Concrete Durability Under Combined Action Of Sulfate Attack And Dry/wet Cycling

Interior and exterior scholars have proceeded a lot of research on the durability ofconcrete suffering from sulfate attack which is a very important aspect in the field ofconcrete durability. However because of differences in experimental methods andcriterions there is a great discrepancy in their research findings, so the study on thedurability of concrete suffering from sulfate attack can be described as totally a mass.The damage is more serious when the concrete is under combined action of sulfateattack and dry/wet cycling compared with that under a single factor. Actually theconcrete is all on multi-factor effects, so it will be more meaningful to proceed aresearch on concrete under combined action of sulfate attack and dry/wet cycling.The experiment method is summarized as follows: the specimens were immersedin Na2SO4solution for7days, and then dried in the air for8days. The experimentlasted for360days. In the course of the experiment the changes of mass, dynamicelastic modulus, compressive strength, damage layer thickness and transmitting speedare detected regularly. The generation of corrosion products is observed by scanningelectron microscope. In the basis of analysis of these results, a damage evolutionequation based on Ed is developed.According to the experiment, the corrosion processes of concrete under combinedaction of sulfate attack and dry/wet cycling is divided into four stages. First stage:Expansion stress caused by sulfate crystallization takes on a bigger role in this stage forthe chemical attack proceeds slower relatively. The expansion stress causes crack inconcrete leading to a decline in density and dynamic elastic modulus drops downcorrespondingly. The compressive strength grows slowly with the hydration of concrete.In the second stage, the corrosion product grows more and more with the progress of chemical attack, which fills in the cracks in concrete partially and the decline of densityis thus delayed. In this stage the mass and compressive strength drops downsimultaneously, but dynamic elastic modulus declined far slowly than the speed inprevious stage. In the third stage, with the aggravation of chemical attack andcrystallization expansion effect, the performance indexes of concrete including mass,dynamic elastic modulus and compressive strength all drops down. The last stage isaccelerated degradation stage, during which surface layer of specimen spalled partially,and some cracks appear in the edge. In this stage the performance indexes of concreteall drops down sharply.Furthermore, the damage of the concrete is analyzed from damage layer thicknessperspective in the5thchapter. The speed of ultrasonic propagation through the damagedlayer was detected according which the damage layer thickness can be calculated. Thevariation tendency of damage layer thickness is similar to that of macroscopicexperimental results.