Experimental And Theoretical Research On Shear Behavior Of Joints In Precast UHPC Segmental Bridges

Precast UHPC segmental bridge is seizing more opportunity to be applied in bridges in recent years due to its lighter structural weight,more convenient construction process and longer service life.Compared with cast-in-place UHPC bridges,one of the major difference of precast UHPC segmental bridges lies in joints between segments,where separation of UHPC at the interface and discontinuity of longitudinal reinforcement leads to negative influences of mechanical properties and durability of UHPC bridges.This paper mainly studied the shear behavior of UHPC joints.34 joint specimens was conducted to investigate several major parameters.Finite element model of concrete joints including different concrete type was established to simulate the structural response of joints at shear loading and to further investigate the influence of major parameters.Based on Mohr circle principle,and considering bending effect on shear plane,existing calculation method on shear strength of concrete joints was modified.Moreover,considering mechanical characteristics of UHPC and fiber contribution at diagonal crack plane,formula for calculating shear strength of UHPC joints was proposed and validated by test results.The main research contents are as follows:1)Tests on shear strength of concrete joints with different concrete strength and fiber ratioThe shear strength,deformation and failure model of concrete joints with different concrete strength,fiber ratio,lateral stress,number and dimensions of shear keys,reinforcement layout and bending moment at shear plane was investigated and analyzed through ultimate shear capacity test of 29 shear-off joint specimens and 5 joint specimens combining shear and bending.Results show that the increase of confining stress,compressive strength of concrete matrix and incorporation of steel fibers can significantly improve the shear strength of concrete joints.Multiple-keyed reduction factor obtained by single-keyed and three-keyed joints is 0.76?0.84.Compared with three-keyed joints,large-keyed joints have a higher shear capacity(9.7%)but a lower stiffness in elastic stage.Reinforcement in key regions can slightly improve elastic stiffness and shear capacity of joints,while straight reinforcement(R1)has a more effective promotion to shear strength of large-keyed joints compared with key-shaped reinforcement(R2).UHPC large-keyed joint has higher shear capacity compared with multi-keyed joint,and is easy to assemble and reinforced.It can be used when shear capacity of precast concrete joints is highly demanding.2)Finite element modelling of shear behavior of UHPC joints and its parametric analysisFinite element model of concrete joints was established and validated by concrete joints conducted by this paper.Parametric analysis of UHPC joints including depth and inclination angle of shear keys,lateral stress,bending moment of shear plane was conducted.Results show simulation of joints explains the observed phenomenon of the tests and reveals loading process of joints:analysis of principal tensile stress,lateral stress,shear stress and deformation of joints explains the stripping phenomenon observed in the experiment;analysis of stress distribution joints explains the reduction effect of multiple-keyed joints and improvement of large-keyed joints.Simulation of shear capacity of joints conform to the test results while prediction of slip is smaller than the test results.The optimal ratio of depth to height of shear key should be greater than the critical value,which is about 0.05?0.1.the optimal inclination angle of shear key should be lower than the critical value,whichi is about 45°?60°.Both values increase with the increase of lateral stress.Bending moment of shear plane has no significant effect on shear capacity of joints at low lateral stress,while at high lateral stress,an increase of bending moment leads to the openness of joints and only part of the keyed joints bear the shear stress,causing a decrease of shear capacity.3)Calculation method on shear strength of concrete jointsExisting calculation method on shear strength of concrete joints was evaluated based on existing test data.Considering bending moment of shear plane of joints,the modified formula of shear strength of concrete joints was proposed and validated by test results.Existing AASHTO formula was also modified considering mechanical characteristics of UHPC.Meanwhile,through analysis on the mechanism of fiber reinforcement in concrete,Formula considering fiber contribution of diagonal crack plane of FRC and UHPC was proposed and validated by test results.Results show that calculated values under maximum principal tensile stress were close to test values,indicating joints fail in tension.Calculated values of formula on shear capacity of UHPC joints agree well with test results compared with AASHTO formula.Formula considering bending effect correlates well with test results in calculating multiple-keyed joints compared with AASHTO formula.Formula considering fiber contribution of FRC and UHPC joints also agrees well with test in this paper,yet its applicability needs to further investigated.