Concrete spalling is a common problem in which part of the surface becomes detached, broken or spalled. It is the result of a weak surface that is susceptible to damage, and is characterised by the appearance of cracks and red rust. The most common causes of spalling are freeze-thaw cycles, the expansive effects of the alkali-silica reaction, exposure to fire, and corrosion of embedded steel reinforcing bars or steel sections. The symptoms of concrete spalling are flaking, chipping and large chunks showing on the surface of the concrete, exposing the coarse, rocky concrete aggregate.
These are the symptoms, but concrete spalling is more complex than just the appearance of the surface. It is a chemical and physical process that occurs within the concrete itself, and can be caused by how the concrete was mixed or environmental conditions. Both conditions create a weak layer of concrete that is prone to breaking upwards when exposed to freezing. The type of repair and the work involved depends on where the concrete spall is located, how deep it is, whether it is on a vertical or horizontal surface, and whether there is any exposed rebar.
Remedies may include specific electrochemical treatment for properties close to the ocean or simply removal of the damaged concrete, cleaning or removal of the affected steel and replacement with new material. ACI 318 (Building Code Requirements for Structural Concrete) prescribes the minimum concrete "cover" for all reinforcing steel within that concrete element. Corrosion of reinforcing steel in concrete is a problem that has the potential to cause a wide range of problems ranging from economic to aesthetic. Concrete resurfacing is used to repair and revitalise concrete surfaces that suffer from things as minor as staining and light corrosion due to the elements, to more structural problems such as spalling.
Concrete spalling can be prevented by ensuring that all reinforcing steel has adequate cover according to ACI 318 standards. Additionally, a waterproofing sealer should be applied one month after the concrete is placed, as well as every few years thereafter. This will help protect against freeze-thaw cycles and other environmental conditions that can cause spalling.