Concrete is widely recognized for its excellent compressive strength but limited tensile resistance, which necessitates reinforcement with high-performance materials for effective structural applications. This study investigates the role of Carbon Fiber-Reinforced Polymer (CFRP) as tensile reinforcement in singly reinforced concrete sections, with emphasis on the contribution of tensile concrete to overall flexural resistance. The elastic behaviour of concrete is first examined, demonstrating stability under low stress levels and progressive deterioration caused by matrix cracking at higher stress states. To capture the structural response, a variable-angle strut model is employed for predicting the load–deflection behaviour of CFRP-reinforced beams subjected to combined flexure and shear. Numerical optimization using Box’s Complex Method is incorporated to refine the stress–strain representation and develop an improved stress diagram that realistically reflects CFRP–concrete interaction. The results highlight that tensile concrete, even after cracking, provides significant resistance through tension stiffening, while CFRP reinforcement remains effective under high load conditions. Furthermore, the optimization process reveals that a neutral axis depth of 0.75d substantially greater than conventional design recommendations, mobilizes nearly 200% additional tensile concrete. This enhanced mobilization improves flexural efficiency and overall load-bearing capacity. The findings of this study provide new insights into the synergistic behaviour of CFRP and concrete, emphasizing that tensile concrete should not be disregarded in design. The proposed framework offers a practical and reliable approach for improving the moment resistance of CFRP-reinforced sections, contributing to safer, more economical, and performance-driven structural design practices.