Seismic performance of soft-burnt brick masonry with fiber-reinforced mortars: Anisotropic strength and joint behavior

Brick masonry constructed with soft-burnt clay bricks remains widely used in many seismic regions due to its low cost and local availability. However, its structural response is typically governed by low tensile capacity, pronounced strength anisotropy, and brittle failure mechanisms. While extensive research has focused on fiber-reinforced coatings and composite overlays for masonry strengthening, the seismic-relevant influence of discrete fiber-reinforced mortars used within masonry joints and assemblages, especially for weak, high-absorption bricks—remains insufficiently quantified. This study presents an experimental investigation into the anisotropic strength and failure mechanisms of soft-burnt brick masonry constructed with conventional mortar and mortars reinforced with hooked-end steel fibers (1.5% by weight) and polypropylene (PP) fibers (0.3% by weight). Masonry prisms and triplet specimens were tested under compression parallel and perpendicular to bed joints, diagonal compression, and bed-joint shear, following ASTM and EN standards. The experimental program reflects material properties and construction practices representative of masonry commonly used in seismic regions such as Pakistan. The results show that fiber reinforcement modifies both peak strength and governing failure mechanisms. Steel fiber-reinforced masonry exhibited consistent strength gains across all loading configurations (up to 25% in shear) and a clear transition from brittle, localized cracking to distributed cracking with improved post-peak response. Polypropylene fibers produced smaller strength increases but significantly enhanced crack control and reduced failure abruptness. The most pronounced improvements were observed in bed-joint shear, indicating that fiber reinforcement primarily enhances masonry performance through increased joint cohesion and crack-bridging rather than through large gains in compressive strength.