Fracture and Pull-Out Behaviour of Basalt Composite Fibres in OSA-Based Cementitious Matrix Assessed by DCB Testing Approach

Abstract The fracture and pull-out behaviour of vertically aligned basalt composite fibres embedded in an oil shale ash (OSA)-based cementitious matrix was investigated using the double cantilever beam (DCB) test. OSA replaced cement at 0 %, 10 %, 15 %, and 35 % to reduce carbon emissions and improve the mechanical properties of fibre-reinforced concrete. The basalt fibres were oriented vertically, perpendicular to the fracture plane, and aligned with the loading direction to facilitate accurate assessment of the pull-out mechanisms during crack initiation and propagation. The DCB test involved two notched concrete beams joined by a thin fibre-reinforced layer, which enabled controlled crack opening. Specimens with varying OSA content were evaluated for peak load, fracture energy, interfacial bond strength, and fibre pull-out. The results indicated that vertical fibre alignment enhanced load transfer and inter-facial resistance, resulting in higher pull-out forces and improved crack-bridging compared to random fibre placement. Incorporating a moderate amount of OSA improved fracture performance by strengthening the matrix–fibre interface and promoting more ductile failure. Specifically, 10–15 % OSA produced notable improvements in fracture resistance and fibre–matrix bonding, shifting the failure mode from brittle, matrix-dominated to a more ductile, pull-out–controlled process. On the contrary, 35 % of OSA reduced the strength of the interfacial bond due to matrix dilution. Force–displacement curves demonstrated that optimally modified mixtures dissipated more energy and delayed crack propagation. Post-test examination of fibres and force–displacement data confirmed a transition from brittle fracture to gradual pull-out, primarily attributed to enhanced fibre–matrix adhesion. In general, OSA-modified matrices with vertically aligned basalt fibres demonstrated significant potential for developing durable, high-strength, and crack-resistant cementitious composites.