Use of Vapor Barriers Under Slab On Grade

8/12/2019 Use of Vapor Barriers Under Slab On Grade

1/4

ISSN 0701 -

PUB

I - .db = t ..... . --4 . ,f#;. $.:,.-.::::: *::--: ,:.:(BUILDINGPRACTICENOTE .+Vky ;.LyL

. ,T bs&Fzg~ . . b ~ * i ~>p7 .,, ..-3 i---:,*-.>. - .

TH USE OF VAWUR ARRIERS U N E RCONCRETE SLABS ON GROUND

byR.G . Turenne

Division of Building Research, National Research Council of Canada

O t t a w a August 1978


2/4

TH USE OF V P OUR BARRIERS UNDERCONCRETE SLMS ON GROUND

R. G, Turenne

The Division bas received a number of inquiries regarding thpossible weakening effe t of placing concrete for floor slabs on plasticsheets instead of directly an the subgrade, These inquiries have beenprompted by the interpretation given to a recent research paper l e . ,that t is practice which deprives the concrete of much needed moisturef r o m the soil, can cause excessive shrinkage cracking and loss ofstrengthDescription of California Tests

In these tests, which were conducted i n southern California,concrete slabs, 7 to 92 mm thick weye placed on different substratessubjected to various curing treatments and their behaviour observedover a period of one year. Cmcrete slump at the t5me of placingvaried between 200 and 229 mm 8 to 9 in. .The substrates used were the following:a m i l thick polyethylene placed on the groundb) a 76 sr thick dampened sand bed on naTive s o i lc ) a pervious non-compacted 76 thi k bed of dampenedcement-treated sand mixture.An area af slab over each of the three substrates received nocuring treatment Other areas were treated as follows:1) sprayed-on membrane forming curing compound2 rolled-on wax base c u ~ i n g ompmd3) trowelled-in colour and rolled-on wax-based curing compoundOf special interest is that Ifthe slabs were cast on a hat and windyday not uncommon i n southern California, and were left totallyunprotected for a period of one year except for t h e curing treatmentsjust described.


3/4

Follawing the one year exposure the researchers reported that theslabs cast directly on palyethylene showed excessive cracking whilethose placed on dampened sand or dampened cement-treated sand had oneshrinkage crack each. Core and cube ompression tes ts indicated thatthe concrete placed on sand was 30 per ent stronger after one year thanthe concrete placed on the polyethylene.Discussion of California.Tests Results

These results mus t be examined in the l i g h t of various designconsiderations and the present construction requirements and practice inCanada.Present pract5ce in anada is to place between the natural soiland the concrete slab a dampproofing layer of granular f i l l These

granular f i l l s are used under most on rete slabs an grade t o providefor subsurface drainage to contra moisture diffusion or as areplacement fo r undesirable materials. Granular fills are also anideal material to set smooth grades and prepare an area to receive auniform thickness of concrete. These fills can be used with or without:a polyethylene film depending ran design or building code requirements.

Several features of the California tests make a direct comparisonwith the Canadian practice d i f f i c u l t such as the constant expasure Tosun and wind in the California t e s t and the apparent absence of propermoist euring at l eas t during the first few days conditions thatnormally do not apply in Canada.Tn residential and even commercial and industrial construction inCanada floors on ground axe often placed after the walls and roofs areerected. Th i s prevents their exposure to direct solar hear and protectsthem from the drying effect of the wind. Goad practice in Canada

requires thgt the surface of fresh concrete be kep-t: continuously moistfor a mtnimum of days and that the concrete be subjected to moistcuring for a t least 7 days. To be conclusive the California tests wouldhave to be repeated under Canadian climatic conditions and the procedureclosely follow recommended anadian practice. It would al so be necessaryto have t he same t s t condition e . g . dampened sand beneath m of theszurrples

Although the study would seem to demmstrate that cast ing a concreteslab on a pervious base minimiaes shrinkage cracking the conditionsunder which the tests were conducted hardly jus t i fy the elimination ofgranular i l l and polyethylene film as a means of controlling subsurfacemoisture .The California study also shows the possible effect of the waterlcementratio on the ultimate strength af concrete. Although much of the excess

w a t e r 220 t 9 slump] use d in mixing concrete placed on a dampened


4/4

sand bed or a cement-treated sand bed in the tests could b l e e d in to thesubstrate {thus lowering the waterlcement r tio of the mix , this w snot the case for concrete placed on a polyethylene film. In this casea l l the excess water [that is w ker not required for hydration] had t oevaporate, which would account for the lower strengths and the excessiveshrinkage. If anything the study demmstrates the importance of usingas low a water/cement ratio s possible in concrete f loors on grade andif great workability is required, consideration of the use ofplaslicizars

The decision to place vapour b rrier under a concrete s lab onground should be based on design factors and performance requirements.The water/cement rat io should be kept as low s possible wns i s t entwith workability requirements.Recommended curing methods s h w l d be followed to ensure that theconcrete develops the desired strength and finish.

REFERENCE1 ob conditions affect cracking and strength of concrete in-place byRichard H. Campbell Wendell Harding, Edward MisenhimerGeo P. Nicholsan and Jack Sisk . ACI Journal, January 1976

Use of Vapor Barriers Under Slab On Grade

Documents

Transcript of Use of Vapor Barriers Under Slab On Grade