Early-age Thermal Crack Control in Concrete

Early-age thermal cracking occurs when the restrained thermal contraction strain exceeds the tensile strain capacity of concrete. The restraint to thermal movement is the product of the coefficient of thermal expansion of the concrete, the temperature fall from a peak level during cement hydration, and a restraint factor. This report discusses each of these factors in turn, and shows how early-age thermal strains can be reduced by taking appropriate measures during the design and construction of reinforced concrete elements. The concrete temperature rise is shown to depend on a number of factors: cement content, type of aggregate, possible use of cement replacement materials, as well as ambient temperature, formwork used and section thickness. Restraint is a function of the construction sequence and the constraint of neighbouring elements. If early-age thermal cracking cannot be prevented, crack width can be controlled with reinforcement. Equations for the control of crack widths are developed, and while they are based on present requirements, they differ in two important respects. First, the concept of a restraint factor is introduced, then it is suggested that the crack widths are only controlled within an effective zone around the reinforcement and not necessarily throughout the section thickness. (TRRL) (TRRL)