Study On The Change Law Of The Dynamic Strength Of Concrete Based On Energy And Microscopic Stress Analysis

As the most commonly used building material,concrete is widely used in various civil and military buildings such as high-rise residential buildings,bridges,tunnels,hydropower stations,and military bunkers.In the design of important buildings,the stability under dynamic loads such as earthquakes,impacts,and explosions must be taken into account.Since the dynamic mechanical properties and the static mechanical properties of concrete materials are quite different,it's necessary to study the mechanical properties of concrete under dynamic loads.In this paper,based on the basic principle of energy conversion,the energy conversion process of concrete during the dynamic failure process of concrete is analyzed.The mechanism of improving the dynamic strength of concrete is proposed.The rationality and applicability of the proposed mechanism are verified by experimental test and numerical simulation.The influence of mesoscopic heterogeneity on dynamic strength of concrete is analyzed.Research work of this paper can be summarized as the following aspects:(1)The micro-mechanism of energy conversion in concrete during the dynamic failure process is analyzed.It is determined that the elastic strain energy is a bridge between the input energy and dissipated energy is determined.The relationship between the rate of elastic strain energy and the strength of concrete is determined by further analyzing the change process of elastic strain energy.It is proposed that the increase of the dynamic strength of concrete is caused by the hysteresis effect of energy release.On this basis,the influence mechanism of mesoscopic heterogeneity on dynamic strength of concrete is discussed.It is considered that the effect of crack encountering aggregate will increase the lag effect of concrete energy release,thus improving the dynamic strength of concrete.(2)The dynamic Brazilian disc test of concrete was carried out by using SHPB apparatus.The dynamic failure process of concrete under different impact velocity is studied,and the applicability of the "lag effect of energy release" to explain the mechanism of improving the dynamic tensile strength of concrete is verified.The effect of aggregate volumetric ratios on the dynamic tensile strength of concrete was studied by the dynamic Brazilian disc test of pure mortar,concrete with different aggregate volumetric ratios and pure aggregate,respectively.It shows that,the higher the impact velocity,the greater the input rate of energy,the expansion of the crack is lagging behind the input of the energy,which leads to the increase of the energy stored in the concrete and the increase of the dynamic tensile strength of the concrete.The higher the aggregate volumetric ratios,the longer the crack propagation path,the crack will occur when the aggregate is encountered,the energy accumulation process is longer,the energy accumulation is more,the dynamic tensile strength is higher.(3)Because the mechanical mechanism of dynamic compression and dynamic compression of concrete is different,the mechanism of dynamic strength of concrete under dynamic compression needs to be studied.The dynamic compression test of concrete was carried out by using SHPB apparatus.The effects of impact velocity and aggregate volumetric ratios on dynamic compression properties of concrete are studied,and the applicability of the "lag effect of energy release" and mesoscopic heterogeneity to explain the mechanism of improving the dynamic tensile strength of concrete is verified.It shows that,due to the effect of friction and failure mode,the energy transferred to kinetic energy is smaller when the dynamic compression is destroyed,resulting in a gentle section of the stress-strain curve.The higher the impact velocity is,the higher the conversion rate of the input energy is,the dissipative energy is lagging behind the input of the energy,so that the energy continues to be stored in the concrete,and the dynamic compressive strength of the concrete is improved.The aggregate volumetric ratio increases,because the stress wave is reflected in the aggregate,which causes some energy to be stored in the concrete and cannot be released in time.Therefore,the stress and strain rise time is later,but the speed of lifting is faster,and the concrete is more difficult to deform.(4)Because the energy in dynamic tests cannot be better statistically analyzed,dynamic cracking tests of concrete were carried out using numerical simulation methods on the basis of dynamic experimental test.The energy conversion process in concrete under different impact velocities was studied.An ideal model with a single aggregate in the center was established.The effect of aggregate size,position,and shape on the dynamic mechanical properties of concrete was established.It shows that,the intersection of input energy conversion rate and consumption energy conversion rate is the same as the peak stress of concrete.The higher the strain rate is,the higher the input energy conversion rate is.Although the conversion rate of dissipated energy is improved,it still lags behind the input of energy,resulting in an increase in concrete strength.When the aggregate volume ratio is fixed,the smaller the aggregate size,the larger the crack area,the higher the dynamic strength of the concrete.(5)In order to better reveal the dynamic failure process of concrete during dynamic Brazilian disc test,two dimensional meso-scale model of concrete specimens was carried out by using CT images.The dynamic Brazilian disc test of the meso-scale concrete model was carried out,and the dynamic failure process of concrete under different strain rate,different aggregate volume ratio and different tensile strength of various mesoscopic components was studied.It shows that,the energy conversion process of concrete under dynamic loading can be divided into four stages,which are the full storage stage of elastic strain energy,crack initiation and the increase of fracture energy consumption,the peak stress and the transformation of some elastic strain energy into kinetic energy stage,the full release stage of loading completion and elastic strain energy.The modulus of elasticity of concrete is determined by the modulus of elasticity of each microscopic component and its volumetric ratio.The higher the strain rate is,the more the crack directly passes through the expansion of the aggregate,the higher the aggregate volume,the more the stop crack,the longer the time it takes to continue the cracking and the higher the strength.The higher the hole volume fraction and the larger the diameter,the closer the center of the specimen is,the earlier the crack initiation time is,and the lower the strength is.In the project,in order to further improve the dynamic tensile strength of concrete,in the process of concrete preparation,the strength of the mortar should be improved first.Secondly,the strength of the material should be ensured.Finally,the tensile strength of the aggregate is only higher than the mortar.It is not necessary to select materials with higher strength and better quality as the aggregate.(6)Dynamic compression tests were carried out on the meso-scale model of reconstruction concrete by CT image.The applicability of the "lag effect of energy release" to explain the mechanism of improving the dynamic compressive strength of concrete is verified from the viewpoint of energy.The effects of different meso-scale components of concrete on dynamic compressive properties of concrete are studied.It shows that,the dynamic compressive strength and elastic modulus of concrete increase with the increase of aggregate volume fraction,and decrease with the increase of void volume fraction.