Background Knowledge - Strengthening Design of Reinforced Concrete with FRP - page 18

23 − f c in psi (Nilson 1997). On the other hand, typical values for split-cylinder

strength f c () for normal weight concrete are 68 − f c in psi and for light weight

concrete are 4− in psi. Furthermore, typical values for modulus of rupture

f () for normal weight concrete are

f c

812 − f c in psi and for light weight concrete

are 6− in psi. The tensile contribution of uncracked concrete is ignored at the

level of ultimate capacity because it is negligible at that stage.

f c

2.2.4.3 Behavior of Reinforcing Steel
Steel reinforcement bars behave similarly in tension and compression. Even though
mild steel has some strain hardening prior to final fracture, the ACI 318 code assumes
a flat plateau after steel yielding, thus conservatively ignoring this strain hardening
effect, as shown in Figure 2.9. On the other hand, higher strength steels have non-
linear strain hardening behavior after steel yielding, as demonstrated in Figure 2.10.
However, typical design computations still model steel as elastic-perfectly plastic to
conservatively simplify the calculations.
The primary parameters that define the idealized stress-strain model of reinforc-
ing steel are

E s : The Young's modulus of elasticity for steel is known to be approximately
equal to 29,000 ksi = 200,000 MPa.
f y : The yield strength of steel, which varies depending on the composition of
steel alloy, ranges between 40-100 ksi (276-690 MPa).

For high-strength steel, ACI 318-11 code specifies f y as the stress at ε s = 0.0035
( f y > 60 ksi, 414 MPa), as shown in Figure 2.10.

Neglect in design

f y

Design stress-strain curve

E s

1

ε y

Strain, ε s
FIGURE 2.9 Actual and idealized stress-strain curve of reinforcing steel. (Courtesy of
Portland Cement Association [2013].)