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Concrete curing techniques

Concrete curing techniques

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  • 1. SCOPEThis data sheet provides information on theimportance of curing and the techniques forundertaking the process on site.INTRODUCTIONCuring is the process of controlling the rate andextent of moisture loss from concrete duringcement hydration. It may be either after it hasbeen placed in position (or during the manufactureof concrete products), thereby providing time forthe hydration of the cement to occur. Since thehydration of cement does take time – days, andeven weeks rather than hours – curing must beundertaken for a reasonable period of time if theconcrete is to achieve its potential strength anddurability. Curing may also encompass the controlof temperature since this affects the rate at whichcement hydrates.The curing period may depend on the propertiesrequired of the concrete, the purpose for whichit is to be used, and the ambient conditions, iethe temperature and relative humidity of thesurrounding atmosphere.Curing is designed primarily to keep theconcrete moist, by preventing the loss of moisturefrom the concrete during the period in whichit is gaining strength. Curing may be appliedin a number of ways and the most appropriatemeans of curing may be dictated by the site or theconstruction method.Apr2006Datasheet>>curing of ConcreteCuring is the process ofcontrolling the rate and extentof moisture loss from concreteduring cement hydration.>
  • 2. Page  of 7 >curing of Concreten Curing by preventing excessive loss of moisturefrom the concrete: either by> leaving formwork in place> covering the concrete with an impermeablemembrane after the formwork has beenremoved> by the application of a suitable chemicalcuring agent (wax etc)> or by a combination of such methodsn Curing by continuously wetting the exposedsurface thereby preventing the loss of moisturefrom it. Ponding or spraying the surface withwater are methods typically employed to this end.EFFECT OF DURATION OF CURING ONPROPERTIES OF CONCRETEThe strength of concrete is affected by a number offactors, one of which is the length of time for whichit is kept moist, ie cured. Figure 1 illustrates this,comparing the strength (at 180 days) of concrete forwhich the surfaces have been:n kept moist for 180 days;n kept moist for various periods of time andallowed to dry out; andn allowed to dry out from the time it was firstmade.As may be seen in this example, concreteallowed to dry out immediately achieves only 40%of the strength of the same concrete water curedfor the full period of 180 days. Even three dayswater curing increases this figure to 60%, whilst28 days water curing increases it to 95%. Keepingconcrete moist is therefore, a most effective way ofincreasing its ultimate strength.Concrete that is allowed to dry out quickly alsoundergoes considerable early age drying shrinkage.Inadequate or insufficient curing is one of mainfactors contributing to weak, powdery surfaces withlow abrasion resistance.The durability of concrete is affected by anumber of factors including its permeability andabsorptivity. Broadly speaking, these are relatedto the porosity of the concrete and whether thepores and capillaries are discrete or interconnected.Whilst the number and size of the pores andcapillaries in cement paste are related directly to itswater-cement ratio, they are also related, indirectly,to the extent of water curing. Over time, watercuring causes hydration products to fill, eitherpartially or completely, the pores and capillariespresent, and, hence, help to reduce the porosity ofthe paste.Figure 2 illustrates the effect of differentperiods of water curing on the permeability ofcement paste. As may be seen, extending the periodof curing reduces the permeability.CURING METHODSGeneralMethods of curing concrete fall broadly into thefollowing categories:n Those that minimise moisture loss from theconcrete, for example by covering it with arelatively impermeable membrane.n Those that prevent moisture loss by continuouslywetting the exposed surface of the concrete.n Those that keep the surface moist and, atthe same time, raise the temperature of theconcrete, thereby increasing the rate of strengthgain. This method is typically used for precastconcrete products and is outside the scope ofthis data sheet.COMPRESSIVE STRENGTH AT 180 DAYSAS % OF CONTINUOUSLY MOIST SAMPLEDays cured (then in air)204060800Contin-uouslymoist28 14 7 3100Contin-uouslyin airFigure 1: Effect of duration of water curing onstrength of concrete.COEFFICIENT OF PERMEABILITY (m/s)CURING PERIOD (days)Cement paste (W/C = 0.51)010-810-910-1010-1110-1210-1310-1410-1510-1610-713 7 14 28 9010-17Figure 2: Effect of duration of water curing on thepermeability of cement paste.
