Steel reinforcement used in reinforced concrete may be of the following types
- Mild steel bars
- Hot rolled mild steel
- Medium tensile steel
- Hot rolled medium tensile steel.
- Hot rolled High Yield Strength Deformed (HYSD) steel.
- Cold-worked steel HYSD bars steel.
- Hard drawn steel wire fabric.
- Rolled steel made from structural steel.
the most important characteristic of a reinforcing bar is its stress strain curve and the important property yield stress or 0.2% proof stress, as the case may be.
The modules of elasticity E for these steel is 2×105 N/mn2.
Mild steel bars have yield strength of 250 N/mm2 and hence it is known as Fe 250.
HYSD bars may be hot rolled high yield strength bars or cold rooked steel high strength deformed bars. The latter are also known as cold twisted deformed bars or Tor steel and are available in different grades
Fe 415 ii) 500 iii) Fe 550 having 0.2% proof stress as 415N/mm2, 500N/mm2 and 550 N/mm2
The reinforcing bars should have sufficient % of elongation.
Its coefficients of thermal expansion should be more or less equal to the cement concrete.
Stress-strain curves for reinforcement
Figure1: Stress-strain curve for Mild steel (idealised) (Fe 250) with definite yield point
Figure2: Stress-strain curve for cold worked deform bar
Figures 1. and 2. show the representative stress-strain curves for steel having definite yield point and not having definite yield point, respectively. The characteristic yield strength fy of
steel is assumed as the minimum yield stress or 0.2 per cent of proof stress for steel having no
2
definite yield point. The modulus of elasticity of steel is taken to be 200000 N/mm .
For mild steel, the stress is proportional to the strain up to the yield point. Thereafter, post yield strain increases faster while the stress is assumed to remain at constant value of fy.
For cold-worked bars (Fig. 2.), the stress is proportional to the strain up to a stress of 0.8 fy.
Thereafter, the inelastic curve is defined as given below:
Stress value
Inelastic strain value
0.80 fy
Nil
0.85 fy
0.0001
0.90 fy
0.0003
0.95 fy
0.0007
0.975 fy
0.0010
1.00 fy
0.0020
Linear interpolation is to be done for the intermediate values. The two grades of cold-worked bars are used as steel reinforcement are Fe 415 and Fe 500 with the values of fy as 415 N/mm2 and 500 N/mm2, respectively.