Introduction
Portland cement consists of several complex chemical compounds (see composition of cement). In the preparation of concrete, the cement acts as a glue which bonds together the aggregates to achieve the final castproduct. The concrete achieves its strength through a series of chemical reactions, known as hydration, which are initiated by the addition of water to the mixture. The rate of the reactions influences the properties of the hardened concrete such as strength, permeability, durability, abrasion resistance and resistance to freezing and thawing. As long as water is present, the hydration will continue for many years. The final strength of the concrete formed in the process will depend on the constituents in the original mixture, and the environment under which the reactions take place.

Curing and its importance
Curing is the process of maintaining satisfactory moisture content and temperature in freshly cast concrete for a definite period of time immediately following placement. The process serves two major purposes:

• It prevents or replenishes the loss of moisture from the concrete;
• It maintains a favorable temperature for hydration to occur for a definite period.

Lack of water also causes the concrete to shrink, which leads to tensile stresses within the concrete. As a result, surface cracking may occur, especially if the stresses develop before the concrete attains adequate tensile strength.

Hydration is an exothermic chemical process, increasing the ambient temperature will increase the rate of hydration, and hence of strength development, while lowering it will have the opposite effect. Too much heat reduces the final concrete strength. Selecting an appropriate curing process helps in temperature control during hydration

Methods of curing
Concrete can be kept moist and often at a favorable temperature by any of three methods:

• Maintaining the presence of mixing water during the early hardening period. Methods used include ponding or immersion, spraying (or fogging) and wet coverings. These methods will also cool the concrete as the water evaporates.
• Preventing loss of mixing waterfrom the surface by sealing. This may be achieved using impervious paper, plastic sheeting, applying membrane-forming compounds, or by leaving the forms in place.
• Accelerating strength gain by supplying heat and additional moisture to the concrete. This may be accomplished using live steam (steam curing), insulating blankets or covers, and various heating techniques including coils and forms.

Curing under different weather conditions
Under normal weather, the key concerns in curing will be the maintenance of a moist environment around the concrete. Temperature variations are not a major problem, provided the concrete temperature is maintained above 5 degrees Celsius. Curing can therefore be achieved either through maintaining mixing water in the concrete during early hardening or by preventing moisture loss from the surface by sealing. The ultimate choice of the particular method to use will take into consideration factors such as economy, esthetics, member shape, etc.

Under hot weather conditions, the high temperatures are likely to result in excessive moisture loss. Maintaining mixing water in the concrete is the major concern. Continuous moist curing should be done for the entire curing period. If this is not possible, the concrete surfaces should be protected from drying out using any of the previously mentioned methods and the surfaces kept damp. Surfaces should dry out slowly after curing to reduce possibility of surface cracking.

Curing in cold weather will be different as in this case the biggest concern will be the maintaining of an adequate and conducive temperature for hydration. For massive members, the heat generated by the concrete during hydration will be adequate to provide a satisfactory curing temperature. For non-massive members, a good alternative is steam curing, which provides both moisture and heat. In any case, a minimum favorable temperature in the range of 10 - 21º C must be maintained in the concrete for the minimum required curing period (see below). Where moist curing is not done, very low temperatures may be avoided by insulating the member appropriately.

Curing period and temperature
The curing period depends upon the type of cement used, mixture proportions, required strength, size and shape of member, ambient weather, future exposure conditions, and method of curing. Since all desirable properties are improved with curing, the period should be as long as practical. For most concrete structures, the curing period at temperatures above 5º C (40º F) should be a minimum of 7 days or until 70% of the specified compressive or flexural strength is attained. The period can be reduced to 3 days if high early strength concrete is used and the temperature is above 10º C (50º F).

References:
Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ, pp. 671.

Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.

Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers, Addison Wesley Longman, Inc.,