  • 3. Page  of 7 >curing of ConcreteImpermeable-membrane CuringFormwork Leaving formwork in place is oftenan efficient and cost-effective method of curingconcrete, particularly during its early stages. Invery hot dry weather, it may be desirable to moistentimber formwork, to prevent it drying out during thecuring period, thereby increasing the length of timefor which it remains effective.It is desirable that any exposed surfaces of theconcrete (eg the tops of beams) be covered withplastic sheeting or kept moist by other means. Itshould be noted that, when vertical formwork iseased from a surface (eg from a wall surface) itseffectiveness as a curing system is significantlyreduced.Plastic sheeting Plastic sheets, or other similarmaterial, form an effective barrier against waterloss, provided they are kept securely in place andare protected from damage. Their effectiveness isvery much reduced if they are not kept securely inplace. The movement of forced draughts under thesheeting must be prevented.They should be placed over the exposedsurfaces of the concrete as soon as it is possible todo so without marring the finish. On flat surfaces,such as pavements, they should extend beyond theedges of the slab for some distance, eg or at leasttwice the thickness of the slab, or be turned downover the edge of the slab and sealed.For flat work, sheeting should be placed on thesurface of the concrete and, as far as practical, allwrinkles smoothed out to minimise the mottlingeffects (hydration staining), due to uneven curing,which might otherwise occur. Flooding the surfaceof the slab under the sheet can be a useful way toprevent mottling. Strips of wood, or windrows ofsand or earth, should be placed across all edges andjoints in the sheeting to prevent wind from lifting it,and also to seal in moisture and minimise drying.For decorative finishes or where colouruniformity of the surface is required sheetingmay need to be supported clear of the surfaceif hydration staining is of concern. This can beachieved with wooden battens or even scaffoldingcomponents, provided that a complete seal can beachieved and maintained.For vertical work, the member should bewrapped with sheeting and taped to limit moistureloss. As with flatwork, where colour of the finishedsurface is a consideration, the plastic sheetingshould be kept clear of the surface to avoidhydration staining.Care must also be taken to prevent the sheetingbeing torn or otherwise damaged during use. Aminimum thickness is required to ensure adequatestrength in the sheet; ASTM C 171 Sheet Materialsfor Curing Concrete specifies 0.01 mm. Figure 3illustrates the lack of effectiveness of plasticsheeting with holes equivalent to only 1.7% of thesheet’s surface area.Plastic sheeting may be clear or coloured.Care must be taken that the colour is appropriatefor the ambient conditions. For example, white orlightly coloured sheets reflect the rays of the sunand, hence, help to keep concrete relatively coolduring hot weather. Black plastic, on the other hand,absorbs heat to a marked extent and may causeunacceptably high concrete temperatures. Its useshould be avoided in hot weather, although in coldweather its use may be beneficial in acceleratingthe rate at which the concrete gains strength.Clear plastic sheeting tends to be more neutralin its effect on temperature (except in hot weather,where it fails to shade the surface of the concrete)but tends to be less durable than the colouredsheets, thereby reducing its potential for re-use.Membrane-forming curing compounds Curingcompounds are liquids which are usually sprayeddirectly onto concrete surfaces and which then dryto form a relatively impermeable membrane thatretards the loss of moisture from the concrete.Their properties and use are described in AS 3799Liquid Membrane-forming Curing Compounds forConcrete.They are an efficient and cost-effective meansof curing concrete and may be applied to freshlyplaced concrete or that which has been partiallycured by some other means. However, they mayaffect the bond between concrete and subsequentsurface treatments. Special care in the choice of asuitable compound needs to be exercised in suchcircumstances. The residue from some productsmay prevent the adhesion of flooring products andtiles onto the concrete surface.STANDARD CURING1-day mould1-day mould+ 6-days fogPLASTIC (sealed)3-days7-days3-days7-daysPLASTIC (1.7% holes)0WATER SORPTIVITY (mm/h0.5)2 4 6 8 10 12 14 16 18 20Figure 3: Effectiveness of plastic sheetingcompared to standard curing.
