Evaluation of the impact of neutral axis depth in flexural performance of reinforced lightweight geopolymer concrete

The widespread deployment of low-carbon and alternative cementitious concretes necessitates a critical evaluation of current design standards, particularly those governing the ultimate compressive strain and its implications for flexural behaviour in reinforced concrete (RC) elements. This study examines the effect of reduced ultimate compressive strain on the position of the neutral axis, the minimum lever arm factor (Kbal), and associated reinforcement requirements in RC beam design. A strain-adaptive analytical model was developed to quantify the shift in neutral axis depth (x) relative to the effective depth (d) and the corresponding variation in Kbal for concretes exhibiting strain capacities below the conventional Eurocode 2 assumption of 0.0035. Results indicate a progressive increase in x/d from 0.36 to 0.90 as strain capacity decreases, with Kbal values ranging from 0.253 to 0.304. Application of the model to a case study demonstrated a potential reduction of up to 680 mm² in required reinforcement for a 6m length of reinforced concrete beam , yielding significant material and embodied carbon savings. This underscores the need for performance-based design methodologies that reflect the evolving mechanical behaviour of sustainable concrete materials, offering a pathway toward more efficient and resilient structural systems. Based on embodied carbon factor of 0.684 kgCO₂e/kg for rebar, this reduction corresponds to a carbon saving of approximately 21.91kgCO₂e per 6 m length of RC beam. These findings highlight the dual structural and environmental benefits of strain-adaptive design and reinforce the need for performance-based methods that account for the evolving mechanical properties of sustainable concrete materials, supporting more efficient, economical, and low-carbon structural solutions.