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.