  • 4. Page  of 7 >curing of ConcreteCuring compounds are generally formulatedfrom wax emulsions, chlorinated rubbers, syntheticand natural resins, and from PVA emulsions. Theireffectiveness varies quite widely, depending onthe material and strength of the emulsion, as isillustrated in Figure 4. Note: Attention should bedrawn to the very poor efficiency of PVA curingcompounds.When used to cure fresh concrete, the timing ofthe application of the curing compounds is criticalfor maximum effectiveness. They should be appliedto the surface of the concrete after it has beenfinished, as soon as the free water on the surfacehas evaporated and there is no water sheen visible.Too early an application dilutes the membrane; toolate results in it being absorbed into the concrete,with a consequent failure of the membrane to form.They may also be used to reduce moistureloss from concrete after initial moist curing or theremoval of formwork. In both cases, the surfaceof the concrete should be thoroughly moistenedbefore the application of the compound to preventits absorption into the concrete.The use of curing compounds will not preventformation of early age thermal cracking.Curing compounds can be applied by handspray, power spray, brush or roller. The type orgrade of curing compound should be matchedto the type of equipment available and themanufacturer’s directions followed. The rate ofapplication should be uniform, with coveragenormally in the range 0.20 to 0.25 litres/m2. Wherefeasible, two applications at right angles to eachother will help ensure complete coverage.Pigmented compounds also help ensurecomplete coverage and are advantageous in helpingconcrete surfaces reflect rather than absorb heat.Figure 5 shows pressure spraying of a white-pigmented curing compound.It is extremely important to check thesubsequent floor finish. Most curing compoundsmust be removed before the application of anyapplied floor finishes such as direct stick carpet andvinyl, epoxy or polyurethane coatings and ceramictile adhesives.Finally, it should be noted when using curingcompounds that are solvent-based adequateventilation must always be provided in enclosedspaces and other necessary safety precautionstaken. Manufacturer’s recommendations shouldalways be followed.Internal curing compounds These are incorporatedinto the concrete as an admixture hence known asinternal curing compounds. They inhibit moistureloss and thereby improve long term strengthand reduce drying shrinkage. Internal curingcompounds are relatively new and care should betaken when utilised. They have been used in tunnellinings and underground mines to provide at leastpartial curing when traditional methods are difficultor even impossible to employ.Water CuringGeneral Water curing is carried out by supplyingwater to the surface of concrete in a way thatensures that it is kept continuously moist.The water used for this purpose should notbe more than about 5°C cooler than the concretesurface. Spraying warm concrete with cold watermay give rise to ‘thermal shock’ that may cause orcontribute to cracking. Alternate wetting and dryingof the concrete must also be avoided as this causesvolume changes that may also contribute to surfacecrazing and cracking.Ponding Flat or near-flat surfaces such as floors,pavements, flat roofs and the like may be curedby ponding. A ‘dam’ or ‘dike’ is erected around theedge of the slab and water is then added to createWAXEMULSIONCHLORINATEDRUBBERHYDROCARBONRESINACRYLICEMULSIONPVAEMULSION100EFFICIENCY INDEX (%)Ranging from wellformulated to adulteratedAS 3799 requirements(≥ 90% after 72 hours)90 80 70 60 50 40 30 20 10 0Figure 4: Comparative efficiency of curingcompoundsFigure 5: Spray application of curing compound
