Experimental Study On The Bond Behavior Between Carbon Fiber Reinforced Polymer Bars And Ultra-High Performance Concrete

Fiber reinforced polymer(FRP)bars have the characteristics of high tensile strength,corrosion resistance,and light weight.Using them instead of steel bars can fundamentally solve the problem of steel corrosion.Ultra-high performance concrete(UHPC)has excellent strength,toughness and durability.The new structure formed by the combination of the two materials has the advantages of higher bearing capacity,lower self-weight and better corrosion resistance,and has a broad application prospect.The premise of FRP bars and UHPC working together is that there is reliable bond and anchorage between them.The study of bond-slip performance between FRP bars and UHPC is of great significance for theoretical analysis and engineering practice.Therefore,in this paper,the bond behaviors between CFRP bars and UHPC was studied by standard pull-out test and beam test,the influence laws of various factors were deeply analyzed,the calculation formula of ultimate bond strength considering the surface characteristics of CFRP reinforcement was proposed,the bond-slip constitutive model was established,and the calculation method of development length was given.The main research contents and conclusions are as follows:(1)The effects of steel fiber volume fraction,CFRP bar diameter,bond length,concrete cover thickness and other factors on the bond behaviors between CFRP bars and UHPC were studied through pull-out tests and beam tests.When concrete cover thickness was increased from 10 mm to 20 mm and 30 mm,the ultimate bond strength increased by 53.9% and 62%,respectively.It is recommended that the cover thickness of CFRP bars in UHPC beams be taken as 20mm(2d: 2 times the diameter of CFRP bars).The bond failure modes and mechanisms under the two test methods were analyzed,the characteristics of each stage of the bond-slip curve were discussed,and the bond properties of CFRP bars and UHPC under different test methods were compared.(2)In the pull-out test,the core-pulling failure of the CFRP bar occurred,and the residual bond strength was extremely low.The failure of the beam specimen showed a certain ductility,and the residual bond strength was about 40%-60% of the ultimate bond strength.The bond-slip curve between CFRP bars and UHPC can be divided into three stages: ascending section,descending section and residual section.For 12 mm diameter threaded CFRP bars,the ultimate bond strength obtained from the pull test was approximately 1.44 times that of the beam test.(3)Based on the comparative analysis of the existing FRP bars and concrete bondslip constitutive models at home and abroad,a bond-slip constitutive model that was in good agreement with the experimental data was proposed.The ultimate bond strength and development length calculation formulae were established.The suggested value of development length of CFRP bars in UHPC was given.(4)Preliminary exploration of the anchoring measures of CFRP bars was carried out.It was found that the bonding anchoring method and the grouting material using expansive cement mortar could achieve a better anchoring effect.When the cementsand ratio was 1.2,the 10 mm CFRP tendons appeared tensile failure.