that will be exposed to freezing temperatures while
saturated should contain entrained air. Slabs in contact with
soil or placed outside should be assumed to be saturated.
Entrained air is not recommended for concrete to be given a
smooth, dense, hard-troweled finish because blistering and
delamination may occur (Suprenant and Malisch 1999a).
Air-entraining admixtures, when used in the concrete as
recommended in this chapter, should meet the requirements
of ASTM C260/tent control of air entrainment is necessary. In most
cases, concrete for trowel-finished interior concrete floors
made with normalweight aggregates should not include an
air-entraining admixture; the maximum total air content for
this concrete should normally not exceed 3 percent at the
point of placement. Air contents in excess of 3 percent make
the surface difficult to finish and can lead to surface blis-tering and peeling during finishing. Troweled concrete with
intentionally added air will typically not retain the proper
bubble size at the surface required to provide scale resis-tance and freezing-and-thawing durability for most applica-tions. Troweling can also reduce the ability of the concrete
mortar at the surface to have adequate protection for resis-tance to freezing and committee recommends that the total air content of the
concrete initially delivered to the job site be tested at the point
of placement for air in accordance with ASTM C173/C173M
or C231/C231M. Air content can be checked or verified by the
use of unit weight testing, in accordance with ASTM C138/C138M. Some combinations of admixtures have been shown
to introduce, or increase, entrained air in concrete.8.8.2 Chemical admixtures—Chemical admixtures should
meet the requirements of ASTM C494/C494M for the
following:a) Type A—water-reducingb) Type B—retardingc) Type C—acceleratingd) Type D—water-reducing and retardinge) Type E—water-reducing and acceleratingf) Type F—high-range water-reducingg) Type G—high-range water-reducing and retardingThe high-range water reducers (Types F and G) should
also meet the requirements of ASTM C1017/C1017M.
Water-reducing and combination admixtures should provide
the additional advantage of increased compressive and
flexural strength at ages less than 6 months. The retarding
admixtures can be useful in delaying initial set and possibly
extending time available for final finishing in hot weather;
however, excessive retardation can cause surface crusting or
plastic shrinkage cracking. Accelerating admixtures increase
the rate of strength gain at early ages and can be useful in cold
weather. High-range water-reducing admixtures meeting
ASTM C494/C494M, Types F or G, and mid-range water-
American Concrete Institute – Copyrighted © Material –
26 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)reducing admixtures meeting ASTM C494/C494M, Type A,
can be used to either reduce the water content required for a
given slump or increase the slump of a given concrete while
maintaining the same total water content. Water-reducing and
high-range water-reducing admixtures used in industrial floor
construction are most effective when the initial slump of the
concrete, before introducing admixtures, is between 2 and 3 in.
(50 and 75 mm). The admixture’s impact on the workability
and setting characteristics of the concrete for floor construc-tion appear to be optimized when they are used in this use of mid- or high-range water-reducing admix-tures will not necessarily reduce the total water content of a
concrete mixture as compared with that required for a Type
A low-range water-reducing admixture. Although these
products have the capacity to reduce water content to a level
below that which would correspond to pre-admixture slumps
of 1 in. (25 mm) or less, the water content should not be
reduced to less than that which would produce a minimum
slump of 3 to 4 in. (75 to 100 mm). Water-reducing admix-tures should not be relied on to reduce concrete g and Dziedzic (1992) suggest that certain water-reducing admixtures can increase concrete shrinkage. If the
goal is to reduce drying shrinkage, the effect of the admix-ture can be measured during development of the concrete
mixture design (Caltrans 1995). The test can be run on the
actual aggregate gradation being considered for the project.
In addition, a reduction in the total water content of the
concrete mixture may be possible by improving the charac-teristics of aggregate used to produce the concrete. Careful
selection of characteristics, such as density, particle shape
and texture, maximum size of the aggregate, and combined
aggregate grading have a profound impact on reducing total
water content, cementitious paste content, and long-term
shrinkage. Considerations influencing a reduction in cement
content should include the amount necessary to properly cut,
trim, finish, and compact the floor slab surface. Reducing the
total cement content could provide more than proportional
reduction in shrinkage, as could the inclusion of certain
pozzolans (Kosmatka et al. 2002b).Although an initial slump of 2 to 4 in. (50 to 100 mm)
is often recommended before the introduction of water-reducing admixtures, design water slump can be increased
to 3 to 4 in. (75 to 100 mm) for lightweight concrete or when
an embedded aggregate type hardener will be applied. When
using a high-range water-reducing admixture, the target
slump at the point of placement can be increased to 6 to 8
in. (150 to 200 mm) without increasing the water content of
the original concrete mixture. A mid-range water-reducing
admixture can be used to increase the target slump at the
point of placement to 4 to 6 in. (100 to 150 mm) without
increasing the water -slump concrete can require less effort to place,
consolidate, and finish compared with lower-slump concrete.
If high slumps are used, excessive internal and external
vibration can promote segregation of the concrete, excessive
fines at the surface, or both, resulting in reduced abrasion
resistance, especially for non-optimized combined aggre-gate grading. Use caution to avoid beginning or continuing
the finishing process before the concrete has achieved a
sufficient degree of stiffness to support the type of finishing
process and equipment to be used. A representative test slab
can be cast at the job site so that the workability, finishability,
and setting time of the proposed mixture can be evaluated by
the project team (ACI 212.3R).8.8.3 Accelerating admixtures—Timely, uniform set
of concrete materials is crucial to producing a flat, evenly
burnished trowel-finished slab with nonraveled saw-cut
joints. Colder ambient or concrete material temperatures will
delay setting and associated finishing operations. With lower
portland cement contents, sometimes reduced when replaced
with fly ash or other alternative cementitious materials, the
setting time of the concrete can also be retarded. Set-accel-erating admixtures can be used to compensate for the set-retarding effects of temperature or mixture many years, the most efficient and cost-effective set
accelerator of concrete has been calcium chloride. Due to
potential and often misunderstood side effects associated
with use of chloride admixtures, they should be approved
before use. While accelerating setting, these admixtures can
also accelerate the rate of early-age concrete strength devel-opment, and may reduce the later-age strengths. Calcium
chloride is not intended for use as an antifreeze to lower
the temperature at which the concrete will freeze. Calcium
chloride is used to accelerate early-age reactions related
to the setting of concrete and early-age strength gain, and
thereby decreases the length of time during which protection
against freezing should be provided. The accelerated hydra-tion continues beyond setting, allowing more rapid finishing
than would occur without used, calcium chloride should be added as a water
solution Type L (ASTM D98) in amounts of not more than 2
percent by weight of cement or less as defined by specifica-tions or other governing documents. Limits on chloride ion
content in fresh concrete mixtures should comply with ACI
222R. Calcium chloride should not be dispensed dry from
bags, because dry-flake material frequently rapidly absorbs
moisture and becomes lumpy. Pellet-type calcium chloride
should be completely dissolved before addition to concrete
to avoid surface popouts associated with undissolved m chloride may cause color variations, typically
darkening or mottling the hardened concrete surface. If
concrete containing calcium chloride is not adequately
cured, the surface can show light and dark spots. A reduction
in bleed water and a slight increase in crazing may be notice-able, depending on the placing addition of chlorides can promote corrosion in rein-forced concrete. The following concrete may require speci-fied chloride limits or restrictions for intentionally added
calcium chloride:a) Prestressed concreteb) Floors over prestressed concrete or galvanized deckc) Floors containing two kinds of embedded metalsd) Conventionally reinforced concrete in a moist environ-ment and exposed to deicing salts or saltwater miste) Parking garage floors in areas where freezing and
thawing should be considered
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 27f) Structures near bodies of saltwaterg) Floors or slabs containing snow-melting electrical
radiant heating systemsh) Floors finished with metallic dry shakesProprietary, nonchloride accelerators consisting of
sulfates, formates, nitrates, and triethanolamine afford users
noncorrosive alternatives for reinforced concrete. These
products are typically less effective at accelerating concrete
set and are more expensive than calcium chloride. The
admixture manufacturer should be able to provide long-term
data for at least a year’s duration to demonstrate noncorro-sivity using an acceptable accelerated corrosion test method,
such as one using electrical potential measurements. Data
from an independent laboratory are accelerated set or high early strength is desired, many
concrete producers increase the portland cement content of
the mixture. A significant decrease in setting time may not
be realized with the increased cement content alone at fixed
w/c. Increasing paste volume and water content can enhance
shrinkage and curling, possibly requiring changes in design
and construction detailing to accommodate increased slab
volume changes. Research has shown that both chloride
and nonchloride accelerators can also increase early-age
shrinkage of concrete materials (Flood 2005).Heated concrete may be required for cold-weather
construction (ACI 306R; ACI 306.1). Chloride-based accel-erators should only be used in nonreinforced concrete and
when specifically permitted by project specifications.8.8.4 Coloring admixtures—Pigments for colored floors
should be either natural or synthetic mineral oxides, or
colloidal carbon. Synthetic mineral oxides can offer more
intensity in color, but are normally more expensive. Pigments
can be purchased alone or interground with a water-reducing
admixture for mixing into the batched concrete to produce
integrally colored concrete. Colored aggregate-type surface
hardeners containing pigments can also be used. These
pigmented mineral aggregates or metallic hardeners contain
mineral oxide pigment, portland cement, a well-graded
mineral aggregate, or metallic hardener and ts for integrally colored concrete should conform
to ASTM C979/C979M and have uniform color. Carbon
black pigments especially manufactured for this purpose
will appear lighter in color at an early age. The prepared
mixtures should only contain pigments that are mineral
oxides. Job proportioning or job mixing of material for
monolithic colored surfaces is not recommended. The use of
these materials is described in ts are also available as a liquid suspension
dispensed by automated weighing equipment. These
systems are proprietary in nature, but all allow the pigment
load to be set by the cement content and, therefore, allow
more uniform color between mixtures. A batch record is also
available from these ng admixtures should be limeproof and contain no
calcium chloride. Curing compounds for these slabs should
be the same as those used on the approved sample panels
(Chapter 11).8.8.5 Expansive cementitious admixtures—Specifically
formulated dry-powder admixtures can be blended with
portland cement at the batch plant to produce shrinkage-compensating concrete. Concrete incorporating the same
materials used for the anticipated project should be tested for
expansion according to ASTM C878/C878M (refer to ACI
223R for full details). The compatibility of the expansive
cementitious admixture and portland cement should conform
to ASTM C806. The anticipated rate and quantity of expan-sion, which is obtained in the field, should be established
before the start of construction. This can be accomplished by
conducting a series of tests using identical materials to those
proposed for the project. These tests should be conducted by
a testing laboratory that is familiar with ASTM C878/C878
procedures.8.9—Concrete mixture analysis8.9.1 Evaluation of concrete mixture—Due to the variables
involved in concrete production, the ultimate evaluation of
the concrete materials is performed when the concrete is
mixed, placed, and finished under the anticipated conditions
of the job site. There are, however, evaluation methods that
can be used to identify potential problem areas of defined
proportioned materials before mixing and placing proper analysis, major emphasis is placed on the
combined aggregates and mortar contents. Optimization
of the combined aggregate materials not only improves
the long-term strength and durability characteristics of the
concrete, but it can also dramatically improve placing char-acteristics during construction (Shilstone 1990). A satisfac-tory mortar content is one that finds the balance between
adequate mortar for placing and finishing of fresh concrete
while minimizing the shrinkage and curling properties of the
hardened material.8.9.2 Aggregate blending—To maximize the uniform
gradation distribution of the combined aggregates, blending
of three or more individual aggregates may be necessary.
Generally, this includes one coarse aggregate, one fine
aggregate, and the addition of a nominal amount of an
intermediate-sized aggregate, typically to compensate for
deficiencies in particles’ sizes retained on the 3/8 in. (9.5
mm) through No. 8 (2.36 mm) size sieves. Occasionally the
addition of a second fine aggregate source is necessary to
supplement deficiencies in the finer aggregate particle l methods are used to blend both coarse and fine
aggregate materials to produce an optimized proportioning
from the largest to smallest particles. ASTM D448 includes
sizes 89 and 9 to provide the opportunity to blend these sizes
with other classifications to obtain improved particle distri-bution. Sizes 89 and 9 are abundant in No. 4 and 8 (4.75 and
2.36 mm) size particles. These size and gradation designa-tions were developed to supplement the intermediate-sized
aggregate that is often missing in a standard single coarse
plus single fine aggregate ng aggregates to meet criteria for a combined grading
is another proportioning method used. Procedures used to
determine proportions and potential concrete characteristics
due to the gradations of combined aggregates include:
American Concrete Institute – Copyrighted © Material –
28 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)a) Percentage of the combined aggregate retained on each
of the standard sievesb) Coarseness factor chartc) 0.45 power chartWhen one of the aforementioned or other similar methods
is used, the specific combined grading to which aggregate
is to be blended, along with tolerances for control, should
be included with the mixture proportion submittal. Details
for these procedures are described in 8.9.2.1 through 8.9.2.3.8.9.2.1 Percent of combined aggregate retained on each
of standard sieves—This procedure provides a tolerance of
acceptable uniformity of distribution of the total combined
aggregate particles found in the mixture. Recommended
combined aggregate gradation tolerance limits are defined
in the ions requiring between 8 and 18 percent for large
top size aggregates (such as 1-1/2 in. [38 mm]) or between
8 and 22 percent for smaller maximum-size aggregates
(such as 1 or 3/4 in. [25 or 19 mm]) retained on each sieve
below the top size and above the No. 100 (150 µm) sieve.
These tolerances have been proven satisfactory in reducing
water demand while providing good workability. A satisfac-tory range for No. 30 and 50 (600 and 300 µm) sieves is
8 to 15 percent retained on each. Often, a third aggregate
is required to achieve this gradation (Shilstone 1990). Typi-cally, 0 to 4 percent retained on the top size sieve and 1.5 to
5.0 percent on the No. 100 (150 µm) sieve will be a well-graded mixture. This particle-size distribution is appropriate
for round or cubically shaped particles in the No. 4 to 16
(4.75 to 1.18 mm) sieve sizes. If the available aggregates for
these sizes are slivered, sharp, or elongated, 4 to 8 percent
retained on any single sieve is a reasonable compromise.