  • 5. Page  of 7 >curing of Concretea shallow ‘pond’ as shown in Figure 6. Care mustbe taken to ensure the pond does not empty due toevaporation or leaks.Ponding is a quick, inexpensive and effectiveform of curing when there is a ready supply ofgood ‘dam’ material (eg clay soil), a supply of water,and the ‘pond’ does not interfere with subsequentbuilding operations. It has the added advantage ofhelping to maintain a uniform temperature on thesurface of the slab. There is thus less likelihood ofearly age thermal cracking in slabs that are curedby water ponding.Sprinkling or fog curing Using a fine spray or fogof water can be an efficient method of supplyingadditional moisture for curing and, during hotweather, helps to reduce the temperature of theconcrete.As with other methods of moist curing, it isimportant that the sprinklers keep the concretepermanently wet. However, the sprinklers do nothave to be on permanently; they may be on anintermittent timer.Sprinklers require a major water supply, canbe wasteful of water and may need a drainagesystem to handle run-off. The alternative is to havea ‘closed’ system where the water is collected andrecycled.Sprinkler systems may be affected by windyconditions and supervision is required to see thatall of the concrete is being kept moist and that nopart of it is being subjected to alternated wettingand drying. This is not easy to achieve.Wet coverings Fabrics such as hessian, ormaterials such as sand, can be used like a ‘mulch’to maintain water on the surface of the concrete. Onflat areas, fabrics may need to be weighed down.Also, it is important to see that the whole area iscovered. Wet coverings should be placed as soonas the concrete has hardened sufficiently to preventsurface damage. They should not be allowed to dryout as they can act as a wick and effectively drawwater out of the concrete.Fabrics may be particularly useful on verticalsurfaces since they help distribute water evenlyover the surface and even where not in contact withit, will reduce the rate of surface evaporation. Careshould be taken however, that the surface of theconcrete is not stained, perhaps by impurities in thewater, or by the covering material.New fabrics can leach fabric stains, pre‑washingshould be essential. Prior to placement of anyfabric – pre moisten to avoid wicking of moisturefrom the concrete that can result in the fabrictexture negatively absorbed into the concretesurface.Figure 6: Ponding method of water curing
  • 6. Page  of 7 >curing of ConcreteSummary Considerations for selecting a curing methodGeneralImpermeablemembranecuringWater curingType of memberEnvironmentRetention of formworkPlastic sheetingCuring compoundsPondingSprinklingWet coverings> Is the member vertical or horizontal? Some methods are affected or excludedby orientation, eg ponding.> Is the member thin or thick? Thick sections such as large columns or massconcrete are mostly ‘self-curing’ but require temperature gradient at outerlayers to be limited.> Is the member insitu or precast? Precast members are suited to low-pressuresteam curing while precast products may benefit from autoclaving.> Does the location affect the availability or cost of some curing materials?eg water in an arid region.> Is the weather likely to be hot or cold? If the temperature is higher than 30°Cor less than 10°C special precautions need to be taken.> Is the site exposed to winds? If so, special precautions may be required toprevent plastic shrinkage cracking; sprinkling methods may be affected;or extra care required when using plastic sheeting.> What is the effect on site operations and construction cycle schedule?> Is there likely to be cold weather? This method allows easy addition ofinsulation.> Is uniform concrete colour specified? If so, a constant stripping time will needto be maintained to avoid hydration colour change.> What is the effect on access and site operations?> Is there a safety consideration? Plastic sheeting may be slippery, and istherefore a hazard in horizontal applications.> Is there likely to be hot or cold weather? Colour of sheeting should be selectedto suit.> Is the situation such that the seal can be maintained with minimum risk ofholing.> Is uniform concrete colour specified? If so, the sheeting must be kept clear ofthe surface to avoid hydration staining.> What are the manufacturer’s recommendations? Both the rate of applicationand the timing are critical for effectiveness. Does the product comply withAS 3799.