There are a number of factors that are involved in determining the composition of the concrete suitable for any given application. The aggregate is considered as the denser stronger phase while the hardened cement is considered as the weaker phase. However, since the aggregate is cheaper, it is usual to pack as much of this as possible into the final concrete composition while still retaining sufficient levels of workability. Workability refers to the ability of the concrete to fill all the space in the form that is required. Much of this ability also comes from the water-cement ratio. A higher water-cement ratio indicates greater workability. However, this leads to a reduction in strength as well as large levels of shrinkage during curing. Typical water-cement ratios are in the range of 0.45-0.55.
The best way to put as much aggregate as possible into a concrete mix is to grade it so that it packs well. If similar sized particles of aggregate are used then a maximum packing density of approximately 60% can be achieved. However, if smaller particles are mixed in to fill the gaps between the bigger particles then a higher relative packing density is possible.