Researches On Damage Evolution Process In Concrete Under Sulfate Attack/Temperature Variation

Concrete is a kind of structural material, and it has been widely used in a lot of importantengineering. The damage evolution may take place due to the action of external loads, temperaturevariation, as well as the chemical action induced by the attack of sulfate and chloride ions. In thisstudy, close attention is paid to the damage evolution in concrete due to sulfate attack and variabletemperature, respectively.On the first part of this thesis, nucleation mechanism of micro-damage of concrete materialunder the action of variable temperature is studied. Concrete materials consist of two components,i.e., cement mortar and aggregate. Generally, coefficient of thermal expansion of cement mortar isgreater than that of aggregate, therefore, the thermal mismatch directly causes the thermal stresseven under the variable temperature. Deformation of the aggregate and the stress distribution at theinterface between cement mortar and aggregates is analyzed by using Eshelby’s equivalentinclusion method, and two kinds of formation mechanism of micro-crack is found. Under heatingcondition, nucleation mechanism of micro-crack is caused by the thermal stress at the interface,such the thermal stress is tensile stress, and the expansion of micro-crack leads to the interfacialdebonding. Under cooling condition, nucleation of micro-crack takes place in cement mortar at theinterface and the direction of micro-crack is perpendicular to the interface. Experiments under thethermal impact are carried out, and the results indicate that the theoretical prediction on thenucleation of micro-crack is correct.On the second part, three aspects of sulfate attack are studied. The surface hardness evolutionof concrete; the damage evolution of concrete and the evolution of mass; and then the expansion ofsurface cracks of concrete under surface crack. Two stages of the surface hardness evolution arefound based on the experimental results. The surface hardness increases in the first stage anddecreases in the second stage. A new model, i.e.,-potential function and its derivative function isproposed to describe the attenuation of surface hardness. First-order reaction rate equation isproposed to describe the mass evolution based on the four groups of sulfate attack experimentalresults. And equation which obeys Weibull’s distribution is suggested to describe the damage ofconcrete based on the variation of dynamic modulus of concrete. In the third aspect, Crackexpansion tests need long-term observation under the sulfate attack. At the same time, microscope is used to capture the growth picture in fixed area, a simple model is suggested to describe theexpansion of crack under sulfate attack.