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Controlling thermal contraction cracks

The basic principle for the control of cracking is to provide a minimum amount of reinforcement such that its tensile capacity exceeds that of the concrete when it cracks. Thus:

pcrit = fct/fy


pcrit = critical (minimum) ratio of area of steel reinforcement to area of concrete
fct = tensile strength of the immature concrete
fy = characteristic strength of the reinforcement.

To control maximum crack widths, the steel ratio required is:

p = (kRT1aD)/(3wmax)


T1 = drop between peak temperature after casting and ambient temperature, C
a = coefficient of thermal expansion, per C
k = modification factor
R = restraint factor
D = bar diameter
wmax = maximum crack width.

Further guidance is given in BS 8007.

Acknowledgement: The Concrete Society

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TR22 Non-structural cracks in concrete

Crack width measurement