Experimental And Analytical Study Of Seismic Performance On Recycled Aggregate Concrete Members Under Cyclic Torsion

With the rapid development of economy in our country and the need of city construction, many existing structures should be removed. In order to relieve the pressure of environmental protection, it is urgent to process these waste concrete in reasonable way. There is one of the most effective measures that treating the waste concrete with the following processes:recycling, crushing, screening and blending etc. These processes can produce recycled aggregate. Using recycled aggregate to replace natural sand and gravel in whole or in part can produce a new concrete, i.e. Recycled Aggregate Concrete (RAC). It is not only solving the problem of environmental protection, but also fitting the need of building a conserving and sustainable society.Existing research are mainly concentrated on the members in tension, pressure, bending and shearing, which are made of recycled aggregate concrete. Research on mechanical behavior and seismic performance of RAC members under torsion is rarely seen. In order to provide experimental and theoretic basis for practical engineering application of RAC, experiments were carried out for better understanding the seismic performance of RAC members under torsion loading or flexure-shear-torsion complex loadings.Based on the pseudo-static experimental study of 2 RAC members and 2 natural aggregate concrete members (NAC), the mechanical properties and destruction process were understand. Some seismic performance indicators such as hysteresis curve, displacement ductility, energy dissipation, degradation of strength and stiffness and etc. The failure process of RAC members can be divided into four stages:elastic stage, cracking stage, yielding stage and failure stage, which is similar to NAC members. The cracking load of RAC members is 17% lower than that of RC members and carring capacity of them are similar. The law of stiffness degradation of RAC members and RC members are basically consistent. The coefficient of displacement ductility of RAC members is 12.3% more than that of RC members and the equivalent viscous damping coefficient is 27.7% more than that of RC members. These experiment results showed that RAC members had good seismic performance.According to the regulation of current specification, replacement of Recycled Coarse Aggregate by mass should be limited between 30% to 50% based on the considerations of durability. Studies showed that fly ash could enhance the density of interfacial transition zone in RAC and improve the bonding effect. Using appropriate fly ash to substitute cement will obtain better durability of RAC, therefor the replacement of Recycled Coarse Aggregate by mass can be increased. This method will affecte the mechanical perfomence of RAC members because of the change of cement amount.On this basis, a study is conducted on the seismic behavior of recycled concrete members under reversed load comprised of bending moment, shearing force and torque. There are 6 members in total,3 of them are modified on the way of replacing 15% of cement by fly ash. The results showed that the failure process of modified RAC members is similar to ordinary RAC members. There are little deference in hysteresis curve, characteristic load, degradation of stiffness and etc. Whereas, cracking load, displacement ductility and energy dissipation of modified RAC members are slightly higher than ordinary RAC members. Consiquently, introducing fly ash into RAC cannot change the seismic performance of it, as well as improve the durability of it.On the basis of these experiments, hysteretic curve mode is developed by analyzing skeleton curves and hysteretic loops. Degradation of stiffness and shearing slide are considered in this mode. It can be used to complete numerical simulation on members under reversed cyclic complex load, such as bending moment, shearing force and torque existed at the same time.Some formulas are established in this paper for calculating cracking load. Under torque and shearing force, shear stress should be calculated according to elastic theory, whereas torsion shear stress according to plastic theory. Stress redistribution produced by softening of concrete are considered. Based on space truss model, this paper unifies reversed torsion and combined torsion, material properties of RAC is considered in it. The unification theory describes the load behavior of torsion members very well. Based the unification theory a torsion bearing capacity equation of combined torsion members is obtained. Material characteristics and loading mode are considered in the equation.Degradation and recovery of stiffness in concrete, which is generated by plastic tensile strain and compressive strain under reversed load, can be simulated by concrete damaged plastic model (CDP) of ABAQUS. Nonlinear static analysis is executed by ABAQUS for the quasi-static tests of both RAC members and RC members. The simulation results of the tests showed that ABAQUS is appropriate for RAC structures’ nonlinear analysis. The calculation results fit the test results well.