AGGREGATES - Materials for Civil and Construction Engineers - page 162

5.4 Aggregate Uses

As mentioned, aggregates are primarily used as an underlying material for
foundations and pavements and as ingredients in portland cement and as-
phalt concretes. Aggregate underlying materials, or base courses, can add
stability to a structure, provide a drainage layer, and protect the structure
from frost damage (Figure 5.2). Stability is a function of the interparticle
friction between the aggregates and the amount of clay and silt “binder” ma-
terial in the voids between the aggregate particles. However, increasing the
clay and silt content will block the drainage paths between the aggregate
particles, thereby inhibiting the ability of the material to act as a drainage
layer.
In portland cement concrete, 60% to 75% of the volume and 79% to
85% of the weight is made up of aggregates. The aggregates act as a filler to
reduce the amount of cement paste needed in the mix. In addition, aggre-
gates have greater volume stability than the cement paste. