Carbonation model for concretes with fly ash, slag, and limestone calcined clay - using accelerated and five - year natural exposure data

Abstract

Supplementary cementitious materials (SCMs) can be used in concrete to enhance sustainability and reduce the concrete industry's carbon footprint. However, some negative perceptions about their long-term carbonation resistance are obstacles for large-scale implementation of such concretes. This study evaluated the carbonation resistance of 34 concretes (with Ordinary Portland Cement, fly ash, blast furnace slag, and limestone calcined clay) in natural tropical exposure conditions (Open and Sheltered) for 5 years and in accelerated exposure conditions (1 and 3% CO2) for 112 days. Using these data and the square root of time function, the carbonation coefficients (KCO2, natl and KCO2, accl) of these concretes were estimated and a good correlation between them could not be observed. Hence, a more generic model (named as “A-to-N model”) to estimate the KCO2, natl using the KCO2, accl, CO2 concentration, and mixture proportion of concrete was developed, for which the mean absolute percent error is about 12% (reasonable accuracy). Using the A-to-N model, the carbonation depth at 50 years was estimated for various concretes. SCM concretes with low water-binder ratio and optimal binder content showed high resistance against carbonation at later ages; such information along with the target cover depth must be used while selecting materials for concrete design. Based on the model developed, a relatively simple ‘service life design chart’ was developed. This chart can be used by engineers to set the target KCO2, natl or KCO2, accl, and select the cover depth and binder type to provide the target service life (i.e., corrosion initiation time). This paper clearly shows that SCMs can be used to design concretes with comparable long-term carbonation depth as OPC concretes.