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Plastic settlement cracks form within 30 minutes to 6 hours of casting the concrete, dependent on the prevailing conditions and mix characteristics.

The mechanism of plastic settlement

If the settlement of solids in the concrete can freely take place without hindrance there will be a reduction in depth and volume of the cast concrete but no cracking. However, any restraint to this movement, e.g. reinforcement, can result in plastic settlement cracks.

Where the solids continue to settle in comparison to those which are prevented from further downward movement, the concrete will ‘break its back’ and a tear appears in the surface as it is forced into tension. Cracks may develop at regular spacing reflecting the reinforcement layout. They often occur in conjunction with voids under the bars as shown in Figure 1. Figure 1(a) shows initiation and Figure 1(b) the condition after a few hours. These crescent-shaped voids may initially be filled with bleed water. The region of bond between the bar and concrete is thus reduced.