As global construction solid wastes surged and to achieve the carbon peak and carbon neutrality goals, the application of fully recycled coarse aggregate concrete (FRCAC, 100% replacement rate) in structural engineering has emerged as a breakthrough solution to addressing resource-environmental constraints and reshaping low-carbon structural systems. However, current codes and standards lack long-term empirical data under actual service conditions, constraining technological updates and large-scale application. This study aims to break technological bottlenecks, drive code upgrades, and promote low-carbon application. It explores the service performance of FRCAC structures using a 30-m-span simply supported beam as the engineering prototype. Under self-weight loading, the maximum compressive stress at the edge of the compression zone of FRCAC is approximately 50% of its axial compressive strength, simulating the stress state under actual loading conditions, thus enabling quantitative analysis of performance evolution mechanisms and low-carbon benefits under long-term coupled effects of mechanical loading and environmental exposure. Through designing comparative beams with equivalent reinforcement and water-to-binder ratios between FRCAC and conventional concrete, we established a comprehensive framework covering material preparation, component behaviors, and carbon benefit quantification. We tracked deformations, crack patterns, and carbonation depth evolution over 4-year service periods, and quantified carbon absorption effects using life cycle assessment models. Results demonstrate that although FRCAC exhibits 7.8%~14% reduced elastic modulus under the same-condition curing, pre-cambering completely compensates deformation discrepancies, satisfying structural requirements. While bending cracks increased by 8% and the crack length increased by 15%, the average width remained comparable to conventional concrete. Considering service-period carbon absorption, FRCAC beams achieved 7.69% reduction in net carbon emissions. This study pioneers engineering prototype validation of FRCAC's feasibility under actual load-environment coupling conditions. The findings are expected to advance the transformation of construction wastes from extensive landfilling to high-value utilization, providing a forward-looking solution for low-carbon structural engineering.