Developing a novel mix design methodology for slow hardening composite cement concretes through packing density approach

Abstract

In the recent decades, partial replacement of Ordinary Portland Cement (OPC) with pozzolanic materials gained more importance compared to plain OPC due to several advantages in terms of performance and sustainability. However, it is well known that the replacement of pozzolans in OPC, results in delayed strength particularly, at early ages due to slow pozzolanic activity compared to cement hydration. However, in case of pozzolanic concrete mixes, a better packing of aggregates is found to improve the compressive strength apart from the beneficial properties in terms of durability and sustainability. In this study, Composite cement (CC), a ternary blended cement with replacement of fly ash (FA) and granulated blast furnace slag (GBFS) is used in design of concrete mixes. The amounts of OPC, FA and GBFS in CC were optimized based on packing density and compressive strength results. Four mixes of CC concrete were prepared with different values of packing densities of aggregates (0.80, 0.75, 0.70 and 0.65) with two combinations each having coarse and fine aggregate dominant and with three water to binder ratios (0.45, 0.50 and 0.55). It was identified that at the same level of packing density, with variation in proportions of different sizes of aggregates within the domain of extreme size range, the difference in compressive strength was found to be insignificant. Hence, a tri-relationship between compressive strength, water to binder ratio and packing density of aggregate system is established for proposing a new concrete mix design approach. Compressible Packing Model (CPM) is used in proportioning the coarse and fine aggregates to achieve the required packing density. As increase in the compactness of aggregate system results in low workability, the correlations were established between the parameters influencing the workability of concrete such as water to binder ratio, coarse to fine aggregate ratio, packing density, binder to aggregate ratio and dosage of superplasticizer. Packing density of aggregate system was found to have a strong positive correlation with compressive strength (R2 = 0.957) and strong inverse correlation with workability of concrete (R2 = 0.815) compared to other parameters. A relationship between workability and packing density is established for different water to binder ratios for designing CC concrete mixes. This approach was found to be more flexible and provides different possible alternatives of packing density, water to binder ratio and workability for the same target compressive strength useful for the concrete designer. Efforts are made to understand the influence of proportioning of different sizes of aggregates in designing the concrete mix. The binder paste content is determined by the volume of voids present in between the aggregates. Tests on slump and compressive strength were carried out on concrete mixes. The concrete mixes having higher aggregate packing densities were found to give higher compressive strength at all ages of curing with lesser binder consumption. Since, the packing density decreases the binder content per unit compressive strength, it was found to have a strong correlation with binder intensity index with R2 = 0.944. It was also noticed that, at 90 days, the compressive strength of all the CC concrete mixes has increased between 1.2 and 1.3 times of 28 days compressive strength due to slow pozzolanic action.