Damages to residential buildings related to pyritic rockfills: field results of an investigation on the south shore of Montreal, Quebec, Canada

Abstract

Numerous cases of garage and basement concrete slab distress at various levels of heaving and deterioration were noticed early after construction in the Montreal south-shore area (Canada). The use of clayey sedimentary rock containing pyrite as aggregates for granular subbase is responsible for most of the observed distress. The heaving of the slabs is related to the oxidation process of the pyrite, followed by sulphate crystallization causing the swelling of the compacted granular subbase. A first inspection survey was carried out in the summer of 1999 in an attempt to assess the extent and magnitude of the damage caused by pyritic rockfills. More than 200 houses showing distress associated with pyrite were investigated in three localities. Damage of the garage floor slab seems to occur earlier after construction and to be more severe likely due to the subbase thickness and to the lower quality of the rockfill. Five houses at various stages of reaction process were monitored. The results suggest that the rockfill swelling is quite fast and continuous. The heaving rate of some garage slabs might be as high as 1.2 mm/month.Key words: pyrite, shales, swelling rockfill, oxidation, sulphate attack, damage.