By Shawn P. Gross¹, Joseph Robert Yost¹,   David W. Dinehart¹, Erik C. Svensen², and Ning Liu²

Fiber reinforced polymer materials (FRP) have much potential as longitudinal reinforcement in slab type concrete structures susceptible to reinforcement corrosion and stressed primarily in bending. Examples of such structural components include bridge decks, footings, floor slabs and wall type structures (abutments, stems and wing walls). In these members, flexural strength is provided by the longitudinal reinforcement and shear strength is provided by the concrete only. As such, where shear reinforcement is absent it is critical that an accurate quantification of concrete shear strength (V_c) be known.

This paper evaluates the limit state shear strength of intermediate length (2.5<a/d<6) simply supported concrete beams subjected to four point monotonic loading and reinforced with deformed glass fiber reinforced plastic (GFRP) reinforcement bars. The main parameters of the study are concrete strength (f'_c) and the amount of longitudinal GFRP reinforcement (r_f). Normal strength and high strength concrete were investigated. With respect to a balanced strain condition (r_b), eleven different overreinforced FRP designs (r_f >r_b) were tested with three identical beams per design. Two underreinforced steel designs with 3 samples per design were tested as a control. No shear or compression reinforcement was used and the shear span to depth ratio (a/d) for all samples was four. Twenty-one of the thirty-nine sample inventory used normal strength concrete (f'_c<5.5 ksi) with the remaining eighteen beams using high strength concrete (f'_c>10 ksi).

All samples (GFRP and Steel) were proportioned so that shear failure would preclude flexural failure. This objective was successful and all samples failed in shear. Laboratory recorded shear strengths at failure are compared with theoretical strengths calculated using traditional steel-reinforced concrete procedures and recently published expressions for beams reinforced with FRP. The study concludes that the large deflections and small compression block depths typical of GFRP reinforced flexural members reduce shear strength relative to sections reinforced with steel on an equal area or equal strength basis. A simplified lower-bound empirical form of shear strength for normal and high strength concrete beams reinforced with GFRP is proposed.

This paper evaluates the shear strength (V_c) of intermediate length (2.5<a/d<6) simply supported concrete beams subjected to four point monotonic loading and reinforced with deformed glass fiber reinforced plastic (GFRP) reinforcement bars. The main parameter of the study is the amount of longitudinal FRP reinforcement (r). Twenty-one beams were tested with respect to a balanced strain condition (r_b). Six different overreinforced FRP designs (r >r_b) were tested with three replicate beams per design, and three samples of a single underreinforced steel design were tested as the control. All samples (FRP and Steel) were proportioned so that shear failure would preclude flexural failure, and all samples failed as a result of diagonal-tension shear. Measured shear strengths at failure are compared with theoretical predictions calculated according to traditional steel-reinforced concrete procedures and recently published expressions for beams reinforced with FRP. Recommendations are made regarding the adequacy of shear strength prediction equations for FRP-reinforced members. The study concludes that shear capacity is significantly overestimated by the ACI 318-99 expression for V_c as a result of the large crack widths, small compression block, and reduced dowel action in FRP-reinforced flexural members. In general, shear strength was found to be independent of the amount of longitudinal FRP reinforcement.

1 Assistant Professor, Department of Civil and Environmental Engineering, Villanova University, Villanova, PA 19085.

² Graduate Assistant, Department of Civil and Environmental Engineering, Villanova University, Villanova, PA 19085.