Bond-dependent coefficient of glass- and carbon-FRP bars in normal- and high-strength concretes

• Investigating the bond-dependent coefficient, kb, of FRP bars in normal and high strength concretes.
• Evaluating the dependency of kb values on FRP bar type, diameter, and concrete type, and strength.
• Evaluating the determined kb values against the current recommendations of FRP design codes and guides.

The design of concrete members reinforced with fiber-reinforced-polymer (FRP) bars is typically governed by serviceability state rather than ultimate state. This necessitates verifying the crack width in FRP-reinforced concrete (FRP-RC) members at service load. Recent developments in the FRP industry led to the introduction of FRP bars with different surface configurations and mechanical properties, which are expected to affect their bond performance. In the absence of test data, the design codes and guides, however, recommend bond-dependent coefficient (kb) values considering the surface configurations. Thus, this study aims at investigating the kb values and verifying the dependency of the kb values on FRP bar type (glass [GFRP] and carbon [CFRP]), diameter, and concrete type, and strength limited to the first critical flexural-crack width. The investigation included 16 beams measuring 4250 mm long × 200 mm wide × 400 mm deep. The beams were reinforced with sand-coated GFRP bars, helically-grooved GFRP bars, and sand-coated CFRP bars and were fabricated with normal- and high-strength concretes (NSC and HSC). The measured first critical-crack widths and measured strains were used to assess the current kb values recommended in FRP design codes and guides. The findings did not support using the same kb value for FRP bars of different types (carbon and glass) with similar surface configurations.