Structural Concrete - Hassoun

2.19 Steel Reinforcement 75
Table 2.19
ASTM 615 M (Metric) for Reinforcing Steel Bars
Bar No. Diameter (mm) Area (mm 2 ) Weight (kg/m)
10 M 11.3 100 0.785
15 M 16.0 200 1.570
20 M 19.5 300 2.355
25 M 25.2 500 3.925
30 M 29.9 700 5.495
35 M 35.7 1000 7.850
45 M 43.7 1500 11.770
55 M 56.4 2500 19.600
Table 2.20
ASTM
ASTM Metric Specifications
Bar Size No.
MPa ksi Grade
A615 M 10, 15, 20 300 43.5
Billet steel 10–55 400 58.0
35, 45, 55 500 72.5
A616 M 10–35 350 50.75
Rail steel 10–35 400 58.0
A617 M 10–35 300 43.5
Axle steel 10–35 400 58.0
A706 10–55 400 58.0
Low alloy
The code defines the type of structure and the recommended type of steel to be used. These are
shown in Table 2.21 and Table 2.22.
2.19.3 Stress–Strain Curves
The most important factor affecting the mechanical properties and stress–strain curve of the steel
is its chemical composition. The introduction of carbon and alloying additives in steel increases
its strength but reduces its ductility. Commercial steel rarely contains more than 1.2% carbon; the
proportion of carbon used in structural steels varies between 0.2 and 0.3%.
Two other properties are of interest in the design of reinforced concrete structures; the first
is the modulus of elasticity, E s . It has been shown that the modulus of elasticity is constant for all
types of steel. The ACI Code has adopted a value of E s = 29 × 10 6 psi (2.0 × 10 5 MPa).
The modulus of elasticity is the slope of the stress–strain curve in the elastic range up to the
proportional limit; E s = stress/strain. Second is the yield strength, f y . Typical stress–strain curves
for some steel bars are shown in Fig. 2.7. In high-tensile steel, a definite yield point may not show
on the stress–strain curve. In this case, ultimate strength is reached gradually under an increase of
stress (Fig. 2.7). The yield strength or proof stress is considered the stress that leaves a residual
strain of 0.2% on the release of load, or a total strain of 0.5 to 0.6% under load.