Curing
The importance of thorough curing is to achieve a low permeability concrete structure. Curing is usually specified to aid moisture retention, to permit proper strength development, prevent steep temperature gradients and maintain satisfactory temperature regimes.
Curing can provided by introducing water to the surface of the concrete and by prevention excessive loss of moisture. Water addition methods include ponding, spraying and the sue of absorbent curing materials such as hessian or sand which are kept moist during the curing period. The loss of moisture due to evaporation can be prevented by covering the concrete with sheet materials such as polyethylene or by coating the surface with a curing compound.
The main factors that determine the rate of evaporation are the temperature of the concrete, the air temperature, relative humidity, and wind velocity of the air adjacent to the concrete.
The length of curing period is usually dependent on the type of cement used and on the ambient conditions during construction. Table 1 compares the curing requirements of various Codes.
Table 1: Comparison of requirements for curing concrete in various Codes
| Code or Reference | Basis for recommendation and/or factors considered | Curing period (s) (days) | Comments |
| ACI 1318-83 | All concretes except RHPC-to keep moist and above 50 Fahrenheit. RHPC concrete | 7 3 |
Accelerated curing permitted but must produce equivalence to the general requirement |
| BS 8110: Part 1: 1985 | Moisture retention; strength development; prevention of temperature gradients; maintenance of satisfactory temperature regime | 3-10 depending on: type of cement; surface temperature; ambient conditions (RH; degree of protection from sun and wind) | No special requirements if RH>80%, and concrete is damp and protected from sun and wind. Minimum of 3d relates to OPC, RHPC or SRPC, with a surface temperature above 1o degrees Celsius and intermediate ambient conditions (50% < RH <80%) |
| Draft Australian Code DR 85137 | Maturity: durability all concrete, except that fully enclosed within a building | 7 | Moist curing at 20 degrees Celsius, specified. Alternative curing allowed only if higher concrete grade is used than normally specified for the particular exposure conditions |
| CEB Bulletin no. 166 | Exposure condition during use; environment during curing; curing sensitivity of the mix (type and amount of cement; w/c ratio) | 10-14 | Approach similar to BS 8110, but less detailed. One difference is the heavy dependence on ‘exposure condition in use’ three curing methods listed (as BS 8110). Curing must be continued after casting |