The two types, plastic settlement and plastic shrinkage cracks, both occur between one and eight hours after placing of the concrete due to the process of bleeding, namely the action of water rising to the surface shortly after compaction. Bleeding is caused by the water being forced upwards when heavier solid particles settle downwards. Bleed water is only seen at the surface when the rate of evaporation is less than the rate of bleeding and should be distinguished from laitance, which is a mixture of water, cement and very fine particles. A wet mix will bleed more than a dry one, so excessive water contents should be avoided.
Plastic settlement cracking
This type of cracking occurs when there is a large amount of bleeding and settlement, and there is some form of restraint or obstruction to free settlement As illustrated in Fig. 3.19, cracks occur mainly at a change in section, in narrow columns and walls due to arching, and in trough or waffle slabs; other locations are formwork tie bolts and fixed reinforcement near the top of the concrete where voids can form underneath the reinforcement. Plastic settlement cracks can be prevented by reducing the bleeding using air entrainment or by incor- porating fibre reinforcement. If possible, a reduction of restraint and re-vibration of the concrete are also other ways to avoid or eliminate plastic settlement cracks.
Plastic shrinkage cracking
Plastic shrinkage is caused by the loss of water by evaporation from the surface of newly laid concrete or by suction of dry concrete underneath. At the surface, plastic shrinkage occurs when the rate of evaporation exceeds the rate of bleeding. Contraction induces tensile stress in the surface layers because they are restrained by the non-shrinking inner concrete. Since concrete has a low tensile strength or low strain capacity in its plastic state, cracking can readily occur.
Plastic shrinkage cracks are most common in slabs, occurring randomly, diagonally and over reinforcement (see Fig. 3.19). Prevention of plastic shrink- age cracking is achieved by covering the surface of the concrete as early as possible and protecting it from the effects of drying winds. Spraying of resin- based curing compounds (unless in emulsion form) cannot be done effectively until the free water has evaporated. It is therefore difficult to ensure that the compound is applied before plastic shrinkage cracks have begun to form. Covering with polythene sheet is the most effective solution. On concrete roads and other surfaces, where the finished texture is vital, the covering must be suspended clear of the surface. The risk of cracking is also reduced by the use of fibre reinforcement, which significantly increases the tensile strain capacity of concrete in its plastic state. In some cases, cracks can be eliminated by re- vibration of the concrete or by power floating and trowelling of flat surfaces.