Determination Of The Characteristic Strength Properties Of Mild Steel Reinforcement
[A CASE STUDY OF ILORIN METROPOLIS]

2.5.2 TYPES OF REINFORCEMENT
Steel is used in two different ways in concrete structures as reinforcing steel and as pre-stressing steel. Reinforcing steel is placed in the forms prior to casting of concrete.
Stresses is the steel, as in the hardened concrete are caused only by the loads on the structure, except for possible parasitic stresses from shrinkage or similar causes.
In contrast, in prestressed-concrete structures large tension forces are applied to the reinforcement prior to letting it act jointly with the concrete in resisting external loads. (William Nash, strength of material 4th Edition)
2.6 PRESTRESSING STEEL
Prestressing steel is available in the form of cold-drawn wire, stranded cable and alloyed steel bars. Wire and strand are produced with ultimate tensile strengths fpu of 250 and 270kips/〖in〗^2 (1724 and 1862 Mpa). Smooth surfaced high strength bars with diameters between ¾ and 13/8  (19 and 35mm) are manufactured with ultimate tensile strengths of 145 and 160 kips/〖in 〗^2 (1000 and 1103MPa).
High strength steels are produced by using alloying elements (manganese, silicon, carbon etc) by cold working, and by heat treating and tempering. Since the manufacturing techniques that produce high strength also reduce ductility and toughness, steels with yield points above 270kips/m^2 (1862MP), which would be extremely brittle are not used as tendons. The stress strain curves for high strength bars and wires indicate that prestress steels lack sharply defined yield point. To establish the beginning of the inelastic range a yield strength fpy for wire and strand is often defined as the stress associated with a 1 percent strain. For high-strength bars, the yield strength is frequently specified as the stress associated with the intersection of the stress strain curve and a line parallel to the initial slope of the stress strain that extends upward from a strain of 0.002 at Zero stress.
High tensioned steels are more vulnerable to corrosion than lightly stressed steels. This stress corrosion is more likely to occur in oil-tempered wire tendons expose directly to a combination of air and moisture. Since calcium chloride, which accelerates the rate at which concrete gains strength, also increases the susceptibility of prestressed tendons to stress corrosion, concrete specifications typically specify that calcium chloride must not be added to cements or mortars in contact with prestressed reinforced.