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Plastic and thermal cracking 2/5

2.2 Plastic cracking

Plastic cracking occurs in the first few hours of the concrete being laid, before it has
gained sufficient tensile strength to resist internal tensile stresses. Because they form in
the unhardened concrete they are fundamentally different from thermal or other cracks.

Plastic settlement cracks typically occur in deeper sections such as walls, columns and
deep beams. Plastic shrinkage cracks are more prevalent on exposed flat slabs. The key
to understanding the mechanism for both types of plastic cracking is bleeding.

Bleeding may be described as the relative upward movement of water within fresh
concrete accompanied by the downward movement of the heavier particles that are suspended
in the concrete matrix. This is caused by the inability of the solid constituents to prevent
water movement as they settle under gravity. Bleeding is effectively a form of sedimentation,
which is arrested as the particles form bridges, interrupting further downward movement,
and as the cement paste hydrates and stiffens. It therefore depends not only on the mix
constituents and section dimensions but also the ambient conditions. A major factor in the
capacity of a mix to bleed is the grading and consistency of the mix. Mixes that bleed
excessively are generally harsh and not cohesive, i.e. contain insufficient fine material.
(This subject is covered in Chapter 1.) It must be noted that all concrete experiences some
bleeding but it is not a sign of incomplete compaction.

When bleed water is seen it appears as clean water on the surface, but on warm or
windy days this may evaporate. It is the combination of the capacity of concrete to bleed
and surface evaporation that causes both forms of plastic cracking. The mechanism is
discussed in the following sections.

2.3 Plastic settlement cracks

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

2.3.1 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 2.2. Figure 2.2(a) shows initiation and Figure 2.2(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.

The nearer to the surface the restraint occurs, the more likely the formation of cracks,
i.e. the less the cover, the greater the chance of cracks. A settlement crack is unlikely to
occur if the depth of cover to the reinforcement is greater than one third of the section

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