An Integrated Approach to Assessing Carbon Efficiency in the Selection of Strengthening Schemes for Existing RC Beams Based on Load-Capacity Increments

Existing studies usually evaluate reinforced-concrete (RC) beam strengthening schemes using different specimens and structural indicators, while embodied-carbon emissions are assessed separately. Consequently, a consistent metric for quantifying the environmental cost of each unit of flexural-capacity gain, together with a reproducible procedure for scheme selection, remains lacking. This study develops a member-level framework that uses one representative deficient RC beam as a unified functional unit and integrates nonlinear finite-element analysis, process-based A1–A5 embodied-carbon accounting, and AHP–entropy multicriteria evaluation. UHPC thin-layer, externally bonded Q235 steel-plate, and externally bonded CFRP sheet strengthening were compared under identical geometry, loading, strengthening length, and accounting boundaries. The calculated peak-load increases were 64.4%, 75.1%, and 54.7%, respectively, and the peak-load secant-stiffness increases were 82.6%, 52.4%, and 29.6%. Total A1–A5 emissions were 21.97, 18.83, and 7.59 kg CO2e, while carbon intensities per unit load-capacity increment were 0.89, 0.66, and 0.36 kg CO2e/kN. CFRP also achieved the best normalized unit-cost and construction-duration scores (0.920 and 0.950) and the highest conditional overall score (0.890). A bond-efficiency check increased its carbon-efficiency index to 0.48 kg CO2e/kN without reversing the ranking. The framework provides a transparent screening tool; however, project-specific bond, anchorage, fire, durability, and code-based verification remain mandatory.