Study On Mechanical Properties And Damage Mechanism Of Polypropylene Fiber Shale Lightweight Aggregate Concrete After High Temperature

Building fires have the characteristics of rapid fire spread and difficulty in fighting,which pose great threat to people's life safety and building structure safety.Concrete is one of the most widely used building materials,and its high temperature resistance has always been the focus of research.Compared with conventional concrete,lightweight aggregates are made by high temperature calcination and have excellent fire resistance properties,thus improving the high temperature resistance of concrete.On the other hand,the lightweight aggregate has a large porosity and a high water absorption rate,which has a great influence on the workability of the concrete mixture.It is usually necessary to pre-wet the lightweight aggregate before mixing,thereby increasing the moisture content of the lightweight aggregate concrete,resulting in an increase in the probability of high-temperature bursting.Domestic and foreign research on the high temperature performance of lightweight aggregate concrete is not as deep as conventional concrete.In view of the above situation,this paper uses different types of polypropylene fibers to be incorporated into lightweight aggregate concrete to improve its high temperature and burst resistance.Through the compressive strength and splitting tensile strength of fiber lightweight aggregate concrete,study on the effects of heating temperature,fiber type and cooling method on the mechanical properties of lightweight aggregate concrete at room temperature and high temperature.The microstructure of lightweight aggregate concrete after high temperature was observed by optical microscopy and Scanning Electron Microscopy(SEM),and the chemical composition of cement was analyzed by X-ray Diffraction(XRD).Study on damage mechanism of high temperature performance damage of lightweight aggregate concrete.The results showed that:(1)The incorporation of different types of polypropylene fibers can significantly improve the high temperature anti-burst performance of lightweight aggregate concrete.(2)The proper amount of polypropylene fiber improves the brittle fracture morphology of lightweight aggregate concrete,and improves the compressive strength and splitting tensile strength at room temperature,but the strength decreases with the increase of fiber volume.(3)When the temperature at 100~300°C,the compressive strength after natural cooling is slightly higher than the compressive strength at room temperature,and the tensile strength is relatively reduced.The strength after adding polypropylene fiber is higher than that of ordinary lightweight aggregate concrete.At 500~900 °C,the compressive strength and splitting tensile strength after natural cooling continue to decrease,and the polypropylene fiber has an adverse effect on the strength after melting.At 100 °C,the residual strength after water cooling is higher than natural cooling.At 300~900 °C,the strength after water cooling is greatly reduced.(4)At 100 °C,the fine microstructure of the cement matrix and ITZ is intact,and the hydration products other than ettringite are not decomposed.At 500~900 °C,the structure of cement matrix and ITZ is gradually loose.Large decomposition of hydration products leads to a decrease in the strength of concrete.Water spray cooling can promote the formation of rehydration products,but rapid cooling damages the concrete structure to a greater extent,resulting in a significant reduction in strength.The fiber lightweight aggregate concrete prepared in this paper has excellent high temperature resistance.The research results can provide a basis for the research and development of high temperature resistant lightweight aggregate concrete.