Bond of FRP bars with different surface characteristics

Abstract

This paper presents an extensive experimental study of the effect of surface characteristics on the bond behaviour of Fibre Reinforced Polymer (FRP) bars in concrete. A wide range of commercially available FRP bars with different surface profiles (total of 13 different surface configurations) were involved. For comparison, ribbed steel bars were used. To investigate whether the effect of surface characteristics is in interaction with other factors, further parameters were included in the study such as concrete compressive strength (35 and 66 MPa), bar diameter (6 to 12 mm) and test type (pull-out [P-O] and direct tension [DT] pull-out). This resulted a total of 200 P-O and 38 DT tests. DT test method was developed by the authors based on a similar test setup from the literature. Surface finish of the investigated FRP bars include sand coated (SC), helically wrapped (HW), helically wrapped and sand coated (HWSC), indented (In) and ribbed (Rb). Most of the applied bars are Glass FRP (GFRP), however, due to the similar physical and mechanical properties Basalt FRP (BFRP) bars were used as well. The bond behaviour of the bars was evaluated by bond strength, bond stress-slip relationship, representative bond stresses and failure mode. It was found that the bond strength as well as bond stress-slip behaviour and failure mode vary considerably depending on surface characteristics. Furthermore, even within the same surface category the bond strength difference can be significant (e.g., SC surface with different sand fineness). The concrete strength influences the bond strength even if it is higher than the limit stated in literature (approximately 30 MPa). Furthermore, the bond strength results of all FRP bars were consistently higher than those of steel bars. The highest slip value to reach the bond strength was observed for HW FRP bars, while the lowest for SC bars. Some results reported in this paper considerably differ from those previously reported in other papers based on testing similar bars. The difference, in terms of bond strength, can be explained by the alteration of surface configuration, the improvements in material properties and fabrication processes. Finally, the test setup was found to affect the measurable bond strength.