Concrete Deterioration Mechanisms under Combined Sulfate Attack and Flexural Loading

The deterioration mechanisms of sulfate attack on concrete under sustained loading and wet-dry cycling were investigated based on micro and macroobservations. The mass fraction of sodium sulfate solution was 6.9%. Three loading levels of 20, 40, and 60% of ultimate flexural load were considered, and the load was mechanically applied to the specimens under four-point bending. Microobservations included the analysis of the chemical products formed using thermal analysis and the determination of the sulfate-ion content profile using the modified barium sulfate gravimetric method (chemical titration). Macroobservations primarily included visual observations and flexural strength changes. Test results showed that under alternate action of wet-dry cycling, concretes are attacked by expansive products such as ettringite and gypsum during the wetting cycle, and crystallization damage, induced by evaporation, is superposed during the drying cycle. Results also showed that the tensile stresses can increase diffusivity by initiating or developing microcracks; the compressive stresses are small compared with the concrete compressive strength, so any effect on ion transport properties is not obvious. Under simultaneous sulfate attack and flexural loading, deterioration is aggravated as the loading level increases, and this is characterized on the macroscale by the increased strength degradation. This research can provide some references for the assessment method of concrete structures under combined sulfate attack and loading action.