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Reinforcement for large area pours

When considering the risk of cracking caused by early thermal movements in large area pours, there are two approaches that can be adopted. The first is to avoid cracking, by limiting the stresses induced in the new concrete by limiting the temperature rise. The second is to assume that cracking will occur and to control crack widths by providing sufficient reinforcement. The former will generally not be practical and hence the suggested design approach is to ensure that there is sufficient reinforcement in critical regions of the slab.

BS 8007, Design of concrete structures for retaining aqueous liquids, indicates that the amount of reinforcement required to control cracks is governed by two criteria. Firstly, to ensure that the cracks are distributed and the steel does not yield, a minimum reinforcement ratio is required, equal to the ratio of the tensile strength of the immature concrete and the tensile strength of the reinforcement. Secondly, to control the crack widths a reinforcement ratio is required that is a function of the temperature change from peak to ambient, the restraint factor (taken as 0.5 for areas of high restraint and 0.2 for other areas), the coefficient of thermal expansion, the bar diameter and the maximum permissible crack width.

Concrete Advice No. 23, Large area pours for suspended slabs gives more information on the approach for determining the area of high restraint in slabs and the determination of the reinforcement required. It also gives a design example to illustrate the method.

Guidance on the prediction of early-age temperature rise may be found in CIRIA Publication C660, Early-age thermal crack control in concrete. Published in 2007, this replaces and updates CIRIA Report R91.

Note copies of Advice Sheets can be downloaded from the Members Area of the Concrete Society web site.

Acknowledgement: The Concrete Society