Introduction of deformed bars:

The various forms in which steel is used as reinforcement in RCC work are round bars, deformed bars, twisted bars, square bars and flats. Sometimes expanded metal fabric or fabric made by welding or weaving steel wire in the form of oblong or square mesh are also used as reinforcement in slabs, shells and concrete roads. For works of large dimensions like massive foundations, etc. sections like channels, rolled steel beams, or angle iron are also used as reinforcement. Mild and medium tensile steel bars of round section are most commonly used in RCC work. The diameter of round bars used in normal building are 6 mm, 8 mm, 10 mm, 12 mm, 16 mm, 18 mm, 20 mm, 25 mm, 28 mm, 32 mm, 36 mm and 40 mm. Bars of greater diameter, i.e. 45 mm and 50 mm are only used in exceptionally heavy foundations, huge girders or counterforts, etc.

With the introduction of deformed bars and twisted bars, the use of plain round bars is gradually decreasing. Deformed bars or high yield strength deformed bars (HYSD) are furnished along with lugs, ribs or other form of surface deformations for the purpose of increasing their bond strength with concrete. It is seen that the process of twisting a plain or deformed bar results in the increase in yield stress, tensile strength and bond strength. Twisted plain or deformed bars not just have high yield stress but also have strength that is 40 % more than that of plain round bars. On account of increased bond strength such bars do not need end hooks and require reduced length for overlaps, etc. through leading to reduction in the cost of reinforcement and labour.