Carbonation models using mix-design parameters for concretes with supplementary cementitious materials

Low-carbon concretes with high-volume supplementary cementitious materials (SCM) can have lower carbonation resistance with an increase in the level of substitution due to the dilution of the clinker effect. Predominantly, this observation is based on carbonation depth (dCO2) measurements from accelerated and natural carbonation studies conducted for a lesser exposure period. Limited works are available to estimate and evaluate the carbonation resistance of low-carbon concretes at later ages. This study focuses on understanding the long-term carbonation behaviour of concretes with SCMs. 8-year dCO2 data was collected from OPC, OPC + Slag, OPC + Fly ash F, and OPC + Fly ash C concretes exposed in tropical climate in south India. Limestone calcined clay concretes were exposed to natural carbonation for six years. The collected data was used to model the progress of dCO2 based on mix design parameters such as paste content, water-to-binder ratio, and age of the concrete. Five mix-design-based (MDi) models were developed by providing higher weightage to the dCO2 data collected at 5, 6, and 8 years of exposure. The dCO2 data from field-exposed concrete made with OPC and OPC + fly ash was collected and used to validate these models. Parametric studies conducted by altering the cover depth in proposed service life framework revealed that a 20-30 % increase in cover depth in low-carbon concretes can enhance the life expectancy by 50-100 % even with high-volume substitutions. A nomogram was proposed to determine the necessary cover depth to achieve the intended service life for concretes made with different binders.