Laboratory and field evaluations of external sulfate attack
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External sulfate attack is a complex process that can result in concrete structures deteriorating prematurely. This dissertation describes a comprehensive evaluation of factors influencing external sulfate attack and includes both laboratory and field investigations. Significant emphasis was placed on evaluating the sulfate resistance of mortar and concrete containing high-calcium fly ash (Class C fly ash as per ASTM C 618). This investigation showed that these fly ashes generally reduced the sulfate resistance of mortar and concrete, but that sulfate resistance was possible through the incorporation of other supplementary cementing materials (silica fume and ultra fine fly ash) into these mixtures. Another key area of emphasis in this project was the correlation between accelerated laboratory tests and outdoor exposure site performance. The behavior of concrete exposed to various sulfate salts (sodium, magnesium, and calcium) was evaluated, both in static immersion tests and in outdoor sulfate trenches. It was found that the distress outdoors was exacerbated by physical sulfate attack, especially when concrete was exposed to sodium sulfate. It was found that lowering the water-tocementitious ratio (w/cm) and incorporating appropriate dosages of suitable supplementary cementing materials improved resistance to both chemical and physical forms of sulfate attack. Lastly, a comprehensive study of bridges throughout Texas discovered concrete structures suffering from possible external sulfate attack. The use of analytical techniques such as x-ray diffraction (XRD) and scanning electron microscopy (SEM) were used throughout the study to determine the presence of hydration products that may form with mortars and concrete specimens. Findings from this research will be implemented in specifications in Texas for the use of Class C fly ash use in concrete in areas that may have sulfate soils and groundwater.