if it is properly cured for an adequate period of time before being placed in 

 service. Proper curing prevents loss of moisture for the time necessary to 

 obtain necessary hydration of the cement. Excess mixing water is allowed 

 to escape; however, the appearance of plastic shrinkage cracks in the 

 surface of the concrete about the time the concrete is ready for finishing 

 indicates that the paste is losing water too rapidly. 



Concrete should be cured by keeping the concrete damp for not less than 

 7 days if made of normal Portland cement, and for not less than Q 3 days if Q 

 made of high (early) strength cement. For each decrease of 2.7 below 21 

 Celsius (5 below 70° Farenheit) , in the average curing temperature, the 

 curing period shall be increased by 4 days for units made of normal Portland 

 cement and by 2 days for units made of high (early) strength cement or 

 until the concrete has attained its designed strength. Where units are 

 cured by high-pressure steam, steam vapor, or other approved processes used 

 to accelerate the hardening of the cement, the curing time may be reduced 

 provided the compressive strength of the concrete is equal to that obtained 

 by damp curing, equal to the 28-day strength. Concrete units shall not be 

 moved from the casting bed until the curing period is complete. 



(1) Methods of Curing . There are two general methods of retaining 

 the required water for hydration furnished by the mixing water in concrete. 



(a) Moist Environment . A moist environment can be maintained 

 through water ponding, water sprays, steam, or saturated cover materials 

 such as burlap or cotton mats, carpets (some carpets may contain certain 

 dyes which inhibit the settling of concrete), earth, sand, sawdust or 

 straw, all of which must be maintained continuously wet. 



(b) Sealing Materials . Curing can also be accomplished by 

 preventing the loss of mixing water by means of sealing materials or 

 curing compounds. Sealing is accomplished by the use of impervious sheets 

 of paper or plastics, or by the application of an impervious membrane- 

 forming curing compound applied to the freshly placed concrete. Compounds 

 consisting essentially of waxes, resins, chlorinated rubber, and solvents 

 of high volatility at atmospheric temperatures are used extensively for 

 curing concrete. The formulations must be such as to provide a moisture 

 seal shortly after being applied and must not be injurious to Portland 

 cement. Compounds should comply with the requirements of ASTM Standard 

 C309. 



Before applying curing compound, tops of joints that are to receive 

 sealant shall be tightly closed with temporary material to prevent entry of 

 the compound and to prevent moisture loss during the curing period. The 

 compound shall be applied on damp surfaces as soon as the moisture film has 

 disappeared. The curing compound shall be applied by power spraying equip- 

 ment using a spray nozzle equipped with a wind guard. The compound shall 

 be applied in a two-coat continuous operation at a coverage of not more 

 than 10 square meters per liter (400 square feet per gallon) for each coat. 

 When applied by hand sprayers, the second coat shall be in a direction 

 approximately at right angles to the direction of the first coat. The 

 compound shall form a uniform, continuous, adherent film that shall not 

 check, crack, or peel, and shall be free from pinholes or other imperfec- 

 tions. Surfaces subjected to rainfall within 3 hours after compound has 



