Concrete is strong in compression, with crushing strengths typically in the range 20–40 MPa, and up to 100 MPa for high-strength concretes. However, the tensile strength of concrete is usually only 10% of the compressive strength. Steel is the universally accepted reinforcing material, as it is strong in tension, forms a good bond and has a similar coefficient of thermal expansion to concrete. The location of the steel within reinforced concrete is critical, to ensure that the tensile and shear forces are transferred to the steel. The longitudinal bars carry the tensile forces while the links or stirrups combat the shear forces and also locate the steel during the casting of the concrete. Links are therefore more concentrated around locations of high shear, although inclined bars may also be used to resist the shear forces. Fewer or thinner steel bars may be incorporated into reinforced concrete to take a proportion of the compressive loads in order to minimise the beam dimensions.
Steel reinforcement for concrete is manufactured, largely from recycled scrap, into round, ribbed, indented or ribbed and twisted bars . Mild steel is frequently used for the plain bars to form bent links. Hot rolled, high-yield steel is used for ribbed and indented bars. British Standard BS 4482: 2005 refers to 250 MPa yield strength steel for plain bars and to the higher grade 500 MPa steel for plain, ribbed and indented reinforcement of diameters between 2.5 and 12 mm. British Standard BS 4449: 2005 specifies highyield steel (grade 500 MPa) with three levels of ductility A, B and C (highest) for ribbed bars of 6–50 mm diameters. Welded steel mesh reinforcement to BS 4483: 2005 is used for slabs, roads and within sprayed concrete.
Austenitic stainless steels may be used for concrete reinforcement where failure due to corrosion is a potential risk. Grade 1.4301 (18% chromium, 10% nickel) stainless steel is used for most applications, but the higher grade 1.4436 (17% chromium, 12% nickel, 2.5% molybdenum) is used in more corrosive environments. Where long-term performance is required
in highly corrosive environments, the duplex grades of stainless steel may be used. The initial cost of stainless steel reinforcement is approximately eight times that of standard steel reinforcement, but in situations where maintenance costs could be high, for example, due to chloride attack from sea water or road salts, the overall life cycle costs may be reduced by its use. In addition, stainless steels have higher strengths than the standard carbon steels. Suitable stainless steels for the reinforcement of concrete are specified in BS 6744: 2001 + A2: 2009.
For reinforced concrete to act efficiently as a composite material the bond between the concrete and steel must be secure. This ensures that any tensile forces within the concrete are transferred to the steel reinforcement. The shape and surface condition of the steel and the quality of the concrete all affect the bond strength.
To obtain the most efficient mechanical bond with concrete, the surface of the steel should be free of flaky rust, loose scale and grease, but the thin layer of rust, typically produced by short-term storage on site, should not be removed before use. The use of hooked ends in round bars reduces the risk of the steel being pulled out under load, but high bond strength is achieved with ribbed or indented bars which ensure a good bond along the full length of the steel. Steel rebars are usually either supplied in stock lengths, or cut and bent ready for making up into cages. Sometimes the reinforcement may be supplied as prefabricated cages, which may be
welded rather than fixed with iron wire as on site. Steel reinforcement, although weldable, is rarely welded on site. Rebar joints can easily be made with proprietary fixings, such as steel sleeves fastened by shear bolts. Spacers are used to ensure the correct separation between reinforcement and formwork.
Good-quality dense concrete gives the strongest bond to the steel. Concrete should be well compacted around the reinforcement; thus, the maximum aggregate size must not bridge the minimum reinforcement spacing.