In situ investigation of the mechanical strength of the mortars: The case of the vaults of the Modena Cathedral

The 12th C. Modena Cathedral is a masterpiece testimony of early Romanesque architecture, art and civic values of the time. Following the inclusion in the UNESCO list in 1997, studies, surveys and maintenance works have started. The Emilia's seismic events of May 2012 have happened when the maintenance operations were already in an advanced phase. The quakes caused damage and re-opening of some historical cracks, in particular in the thin vaults of the naves, near the façade and at the junction between naves and choir and crypt. Fragments of brick, mortar from the joints between bricks and even limited portions of a diagonal rib have fallen to the ground. In view of seismic assessment and strengthening for improvement of the structural behaviour, detailed studies of the vaults' mortar became necessary. The investigation approach was minimally invasive, combining on-site close-up visual inspections and micro-destructive testing of the mortar joints by mortar penetrometer. For this purpose, openings of 0.25×0.25 m2 in the render layer of the 23 vaults were created. The outcome has allowed differentiating between repair mortars of different strength, used in different historic periods. The mortar resistance was very low but different for lime mortar and gypsum. Values, divided in 3 classes of strength, were found to correlate well with the location and severity of the crack pattern and damage map in the vaults. The results were useful for a correct design (minimal and reversible) of the cathedral strengthening intervention.