Mixture proportions should be adjusted whenever individual
aggregate grading varies during the course of the work.A deficiency in particles retained on the No. 8, 16, and 30
(2.36 mm, 1.18 mm, and 600 µm) sieves and an excess of
particles retained on the No. 50 and 100 (300 and 150 µm)
sieves occur in many areas of the United States, leading to
problems associated with cracking, curling, blistering, and
spalling of tions in locally available material may require some
deviations from the aforementioned optimum recommenda-tions. The following limitations should always be imposed:a) Do not permit the percent retained on two adjacent
sieve sizes to fall below 5 percent.b) Do not allow the percent retained on three adjacent
sieve sizes to fall below 8 percent.c) When the percent retained on each of two adjacent sieve
sizes is less than 8 percent, the total percent retained on
either of these sieves and the adjacent outside sieve should
be at least 13 percent. For example, if both the No. 4 and
No. 8 (4.75 and 2.36 mm) sieves have 6 percent retained on
each, then: 1) the total retained on the 3/8 in. and No. 4 (9.5
and 4.75 mm) sieves should be at least 13 percent; and 2) the
total retained on the No. 8 and No. 16 (2.36 and 1.18 mm)
sieves should be at least 13 gradation limits for percent of aggregate retained on
each sieve discussed previously should only be used as a
Fig. 8.9.2.2—Coarseness factor chart for evaluating poten-tial performance of for analysis, as they are rarely wholly attainable using
practically available resources and should not be specified.
Weymouth (1933) described the importance of clusters versus
individual sieve sizes. If there is a deficiency on one sieve but
excess on an adjacent sieve, the two sizes are a cluster and
they balance one another. When there is a deficiency in parti-cles on each of two adjacent sieve sizes, but abundance on
the sieves adjacent to each, the adjacent sizes tend to balance
the two-point valley. If there are three adjacent deficient sizes,
there is a problem that should be corrected.8.9.2.2 Coarseness factor chart—Figure 8.9.2.2 illustrates
an alternative method of analyzing size and uniformity of
the combined aggregate particle distribution, balanced with
the fine aggregate content of the mixture. The x-axis, labeled
as coarseness factor, defines the relationship between the
coarse and intermediate particles (for example, 33 percent
retained on the 3/8 in. [9.5 mm] sieve/45 percent retained on
the No. 8 [2.36 mm] sieve = 73.3 percent). The percent of the
combined aggregate that is retained on the No. 8 (2.36 mm)
sieve is also retained on the 3/8 in. (9.5 mm) y-axis represents the percent of the combined aggre-gate that passes the No. 8 (2.36 mm) sieve. A correction
based on cementitious material content should be made. This
chart was developed for a cementitious material content of
564 lb/yd3 (335 kg/m3) of concrete. When a mixture contains
564 lb/yd3 (335 kg/m3) of cementitious materials, there is no
correction factor. With respect to the workability factor, the
impact of 94 lb (43 kg) of portland cement is approximately
equal to a similar adjustment of 2.5 percent in the amount
of fine aggregate. As cementitious materials are increased,
the fine aggregate content should be reduced to maintain
the same workability factor W. The same applies in reverse.
An increase or decrease in either the cementitious material
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 29content or fine aggregate content without a compensating
adjustment in the other of these two components will impact
the workability of the diagonal trend bar defines a region where combined
rounded or cube-shaped crushed stone and well-graded
natural sand are in near perfect balance to fill voids with
aggregate. Variations in shape and texture of the coarse and
fine aggregates that allow the combined mixture to fall within
this region reflect maximum packing of aggregate within the
concrete volume. Mixtures with aggregate combinations that
fall in or near this region should be placed by bottom drop
buckets or paving zones are used to identify regions above the diagonal
trend bar where variation in combined aggregate grading
is indicative of certain general characteristics based on the
following field experience (8.9.2.2.1 through 8.9.2.2.5)
(Shilstone and Shilstone 2002).8.9.2.2.1 Zone I—A mixture is seriously gap-graded and
will have a high potential for segregation during placement
or consolidation due to a deficiency in intermediate parti-cles. These mixtures are readily identified in the field when
placed by chute. They are not cohesive, so a clear separation
between the coarse particles and the mortar can be seen as the
concrete is deposited from the chute. Segregation is a major
problem, especially for floor slabs and paving. Slab mixtures
plotting in this zone and three points above the trend bar
described in previous paragraphs segregated at a 1 in. (25
mm) slump. These mixtures lead to blistering, spalling, and
scaling. As mixtures approach Zone IV, they can experience
additional problems, as described in the following.8.9.2.2.2 Zone II—Zone II is the optimum zone for
mixtures with nominal maximum aggregate size from 1-1/2
to 3/4 in. (37.5 to 19 mm). Mixtures in this zone generally
produce consistent, high-quality concrete. Field observa-tions with multiple materials and construction types have
produced outstanding results when the coarseness factor is
approximately 65 to 70 and the adjusted workability factor
W is approximately 35 to 38. Mixtures that plot close to
the trend bar require close control of the aggregate. Varia-tions in grading can lead to problems caused by an excess of
coarse particles. Mixtures that plot close to Zone IV should
be placed with special care or they can experience problems
found in that zone. Mixtures with slivered or flat interme-diate aggregate require more fine sizes, as their shape creates
mobility problems.8.9.2.2.3 Zone III—Zone III is an extension of Zone II
for maximum aggregate size equal to or smaller than 1/2 in.
(12.5 mm).8.9.2.2.4 Zone IV—Excessive fines lead to a high poten-tial for segregation during consolidation and finishing. Such
mixtures will produce variable strength, have high perme-ability, and exhibit shrinkage, which generally contributes to
the development of cracking, curling, spalling, and scaling.
They are undesirable.8.9.2.2.5 Zone V—The aggregates are too coarse—that is,
nonplastic; therefore, an increase in fines content is necessary.8.9.2.3 0.45 power chart—The 0.45 power chart (Fig.
8.9.2.3) is similar to a semi-log graph, except the x-axis is
Fig. 8.9.2.3—The 0.45 power chart for determining best
combined gradation of sieve opening in microns to the 0.45 power. This chart
is widely used by the asphalt industry to determine the best
combined grading to reduce voids and the amount of asphalt
in a mixture. A straight line on this chart defines the densest
grading for aggregate for asphalt. Because asphalt includes
fine mineral filler while concrete includes cementitious
materials, fewer fine particles passing the No. 8 (2.36 mm)
sieve are necessary for concrete mixtures. This chart, histor-ically used to develop uniform gradations in the asphalt
industry, can also be adapted for use with concrete mate-rials. To create a 0.45 power curve, plot the mathematically
combined percent passing for each sieve on a chart having
percent passing on the y-axis and sieve sizes raised to the
0.45 power on the x-axis. Plot the maximum density line
from the origin of the chart to the sieve one size larger than
the first sieve to have 90 percent or less deviations from the optimum line help identify the
location of grading problems. Gradings that zigzag across
the line are undesirable. A gap-graded aggregate combina-tion will form an S-shaped curve deviating above and below
the maximum density line.8.9.3 Mortar fraction—Mortar fraction is an extension of
the coarseness factor chart. The mortar fraction consists of
all materials passing the No. 8 (2.36 mm) sieve (fine aggre-gate and paste) and is often at the center of conflicting inter-ests. With reasonably sound and properly distributed aggre-gate, it is the mortar fraction of the concrete mixture that has
a major effect on the designer’s interest in strength, drying
shrinkage, durability, and creep. The mortar fraction also
provides the contractor with necessary workability, pump-ability, placeability, and finishability. Neither should domi-nate. A mixture that is optimized for strength and shrinkage
but cannot be properly placed and consolidated will perform
poorly regardless of the w/cm mortar factor needed for various construction types
varies. A mat foundation with the concrete placed by chute
requires less mortar than the same-strength concrete cast in
a thin slab to be trowel-finished. Unless aggregate propor-tions are adjusted to compensate for differing needs, changes
American Concrete Institute – Copyrighted © Material –
30 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)Table 8.9.3—Mortar fractions for various
construction methodsConstruction
classificationPlacing and construction methodSteep sided bottom-dropped
, conveyor, or paving
machineBottom-drop bucket or chute in
vertical constructionChute, buggy, or conveyor in an
3.8 in. (200 mm) or deeper slab1-1/2 in. (37.5 mm) maximum
size aggregate mixture receiving
high-tolerance finish4.5 in. (125 mm) or larger pump
for use in vertical construction,
thick flat slabs and larger walls,
beams, and similar elements3/4 to 1 in. (19 to 25 mm)
maximum size aggregate mixture
receiving high tolerance finish5 in. (125 mm) pump for pan
joist slabs, thin or small castings,
and high reinforcing steel density4 in. (100 mm) pumpLong cast-in-place piling shellsPump smaller than 4 in. (100 mm)Less than 4 in. (100 mm) thick
toppingFlowing fillApproximate mortar
fractions (percent
volume of concrete)48 to 5050 to 5251 to 5352 to 545.53 to 556.7.8.9.10.55 to 5756 to 5858 to 6060 to 6263 to 66Table 8.9.4a—Mixture design exampleDensitylb/ft3kg/m3196.63150146.72350164.22630164.22630162.3260062.410000.00————————————Original mixture (gap graded)MassVolume3lbkgftm3480217.72.440.078438.10.570.021706773.810.390.2900.00.000.001380626.08.500.24292132.44.680.1300.00.420..127.000.76——————————5.1630588440—————Percent
by
volume9.02.138.50.031.517.31.6100.0—————Adjusted mixtureMassVolume3lbkgftm3440199.62.240.067734.90.520.011223554.77.450.21658298.54.010.111292586.07.960.23275124.74.410.1200.00.420..527.000.76——————————5.—————Percent
by
volume8.31.927.614.829.516.31.6100.00—————Mixture componentsPortland cementPozzolan1 in. (25 mm) aggregate3/8 in. (9.5 mm) aggregateFine aggregateWaterAirTotalsCombined fineness
modulusPaste + air fraction,
percentMortar fraction, percentCoarseness factor, percentWorkability factor, percent
American Concrete Institute – Copyrighted © Material –
in slump to increase mortar content through the addition of
water is the only option open to the contractor. Construction
requirements that affect the amount and quality of mortar
necessary to properly place and finish the concrete materials
should be considered when optimizing a mixture. Mortar
fractions are influenced by aggregate particle shape, texture,
and distribution, and will vary with each mixture. Approxi-mate mortar fractions for 10 construction classifications are
shown in Table 8.9.3 (Shilstone 1990).8.9.4 Example slab mixture analysis—Table 8.9.4a shows
a concrete slab mixture design example that compares an
original gap-graded mixture that is high in fine content with
an adjusted mixture that uses an intermediate-size aggregate
material to improve the uniformity of the combined aggre-gate particle size distribution. The total combined aggre-gates in the original mixture consist of blending a 1 in. (25
mm) maximum-sized stone source with a single sand source.
The adjusted, or optimized, mixture adds a 3/8 in. (9.5 mm)
intermediate aggregate to supplement the particle sizes that
were previously lacking in the combined gradations of the
original mixture (Table 8.9.4b).Improving the uniformity of aggregate particle size distri-bution (Fig. 8.9.4a and 8.9.4b) will reduce the amount of
paste necessary to coat the particles, resulting in a reduction
in the amount of water necessary to produce the same work-ability and finishability in the mixture. A reduction in paste
also reduces the cost of producing the concrete mixture while
maintaining its equivalent strength and improved durability.
The potential for cracking and curling is reduced because
shrinkage is affected changes in Coarseness Factor Chart are as
follows:a) The blending of the 3/8 in. (9.5 mm) aggregate changes
the coarseness factor (Fig. 8.9.4c) from 84 to 60 percent,
moving the mixture optimization indicator (MOI) to the
right. At this point, the total cementitious materials content
is equal to 564 lb/yd3 (335 kg/m3). Both the MOI and the
MOI-Adj, which is influenced by the adjusted workability
factor W-Adj, are therefore the same.b) The fine aggregate content is reduced, changing the
workability factor from 40 to 36 percent optimized for the
new coarseness factor of 60 percent.c) Due to the improved uniformity of the gradations, it is
determined that less cementitious material is necessary to
produce the same flexural strength without degrading the
finishing characteristics of the mixture. This is due to a decrease
in void space between the combined aggregates. The amount
of fine paste materials necessary to fill these void spaces is
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 31Table 8.9.4b—Aggregate gradation exampleIndividual percent retained1 in. 3/8 in.
Sieve size(25 mm)(9.5 mm)Fine1-1/2 in. (37.5 mm)———1 in. (25.0 mm)2.0——3/4 in. (19.0 mm)18.0——1/2 in. (12.5 mm)52.0——3/8 in. (9.5 mm)20.012.0—No. 4 (4.75 mm)6.072.00.9No. 8 (2.36 mm)0.713.011.7No. 16 (1.18 mm)0.41.716.3No. 30 (600 μm)0.91.324.1No. 50 (300 μm)——34.2No. 100 (150 μm)——11.6No. 200 (75 μm)——1.2Combined fineness modulusCombined percent
retainedOriginal—1.19.928.611.03.75.77.611.315.45.20.55.16Adjusted—0.86.919.910.117.57.77.210.514.04.80.55.09Fig. 8.9.4b—The 0.45 power chart showing change in
combined gradation of aggregates after blending of 3/8 in.
(9.5 mm) aggregate.
