Study On Frost Resistance And Damage Constitutive Relationship Of Concrete With Different Phase Transformation Temperature Gradients

Concrete often freezes and thaws in cold regions.This is also the main reason why concrete is damaged.Therefore,the freeze-thaw resistance of concrete itself is not only one of the key evaluation indicators of concrete durability in cold regions,but also a hot topic in hydraulic concrete research.The purpose of improving the frost resistance of cement-based materials is to use the heat compensation of phase change materials to alleviate the water freezing in cement-based materials.It will be of great significance to prolong the service life of hydraulic structure concrete.In this paper,microencapsulated phase change materials(m PCMs)with different phase change temperatures(5°C and-5°C)and different contents(0%,5% and 10%)are added into hydraulic concrete to form m PCMs modified concrete with different temperature gradient combinations(0%,5% phase change temperature 5°C,5% phase change temperature-5°C,5% phase change temperature 5°C+5% phase change temperature-5°C,10% phase change temperature 5°C and 10% phase change temperature-5°C).By studying the temperature control performance and frost resistance of m PCMs modified concrete with different phase change temperature gradient combinations,the results show that:(1)The use of phase change materials can delay the change of the internal temperature of the concrete with the ambient temperature;(2)With the increasing amount of m PCMs,the loss rate of compressive strength of concrete is accelerating.The concrete strength is also obviously weakened;(3)The larger the content of m PCMs(5°C)is,the stronger the frost resistance of concrete becomes;With the increase of m PCMs(-5°C),the frost resistance of concrete gradually decreases;The composite phase change concrete with MPCMS(5°C)and MPCMS(-5°C)has the strongest frost resistance.Considering the above conditions,the optimal content of phase change materials is determined to be 5% m PCMs(5°C)+5% m PCMS(-5°C).When the content of phase change material reaches the optimum,the strength of concrete without freezing and thawing is 33.9MPa.Based on the CT non-destructive scanning test results of concrete under different freezethaw cycle times,it can be seen that the more m PCMs is added,the faster the pore growth rate in the test piece continues to accelerate during the freeze-thaw cycle of concrete until the test piece is destroyed.According to the analysis results,it can be concluded that the pore growth rate with the optimal content is the largest.At this time,the pore growth rate is as high as 242.13%;By comparing the two-dimensional slices of the specimens at different depths,the results show that the freeze-thaw damage of concrete is from the outside to the inside;By comparing the two-dimensional slices of the specimens under different freeze-thaw cycles,the fractal dimension of the specimens gradually increases with the increase of the freezethaw cycles.Based on the fractal theory and damage mechanics theory,a damage constitutive model of concrete under uniaxial compression is established,which uses fractal dimension to represent the homogeneity of materials.The reliability of the model is verified by comparing the experimental results of phase change concrete with the optimal content in freeze-thaw environment.