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:
p_{crit} = f_{ct}/f_{y}
where:
p_{crit} = critical (minimum) ratio of area of steel reinforcement to area of concrete f_{ct} = tensile strength of the immature concrete f_{y} = characteristic strength of the reinforcement.
To control maximum crack widths, the steel ratio required is:
p = (kRT_{1}aD)/(3w_{max})
where:
T_{1} = 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 w_{max} = maximum crack width.
Further guidance is given in BS 8007.
