Direct Shear Behavior Of Castellated UHPC Joints In Precast Concrete Segmental Bridges

Ultra-high performance concrete（UHPC）is a new type of cement-based composite material with ultra-high compressive strength,high tensile strength,high toughness and excellent durability,which has great application prospects in the field of precast segmental bridges.Precast segmental UHPC bridges will further improve the short-and long-term performance of the precast structure,which provides an alternative for precast segmental concrete bridges with slenderer sections,higher performance,potentially cost-savings applications and faster construction.Compared with the traditional cast-in-place bridge,one of the differences of precast segment UHPC bridge is that there are joints between the precast segments.The longitudinal reinforcement of joints is discontinuous,and the transverse connection only depends on the shear key and prestressed tendons.The performance of the joints is an important factor of affecting the behavior and performance of the whole bridge,which are the key parts and susceptible to shear off.Recently,there are still few studies on UHPC joints with compressive strength between 110 MPa ～ 140 MPa in China and abroad,and the relevant specifications are lagging behind,which is necessary to carry out direct shear test on UHPC joints.In this study,shear performance of the UHPC joints is investigated through the push-off test and finite element simulation.The main research contents are as follows:（1）The push-off tests of 25 ‘Z-shape’ specimens are carried out with parameters of the key number,confining stress,steel fiber content and joint type.The experimental data such as cracking patterns,cracking strength,ultimate strength and residual strength were observed and recorded.The causes of crack patterns and failure mechanism of UHPC joint were analyzed,and the influence of test parameters on the direct shear performance of joints in precast segmental UHPC bridges were summarized.（2）Based on the test,the numerical models were established by the use of finite element software ABAQUS.The direct shear capacity,failure patterns and load-displacement curves of the model were compared with the experimental results in order to calibrate the model.And the calibrated model was used for subsequent extended parametric research,which parameters including key height,key depth and confining stress.Finally,the numerical simulation results were analyzed,and the design suggestions suitable for joints in precast segment UHPC bridge were proposed.（3）The experimental results of joints with steel fiber in previous research were collected and summarized.With the test results in this study and in other literatures as reference,the accuracy of AASHTO formula,JSCE formula,Turmo formula,Sun Li formula,Rombach formula and Liu Tongxu formula were evaluated,and the applicability of those formulae was analyzed.（4）Based on the diagonal strut model of joints,two different failure criteria were used to derive the formulae of calculating direct shear capacity of UHPC joints respectively,which taking the Mohr’s circle as the analysis method.The average ratio,standard deviation and coefficient of variation of the formula based on the maximum tensile stress failure criterion were 0.996,0.171 and 0.172 respectively.The average ratio,standard deviation and coefficient of variation of the formula based on Mohr-Coulomb failure criterion were 0.748,0.126 and0.168,respectively.The formulae both have good accuracy and reliability,which can well predict the test direct shear capacity of UHPC joint.In addition,the formula derived based on the maximum tensile stress failure criterion could more accurately predict the direct shear capacity of UHPC joints after introducing the strength reduction factor of multi-keyed joints（0.7）,which the average ratio,standard deviation and variation coefficient were 0.904,0.093 and 0.103,respectively.（5）The theory of “shear-friction” was briefly introduced.According to the characteristics of UHPC joints with cracking,the regression analysis of the experimental results of joints with steel fiber was carried out based on “shear-friction” theory.And simple formulae for predicting the shear residual strength of UHPC joints under different fiber content was obtained.