Cracking and heaving are common flaws in concrete. Because concrete is a mixture of cement, gravel and sand, it has a finite tensile strength that is prone to fracturing under environmental or structural stress. Concrete was used in natural forms by the Ancient Egyptians and Chinese. The Romans then invented synthetic concrete by mixing volcanic ash with lime mortar, sand and gravel. Modern or "Portland" cement was invented in the 19th century and is a mixture of chalk and clay which has been baked in a lime kiln. By the late 19th century concrete being was being reinforced with steel and iron.
Cracking from Shrinkage
Because hydration is required to bond concrete's cementitious content, evaporation shrinks drying concrete. When this happens stresses develop, leading to cracks. Higher amounts of water in the concrete mix raises tensile stress during drying as more shrinkage occurs. Surface cracks are common on drying concrete, but these are usually cosmetic. However, if water on the drying concrete surface evaporates at a quicker rate than it can be replaced by bleed water from the interior, tensile stresses result in deeper cracks.
Cracking also occurs because of temperature differentials between concrete's interior and surface. Stresses are caused by the cementitious materials expanding and contracting at different levels of the concrete, resulting in structural deformation and cracking. The severity of the cracks depends on the concrete's content, the temperature differences and other factors such as the presence of reinforcing steel.
Structural failure in concrete can also result from subsidence in the underlying support which the concrete lies on. Soil may sink due to underground water erosion or natural settling. The concrete itself may also be subject to internal stresses because of poor consolidation after being poured. This makes it prone to damage from ground vibrations.
Concrete that has been reinforced with iron rebar or mesh is susceptible to cracking if the iron is exposed to water corrosion. When iron rusts its structure expands, putting stress on the concrete surrounding the metal. Eventually cracks will form and the damage is compounded by more water being able to enter the concrete and corrode additional rebar or mesh.
Frost heaving occurs in a climatic zone where freezing temperatures can penetrate deep beneath soils. Frost formation in soil can result in damage to concrete on the surface. This is due to water volume increasing by 9 percent when it crystallizes as ice. As water freezes in soil underneath concrete volume expansion occurs, displacing the concrete's support. Additional stress may result if the ice grows from a continuous supply of water.
- Photo Credit concrete image by Empath from Fotolia.com
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