> What is the concrete surface texture? Coarse textures require higherapplication rates.> Can a uniform application be achieved in the particular situation? Twoapplications at right-angles help. Sites exposed to wind create problems.> Is there likely to be hot or cold weather? A suitably pigmented compound canhelp.> Are there to be applied finishes (render, tiles, etc)? Residue from compoundscan affect the ‘bond’ of later applied finishes.> Is there a health consideration? Compounds may be toxic, and their use inenclosed situations may therefore be hazardous.> What is the effect on access and site operations?> Is suitable ‘dam’ material available? A clay soil is the most suitable.> Is there likely to be hot weather? Ponding is an efficient means of maintaininga uniform temperature on slabs.> Is concrete colour or appearance a consideration? ‘Dam’ materials,particularly clay, tend to stain. > What is the effect on site operations?> Is there an adequate water supply?> What is effect of run-off? Usually some form of drainage is required.> Will required volume/timing be such as not to damage the concrete surface?> Can application be maintained continuously? Intermittent wetting and dryingcan be deleterious.> Is site exposed to winds? This makes continuous application very difficult.> What is the effect on site operations?> Can they effectively cover all surfaces?> Is site exposed to wind? Wet coverings are easier to keep in place than plasticsheeting.> Is concrete colour or appearance a consideration? If so, sand should have lowclay content; fabrics and water should contain no impurities.> In the case of sand, its supply or removal a problem?> Can coverings be kept continuously moist? Intermittent wetting and drying canbe deleterious.
  • 7. Page  of 7 >curing of ConcreteCCAA OFFICESSYDNEY OFFICE:Level 6, 504 Pacific HighwaySt Leonards NSW Australia 2065POSTAL ADDRESS:Locked Bag 2010St Leonards NSW 1590TELEPHONE: (61 2) 9437 9711FACSIMILE: (61 2) 9437 9470BRISBANE OFFICE:Level 14, IBM Building348 Edward StreetBrisbane QLD 4000TELEPHONE: (61 7) 3831 3288FACSIMILE: (61 7) 3839 6005MELBOURNE OFFICE:2nd Floor, 1 Hobson StreetSouth Yarra VIC 3141TELEPHONE: (61 3) 9825 0200FACSIMILE: (61 3) 9825 0222PERTH OFFICE:45 Ventnor AvenueWest Perth WA 6005TELEPHONE: (61 8) 9389 4452FACSIMILE: (61 8) 9389 4451ADELAIDE OFFICE:Greenhill Executive Suites213 Greenhill RoadEastwood SA 5063POSTAL ADDRESS:PO Box 229Fullarton SA 5063TELEPHONE: (61 8) 8274 3758FACSIMILE: (61 8) 8373 7210EXTRACTIVE INDUSTRIES OFFICEPO Box 243Henley Beach SA 5022TELEPHONE: (61 8) 8353 8151FACSIMILE: (61 8) 8353 8151TASMANIAN OFFICE:EXTRACTIVE INDUSTRIES OFFICEPO Box 246Sheffield TAS 7306TELEPHONE: (61 3) 6491 2529FACSIMILE: (61 3) 6491 2529WEBSITE: www.concrete.net.auEMAIL: info@ccaa.com.auLayout: Helen Rix DesignDisclaimer: Cement Concrete & AggregatesAustralia is a not for profit organisation sponsoredby the cement concrete and aggregate industriesin Australia to provide information on the manyuses of cement and concrete. This publicationis produced by CCAA for that purpose. Sincethe information provided is intended for generalguidance only and in no way replaces the servicesof professional consultants on particular projects, nolegal liability can be accepted by CCAA for its use.CCAA respects your privacy. Your details havebeen collected to provide you with information onour activities, publications and services. From timeto time your details may be made available to thirdparty organisations who comply with the PrivacyAct such as affiliated associations, sponsors ofevents and other reputable organisations whoseservices we think you may find of interest. If you donot wish to receive information from CCAA or wishto be taken off the database please write to thePrivacy Officer, CCAA, Locked Bag 2010,St Leonards, NSW, 1590Apr2006FURTHER INFORMATIONFurther information on good concreting practicescan be downloaded from the Cement Concrete& Aggregates Australia website atwww.concrete.net.au.