Research On The Performance Of PP-ECC Beams Under The Coupling Action Of Bending Load And Freeze-Thaw

Engineering cementitious composite(ECC)is a special type of high-performance fiber-reinforced cementitious composite that can effectively control the development of cracks,exhibit excellent tensile strain hardening properties,and exhibit good tensile strength and toughness,ductility,and crack resistance.At present,most researches focus on the mechanical properties of ECC components in normal environment,and the research on the performance of ECC components in freeze-thaw environment is insufficient.Structures serving in cold regions are often subjected to the dual effects of freeze-thaw conditions and loading.If the frost resistance of the material is insufficient,it will cause major safety hazards and economic losses.In view of the above deficiencies,this paper carried out research on the frost resistance of PP-ECC materials,and analyzed the flexural performance of steel-reinforced PP-ECC beams under the action of a single freeze-thaw cycle and bending load freeze-thaw coupling.The main contents of the research are as follows:(1)Study on frost resistance of PP-ECC material.PP-ECC test blocks for the freeze-thaw test were prepared.The freeze-thaw environment was consistent with the freeze-thaw environment of the test beam.The freeze-thaw damage of the material was evaluated through the loss of mass loss,relative dynamic elastic modulus,compressive and tensile strength,and other mechanical properties.The formula describing the development of compressive and tensile properties of PP-ECC materials under the freeze-thaw condition was established based on the experimental data.The microstructure of the sample is analyzed by electron microscope(SEM)and mercury intrusion(MIP)test.The results showed that freeze-thaw damage was a gradual process from the outside to the inside.With the increase in freeze-thaw cycles,the indicators of PP-ECC continued to decrease,and the damage gradually increased.The reasons for the deterioration of various indicators analyzed by SEM and MIP are: with the increase of freeze-thaw cycles,a large number of holes and micro-cracks appear in the sample,the cracks continue to develop and extend and finally form a network distribution,and the average pore size also continues to increase.The pores began to increase,and the structure became looser.(2)Study on the deterioration process of the matrix material in different stress regions of PP-ECC beams under the action of bending loads during the freeze-thaw process.First,the PP-ECC beams were prepared.Secondly,continuous bending loads of 0,25%,and 50%were applied and the freeze-thaw test was performed simultaneously.Ultrasonic flaw detection technology was applied to test the ultrasonic wave velocity in different stress areas of the beam body and to evaluate the damage in the beam.The results show that with the increase of freeze-thaw cycles,the freeze-thaw damage in each area of the beam body continues to intensify.Under the coupling action of bending loads and freeze-thaw cycles,the ultrasonic wave velocity of the beam body is different in the compression zone,tension zone and bending shear zone.The freeze-thaw damage is the most serious in the tension zone,followed by the compression zone and the bending shear zone.This shows that tensile stress will accelerate the degradation of the strength of the matrix material in the tension zone.Moderate compressive stress can inhibit the degradation of the strength of the matrix material in the compression zone,and excessive compressive stress will accelerate the degradation of the strength of the matrix material in the compression zone.However,the stress level of the matrix material in the bending shear zone is relatively low,which is close to the degradation degree of the compressive strength of the cube.(3)The effects of freeze-thaw cycles,bending loads and freeze-thaw cycles on the performance of PP-ECC beams were investigated.The failure phenomenon,stress-strain curve,load-deflection curve,and stiffness were analyzed.It was found that as the freeze-thaw damage of the specimen beam intensifies,the number of cracks in the beam body decreases continuously,the maximum crack width increases,and the ability to control cracks gradually weakened.Under the action of a single freeze-thaw cycle,the stiffness of the beam gradually decreases.An appropriate bending load can restrain the stiffness degradation of the beam under the load-freeze-thaw coupling action.An excessive bending load can accelerate the stiffness degradation of the beam.(4)The flexural bearing capacity of PP-ECC beams under freeze-thaw cycle,bending load-freeze-thaw coupling was studied.The calculation model of the flexural bearing capacity of PP-ECC beams under freeze-thaw conditions was deduced based on the computational model of the flexural bearing capacity of PP-ECC beams under normal conditions.In addition,the load influence coefficient γ was introduced to establish the calculation model of the flexural bearing capacity of the PP-ECC beam under the load-freeze-thaw coupling effect,and the test data was used to verify it.The results showed that the bearing capacity of the beam decreased with the increased freeze-thaw cycles;the bearing capacity first increases and then decreases with the increased bending load;the established model has high calculation accuracy.