Characteristics of reinforcing steel bars

Reinforcing steel is used in RCC to carry tension (and compression) and to control cracking. IS 1786 covers HYSD/TMT bars and IS 432 covers mild steel bars.

Main characteristics

1. High tensile strength: Fe250 ($f_y=250$), Fe415 ($f_y=415$), Fe500 ($f_y=500$) and Fe550 ($f_y=550$ N/mm$^2$).
2. Ductility: elongation of at least 23% (Fe250), 14.5% (Fe415) and 12% (Fe500); ductile steel gives warning before failure. Fe500D and Fe550D give higher ductility for seismic use.
3. Modulus of elasticity: $E_s=2\times10^5$ N/mm$^2$ for all grades.
4. Yield behaviour: mild steel has a definite yield point; HYSD/TMT bars have no sharp yield point, so the 0.2% proof stress is taken.
5. Good bond: ribs/deformations on the surface give better bond with concrete (the design bond stress is increased by 60% for deformed bars).
6. Thermal compatibility: the coefficient of expansion is about $1.2\times10^{-5}$ per $^\circ$C, nearly equal to that of concrete, so no significant internal stress develops with temperature change.
7. Weldability and bendability: bars should bend without cracking (bend and rebend tests); the carbon equivalent is limited for weldability.
8. Ultimate to yield ratio: $f_u/f_y\ge1.10$ (1.08 for Fe500) to ensure reserve strength.
9. Corrosion resistance: TMT bars have a tough outer surface and a softer core, giving better corrosion resistance than cold-twisted bars.
10. Density: 7850 kg/m$^3$; bars are available in sizes 6, 8, 10, 12, 16, 20, 25 and 32 mm.