1 Curing is the maintenance of a satisfactory moisture con-tent and temperature in concrete for a period of time imme-diately following placing and finishing so that the desiredproperties may develop (Fig. 12-1). The need for adequatecuring of concrete cannot be overemphasized. Curing has astrong influence on the properties of hardened concrete ;proper Curing will increase durability, strength, watertight-ness, abrasion resistance, volume stability, and resistance tofreezing and thawing and deicers. Exposed slab surfacesare especially sensitive to Curing as strength developmentand freeze-thaw resistance of the top surface of a slab canbe reduced significantly when Curing is portland cement is mixed with water, a chemicalreaction called hydration takes place.
2 The extent to whichthis reaction is completed influences the strength and dura-bility of the concrete . Freshly mixed concrete normally con-tains more water than is required for hydration of the ce-ment; however, excessive loss of water by evaporation candelay or prevent adequate hydration. The surface is partic-ularly susceptible to insufficient hydration because it driesfirst. If temperatures are favorable, hydration is relativelyrapid the first few days after concrete is placed; however, itis important for water to be retained in the concrete duringthis period, that is, for evaporation to be prevented or sub-stantially proper Curing , concrete becomes stronger, moreimpermeable, and more resistant to stress, abrasion, andfreezing and thawing.
3 The improvement is rapid at earlyages but continues more slowly thereafter for an indefiniteperiod. Fig. 12-2 shows the strength gain of concrete withage for different moist Curing periods and Fig. 12-3 showsthe relative strength gain of concrete cured at 12 Curing ConcreteFig. 12-1. Curing should begin as soon as the concretestiffens enough to prevent marring or erosion of the sur-face. Burlap sprayed with water is an effective method formoist Curing . (69973)60504030201000 7 289036586420 Age at test, daysMoist-cured entire timeIn air after 28 days moist curingIn air after 7 days moist curingIn laboratory air entire timeCompressive strength, MPaCompressive strength, 1000 psiFig.
4 12-2. Effect of moist Curing time on strength gain ofconcrete (Gonnerman and Shuman 1928). 219 HOMEPAGEThe most effective method for Curing concretedepends on the materials used, method of construction,and the intended use of the hardened concrete . For mostjobs, Curing generally involves applying Curing com-pounds, or covering the freshly placed and finished con-crete with impermeable sheets or wet burlap. In somecases, such as in hot and cold weather, special care usingother precautions is mixtures with high cement contents and lowwater-cement ratios (less than ) may require specialcuring needs.
5 As cement hydrates (chemically combiningwith water) the internal relative humidity decreases caus-ing the paste to self-desiccate (dry out) if no external wateris provided. The paste can self-desiccate to a level wherehydration stops. This may influence desired concreteproperties, especially if the internal relative humiditydrops below 80% within the first seven days. In view of220 Design and Control of concrete Mixtures EB001this, membrane-forming Curing compounds may notretain enough water in the concrete .
6 Therefore, foggingand wet Curing become necessary to maximize hydration(Copeland and Bragg 1955). Fogging during and afterplacing and finishing also helps minimize plastic crackingin concretes with very low water-cement ratios (especiallyaround or less).When moist Curing is interrupted, the development ofstrength continues for a short period and then stops afterthe concrete s internal relative humidity drops to about80%. However, if moist Curing is resumed, strength devel-opment will be reactivated, but the original potentialstrength may not be achieved.
7 Although it can be done ina laboratory, it is difficult to resaturate concrete in thefield. Thus, it is best to moist-cure the concrete continu-ously from the time it is placed and finished until it hasgained sufficient strength, impermeability, and of water will also cause the concrete to shrink,thus creating tensile stresses within the concrete . If thesestresses develop before the concrete has attained adequatetensile strength, surface cracking can result. All exposedsurfaces, including exposed edges and joints, must be pro-tected against moisture proceeds at a much slower rate when theconcrete temperature is low.
8 Temperatures below 10 C(50 F) are unfavorable for the development of earlystrength; below 4 C (40 F) the development of earlystrength is greatly retarded; and at or below freezingtemperatures, down to -10 C (14 F), little or no recent years, a maturity concept has been introducedto evaluate the development of strength when there is vari-ation in the Curing temperature of the concrete . Maturity isthe product of the age of the concrete and its average curingtemperature above a certain base temperature. Refer toChapter 14 for more information on the maturity concept.
9 Itfollows that concrete should be protected so that its tem-perature remains favorable for hydration and moisture isnot lost during the early hardening METHODS AND MATERIALSC oncrete can be kept moist (and in some cases at a favor-able temperature) by three Curing methods:1. Methods that maintain the presence of mixing waterin the concrete during the early hardening include ponding or immersion, spraying orfogging, and saturated wet coverings. These methodsafford some cooling through evaporation, which isbeneficial in hot Methods that reduce the loss of mixing water fromthe surface of the concrete .
10 This can be done by cov-ering the concrete with impervious paper or plasticsheets, or by applying membrane-forming temperature, C ( F)125105856545250204060 Age, daysCompressive strength, % 28-day strength23/23 (73/73)32/32 (90/90)10/10 (50/50)23/10 (73/50)Casting/ Curing temperature, C ( F)12010080600204060 Age, daysCompressive strength, % 23 C (73 F)23/23 (73/73)32/32 (90/90)10/10 (50/50)23/10 (73/50)Fig. 12-3. Effect of Curing temperature on strength gain(top) relative to 28-day strength and (bottom) relative to thestrength of concrete at 23 C (73 F) (Burg 1996).