"In reinforced concrete, concrete cover is the shortest distance between the surface of embedded reinforcement and the exterior surface of the concrete (ACI 130)." The concrete cover protects the reinforcement from corrosion, insulates it from intense heat such as fire, and guarantees that the reinforcement may be actively repositioned. Concrete cover is also important for aesthetic reasons. The reinforcement used in reinforced concrete consists of bars or wires of metal (usually steel) that are imbedded at regular intervals in the concrete. These reinforcements serve to give strength and support to the concrete, just as the rebars do in a traditional building structure. The amount of reinforcement used depends on the quality of the concrete and its intended use. Low-quality concrete with no reinforcement in itself can be strengthened by using plain concrete in active areas where stress levels are low. Higher-quality concrete with steel reinforcing will require less reinforcement than lower-quality concrete because the additional strength provided by the reinforcement is taken up by the concrete itself. The concrete cover is applied in two main forms: external and internal. In an external cover, the outer surface of the concrete is exposed. This means that if anything breaks the cover, such as a crack in the sidewalk due to excessive load bearing, water can enter the reinforcement and cause corrosion or other damage. Corrosion can lead to the loss of reinforcement, which could then need to be replaced. Internal covers have an inner layer of concrete that is bonded to the reinforcement beneath it.