| Precast concrete structures have advantages of good component quality,rapid construction,and environment conservation,which have been a development tendency of Chinese building construction.Precast concrete structures are constructed by assembling the prefabricated components and pouring concrete in site.Some investigations demonstrated that precast concrete structures often suffered more serious damage than the cast-inplace concrete structures under the earthquakes.Therefore,a reliable connection between the prefabricated components should be adopted to enable the precast concrete structures achieving satisfactory integrity and seismic performance.ECC is an advanced cementitious composite,which is characteristic in strain hardening and multi-cracking behavior.ECC has advantages of high ductility,high energy dissipation,and high damage resistance.Using ECC in engineering structures can effectively improve seismic performance.This paper proposes to apply ECC in the key parts of precast concrete frames with cost-effectiveness in mind,as ECC costs approximately four times more than concrete.ECC is firstly applied in the key parts of prefabricated components to form RC/ECC composite components.The prefabricated components are then assembled with reliable connection methods.ECC can be poured in the post-casting areas to form RC/ECC composite frame.Thus the precast composite frame can achieve an improved seismic performance within a reasonable construction cost.This paper investigated the seismic performance of the precast frame structures in detail with the experiments and finite element analysis.The main research content of the paper is as follows:(1)The existing connection methods of precast frame structures were summarized.Corresponding connection methods according to three different separation schemes of precast frames were proposed.Besides,ECC was innovatively applied in the prefabricated components and post-pouring areas of the structures.The specific positions of ECC in the composite frames were clarified.(2)The seismic performance of precast RC column and RC/ECC composite column with grouted sleeve connection was studied.Cyclic load tests were conducted on a cast-in-place column,a precast RC column,and a precast RC/ECC composite column.The results showed that the skeleton curves of the specimens were similar.Serious concrete spalling was observed in the cast-in-place column and the precast RC column,while the precast RC/ECC composite column kept its integrity.The precast RC/ECC composite column exhibited superior deformation capacity,damage resistance capacity and energy dissipation capacity compared to other tested columns.The cumulative energy dissipation of the precast RC/ECC composite column was 7% and 34% higher than that of the cast-in-place column and the precast RC column respectively,which greatly enlarging the application of grouted sleeve connection in high-intensity seismic zones.The mechanical properties of precast RC columns and RC/ECC composite columns were investigated with a nonlinear finite element method based on the simplified constitutive relations and analysis methods.Good agreements were achieved between the experimental and simulation results.Parametric analysis was conducted on the precast columns to investigate the effect of axial load,concrete strength,grouting layer strength,sleeve stiffness and grouting layer placement on the mechanical properties of the columns.The mechanical properties of cast-in-situ columns,precast RC columns and RC/ECC composite columns were compared under low and high axial load respectively.(3)The seismic performance of precast RC joints and precast RC/ECC composite joints was studied.Seven precast joints and two cast-in-place joints were designed according to the proposed assembly schemes and tested under cyclic loads.The test results showed that all the joints had good ductility and loading capacity.The connection methods were reliable.The slight pinching effect was observed in the precast RC joints.The cast-in-place joints showed a larger ductility,larger energy dissipation,and better damage resistance capacity than the precast RC joints.Using ECC in the key areas of the precast joints could prevent premature matrix crushing and spalling,and improve the lateral stiffness and damage resistance of the joints.Besides,ECC effectively prevented the formation of splitting cracks,and increased energy dissipation of the precast joints.The mechanical properties of precast RC joints and RC/ECC composite joints were investigated with a nonlinear finite element method based on the simplified constitutive relations and analysis methods.Parametric analysis was conducted on the precast joints to investigate the effect of strength of later pouring concrete,reinforcement diameter and bonding strength between old and new interfaces on the yielding and peak load of the joints.The effect of reinforcement lap scheme on the mechanical properties of precast joints was also investigated by the parametric analysis,and an appropriate lap length of reinforcement was proposed.(4)The length of plastic hinge zones and the deformation capacity of RC beams and RECC beams were analyzed with a nonlinear finite element method.The influence of various parameters on the length of plastic hinge zones for RECC beams was investigated.The analysis results showed that RECC beams had larger deformation capacity,better ductility,better energy dissipation capacity,and longer plastic hinge lengths than RC beams.The empirical formula for calculating the plastic hinge length of RECC beams proposed in this paper contained most parameters and provided a reasonable prediction for the rebar yielding area of the RECC beams.The deformation of the plastic hinge region accounted for 80-90% of the total deformation of the beams,which indicated applying ECC in the plastic hinge zone of RC/ECC composite members could effectively improve the deformation and energy dissipation capacity.(5)The seismic performance of a precast RC frame and a precast RC/ECC composite frame was studied.A precast RC frame and a precast RC/ECC composite frame were designed according to the proposed separation and assembly scheme.Shaking table tests were conducted on the two frames.The test results showed that the connection zones of the frames maintained the integrity and no connection failure occurred after the excitation of seismic waves.The precast RC frame exhibited concrete crushing at some beam ends.While the precast RC/ECC composite frame had large cracks at the beam end,which indicated the assembly method adopted in the specimens was reliable.The precast RC/ECC composite frame had less diagonal shear cracks in the joint zones than the precast RC frame.The application of ECC could significantly improve the shear resistance and reduce the damage.The precast RC/ECC composite frame had lower inter-story drift and lower acceleration response in the roof than the precast RC frame.Besides,the equivalent damping ratio and natural vibration frequency of the precast RC/ECC composite frame were 21% and 30% larger than that of the precast RC frame respectively,which indicated the precast RC/ECC composite frame had better seismic performance.(6)Pushover analysis were conducted on seven four-story,single-span plane frames with nonlinear finite element method.The performance points at different seismic levels of the frames were obtained by the pushover analysis to evaluate their seismic performance.The results showed that the RC frames with the first and second assembly schemes had higher loading capacity,higher ultimate displacement,and lower ductility than the cast-inplace frame.The RC frame with the third assembly scheme had similar mechanical properties to the cast-in-place frame.The application of ECC in the precast frames could significantly improve the shear capacity of the joint zone,which greatly increased the loading capacity,deformation capacity and ductility of the frame.The elastic and elasticplastic inter-story drifts of the frames met the specification requirements.All the frame structures showed similar roof displacements and inter-story drifts at the same performance point,which indicated the precast frames could achieve the same seismic performance as the cast-in-place frame. |
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