Study On Seismic Behavior Of The Precast Concrete Frame Joints Strengthened Locally By Prefabricated UHPC Shells

In recent years,with the development of economy,society and urbanization,China’s construction industry is developing towards the direction of green environmental protection,improving quality and efficiency,and construction industrialization has become the trend of future development.Precaste concrete structure can reduce the amount of wet work and labor,improve the construction efficiency,and ensure the construction quality.At present,a common connection mode of the beam and column of the assembled frame structure is the U-shaped trough connection at the beam end.In previous studies,such joints tend to break at the beam end under the earthquake load and cause the rebar to bend,resulting in the rapid decline of the bearing capacity.A large number of cracks in the joint area often accompany with the earthquake load.Ultra-high performance concrete(UHPC)is a new type of cement-based material with ultra-high strength,high toughness and high durability.Due to the existence of steel fiber,the tensile strength of UHPC is much higher than that of normal concrete,and it can withstand tensile stress of a certain strength after cracking,which is expected to enhance the energy dissipation capacity as well as the structural strength.However,UHPC is difficult to be widely used in construction engineering due to its high price and high requirements for pouring and curing.This paper is based on the Natural Science Foundation — "Seismic Behavior and Design Method of the Precast Concrete Frames Strengthed Locally by Prefabricated UHPC Shells"(5180081127)and Jiangsu Natural Science Foundation Youth Fund Project — "Seismic Performance and Design Method of Prefabricated Concrete Frame Structure Based on Small Prefabricated UHPC Shell"(BK20180385).In view of the phenomenon of beam end damage and joint damage in previous studies,two methods,beam-end UHPC shell and joint UHPC shell,were adopted to enhance the beam-column joints.Without significant cost increase,prefabrication technology is combined with UHPC to improve the seismic performance of the beam-column joints of precast concrete frame structures.The main contents and achievements of this paper are as follows:(1)Study on the shear performance of interface between UHPC and normal strength concrete(NSC):The push-off test was conducted on 13 specimens.Variables of the specimens included the treatment of the interface(flat surface,water-jet surface,bubble groove and castellated key)and the pouring sequence of UHPC and normal strength concrete.The experimental results show that: for the UHPC substrate-NSC overlay interface,the treatment of castellated key and bubble groove is more effective,while the strength of flat surface and water-jet surface is low.The shear strength of the interface of the UHPC substrate-NSC overlay interface is not as good as that of the the NSC substrate-UHPC overlay interface when treated in the same way.(2)The quasi-static tests were carried out on two precast concrete beam-column joints with different lengths of UHPC shells at the beam ends.The test results were compared with a monilithic joint.The results showed that setting UHPC shell in the broken concrete area at the bottom of the beam will cause the broken area to move out,and when the length of UHPC shell is larger than the broken concrete area length at the bottom of the beam,the concrete at the bottom of the beam will not be broken.The strength of beam-column joints strengthen by UHPC shells at the beam ends are equivalent to that of monilithic joint,but their ductility is greatly improved compared with the monilithic joint so their seismic performance is better.(3)The quasi-static tests were carried out on a monilithic beam-column joint and three precast concrete joints with different UHPC shells at the joint core area.Variables of the UHPC shells included the thickness and the configuration of stirrups.The experimental results were analyzed and compared.The results showed that the UHPC shell could enhance the shear strength of the the joint core area and effectively reduce the cracks.However,as the UHPC shells blocks the concrete between beam end and joint area,the integrity of beam and joint area is weakened.So,the beam end is destroyed earlier under cyclic loading and the seismic performance of the joint is reduced.This kind of beam-column joint needs further study.(4)ABAQUS was used to simulate the behaviour of the new precast concrete beam-column joint.The simulation results were in good agreement with the test results.The parametric analysis is carried out and the parameters included the length and the thickness of UHPC shell and the strength of UHPC.Finally,the research contents and results of this paper were summarized.Suggestions and prospects for improvement of this type of beam-column joints were also proposed.