They have smaller members. Components that have larger dimensions trap more heat than smaller sections, while smaller sections are more willing to lose heat.
Also, during heat exchange with the environment, wooden molds have more thermal insulation properties than steel molds, so the maximum temperature is higher in wooden molds.
However, when concrete is heated due to hydration, it begins to expand. If there is any resistance to this expansion (for example, from previously cast parts) compressive stresses are produced. After reaching the maximum temperature, the concrete starts to cool down and its volume decreases. Resistance to this shrinkage causes tensile stresses. At this stage, which usually starts a few days after pouring the concrete, the concrete has little resistance and is not yet ready to bear these tensile stresses, so there is a possibility of cracking due to shrinkage.
Among the types of thermal cracks, we can mention the cracks that appear in the walls and on the tape foundations that have been in place for a few days. In this case, the vertical cracks start from the base of the walls in the middle of the wall and pass through its section and have an angle of 45 degrees on the side of the wall. The width of the crack and its depth depends on the amount of rebars, and usually in the design methods, minimum rebar control is done in order to control these cracks.
Cracks caused by shrinkage due to long-term drying
Usually, the amount of water in the concrete mixture is more than what is needed for hydration. If the concrete is placed in a condition where the relative humidity of the environment is low, the humidity of the concrete will decrease through the surface.
The loss of moisture causes a decrease in volume, which is interpreted as shrinkage caused by drying. If the action of shrinking and shrinking is faced with an external or internal limitation, the tensile stresses created will cause the concrete to crack. Thin components with large surfaces, such as slabs, are susceptible to this type of cracking. The appearance time of this cracking depends on the amount of drying which is also determined by the environmental conditions. But it usually happens several months after pouring concrete, and this cracking appears on the concrete surface, because drying takes place from the surface layer.
The drying of the surface layer and the reduction of its volume and the resistance to volume change by the lower layers cause cracks in the surface layer in slabs with large surfaces. Usually, the first cracks appear in the corners of the slabs, because drying takes place from 3 sides. There is no specific pattern of cracks caused by shrinkage due to drying. The width of these cracks also depends on the amount of concrete drying, the geometry of the member and the distance between the external constraints. For example, in slabs that are placed on the ground and one dimension is larger than the other, cracks are formed in the middle part and parallel to the smaller dimension, and cracks are also created diagonally in the corners. This situation also occurs in one-sided or two-sided slabs. There are cracks in the corners of the empty parts of the slab that are intended for example for stairs.
This post is written by Naeimi_Behzad