PSI - Issue 78

Francesco Ascione et al. / Procedia Structural Integrity 78 (2026) 1334–1341

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and hoop strain versus axial stress. Note that the axial strain was evaluated by dividing the measured axial shortening — calculated as the mean value of the vertical LVDT readings — by the corresponding LVDT gauge length. The hoop strain was obtained by averaging the readings from the four horizontal LVDTs and normalising this value by their gauge length. Generally, three distinct stages were identified across all tested columns. At low load levels, both unconfined and confined specimens displayed a quasi-linear elastic behaviour. In this initial stage, the columns remained structurally intact, and the confinement system had not yet been mobilised. As the axial load increased, lateral expansion of the masonry initiated the activation of the TR jacket, resulting in a gradual increase in tensile stress within the composite and a corresponding rise in the lateral confining pressure. After reaching the maximum load-bearing capacity, a progressive reduction of axial stress can be observed until failure.

(a)

(b)

Fig. 2. Experimental results: (a) axial stress vs. axial strain and (b) axial stress vs. hoop strain curves.

As reported in Fig.2.a, it is evident that the stress – strain response of the confined columns is not entirely linear up to the peak load. Rather, this phase is preceded by strain hardening, which distinguishes the response from that of the unconfined specimens. It is also worth mentioning that, due to the passive nature of the confinement, the initial linear branch of the curves showed minimal deviation between confined and unconfined configurations- divergence between the curves becomes apparent only with increasing load. Table 2 reports the key parameters considered in this study, including the peak load ( P u ), the axial strength ( f c ), the axial strain at peak ( ε c ), and the ultimate axial strain ( ε cu , defined as the strain measured when the applied load reduced by 20% from its maximum value). In general, the axial load ranged from a maximum of 814.65 kN for the column wrapped with the commercial S-TR system, to a minimum of 449.15 kN for the unconfined column. The column confined with the “ green ” mortar and basalt fibre system achieved a maximum load approximately 1.3 times greater than that of its unconfined counterpart. In comparison to the load-bearing capacity of the column confined with a commercial B-TR system, a reduction of approximately 12% was observed. This difference is considered acceptable and can reasonably be attributed to the inherent variability associated with these types of systems. When comparing confined S-TR and B-TR systems strengthened columns, it can be observed that those retrofitted with a S-TR system exhibited a significant improvement in load capacity. Specifically, an increase of approximately 80% was achieved using the commercial S-TR system (compared to the unconfined specimen), while the “ green ” S-TR system resulted in a more modest gain of 17%. These outcomes highlight two main points: (i) the notable potential of the commercial S-TR systems to enhance the mechanical performance of clay brick masonry columns; (ii) the limited composite action offered by the “ green ” S-TR configuration proposed in this study (as already observed in the DT tests carried out on the composite). As reported in Fig.2.b, hoop strains became relevant only after the confinement mechanism was activated- during the early loading stages, unconfined columns showed a response almost indistinguishable from confined ones, confirming that the TR jacket was still inactive.