Mechanical Performance Of RC Beam-column Joint Cores Strengthened Using UHPC Panels

Reinforced concrete(RC)frame structures are widely used in the construction industry and are one of the most common structural forms.Under the action of horizontal earthquakes,the frame beam-column joint cores are subjected to complex forces,especially in old buildings with defects such as insufficient configuration of hoop reinforcement in the node core area,which are prone to brittle damage,thus leading to the overall destruction and collapse of the building.Therefore,it is of great practical significance to adopt efficient and environmentally friendly seismic strengthening means to strengthen and diagnose them.Ultra High Performance Concrete(UHPC)has the advantages of high strength,high toughness,high durability and strain-hardening characteristics,which is a more ideal reinforcement material.Its combination with fiber reinforced composite material(FRP)slats,which have high tensile strength but insufficient brittleness and heat resistance,can achieve the effect of complementary advantages.At present,there are few studies on the seismic performance of frame joint reinforced with UHPC materials at home and abroad,and there are no relevant code provisions.UHPC plate reinforcement has the advantages of wide range of application,high reinforcement efficiency and low maintenance cost,but its reinforced force mechanism and shear contribution to the core area of the node need to be clarified.In view of this,based on the force characteristics of frame joint,this paper designs and carries out experimental research and numerical simulation analysis on the diagonal tensile performance of UHPC plate reinforced footprint node core specimens,aiming to investigate the interaction mechanism between UHPC plate and substrate,quantitatively determine the contribution of reinforcement materials to the node core area force performance improvement,and provide theoretical basis for the reinforcement design of related frame structure beam-column joint cores.On this basis,the prediction equation of the bearing capacity of the core zone of RC joint reinforced by UHPC plates is proposed to explore the contribution of UHPC plates to the core zone of joint.The main research contents and results are as follows:(1)The experimental study of the diagonal tensile performance of 12 sets of UHPC plate reinforced footprint node core area specimens was carried out.The UHPC plate thickness,FRP slat reinforcement rate and UHPC plate installation method were used as the main research parameters to focus on their effects on the strength,stiffness and ductility of the specimens.The test results showed that the strength and stiffness of the specimens in the node core area increased with the increase of UHPC plate thickness and reinforcement rate.Strength increased by 38.4%-58.2%,ductility increased by 7.5%-135%.The mechanical anchoring of the tensile bolts can improve the ductility of the specimens,and the UHPC plate has a certain inhibitory effect on the deformation and cracking of the matrix,which can delay the development of cracks in the matrix.(2)The study of the influence law of the main reinforcement design parameters was carried out based on the finite element model calibrated from the experimental data.The nonlinear finite element model of the specimen was established using DIANA finite element analysis software,and it was calibrated and optimized based on the experimental data.Meanwhile,based on this model,the influence law of parameters such as strength of matrix concrete,cross-sectional area of CFRP slats,and spacing of CFRP slats were supplemented and analyzed.The results show that the finite element model can simulate the strength,stiffness and damage mode of the specimen more accurately;the strength of the specimen increases with the increase of the strength of the matrix concrete and the cross-sectional area of the CFRP slats,and the influence of the slat spacing on its performance is not obvious.(3)The contribution of UHPC plates is considered in the form of nominal tensile strength with the theoretical background of the concrete bi-directional tensile damage criterion,and the load capacity prediction equation of UHPC plate reinforced RC node core zone specimens is proposed to quantify the contribution of UHPC plates and CFRP bars to the load capacity of reinforced specimens.The verification by experimental and numerical simulation results shows that the theoretical formula has high accuracy and is biased towards safety.