Study On Mechanical Properties And Impact Response For Concrete Beam In The Northwest Freezing-thaw Environment

Rockfall impact causes great hidden danger to bridge structure and vehicle safety in western mountainous area.The loading rate of gravity,vehicle load,preloading and other loads is slow,which can be analyzed according to the quasi-static loading.The impact of the collapse rock on the bridge structure will transfer a lot of energy in a short time.In this case,the bridge structure shows a completely different structural response from that under quasi-static loading.Meanwhile,the freezing-thawing durability of concrete bridge structures in the cold region of western China is becoming more and more serious.Therefore,it is of great theoretical significance and practical application value to study the mechanical properties and impact structural response of box concrete beams under the freeze-thaw cycle environment.In this paper,theoretical analysis,laboratory test and numerical simulation are used to carry out the research.This paper mainly studies the degradation law of mechanical properties of concrete materials after freeze-thaw cycle,the plastic constitutive model of concrete component damage,and the impact response of box concrete beam structure.The specific research content is described below.(1)The quasi-static mechanical properties of concrete after freeze-thaw cycle were studied.The quasi-static uniaxial compression tests were carried out on concrete samples without freeze-thaw cycles of 25 times,50 times and 75 times,respectively.The relationship between the number of freeze-thaw cycles and the mechanical properties of concrete such as uniaxial compressive strength and splitting tensile strength is discussed.By constructing freeze-thaw damage variables in Loland form,the functional relationship between concrete stress,strain and freeze-thaw damage factors under uniaxial compression was established.(2)Study the dynamic compressive strength and failure mode of concrete after freeze-thaw cycle.Compared with quasi-static loading,the strength of concrete material under high strain rate is obviously improved.The conditions of separated Hopkinson pressure bar are verified.Dynamic impact test of concrete samples after freezing-thawing cycle was carried out.The dynamic compression failure mode of concrete is analyzed.The growth law of dynamic strength growth factor of concrete compression strength under freeze-thaw cycle is obtained.(3)Research on the damage plastic model of freezing-thawing concrete members.Based on the test results,the damage plasticity model of concrete was modified.ABAQUS finite element software is used to verify the effectiveness of the improved concrete damage plastic model.ABAQUS software is used to analyze the failure process of concrete components under quasi-static compression and the corresponding stress-strain curves.The three-dimensional finite element model of concrete component under hopkinson pressure bar loading is established.The dynamic strength growth factor of freezing-thawing concrete member is analyzed(4)Experimental study on static and dynamic loading of box concrete beams with threepoint bending.Static three-point bending loading test was carried out for box concrete beams without freeze-thaw cycles of 25 times and 50 times.The effect of the number of freeze-thaw cycles on the bearing capacity of the beam is studied.The effect of freeze-thaw damage on its static bearing capacity is analyzed.The crack propagation rule of box concrete beam under impact load is analyzed by three point bending test of drop hammer impact.By comparing the impact force,mid-span deflection and acceleration of the beam under different impact heights,the influence of freeze-thaw damage on the dynamic bending energy dissipation of the beam is analyzed.(5)Numerical simulation study on impact response of box concrete beams in freeze-thaw environment.ABAQUS finite element analysis program is used to complete the whole process analysis of falling hammer impact box concrete beams.In the analysis,truss element and double diagonal constitutive model are used for reinforcement,and three-dimensional solid element and damage plastic constitutive model are used for concrete.The stress and deformation process of box concrete beam are calculated.The validity of the numerical simulation results is verified by comparing with the experimental results.Then,the impact response of box concrete beam under impact load is analyzed.The effects of freeze-thaw damage and reinforcement ratio on the impact force,mid-span deflection and reinforcement strain of the beam are discussed.