Confinement of rubble stone masonry columns with steel-FRCM jackets: Experimental evidence and analytical validation

The structural assessment and retrofitting of irregular stone masonry structures, frequently found in historic urban centres, remain particularly challenging due to its intrinsic heterogeneity and vulnerability. Although Fabric-Reinforced Cementitious Matrix (FRCM) systems were widely investigated for brick and squared-stone masonry, only limited evidence is available for rubble stone masonry. The present study addressed this gap by presenting both an experimental programme and an analytical evaluation of the compressive behaviour of rubble stone masonry columns confined with stainless-steel FRCM jackets. Nine square-section columns (250 × 250 × 700 mm) were tested under axial compression: three were left unconfined as reference specimens and six were confined with one or two continuous reinforcement layers. The results showed that FRCM confinement substantially increased both the peak load capacity (up to ∼27%) and ductility (over twofold), while confirming the non-linear efficiency of confinement, as the second reinforcement layer provided a smaller strength increment but a clear gain in deformability. Failure modes were governed by progressive multi-cracking and edge damage of the jackets, whereas the masonry core determined the ultimate capacity. Additionally, experimental results were benchmarked against existing confinement models and a new analytical formulation developed in this study. The proposed model, explicitly accounting for the frictional interaction between stone and mortar, provided close agreement with the tests, predicting the confined strength within ±10%. • Steel-FRCM jackets markedly enhanced strength and ductility of rubble stone columns. • Double-layer confinement improved deformability more than peak compressive strength. • Failure was governed by masonry-core degradation rather than jacket rupture. • Existing models underestimated confined strength when applied to rubble stone masonry. • A friction-based model predicted confined strength within ±10% of experiments.