The curling in the concrete slab refers to the distortion of a slab into a curved shape by upward or downward bending of the edges. This distortion can lift the edges of the slab from the base leaving an unsupported edge or corner which can crack when heavy loads are applied. Curling typically occurs some months after concrete placement but can occur much earlier. The extent of movement is usually minimal and it can be as much as 20–25 mm maximum. When the edges or corners of a slab curl up, often at construction joints, it’s caused by concrete at the top of the slab shrinking or contracting faster than concrete at the bottom. Water leaves the top surface of the slab faster than it leaves the bottom, creating shrinkage forces that can raise slab edges or corners.
Causes for curling of concrete slab
There are many causes for corner curling. The significant causes of curling in concrete are drying shrinkage, distance between joints and temperature differences.
The most common cause of curling is drying of the top surface which then shrinks to the bottom. This upward curling of the edges of a slab is caused by shrinkage or contraction of the top relative to the bottom. When one surface of the slag changes sizes more than the other, the slab will warp its edges in the direction of relative shortening. This curling is most noticeable at the sides and corners. Changes in slab dimensions that lead to curling are most often related to moisture and temperature gradients in the slab. The curling of a slab is most likely related to poor curing and rapid surface drying and anything that increases drying shrinkage will tend to increase curling.
Thin slabs and long distance of joints often increase curling. Therefore, thin unbonded toppings need to have a fairly close joint spacing. In industrial floors, close joint spacings may be undesirable because of the increased number of joints and increased joint maintenance problems. However, this must be balanced against the probability of intermediate random cracks and increased curling at the joints.
The temperature differences between the top and bottom of the slab can also cause curling. The top part of the slab exposed to the sun will expand relative to the cooler bottom causing downward curling of the edges. Alternatively, during a cold night when the top cools and contracts concerning a warmer subgrade, the curling due to this temperature differential will add to the upward curling caused by moisture differentials.
Preventing curling of concrete slab
All concrete shrinks to some extent, so you have to be cautious about your preventive measures implemented. It can be done by preventing drying shrinkage and bleeding. The following practices will help to minimize the potential for curling.
