Retrofit or Replacement? Experimental and Environmental Assessment of Low-Carbon Strategies for Reinforced Concrete Members

Reducing the embodied carbon of reinforced concrete (RC) structures requires consideration not only of material composition but also of structural efficiency and the potential to retain and strengthen existing members. This study combines experimental testing and environmental assessment to evaluate alternative low-carbon strategies for RC flexural members and subsequently examines when retrofit is environmentally preferable to demolition and optimised replacement. Five large-scale beams were designed for comparable flexural resistance and tested under three-point bending, representing conventional concrete, eco-concrete, geometry-optimised eco-concrete, ultra-high-performance concrete (UHPC), and textile-reinforced concrete (TRC) strengthening. Their structural performance was combined with a global warming potential (GWP) assessment based on equivalent load-bearing capacity. All tested solutions exceeded the target design resistance. Compared with the conventional reference beam, binder substitution reduced embodied carbon by 15.4%, geometry optimisation by 36.0%, and the UHPC solution by 41.7%, although the latter result was sensitive to the reinforcement emission factor. The TRC-strengthened beam attained the highest measured flexural resistance of the investigated configurations, reaching 414.6 kNm, while the strengthening materials had a cradle-to-gate GWP of 71.0 kg CO2-eq. A parametric study subsequently compared TRC strengthening with demolition and replacement for different existing reinforcement ratios and required capacity increases. Within the investigated design space, TRC strengthening remained environmentally preferable wherever it was structurally feasible, providing approximately 27–90% lower GWP than replacement. No environmental crossover between the two strategies was identified before the structural feasibility limit of the TRC system was reached. The results demonstrate that low-carbon structural decisions should be based on equivalent structural performance and that retaining and strengthening existing RC members can provide substantial carbon savings compared with reconstruction when the required capacity increase can be achieved through retrofit.