Computer simulation of interfacial packing in concrete

Computer simulation of particle packing against an aggregate surface was undertaken to show the effects of four variables on interfacial porosity profiles. The variables in order of significance and their assumed physical meaning are: sticking probability (tendency of cement particles to flocculate), amplitude of particle motion (energy of mixing), travel distance of particle to surface (thickness of water film surrounding aggregate), and original particle density (roughly related to water/cement ratio). In all cases, simulations demonstrated that interface porosity decreased from nearly 100% directly at the interface to that of the bulk paste at two to three particle diameters. Flocculation (sticking probability) was found to be the single most-significant variable. Highly flocculated systems produced very porous interfaces. When flocculation was reduced, packing became more efficient. It was also found that energy of mixing (amplitude of motion), was not an entirely independent variable. The simulation showed that, if the tendency to flocculate was high, gentle mixing (low amplitude of motion) was found to result in better packing and a less porous interfacial zone. If, on the other hand, flocculation was low, then vigorous mixing (high amplitude of motion) promoted better packing near the interface. The thickness of the water film surrounding the aggregate (travel distance) was found to have only a minor effect on the outcome of simulations, while original packing density (w/c) resulted in no significant differences at all.