The present paper investigates the flexural performance of reinforced concrete (RC) slabs incorporating waste crumb rubber (CR) and glass powder (GP). Seven concrete mixes were created with varying CR as a replacement for fine aggregate (0%, 10%, and 20% by volume), and GP as a cement substitute (0%, 10%, and 20% by weight). Steel fibers were added in order to enhance ductility and mitigate brittleness. Seven simply supported one-way RC slabs (2200 × 800 × 100 mm) were tested under two-line loads, using a mix designed for a strength of 40.3 MPa. The experimental results showed that substituting fine aggregates with 10% CR and cement with 10% or 20% GP leads to satisfactory slab performance. The 10% CR replacement emerged as optimal. Additionally, steel fibers significantly improved the performance, stiffness, and toughness of the slabs containing CR and GP. The lab analysis included cracking and ultimate loads, deflections, crack patterns, load-deflection relationships, stiffness, ductility, and toughness. At a 20% crumb rubber (CR) replacement, splitting and compressive strengths dropped by 16.2% and 12.2%, respectively, compared to normal concrete. Nonetheless, both 10% and 20% CR replacements maintained acceptable performance in reinforced concrete (RC) slabs, with 10% being the most effective. Similarly, glass powder (GP) substitution at 10% and 20% levels yielded satisfactory results, with 10% GP showing an 8.8% increase in failure loads. In contrast, 20% GP resulted in minor reductions in failure and crack loads by 14.7% and 5.9%.