The pyrite heave problem: new insights from trace-element analysis

Over 12 000 houses built in Ireland between 2002 and 2008 are estimated to have sustained structural damage due to swelling initiated by pyrite in crushed rock aggregate beneath floor slabs. The Irish Standard I.S. 398-1 presented a categorization protocol for sub-floor fill material suspected of causing pyritic expansion. While at the extremes, the current risk characterisation protocol is definitive, there is a broad range of conditions where the findings are inconclusive, and the risk assessment requires expert interpretation. In this study, the possibility of using trace-element analysis to better quantify risk from reactive pyrite has been explored. The form and amount of pyrite in sedimentary rock are determined by the depositional environment, which also determined the concentration of trace elements such as molybdenum (Mo) and uranium (U). In this study, a range of high- and low-risk aggregates has been analysed. The results show very good correlation between the empirically derived risk for pyrite expansion categories and molybdenum and uranium enrichments and the ratio between these concentrations. The results to date are very promising and suggest that further research and testing of more samples will help to confirm the basis of the analysis and establish numerical risk thresholds.