This study investigates the flexural behavior of encased steel–concrete composite sections incorporating self-compacting concrete (SCC) modified with rubber powder. The primary objective is to assess the structural performance of these composite beams under bending loads, focusing on their mechanical properties and overall efficiency. By integrating recycled rubber powder from waste tyres into SCC, the research aims to enhance material workability, increase ductility, and improve energy absorption while reducing brittleness. Experimental testing was conducted on specimens with varying rubber powder content to evaluate load–deflection response, failure mechanisms, and flexural strength. The findings reveal that rubber-modified SCC enhances flexural performance compared to conventional concrete, with the encased steel section significantly contributing to higher load-bearing capacity and structural resilience. This study underscores the potential of sustainable materials, such as recycled rubber, in promoting durable and environmentally friendly construction practices, encouraging further innovation in composite material applications.