Fig. 8.9.4a—Material sieve analysis showing change in
aggregate distribution with blending of 3/8 in. (9.5 mm)
d, thus reducing the amount of paste necessary for the
mixture. The reduction in cementitious materials reduces the
W-Adj value, thus moving MOI-Adj down the R 9—BATCHING, MIXING, AND
TRANSPORTINGDetailed provisions relating to batching, mixing, and
transporting concrete are available in ASTM C94/C94M,
ASTM C1116/C1116M, and ASTM C685/C685M. Chapter
9 is intended to give guidance to the concrete installer and
provider in delivering a consistent and suitable material.9.1—BatchingWhether the concrete is centrally mixed on-site or in a
ready mixed concrete operation, the materials should be
batched by mass within the following ranges of the target
batch weights:Fig. 8.9.4c—Coarseness factor chart showing the impact of
blending 3/8 in. (9.5 mm) aggregate.a) Cement: ±1 percentb) Added water: ±1 percent of the total mixing waterc) Fine and coarse aggregate: ±2 percent if cumulative and
±1 percent if individuald) Admixtures: ±3 percente) Pigments: ±0.25 percentNote: Per ASTM C94, the tolerances are not as restrictive
for smaller batches (less than 30% of scale capacity).Except for site mixing on small jobs, cement should be
weighed on a scale separate from that used for weighing
aggregates. If batching is by the bag, no fractional bags
American Concrete Institute – Copyrighted © Material –
32 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)should be used. Aggregate should be batched by weight or
mass. Batching by volume should not be permitted, except
with volumetric batching and continuous-mixing equipment.
Batch weights should be adjusted to compensate for absorbed
and surface moisture. Moisture determinations should be
made daily or more frequently when an obvious change
has occurred in the appearance of the aggregate. When the
concrete mixture contains special aggregates, particular care
should be exercised to prevent segregation or can be batched by weight, mass, or volume. The
measuring device should have a readily adjustable positive
cutoff and provisions for calibration. Water from washing
out the trucks should not be incorporated into subsequent
batches of concrete, unless the water is treated to reduce the
suspended solids and the amount can be measured to the
accuracy required previously. Water should never be left in
the drums from washing or storage. One way to accomplish
this goal is to require that truck drivers reverse their drums
prior to taking on a new te batching of admixtures and colored pigments is
critical because they are used in relatively small quantities.
Admixtures should be accurately batched at the concrete
plant. Admixtures that are designed to be added to the
concrete at the job site should be incorporated in accordance
with the manufacturers’ recommendations. When more than
one admixture is batched, each should be batched separately
and in such a way that the concentrated admixtures do not
come into contact with each other. Care should be taken to
avoid the freezing of admixtures in cold weather, as this
can damage some of them. Preferably, purchase pigments
or colored admixtures prepackaged in batch-sized quanti-ties. Powdered admixtures should be batched by weight, and
paste or liquid admixtures by weight or -borne pigments should be batched before batching
the concrete and the water used accurately determined and
accounted for while batching the concrete. The allowable
tolerance for liquid pigments should be a maximum of ±0.25
percent or 0.25 lb (0.11 kg), whichever is less. Care should
be taken using liquid pigments, as they contain a dispersant
to help keep the pigment in suspension. With certain cement
chemistries, this may result in unintentional air entrainment
or in increases in slump. The volume of admixture batched
should not be controlled by timing devices. Liquid admix-tures are preferred but can require agitation prior to batching
to prevent the settling of solids.9.2—Mixing9.2.1 Ready mixed concrete—Mixing should be performed
in accordance with ASTM C94/C94M or ASTM C1116/C1116M. The mixture should be proportioned to produce the
required slump and air content without exceeding the speci-fied w/cm. Close attention should be given to the moisture
content of the aggregate and to the necessary adjustments to
batched weights. Concrete trucks should be in compliance
with requirements of the project specification. One way to
assure uniformity of concrete trucks is to require that they be
certified to deliver concrete to department of transportation
projects for the state.
Some water may be held out during batching. The amount
of withheld water should be indicated on the ticket. The
truck should then leave the plant with a full water tank to
allow the addition of water on site. Any on-site addition
should be monitored and controlled, with the amount of
water added accurately measured and recorded. Wherever
possible, the concrete should not be adjusted on site. After
the addition of any materials to the truck, the mixer should
be turned at the maximum speed for 5 minutes or longer as
required by the concrete truck manufacturer. If slump adjust-ment is required after the maximum w/cm has been reached,
it should be done with an ASTM C494/C494M Type A or
Type F admixture. Care should be taken in using these mate-rials, as they may affect the setting or finishing characteris-tics of the concrete. The added fluidity can also affect the
air content of the concrete and create air contents in excess
of the normal 2 to 3 percent entrained air contents in the
absence of air-entraining agents. The addition of large quan-tities of water can affect the setting characteristics and air
content of the concrete. The sequence of addition of admix-tures and cementitious materials can impact the properties
of the concrete.9.2.2 Site mixing—Mixers that produce a volume of
concrete requiring less than one bag of cement should not
be used. For small quantities of concrete, packaged prod-ucts meeting ASTM C387/C387M are more convenient and
can be more accurately proportioned. Mixing time should
be sufficient to produce uniformly mixed concrete with the
required slump and air content. Site mixers less than 1 yd3
(0.75 m3) in capacity should mix for no less than 3 minutes;
ordinarily, 15 seconds should be added for each additional
cubic yard (0.75 m3) of capacity or fraction thereof, unless
a turbine mixer is used. A longer mixing time is required for
concrete with a slump of less than 3 in. (75 mm). Equipment
for volumetric batching and continuous mixing at the job
site is available. Concrete produced in this manner should
comply with ASTM C685/C685M.9.2.3 Architectural concrete—When special architec-tural concrete is produced using special aggregates, white
cement, special cements, or pigments, mixer drums and
equipment should be kept clean and any wash water should
be disposed of before a new batch is introduced. Identical
ingredients and quantities of materials should be used, using
no less than one-third of the capacity of the mixing drum,
a minimum of 3 yd (2.3 m3) in a 9 yd (6.9 m3) drum, and
should always be in full yard increments. Refer to ACI 303R
for additional details.9.2.4 Shrinkage-compensating concrete—When expan-sive cement or an expansive component-type admixture
specifically designed for producing shrinkage-compensating
concrete is required, refer to ACI 223R for details.9.3—Transporting9.3.1 Fluid concretes—The delivery of fluid concretes,
such as shrinkage-compensating concretes, can result
in problems unless care is taken during hauling. Trucks
should not be filled above their mixing capacity even when
the concrete is central mixed. Delivery routes that contain
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 33significant grades should be carefully surveyed to ensure
that material will not flow out of the mixer.9.3.2 Discharge time—Concrete mixed or delivered in
a truck mixer should be completely discharged while the
concrete still has sufficient workability to respond properly
during the placing and finishing operations. The period after
arrival at the job site during which the concrete can be prop-erly worked will generally vary from less than 45 minutes
to more than 90 minutes, depending on the weather, the
concrete mixture proportions, and travel time from the batch
plant to the job site. Prolonged mixing accelerates the rate of
stiffening and can greatly complicate placing and timing of
finishing operations.9.3.3 Job-site slump control—The specified slump and
air content of the concrete should be measured at the point
of discharge from the pump. When concrete arrives at the
point of placement with a slump below what will result in
the specified slump at the point of placement and is unsuit-able for placing at that slump, the slump may be adjusted
to the required value by adding water up to the amount
allowed in the accepted mixture proportions. The addition
of water should be in accordance with ASTM C94/C94M. If
additional slump is required, then an admixture conforming
to ASTM C494/C494M Type A, F, or G should be used.
The mixer should be turned at the mixing speed for at least
5 minutes to fully incorporate the admixture. The specified
w/cm or slump should not be exceeded. Water should not be
added to concrete delivered in a concrete truck or similar
equipment acceptable for mixing. Test samples for compres-sive strength, slump, air content, and temperature should be
taken after any necessary adjustment. Refer to ACI 301 for
further details.9.3.4 Delivery to point of discharge—Concrete for floor and
slab placement can be delivered to the forms directly from a
concrete truck chute or by pump, belt conveyor, buggy, crane
and bucket, or a combination of these methods. Delivery of
concrete should be at a consistent rate, appropriate to the size
of the placement, and should be deposited as close as possible
to its final location. Concrete should not be moved horizon-tally by vibration, as this contributes to segregation. Refer to
ACI 304R for recommended R 10—PLACING, CONSOLIDATING, AND
FINISHINGMost of Chapter 10 applies to both normalweight and light-weight concrete. The proper procedures for finishing light-weight concrete floors differ somewhat from finishing normal-weight concrete. Procedures specific to finishing lightweight
concrete are discussed separately in 10.11. This chapter also
contains information on finishing fiber-reinforced concrete.
For additional information, refer to ACI s finishing procedures should be executed sequen-tially and within the proper time period, which is neither too
early nor late in the concrete-hardening process. This time
period is called the window of finishability. It refers to the
time available for operations taking place after the concrete
has been placed, consolidated, and struck off. Surface
finish, surface treatment, and flatness and levelness require-
Fig. 10.1.1.1a—Laser-controlled dictate the type and number of finishing operations.
All should take place within the proper time period. If the
floor slab is placed during a time period of rapid hardening,
this window becomes so small that it can present consider-able difficulties to the floor contractor. The preconstruction
meeting should include discussion of the measures neces-sary to ensure a satisfactory window of finishability. There
is a preconstruction checklist available from NRMCA that
provides an outline of topics that might be considered for
inclusion in the meeting agenda. ACI 311.4R also contains
guidance that could be valuable.10.1—Placing operations10.1.1 Caution—All concrete handling operations should
minimize segregation because it is difficult to remix concrete
after it has been placed.10.1.1.1 Placing sequence—Large block placements are
the most efficient way to place concrete in large areas. Laser-guided equipment is most often used for this configuration
(Fig. 10.1.1.1a). Laser screeds provide accurate strike-off
between construction placements are an acceptable alternative to block
placements if a laser screed is not available or access is inad-equate. Long alternating strips are the best option if poor
subgrade or multiple layers of reinforcement are encoun-tered. Strip placements are also the best option for high-tolerance, defined traffic floors. Interior contraction joints
will be required. These joints should be installed at specific
intervals in a timely manner. Contraction joints are not
needed if shrinkage-compensating concrete is used.A checkerboard sequence of placement with side dimen-sions of 50 ft (15 m) or less, as shown in Fig. 10.1.1.1b, has
been used in the past in an effort to permit earlier placements
to shrink and to obtain minimum joint width. Experience has
shown, however, that shrinkage of the earlier placements
occurs too slowly for this method to be effective. Access is
more difficult and expensive, and joints may not be as smooth.
A checkerboard sequence of placement should not be used.10.1.1.2 Placing sequence for shrinkage-compensating
concrete—The use of shrinkage-compensating concrete is
American Concrete Institute – Copyrighted © Material –
34 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)Fig. 10.1.1.1b—Placing sequence: long-strip construction (left) is recommended; checker-board construction (right) is not actory for block placements. Neither the strip method
nor the checkerboard method described in 10.1.1.1 should
be used with shrinkage-compensating concrete. Refer to
ACI 223R for specific recommendations concerning place-ment configuration and sequence.10.1.2 Discharge rate of concrete—The rate of discharge
of concrete from a concrete truck can be controlled by
varying the drum speed.10.1.3 Job-site transfer—Chutes should have rounded
bottoms and be constructed of metal or be metal-lined. The
chute slope should be constant and steep enough to permit
concrete of the slump required to flow continuously down
the chute without segregation. Long, flat chutes should
be avoided because they encourage the use of high-slump
concrete. A baffle at the end of the chute helps prevent segre-gation. The discharge end of the chute should be near the
surface of previously deposited concrete. When concrete is
being discharged directly onto the base, the chute should be
moved at a rate sufficient to prevent accumulation of large
piles of concrete. Allowing an excessively steep slope on
chutes can result in high concrete velocity and less of the method of transportation and discharge,
the concrete should be deposited as near as possible to
its final position and toward previously placed concrete.
Advance planning should include access to and around the
site, suitable runways, and the use of other devices to avoid
the use of concrete with a high w/cm or excessive delays.10.1.4 Placing on base—Mixing and placing should be
carefully coordinated with finishing operations. Concrete
should not be placed on the base at a faster rate than it can be
spread, bull floated or darbied, and restraightened because
these latter operations should be performed before bleeding
water has an opportunity to collect on the surface.A properly sized finishing crew, with due regard for the
effects of concrete temperature and atmospheric condi-tions on the rate of hardening of the concrete, will assist
the contractor in obtaining good surfaces and avoiding cold
joints. If construction joints become necessary, they should
be produced using suitably placed bulkheads, with provi-sions made to provide load transfer between current and
future work (5.2.9).10.2—Tools for spreading, consolidating, and
finishingThe sequence of steps commonly used in finishing
unformed concrete floor surfaces is illustrated in Fig. 10.2.
Production of high-quality work requires that proper tools be
available for placing and finishing operations. The following
is a list and description of typical tools that are commonly
available. Refer to 10.3 for suggestions and cautions
concerning tool usage.10.2.1 Spreading—Spreading is the act of extending or
distributing concrete or embedding hardeners, which are
often called shake-on or dry-shake, or other special-purpose
aggregate over a desired area.10.2.1.1 Spreading concrete—Level the concrete and
prepare the surface for strike-off.10.2.1.2 Hand spreading—Short-handled, square-ended
shovels, or come-alongs, which are hoe-like tools with
blades approximately 4 in. (100 mm) high, 20 in. (500 mm)
wide, and curved from top to bottom, should be used for the
purpose of spreading concrete after it has been discharged.10.2.1.3 Spreading dry-shake hardeners, colored dry-shake hardeners, or other special-purpose material—The
goal of spreading operations for these materials is to provide
an even distribution of product over the desired area. Gener-ally, hand application should be used for distribution of these
materials in areas where a mechanical spreader cannot be
used.10.2.1.4 Mechanical spreaders—Mechanical spreaders
are the best method of uniformly applying dry-shake hard-eners, colored dry-shake hardeners, or other special purpose
materials to concrete during the finishing process. These
devices generally consist of a bin or hopper to hold the mate-rial, a vibrator or motorized auger to assist in distribution
of the material, and a supporting framework that allows the
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 35Fig. 10.2.1.4—Mechanical 6 in. (100 to 150 mm) deep. Tools specifically made for
screeding, such as hollow magnesium straightedges, should
be used instead of randomly selected lumber.10.2.3.2 Mechanical screeding—Various types of surface
vibrators, including vibrating screeds, vibratory tampers,
and vibratory roller screeds, are used mainly for screeding
slab-on-ground construction. They consolidate concrete
from the top down while performing the screeding function.
