Experimental Study On The Bond Behavior Between BFRP Bars And Recycled Aggregate Concrete

In order to reduce the damage and dependence of modern society on the natural environment,the use of recycled aggregate and fine aggregates from abandoned buildings or structures as new concrete materials conforms to the sustainable development concept of human society.Compared with natural concrete,recycled concrete made from recycled aggregate has higher porosity and more interface transition zone(ITZ).Although there are differences in the test results,most research results show that the mechanical properties of recycled aggregate concrete are not significantly reduced when the replacement rate is low,and the performance is significantly reduced when the replacement rate is high.Currently,the application of fiber-reinforced composite materials in civil engineering mainly includes two forms of chopped fiber yarn and FRP bars.The incorporation of the former has the effect of toughening and cracking resistance in concrete,especially when combined with recycled concrete,it can effectively make up for the defects of mechanical properties compared with ordinary concrete.The latter uses its corrosion resistance,light weight,high strength,and ionization resistance to replace steel bars,avoiding the durability of reinforced concrete caused by harsh environments such as corrosion.At present,domestic and foreign scholars have carried out various researches on fiber recycled concrete,but the research on its high temperature resistance and freeze-thaw cycle resistance is still immature.This paper discusses the basic mechanical properties of basalt-cellulose hybrid fiber recycled concrete after high temperatures and the bond performance of basalt-cellulose hybrid fiber recycled concrete and BFRP tendons at room temperature,high temperature and freeze-thaw cycles through central pull-out tests.The main research work is as follows:(1)Through the cube compressive strength test and splitting tensile strength test,the basic mechanical properties of basalt-cellulose hybrid fiber recycled concrete after high temperature are studied.The results show that the mixed fiber can reduce the surface peeling and quality loss of recycled concrete;the ductility of recycled concrete is improved by proper amount of hybrid fiber,and the failure form is ductile failure;when the fiber content of cellulose fiber is 0.15% and basalt fiber is 0.15%,the compressive resistance of recycled concrete is higher,the fiber content of cellulose fiber is 0.15%,and the content of basalt fiber is 0.10% or 0.15%,the recycled concrete is mixed The tensile strength of the splitting of the condensate is higher.(2)Through the center pull test,the bond performance of BFRP tendons and hybrid fiber recycled concrete after high temperature is studied.Choose the volume mixing amount of0.15% basalt fiber and 0.15% cellulose fiber to mix in the recycled concrete,and the temperature is 20°C,200°C,400°C,600°C and 800°C.The result shows: At different temperatures,the hybrid fiber can improve the bond strength between recycled concrete and BFRP bars.With the increase of temperature,the elastic modulus of bond between the two decreases gradually.The change of temperature will change the constitutive model of recycled concrete and BFRP reinforcement,and the incorporation of hybrid fiber can delay the transition temperature.(3)Through the central pull-out test,the bond performance of BFRP bars and hybrid fiber recycled concrete after freeze-thaw cycles is studied.Choose the volume mixing amount of 0.15% basalt fiber and 0.15% cellulose fiber to mix in the recycled concrete,and select 25,50,75,100,125,150 and 175 for the number of freeze-thaw cycles.The result shows: With the increase of freeze-thaw cycles,the failure mode of central pull-out specimen gradually changes from pull-out failure to splitting failure.Mixing cellulose fiber and basalt fiber can significantly improve the bond elastic modulus between BFRP reinforcement and recycled concrete,and effectively improve the peak bond strength in the bond slip curve between BFRP reinforcement and recycled concrete.(4)Through the pull-out test,the influence of the different volume content of single fiber and the different volume content of mixed fiber on the bonding performance of BFRP bar and basalt-cellulose hybrid fiber recycled concrete was explored at room temperature.The results showed: Compared with the recycled concrete specimens,the peak bond strength between the recycled concrete and BFRP bars can be improved by adding basalt fiber and mixed fiber,while the bond stress between the recycled concrete and BFRP bars can be reduced by adding basalt fiber.Low content of cellulose fiber or high content of hybrid fiber is unfavorable to the bond stress between recycled concrete and BFRP reinforcement.The optimal volume content of hybrid fiber is 0.15% basalt fiber and 0.20% cellulose fiber.