When Do Shrinkage Cracks in New Concrete Occur?

Plastic Shrinkage

• Plastic shrinkage cracks occur prior to drying, when the concrete is still in its plastic state and is full of water. They occur when the moisture within the concrete evaporates at the surface faster than the water beneath can rise and replenish it. If the concrete has not achieved high-enough tensile strength when this happens, the shrinkage at the surface can cause cracking. The cracks typically occur across the surface and are usually found in horizontal surfaces.

Drying Shrinkage

• Drying shrinkage cracks form after the concrete has hardened, and are usually caused by thermal contraction. As it dries, hardened concrete shrinks; this places stress on the slab, and cracks occur to relieve this stress. The higher the water content in the new concrete, the more likely it is to develop drying shrinkage cracks. High sand content and fine aggregate can also increase the chances of cracks occurring, because they provide less internal resistance to shrinkage.

Identification

• Plastic shrinkage cracks usually occur parallel to each other and are shallow, running one to three feet apart. They commonly occur in hot weather as early as a few hours after pouring and finishing.The time at which drying shrinkage cracks occurs depends on the rate of drying. Typically they occur several months after casting, but may occur up to four years later. Drying shrinkage occurs on the surface of the concrete, and corners are particularly vulnerable to this type of cracking. However, there is no typical pattern, and cracks may develop at any location.

Minimizing Shrinkage Cracks

• Shrinkage cracks in new concrete do not usually impair the concrete’s durability or strength. While they can’t be prevented completely, they can be minimized or reduced. Concrete masons typically create a line in concrete slabs during finishing or immediately after pouring to control where cracks will occur in new concrete. This groove in the slab reduces the thickness and makes it easier for the concrete to crack at that location. These are called control joints and should be a minimum of 1/4 of the thickness of the slab to be effective. Reinforcing steel or wire mesh placed within the concrete helps prevent structural problems from occurring by holding the concrete together when it does crack. Shrinkage cracking can be reduced by reducing the total water in the mix and pouring and finishing concrete when weather is not hot, dry or windy.