Study On The Bond Performance Of BFRP-concrete Interface Under The Effects Of Freeze-thaw And The Fatigue Load Coupling

China is a vast region,in China’s high latitude areas such as Qinghai and northeast,the climate is very special.The average winter temperature in such areas is about-20℃,while the summer temperature is generally around 20℃.The annual temperature variation range is large,resulting in many concrete buildings are damaged by freeze-thaw cycle,and many buildings are in disrepair,the damage of freeze-thaw is more obvious.For viaduct,production workshop and other buildings under long-term fatigue load,reinforcement is more important.Among them,basalt fiber reinforced composites(BFRP),with its high tensile strength,acid and alkali corrosion resistance,not easy to oxidation and other advantages,has a wide development prospect in the field of FRP reinforcement technology.In this paper,the static load performance of BFRP-concrete interface under freezing-thawing condition was analyzed based on the double-shear test,and then the bond performance under fatigue load was studied in depth.The main research contents are as follows:(1)Mechanical tests were carried out on BFRP sheet,epoxy resin and concrete cube specimens under freezing-thawing cycle,and the tensile strength,elastic modulus and elongation of BFRP sheet and epoxy resin specimens under freezing-thawing cycle were obtained,as well as the weight loss rate and compressive strength of concrete cube specimens.It provides theoretical support for double shear test.The experimental results show that the stress of BFRP sheet is uniform during the tensile process,and it shows good cooperative performance of fiber and impregnation adhesive.The failure mode and mechanical properties of the freezing-thawing specimens do not change greatly,while the elongation of epoxy resin specimens decreases after freezing-thawing.For concrete specimens,the freeze-thaw cycle reduces the compressive strength of concrete and increases the weight loss rate.(2)Under the premise of four variables,freezing-thawing cycle and static load double shear tests were carried out,and the strain values of each load were measured by strain acquisition instrument.The ultimate load is recorded by load sensor.The relative slip of concrete blocks is recorded by displacement meter.The experimental data show that the change of concrete strength and bond length has great influence on ultimate load and maximum strain,but the change of adhesive thickness has no obvious influence.With the increase of freezing-thawing times,the ultimate load and strain values of all specimens decrease.The shear stress was calculated and analyzed by the strain values under various loads.It was found that the shear stress distribution in the front part of the boundary was better than that in the back part,and the overall shear stress loss of the interface was serious after 200 times of freeze-thaw.(3)In the double shear test under freeze-thaw cycle and fatigue load,70% of the ultimate load under static load was taken as the upper limit of fatigue cycle load,and the stress ratio was 0.2.The cycle times and strain values under different fatigue times of each specimen under four variables were recorded and analyzed.The results show that with the increase of adhesive thickness,concrete strength and bonding length,the fatigue cycles and the maximum strain increase,but the fatigue cycles decrease after freezing-thawing.The bond-slip hysteresis curve was drawn by shear stress and slip value and compared with the static load model.The dynamic bond-slip curve was established by summarizing the curve law.The stiffness degradation curve was drawn and the S-N formula and fitting curve were given by s-N curve method.