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Plastic settlement cracks in concrete are formed during initial setting of concrete. Mechanism, appearance and prevention of plastic settlement cracks are discussed below.Contents:
Mechanism of Plastic Settlement Cracks in Concrete
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. Cracks in concrete 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.
Fig. 1: Formation of plastic settlement crack (initial and final)
Appearance of Plastic Settlement Cracks in Concrete
Visual appearance of Plastic Settlement Cracks The most common restraint in slabs is from the reinforcement. The cracks occur on the top surface and usually follow the line of the uppermost bars, giving a series of parallel cracks; there may also be shorter cracks at right angles over the bars running in the opposite direction. Plastic settlement cracks are typically 1 mm wide and usually run from the surface to the bars (see Figures 2 and 3). The settlement may also result in visible undulations on the concrete surface, with the high points over the top reinforcing bars.
Fig. 2: General Plan View of Cracks
Fig. 3: Section showing undulations
In some cases where the bars in the top layer of reinforcement are close together, the whole surface layer of the concrete may be ‘suspended’ on the reinforcement while the concrete below settles. This can lead to a horizontal discontinuity beneath the bars, resulting in a loss of bond and with time delamination of concrete cover that protects the reinforcing steel against corrosion. Unlike cracks in hardened concrete, due to overloading for instance, these cracks form at a very early age and pass through the cement paste and do not pass through aggregate particle pieces. The path is therefore more tortuous. This form of crack can be potentially serious as it passes longitudinal with the reinforcement and extends to the steel, negating the resistance to corrosion provided by the concrete. Fine cracks can occur in relatively narrow formed surfaces such as columns. The concrete may arch between the containing form faces. Settlement below the restrained concrete results in a crack being formed, generally coinciding with the links. It is sometimes possible for plastic settlement cracks to form on a vertical face where reinforcement has restricted the free flow of concrete within the formwork. In such cases it is possible that the cracks are formed between the lines of the reinforcement. The concrete can also be supported by the formwork face. This causes restraint to the concrete between connected members and is especially evident where changes in section cause differential settlement, the concrete in the deeper section settling more than the shallower section resulting in a crack. This is noticeable in the transition between a flared column head (mushroom) and the plain column, and in trough and waffle slabs where more settlement takes place in the web than the comparatively thin flange (see Figures 4 and 5). The cracks may pass through the flange and appear similar to shrinkage cracks.
Fig. 4: (a) Arching near the top of column, cracking coincides with links. (b) Cracks at change of section.
Fig. 5: Cracks at change of section in trough and waffle slabs.
It can also occur at other locations, such as under spacer blocks. Cracks at mushroom heads of columns are generally horizontal. They are also typically 1 mm wide and can cross the full section. If the sub-base or other material against which the concrete is placed has a high absorbency (dry soil, permanent forms) the settlement can be exaggerated, again the cracking following the reinforcement layout.Prevention of Plastic Settlement Cracks in Concrete
The restraints that cause plastic settlement cracking are inherent in the construction and generally cannot be avoided. Abrupt changes in section depth could be avoided at the design detailing stage but the main reduction of risk is through mix design and suitable cohesion of the concrete to reduce bleeding. In simple terms prevention of plastic settlement cracks in concrete can be achieved by increasing the sand content. However, there is a limit to this at which the bleeding will increase. Very clean (marine-dredged) sand tend to assist water movement, so blending with a ‘dirtier’ sand with a higher fines (<150 micron) content can be beneficial. A tendency to bleed largely depends on the properties of the cement. Fineness is a controlling factor, possibly because the finer particles hydrate more quickly thereby reducing the rate of sedimentation. Rich mixes are less prone than lean mixes. Pozzolanic additions may also help as they reduce water content and add to the fines. High GGBS contents should be avoided as slower setting times allow for the bleeding to continue longer. Air entrainment admixtures can be used, the entrained air stabilizing the matrix and reducing the water movement. A similar effect is claimed for polypropylene fibres within concrete (British Board of Agreement, 1995).
