Effects of the expansion mechanisms on the pyrrhotite-induced deterioration of concrete foundations

Using pyrrhotite containing aggregates in concrete foundations can lead to premature deterioration visibly identifiable by extensive map cracking, expansion and deformation of the concrete foundation. The two main expansion mechanisms are: (1) aggregate expansion induced directly by the oxidation of pyrrhotite inclusions and (2) matrix expansion due to the consequent internal sulfate attack (ISA). The relative contribution of these two mechanisms of expansion were investigated in this study. Theoretical calculations based on the practical combinations of oxidation products and degrees of oxidation indicated that aggregate expansion alone may not be sufficient to cause severe deterioration of the concrete incorporating pyrrhotite-bearing aggregate. Observations and this investigation show that this mechanism leads to microcracking which initiates the deterioration process. The combination of microcracking and the release of sulfate facilitates and intensifies the local ISA. The strain derived from the ISA is substantially higher than the typical ultimate tensile strain of a typical matrix used on foundation walls, thus adding additional mechanical stress on the material, and allowing the development of the typical map cracking throughout the concrete.