Experimental And Theoretical Research On The Diagonal Cracking Behavior Of UHPC-T Beams

UHPC is a new type of civil engineering with super mechanical properties and excellent crack control capabilities.Therefore,more and more engineering applications of UHPC beams appear in China.Compared with ordinary concrete beams,the section size of UHPC beams is greatly reduced,and the web is naturally thinned.However,the thin web structure will bear higher stress levels and more complex forces in the diagonal section.Currently,many researchers are more concerned about the strength and crack resistance of UHPC beams’ normal section instead of the diagonal section.In particular,the diagonal crack in the web are prominent problems in the operation stage of the bridge.Thus,it is urgent to pay attention to the stress change and crack development of UHPC bridges’ diagonal section,and evaluate its crack resistance.In order to provide data and theoretical basis support for promoting the wider application of UHPC bridges,this paper combines model experiment,finite element analysis and theoretical analysis to explore the diagonal crack resistance and shear bearing capacity of UHPC beams.The main tasks are as follows:(1)The model test of 5 prestressed UHPC-T beams were designed and completed.The test parameters included steel fiber volume,shear-span ratio and stirrup ratio.Experimental results included load,crack pattern,strain response,and et al.were gained.It can be seen that the increase of shear-span ratio decreased the cracking load and increased the crack width development.The stirrups showed little effect on the cracking load,but inhibited the propagation of diagonal cracks.The increase of the fiber volume increased the cracking load and inhibited the crack width development.(2)Based on the ABAQUS software,the test beam models were established by using two methods to contact relationships between unbonded prestressed ribs and UHPC,which are rigid spring and local coordinate coupling.The parameter analysis on diagonal cracking behavior and shear performance of UHPC beam were completed,which included prestressed tensile stress,longitudinal reinforcement ratio and stirrup ratio.The results show that the simulation results are consistent with the test results.The simulation results of the whole process force of the test beam with bending failure are more accurate.Prestress can effectively increase the cracking load of UHPC beam,and the longitudinal reinforcement ratio to the improvement of cracking load is small.The stirrup ratio has almost no effect on the cracking load.The increase of prestressed tensile stress,longitudinal reinforcement ratio and stirrup ratio all improve the shear bearing capacity of the structure,of which the effect of the stirrup ratio is the most significant.(3)Based on the mechanics of materials,the formula for shear cracking force was derived.In addition,the formula for diagonal crack width in serviceability limit states were proposed using the limit balance method.By calculating the principal tensile stress under the measured cracking shear force,it is found that the principal tensile stress of specimens during cracking exceeds the tensile strength of UHPC,which reflects the strain hardening characteristics of UHPC and the good diagonal crack resistance of UHPC beams.Compared with the design specifications of different countries,the allowable stress in Chinese normative proposal draft was relatively conservative.Considering the unevenness of the fiber distribution,it is recommended to take 0.6 times the elastic ultimate tensile strength as the principal tensile stress limit of the prestressed UHPC beam.(4)The shear bearing capacity of the test beams in this paper were calculated by using the design specifications of different countries.Based on the modified compression field theory,different concrete strength criteria were used to establish UHPC damage criteria in the compression zone,and the calculation program of the shear bearing capacity of UHPC beam was compiled.The results show that the calculation of various specifications is relatively conservative.The calculation results of the Mohr-Coulomb criterion are closer to the experimental values than those of the Rankine criterion.Because the test beam with a small shear-span ratio is in a bending and shearing state,the Mohr-Coulomb criterion can consider the influence of normal and tangential stresses on the compression zone,where the simulation results are closer to the reality.