Comparative study of orthogonal and diagonal NSM-TRM strengthening using GFRP meshes for flat slabs

Abstract Reinforced concrete (RC) flat slabs are widely used in modern structures due to their architectural flexibility, but they often require strengthening to enhance their flexural performance and service life. This study investigates the structural behavior of RC flat slabs externally strengthened using near-surface-mounted textile-reinforced mortar (NSM-TRM) systems incorporating glass fiber-reinforced polymer (GFRP) meshes. The experimental program included seven slab specimens: one unstrengthened control and six strengthened slabs with varying GFRP layer numbers, reinforcement orientations (orthogonal or diagonal), and installation methods (epoxy bonding or mechanical anchorage). The results revealed that both orientations substantially enhanced flexural performance relative to the unstrengthened slab, with the orthogonally strengthened specimen achieving the highest ultimate load increase (24%) among all tested configurations. This behavioral advantage is attributed to the alignment of orthogonal GFRP strips perpendicular to the dominant tensile crack directions emanating from the slab center, which maximizes crack-bridging efficiency and stress transfer capacity. A three-dimensional finite element model (FEM) developed in ABAQUS was validated against experimental results with coefficients of variation not exceeding 5.86%. The findings confirm that reinforcement orientation, number of GFRP layers, and anchorage method are the key factors governing the flexural performance of GFRP-based NSM-TRM systems for flat slab strengthening, as collectively quantified through ultimate load capacity, post-cracking stiffness, and energy absorption.