**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.