Investigation Of Damage And Transport Performance Of Ultra-high Performance Concrete Under The Coupling Of Stresstemperature-osmotic Pressure

The penetration of chloride ions and moisture in cement-based materials is the core issue of durability.However,due to the extremely complex working conditions in actual engineering,the damage process and transport mechanism of ultra-high performance concrete（UHPC）are more complicated.Taking ultra-high performance concrete as the research object,this study investigated the damage process of ultra-high performance concrete under the coupling action of stress,temperature and osmotic pressure,and the transmission process of moisture/chloride ions.A microscopic view of the thermal conductivity under the coupling effect of load and temperature and theoretical model of chloride ion transport under load-temperature-osmotic pressure coupling were established.This study provides a theoretical basis for the durability and life prediction of UHPC under the coupling effect of multiple factors.The following innovations were achieved:（1）It reveals the damage process of UHPC and the law of chloride ion transmission under load-temperature coupling.The test results show that the damage degree of UHPC increases with the increase of load,temperature and water-to-binder ratio.The fiber content has a special effect on UHPC damage,and there is an optimal content at which the damage of UHPC is the smallest,and UHPC without fiber content has the most serious damage.Load,temperature and water-binder ratio have the greatest impact on the transmission performance of UHPC.The chloride ion diffusion coefficient increases significantly with the increase of the three,while the fiber content has little effect on its transmission performance.In addition,on the basis of the parallel model,a meso model of the thermal conductivity of UHPC under load-temperature coupling is established,and the predicted value is in good agreement with the measured value.（2）The experiment researched the damage process and the distribution of chloride ion content of UHPC under the coupling effect of load-temperature-osmotic pressure.The experimental results show that with the existence of osmotic pressure,the damage of UHPC is improved to a certain extent,and the degree of improvement increases with the increase of load,temperature,osmotic pressure and water-binder ratio,but decreases with the increase of fiber content.With the increase of load,temperature,osmotic pressure and water-binder ratio,the chloride ion concentration and penetration depth at different depths increase,but the chloride ion concentration distribution under different fiber content is very small.The impact on UHPC chloride ion transmission performance from large to small are:osmotic pressure,load,temperature,water-to-binder ratio and fiber content.A diffusion-convection chloride ion transport equation under the coupling effect of load-temperature-osmotic pressure is established.After testing,the calculated value of the model is close to the experimental value,and the model has certain applicability.（3）The microstructure evolution of UHPC under chloride erosion was studied by means of SEM and XRD.The experimental results show that with the increase of stress,the microcracks of UHPC propagate to a certain extent and the damage is aggravated.The number,width and length of UHPC microcracks decrease with the increase of temperature and osmotic pressure,and the damage is improved.The structure of plain UHPC is loose and there are a large number of microcracks,but the addition of fiber can effectively restrain the crack propagation and significantly improve its structure.With the increase of load,temperature,osmotic pressure and water-to-binder ratio,the content of Ca（OH）₂ decreased and the content of Freidel salt generated increased,while the content of Ca（OH）₂ of plain UHPC was lower than that of mixed The UHPC incorporated into the fiber,and the Freidel salt content of the product is significantly higher than that of the UHPC incorporated into the fiber.（4）Using NMR technology to study the water transmission law of low water-binder ratio mortar under the coupling effect of osmotic pressure-confining pressure-temperature.The test results show that the pore size of low water-binder ratio mortio is nearly an order of magnitude lower than ordinary mortar,and it greatly slows down the water penetration rate.The time to penetrate the specimen is 80%longer than that of ordinary mortar.Osmotic pressure and temperature can significantly accelerate the speed of water transmission,which has an extremely adverse effect on the water transmission of mortar.The appropriate confining pressure can effectively slow down the rate of water penetration,and has a positive impact on the water transmission of the mortar.The degree of influence on the water transmission performance of the low water-binder ratio mortar is as follows:osmotic pressure,temperature and confining pressure.This paper has a total of 108 figures,15 tables and 135 references.