Shear Strength of FRP-Reinforced Concrete Beams without Transverse Reinforcement

The behavior and shear strength of concrete slender beams reinforced with fiber-reinforced polymer (FRP) bars were investigated. A total of nine large-scale reinforced concrete beams without stirrups were constructed and tested up to failure. The beams measured 3250 mm long, 250 mm wide, and 400 mm deep and were tested in four-point bending. The test variables were the reinforcement ratio and the modulus of elasticity of the longitudinal reinforcing bars. The test beams included three beams reinforced with glass FRP bars, three beams reinforced with carbon FRP bars, and three control beams reinforced with conventional steel bars. The test results were compared with predictions provided by the different available codes, manuals, and design guidelines. The test results indicated that the relatively low modulus of elasticity of FRP bars resulted in reduced shear strength compared to the shear strength of the control beams reinforced with steel. In addition, the current ACI 440.1R design method provided very conservative predictions, particularly for beams reinforced with glass FRP bars. Based on the obtained experimental results, a proposed modification to the current ACI 440.1R design equation is presented and verified against test results of other researchers.

Keywords: beams; fibers; polymers; shear; strength.

(ProQuest-CSA LLC: ... denotes formulae omitted.)

INTRODUCTION

The long-term durability of reinforced concrete structures has become a major concern in the construction industry. One of the main factors reducing durability and service life of reinforced concrete structures is the corrosion of steel reinforcement. Many steel-reinforced concrete structures exposed to deicing salts and marine environments require extensive and expensive maintenance. Recently, the use of fiber-reinforced polymer (FRP) as an alternative reinforcing material in reinforced concrete structures has emerged as an innovative solution to the corrosion problem. In addition to the noncorrosive nature of FRP materials, they also have a high strength-to-weight ratio that makes them attractive as reinforcement for concrete structures.

Extensive research programs have been conducted to investigate the flexural behavior of concrete members reinforced with FRP reinforcement. On the other hand, the shear behavior of concrete members reinforced longitudinally with FRP bars has not yet been fully explored. Due to the difference in mechanical properties between FRP and steel reinforcement, particularly the modulus of elasticity, the shear strength of concrete members reinforced longitudinally with FRP bars may differ from that of members reinforced with steel. In previous flexure tests conducted by El-Salakawy and Benmokrane,1 Deitz et al.,2 and Michaluk et al.,3 shear failures were reported for members reinforced longitudinally with FRP bars.

The applied shear stresses in a cracked reinforced concrete member without transverse reinforcement are resisted by various shear mechanisms. Joint ACI-ASCE Committee 445, Shear and Torsion,4 identified the following five mechanisms of shear transfer: 1) shear stresses in uncracked concrete; 2) interlocking action of aggregate; 3) dowel action of the longitudinal reinforcing bars; 4) arch action; and 5) residual tensile stresses transmitted directly across the cracks. Aggregate interlock results from the resistance to relative slip between two rough interlocking surfaces of the crack, much like frictional resistance. As long as the crack is not too wide, this action can be significant.5 Dowel forces generated by longitudinal bars crossing the crack partially resist shearing displacements along the crack. Arching action occurs in deep members or in members in which the shear span-to-depth ratio (a/d) is less than 2.5. This is not a shear transfer mechanism in the sense that it does not transmit a tangential force to a nearby parallel plane, but permits the transfer of a vertical concentrated force to a reaction, thereby reducing the contribution of the other types of shear transfer. The basic explanation of residual tensile stresses is that when concrete first cracks, a clean break does not occur. Small pieces of concrete bridge the crack and continue to transmit tensile force up to crack widths4 in the range of 0.05 to 0.15 mm.

Due to the relatively low modulus of elasticity of FRP composite material, concrete members reinforced with FRP bars will develop wider and deeper cracks than members reinforced with steel. Deeper cracks decrease the contribution to shear strength from the uncracked concrete due to the lower depth of concrete in compression. Wider cracks in turn decrease the contributions from aggregate interlock and residual tensile stresses. Additionally, due to the relatively small transverse strength of FRP bars and relatively wider cracks, the contribution of dowel action can be very small compared to that of steel. Finally, the overall shear capacity of concrete members reinforced with FRP bars as flexural reinforcement is lower than that of concrete members reinforced with steel bars.