Efficiency of Strength Enhancement in Strengthened Singly Reinforced Concrete Sections: A Strain-Based Strength–Ductility Assessment

This study examines how different strengthening modes influence the interaction between flexural resistance and strain-based ductility reserve in singly reinforced concrete sections. Rather than assessing strengthening effectiveness solely in terms of strength increase, the analysis evaluates how efficiently strength gains are achieved relative to changes in post-yield deformation capacity. A reference section, designed modestly above the Eurocode 2 minimum longitudinal reinforcement requirement, is analysed using strain-based sectional mechanics consistent with ultimate limit state assumptions. Three strengthening strategies—tension-side, compression-side, and balanced strengthening—are examined. Ductility reserve is quantified using a normalised strain-based metric, and a simple efficiency index is introduced to relate strength enhancement to changes in deformation capacity. The results show that strengthening mode governs both the strength–ductility interaction and the efficiency of strength enhancement. Tension-dominated strengthening achieves large strength gains but rapidly depletes deformation reserve, while compression-side strengthening substantially enhances ductility with limited strength increase. Balanced strengthening provides moderate strength enhancement while largely preserving deformation capacity. These findings demonstrate that sectional strengthening performance cannot be adequately characterised by strength-based assessment alone and highlight the value of explicitly considering deformation reserve when comparing strengthening strategies.