Research On The Relationship Between Concrete Testing Strength And Strain Rate

Concrete is a widely used engineering material in the world at present.In the practical engineering field,it is not only subjected to static load,but also inevitably affected by uncertain dynamic impact load,such as airports,dams,bridges and so on.There have been more than 100 years of research on concrete materials,and a large number of research results have been obtained.However,the understanding of the true strain rate effect of concrete materials has not been unanimously recognized.At present,the majority of researchers in the world think that concrete materials have real strain rate effect,and that the strength increases with the increase of strain rate,but the strength should be the inherent property of the material itself.How does it change with the speed at which it is loaded by the outside world?However,according to the difference of design strength and material,the compressive peak strain of concrete specimen is only a few to ten thousand percent,but under high speed impact,the dynamic strength of the specimen can exceed its static strength several times.This clearly does not conform to the definition of material strength.Based on this,a large number of static and dynamic compression tests have been carried out,and the relationship between concrete strength and strain rate has been deeply analyzed,including the following aspects,(1)Analysis and research of quasi-static compression test method for concrete material.The influence factors of strength change of mortar specimens in quasi static compression tests were studied.The experimental results show that there may be a large number of original defects such as uneven density and step density in axial direction.There are original microcracks in the specimen,and the failure of specimens is a continuous process.During the test process,the crack continues to expand and extend with the loading.Based on the analysis of the end friction,it is found that the existence of the end face friction in the static compression test can change the stress situation of the specimen and increase the bearing capacity due to the multi-axial stress state in the center,and increase the bearing capacity,thus increasing the strength of the whole specimen.The internal defect and aggregate size of the specimen will affect the bearing strength of the specimen and make the test results of the same specimen showing great difference.However,when the internal defect is less,the specimen with uniform internal density will still be affected by the loading speed.And the strength increases with the increase of loading speed.(2)Analysis and research on dynamic compression test of concrete material.The dynamic mechanical response of concrete materials under high strain rate loading was studied by using a split Hopkinson pressure bar(SHPB)experimental device.It was found that the aggregate has an obvious influence on the strength of specimen.The size of aggregate,the ratio and distribution of volume will affect the strength of the specimen,and the more uniform the distribution inside the specimen,the higher the strength.The more uniform the specimen,the less the internal defects,the smaller the axial strain of specimen,the smaller the strain rate of specimen and the higher the dynamic strength of specimen.At the samespeed,the same size specimens with different strength reach the peak strain at the same time.The end friction has no obvious effect on the results in the dynamic compression test and can be neglected.With the increase of strain rate,the strength of concrete does not increase.But under high speed impact,the specimen has a special failure principle and damage mechanism,resulting in random damage of specimen,and a strong change in the test strength of specimen.(3)Dynamic and static of numerical simulation of concrete materials.In quasi-static numerical simulation analysis of concrete materials,the effect of end friction on the test cannot be ignored,and the strength of specimen increases with the increase of friction factor between the specimen and the loaded end surface.The end friction will increase the strength of concrete in quasi-static compression test,but it will not increase with the increase of loading speed.However,the end friction has little effect on the experimental results in high-speed impact test.In the dynamic compression numerical simulation analysis of concrete materials,the external loading speed does not cause the increase of strength,only when the expansion angle of the parameters related to the plastic flow is input.Under the influence of the friction of the end face,concrete material will show a strong strain rate effect.(4)Analysis of dynamic compression test of hollow specimens.Based on the shortage of dynamic and static test samples,hollow test samples are designed,which can effectively reduce the effect of end friction,lateral circumference pressure and radial inertia effect.The influence of the pressure,design strength,aspect ratio and the inner diameter of hollow specimens on the experiment were analyzed by orthogonal test.