The article presents an original method to assess the sustainability of concrete. The method uses three parameters, namely, performance, lifetime and environmental impact, to calculate a sustainability index. The originality and simplicity of the proposed method is shown when finding the sustainability index, where the first two factors (service life and performance) are kept constant. This approach is possible within the context of the new proposals that specify the durability of structural concrete in EN 1992 and EN 206. It allows the classification of concrete according to its performance, through environmental action resistance classes (ERC). For this purpose, specific experimental methods were used in order to determine the performance of concrete exposed to carbonation. The concretes were prepared with two cement types with additions (CEM II/A-S and CEM II/A-M (S-LL)). Based on the carbonation resistance classes (first parameter—performance) and exposure classes, the thickness of the concrete cover layer was determined to ensure a certain service lifetime (second parameter—service lifetime). Lastly, the global warming potential was calculated for each composition, allowing the users of the method to select the compositions with the lowest environmental impact.