Deterioration Mechanism And Life Prediction Of Concrete Under Combined Freeze-thaw Cycle And Sulfate Dry-wet Erosion

The durability of concrete is affected by many factors.In the northwestern part of China,the durability of the concrete subjected to sulfate attack combined with freeze-thaw cycles,dry-wet cycles and so on is seriously threatened.At present,the research on concrete durability mostly focuses on single factor,while composite multi-factor research is still relatively rare.In addition to traditional concrete design generally only considering structural bearing capacity but neglecting environmental factors,a large number of concrete structures has not reached design life causing huge economic losses.Therefore,it is necessary to systematically study the deterioration law and life prediction of concrete under the complex environmental factors.Aiming at the durability problem of concrete in the saline land area of northwest China,this paper simulated freeze-thaw cycle,sulfate attack with dry-wet cycle by the rapid freeze-thaw method and the dry-wet cycle method in the solutions of 5%Na₂SO₄ and carried out the following research based on mortar specimens:（1）Optimization of concrete mix ratio:According to the incorporation of mineral admixture can effectively improve the anti-corrosion performance of concrete,especially the compounding technology.Under the same compressive strength of the test block,20%,40%and 60%fly ash and slag powder were mixed（fly ash:slag powder=1:1）to explore the anti-corrosion properties of different admixture concrete.The results show that the compressive strength of the three kinds of mortars reaches to C50,and the slump of fresh mortar meets the design requirements.（2）Deterioration mechanism of concrete under composite multi-factors:The degradation mechanism of different anti-corrosion concrete was studied by the appearance change,mass loss rate,relative dynamic elastic modulus and compressive strength loss rate under the freeze-thaw cycles,sulfate attack,erosion after freeze-thaw cycles and the alternate test.The results show that the degradation process of specimens under the combined action of freeze-thaw and sulfate attack is not a simple superposition of effects,but promotes each other.The deterioration degree of concrete under alternating test is highest,followed by the erosion after freeze-thaw test,both of which are larger than the single factor degradation effect;With the same compressive strength,the ability to resist freeze-thaw damage and sulfate dry-wet erosion is improved most effectively by mixed 20%fly ash+20%slag powder.（3）Deterioration mechanism of concrete under composite multi-factors:The water kinetics and scanning electron microscopy（SEM）were used to test the pore structure parameters and hydration product of concrete under different environmental conditions.The results show that the freeze-thaw cycles will cause damage to the concrete pore structure,which will make the concrete deteriorate faster and more severely when attacked by sulfate;The admixture can make the pore structure better and improve the corrosion resistance of concrete.Among them,20%fly ash+20%slag powder has the best effect on pore structure improvement;Under the standard curing conditions,concrete structure is dense,reticulated calcium silicate is the main hydration product,calcium hydroxide is basically invisible;After240 times of dry-wet cycle in the solutions of 5%Na₂SO₄,the concrete structure becomes loose,and the type of sulfate attack is mainly ettringite crystal type;The erosion of concrete after freezing-thawing is more serious than the single sulfate attack,and the crack generated by the freeze-thaw cycle will aggravate the degree of sulfate attack.（4）Concrete life prediction and evaluation under composite multi-factors:The concept of concrete material damage degree D was established in this paper,which is used to simulate the possible material deterioration process.The material damage degree data of single freeze-thaw,single sulfate attack and freeze-thaw-erosion composite test were numerically fitted to establish a deterioration model that can simultaneously satisfy different admixture concrete.Based on the model and the interrelationship of the natural environment with indoor accelerated tests,the basic equations for quickly predicting concrete life under complex conditions were compiled,and the durability life of concrete was evaluated.