Experimental Study On Seismic Behavior Of Special-shaped Column Frame Joints Strengthened By MRPC After Seismic Damage

For reinforced concrete frame structure,it is widely used because of its advantages of simple structure,clear transmission force and large usable space.However,due to the improper design of the early structural designers and the improper construction of the construction personnel,there are a large number of "strong members and weak nodes" of the frame structure in the earthquake area,so it is particularly important to reinforce the frame joints after the earthquake damage.This article in view of the current domestic and external reinforcement of blank area of earthquake damage of frame nodes,modification of reactive powder concrete(MRPC)using the advantages of the high ductility,high strength and energy intensive,respectively for seismic damage of cast-in-situ frame with special-shaped columns intermediate node CKJ-1,the earthquake damage of special-shaped column frame intermediate node AKJ-1,the seismic damage of cast-in-situ special-shaped column frame edge node CKJ-2,and earthquake damage of special-shaped column frame edge node AKJ-2 on the earthquake damage of reinforcement,and then after the above four seismic damage of reinforced zone plate specialshaped column frame under low reversed cyclic cyclic loading test research.The results show that:(1)After the MRPC reinforcement,the crack development mode of the frame node of the earthquake-damaged special-shaped column has significantly changed.Due to the drawing action of steel fiber and polypropylene fiber on concrete,the crack development is both dense and fine,and the energy dissipation performance of the node is significantly improved.However,the failure form of some nodes is the failure of the junction of old and new materials at the lower column end.This is because:(1)The steel fiber is deposited downward in the MRPC during the casting and vibration process,which reduces the contact area between the cementing material in the MRPC at the lower column end and the original concrete;(2)The cutting length of the lower column end is too short,resulting in the old and new materials at the junction of the moment is large,so easy to crack.(2)After the MRPC reinforcement,the ultimate bearing capacity and ultimate displacement of each earthquake-damaged special-shaped column node were significantly increased compared with those before the damage.The ultimate bearing capacity of CKJ-1 specimen,AKJ-1 specimen,CKJ-2 specimen and AKJ-2 specimen were increased by 10.23%,21.08%,26.22% and 15.93% respectively.Therefore,the limit displacements of CKJ-1,AKJ-1 and CKJ-2 were only measured to be 35.16%,24.10% and 21.69% higher than those of the pre-damage joints.(3)The hysteretic characteristics of the joints strengthened by the MRPC are better.The hysteretic loop of the joints strengthened by the MRPC can obviously envelope the hysteretic loop of the nodes before the earthquake damage,and the "inverted S" shape of the skeleton curves of the joints strengthened by the earthquake damage shows a trend of amplification.Comparing the hysteretic loop of the assembly intermediate node after seismic damage reinforcement and the cast-in-situ intermediate node before seismic damage reinforcement,it can be found that the former can basically cover the latter,indicating that the seismic performance of the assembly node after MRPC reinforcement can basically reach the result of "better than cast-in-situ".(4)MRPC to improve the seismic damage of special-shaped columns node strength degradation and stiffness degradation is mainly manifested in the late loading,this is due to the earthquake in the late loading damage area of normal concrete aggregate separate the strength and stiffness degradation is obvious,and the steel fiber and polypropylene fiber on concrete in MRPC drawing function makes the node earthquake damage area is relatively intact,not separation,so after the seismic damage of reinforced node in the late strength and stiffness degradation also relatively moderate before earthquake damage.(5)The cumulative energy consumption of using MRPC to reinforce the joints of specialshaped columns with seismic damage is far higher than that of the joints before seismic damage.The most significant increase in the cumulative energy consumption is that of CKJ-1 specimens.The overall cumulative energy consumption is about twice that of the joints before seismic damage.Except for the early end of the AKJ-2 test,the overall cumulative energy consumption of AKJ-1 and CKJ-2 specimens was 1.88 and 1.73 times of that of the pre-seismic node.(6)The ductility coefficients of CKJ-1,AKJ-1 and CKJ-2 were 1.47 times,1.33 times and 1.23 times of those before the earthquake damage,except that the failure of the hinge support of AKJ-2 specimens made the ductility coefficients impossible to obtain.(7)Because the test conditions of special-shaped column nodes in the actual state are different from those in the ideal state;Therefore,the shear bearing capacity and the bearing capacity of beam and column in the core area of the special-shaped column joints strengthened by seismic damage are higher in theoretical calculation than in actual test.