Concrete is made up of an aggregate of sand and gravel, held together with cement. Placed in forms as a semi-liquid material, it cures over time to become a hard, stone-like material. Concrete is very strong under compression loads. What that means is that a typical concrete slab on a well-compacted base will resist more than a two-ton load per square inch resting on it without any damage. Concrete also resists surface damage, due to its hard surface.
On the other hand, concrete has much less strength in tension. That means that forces that tend to pull it apart or bend it can easily cause the bond between the cement and the aggregate to break, creating a crack. The cement that binds the aggregate is relatively brittle. Some of the tension and bending forces that can crack concrete include:
- Movement or Sagging of the Base – If the base material under the concrete swells, moves, or sags, the weight of the concrete itself, along with the weight of object, like cars, on the concrete can create a bending force. This can cause cracks to form.
- Expansion and Contraction – As temperatures change, concrete expands and contracts, like all other substances. In Minnesota, where the range of temperatures is extreme, it can create tension forces that can cause cracks in concrete.
- Hydraulic Pressure – Water that collects under concrete, roots growing under concrete, and the expansion of water when it freezes, can also create bending forces, which can crack concrete. Once a crack develops, too, it allows water infiltration and the freeze/thaw cycle and hydraulic pressure may increase.
- Edge Loads – Heavy weights on the edges of a concrete slab can also cause bending forces that can crack concrete, especially if the base near the edges has been infiltrated with soil.
- Shrinkage During Curing – Excessive water loss during the curing stage of concrete can result in the slab shrinking, which can cause cracks. A curing agent applied to the concrete after finishing helps to minimize this.