The Job Isn't Done Until You've Adequately Cured It!

By: Bob Toth, Technical Service Manager
Keystone Cement Co., Bath, Pa.

Although curing of concrete is not usually an integral part of placing and finishing concrete, especially flatwork, it is an important step in producing an acceptable finished produce. Basically, curing is maintaining a satisfactory moisture content and temperature in the concrete. Curing begins after finishing so the concrete placed may develop the strength and durability desired. Properly cured concrete often gains more than its design stength. Drying will remove the water needed for the chemical reaction called "hydration" or the hardening of the concrete.

Temperature is an important factor in proper curing since the rate of hydration is temperature dependent. Concrete placed under high temperature conditions will gain strength quickly but later strengths may be reduced. Concrete placed in cold weather will take longer to gain strength, delaying form removal and placing the structure in service.

There are numerous ways to cure concrete surfaces. Ideal curing methods, or keeping the surface continuously wet, should be maintained for several days, usually seven to ten days. The easiest methods to maintain moisture in the concrete include:
1. A regular schedule of wetting the concrete with a hose or sprinkler. Timing of the intervals of wetting will depend on whether the concrete is in direct hot sun or shade and the ambient temperature. The concrete should not be allowed to dry.
2. Burlap or cotton mats used with a soaker hose or sprinkler. Caution must be taken not to allow the coverings to dry out and absorb water from the concrete.
3. Cover the concrete with wet straw. The layer of wet straw should be at least six inches thick and can be covered with plastic. The plastic sheeting should be at least four mils thick and either clear or white in color. Black or dark colored induces heat into the concrete. The concrete should not be allowed to dry out.
4. Ponding the concrete with water is an excellent method of curing..the water should not be more than 20F cooler than the concrete and the dike around the pond must be secure from leaks.
5. Damp earth, sand or sawdust will cure concrete, especially floors. They should not contain organic or iron staining materials. These materials should not be allowed to dry out during the curing period.
6. Liquid membrane-forming compounds that conform to A.S.T.M. C 309, "Liquid Membrane Forming Compounds For Curing Concrete". This specification describes rate of application of the sprayed-applied material. Apply the material approximately one hour after finishing. Do not apply the material to concrete that is still bleeding, or it has a visible water sheen on the surface. While clear pigment may be used, a white pigment will give reflective properties and allow for inspection of coverage. A single coat may be adequate but when possible, a second coat should be applied at right angle to the first, thus guaranteeing even coverage.

*Information for this article was obtained from articles supplied by the National Ready Mixed Concrete Association. A complete review of concrete products, procedures, problems and remedies are available from the N.R.M.C.A. through your ready mix supplier.