In the past year, we’ve heardfrom five contractors who havebeen expected to pay for floor-slab delamination repairs relatedto high entrained-air contents.

In all of these slabs, the hardened-concrete air content exceeded 10%.Since ACI 302.1R-96, “Guide forConcrete Floor and Slab Construc-tion” (Ref. 1), recommends an aircontent less than 3% for floors re-ceiving a hard-trowel finish, are con-tractors liable for the cost of thesedelamination repairs? And why wasair-entrained concrete specified foran interior floor?

The designers’ rationale

Under service conditions, most in-terior slabs don’t have a highmoisture content or a chance tofreeze, so air-entrained concreteisn’t needed to ensure a durablefloor surface. But what aboutconcrete exposed to freezingduring construction?

Non-air-entrained concretewith a moisture content nearsaturation is susceptible to sur-face scaling caused by freeze-thaw cycles. Because of this, de-signers often require interiorconcrete slabs to be air-en-trained if the slabs will be ex-

posed to a cold, wet environmentduring construction. In addition torequiring entrained air, designersmay also specify a lower water-ce-ment ratio and higher compressivestrength, which, in some regions,can increase concrete costs by asmuch as $5 per cubic yard.

Designers take these precautionsto minimize the owner’s risk of adamaged slab surface and the subse-quent cost of repairs. If they didn’trequire air-entrained concrete andthe slab was exposed to a freezingconstruction environment for onewinter, scaling could damage theslab surface in areas where waterponds, requiring local repairs. In ex-treme cases, the entire slab surfacecould scale, requiring a full-surface

topping. But owners who opt to use air-en-

trained concrete to reduce the risk ofsurface scaling and the cost of anyresulting repairs may face anotherthreat—the potential for delamina-tions when air-entrained concrete ismachine troweled.

Balancing the risk: scalingvs. delamination potential

Because of the risk of delamina-tions, ACI 302.1R and ACI 301-96,“Specifications for Structural Con-crete” (Ref. 2), recommend againstincorporating entrained air in nor-malweight concrete slabs requiring amachine-troweled finish. A field trial(Ref. 3) and our studies of petrographic reports related to

Beware of trowelingair-entrained concrete floors

BY BRUCE A. SUPRENANT AND WARD R. MALISCH

Some designers specify air-entrained concrete for interior floors subjectto a few freeze-thaw cycles during construction. But be aware that thisshifts the risk of surface damage from the designer to the contractor.

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Machine-troweled surface Machine-troweled surface

ScalingDelamination

Scaling caused by freezingand thawing of wet, non-air-entrained concrete

Delamination caused by anaccumulation of air andwater beneath the troweledsurface

OR

Which is more likely—scaling of non-air-entrained concrete exposed to freezing andthawing, but only during construction, or delamination of machine-troweled, air-en-trained concrete? Designers, owners and contractors should consider this question whenmaking decisions on specification requirements for interior concrete floors.

2 • CONCRETE CONSTRUCTION / MONTH 1999

delaminations indicate that en-trained air increases the likelihoodthat machine-troweled floors will de-laminate.

When designers recommend re-ducing the owner’s risk of slab dam-age due to freeze-thaw exposure byspecifying air-entrained concrete,they should also point out the in-creased likelihood of delamination.In cases where the use of air-en-trained concrete increases materialscosts, owners may decide the cost of

air entrainment and the increasedrisk of delaminations doesn’t justifythe added protection against scaling.If the slab does scale, owners wouldthen be expected to bear the cost ofrepairs because they accepted therisk of scaling when they elected touse non-air-entrained concrete. Butthe cost of localized surface repairsmight still be less than the extra costfor air-entrained concrete.

However, if the owner decides touse air-entrained concrete, and the

slab delaminates, the cost burdenshifts to the contractor. Because de-laminations are considered to be aconstruction defect, the contractor isexpected to pay for the repairs, eventhough the designer specified theuse of the air-entrained concretethat increased the delamination risk.By expecting finishers to hard trowelan air-entrained floor without de-laminations occurring, designerstransfer risk from the owner to thecontractor. To be properly compen-sated for this risk, contractors shouldbe paid a higher price per squarefoot of floor finished. However,that’s seldom done.

Before requiring air-entrained con-crete for interior slabs with a ma-chine-troweled finish, designersshould assess:n The risk of concrete scaling due to

cold-weather exposuren The cost of air-entrained concrete

vs. non-air-entrained concreten The cost of repairing scaling or

other freeze-thaw damagen The risk of delaminations when

air-entrained concrete slabs aremachine troweled

n The cost of repairing slab delami-nations These risks and associated conse-

quences should be explained to theowner before the project begins.Contractors who are expected toplace and finish air-entrained floorsshould also be aware of the risks.

References

1. ACI 302.1R-96, “Guide for ConcreteFloor and Slab Construction,” AmericanConcrete Institute, Farmington Hills,Mich., 1996.

2. ACI 301-96, “Standard Specifica-tions for Structural Concrete,” ACI,1996.

3. Bruce A. Suprenant and Ward R.Malisch, “Diagnosing Slab Delamina-tions,” Concrete Construction, January1998, pp. 29-35.

BEWARE OF TROWELING continued

Although finishers can’t preventall delaminations, they can mini-mize them if they can bend therules for what is considered to begood finishing practice. David Ros-setti of DJ Rossetti Inc., Schenec-tady, N.Y., and Dennis Phillips ofBaker Concrete Inc., Monroe,Ohio, both suggest the followingapproach, based on their experi-ences:

1. Delay floating until all bleedwa-ter has evaporated and the con-crete is sufficiently stiff.

2. Make two passes with floatblades or pans, waiting the nor-mal time interval betweenpasses. Make the second pass ata right angle to the first pass.

3. Delay troweling until the sur-face is too stiff to close up. Thisallows the concrete to continuebreathing. Here’s a good rule ofthumb for deciding when tostart troweling passes: If youcan close up the surface with-out misting it with water orevaporation retarder, it’s toosoon to be troweling.

4. Make two passes with trowelblades (or combination blades,if you prefer), lightly mistingahead of the power trowel toclose up the surface. Again,wait the normal interval be-tween passes, and make the

second pass at a right angle tothe first.

5. If a burn finish is required,make additional trowel passeswith the blades tilted.

Before using this finishingapproach to solve the delami-nation problem, be aware ofthe possible effects on floorquality. Because final trowelingis delayed until the surface sets,floor flatness may suffer. If highfloor flatness numbers are re-quired, the delay in trowelingmay cause an out-of-spec value.Adding water to the surfacemay also reduce abrasion resis-tance. But this water is a neces-sary consequence of allowingthe concrete to breathe until itsets.

“Although you can attain agood, lasting finish using thisprocedure,” says Rossetti, “itwill be risky because finishersmay try to use conventional fin-ishing methods that they’velearned over the years. The bestpolicy is to get involved in themix design before approvals bythe engineer and seek to elimi-nate the requirement for air-en-trained concrete. If this can’t beachieved, insist on a release ofliability for any delamination re-pairs.”

Bending the rules and accepting trade-offs

Publication #C99B035Copyright© 1999, The Aberdeen Group

a division of Hanley-Wood, Inc.All rights reserved