Experimental Study On Mechanical Properties Of Steel-BFRP Bars Reinforced Concrete Composite Beams After High Temperature

Basalt Fiber Reinforced Polymer bar(BFRP bar)is a new type of green polymer material,which has the advantages of high specific strength,corrosion resistance,and insulation,and has a wide application prospect in engineering.The application of BFRP bars in steel-concrete composite beams can solve the durability problem of bridge decks caused by steel corrosion.However,bridge fire will lead to the deterioration of BFRP bars,affect the durability and bearing capacity of the overall structure,and threaten the safety of the structure.In this paper,the mechanical properties of steel-BFRP bars reinforced concrete composite beams after high temperatures are studied by means of experimental research and finite element simulation.The main research contents and conclusions are as follows:(1)According to the heating conditions of the test beam,the high temperature and mechanical properties tests of the materials were designed,and the damage mechanism of each material under separate temperatures was studied.The results show that:(1)The high temperature within 600℃ has little effect on the properties of steel bars and steel.(2)With the increase of temperature,the strength of BFRP bars decreases by 84.1% after 400℃.(3)At600℃,the strength of concrete decreases by 33.1% at most.The temperature deterioration mainly causes the increase of internal stress and the dehydration of hydrate.(2)The temperature field of steel-BFRP bars reinforced concrete composite beams was studied by the combination of experimental measurement and finite element simulation,and the temperature distribution law of composite beams at high temperature was clarified.The results show that:(1)The temperature of concrete slab is slower than that of the steel beam,and the cooling is slower,showing thermal hysteresis;(2)During the heating process,the lower concrete temperature will lead to a decrease in the temperature of the upper flange of the steel beam.(3)The thermal conductivity of BFRP bars is low,which affects the temperature rise of the surrounding concrete.(3)Three-point loading tests were carried out on two kinds of steel-concrete composite beams with different reinforcements after high temperatures to clarify the failure mode of composite beams and explore the influence of reinforcements on the bearing capacity of steel-concrete composite beams.The results show that:(1)The failure modes of the two composite beams are similar,which are divided into elastic stage,elastoplastic stage and failure stage.(2)The stiffness of steel-BFRP bars reinforced concrete composite beams is small.(3)At 600℃,the residual bearing capacity of ordinary reinforced steel-concrete composite beams is 14.7% higher than that of steel-BFRP bars reinforced concrete composite beams.(4)The catastrophic mechanism of BFRP bars on steel-concrete composite beams is studied,and the reduction coefficient of the concrete slab is proposed.Based on the strength reduction formula of each material after high temperature and the influence coefficient of BFRP bars,the prediction formula of residual bearing capacity of steel-BFRP bars reinforced concrete composite beams after high temperature is derived by referring to national codes.The results show that the calculated values of the formula are in good agreement with the experimental values.(5)According to the actual situation of the test,the ABAQUS finite element model of steel-BFRP bars reinforced concrete composite beams is established,and the bearing capacity,temperature field and thermal stress field are simulated and analyzed.The results show that:(1)The load-displacement curve of the finite element model is close to the measured value,and the bearing capacity of the steel-BFRP bars reinforced concrete composite beams can be calculated by this model.(2)The temperature field and stress field results of the thermal-stress coupling model are consistent with the experimental phenomena,and the temperature stress of the composite beam at high temperature can be studied by this model.(3)Through the analysis of fire surface parameters,it is found that the fire under the bridge is the most unfavorable condition.(4)The larger the diameter of the BFRP bar,the greater the stress of the concrete slab and the smaller the stress of the steel beam.