| The composite FRP rebars research at Oregon State University was initiated in 1993 principally to develop a non-metallic hollow reinforcement. It was recognized that the tensile properties of such reinforcement are unquestionably superior to steel, but its performance in concrete could be problematic. The bond between FRP rebars and concrete was identified as a critical area of concern.; The purpose of this study is (i) to analyze a variety of FRP and steel reinforcing units; (ii) to advance the knowledge of bond mechanism, failure modes, and parameters influencing the bond strength; (iii) to compare composite rebars to conventional steel and to assess their applicability as reinforcing members. Commercially available FRP rebars were investigated. Particular emphasis was given to a hollow glass FRP rod designed at Oregon State University.; Several parameters were investigated, including: failure mode, concrete compressive strength, rebar diameter and circumference/cross section ratio, embedment length, concrete cover, and microstructure of the composite rebars.; It was recognized that the ASTM C234-90 pull-out standard is test of concrete strength. Therefore, a modified pull-out test was developed for evaluating the bond strength behavior. A newly developed European bond test procedure was compared with locally modified version of the pull-out method. The new procedure was used for the first time in the United States.; The study demonstrated a phenomenon, not reported in the published research at this time, defined as a size effect. The size effects result in lower bond strength with increasing area of the interface between FRP bars and concrete.; The next phase of the research was dedicated to the hollow glass FRP rebar. The goal was to compare its bond properties to conventional steel and solid FRP bars. The study led to two new phenomena not described in the literature previously. Results showed that the concrete compressive strength does not significantly affect the bond strength. This observation was in contradiction with the bond strength theory which considers the concrete strength as a major variable. The second observation revealed significant difference in bond performance between bars with different microstructures. It is recommended that microstructure of the FRP bars be considered as a variable when investigating bond strength. |
