The Roman mortars used in the construction of the Ponte di Augusto (Narni, Italy) – A comprehensive assessment

The ruins of the Ponte di Augusto bridge, built in 27 BC close to Narni, have been investigated in recent years in order to assess the seismic resistance of the bridge. The inner structure of the existing pier was surveyed by means of four vertical core drillings, which enabled an assessment of its structural integrity, and provided material samples of its cast mortar masonry. The article presents results of tests that aim to understand the particular function of the mortars that were used. Alongside the macroscopic observations, the following instrumental methods were applied in a study of the extracted material: a macroscopic description of the mortar, thin section optical microscopy, binder SEM–EDS analysis on broken mortar samples, determining the basic physical characteristics, thermal analyses (TGA) and (DTG), and bending and compressive strength testing according to a special methodology for testing very small non-standard specimens. The measured mechanical characteristics (strength and stiffness) vary in a rather large range of values, which may indicate that the cast mortars were not set with suitable technological care. Although it cannot be stated unequivocally that the different compositions of the Ponte di Augusto pier mortars were designed intentionally to take into account changes in the load carrying capacity, from the point of view of the statics of the structure, the strongest mortar was used logically and it was probably also intentionally designed. This strongly interdisciplinary study contributes substantially to our understanding of the work of ancient Roman engineers.