UPDATE ON THE DETERIORATION OF CONCRETE BLOCKS INCORPORATING SULFIDE-BEARING AGGREGATES

In the Trois-Rivières area (Québec, Canada), several hundred foundations of single-family homes, apartments and commercial buildings were built using concrete incorporating an anorthositic gabbro coarse aggregate containing sulfide minerals including pyrite and pyrrhotite. A large number of those concrete structures developed serious deterioration soon after their construction, which triggered, in many cases, their demolition and foundation replacement. The cause of the deterioration is due to the oxidation of sulfides which gave rise to an internal sulfate attack of the concrete.

Concrete blocks measuring 1 m x 1 m x 0.225 mm were extracted from the foundations of the housing just before their demolition. The blocks were placed on an outdoor exhibition site in 2010. The blocks are monitored regularly to measure volumetric changes to determine expansion rates. The evolution of cracking is also monitored. These measurements help to understand the deterioration mechanisms and conditions most prone to the development of premature damage, as well as to quantify the rate of deterioration. In parallel, more than three hundred cores were extracted at different depths in eight housing foundations. A variety of tests, including in situ relative humidity measurements, petrographic, physical, mechanical and chemical tests on these cores were performed for a complete assessment of the condition of the concrete.

A good correlation was observed between the monitoring of crack and the development of expansion in concrete foundation walls of different initial damage degree. Deterioration was also found to be largely influenced by the exposure conditions and RH conditions within the concrete. Deterioration measured increased dramatically over the 10-year follow-up period. Some excessively weakened blocks broke while the site was reconditioned.

Keywords: Pyrrhotite, Sulfide-Bearing Aggregate, Oxidation, Expansion, Cracking, Relative Humidity.