Integrating Carbonation Durability and Cover Scaling into Low-Carbon Concrete Design: A New Framework for Sustainable Slag-Based Mixtures

Conventional low-carbon concrete design approaches have often overlooked carbonation durability and the progressive loss of cover caused by surface scaling, both of which can increase the long-term risk of reinforcement corrosion. To address these limitations, this study proposes an improved design framework for low-carbon slag concrete that simultaneously incorporates carbonation durability and cover scaling effects into the mix proportioning process. Based on experimental data, a linear predictive model was developed to estimate the 28-day compressive strength of slag concrete, achieving a correlation coefficient of R = 0.87711 and a root mean square error (RMSE) of 7.55 MPa. The mechanism-based equation exhibits strong physical interpretability, as each parameter corresponds to a clear physical process, satisfying the requirements of design codes for physical significance. By integrating the strength and carbon-emission models, the carbon-emission efficiency was further analyzed. Across all water–binder ratios (0.3, 0.4, 0.5), CO2 emissions per unit strength decreased steadily with increasing slag content, indicating that carbon efficiency is primarily governed by slag replacement rather than the water/binder ratio. Four design cases, all with a design strength of 30 MPa, were then evaluated to illustrate the combined effects of carbonation and scaling. In Case 1, without considering carbonation durability, the carbonation depth after 50 years exceeded the 25 mm cover, leading to potential corrosion. In Case 2, when carbonation durability was considered, the required actual strength increased to 31.28 MPa. When mild cover scaling of 3 mm was introduced (Case 3), the required strength rose to 34.59 MPa, and under severe scaling of 10 mm (Case 4), it increased to 45.73 MPa. These results indicate that intensified scaling demands higher strength and lower water/binder ratios to maintain durability. Overall, the proposed framework quantitatively balances strength, durability, and embodied carbon, supporting sustainable low-carbon concrete design.