Bond-Slip Behavior Between Ribbed Cfrp Bars And Ultra-High-Performance Concrete

Carbon fiber-reinforced polymer（CFRP）bars and ultra-high-performance concrete（UHPC）feature high strength,high elastic modulus,and excellent durability,which can be combined and applied to high-load or corrosive environments.The bond behavior,which is affected by many complicated variables,are essential mechanical properties of the structure.There are few studies on the bond properties between CFRP bars and UHPC,and the research is unsystematic.In this study,the evolution of the bond behavior between CFRP bars and UHPC under different factors was investigated by using bond tests,and the bond failure mechanism was analyzed.The distribution and transfer mechanisms of anchorage variables were further clarified.A bond strength and bod-slip models that can be used for the bond between CFRP bars and UHPC was proposed.The main work and research results are as follows:（1）The interfacial bond mechanism between CFRP bars and UHPC was analyzed,and the effects of CFRP bars diameter,anchorage length and UHPC cover thickness on the bond behavior were clarified based on 42 CFRP bars-UHPC pull-out specimens.The critical cover thickness of CFRP bars-UHPC was obtained.The results showed that the failure modes included pull-out,splitting,and splitting-pull-out failure.Interfacial failure was caused by wear to the surface of the CFRP bars or by delamination between the fibers and the resin matrix.The bond-slip curve consisted of four stages:the micro-slip stage,the slip stage,the descending stage and the residual stage.The bond strength increased with increasing concrete lug ratio（CLR）and relative rib area(R_r).As the diameter increased from 8mm to 12 mm,the bond strength increased from17.66MPa to 36.41 MPa.With an increase in anchorage length,the ultimate external load of the specimens gradually increased,and a slight decrease in bond strength occurred（except for the splitting specimen）.As the cover thickness increased from one to three times the diameter,the bond strength increased from21.43 MPa to 34.06 MPa,the critical cover thickness was about three times the bar diameter.（2）Through the pull-out test conducted on 12 specimens with strain gauges attached to the CFRP bar surface,the distribution of bond stress and relative slip along the anchorage length was revealed,and the position function was established to modify bond-slip curves.The bond stress and relative slip showed a non-uniform distribution over the anchorage length.The peak point of the local bond stress occurred near the loaded end and the relative slip decreased non-linearly from the loaded end to the free end.The anchorage length of CFRP bars broken in UHPC was 20～30 times the bar diameter.（3）The bond strength of CFRP bar-UHPC specimens was calculated using the strength prediction models proposed by codes or previous studies.A bond strength prediction model for ribbed CFRP bars embedded in UHPC was proposed,in which the geometric CFRP bar surface ratios（CLR and R_r）,diameter,anchorage length and cover thickness were incorporated.The predicted values calculated by the model were in good agreement with the experimental data.（4）A comparative analysis was carried out using Malvar,MBPE,CMR and continuous curve model curves with slip curves of CFRP bar-UHPC specimens.The results showed that the Malvar model was suitable for predicting the ascending stage,and the descending stage could be predicted using a non-linear equation.The Malvar model and the non-linear equation matched the test curves well in this study.