Refer to ACI 309R for a detailed discussion of equipment
and parameters for proper ing screeds generally consist of either hand-drawn
or power-drawn single-beam, double-beam, or truss assem-blies. They are best suited for horizontal or nearly horizontal
surfaces. Vibrating screeds should be of the high-amplitude
low-frequency type, at 3000 to 6000 vibrations per minute
(50 to 100 Hz), to minimize wear on the machine and
provide adequate depth of consolidation without creating an
objectionable layer of fines at the surface. Frequency and
amplitude should be coordinated with the concrete mixture
designs being -controlled variations of this equipment can be used
to produce finished slabs-on-ground with improved level-ness over that which might otherwise be achieved. Laser-controlled screeds can ride on supporting forms or they
can operate from a vehicle using a telescopic boom (Fig.
10.1.1.1a). Laser screeds have the ability to strike off lower
slump concrete than other screed types -tamper screeds are vibratory screeds that are
adjusted to a lower frequency and amplitude. Tamper
screeds work best on very stiff concrete. These screeds are
generally used to embed metallic or mineral aggregate hard-eners. The contractor is cautioned that improper use of this
screed could embed the hardener too deeply and negate the
intended ory-roller screeds knock down, strike off, and
provide mild vibration. They can rotate at varying rates up
to several hundred revolutions per minute, as required by the
consistency of the concrete mixture. The direction of rota-tion of the rollers on the screed is opposite to the screed’s
direction of movement. These screeds are most suitable for
concrete mixtures with higher . 10.2—Typical finishing procedures (subject to numerous
conditions and variables).hopper to move smoothly over the concrete surface while
distributing the material (Fig. 10.2.1.4).10.2.2 Tools for consolidating—Consolidation is the
process of removing entrapped air from freshly placed
concrete, usually by vibration. Internal vibration and surface
vibration are the most common methods of consolidating
concrete in supported slabs and slabs-on-ground. Refer to
ACI 309R for additional discussion of topics related to the
consolidation of concrete.10.2.2.1 Internal vibration—Internal vibration employs
one or more vibrating elements that can be inserted into
the fresh concrete at selected locations. Internal vibration
is generally most applicable to supported cast-in-place
construction.10.2.2.2 Surface vibration—Surface vibration employs a
portable horizontal platform on which a vibrating element is
mounted. Refer to 10.2.3.2 for additional discussion.10.2.3 Tools for screeding—Screeding is the act of
striking off concrete lying above the desired plane or shape
to a predetermined grade. Screeding can be accomplished
by hand, using a straightedge consisting of a rigid, straight
piece of wood or metal, or by using a mechanical screed.10.2.3.1 Hand screeding—Hollow magnesium or solid
wood straightedges are commonly used for hand-screeding
of concrete. The length of these straightedges varies up to
approximately 20 ft (6 m). Straightedge cross-sectional
dimensions are generally 1 to 2 in. (25 to 50 mm) wide by 4
American Concrete Institute – Copyrighted © Material –
36 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)10.2.4 Tools for floating—Floating is the act of consoli-dating and compacting the unformed concrete surface in
preparation for subsequent finishing operations. Initial
floating of a concrete floor surface takes place after screeding
and before bleed water comes to the surface and imparts a
relatively even, but still open, texture to the fresh concrete
surface. After evaporation of bleed water, additional floating
operations prepare the surface for troweling.10.2.4.1 Bull floats (long-handled)—Bull floats are used
to consolidate and compact unformed surfaces of freshly
placed concrete immediately after screeding operations
while imparting an open texture to the surface. They are
usually composed of a large, flat, rectangular piece of wood
or magnesium and a handle. The float part of the tool is
usually 4 to 8 in. (100 to 200 mm) wide and 3.5 to 10 ft (1 to
3 m) long. The handle is usually 4 to 20 ft (1.2 to 6 m) long.
The handle is attached to the float by means of an adjustable
head that allows the angle between the two pieces to change
during operation.10.2.4.2 Darby—A darby is a hand-manipulated float,
usually 3-1/2 in. (90 mm) wide and 3 to 8 ft (1 to 2.4 m) long
that is used in early-stage floating operations near the edge
of concrete placements.10.2.4.3 Hand floats—Hand tools for basic floating opera-tions are available in wood, magnesium, and composition
materials. Hand float surfaces are approximately 3-1/2 in.
(90 mm) wide and vary from 12 to 20 in. (300 to 500 mm)
in length.10.2.4.4 Power floats—Also known as rotary floats, power
floats are engine-driven tools used to smooth and to compact
the surface of concrete floors after evaporation of the bleed
water. Two common types are heavy, revolving, single-disk-compactor types that often incorporate some vibration, and
troweling machines equipped with float blades. Most trow-eling machines have four or more blades mounted to the
base and a ring diameter that can vary from 36 to 46 in. (1 to
1.2 m); mass generally varies from approximately150 to 250
lb (70 to 110 kg).Two types of blades can be used for the floating opera-tion. Float blades are designed to slip over trowel blades and
are generally 10 in. (250 mm) wide and 14 to 18 in. (350 to
450 mm) long. Both the leading edge and trailing edges of
float blades are turned up slightly. Combination blades are
usually 8 in. (200 mm) wide and vary in length from 14 to
18 in. (350 to 450 mm). The leading edges of combination
blades are turned up r attachment that is available to assist in power
float operations is a pan with small brackets that slides over
the trowel blades. These pans are normally used on double-
or triple-platform ride-on machines and are very effective
on concrete surfaces requiring an embedded hardener or
coloring agent. The use of mechanical pans (Fig. 10.2.4.4)
can also improve flatness of the finished floor. Pans can be
used for floating, but some contractors believe there are
advantages to initially floating with clip-on float blades.10.2.5 Tools for restraightening—Straightedges are used
to create and to maintain a flat surface during the finishing
process. Straightedges vary in length from 8 to 12 ft (2.4 to
Fig. 10.2.4.4—Double-riding trowel with clip-on . 10.2.5—Modified highway straightedge.3.7 m) and are generally rectangular in cross section, although
designs differ among manufacturers. When attached to a bull-float handle with an adjustable head, these tools are frequently
called modified highway straightedges (Fig. 10.2.5).10.2.6 Tools for edging—Edgers are finishing tools used
on the edges of fresh concrete to provide a rounded edge.
They are usually made of stainless steel and should be thin-lipped. Edgers for floors should have a lip radius of 1/8 in. (3
mm). Edgers are not recommended for slabs when wheeled
traffic is anticipated.10.2.7 Tools for troweling—Trowels are used in the final
stages of finishing operations to impart a relatively hard and
dense surface to concrete floors and other unformed concrete
surfaces.10.2.7.1 Hand trowels—Hand trowels generally vary from
3 to 5 in. (75 to 125 mm) in width and from 10 to 20 in. (250
to 500 mm) in length. Larger sizes are used for the first trow-eling to spread the troweling force over a large area. After
the surface has become harder, subsequent trowelings use
smaller trowels to increase the pressure transmitted to the
surface of the concrete.10.2.7.2 Fresno trowels—A fresno is a long-handled
trowel that is used in the same manner as a hand trowel.
Fresno’s are useful for troweling slabs that do not require a
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 37hard-troweled surface. These tools are generally 5 in. (125
mm) wide and vary in length from 24 to 48 in. (0.6 to 1.2 m).10.2.7.3 Power trowels—Power trowels are engine-driven
tools used to smooth and compact the surface of concrete
floors after completion of the floating operation. Ring diam-eters on these machines generally vary from 36 to 46 in. (0.9
to 1.15 m); their mass generally varies from approximately
150 to 250 lb (70 to 110 kg). Trowel blades are usually 6
in. (150 mm) wide and vary in length from 14 to 18 in.
(350 to 450 mm). Neither the leading nor the trailing edge
of trowel blades is turned up. Power trowels can be walk-behind machines with one set of three, four, or more blades
or ride-on machines with two or three sets of four blades.10.2.8 Tools for jointing—Jointing tools are used for the
purpose of creating contraction joints, or influenced separa-tion points, in slabs. The preferred method is by saw cutting.
In tight areas, groovers, also called jointers, can be used.10.2.8.1 Groovers—Groovers can be handheld or walk-behind. Stainless steel is the most common material. Hand-held groovers are generally from 2 to 4-3/4 in. (50 to 120
mm) wide and from 6 to 7-1/2 in. (150 to 190 mm) long.
Groove depth varies from 3/16 to 1-1/2 in. (5 to 38 mm).
Walk-behind groovers usually have a base with dimensions
from 3-1/2 to 8 in. (90 to 200 mm) in width and from 6 to 10
in. (150 to 250 mm) in length. Groove depth for these tools
varies from 1/2 to 1 in. (13 to 25 mm).10.2.8.2 Saw-cutting—The following three types of tools
can be used for saw-cutting joints: conventional wet-cut
(water-injection) saws; conventional dry-cut saws; and
early-entry dry-cut saws. Timing of the sawing operations
will vary with manufacturer and equipment. The goal of
saw-cutting is to create a weakened plane to influence the
location of shrinkage crack formation as soon as the joint
can be cut, preferably without creating spalling at the types of dry-cut tools are either electrically or gaso-line powered. They provide the benefit of being generally
lighter than wet-cut equipment. Early-entry dry-cut saws
do not provide as deep a cut—generally 1-1/4 in. (32 mm)
maximum—as can be achieved by conventional wet-cut and
dry-cut -entry dry-cut saws use diamond-impregnated
blades and a skid plate that helps prevent spalling. Timely
changing of skid plates is necessary to effectively control
spalling. Change skid plates in accordance with manufac-turers’ recommendations. Conventional wet-cut saws are
gasoline powered and, with the proper blades, are capable of
cutting joints with depths of up to 12 in. (300 mm) or more.10.3—Spreading, consolidating, and finishing
operationsThis section describes the manner in which various
placing and finishing operations can be completed success-fully. The finishing sequence used after completion of the
initial screeding operation depends on a number of variables
related to project requirements or to the concrete finishing
t variables are generally controlled by require-ments of the owner and are specified by the designer. Some
examples are the choice of admixtures used in concrete, the
requirement for an embedded hardener, and the final finish
les subject to the environment include such items as
setting time of the concrete, ambient temperature, timeliness
of concrete delivery, consistency of concrete at the point of
deposit, and site accessibility. Figure 10.2 is a flow chart
that illustrates the normal sequence of steps in the finishing
process.10.3.1 Spreading and consolidating—Concrete, whether
from a truck mixer chute, wheelbarrow, buggy, bucket, belt
conveyor, pump, or a combination of these methods, should
be delivered without segregation of the concrete compo-nents (10.1). Spreading—the first operation in producing a
plane surface—should be performed with a come-along or
a short-handled, square-ended shovel. A plane surface is not
necessarily a level surface because, in many cases, it can be
sloped for surface drainage (10.2.1.1).Long-handled shovels, round-ended shovels, or garden-type rakes with widely spaced tines should not be used to
spread concrete. Proper leverage, of prime importance for
manipulating normalweight concrete, is lost with a long-handled shovel. Round-ended shovels do not permit proper
leveling of the concrete. The tines of garden-type rakes can
promote segregation and should not be used in any l consolidation of concrete in floors, with the excep-tion of heavily reinforced slabs, is usually accomplished
in the first operations of spreading, vibrating, screeding,
darbying or bull floating, and restraightening. The use of
grate tampers or mesh rollers is usually neither desirable
nor necessary, unless the concrete slump is less than 3 in.
(75 mm). If grate tampers are used on lightweight-concrete
floors, only one pass over the surface with a very light
impact should be permitted. Spreading by vibration should
be minimized. Refer to ACI 309R for detailed discussion.10.3.1.1 Cold joint precautions—Any interruption or
inconsistency in concrete delivery and placement can
contribute to the development of a cold joint or potential
plane of weakness and slab surface discoloration. Cold joints
are formed primarily between two batches of concrete where
the delivery and placement of the second batch has been
delayed and the initial placed and compacted concrete has
started to set. The full knitting together of the two batches
of concrete under vibration to form a homogeneous mass is
therefore not possible, unlike the compaction of two fresh,
workable batches of concrete. Covering the surface edge
of the exposed concrete immediately upon interruption of
concrete placement with polyethylene will keep the leading
concrete material edge plastic for a longer period than if left
exposed to the elements.10.3.1.2 Structural floors—Structural floors, both
suspended and on ground, can be reinforced with relatively
large deformed reinforcing bars or with post-tensioning
tendons and typically contain other embedded items such as
piping and conduit. Proper consolidation around reinforcing
steel, post-tensioning anchorages, and embedded elements
requires internal vibration, but care should be taken not to
use the vibrator for spreading the concrete, especially in
American Concrete Institute – Copyrighted © Material –
38 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)deeper sections where over-vibration can easily cause segre-gation. Restraint of concrete movement by embedded items
such as piping and conduits can result in crack formation as
the concrete vibrator head should be completely immersed during
vibration. Where slab thickness permits, it is proper to insert
the vibrator vertically. On thin slabs, the use of short 5 in.
(125 mm) vibrators permits vertical insertion. Where the slab
is too thin to allow vertical insertion, the vibrator should be
inserted at an angle or horizontally. The vibrator should not
be permitted to contact the base because this might contami-nate the concrete with foreign materials.10.3.2 Screeding—Screeding is the act of striking off the
surface of the concrete to a predetermined grade, usually set
by the edge forms. This should be done immediately after
placement. When hand strike-off is used, a slump of 5 in.
