Study On Deterioration Of Concrete Performance Under The Coupling Of Sulfate Attack And Dry-wet Cycle

Sulfate attack on concrete is a complex physical and chemical reaction process,and sulfate attack is the main reason for the concrete to reach the service life in advance.At present,a large number of research results have been achieved at home and abroad,but due to the inconsistency of test systems and evaluation standards,the comparability of research results is poor.Moreover,the damage degree of concrete structures in coastal areas and salty areas is further aggravated due to the double erosion of wet and dry cycles and sulfate.Therefore,it is of great theoretical and practical significance to study the deterioration of concrete performance under the coupling environment of wet and dry cycles and sulfate attack.In this thesis,concrete specimens with different water cement ratios（W/C=0.61,0.50and 0.45）were placed in Na₂SO₄ solution with 5%mass fraction.The durability tests of concrete were carried out under two corrosive environments（sulfate soaking at room temperature and coupling of dry-wet cycling and sulfate）.The total time length of the tests was 120 days.The compressive strength,splitting tensile strength and flexural strength of the concrete blocks were tested every 40 days.The relative values of compressive strength,and splitting tensile strength as well as loss rate of flexural strength were taken as evaluation indexes to discuss and analyze the deterioration of macroscopic mechanical properties of concrete.The results show that:（1）With the increase of erosion period,the mechanical properties of concrete show a trend of rising first and then decreasing,and under the same erosion period,the smaller the water cement ratio,the smaller the amplitude variation of mechanical properties,the better the sulfate resistance of concrete.（2）Dry-wet cycling and sulfate erosion accelerate the damage of concrete,and change the pore structure of concrete to a certain extent.（3）In general,when sulfate attack lasts 40 days,the splitting tensile strength and flexural strength of concrete reach the peak value,while the compressive strength reaches the maximum value after sulfate attack for 80 days.That is to say,the splitting tensile strength and flexural strength of concrete are more sensitive to sulfate attack than the compressive strength.In this thesis,ICT and NMR techniques are used to test changes of the pore structure in concrete under dry-wet cycling and sulfate erosion.Considering the influence of water cement ratio,erosion period and erosion environment on sulfate corrosion of concrete,the damage analysis of concrete is carried out by combining the results of mechanical properties and the characteristics of pore changes:（1）The proportion of small pore in concrete is the largest.With the increase of dry-wet cycles,the number of small pore decreases gradually,while the amount of relatively large increases gradually,showing a trend of pore evolution,and thus the volume of pore in concrete increases gradually.（2）At the initial stage of sulfate attack（0d⁸4d）,with the increase of erosion days,the size distribution of pore fluid in concrete gradually shifts from the small to the large.The larger the water cement ratio,the wider the range of the lateral relaxation time T₂ spectrum of pore fluid,and the stronger the signal intensity corresponding to large pore.（3）Grey relational analysis model is established for the ratio of compressive strength to pore volume of concrete under dry-wet cycling.The ratio of pore volume over 0.5mm³has the greatest influence on compressive strength,and the greater the water-cement ratio,the greater the correlation between them.By studying the deterioration law of concrete performance under the action of wet-dry cycle and sulfate erosion,this thesis provides a basis for predicting the durability of sulfate-eroded concrete,and a theoretical reference for the maintenance and construction of existing concrete structures.