Concrete optimisation with regard to packing density and rheology

In Civil Engineering granular materials, mixtures of particles with a wide grading span are used commonly. At the dry state, the sake of a minimum porosity or the maximum packing density is generally considered as a good strategy to get an optimised mix. However, modern concrete technology is now offering a variety of mixes suitable for different applications, that should match complex sets of specifications. These specifications ensure the ease of placement, the attainment of sufficient mechanical properties at the hardened state and a satisfactory durability in a certain environment. Rheology becomes a key issue in some mix categories, like Self-Compacting Concrete (SCC). In such a case, how the optimal aggregate gradation compares with the one of a dry mixture ? The paper discusses this old question through an example where a dry aggregate mix (in bulk or confined conditions), a Roller-Compacted Concrete, a Normal-Strength Concrete and a Self-Compacting Concrete are optimised with the same constituents. The effect of granular interactions, as wall effect and loosening effect, is highlighted.