(125 mm) or higher should be used to facilitate strike-off
and consolidation of concrete without mechanical methods.
Refer to 10.2.3 for screeding tool all the floor-placing and finishing operations, form
setting and screeding have the greatest effect on achieving
the specified grade. Accuracy of the screeding operation is
directly impacted by the stability and the degree of levelness
of the edge forms or screed guides selected by the contractor.
Consequently, care should be taken to match the forming
system and the screeding method to the levelness tolerance
forms for slab-on-ground and suspended-slab place-ments are normally constructed of wood or metal. The
spacing between edge forms and the support provided for
them will influence the accuracy of the screeding operation.
Where edge-form spacing exceeds the width of the screed
strip, intermediate screed guides can improve the accuracy
of the screeding operation. The width of these screed strips
will generally vary between 10 and 16 ft (3 and 5 m) and will
be influenced by column spacings. Generally, screed strips
should be equal in width and should have edges that fall on
column general, slab-on-ground placements are either block
placements or strip placements. Block placements generally
have edge dimensions that exceed 50 ft (15 m). Strip place-ments are generally 50 ft (15 m) or less in width and vary in
length up to several hundred feet (meters). Suspended-slab
placements are usually block placements. Where wood is
used for edge forms, the use of dressed lumber is recom-mended. The base should be carefully fine-graded to ensure
proper slab ion of the type of screed guide to be used for
screeding operations is somewhat dependent on placement
configuration. The maximum practical width of screed strips
for hand screeding is approximately 20 ft (6 m). Where strict
elevation tolerances apply, it is wise to limit the width of
screed strips. The length of the hand screeding device should
not be longer than 16 ft (5 m) and should overlap previ-ously placed strips of fresh concrete a minimum of 2 ft (600
mm). Screeding of strip placements for slabs-on-ground is
generally completed using some type of a vibrating screed
supported by edge forms. Screeding of block placements for
slabs-on-ground is usually accomplished using wet-screed
guides, dry-screed guides, a combination of these two, or
some type of laser-guided screed. For slabs-on-ground, an
elevation change no greater than 3/8 in. (9.5 mm) in 10 ft (3
m), approximately FL35, can be achieved routinely through
use of laser-guided screeds. Refer to 10.15 for discussion of
floor flatness and levelness. Screeding of block placements
for suspended slabs is usually accomplished using either wet-screed guides, dry-screed guides, or a combination of the -screed guides, when used between points or grade
stakes, are established immediately after placement and
spreading (refer to 6.4 for setting of dry-screed guides).
At the time of floor placement, before any excess mois-ture or bleed water is present on the surface, a narrow
strip of concrete not less than 2 ft (600 mm) wide should
be placed from one stake or other fixed marker to another,
and straightedged to the top of the stakes or markers; then
another parallel strip of concrete should be placed between
the stakes or markers on the opposite side of the placement
strip. These two strips of concrete, called wet-screed guides,
establish grade for the concrete located between the guides.
Immediately after wet-screed guides have been established,
concrete should be placed in the area between, then spread
and straightedged to conform to the surface of the wet-screed
guides. The contractor should confirm that proper grade has
been achieved following strike-off. High spots and low spots
should be identified and immediately corrected. Low spots
left behind should be filled by placing additional concrete in
them with a shovel, carefully avoiding segregation. Noncon-forming areas should then be rescreeded. Difficulty in main-taining the correct grade of the floor while working to wet-screed guides is an indication that the concrete mixture does
not have the proper consistency or that vibration is causing
the guides to ion stakes placed at regular intervals are one method
of establishing grade for wet-screed guides in slab-on-ground construction. As screeding progresses, the stakes can
be driven down flush with the base if expendable or pulled
out one at a time to avoid walking back into the screeded
concrete. This early removal of stakes is one of the signifi-cant advantages in the use of wet screeds; in addition, grade
stakes are much easier and faster to set than dry screeds.
Screeding should be completed before any excess moisture
or bleed water is present on the benefits of using wet-screed guides include economical
and rapid placement of the concrete. The successful use of
wet-screed guides, however, requires careful workmanship by
craftspeople who strike off the concrete because vibration can
change the elevation of the wet screed. Wet-screed guides are
difficult to use when varying surface slopes are required and
can produce inconsistent results when variations in slab thick-ness are required to compensate for deflection of a suspended
slab. Special care is necessary to avoid poor consolidation or
cold joints adjacent to wet-screed -screed guides should not be used in suspended-slab construction, unless the finished floor surface is level
and formwork is shored at the time of strike-off. During
construction activity, vibration of reinforcing steel and the
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 39supporting platform may result in an incorrect finished grade
when wet-screed guides are used. It is imperative, therefore,
that grade is confirmed after strike-off and that errors are
corrected at that time by restriking the -screed guides should be used only for surfaces where
floor levelness is not critical. For slabs-on-grade where floor
levelness requirements are important, dry-screed guides or
laser-guided equipment should be used instead of wet-screed
guides. In general, surfaces produced using wet-screed guides
will exhibit maximum elevation changes of at least 5/8 in. (16
mm) in 10 ft (3 m). This corresponds to an FL20 ion variation of surfaces produced using dry-screed
guides depends on placement-strip width and the accuracy
the guides are installed with. Generally, the maximum eleva-tion changes that can be anticipated will be reduced as the
dry-screed guides are moved closer suspended-slab construction, the desirability of using
dry-screed guides on both sides of each placement strip
is diminished by the damage done when the contractor
retrieves the guide system. For this reason, a combination of
dry-screed guide and wet-screed guide techniques should be
employed on suspended first placement strip should always start against a bulk-head or edge of the building. Strike-off on the interior side
of the strip should be controlled through the use of move-able dry-screed guides, which will provide positive control
over the surface elevation along that line. The concrete edge
along the moveable guide should be kept near vertical and
straight. As concrete is placed and struck off, these guides
are removed. When the next strip is placed, preferably in the
same direction as the initial strip, the prior strip will normally
have been in place for 30 minutes or more. The contractor
can extend the straightedge 2 ft (600 mm) or more over the
previous partially set placement to control grade of strike-off
on that side of the strip and use moveable dry-screed guides
to control grade on the side of the strip not adjacent to previ-ously placed suspended-slab construction, the procedure described
in the previous paragraph has several advantages over
unmodified wet screed techniques or those techniques that
use dry-screed guides on both sides of each placement strip:a) Where previously placed concrete is used as a guide for
strike-off, it provides a relatively stable guide because it will
have been in place for some time before it is used.b) Retrieval of the dry-screed guide from areas surrounded
by previously placed concrete is unnecessary because dry-rigid guides are not used in these locations.c) Moveable dry-screed guides should be used to establish
grade on any suspended slabs that are unlevel and shored
at the time of strike-off, and for any suspended slab where
increases in local slab thickness might be used to compen-sate for anticipated or identified differential deflection of the
structure. When an increase in local slab thickness is used to
compensate for differential floor deflection, it is likely that
the resulting slab will be more than 3/8 in. (9.5 mm) thicker
than design thickness. The contractor should secure permis-sion from the designer to exceed the plus tolerance for slab
thickness before beginning construction. Refer to 5.3 for a
discussion of suspended slab deflection and recommended
construction construction of slabs-on-ground, the use of vibrating
screeds where edge forms or screed-guide rails can be used
will facilitate strike-off operations. By using a vibrating
screed or laser screed, crews can place concrete at a lower
slump than might be practical if screeding was done by
hand. Suspended slabs are seldom both level and supported
at the time of construction. Vibrating screeds and roller
screeds similar to those used for slab-on-ground strip place-ments are generally not appropriate for use in suspended-slab construction because of the probability that their use
will result in slabs that are too thin in localized areas. Main-tain minimum slab thickness at all locations on suspended
slabs to meet compliances with contract documents and fire
safety up to 5 in. (125 mm) are often recommended for
concrete consolidated by vibrating screeds. Where slumps in
excess of 4 in. (100 mm) are used, the amplitude of vibration
should be decreased in accordance with the consistency of
the concrete so that the concrete does not have an accumula-tion of excess mortar on the finished surface after ing screeds can be used to strike-off and straight-edge the concrete in addition to providing consolidation. To
perform significant consolidation, the leading edge of the
blade should be at an angle to the surface, and the proper
surcharge, which is the height of unconsolidated concrete
required to produce a finished surface at the proper eleva-tion, should be carried in front of the leading ing screeds should be moved forward as rapidly
as proper consolidation allows. If not used in this manner,
too much mortar will be brought to the surface in normal-weight concrete; conversely, too much coarse aggregate will
be brought to the surface in lightweight concrete. For fiber-reinforced concrete, both vibrating and laser screeds can be
used to strike off and consolidate the concrete. The vibration
of both pieces of equipment will embed and orient the fibers
in a horizontal plane near the surface. When a laser screed is
used, the operator should adjust the magnitude of vibration
and control the speed of the reacting leveling head to ensure
adequate embedment of the fibers.10.3.3 Floating—The term “floating” describes compac-tion and consolidation of the unformed concrete surface.
Floating operations take place at two separate times during
the concrete finishing first floating, generally called bull floating, is by hand
and takes place immediately after screeding. Initial floating
should be completed before any excess moisture or bleeding
water is present on the surface. Any finishing operation
performed while there is excess moisture or bleed water on
the surface may cause dusting or scaling. This basic rule
of concrete finishing is essential. The first floating opera-tion is performed using a wide bull float, darby, or modified
highway straightedge. The second floating operation takes
place after evaporation of most of the bleed water and is
usually performed using a power trowel with float blades or
a pan attached. The second floating operation is described
in 10.3.9.
American Concrete Institute – Copyrighted © Material –
40 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)10.3.3.1 Bull floating—One of the bull float’s purposes
is to eliminate ridges and fill voids left by screeding opera-tions. Bull floating should embed the coarse aggregate only
slightly. This process prepares the surface for subsequent
edging, jointing, floating, and the specified finished floor flatness, using the
F-number system, restricts the difference between succes-sive 1 ft (300 mm) slopes to a maximum of 1/4 in. (6 mm)—approximately FF20 (10.15)—a traditional-width bull float
of 4 to 5 ft (1.2 to 1.5 m) can be used to smooth and consoli-date the concrete surface after screeding. The use of this bull
float, however, can adversely affect floor flatness and make
it extremely difficult to achieve flatness F-numbers higher
than 20. When the magnitude of difference between succes-sive 1 ft (300 mm) slopes is limited to less than 1/4 in. (6
mm) with a floor flatness greater than FF20 (10.15), an 8
to 10 ft (2.4 to 3 m) wide bull float is useful in removing
surface irregularities early in the finishing process. This is
particularly true for suspended-slab construction, where
local irregularities caused by form- or metal-deck deflection
and concrete leakage can be contractors use an 8 to 10 ft (2.4 to 3 m) wide bull
float or modified highway straightedge after initial strike-off
to restraighten any local irregularities that may be present.
Use of a traditional 4 to 5 ft (1.2 to 1.5 m) wide bull float
will provide little assistance to the finisher in correcting
these irregularities. Using the wider bull float or modified
highway straightedge allows the finisher to recognize and
correct irregularities at a time when significant amounts
of material can be moved with relatively little effort. This
simple substitution of tools can routinely produce up to a 50
percent increase in floor block placements for slabs-on-ground, and for suspended-slab placements, a wide bull float or modified highway straight-edge can also be used. Applied at an angle of approximately
45 degrees to the axis of the placement strip and extending
across the joint between the current strip and the strip previ-ously placed, these tools can remove many irregularities that
would otherwise remain if they were used only in a direction
perpendicular to the axis of the placement strip.A magnesium bull float can be used for lightweight concrete
and sticky mixtures or where a partially closed surface is
desired until it is time to float. The magnesium face of the bull
float slides along the fines at the surface and thus requires less
effort and is much less likely to tear the an embedded hardener or other special-purpose
aggregate is required and rapid stiffening is expected, the
use of a bull float, preferably wooden, can be helpful in
initially smoothing the surface after the aggregate is applied
and before the modified highway straightedge is used in the
initial cutting and filling operation. Inevitable variations in
the uniformity of coverage when an embedded hardener or
other special-purpose aggregate is applied will create slight
irregularities in the slab surface. Restraightening opera-tions necessary to remove these irregularities will remove
embedded material in some locations while adding to the
thickness of embedded material in other locations. Experi-ence has shown that some variations in the uniformity of
embedded material coverage do not adversely impact the
floor’s bull floats are preferable for use on normalweight
concrete that receives an embedded hardener. The wood’s
coarse texture enables it to evenly spread the mortar mixture
of cement and fine aggregate across the surface, leaving
the surface of the concrete open and promoting uniform
bleeding. If a magnesium bull float is used for normalweight
concrete, the embedded hardener should first be forced into
the concrete using a wooden float. This brings moisture to
the surface and ensures proper bond of the hardener to the
base slab. This is particularly important where dry shakes will
be applied for color, increased wear resistance, or both. For
fiber-reinforced concrete, wood floats are not recommended
because they tend to tear the surface and expose fibers.10.3.3.2 Darbying—Darbying serves the same purpose as
bull floating, and the same rules apply. Because bull floating
and darbying have the same effect on the surface of fresh
concrete, the two operations should never be performed on
the same surface. Because of its long handle, the bull float is
easy to use on a large scale, but the great length of the handle
detracts from the attainable leverage, so high tolerances are
more difficult to achieve. A darby is advantageous on narrow
slabs and in restricted spaces. Long-handled darbies should
be used for better leverage and control of level. Metal darbies
are usually unsatisfactory for producing surfaces meeting
high tolerance requirements. The same principles regarding
the use of wooden or magnesium bull floats (10.3.3.1) apply
to darbies because both darbies and bull floats are used for
the same purpose following screeding.10.3.3.3 Hand floating—Wooden hand floats encourage
proper workmanship and timing. If used too early on any
type of concrete, they stick, dig in, or can tear the surface.
