Introduction
Portland cement consists of several complex chemical compounds (seecomposition of cement). In thepreparation of concrete, the cement acts as a glue which bonds togetherthe aggregates to achieve the final castproduct. The concrete achievesits strength through a series of chemical reactions, known as hydration,which are initiated by the addition of water to the mixture. The rate ofthe reactions influences the properties of the hardened concrete such asstrength, permeability, durability, abrasion resistance and resistanceto freezing and thawing. As long as water is present, the hydration willcontinue for many years. The final strength of the concrete formed in theprocess will depend on the constituents in the original mixture, and theenvironment under which the reactions take place.
Curing and its importance
Curing is the process of maintaining satisfactory moisture contentand temperature in freshly cast concrete for a definite period of timeimmediately 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 definiteperiod.
The most crucial time for strength gain of concrete is immediatelyfollowing placement. In field conditions, heat and wind can dry out themoisture from the placed mixture. The accompanying figure shows how concretestrength varies with curing conditions. Concrete that is allowed to dryin air will gain only 50% of the strength of continuously moist-cured concrete.
Lack of water also causes the concrete to shrink, which leads to tensilestresses within the concrete. As a result, surface cracking may occur,especially if the stresses develop before the concrete attains adequatetensile strength.
Hydration is an exothermic chemical process, increasing the ambienttemperature will increase the rate of hydration, and hence of strengthdevelopment, while lowering it will have the opposite effect. Too muchheat reduces the final concrete strength. Selecting an appropriate curingprocess helps in temperature control during hydration
Methods of curing
Concrete can be kept moist and often at a favorable temperature byany of three methods:
- Maintaining the presence of mixingwater during the early hardening period. Methods used include pondingor immersion, spraying (or fogging) and wet coverings. These methods willalso cool the concrete as the water evaporates.
- Preventing loss of mixing waterfromthe surface by sealing. This may be achieved using impervious paper, plasticsheeting, applying membrane-forming compounds, or by leaving the formsin place.
- Accelerating strength gainby supplying heat and additional moisture to the concrete. This may beaccomplished 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 maintenanceof a moist environment around the concrete. Temperature variations arenot a major problem, provided the concrete temperature is maintained above5 degrees Celsius. Curing can therefore be achieved either through maintainingmixing water in the concrete during early hardening or by preventing moistureloss from the surface by sealing. The ultimate choice of the particularmethod to use will take into consideration factors such as economy, esthetics,member shape, etc.
Under hot weather conditions, the high temperatures are likely to resultin excessive moisture loss. Maintaining mixing water in the concrete isthe major concern. Continuous moist curing should be done for the entirecuring period. If this is not possible, the concrete surfaces should beprotected from drying out using any of the previously mentioned methodsand the surfaces kept damp. Surfaces should dry out slowly after curingto reduce possibility of surface cracking.
Curing in cold weather will be different as in this case the biggestconcern will be the maintaining of an adequate and conducive temperaturefor hydration. For massive members, the heat generated by the concreteduring hydration will be adequate to provide a satisfactory curing temperature.For non-massive members, a good alternative is steam curing, which providesboth moisture and heat. In any case, a minimum favorable temperature inthe range of 10 - 21º C must be maintained in the concrete for theminimum 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 exposureconditions, and method of curing. Since all desirable properties are improvedwith curing, the period should be as long as practical. For most concretestructures, the curing period at temperatures above 5º C (40ºF) should be a minimum of 7 days or until 70% of the specified compressiveor flexural strength is attained. The period can be reduced to 3 days ifhigh early strength concrete is used and the temperature is above 10ºC (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 ofConcrete Mixes, Portland Cement Association, Skokie, Ill. pp. 205.
Michael Mamlouk & John Zaniewski (1999): Materials for Civil andConstruction Engineers, Addison Wesley Longman, Inc.,
Information compiled by:
Department of Civil & Environmental Engineering
The Pennsylvania State University
University Park, PA 16802
