4.6 Concluding remarks
The packing density (the complement of the porosity) of a granular skeleton determines
the amount of cement paste that is required to fill the interstices. Any surplus of paste
contributes to a better workability by reducing the friction between fibres and
aggregates. Minimising the porosity also reduces material costs and affects paste-related
aspects of concrete like shrinkage. Steel fibres increase the porosity of the granular
skeleton; the degree of the increase depends on the relative size of the aggregate grains
to the fibre length. In order to predict the packing density of the granular skeleton, the
Compressible Packing Model (CPM) was applied. Five approaches were tested and
compared with results of experiments to obtain the best accuracy. To apply the CPM,
the initial packing density of the components, the wall-effect of the fibres and the
compaction index of the chosen compaction method were determined. The accuracy of
the predictions depends on the composition of the aggregates. The simulations had an
average error close to 2% for optimised mixtures with aggregates up to 8 or 16 mm;
predictions of the packing density of mixtures with smaller maximum aggregate sizes are
less accurate.