Cracks in concrete are a common occurrence, and they can happen for a variety of reasons. Even with the best floor design and proper construction, cracks in concrete are often unavoidable. Fortunately, they can usually be repaired. Fine cracks are commonly seen in newly laid concrete and are caused by plastic shrinkage.
These cracks are very small, about 0.003 inches (0.08 mm) wide and can be very shallow. They often resemble cobwebs or broken glass. When the top of a concrete slab loses moisture too quickly, these cracks are likely to appear. While they may be unsightly, they do not pose a structural problem. When a homeowner notices a crack in their slab or wall, especially if the concrete is relatively new, they may assume something is wrong.
However, some types of cracks are inevitable. The best way to prevent them is to properly prepare the subbase, make sure that the concrete is not too wet, use reinforcement where necessary, and correctly place and space the crack control joints and expansion joints. Despite these precautions, cracks may still occur. Fine cracks usually develop within the first month and most foundation cracks appear within the first year. This is because almost all concrete contains more water than is needed for the hydration of the cement.
If you see cracks or curls, it is possible that the concrete dried out faster than it should during pouring. Diagonal cracks emanating from a corner of a window or other openings are known as re-entrant cracks and are usually caused by stress buildup in the corner. In order to prevent cracking, contractors often use control joints to control where cracks will occur when they cannot avoid them. When the seal is embedded, it separates the surface near the stamped joints and causes small cracks around the outer edges of the “stones”. Other cracks that occur after hardening can be caused by insufficient depth of concrete over the bends in the reinforcement, lack of expansion and contraction joints, or nesting of reinforcing steel in concrete. Cracking cracks after hardening can result from excessive flotation that tends to attract water and cement to the surface, which then comprises weak concrete subject to high shrinkage stresses.
Along with traditional curing methods, concrete additives and curing compounds can help concrete cure faster and resist weathering.