Used too late, they roll the coarser particles of fine aggregate
out of the surface, at which time use of a magnesium float
held in a flat position would be preferable. Wooden floats
more easily fill in low spots with mortar; they should also
be used in areas where embedded hardeners or other special-purpose aggregates will be applied, floated, and finished
by hand only. The use of wooden hand floats has declined
largely due to the need for periodic replacement because of
wear or breakage, and the greater effort and care in timing
required in using them. Used at the proper time, their floating
action is unequaled by other hand ium hand floats require less effort. Like magne-sium bull floats, they slide along largely on fines. They can
be used on concrete from the time of placement to beyond
the point of stiffening when a wooden float cannot be used.
Magnesium floats are best used in the initial smoothing of
the surface near screeds, walls, columns, or other projec-tions, and during placing, screeding, and bull floating when
a wooden float would dig in or tear the surface. Magnesium
floats can also be used on air-entrained concrete that is not
to receive a troweled finish, or following wooden or power
floating to produce a more uniform swirl finish not quite as
roughly textured. Well-worn magnesium floats develop an
edge almost as sharp as a steel trowel, so care should be
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 41exercised to use them flat to avoid closing the surface too
early or causing ition hand floats using resin-impregnated canvas
surfaces are smoother than wooden floats and only slightly
rougher than magnesium floats. They are similar to magne-sium hand floats and should be used in the same manner.10.3.4 Highway-type straightedging—The use of a modi-fied highway straightedge for restraightening of the surface
varies with the type of slab being installed. Experienced
finishers can use this tool early in the finishing process
instead of an 8 to 10 ft (2.4 to 3 m) wide bull float. Care is
needed, however, because the straightedge tends to dig into
the concrete if it is used improperly. Initial restraightening
with the modified highway straightedge should immediately
follow screeding. Restraightening should be completed
before any excess moisture or bleed water is present on the
surface. When specified differences between successive 1 ft
(300 mm) slopes are 3/16 in. (5 mm) or less with flatnesses
higher than FF20 (Fig. 8.6), a modified highway straight-edge is recommended to smooth and restraighten the surface
after power floating or any floating operation that generates
significant amounts of mortar. A weighted modified highway
straightedge can also be used after power-trowel operations
to scrape the surface, reducing local high spots. Filling of
low spots is generally not appropriate after scraping with a
weighted modified highway flatness exhibited by any concrete floor will be deter-mined almost exclusively by the effectiveness of corrective
straightedging employed after each successive strike-off,
floating, and troweling step. Without restraightening, each
step performed in a conventional concrete floor installation
tends to make the surface less flat. Straightedges are capable
of restraightening or reflattening the plastic concrete because
they alone contain a reference line the resulting floor profile
can be compared against. Restraightening operations are
most effective when new passes with the modified highway
straightedge overlap previous passes by approximately 50
percent of the straightedge width. In contrast, traditional
4 to 5 ft (1.2 to 1.5 m) wide bull floats, power floats, and
power trowels are wave-inducing devices. To the extent that
further straightedging can only reduce floor-wave ampli-tudes and enlarge floor-wave lengths, floor surface flatness
can be further improved until Class 9 floor surface quality
is modified highway straightedge is used in a cutting
and filling operation to achieve surface flatness. When using
this or any restraightening tool, it is desirable to overlap
previous work with the tool by at least 50 percent of the
tool width. The tool should be used in at least two direc-tions, preferably in perpendicular directions to each other.
For strip placements, this can be accomplished by using the
straightedge at a 45-degree angle to the axis of the strip and
toward the end of the strip, followed by use of the straight-edge at a 45-degree angle toward the beginning of the strip.
The cutting and filling operation taking place in these two
directions from the edge of a placement strip will enable the
straightedge passes to cross at right angles and to produce a
flatter, smoother floor. Straightedging in a direction parallel
to the strip-cast operation and to the construction joints is
possible but less desirable because this would require the
finisher to stand in the plastic concrete or on a bridge span-ning the strip. This cut-and-fill process can also be performed
after power-floating operations (10.3.9) to further improve
the floor’s slabs-on-ground with an embedded metallic or
mineral hardener, coloring agents, or other special-purpose
material, the use of a modified highway straightedge plays
an important part in reestablishing surface flatness after
application of the material. These products are generally
applied after initial screeding or strike-off, and even the best
of applications will create irregularities in the surface. After
the hardener or special-purpose material has been worked
into the surface of the concrete using a wooden bull float,
a follow-up pass using the modified highway straightedge
is desirable to restraighten the surface after the embedded
metallic, mineral, and special-purpose material or its coating
has absorbed sufficient embedded metallic dry-shake hardeners and colored
dry-shake hardeners are applied immediately after the initial
power float pass (10.3.10). When these materials are rela-tively fine, it is necessary to wait until this point in the
finishing operation to begin their application. When applied
too early in the finishing process, they tend to be forced
below the surface by finishing operations. The use of a
modified highway straightedge to embed these materials and
to restraighten the surface after their application is a critical
component of the finishing ical spreaders should be used in the application of
metallic or mineral hardeners, colored dry-shake hardeners,
or other special-purpose materials. Hand spreading often
results in an inadequate and uneven application of the mate-rial, unless applied by highly skilled craftspeople.10.3.5 Waiting—After initial floating and restraightening
have been completed, a slight stiffening of concrete is neces-sary before proceeding with the finishing process. Depending
on job conditions, it may be necessary to wait for this stiff-ening to occur. Waiting time can be reduced or eliminated by
the use of dewatering techniques. No subsequent operation
should be done until the concrete will sustain foot pressure
with only approximately 1/4 in. (6 mm) indentation (10.3.10).10.3.6 Edging—Edging is not required or recommended
on most floors. Edgers should be used only when specifi-cally required by the project documents. Where edging is
required, use of walk-behind edgers is discouraged because
their use can yield inconsistent results. If the floor is to be
covered with tile, an edger should not be used. If edging
is required by the project documents, a 1/8 in. (3 mm) or
smaller radius edge should be used for construction joints
subjected to regular vehicular traffic, although saw-cutting
is the preferred method for this type of edger is used to form a radius at the edge of the slab
(10.2.6). Edging, or stoning when the placement is finished
flush with the edge forms, will also allow construction joints
to be readily visible for accurate location of sawing, when
used. The second placement at a construction joint will often
bond to the first placement. Sawing this joint encourages
American Concrete Institute – Copyrighted © Material –
42 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)development of a clean, straight crack at the construction
joint. Edging is most commonly used on sidewalks, drive-ways, and steps; it produces a neater looking edge that is
less vulnerable to chipping. Edging should not commence
until most bleed water and excess moisture have left or been
removed from the surface. Instead of being edged, construc-tion joints of most floor work can be finished flush with the
edge forms, and then lightly stoned to remove burrs after the
bulkheads or edge forms are stripped and before the adjacent
slab is placed.10.3.7 Hand-tooled joints—Slabs-on-ground are jointed
immediately following edging, or at the same time, unless
the floor is to be covered with hard or soft tile. If the floor
is to be covered with tile, jointing is unnecessary because
random cracks are preferable to tooled joints under tile. For
floors to be covered with quarry tile, ceramic tile, terrazzo
pavers, or cast-in-place terrazzo, the joints in slabs-on-ground should be aligned with joints in the rigid cutting edge or bit of the jointing tool creates grooves
in the slab, called contraction joints (5.2.9.3). For contrac-tion joints, the jointing tool should have a bit deep enough to
cut grooves that are one-fourth the thickness of the slab. This
forms a weak plane of weakness along which the slab will
crack when it contracts. Jointers with worn-out or shallow
bits should not be used except for forming decorative,
nonfunctional groves in the concrete surface. The jointer
should have a 1/8 in. (3 mm) radius for floors. Because of
limitations on bit length, hand-tooled joints are not practical
for slabs greater than 5 in. (125 mm) thick where the groove
depth is one-fourth of the slab is good practice to use a straight 1 x 8 in. or 1 x 10 in. (25
x 200 mm or 25 x 250 mm) board as a guide when making the
joint or groove in a concrete slab. If the board is not straight, it
should be planed true. The same care should be taken in running
joints as in edging because a hand-tooled joint can either add
to or detract from the appearance of the finished slab.10.3.8 Preformed joints—Preformed plastic and metal
strips are also available as an alternative to the use of jointers
or saw-cuts for making contraction joints. If used, they are
inserted in the fresh concrete at the time hand-tooled jointing
would take place. Proper performance of these strips is
extremely sensitive to installation. Plastic or metal inserts
are not recommended in any floor surface subjected to
wheeled traffic (5.2.9.3).10.3.9 Power floating—After edging and hand-jointing
operations, if used, slab finishing operations should continue
with use of either the hand float or the power float. Power
floating is the normal method selected. The purposes of
power floating are threefold:a) To embed the large aggregate just beneath the surface
of a mortar composed of cement and fine aggregate from the
concreteb) To remove slight imperfections, humps, and voidsc) To compact the concrete and consolidate mortar at the
surface in preparation for other finishing operationsIn the event that multiple floating passes are required,
each floating operation should be made perpendicular to the
direction of the immediately previous pass.
Nonvibrating, 24 to 36 in. (600 to 900 mm) diameter steel
disk-type floats are usually employed to float low-slump
or zero-slump concrete or toppings. They can also be used
for additional consolidating or floating following normal
floating operations when the surface has stiffened to a point
where it can support the weight of the machine without
disturbing the flatness of the ing machines equipped with float blades or pans
slipped over the trowel blades can be used for floating. Float
blades are beneficial when steel fiber reinforcement, surface
hardeners, or both, are used. Troweling machines with
combination blades could be used but are not recommended.
Floating with a troweling machine equipped with normal
trowel blades should not be permitted. Contract documents
should also prohibit the use of any floating or troweling
machine that has a water attachment for wetting the concrete
surface during finishing of a floor. Application of water by
brush or machine during finishing promotes dusting of the
floor surface and should be done only to overcome adverse
conditions. This should be discussed in the context of the
placement environment at a preplacement variables, including concrete temperature, air
temperature, relative humidity, and wind, make it difficult to
set a definite time to begin floating. The concrete is gener-ally ready for hand floating when the water sheen has disap-peared or has been removed, and the concrete will support
a finisher on kneeboards without more than approximately
a 1/8 in. (3 mm) indentation. The slab surface is ready for
machine floating with the lightest machine available when
the concrete will support a finisher on foot without more
than approximately a 1/4 in. (6 mm) indentation, and the
machine will neither dig in nor disrupt the levelness of the
surface. Mechanical pan floating should not begin until the
surface has stiffened sufficiently so that footprints are barely
perceived on the concrete ly, concrete will be ready for power floating in the
same order it was placed in. On a given placement, however,
certain areas can become ready for power floating before
others. The areas that should be floated first generally include
surfaces adjacent to screed guides, edge forms, blockouts,
walls, and columns. Areas exposed to sun tend to set more
quickly than those protected by shade; surfaces exposed to
wind also require attention before those protected from the
wind. One or more finishers should be assigned to look after
those areas that will set faster than the overall slow-setting conditions when flatness tolerances
are not high, power floating should be started as late as
possible; this is indicated by minimum machine indentation
or when a footprint is barely perceptible. Under fast-setting
conditions or when high-flatness tolerances are required,
and with the understanding that abrasion resistance of the
slab can be reduced, floating should be started as soon as
possible; the maximum practical indentation is approximately
1/4 in. (6 mm). When higher-flatness quality is required, the
floating operation should generate sufficient mortar to assist in
restraightening operations with the modified highway straight-edge. Flatness and levelness tolerances can require restraight-ening of the surface before and after the floating operation.
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 43The marks left by the edger and jointer should be removed
by floating, unless such marks are desired for decoration,
in which case the edger or jointer should be rerun after the
floating lly, when the floating operation produces sufficient
mortar, restraightening after the floating operation is benefi-cial. After the initial power-float pass, and while the surface
mortar is still fresh, the modified highway straightedge can
be used to restraighten the slab surface by removing the
troughs and ridges generated by the power float with float
blades or combination blades attached. This is accomplished
by cutting down the ridges and using that mortar to fill the
troughs. These operations should be completed during the
window of floating tends to create troughs under the center of
the machine in the direction of travel, with ridges of mortar
occurring just outside the perimeter of the blades. Around
projections such as columns and sleeves, the power float
tends to push mortar up against the projection. If this mortar
buildup is not removed by the hand finisher, it will remain
when the concrete hardens and the surface will be at a higher
elevation than desired. The use of float pan attachments on
riding machines reduces the tendency to create method that allows proper grade to be maintained
at projections is to place a benchmark a specified distance
above design grade on the projection for subsequent use by
the finisher. While completing hand work around the column
or sleeve, the finisher can use a template to confirm that
proper grade has been maintained. Excess material can then
be removed as required.10.3.10 Panning—Pans with ride-on trowel machines help
produce a flatter, more durable floor if used at the proper
time. Panning is an intermediate finishing step performed
between floating and finishing. Pans can be used for initial
floating. Panning should not begin until the surface has
stiffened sufficiently so that footprints are barely perceived
on the concrete surface. Panning may continue until the
concrete stiffens enough for troweling to begin.10.3.11 Troweling—The purpose of troweling is to produce
a dense, smooth, hard surface. Troweling is done following
floating and or panning; no troweling should ever be done
on a surface that has not been floated by power or by hand.
