Bond-slip And Flexural Behaviors Of CFRP Bars Embedded In UHPC At Elevated Ambient Temperatures

The combination structure of carbon fiber reinforced polymer(CFRP)bars and ultra-high-performance concrete(UHPC)has excellent mechanical properties and good application prospects in civil engineering.However,the significant decrease in mechanical properties of CFRP bars under high ambient temperature is a concern.In hot weather with direct sunlight,the surface temperature of buildings can reach 70 ℃ or even higher.Therefore,when using CFRP-reinforced UHPC structures,the impact of high ambient temperature on their structural performance should be considered.In addition,the bond failure mode and bond-slip law between CFRP bars and UHPC are essential theoretical foundations for the design of CFRP bars UHPC structures.Based on the above background,this article investigates the bonding behavior between CFRP bars and UHPC under high ambient temperatures and the bending performance of CFRP bars UHPC beams.The main work carried out is as follows:(1)We conducted bond tests on CFRP bars with two diameters(10mm and12mm)and UHPC cubes at different environmental temperatures(20 ℃,40 ℃,60 ℃,and 80 ℃).The key parameters,such as bond strength and bond-slip relationship of each pull-out specimen,were obtained from the experiment to explore the bond stress and failure mode between CFRP bars and UHPC.The formation and evolution process of bond force between CFRP bars and UHPC were analyzed,and the effects of CFRP bar diameter and environmental temperature on bond strength were clarified.The results show that the failure mode of all pull-out specimens is the pull-out failure of CFRP bars,with the damage of 10 mm diameter CFRP bar bonded specimens occurring on the surface of the bars,while the failure mode of 12 mm diameter CFRP bar bonded specimens is a core pulling failure,which means relative slip occurs between the surface and core of CFRP bars.In addition,when the ambient temperature increased from 20 ℃ to 80 ℃,the bonding strength between 10 mm and 12 mm diameter CFRP bars and UHPC decreased by 31.9% and 55.7%,respectively.(2)A bond-slip model was established for CFRP bars and UHPC,considering the influence of environmental temperature.Based on the existing bond-slip model between FRP bars and concrete,a bond-slip model including environmental temperature parameters is proposed,And the accuracy between this model and commonly used bond-slip models was compared and analyzed.The results indicate that the bond-slip model proposed in this paper for CFRP bars and UHPC agrees more with the experimental curve.In addition,this article also establishes a bond strength prediction model between CFRP bars and UHPC.Based on the prediction formula for the bond strength between FRP bars and concrete in existing specifications,a bond strength prediction model considering environmental temperature and the surface form of CFRP bars is proposed.The results show that the bond strength prediction model proposed in this article can effectively predict the bond strength between CFRP bars and UHPC under different ambient temperatures,and the prediction error is within 10%.(3)The influence of high environmental temperature and reinforcement ratio on the bending performance of CFRP-reinforced UHPC beams was numerically analyzed.A finite element validation model for FRP-reinforced UHPC beams was established based on Abaqus software,and the influence of high environmental temperature,CFRP bars diameter,and bars ratio on the bending performance of CFRP-reinforced UHPC beams was studied using this model,considering the bond-slip behavior between CFRP bars and UHPC.The results indicate that the bending stiffness and ultimate load of CFRP-reinforced UHPC beams decrease with the increase of environmental temperature,and all increase with the increase of CFRP bars and bars ratio.