Concrete is a strong and durable material, but it is not immune to cracking. Cracks in concrete can range from being non-structural and unsightly, to being detrimental to the structural integrity and safety of a building. There are several causes of cracks in concrete, including plastic shrinkage, thermal stresses, and external load conditions. To prevent cracking, it is important to understand the causes and take steps to reduce the risk.
The most common cause of early cracks in concrete is plastic shrinkage. When concrete is still in its plastic state (before hardening), it is filled with water. As the slab loses moisture during curing, it becomes a little smaller. This shrinkage creates stress in the concrete slab, which can cause it to crack if the slab cannot move freely with the ground.
Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab has been poured and finished. The gradual and slow time-dependent deformation of the concrete structure under sustained loads is known as creep. It can generate excessive stress and provoke the development of cracks. These plastic shrinkage cracks are usually shallow and usually 1 to 2 mm wide, which means they cannot be repaired with the injection method.
Thermal stresses often cause cracks in mass concrete structures, the main cause of temperature differentials being the influence of the heat of hydration on the change in volume. If a tree is located too close to a concrete slab, growing roots can lift and crack concrete. The pressure causes the concrete to form cracks near the steel which over time will lead to more extensive cracking as rust builds up until the concrete begins to peel off the reinforcing steel bars (peeling of the concrete) and exposing the corroded reinforcing steel rods. Due to the alkaline nature of cement, it reacts with carbon dioxide (CO) present in the atmosphere, resulting in an appreciable increase in the volume of materials, which ultimately leads to cracking.
Because concrete cannot shrink in a corner, stress will cause concrete to crack from the point of that corner. To prevent cracking, it is important to design structures taking into account the degree of restriction during the drying or cooling of concrete. Use materials known to have a good service history with respect to cracking, regardless of shrinkage or other evidence in single-contributing causes. Use minimum cement content consistent with design requirements and use minimum water content necessary for workability; do not allow consistencies in excess of moisture.
Place concrete evenly and take into account early settling in forms, around reinforcement, on slopes and elsewhere. Cure wet or sealed concrete, starting very early and avoid extreme temperatures. The key to successful crack repair is to understand the causes of cracks and also whether the cracks are dormant or active. The product reacts quickly with water, chasing the water present in the crack and begins to foam and expand, filling the entire crack, resulting in a strong bond with concrete and a flexible waterproof seal that prevents future water leakage. Eliminating or limiting any of these conditions eliminates or reduces cracking in hardened concrete.