The use of a bull float or darby without following by hand or
machine floating is not troweling is done by hand, it is customary for the
concrete finisher to float and then steel trowel an area before
moving kneeboards. If necessary, tooled joints and edges
should be rerun before and after troweling to maintain
uniformity and true trowels that are short, narrow, or of inferior construc-tion should not be used for first troweling. Mechanical trow-eling machines can be used. The mechanical trowel can be
fitted with either combination blades or with those intended
specifically for the troweling the first troweling, whether by power or by hand, the
trowel blade should be kept as flat against the surface as
possible; in the case of power troweling, use a slow speed. If
the trowel blade is tilted or pitched at too great an angle, an
objectionable washboard or chatter surface will result. With
fiber-reinforced concrete, tilting of the blades at too great
an angle may expose fibers on the surface. A trowel that has
been properly broken in can be worked quite flat without the
edges digging into the concrete. Each subsequent troweling
should be made perpendicular to the previous pass. Smooth-ness of the surface can be improved by restraightening oper-ations with the modified highway straightedge and by timely
additional trowelings. There should be a time lapse between
successive trowelings to permit concrete to become harder.
As the surface stiffens, each successive troweling should be
made with smaller trowel blades or with blades tipped at a
progressively higher angle to enable the concrete finisher
to apply sufficient pressure for proper finishing. Additional
troweling increases the compaction of fines at the surface
and decreases the w/cm of concrete near the slab surface
where the trowel blades agitate surface paste and hasten the
evaporation rate of water within the paste; this process results
in increased surface density and improved wear resistance.
Extensive steel-troweling of surfaces receiving a colored dry-shake hardener can have a negative impact on the uniformity
of color. Refer to 10.6.2 for a detailed formation of blisters in the surface of the concrete
during troweling can be the result of entrained air or exces-sive fines in the concrete mixture, of early troweling, or of an
excessive angle of the trowel blades. Air-entrained concrete
should never be used in any normalweight concrete floor
slab that is to receive a hard-troweled finish. By hindering
the passage of bleed water to the surface, such purposeful
air entrainment can compel the finisher to start the troweling
process too quickly, leading to the entrapment of a liquid
water layer immediately beneath the prematurely closed
surface. Unfortunately, the concrete will appear to behave
normally in the initial troweling stages, so there is no way
for the finisher to know that the slab is being the air content is acceptable, then blister formation is an
immediate indication that the angle of the trowel blade is too
great for the surface in that area at that particular time for the
concrete and job conditions ive steel-troweling leaves the concrete surface with
a very high sheen. Such surfaces become quite slippery when
wet and should be slightly roughened to produce a nonslip
surface if they are to be exposed to the weather. A smooth-textured swirl finish can be produced by using a steel trowel
in a swirling motion, which is also known as a sweat finish,
or by brooming the freshly troweled surface.A fine-broomed surface is created by drawing a soft-bris-tled broom over a freshly troweled surface. When coarser
textures are desired, a stiffer bristled broom can be used after
the floating operation. A coarse-textured swirl finish can be
created after completion of the power float pass and subse-quent restraightening using a modified highway straightedge.
A coarse swirl pattern is normally created using a hand-held
wood or magnesium float (10.13.4). When a broomed fiber-reinforced concrete surface is required, the broom should be
held at a small angle to the surface to minimize excessive
fiber exposure. Never pull the broom in the opposite direc-tion or across the established pattern.
American Concrete Institute – Copyrighted © Material –
44 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)During periods of hot, dry, and windy weather, troweling
should be kept to the minimum necessary to obtain the desired
finish. When ambient conditions create high water loss due
to slab evaporation, fog spraying above the concrete or use
of an evaporation retardant is necessary. After finishing, any
delay in protecting the slab with curing compounds or other
water-retaining materials can result in an increase in plastic
shrinkage cracking, crazing, low surface strength, dusting,
and early deterioration.10.3.12 Saw-cut joints—On large, flat concrete surfaces,
rather than hand-tooling joints, it can be more convenient
to cut joints with an electric or gasoline-driven power saw
fitted with an abrasive or diamond blade using one of the
following three types of saws: conventional wet-cut, conven-tional dry-cut, or early-entry early-entry dry-cut process is normally used when
early sawing is desired. Early-entry dry-cut joints are formed
using diamond-impregnated blades. The saw-cuts resulting
from this process are not as deep as those produced using the
conventional wet-cut process, and are typically no more than
1-1/4 in. (32 mm). The timing of the early-entry process,
however, allows joints to be in place before development
of significant tensile stresses in the concrete; this increases
the probability of cracks forming at the joint when sufficient
stresses are developed in the concrete. Care should be taken
to make sure the early-entry saw does not ride up over hard or
large coarse aggregate. The highest coarse aggregate should
be notched by the saw to ensure the proper function of the
contraction joint. State-of-the-art early-entry saws have an
indicator that shows the operator if the saw-cut becomes too
lly, joints produced using conventional processes
are made within 4 to 12 hours after the slab has been finished
in an area; 4 hours in hot weather to 12 hours in cold weather.
For early-entry dry-cut saws, the waiting period will typi-cally vary from 1 hour in hot weather to 4 hours in cold
weather after completing the finishing of the slab in that
joint location. Longer waiting periods can be necessary for
all three types of sawing for floors with steel-fiber reinforce-ment or embedded-mineral-aggregate hardeners with long-slivered particles such as depth of saw-cut using a conventional saw should be
at least one-fourth the slab depth or a minimum of 1 in. (25
mm), whichever is greater. The depth of a saw-cut using an
early-entry dry-cut saw should be 1 in. (25 mm) minimum
for slab depths up to 9 in. (225 mm). This recommendation
assumes that the early-entry dry-cut saw is used within the
time constraints noted previously. For steel fiber-reinforced
slabs, the saw-cut using the conventional saw should be one-third the slab depth. Typically, when timely cutting is done
with an early-entry saw, the depth can be the same as for
concrete without steel less of the process chosen, saw-cutting should
be performed before concrete starts to cool, as soon as the
concrete surface is firm enough not to be torn or damaged by
the blade, and before random drying-shrinkage cracks can
form in the concrete slab. Shrinkage stresses start building
up in the concrete as it sets and cools. If sawing is unduly
delayed, the concrete can crack randomly before it is sawed.
Additionally, delay can generate cracks that run off from the
saw blade toward the edge of the slab at an obtuse or skewed
angle to the hot, dry, or windy conditions, especially when
placing exterior slabs, initial cracking can occur before
final troweling. These random cracks can also appear hours
or days after saw-cutting. The tendency for these cracks to
form can be reduced by fogging the air over the concrete,
using a monomolecular film, and starting the placement
at night to minimize the impact of temperature, wind, and
exposure to direct sunlight. When these conditions occur, it
may be prudent to stop floor placement until a time when
conditions are more favorable. Project delay may be more
desirable than random out-of-joint cracking.10.4—Finishing Class 1, 2, and 3 floorsClass 1, 2, and 3 floors (Table 4.1) include tile covered,
offices, churches, schools, hospitals, and garages. The
placing and finishing operations described under 10.3 should
be followed. Multiple restraightening operations and two
hand or machine trowelings are recommended, particularly
if a floor is to be covered with thin-set flooring or resilient
tile. This will give closer surface tolerances and a better
surface for application of the floor use of silica fume concrete for parking garage
construction lends itself to a one-pass finishing approach.
After initial strike-off and bull floating have been completed,
the concrete placement strips can be textured using a broom.
Normally, a light broom with widely spaced stiff bristles will
be satisfactory for this e silica fume concrete exhibits virtually no
bleeding, it is necessary to keep the surface moist during
concrete finishing operations to prevent plastic shrinkage
cracking. This normally requires use of an evapora-tion retarder or a pressure fogger with a reach capable of
covering the entire surface. Fogging should be performed
continuously between finishing operations until the surface
has been textured. The goal of the fogging operation should
be to keep the concrete surface moist but not wet. Curing
operations should commence as quickly as possible after
texturing has been completed (ACI Committee 226 1987). If
decorative or nonslip finishes are desired, refer to the proce-dures described in 10.13.10.5—Finishing Class 4 and 5 floorsClass 4 and 5 floors (Table 4.1) may be light-duty indus-trial or commercial. The placing and finishing operations
described in 10.3 should be followed. Three machine trow-elings can be specified for increased wear resistance.10.6—Finishing Class 6 floors and monolithic-surface treatments for wear resistanceIndustrial floors using embedded mineral or metallic hard-eners are usually intended for moderate or heavy traffic and, in
some cases, to resist impact. These hardeners should be prop-erly embedded near the top surface of the slab to provide the
required surface hardness, toughness, and impact resistance.
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 45The total air content of normalweight concrete should
exceed 3 percent only if the concrete is subject to freezing-and-thawing cycles under service conditions and the concrete
floor slab is not to receive a hard-troweled finish. As with
any commercial or industrial floor subjected to wheeled
traffic, special care should be exercised to obtain flat and
level surfaces and joints. Metallic hardeners should not be
placed over concrete with intentionally added chloride. The
proposed mixture proportions should be used in the installa-tion of any test panel or test placement. If adjustments to the
concrete mixture are required, they can be made at that time.10.6.1 Embedded mineral-aggregate hardener—The
application and finishing of embedded mineral-aggregate
hardeners should follow the basic procedures outlined in the
following. Concrete installations are subject to numerous
conditions and variables. Experience is necessary to deter-mine proper timing for the required procedures. These
procedures should be discussed and agreed upon at the
preconstruction meeting:a) Place, consolidate, and strike off concrete to the proper
grade.b) Compact and consolidate the concrete surface using a
bull float.c) Restraighten the surface using a modified highway
straightedge. Occasionally, compacting, consolidating, and
restraightening are accomplished in one step by using a
wide bull float or a modified highway straightedge with the
straightedge rotated so its wide dimension is in contact with
the surface.d) Evenly distribute approximately two-thirds of the spec-ified amount of mineral-aggregate hardener immediately
following strike-off, and before the appearance of bleed
water on the slab surface. The first application generally
consists of a larger, coarser material than will be used in the
final application. Distribution of the hardener by mechan-ical spreader is the preferred method. The concrete mixture
should have proportions such that excessive bleed water
does not appear on the surface after the hardener is applied.e) As soon as the hardener darkens slightly from absorbed
moisture, a long float with rounded edges should be used
to embed the hardener and remove any irregularities in the
surface.f) Wait until the concrete sets up sufficiently to support
the weight of a power trowel with float blades. Combina-tion blades should not be used. The float breaks the surface
and agitates concrete paste at the surface of the slab. The
first power-float passes should be across the placement strip
in the short direction. This will ensure that irregularities
resulting from the power floating can be easily identified and
corrected in subsequent operations.g) Apply the remaining one-third of the specified mineral
aggregate, preferably at right angles to the first application.
This material generally consists of finer-size aggregate and
may be broadcast evenly over the slab surface by hand.h) Restraighten the surface using a modified highway
straightedge. Remove irregularities and move excess mate-rial to low spots.
i) Embed the mineral-aggregate fines using a power trowel
with float blades or a pan attached.j) Restraighten the surface following the power-floating
operation using a weighted modified highway straightedge if
its use is seen to be effective or necessary to achieve required
surface tolerances. One method of increasing the weight of a
modified highway straightedge is to wedge a No. 11 bar (35
mm) inside the rectangular section of the straightedge.k) Continue finishing with multiple power trowelings as
required to produce a smooth, dense, wear-resistant surface
(10.3.11). Provide a burnished, or hard-troweled, surface
where required by specification.l) Cure immediately after finishing by following the curing
material manufacturer’s recommendations. Curing methods
should be in accordance with those used and approved in
construction of any test panel.10.6.2 Metallic dry-shake hardener and colored dry-shake hardeners—Metallic dry-shake hardeners and colored
dry-shake hardeners can be finer in texture than uncolored
mineral-aggregate dry-shake hardeners. This difference,
along with the fact that the metallic dry-shake hardener has
a higher specific gravity, dictates that the material normally
be embedded in the concrete later in the setting process than
is common for uncolored mineral-aggregate dry-shake hard-eners. Some metallic and colored dry-shake hardeners are
designed by their manufacturers to allow all the hardener
application at one time. When such procedures are used,
however, caution should be exercised to ensure that manu-facturer’s recommendations are followed and that the mate-rial is thoroughly wetted-out, because a one-time application
significantly increases the possibility of surface delamina-tion or related finishing problems. Typical installation tech-niques for metallic dry-shake hardeners and colored dry-shake hardeners are similar to those described in 10.6.1, but
the following sequence is recommended (10.13.1):a) Place, consolidate, and strike off concrete to the proper
grade.b) Compact and consolidate the concrete surface using a
bull float.c) Restraighten the surface using a modified highway
straightedge. A wide bull float or a modified highway
straightedge can be used to accomplish both steps in one
operation.d) Open the surface to promote movement of bleed water
to the top of the slab by using a wooden bull float. Steps 3
and 4 can be accomplished in one operation if the wide bull
float or modified highway straightedge is made of wood.e) Wait until the concrete sets up sufficiently to support the
weight of a power trowel.f) Break the surface using a power trowel with float blades
or a pan attached.g) Evenly distribute approximately two-thirds of the spec-ified amount of metallic dry-shake hardener or colored dry-shake hardener. Application of the material by mechanical
spreader is the preferred method.h) Restraighten the surface after application of the metallic
dry-shake hardener or colored dry-shake hardener to remove
irregularities. Some contractors find that embedding the
American Concrete Institute – Copyrighted © Material –
46 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)materials and restraightening can be accomplished in one
step using a modified highway straightedge.i) Complete initial embedment and prepare the surface for
additional material by using a power trowel with float blades
or a pan attached.j) Apply the remaining one-third of the specified amount
of metallic dry-shake hardener or colored dry-shake hard-ener, preferably at right angles to the first application.k) Embed metallic dry-shake hardener or colored dry-shake hardener using a power trowel with float blades or a
pan attached. Thorough embedment and integration of the
metallic dry-shake hardener or colored dry-shake hardener
with the concrete by floating is very important. Failure to
accomplish this goal can result in blistering or delamination
of the slab.l) Restraighten the surface following the power-floating
operation using a weighted modified highway straightedge,
if effective.m) Continue finishing with multiple power trowelings as
required to produce a smooth, dense, wear-resistant surface
(10.3.11). Proper and uniform troweling is essential. Colored
surfaces should not be burnished, or hard-troweled; the
result would be uneven color and a darkening of the surface.n) Cure immediately after finishing by following the curing
material manufacturer’s recommendations. Curing methods
should be in accordance with those used and approved in
construction of any test panel. Colored floors should not be
cured with plastic sheeting, curing paper, damp sand, or wet
burlap. These materials promote uneven color, staining, or
efflorescence.10.7—Finishing Class 7 floorsThe topping course of heavy-duty industrial floors should
have a minimum thickness of 3/4 in. (19 mm). The concrete
topping used should have a maximum slump of 3 in. (75
mm), unless a water-reducing admixture or high-range water-reducing admixture is used to increase the slump, or unless
dewatering techniques are used. Because of the relatively small
amount of concrete in the topping course and the low slump
required, concrete for the topping could be mixed on ed metallic dry-shake hardeners, mineral-aggre-gate dry shakes, and colored dry-shakes can be applied to
produce the desired combination of increased wear resis-tance or color as described in 10.6.1 and 10.6.2, base course should be screeded and bull floated;
close maintenance of the elevation tolerance for the base
course surface is important. Class 7 floors (Table 4.1) can be
constructed in two ways: (1) the topping installation can be
bonded monolithically to the base slab before the base slab
has completely set; or (2) the topping can be deferred for
several suspended slabs, the deferred bonded approach should
be used. This will allow the structure to deflect under its own
weight before application of the topping. The additional
weight of the topping will have little impact on subsequent
deflection of the slab.10.7.1 Bonded monolithic two-course floors—In most
cases, wet curing is recommended for the bonded topping.
