Plastic settlement cracks
Cracks caused when the settlement of fresh concrete is restrained by reinforcement or formwork. Plastic settlement cracks can form in young concrete, within the first few hours after placing. As water moves upward through the mixture, the denser constituents move downward. This downward movement may be obstructed by the top layer of reinforcement or by the shuttering.
The plastic concrete may arch over the top of individual reinforcing bars, bringing the surface into tension. Cracks may develop at regular spacing 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.
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 shuttering, causing restraint to the concrete in connected members. This typically happens at mushroom-heads on columns but 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.