Research On Evolution Law And Deterioration Mechanism Of Mechanical Properties Of Concrete Under Low Vacuum And High Temperature

As a new type of rail transportation tool,Ultra-high-speed train operated in vacuum tube uses the "maglev+low vacuum pipeline" technology to realize the ultra-high-speed running in the mode of low mechanical friction,low air resistance and low noise.However,the pipeline infrastructure during operation not only needs to withstand extremely low air pressure,but also experiences high temperatures(electromagnetic induction).As an important part of the low vacuum tube infrastructure for ultra-high-speed trains,the mechanical performance stability of concrete materials under low vacuum and high temperature condition is essential for safe operation.Therefore,studying the evolution and mechanism of concrete mechanical properties under low vacuum and high temperature condition will provide technical support and theoretical basis for the application of concrete materials in the low vacuum tube infrastructure for ultra-high-speed trains,which is of great significance for promoting the development of ultra-high-speed trains.This study focused on cement paste,mortar,and concrete,and through environmental exposure tests,the spatiotemporal evolution law of mechanical properties such as compressive strength,flexural strength,and impact resistance of concrete under low vacuum and high temperature condition were clarified.At the same time,detection techniques such as X-ray diffraction(XRD),scanning electron microscopy(SEM),mercury intrusion porosimetry(MIP),and X-ray computed tomography(X-CT)were used to explore the evolution characteristics of the phase composition,microstructure,and multiscale pore structure of the matrix under low vacuum and high temperature environments,and the correlation between the microstructure and macroscopic mechanical properties was established,revealing the evolution mechanism of concrete mechanical properties in this environment.Finally,a Markov grey residual model was established to predict the relative dynamic elastic modulus of concrete under a low vacuum environment.The main conclusions are as follows:(1)Compared with normal air condition,the dynamic elastic modulus of paste specimens at different ages is relatively low after low vacuum exposure;for early-age cement paste,shortterm(28d)low vacuum exposure would significantly delay the improvement of their compressive strength and flexural strength,and with the increase of exposure time,low vacuum condition would cause a deterioration of the flexural strength of specimens.For mature specimens,short-term low vacuum exposure would promote the improvement of their compressive strength,but long-term low vacuum exposure is unfavorable to the maintenance and development of strength.(2)Compared with the normal air condition,the dynamic elastic modulus of mortar is relatively low after low vacuum exposure;and after 1～3 months of exposure,the strength of the specimens is relatively high,while after 4～6 months of exposure,the strength of the specimens is relatively low.The mechanical properties of mortar are positively correlated with the exposure depth and negatively correlated with the exposure air pressure.The high temperature after short-term(6 months)exposure can lead to varying degrees of improvement in the mechanical properties.(3)The strength of concrete under low vacuum condition generally increases with the increase of exposure time,but compared with normal air condition,short-term(1 month)low vacuum exposure would promote the improvement of compressive strength of concrete(negatively correlated with the exposure air pressure),while long-term low vacuum exposure would limit the development of concrete strength.The strength of concrete under low vacuum environment is positively correlated with the exposure depth and the curing age,but negatively correlated with the water-cement ratio.Long-term(12 months)high-temperature exposure under low vacuum condition can limit the development of concrete strength.(4)After low vacuum exposure,the specimens showed more obvious brittle fracture in the flexural test,and under impact load,the degree of fragmentation of the specimens after low vacuum exposure was significantly higher than that of the specimens before exposure and in normal air condition,and the specimens under low vacuum exhibited the lowest dynamic compressive strength and toughness.(5)Low vacuum exposure does not change the types of phases in hardened cement paste,but high-temperature exposure can cause the decomposition of Aft in the matrix.The total amount of main hydration products(C-S-H gel,Aft,calcite,etc.)in the matrix after low vacuum exposure is highly correlated with the law of concrete strength.The temporal and spatial development rules of concrete mechanical properties under a low vacuum and high-temperature condition can be reasonably explained based on the microstructure of the interfacial transition zone(ITZ)and multiscale pore structure.(6)The Markov grey residual GM(1,1)model was used to predict the relative dynamic elastic modulus of concrete under low vacuum condition.After verification,the model showed high accuracy(average error of 2.6439%),and the life prediction results showed that compared with the normal air condition,the time for concrete to reach its life limit under low vacuum condition would be reduced by 5.7%.