Research On Static And Dynamic Compressive Mechanical Properties And Constitutive Model Of Concrete-granite Composite After High Temperature

Due to the rapid development of tunnel engineering in China,the risk of tunnel fire accidents(TFA)is increasing.In the event of a fire,temperatures inside the tunnel can quickly reach over 1000℃.After the composite structure of concrete lining and surrounding rock is subjected to high temperature,its post-disaster repair requires stability analysis,seismic and impact resistance evaluation.These engineering and technical problems are based on the compressive mechanical properties of concrete and surrounding rock after high temperature.The mechanical properties of concrete and rock under different high temperatures can vary significantly.Furthermore,the static and dynamic properties,mechanical damage constitutive relationship,damage and failure mechanism of the surrounding rock near the concrete lininginterface still require further study,given the differences in the bonding interface and media materials on both sides.This is of great significance for the safety assessment and long-term operation of tunnels after a fire.Therefore,this paper takes the concrete-granite composite with different roughness interfaces after heat treatment as the research object,and conducts a series of studies using micro and macro mechanical tests,numerical calculations,and theoretical analysis.The main conclusions are as follows:(1)The layered test results based on spin echo single point imaging(SE-SPI)sequence in nuclear magnetic resonance(NMR)technology show that the nuclear magnetic signal intensity of the concrete layer was the largest under different temperatures,followed by the interface layer,and the granite layer had the smallest signal intensity.The temperature thresholds corresponding to the growth rate of micropores in the three material layers are different.Furthermore,conventional triaxial compression tests and discrete element analysis were carried out on composite specimens after high temperature.The interaction between temperature,confining pressure,and interface roughness on the compressive mechanical parameters of the composite was studied.Based on these findings,the influence of temperature,confining pressure,and the mechanical property differences between concrete and granite under the same temperature on the failure mode of the composite was analyzed.(2)The uniaxial(triaxial)compressive strength of the composite under different temperatures is affected by the size effect of the material and the interface constraint effect.The roughness of the interface improves the compressive strength of the composite,but this effect is weakened by an increase in temperature and confining pressure.Based on the test results and the concept of strain energy equivalence,a compressive shear strength failure criterion for the composite considering the joint roughness coefficient(JRC)was derived.The effects of JRC,the height ratio of concrete to granite,and confining pressure on the compressive strength of the composite were analyzed.The applicability of the model was verified through the comparison and analysis of theoretical data and experimental data.The theoretical results showed that the fundamental reason why the roughness improves the overall compressive strength of the composite was that the greater the JRC,the greater the cohesion of the concretegranite interface,which improved the minimum strength level of the composite under the limit equilibrium state.(3)The SHPB test results of the composite specimen under uniaxial and triaxial loading conditions showed that the increase of confining pressure increased the strain rate effect of the composite specimen,and the increase of strain rate increased the confining pressure effect of the composite specimen.The elastic modulus of the composite was found to be insensitive to confining pressure and strain rate.Additionally,the critical strain of the composite hardly changed in the high strain rate range(uniaxial).In the middle strain rate range(confining pressure condition),the critical strain of the composite increased with the increase in strain rate,but the correlation with the confining pressure was not strong.The intervention of roughness increased the strain rate effect of the compressive strength of the composite and reduced the confining pressure effect.Based on these observations,an empirical equation for the strength criterion of the composite body considering strain rate and interface roughness was proposed.(4)The threshold temperature at which the dynamic compressive strength of the composite and concrete suddenly change was 400℃.The compressive strength of granite did not change much within 600℃.The deterioration of composite strength due to the increase in temperature and the reinforcing effect of strain rate on strength was mainly dominated by the former.The influence of temperature on the strain rate of the composite was more complicated.The higher the temperature,the less the contribution of granite to the compressive strength of the composite,especially when the interface roughness was smaller.Compared with confinement pressure and strain rate,the deformation parameters of the composite were more sensitive to temperature.(5)There exists a critical impact velocity when the temperature changes from 25℃ to600℃.Below this critical value,the order of dissipated energy is concrete > composite > granite,while above this value,the order of dissipated energy is granite > concrete > composite.The transition temperature of the arrangement of the average crushing size of concrete,granite,and composite is 600℃.The dissipated energy and degree of fragmentation of the concrete component are significantly higher than those of the concrete monomer under the same impact conditions,while that of granite is the opposite.Moreover,the decrease in energy dissipation capacity of the granite component relative to the monomer is greater than the increase in the dissipated energy of the concrete component relative to the monomer,resulting in a lower dissipated energy for the composite compared to concrete.And based on the elastic-plastic theory,a dynamic constitutive model of concrete in the concrete-granite composite after high temperature was constructed.The comparison between theoretical and experimental results indicates that the dynamic constitutive model has good effectiveness.