Abstract
This study investigates the use of recycled steel fibers (RSF) derived from waste tires as reinforcement in cementitious mortar composites, assessing both mechanical performance and internal structural integrity. Four mortar mixtures containing 0%, 0.5%, 1.0% and 1.5% RSF by volume were prepared and tested at 28 days using compressive and flexural strength testing alongside a suite of ultrasonic diagnostic methods including ultrasonic pulse velocity (UPV), the nonlinear parameter β, the sideband peak count index (SPC-I) and the spectral dissipation index (SDI). UPV measurements remained largely unchanged across all mixtures at 3200 to 3400 m/s revealing its limited sensitivity to fiber-induced microstructural changes. In contrast the nonlinear parameter β and hybrid parameter SDI both exhibited a distinct U-shaped trend with minimum values recorded at 1.0% RSF signaling optimal fiber distribution and matrix integrity at that dosage. At 1.5% RSF both parameters increased, suggesting the onset of fiber clustering and microstructural deterioration. SPC-I showed a moderate correlation with compressive strength while its minimum at 0.5% RSF reflects defect-dominated behavior at low fiber content rather than an optimal dosage. Statistical analysis confirmed there was no significant difference in compressive strength between the 1.0% and 1.5% dosages with a p-value of 0.51. The convergence of near-maximum mechanical strength and minimum nonlinear ultrasonic response at 1.0% RSF collectively identifies this as the optimal dosage. One methodological consideration worth noting is that different material batches from the same supplier were used for each dosage level to reflect real-world commercial variability. This introduces a batch-dosage confounding effect that may limit the broader generalizability of the findings. Nevertheless, convergent evidence from multiple independent measurement techniques supports the conclusion that dosage is the primary influencing factor within the tested material range.