Study On Mechanical Properties And Durability Of High Toughness Cement-based Composites After Ultra-low Temperature

With China’s constant emphasis on clean energy,a large number of civil engineering construction in low-temperature and ultra-low-temperature environment is on the rise,and concrete is more and more widely used in low-temperature and ultra-low-temperature fields.As a new type of high performance concrete,high toughness cement-based composite material has good mechanical properties and durability under conventional environmental conditions.Such as crack control,energy absorption and frost resistance.In order to expand the application field of high toughness cement-based composites,this dissertations mainly studies the influence of low temperature and ultra-low temperature on mechanical properties and durability by testing high toughness cement-based composites after ultra-low temperature and deep cooling environment.The main contents and conclusions are as follows:(1)In order to study the strength changes of high toughness cement-based composites after different temperatures,the compressive strength of nine groups of specimens with different mixing ratios was measured at normal temperature(20℃),0℃,-50℃,-100℃,-150℃ and-196℃.The results show that the compressive failure of cube specimens of high toughness cement-based composites under ultra-low temperature can show a good shape.With the continuous decrease of temperature,the compressive strength of high toughness cement-based composites has been improved to varying degrees,and the lower the temperature,the greater the strength improvement.The lower the temperature,the greater the influence of fiber on compressive strength.The main reason is that after ultra-low temperature action,the pore water in high toughness cement-based composites will freeze into ice with different structures at different low temperatures,so that the adhesive property between the fiber and the matrix is enhanced,and the strength of the material is improved.(2)In order to study the influence of ultra-low temperature on the bending properties of high toughness cement-based composites,nine groups of specimens with different mixing ratios were tested by four-point bending test after ultra-low temperature.The results show that the good bending properties of specimens at normal temperature show obvious degradation after low temperature.After ultra-low temperature,the peak bending load of beam specimen generally increases,but the effect of deflection and toughness decreases.With the decrease of temperature,the failure mode of specimen changes from ductile failure to brittle failure.After ultra-low temperature,the energy absorption of the specimen increases first and then decreases,while the energy absorption capacity of the specimen decreases after too low temperature.(3)Ultra-low temperature will aggravate the durability damage of materials.The carbonation performance test of high toughness cement-based composites after ultra-low temperature shows that the carbonation depth of specimens after ultra-low temperature is greater than that of normal temperature,and the lower the temperature,the more serious the effect is,and the deepest carbonation depth of specimens after-196℃;The carbonation resistance of the material can be improved by adding fiber,and the optimum fiber content is between 1.0% and 1.5%.With the decrease of temperature,the greater the water-binder ratio,the greater the carbonation depth of the specimen;The carbonation depth increases with the increase of fly ash substitution rate.The reason for the deterioration of durability is that the internal damage of materials caused by rewarming at ultra-low temperature is irreversible,the porosity and permeability of materials increase after rewarming,which leads to the decrease of durability of materials.(4)After ultra-low temperature,the anti-corrosion ability of high toughness cement-based composites is significantly reduced.The corrosion resistance test of high toughness cement-based composites after ultra-low temperature shows that ultra-low temperature will aggravate the corrosion of steel bars inside the materials,and the corrosion rate of steel bars after-196℃ is 5.36 times that under normal temperature.The weight loss rate of steel corrosion decreases with the increase of fiber content;After the same temperature,the higher the fiber content,the lower the corrosion degree of steel bars;The greater the water-binder ratio,the greater the corrosion rate of steel bars;The smaller the replacement rate of fly ash,the smaller the corrosion rate of reinforcement.