Minimum thickness of concrete members reinforced with fibre reinforced polymer bars

Abstract

To avoid excessive deflection most design codes specify the ratio (l/h) s , the span to minimum thickness of concrete members without prestressing. Use of the values of (l/h) s specified by the codes, in selecting the thickness of members, usually yields satisfactory results when the members are reinforced with steel bars. Fibre reinforced polymer (FRP) bars have an elastic modulus lower than that of steel. As a result, the values of (l/h) s specified in codes for steel-reinforced concrete would lead to excessive deflection if adopted for FRP-reinforced concrete. In this paper, an equation is developed giving the ratio (l/h) f for use with FRP bars in terms of (l/h) s and (ε s /ε f ), where ε s and ε f are the maximum strain allowed at service in steel and FRP bars, respectively. To control the width of cracks, ACI 318-99 specifies ε s = 1200 × 10 –6 for steel bars having a modulus of elasticity, E s , of 200 GPa and a yield strength, f y , of 400 MPa. At present, there is no value specified for ε f ; a value is recommended in this paper.Key words: concrete, cracking, deflection, fibre reinforced polymers, flexural members, minimum thickness.