Curing Concrete
Strength development and durability of any concrete mix, regardless of the type of cement used, depends on the degree of hydration of the cement. Strength and durability depend on the production of a dense matrix of low permeability that is resistant to the passage of fluids. It follows that the chemical reaction between cement and water must continue and if concrete is allowed to dry out, this reaction ceases and inhibits strength and potential durability development. Premature loss of water must be prevented for the full benefits of cement hydration on the properties of hardened concrete to be realised.
GUIDE TO CURING IN-SITU CONCRETE
MINIMUM MOIST CURING PERIOD (days)
THE DRYING RATE DECREASES RAPIDLY AFTER THE FIRST 24 HOURS AND REACHES AN ALMOST INSIGNIFICANT VALUE WITHIN THREE OR FOUR DAYS. THE FIRST 24 HOURS ARE THEREFORE THE MOST CRITICAL. PROTECTION OF THE CONCRETE ACTUALLY STARTS FROM DISCHARGE, THE REASON BEING THAT SOME 20% OF CONCRETE (BY VOLUME) IS WATER AND IS SUBJECT TO EVAPORATION.
In the case of slabs which are to be trowelled, protection from drying may be required after casting but before finishing begins.
Most curing compounds are not recommended for surfaces that are to receive finishes such as a screed, plaster or paint because of the likelihood of reduced bond.
Curing by ponding or continuous spraying is seldom used because of the practical problems associated with supply, containment and the ultimate disposal of water used for curing. Care must be taken to avoid sudden temperature changes caused by the use of cold curing water in warm concrete. Cycles of wetting and drying should be avoided. Both of the above may lead to micro-cracking of the surface.Water curing is only suitable when the temperature is well above freezing. Materials such as wet hessian or wet sand are sometimes used but they must be kept continuously wet and not be allowed to dry out. Damp materials may stain concrete.
Black polythene sheeting should be avoided in hot weather but has its advantages in cold weather.
APPLICATION FOR CURING COMPOUNDS
HAPHAZARD APPLICATION OF CURING COMPOUNDS REDUCE EFFECTIVENESS:
Further information refer:
SANS 2001 – CCI.
British Concrete Society Digest No.3 – Curing Concrete.
Concrete Society of Southern Africa – Concrete Curing
Description Method and Control.
Concrete Testing Services.