| The traditional concrete material has features of low tensile strength,poor deformation,and it is difficult to guarantee the quality of concrete pouring because of the tight arrangements of steel bars in the beam-column joints as well as adjacent beams and columns.As a result,the joints are prone to interface splitting tensile failure or shear failure with peeling of the protective layer,and the steel bars at the beam end or column end of the concrete frame structure are damaged when they reach the yielding state first,which leads to large residual deformation of the structure after the earthquake,and even worse the related functions are difficult to restore.Engineered Cementitious Composites(ECC)is a new type of high performance fiber reinforced composite material with superior ductility,micro-crack control characteristics and energy absorption capacity.In order to improve the seismic performance and repairability of the beamcolumn joints,high ductility ECC material was developed and applied to the core area of frame beam-column joints,adjacent beam-column ends and plastic hinge areas.The plastic hinge area at the end of the new joint beam adopts "X" shaped longitudinal bars.Through seismic test,the seismic performance of the new type of joint was preliminarily investigated,and then the effects of the ECC length at the beam end and the longitudinal reinforcement ratio on the seismic performance of the new type of joint were studied by numerical analysis.The main work and findings can be shown as follows:1.The ECC material ratio with large fly ash content was optimized and the influences of water-binder ratio,age,size effect and curing regime on the tensile and compression properties of ECC were investigated through basic mechanical properties tests.The results show that:(1)with the decrease of the water-binder ratio,the compressive strength and tensile strength of the ECC cube increase,while the ultimate tensile strain and the number of cracks decrease.This trend will weaken with the increase of the age.The ECC cube compressive strength at the age of 120-day is 18.37% higher than that at the age of the 90-day and 58.34% higher than that at the age of the 28-day,because the fly ash content in the ECC mix ratio accounts for 70% of the cementified material;(2)based on the results of long-age compressive strength tests of ECC specimens of different sizes,it is recommended that the size effect conversion coefficient of the compressive strength of 70.7mm cube and 100 mm cube is taken as 0.89,and the shape effect conversion coefficient of the compressive strength of prisms and cubes is taken as 0.91 under standard curing conditions of 28days;(3)high-temperature steam curing is conducive to improve the compressive strength of ECC.Under the condition of steam curing at 70℃ for 8hours,the compressive strength of ECC cubes can reach 90% of the compressive strength of cubes under the standard curing condition of 28 days,which can meet the preparation and curing requirements of prefabricated components in actual engineering projects.As the steam curing time increases,the size effect of compressive strength of 100 mm cube and 70.7mm cube,as well as the shape effect of the axial compressive strength of prism and cube gradually weaken.2.Seismic tests were carried out on one reinforced concrete beam-column joint and three ECC/RC composite beam-column joints with ECC materials for plastic hinge areas at beam ends,and therefore the influences of plastic hinge areas materials,plastic hinge positions and assembly forms on the seismic performance and repairability of ECC/RC composite beamcolumn joints were investigated.The results shows that:(1)compared with the traditional reinforced concrete specimens,the composite joints with ECC material show the ductile failure characteristic of multiple crack,and the damage degree is low.The peak bearing capacity is increased by 15.55%,and the ductility,damage resistance and energy dissipation capacity are improved significantly.The peak bearing capacity is increased by 15.55%,and the ductility,damage resistance and energy dissipation capacity are improved significantly;(2)the plastic hinge length and deformation capacity of the beam end of ECC composite joint specimen are better than that of concrete specimen under low cyclic load.(3)The plastic hinge of the beam end of the ECC composite joint reinforced with "X" longitudinal bars is transferred to the section where the intersection of the "X" longitudinal reinforcement is located.The number and width of cracks in the core area of the joint and the beam end near the cylinder are significantly reduced,which can ensure the integrity of the critical area of the joint,and play an important role in reducing the damage degree of the critical area of the beam-column joint under the earthquake and the post-earthquake repair work.The "X" longitudinal joint specimens bear the load and deformation of the whole structure mainly through the ECC material in the "X" longitudinal reinforcement area,and still show the ductile failure characteristic of multi-crack crack.The bearing capacity of the joints can reach the level of the reinforced concrete joints with higher reinforcement ratio.(4)the "T"-shaped assembled joint form proposed in this paper is reasonable and feasible since it has seismic qualities of good bearing capacity,hysteretic characteristics,ductility,energy dissipation capacity and more importantly the peak bearing capacity is more than 90% of the cast-in-place specimens.3.Based on the finite element software ABAQUS,a numerical model of beam-column joints using ECC in the plastic hinge area at the beam end was established,and the influence of the ECC pouring length at the beam end and the ratio of longitudinal tensile reinforcement on the seismic performance of such beam-column joints were investigated.Results shows that:(1)within the range of 300 mm to 600 mm,the length of the beam end pouring ECC has little effect on the peak load of the joint,which can be appropriately reduced under the premise of covering the plastic hinge area and ensuring the bearing capacity and deformation capacity of the interface between ECC and concrete.Based on the seismic tests and finite element simulation results in this paper,it is suggested that the ECC casting length of beam-column joint beam can be 1.2 times to 1.5 times the beam height;(2)as the reinforcement ratio of the longitudinal tensile reinforcement increases,the ductility of the beam-to-column joints with ECC in the plastic hinge area at the beam end under reciprocating load decreases,the failure area becomes more concentrated and the plastic deformation capacity decreases. |
