Study On Bending Behavior Of Precast Segmental UHPC Beam

Precast segmental bridges have the advantages of short construction period and high quality,but traditional Normal Concrete(NC)segmental bridges face problems such as high self-weight and poor durability.Ultra-High Performance Concrete(UHPC)is a type of cementitious composite neo-material with excellent mechanical properties and durability,which can be applied to precast segmental bridges to effectively reduce structural weight and improve durability.At present,there are few studies on such structures.In order to promote the application of precast segmental UHPC bridges with external prestressing,the in-depth study of their mechanical properties becomes an important issue to be faced.Therefore,the main work is completed as follows:(1)Four-point bending model tests were completed for three external prestressed UHPC beams,including one monolithic beam and two segmental beams.The main test results were obtained,such as damage modes,crack pattern,and the relationship between load and deflection,strain,joint opening,and prestressing tendon stress.The results showed that the joint opening of the epoxy-joint segmental UHPC beam was caused by the failure of the epoxy,which was different from that of the epoxy-joint segmental NC beam caused by the failure of the NC near the joint;the stresses in the unbonded prestressing tendons at the top of the beams from the beginning to the failure had a small change,while the stresses in the external prestressing tendons at the bottom of the beams had an increase of more than 700 MPa.(2)Considering the sectional pre-compression stress and the flexural tensile strength of epoxy,the calculation formula of joint opening moment was proposed and the difference between the calculation result and the experimental result was less than1%,which was in good agreement.In order to accurately calculate the flexural capacity of segmental UHPC beams with external tendons,the Rod-Spring Hinge model that regards segments into rods and joints into spring hinges was proposed.Based on this model,the analytical method was used to derive the calculation formulae of stress variation and secondary effect of external tendons.The difference between the theoretical and experimental values of tendon stress was less than 2.3%,which was in good agreement.Considering the uncertainty of segmental beams’ joint opening,the concept of flexural capacity domain was proposed,i.e.the envelope range of structural flexural capacity under different joint opening conditions.The difference between the lower limit of the flexural capacity domain and the experimental values were less than 6%,so the lower limit is suggested as the reference value of the structural flexural capacity.(3)Considering the triple nonlinearity,a solid finite element model was established based on the CDP model and the constitutive model of epoxy which can accurately simulate the behavior of the precast segmental UHPC beams with external prestressing under bending load.Based on the finite element model,a series of parametric study were completed and the following conclusions were obtained: the compressive strain corresponding to the peak point in the UHPC compressive constitutive model is positively correlated with the structural ductility;increasing the epoxy flexural strength can improve the joint opening moment;increasing the prestressing tendon reinforcement rate can effectively improve the joint opening moment and ultimate bearing capacity,but too high reinforcement rate will lead to over-reinforcement damage;increasing the prestressing tendon tensioning stress can effectively improve the joint opening moment,and the structure has good ductility when the permanent stress of prestressing tendon at the bottom of the beam is acting around 1000 MPa.Combined with the experimental results and parameter study results,the relevant rational design suggestions are summarized.