Special precautions should be taken to prevent premature
drying of the edges because curling of the topping and
delamination from the base slab can these floors, the topping course is placed before the
base course has completely set. Any excess moisture or
laitance should be removed from the surface of the base
course, and the surface floated before the top course is
placed. When the topping is being placed, the concrete in
the base slab should be sufficiently hard that footprints are
barely perceptible. The use of a disk-type power float can
be necessary to bring sufficient paste to the surface to allow
restraightening to take place. The power-floating operation
should be followed by a minimum of two power trowelings.
This method of topping application is generally not appro-priate for a suspended slab.10.7.2 Deferred bonded two-course floors—Bonding of
two-course floors is a highly critical operation requiring the
most meticulous attention to the procedure described. Even
with such care, such bonding has not always been successful.
As a result, contractors using this type of construction for
heavy-duty industrial applications should be experienced
and familiar with the challenges ons of joints in the base course should be marked
so that joints in the topping course can be placed directly
over the base course has partially set, the surface should
be brushed with a coarse-wire broom. This removes laitance
and scores the surface to improve bond of the topping te base courses should be wet-cured a minimum
of 3 days (11.2.1 and 11.2.2). Shrinkage-compensating
concrete base courses should be wet-cured a minimum of 7
to 10 days, and preferably until the topping is applied. Refer
to ACI 223R for additional the topping is to be applied immediately after the
minimum 3-day curing time has elapsed, the curing cover
or water should be removed from the slab, and any collected
dirt and debris should be washed or hosed off. After most
free water has evaporated or has been removed from the
surface, a bonding grout should be scrubbed in. The bonding
grout should be composed, by volume, of one part cement,
1.5 parts fine sand passing the No. 8 (2.36 mm) sieve, and a
sufficient amount of water to achieve the consistency of thick
paint. The grout should be applied to the floor in segments,
keeping only a short distance ahead of the concrete topping
placing operations that follow it. While the bonding grout is
still tacky, the topping course should be spread and screeded.
The use of a disk-type power float is recommended, followed
by a minimum of two power 3 to 7 days are to elapse between placing the base and
the topping course, the surface of the base course should be
protected from dirt, grease, plaster, paint, or other substances
that would interfere with the bond. Immediately before
placing the topping, the base course should be thoroughly
cleaned by scrubbing with a brush and clean water. Most
excess water should be removed and a thin scrub-coat of
grout applied. While this grout is still tacky, the topping
course should be spread and screeded.
American Concrete Institute – Copyrighted © Material –
GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15) 47If the floor is to be subjected to construction activities after
curing and before application of the topping, more thorough
cleaning may be necessary. One method of cleaning the base
slab is to scrub the surface with water containing detergent.
If oil or grease has been spilled on the floor, a mixture of
sodium metasilicate and resin soap is useful. If this method
is used, the floor should then be rinsed thoroughly with
water. Shot-blasting, sandblasting, or mechanical scarifica-tion by scabbling can also be employed instead of cleaning
with detergent to achieve a bondable some circumstances, it can be convenient or desirable
to bond the topping with an epoxy adhesive appropriate for
the particular application. A standard specification is given
in ACI 503.2. Joints in the topping above the joints in the
base slab should be saw-cut to a depth equal to twice the
thickness of the topping and should match the location of
joints in the base slab, where applicable.10.8—Finishing Class 8 floors (two-course
unbonded)The unbonded topping for two-course unbonded floors
should be a minimum of 4 in. (100 mm) thick. An unbonded
topping thickness of 3 in. (75 mm) has been used with some
success for Class 3 floors, but thickness for strength and
control of curling is less important for a Class 3 slab because
of its duty, loading, and because it may also be covered. A
Class 8 floor (Table 4.1) is intended for industrial applica-tions where strength and control of curling is more impor-tant. The base course, whether old or new, should be covered
with plastic sheeting, felt, a sand cushion, or other approved
bond-breaker, and spread as wrinkle-free as topping slab should contain sufficient steel reinforce-ment to limit the width of shrinkage cracks in the topping
and the displacement of the topping concrete on either side
of any cracks that might form. Steel fiber and high-volume
synthetic fibers in proper quantities may be used effectively
to minimize crack-opening widths. Concrete for the top
course should comply with the requirements of Table floats and power trowels are recommended and
usually required. The practice of completing troweling by
hand is counterproductive because hand troweling is less
effective than power troweling in consolidating the ed mineral-aggregate hardeners for increased
wear resistance can be applied as described in 10.6.1.
Embedded metallic dry-shake hardeners and colored dry-shake hardeners can be applied as described in 10.6.2.10.9—Finishing Class 9 floorsClass 9 floors (Table 4.1) may be superflat or require crit-ical surface tolerance. Floor surfaces of this quality can be
subdivided by function into two separate groups. Refer to
10.9.1 for special considerations dealing with construction
of Class 9 floor more common group of these floor surfaces should
support vehicular traffic along paths that are defined before
construction and that do not change during the life of the
floor surface—that is, defined traffic. A typical example of a
defined-traffic floor would be a distribution center that uses
very narrow aisles and high-bay racking systems. In this
type of facility, tolerances across aisles and the joints that
parallel them are less critical than those along the axis of
the aisle. This type of floor surface is often called surfaces in the second group are less common
but should support traffic in all directions, that is, random
traffic. A typical example of a random-traffic floor would be
a gymnasium, ice rink, or television or movie studio. The
random nature of traffic in these facilities requires that toler-ances across placement strips and their joints should match
those achieved parallel to the axis of the ing procedures required to produce Class 9 floors
represent the most rigorous and demanding floor installation
technology now being performed. However, if discipline
and preplanning are a part of the overall process, installa-tion of Class 9 floors is neither complex nor especially diffi-cult. Proper timing and execution of various procedures will
usually ensure that the floor produced is of a predictable
9 defined-traffic floor construction requires that:a) Slabs be constructed in long strips less than 20 ft (6 m)
in widthb) Concrete slump be adjusted on-site to within ±1/2 in.
(±13 mm) of the target slumpc) Slump at point of deposit be sufficient to permit use of
the modified highway straightedge to close the floor surface
without difficulty after the initial strike-offd) Window of finishability be sufficient for the concrete
contractor to perform the necessary finishing operationse) Concrete supplier use enough trucks to ensure an unin-terrupted concrete supplyIn addition, because environmental factors can signifi-cantly alter the setting rate of concrete, an effort is usually
made to construct Class 9 floors out of the weather and under
environmentally controlled conditions where Class 9 defined-traffic floors, construction joints
between placement strips are located out of the traffic pattern
where racks abut each other. These surfaces are evaluated by
taking measurements only in locations matching the wheel
paths of the vehicles that will eventually use the floor. The
part of the floor surface falling under racks is not s the same construction techniques can be used to
produce Class 9 random-traffic floors, such as for television
studios or similar surfaces, the entire floor surface should
be evaluated because the entire surface will be subjected
to traffic. The contractor is cautioned that grinding of the
entire length of the joints will be necessary to produce Class
9 quality across the width of concrete placement most projects with Class 9 defined-traffic floors,
surfaces are measured for flatness and levelness imme-diately following the final troweling of each placement;
placements are frequently scheduled for consecutive days.
Where Class 9 random-traffic quality is required across
multiple placement strips, initial testing should take place as
each strip is placed, but final testing should be deferred until
the installation is eless, it is imperative that surface-profile testing and
defect identification be accomplished on each new slab as soon
American Concrete Institute – Copyrighted © Material –
48 GUIDE TO CONCRETE FLOOR AND SLAB CONSTRUCTION (ACI 302.1R-15)as possible. To maintain satisfactory results, the contractor
requires continuous feedback to gauge the effectiveness of
construction techniques against ever-changing job conditions
(10.9.1). Refer to Figure 10.9 for additional ing Class 9 quality levels on suspended slabs is
impractical in a one-course placement. Deflection of the
surface between supports occurs after removal of supporting
shores. If the surface was to meet Class 9 requirements in a
shored condition, it is likely that the deflected surface, after
shores are removed, would be less level than is required to
meet Class 9 requirements. Two-course placements using
methods similar to those discussed for Class 7 and 8 floors
provide the best opportunity for achieving Class 9 quality
levels on suspended floors.10.9.1 Special considerations for construction of Class
9 floor surfaces—Certain specialized operations—narrow-aisle warehouses, gymnasiums, ice rinks, television studios,
and air-pallet systems—require extraordinarily flat and level
floors for proper equipment performance. Such superflat
floors, if random traffic, exhibit FF numbers at or above 60
and FL numbers at or above 40 in the direction of travel for the
particular application. Refer to 10.15 for additional floor-finish tolerance employed in the contract speci-fication should meet the equipment supplier’s published
requirements unless there is reason to doubt the validity
of such requirements. In any case, written approval of the
contract floor tolerance should be obtained from the appro-priate equipment supplier before finalizing the bid package.
In this way, equipment warranties will not be jeopardized,
and the special superflat nature of the project will be identi-fied to key parties from the lat floors have very specific design requirements.
Chief among these is the limit imposed on placement
width. In general, defined-traffic superflat floors cannot be
produced if the placement strip width exceeds 20 ft (6 m).
Because hand-finishing procedures and curling effects are
known to make floors in the vicinity of construction joints
less flat than in the middle of the slab, joints should be
located out of the main traffic areas, or provisions should be
made for their correction. Contraction joints oriented trans-verse to the longitudinal axis of a Class 9 placement strip can
curl and reduce surface flatness along aisles. Limited place-ment width, consequent increased forming requirements,
and reduced daily floor production are primary factors that
increase the cost of Class 9 prebid meeting is an essential component of any
superflat project. Because floor flatness/levelness is one of
the primary construction requirements, a thorough prebid
review of the design, specification, and method of compli-ance testing is required. This will enable the prospective
contractor to price the project realistically, thereby avoiding
costly misunderstandings and change orders, and will greatly
increase the chances of obtaining the desired results at the
lowest possible further reduce the risk of problems, the installation
of test slabs has become a standard part of superflat floor
construction. If the contractor is inexperienced with super-flat construction or with the concrete to be used, at least
two test slabs should be installed and approved before the
contractor is permitted to proceed with the balance of the
superflat floor d traffic superflat floor tolerances should be
measured and reported within 4 hours after slab installation.
This eliminates the possibility of large areas being placed
before any tolerance problem is discovered. In narrow-aisle
warehouses, tolerances are measured using a continuous
recording floor profilograph or other device. In these facili-ties, floor tolerances are based on the lift-truck wheel dimen-sions, and compliance measurements and corrections are
required only in the future wheel television studios and other similar random-traffic
installations, the use of FF and FL to specify the floor-surface
tolerances is appropriate. Measurements for compliance
should be made in accordance with ASTM E1155 (10.15),
except that measurements should extend to the joints.10.10—Toppings for precast floorsMany types of precast floors require toppings. These
include double tees, hollow-core slabs, and other kinds of
precast floor elements. When these floors are to be covered
with bonded toppings, the procedures in 10.7.2 or 10.8
should be followed, as appropriate. High-strength concrete
is often used for precast floor elements; roughening of the
surface of such members can be difficult if delayed too long.
Rather than use strip placements on this type of floor, it may
desirable to make a block placement using closely-spaced
grade pins to control the accuracy of initial strike-off. The
spacing of the grade pins should not exceed 10 ft (3 m) in
perpendicular directions. The elevation of the top of the
grade pins should be closely controlled to ±1/16 in. (±1.5
mm) using an optical level.10.11—Finishing lightweight concreteThis section concerns finishing lightweight concrete
floors. Finishing lightweight insulating-type concrete,
having fresh density of 60 lb/ft3 (960 kg/m3) or less, that is
sometimes used below slabs, generally involves little more
than ural lightweight concrete for floors usually contains
expanded shale, clay, slate, or slag coarse aggregate—expanded shale is most common. The fine aggregate can
consist of fine lightweight aggregate, natural sand, or a
combination of the two, but natural sand is most common.
The finishing procedures differ somewhat from those used
for a normalweight concrete. In lightweight concrete, the
density of the coarse aggregate is generally less than that of
the sand and cement. Working the concrete has a tendency
to bring coarse aggregate rather than mortar to the surface.
This should be taken into account in the finishing ing the following recommendations will help
control this tendency so that lightweight concrete can be
finished similar to normalweight concrete, provided the
mixture has been properly proportioned:a) The mixture should not be over-sanded in an effort to
bring more mortar to the surface for finishing. This usually
will aggravate, rather than eliminate, finishing difficulties.
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