Study On Salt-freezing Durability Of Concrete After Pre-loading

The environmental water in saline area has high salinity and freeze-thaw cycle damage conditions,which directly affect the durability life of concrete structures in this geological environment.In the research of concrete durability,environmental factors have become more and more worthy of attention.At the same time,concrete often works under the condition of holding load(with certain micro cracks),but the current research has not paid much attention to this factor.In this paper,considering the environmental factors and the working state of concrete,according to the constitutive relationship of concrete,four kinds of horizontal compressive stress specimens are designed in advance to study the salt frost resistance durability of concrete under the combined action of load,freeze-thaw and salt corrosion,and explore its influence on the salt frost resistance of concrete.The main work is as follows:(1)The concrete specimens with three strength grades of C20,C40 and C50 were prepared,and the preloading system was designed.The compressive stress of 0%f_c,25%f_c,50%f_c and75%f_c was applied respectively to simulate the pore structure of concrete in the real working state.The change of internal pore structure was verified by water absorption test and SEM test.The results show that the designed loading system is effective,and the pore structure changes in different degrees.(2)The salt corrosion test of concrete in freeze-thaw was carried out,and the mass loss rate and relative dynamic elastic modulus of concrete in different strength grades under different salt solution corrosion conditions were obtained.The results show that when the compressive stress level is between 0%and 50%,the higher the compressive stress level is,the smaller the mass loss rate is,the better the salt frost resistance of concrete is,and the quality improvement rate is more than 5%;when the compressive stress level is between 50% and 80%,the higher the compressive stress level is,the greater the mass loss rate is,and the worse the salt frost resistance is.At the same time,the mass loss rate in sodium chloride solution is more than 5%higher than that in sodium sulfate solution.(3)The influences of freeze-thaw cycles,stress level,strength grade and salt solution type on the salt frost resistance of concrete are analyzed.The results show that the salt frost resistance of concrete decreases with the increase of freeze-thaw cycles;compared with the specimens with other stress levels,the compressive stress of 50%FC can effectively improve the salt frost resistance of concrete;the higher the strength grade,the better the salt frost resistance;the effect of chloride on the salt frost resistance of concrete is greater than that of sulfate.(4)The regression equations between concrete mass loss rate,relative dynamic elastic modulus and stress level,freeze-thaw cycle times and strength grade are established respectively,and the relationship between three factors and salt frost resistance is obtained.Through range analysis,the influence degree from large to small is freeze-thaw cycle times,stress level and strength grade.(5)Based on the test data,a BP neural network prediction model with four input values and two output values is established for the salt frost resistance of concrete with pre applied compressive stress.The results show that,through the model accuracy and gradient analysis,it is found that the fitting degree of the model is high,and the error relationship between the experimental value and the predicted value is found to be between 0%and 1%.The validity and feasibility of the model are determined,which can be used to realize the prediction of concrete salt frost resistance durability.