Bonding behavior of GFRP bars and macro fiber reinforced concrete (MFRC): Tests and analytical modeling

A novel environmentally friendly concrete has been recently developed in the authors’ group by incorporating macro fibers processed from waste glass fiber reinforced polymer (GFRP) composites into ordinary concrete. Owing to their superior anti-corrosion performance, the combined utilization of FRP composites and MFRC in structural construction is seen as a tremendous demand for coastal and marine construction as well as the potential for exploitation of marine resources. To ensure the compatibility between FRP bars and MFRC, superior bond performance is a prerequisite for the design and application of FRP reinforced MFRC structures. This paper has therefore conducted for studying the bond behaviors between GFRP/steel bars (specifically refers to GFRP bar or steel bar, rather than a hybrid of steel and GFRP bars) and MFRC through 72 pull-out tests with GFRP/steel bars embedded in macro fiber concrete. The effects of GFRP/steel bar diameters (8 mm, 12 mm, and 16 mm), bar embedded lengths (1d, 3d, and 5d), concrete strength (C30 and C50), and macro fiber contents (0.0 vol%, 0.5 vol%, 1.0 vol%, and 1.5 vol%) on the failure patterns, bond mechanism, and bond-slip curves of specimens were systematically analyzed. The test results indicate that the bond stress-slip curves of GFRP bars exhibit a sinusoidal decay after reaching the peak value, in contrast to the direct decline observed in conventional steel bars. With the increased rib height and decreased bar diameter, the bond strength between MFRC and FRP bars under pull-out conditions is enhanced accordingly. Increasing bonding lengths leads to a gradual decrease in the bond strengths, while the increase in concrete strengths and macro fiber contents generally improves the bond strengths. Within a reasonable range, the improvement of bond strengths will become more pronounced when the volume contents of macro fibers exceed 1.5 vol%. Overall, the influential parameters have a more significant impact on bond strengths between steel bars and macro fiber concrete compared to GFRP bars. Subsequently, this study also evaluates the accuracy and applicability of existing analytical models in predicting bond strengths between GFRP/steel bars and ordinary concrete, and then proposes corresponding modified prediction models for quantifying the bond-slip behavior between GFRP/steel bars and MFRC, resulting in more accurate predictions based on test results.