Durability Research And Life Prediction Of Hydraulic Concrete Under Freeze-Thaw And Sulfate Compound Action

With the current development needs of our country,under the implementation of the western development and the Belt and Road policy,our project construction is moving towards cold and high cold areas.At the same time,due to the nature of the land in these alpine areas,there are a large number of saline land,many soil and groundwater widely distributed sulfate ions.And because the western region is located in the hinterland of the mainland,the temperature varies greatly throughout the year.Due to the superimposed deterioration of salt erosion and freezethaw cycle factors,the failure and failure of concrete buildings serving in this environment are very rapid.It is of great significance to reveal the damage deterioration and failure mechanism of concrete structures in view of the concrete components in service under such complex climate and harsh environment.Therefore,this paper takes concrete with different fly ash content（0,10%,20%,30%）as the research object,and designs freeze-thaw tests of clean water and different sulfate concentrations（2%,5%,8%）,mainly by analyzing macroscopic mechanical indexes such as appearance,quality,compressive strength,anti-splitting strength and dynamic elastic modulus.The influence of different fly ash content and sulfate concentration on concrete durability was explored.The main research results are as follows:（1）When concrete is subjected to freeze-thaw damage,its appearance changes mainly from the initial surface becomes rough,to the surface cracks and mortar shedding,to the large area of skin falling off and part of aggregate exposed.Compared with the concrete under freeze-thaw in a single clear water,the appearance deterioration rate of concrete under salt-freeze environment is more rapid and intense.（2）The mechanical properties of concrete with different fly ash contents（0,1 0%,20%,30%）,such as compressive strength,splitting strength and dynamic elastic modulus,were tested through the indoor fast freeze-thaw cycle test of clean water.The results show that:When fly ash is added,the mechanical properties such as initial compressive strength,split strength and dynamic elastic modulus of concrete will decrease.10%fly ash content is close to that of ordinary concrete,and when it reaches 150 cycles,the relative mass loss rate,compressive strength and relative dynamic elastic modulus are 3.43%,82.43%and 72.67%,respectively.However,the mechanical properties of concrete decrease greatly at 20%and 30%fly ash content.（3）The concrete subjected to freeze-thaw cycle in Na₂SO₄ solution is studied.The results show that:at the beginning of freeze-thaw cycle,the mechanical indexes such as compressive strength,anti-splitting strength and dynamic elastic modulus decline at a relatively slow rate.With the increase of freeze-thaw cycle time,the decline rate becomes faster and faster,and the damage rate increases with the increase of fly ash content.The durability of concrete is negatively correlated with fly ash content and Na₂SO₄ solution concentration.After 150 freeze-thaw cycles,the mechanical properties of the concrete with 10%fly ash content are the best,and the mass loss rate,compressive strength and relative dynamic elastic modulus are 3.99%,79.70%and 67.01%,respectively.（4）The damage degree was established by using the compressive strength as the index,and the deterioration degree of concrete was compared under three working conditions,water freezethaw,sulfate dry-wet cycle and sulfate solution freeze-thaw cycle.It was found that before 125 cycles,the damage degree of concrete under the combined action of salt and freezing was greater than the sum of the two single factors,and at 125 cycles,The damage degree of the four kinds of concrete under freeze-thaw in clear water is 0.51,0.57,0.63,0.69,respectively.The compressive strength of the four kinds of concrete after dry-wet sulfate cycle is 0.19,0.18,0.16,0.31,respectively.The damage degree of the four kinds of concrete under salt-freeze combined action is 0.60,0.68,0.69,0.82,respectively.At this time,the damage degree under the combined action of salt and freezing is greater than that under the action of two single factors,but less than the sum of the two values.This indicates that the damage effect of salt-freeze combination on concrete at the early stage of freeze-thaw cycle is far greater than the numerical superposition of the damage effect of single factor,which is a more rapid damage to concrete.In the later stage of freeze-thaw cycle,although the damage speed decreases slightly,it is still greater than the deterioration rate of concrete under the action of single factor.（5）Based on Weibull model,a life prediction model was established under the combination of clean water and salt-freeze with three indexes,including mass,compressive strength and dynamic elastic modulus.It was found that the life of concrete with 10%fly ash content was the best under the freeze-thaw cycle of clean water and Na₂SO₄ solution.In clear water,the life expectancy based on mass,compressive strength and dynamic elastic modulus is 227 times,238 times and 237 times,and in 5%Na₂SO₄ solution,the life expectancy based on mass,compressive strength and dynamic elastic modulus is 186 times,171 times and 175 times.