Mechanical Properties Of Post-fire Recycled Aggregate Concrete Filled Steel Tubes Under The SHPB Impact Load

As a kind of green material,recycled aggregate concrete(RAC)has aroused wide concern about it.Promoting the application of RAC can solve the environmental problems caused by waste concrete and relieve the pressure of supplying of aggregate in construction industry,avoiding over exploitation of natural resources.The composite member-recycled aggregate concrete filled steel tubes(RACFST)can reduce adverse effects caused by the natural disadvantage of recycled aggregates.So,it has been regarded as a good form to promote the usage of RAC.Some scholars have devoted to investigate the properties of RACFST under quasi-static and under ambient temperature.Some important conclusions were obtained in these areas.However,the research about the residual mechanical properties of RACFST after exposure to high temperature and study about the dynamic performance of RACFST under impact load are rare to see.In this paper,devices such as press test machine and 100mm diameter split Hopkinson pressure bar(SHPB)in Hunan University were used to carry out series tests(loading rate ranging from quasi-static to dynamic impact)for RACFSTs which had suffered from high temperatures of 20?,200?,500? and 700? before and had different recycled coarse aggregate(RCA)contents(0%,50%,100%).In addition,finite element software-ABAQUS was used to conduct the further numerical simulation analysis.This paper was organized by mainly focusing on the following aspects:(1)Quasi-static uniaxial compressive properties of RACFSTs with different percentages of RCA and suffered from different temperatures before were experimentally researched.This paper explored and compared the influences of high temperature and RCA replacement ratio on some mechanical indexes of RACFST such as the failure mode,compressive stress-strain curve,compressive strength,axial compression stiffness and peak strain.Moreover,the performance evaluation formulas of RACFST after high temperature were obtained by regression analysis.(2)Dynamic impact properties of RACFSTs with different percentages of RCA and suffered from different temperatures before were experimentally studied by SHPB system.These works analyzed the impact of high temperature and RCA replacement ratio on several mechanical parameters of RACFST such as failure pattern,impact stress-strain curves,impact strength,impact modulus,remaining impact strength and dynamic increase factor(DIF).Comparative analyses were also conducted for post-fire performance degradation of RACFST under quasi static loading and dynamic loading.(3)Superposition method which was adopted by many researchers before to evaluate the DIF of RACFST was discussed in this paper.Research results in this experiment were used to verify the accuracy of the superposition method and force analysis in the impact process was listed to provide relevant explanation.In view of the theoretical defects of superposition method and unsatisfactory assessment results by it,a new calculate formula which considers the confinement coefficient was put forward.Calculation results by this formula agreed well with the experimental value.(4)The finite element software ABAQUS was used to simulate the impact tests of post-fire RACFST.The numerical results were in good agreement with the experimental results.Further parameter analyses based on it were carried out to explore the influence of concrete strength,steel strength and steel ratio on the mechanical behavior of RACFST.