Concrete spalling
Spall refers to small flakes that break off of the parent material. When concrete is applied poorly it is more susceptible to spalling. Spalling, also known as concrete cancer, damages the structural integrity of concrete. It can be the result of several processes
- salt damage
- rusting of the internal steel reinforcing bar
- extreme heat
- pressure
Salt is extremely damaging to concrete which is naturally porous. Dissolved salt is carried into the material and the crystals are left behind when the water evaporates. Over time the salt crystals grow and create shear stresses that break off pieces of concrete from the surface. Sometimes penetrating sealants are used to protect the concrete, but these can create problems of their own by preventing water from evaporating properly.
A common cause of concrete spalling is rusting of the reinforcing rod inside the concrete. When the rusted steel rod expands it causes the surrounding concrete to flake. This comprises the structural strength of the construction member.
Extreme heat might damage concrete, for example when a car fire occurs on a bridge.
Tunnels can experience spalling simply from the pressure imposed by the weight of the overlying rock and soil.
Spalling in ordinary concrete applications is usually the result of poor quality concrete or bad application. But high performance concrete structures such as tunnels, bridges, or piers may be subjected to additional stresses that can damage even properly mixed and applied concrete.
Spalling must be repaired early to prevent large expenses. The loose concrete must be removed. If the re-rod is severely rusted it must be replaced. New concrete is applied, the type depending on the particular site. And finally, the surface may be refinished to match the original.