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Plastic settlement cracks

Plastic settlement occurs as a result of restraining the settlement of concrete. Plastic settlement cracks are so-called because it is formed while the concrete is still plastic. Plastic settlement cracks are different from shrinkage cracks by its distinguished pattern. Plastic settlement cracks tend to mirror the restraining elements, such as reinforcing steel. Plastic settlement cracks may become noticeable at an early stage. However, the cracks often noticed after a few hours of concrete placing. The cracks occur while the concrete is plastic, and the bleeding water is rising. These cracks can seriously affect reinforced concrete that exposed to harsh environments. These cracks will facilitate the ingress of aggressive chemicals such as chloride and sulfate. The ingress of chloride will result in corrosion of steel reinforcement, which may reduce the service life of the structure.
Figure 1

Following the concrete placing, concrete water tends to rise up while the solid materials go down. In the absence of restraining elements, the concrete level will go down slightly. Clearly, the presence of restraining elements such as reinforcement will locally restrain the movement of concrete paste while the adjacent concrete continues to settle, which generates plastic settlement cracks.

Plastic settlement cracks can be reduced by adopting practices such as using concrete with low bleeding characteristics (low slump and more cohesive mixes) and increase the cover to bar diameter. This can be achieved by increasing the concrete cover or reducing the reinforcement bar diameter.

The repairing of plastic settlement cracks depends on the concrete state. For plastic concrete, cracks can be repaired by re-vibrating the concrete and re-finishing the surface while concrete in a plastic state. Timing is important to ensure the concrete re-liquifies under the vibrating so the cracks will fully close. Cracks may form again if the concrete re-vibrated too early. Besides, late re-vibration will impair the bond between concrete and reinforcement. For hardened concrete, the cracked area should be chased out and filled with appropriate repairing materials. Proper repairing of crack will ensure concrete durability, and it will reduce the need for maintenance.


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