Anchorage And Structural Performance Of Prestressed UHPC Beam With Pre-tensioned And Post-tensioned Tendons

The pre-stressing effect in UHPC bridges is beneficial for preventing concrete from cracking,improving structural stiffness,reducing the amount of reinforcement,increasing shear capacity,and improving the spanning capacity of the structure,which is one of the significant ways to achieve long-life,lightweight and ultra-high performance of bridge.Because of the ultra-high mechanical properties of UHPC,the section size of prestressed UHPC beam is greatly reduced in comparison with the traditional prestressed concrete beam,showing a thin-walled trend,which puts forward a higher demand and challenges in the layout of prestressed tendons and the application of anchor loads.Thus,the traditional prestressing and anchorage system may not fully match against the thin-walled structure and make full use of the excellent mechanical properties of UHPC.In this paper,a lightweight UHPC anchorage system without an anchor plate is innovatively proposed.Its local bearing capacity and overall crack resistance are tested and studied,and then the calculation method of bearing capacity and crack resistance of the UHPC anchorage zone are established.In addition,the hybrid prestressing system with large-diameter pre-tensioned tendons and retard-bonded post-tensioned tendons was proposed.Then the bending and shear behavior of pre-tensioned and post-tensioned prestressed UHPC-NC composite beams is experimentally studied,and an efficient and accurate flexural and shear design method for UHPC beams was proposed.Moreover,the shear strength prediction method of the UHPC keyed dry joint considering the bridging effect of steel fibers was proposed.Specific content and the results are as follows:(1)The local bearing performance and capacity of lightweight UHPC anchorage local zoneTwenty local anchorage zone of traditional anchorage systems and lightweight UHPC anchorage systems were fabricated and tested.The effect of the anchorage configuration(anchor plate,spiral reinforcement,and skin reinforcement),fiber volume ratio,and spiral reinforcement ratio(spiral reinforcement diameter,restraint diameter,and height)on their bearing performance and capacity are investigated.Based on test results,the local bearing and failure mechanism of the UHPC anchorage zone were illustrated,and the calculation formula of bearing capacity of UHPC local anchorage zone was proposed considering the bearing contribution of compressive strength of UHPC,the restraining strength of spiral,and the fiber bridge contribution after cracking.The reliability and accuracy of this formula were then verified by experimental results,with a mean between the predicted and experimental shear strength of 0.96.(2)The bursting resistance and reinforcement calculation method of lightweight UHPC anchorage zoneThe cracking resistance of the general anchorage zone was investigated using 15 specimens.The influence of fiber volume fraction,amount of splitting reinforcement,a/h,plate thickness,and reinforcement arrangement on the bursting resistance of the UHPC anchorage zone were analyzed.The cracking failure process and mechanism of the general anchorage zone were uncovered.Based on test results,the fiber volume fraction directly influences the bursting resistance of the UHPC anchorage zone;the bursting resistance increases by 46.7%,when the fiber content increases from 0%to 1%.According to the"local punching-splitting"joint failure,the plastic equilibrium limit model is established considering the failure equilibrium between the splitting and the local failure and the post-cracking bridge effect of the steel fibers.(3)Experimental study on flexural and shear behavior of prestressed composite UHPC-NC beams with pre-tensioned and post-tensioned tendonsTen prestressed composite UHPC-NC beam specimens were fabricated and designed to investigate their shear and flexural performance through a four-point bending test.Firstly,the influence of fiber volume fraction on flexural performance was analyzed.Then,the shear performance of the UHPC beam without web reinforcement(V_f=2.0%)and with web reinforcement(V_f=1.0%,V_f=0.75%)was compared,and the equivalent relationship between fiber and stirrup is quantified.Moreover,the effects of the shear span ratio and web thickness on the shear behavior of the UHPC beam were studied.It is found that the beam presents ductile failure in bending and shear,and the steel fiber can replace a part of stirrups when the fiber volume fraction is higher than 1.0%.The shear bearing capacity of beams without web reinforcement(V_f=2.0%)is substantially the same as that with web reinforcement(stirrup reinforcement ratio is 0.25%,V_f=1.0%).(4)Calculation method of shear bearing capacity of UHPC beams based on compression failure mechanismThe shear test database of 247 UHPC beams is established to further study the influence of more design parameters on their shear performance and the failure mechanism,and evaluate the shear strength formula in current UHPC design codes and standards.According to the failure mechanism of the compression zone and the characteristic of the diagonal shear critical crack,the shear bearing capacity calculation formula of UHPC beams with or without web reinforcement was proposed,which can further accurately predict the shear bearing capacity of UHPC beams.(5)The bending capacity calculation method considering the bridging effect of steel fibers under serviceability and ultimate limit states.It is revealed that the bridging effect of some fibers in the serviceability limit states may disappear due to the fiber debonding effect under the cyclic load.Based on the influence of strain hardening and softening of UHPC on the flexural capacity and failure characteristics,and considering the degradation of fiber bridging effect at a specific crack width in the serviceability limit state,a reasonable calculation method of the flexural capacity of UHPC beams was proposed.The verification results show that the formula can accurately predict the bending capacity of strain hardening and softening UHPC beams.(6)Shear strength prediction method of UHPC keyed dry joints based on the modified compression-field theoryA new shear strength model for UHPC-keyed dry joints incorporating transferring mechanisms of three primary shear components based on previous experimental results and numerical simulation.Based on the modified compression-field theory,the shear contribution by the matrix in UHPC keys is mainly influenced by normal compressive stress and the area of all bases of keys.Based on the mesoscale fiber-matrix discrete model,the shear contribution by steel fibers was affected by the steel fiber length,diameter,type,and volume fraction.The reliability and accuracy of these results were then verified by experimental results of 27 push-off tests available,with a mean between the predicted and experimental shear strength of 0.98.