External sulfate attack of concrete : an accelerated test method, mechanisms, and mitigation techniques
Sulfate attack of concrete is perhaps the least understood of the major durability mechanisms plaguing reinforced concrete infrastructures. Many studies have attempted to better understand the underlying mechanisms in which the various modes of deterioration by sulfate attack manifest; however, several controversies still exist. Moreover, ASTM C 1012 (2012), which is the most commonly referred standardized laboratory test method to determine sulfate resistance of blended portland cement mixtures, does not always link well to field performance and may take up to 18 months to complete. The research program presented in this dissertation investigates various issues pertaining to the mechanisms, testing methods, and factors influencing external sulfate attack. The primary focus of this research study was to investigate and propose a reliable, and innovative accelerated test method to evaluate the sulfate performance using concrete specimens. The research program was divided into the following four key components: (1) design a method that can obtain results within a reasonable timeframe (less than six months); (2) design a method that uses concrete specimens and thus links more closely to field performance; (3) develop a better understanding on the role and mechanisms of sulfate attack on concrete through a comprehensive research program including field and laboratory investigations; and (4) investigate the use of calcium sulfate (gypsum) used as an additive to mitigate the potential of sulfate attack in blended portland cement mixtures using high-calcium fly ash. The findings in this dissertation led to the development of a potential accelerated test method for determining sulfate resistance by vacuum impregnating concrete (or mortar) samples with sulfate solution to accelerate the ingress and onset of chemical reactions between the hydrated cement paste and sulfate ion (SO42-). The effects of binder type, water-to-cementitious ratio (w/cm), curing regime, sulfate type and concentration are examined. In comparison to the conventional ASTM C 1012 method, results showed a higher rate of expansion with significant distress observed in samples subjected to the accelerated test method and placed in a 5% Na2SO4 solution. Similar trends, but at a relatively lower expansion rate, were also observed in samples placed in a 0.89% Na2SO4 solution. Physical measurements, chemical analysis and microstructural studies were performed periodically on the specimens.