Confinement Mechanism of Basalt TRM-Confined Concrete: The Role of the Mortar Matrix

Textile-reinforced mortar (TRM) confinement enhances the compressive strength and ultimate axial strain of concrete. This research investigated the confinement mechanism of TRM, focusing on the role of the mortar matrix. Forty-eight compression tests were conducted on concrete columns, comparing those without jackets to columns encased in basalt TRM (BTRM). The variables included the number of textile layers (ranging from 0 to 4) and mortar matrix strengths (from Low-grade M1 to High-grade M3). Low-grade mortar was found to reduce the effective confinement stiffness of the BTRM jackets, evident from the less steep strain-hardening phase. Additionally, increasing mortar strength corresponded to a decrease in both the hoop rupture strain of the BTRM jackets and the ultimate axial strain of the confined concrete. This study extends the existing analysis-oriented stress–strain model for fiber-reinforced polymer-confined concrete to incorporate the identified confinement mechanism of BTRM. The adaptation focuses on the influence of mortar, introducing a coefficient km to quantify its impact on the confinement stiffness in predicting the stress–strain behavior of BTRM-confined concrete. The model inherently incorporates the effect of mortar grade on the hoop rupture strain using experimentally determined values as the modeling endpoint. Predictions were validated against new test data on ultimate axial strain and axial stress–strain curves, demonstrating satisfactory agreement.