(CS1.1) Notre-Dame Basilica of Montreal - Repair of the towers and bell towers

Over the past forty years, the basilica and its bell towers have undergone several restoration projects, particularly to counter the problem of cracking limestone. Fifteen years separate the construction of the basilica, completed in 1829, from that of the two towers, completed in 1843. The origin of the stones used in these two stages of the basilica's construction differed. As far back as 1929, Abbé Olivier Maurault commented on the inferior quality of the stone used in the towers. We were able to confirm this during our inspections of the towers. From a certain level upwards, a large number of stones are laid off-course, with a significant number of micro-cracks and fractures. The problem was undoubtedly noted during conservation work in the 1980s, when epoxy was injected into the stones, and then removed in the early 2000s by bush-hammering the sealed cracks. Careful inspection revealed that the 180-year-old stones in the towers had reached a level of degradation that necessitated their replacement at great heights. In addition, the severe environmental conditions to which the towers are exposed have accentuated the erosion and damage caused by water infiltration into the cracks and mortar joints. Restoration work on the West Tower, including the replacement of a large number of stones, revealed increased fragility and a network of micro-cracks within the stones. Inspection of the east tower revealed even greater deterioration. Hypotheses were put forward as to the effect of structural movements emanating from wind pressure or bells. The use of a mortar more resistant than stone was also considered to be the cause. Samples were analyzed and resonance tests conducted. The mathematical model of the facade and towers has enabled us to assess their structural behaviour, without however being able to identify an explanation for the damage to the masonry. At the present stage of our investigations, we have identified multiple factors, possibly including the composition of the limestone in the towers, which contains a high level of allochems (foreign and fossil elements within a rock) that could affect the structural behaviour of the stone and its hardness. The presentation will highlight how the contribution of in situ and on-site observations, as well as theoretical and laboratory analyses, have enabled us to develop a plausible hypothesis of the causes of the deterioration specific to the basilica's two towers.