Optimized Mix Design Regarding Cost and Sustainability

The authors developed a model to optimize the design of concrete mixes in terms of their raw materials’ cost. The model simulates the effect of the characteristics and proportions of raw materials on the behaviour of concrete in relation to: consistency and cohesiveness of the fresh mix and compressive strength of the hardened concrete. Different alternative sets of constituents can be analysed simultaneously, with the model being calibrated with data of existing concrete mixes. For each set, the mix - that complies with the specifications - of lowest cost is obtained, applying an optimization engine based on the CobyLa2 simplex method, subjected to specification boundaries (e.g., w/cmmax, entrained air, etc.). The suitability of the software was validated during successful applications across the Holcim world. Currently, the authors are in the process of upgrading the software to extend the model to find the mix with the lowest CO2-Footprint (and/or other sustainability indices) and to improve the software’s user-friendliness (capturing the required data directly from the concrete plant databases). The suitability of the software is demonstrated by optimizing several mixes, using real data of nine alternative industrial cements, with contents of addition between 0 and 69% (Silica Fume, GBFS, PFA, Limestone Filler) and CO2-Footprints between 300 and 900 kgCO2 per ton of cement. It is shown that the cement providing the most sustainable solution depends, on a case-by-case basis, on the mix specifications. The paper describes the background of the model and presents and discusses the results of the optimizations.