Masonry arches and vaults represent distinctive structural elements of historical masonry monuments. While in most cases they were conceived and empirically optimised to bear vertical loadings, e.g., in vaulted floors, arch bridges, or triumphal arches, in seismically active regions their response to horizontal loading may represent a significant fragility indicator, as testified by several collapses observed after recent earthquakes. Retrofitting of curved structures is thus crucial to preserve and enhance historical heritage, and as such, it must obey to the principles of compatibility with the ancient materials.

In this paper, retrofitting of masonry circular arches by means of Fibre-Reinforced Lime-Based Mortar (FRLBM) as additional internal or external layer is experimentally and numerically investigated. Reduced-scale yellow tuff arches are subjected to monotonic horizontal loading while subjected to constant vertical loads. The effect of various configurations of retrofitting (intrados/extrados) is assessed and compared to the unreinforced arch response, and the enhancement provided by the increased ductility of mortar with respect to ordinary mortar is clearly shown. Numerical simulations are also developed and validated with the aim of better understanding the relationship between the characteristics of the retrofitting material and the overall increase in seismic strength of the arch.