Experimental And Analytical Study On Seismic Behavior Of Modified Recycled Aggregate Concrete Frame Joints

As a new type of green building material recycled concrete can not only solve the problem of the waste building occupation the landfill area and the environmental pollution,but also can reduce the damage to the natural ecological environment caused by the mountain quarrying.Which is accordance with national strategy of conserving resources,protecting the environment and sustainable development.Because of the original cement mortar attached to the interface of the recycled aggregate,the aggregate interface has high porosity and strong water absorption,and its durability is worse than that of natural aggregate concrete。At present,it is mainly used as a solidified material for highway construction and non-structural components of building structures.In order to use the recycled aggregate concrete as the load-bearing structural components in building structures,Adding a proper amount of fly ash into the recycled concrete to permeate and fill the pores and microcracks of the recycled aggregate,which can enhance the compactness of the interface,strengthen the bonding interface and improve the durability of the recycled concrete.Replacing cement with equivalent fly ash will affect the strength of recycled aggregate concrete,therefore further study on the mechanical properties of structural members is needed.In this paper,the beam-column joints of a frame structure are selected as the research objects.Seven beam-column joint specimens are manufactured in three different types,with 100%replacement rate of the recycled aggregate.Some of the specimens were modified by 15%fly ash instead of cement.Based on the quasi-static test,the damage behavior,failure mode,bearing capacity,hysteresis performance,stiffness degradation,displacement ductility and energy dissipation capacity of the specimens were studied.The shear deformation,stirrups strain and bond stress degradation of longitudinal reinforcement in core zones of the joints were also discussed.The main contents and achievements about seismic performance of joints are as follows,(1)The results show that the failure process of interior-joints and exterior-joints of recycled aggregate concrete under cyclic loading underwent the initial cracking stage.the fully cracking stage,the ultimate stage and the failure stage.In the final failure,the concrete in the joint core area was crisp and the brittleness was obvious.The recycled aggregate concrete knee-joints were subjected to" open moment "and"closed moment"under cyclic reverse loading.This stress state of the joint was similar to that of a bending folded beam.Under the"closed moment",the concrete protective layer on the outside corner of the knee-joint is the first to peel off and the top surface of the joint cracks,which is degraded by the tension bars and loses the bearing capacity.The failure mode,displacement ductility and energy dissipation capacity of modified recycled aggregate concrete joints with fly ash have little difference from those of unmodified recycled aggregate concrete joints.(2)The load-displacement hysteric curves of the interior and exterior joints were plump.The bond slip degradation between the steel bars and concrete at the later stage caused "pinching effect"and the shape of the hysteretic curve tends to be bow-shaped.Under the influence of "closed moment",the bending stiffness of the knee-joints is degraded,and the hysteretic curve is pinched,and the area of the hysteric loop was smaller.In the "open moment" function,the the the reinforcement of knee-joints inside corner fully tension,joint core concrete is still under pressure,the load-displacement hysteretic curves tend to be plump.The load-displacement hysteretic curve of the knee-joints were asymmetry under positive and negative loading.(3)The ductility of the interior-joint and exterior-joint was better than the knee joint.The ductility factor of the modified recycled aggregate concrete joints was not obvious different from that of the unmodified ones.The energy dissipation capacity of the exterior-joint specimens were similar to that of the interior-joint,and the energy dissipation capacity of the modified recycled aggregate concrete exterior-joint specimens were slightly weaker than that of the unmodified ones.The equivalent viscous damping coefficient of the knee-joints in each loading stage was smaller than that of the interior-joints and exterior-joints,and its overall energy dissipation capacity was relatively poor.(4)The longitudinal reinforcement throughout the recycled aggregate concrete joints had relatively stable bond stress under full crack stage and ultimate stage,and the relative bond strength was between 0.213 and 0.246.The bond stress declined sharply after the joints entered the failure stage.Take the full crack stage as the basis for calculating the bearing capacity of the joint,the bond strength was able to meet the requirements of stress transfer and deformation coordination between steel bar and concrete.Under the same conditions,the greater the diameter of steel bar,the more serious the loss of bond strength.Adding fly ash improved the bonding properties between the steel bar and concrete,In the modified recycled concrete longitudinal steel bar bond strength increased by 8.5%comparing with the unmodified recycled aggregate concrete at the full crack stage.(5)The shear angle and the load-maximum shear angle curve of the core area under the cyclic loading at different stages were obtained,by two displacement gauges were installed diagonally across the core area of the joint.The strain distribution of the horizontal stirrups in the core area was measured,and the hysteresis curve of the load-strain of the stirrups under each cyclic loading were obtained.The stress state at different stages and the contribution of the stirrups to the shear strength in the joint core region were investigated.(6)Based on the experiment,the formula for calculating the shear force in the joint core area and the formula for calculating the shear capacity of the beam-column joints are revised.The design method of shear resistance for recycled concrete joints was proposed.The shear bearing capacity of the recycled concrete joints were calculated by the general model,softened strut and tie model,and the modified formula proposed in this paper and compared with the test results separately.The results show that the shear bearing capacity of the joints calculated by the modified formula agrees well with the calculated values,and the dispersion is smaller.(7)On the basis of the experimental study,the Concrete Damaged Plasticity(CDP)model was used to simulate the elastic stiffness degradation and recovery of the concrete material caused by tensile or compressive plastic strains under the low cyclic loading.The calculation of recycled aggregate concrete joints under low cyclic loading is carried out.The results show that ABAQUS software can well simulate the stress state of recycled aggregate concrete joint under low cyclic loading,and the finite element simulation results are in good accord with the experimental results.(8)Using the finite element software OpenSees to simulate the quasi-static test of interior-joints and exterior-joints of recycled aggregate concrete.Based on the beam-column fiber model and beam-column joint macroscopic model,OpenSees can better simulate the nonlinear behavior of the recycled aggregate concrete joints.The simulated hysteresis curves agree well with the experimental results,which better reflects the stiffness degradation and bearing capacity degeneration rule of the specimens.