2) Cracking due to restraint from the old concrete
This type of cracking usually happens during the cooling down of the concrete. When cooling down, the newly placed concrete reduces its volume or shrink. If the shrinkage is not restrained, there will be no crack. On the other hand if the cool-down concrete is restrained, usually by the previously placed or old concrete, cracks can be formed in the newly placed concrete in the direction normal to the plane of the connection. During heating up, the newly placed concrete is under compression, so there is no crack.
In general, the location of maximum temperature difference is the most subjective location of develop temperature cracks. Consequently, the exposed surfaces of concrete, either to the air or the underground soils, are likely to have temperature cracks of the self-restraint category.
In case of external restraint, cracks are formed in the newly placed concrete at the location of the connection between the new and old concrete. The cracks are formed in the direction normal to the plane of connection as in Fig.3.3.