Micro-Macro Degradation Regularity Of Sulfate Attack On Concrete Under Complex Environments

Degradation of concrete exposed to sulfate salts is the result of sulfate ions transport through the pore system, chemical reaction with the hydration product phases present or crystallization, generation of stress due to the creation of the expansive reaction products, and the mechanical response (typically cracking, spalling, strength loss, et al.) of the bulk material due to these stresses. The degradation is a very complex micro-macro evolution process. According to field observations, degradation of concrete is more serious under wet-dry cycling than under continuous immersing. But many laboratory researches under wet-dry cycling often accelerate drying by rising temperature under the dry cycling. When doing so, on the one hand, it is hard to simulate the field-like conditions, and on the other hand, rising temperature will change the attack mechanisms of reaction products. In this paper, experimental study on deterioration mechanisms of sulfate attack on concrete under wet-dry cycling was conducted, in which the specimens were naturally drying in the air under the dry cycling. Based on this, the effect of load was further considered. And the compound environments action cases were also studied, mainly including the alternate action of repeat sub-high temperature/cooling by water and sodium sulfate solution attack, and the alternate action of carbonation and sodium sulfate solution attack.By micro-analysis and macro-analysis in the tests, the main results were achieved as follows:(1) The attack products were analyzed by DSC method, combined with SEM and XRD techniques. Under alternate action of wet-dry cycling, concretes are attacked mainly by ettringite during the wet cycling, and Na2SO4 and Na2SO4 10H2O crystallization damage, induced by evaporation, is superposed during the dry cycling. And the Na2SO4 crystallization damage has a dominant control during the dry cycling.(2) The sulfate-ion content profiles in the concrete were determined by the modified barium sulfate gravimetric method (chemical titration). The effects of W/C, concentrations of attack solution, loading levels, action of the repeat sub-high temperature/cooling by water and action of the carbonation on the sulfate ions transport properties were considered respectively.(3) The macro-mechanical behavior changes including compressive strength, splitting strength and flexural strength were determined. The compressive failure of attacked concretes shows the mode of layered scalling. The splitting strength appears to be more susceptive to the damage resulted from sulfate attack. And after the 40% ultimate flexural load, the effect of increased load on the flexural strength is remarkable.Based on sulfate attack features on concrete during the wet cycling and the dry cycling respectively, two modelling studies were conducted as follows:(1) Based on the diffusion-reaction problem with a moving boundary, the micro-macro damage evolution model of sulfate attack on concrete was constructed. On the micro-level, the simulation on the sulfate ions transport properties in the cracking concrete was resolved, and on the macro-level, the simulation on the compressive strength degradation was realized. The contrast between the model calculations and the tests showed the simulations on the micro-macro levels are reasonable.(2) Combined with some theoretical models present, the sodium sulfate crystallization attack evolution process, induced by evaporation action during the dry cycling, was analyzed. The crystallization stresses generated by Na2SO4 and Na2SO4 10H2O crystals growth were calculated, and the tensile hoop stresses induced by the crystallization stresses were also calculated.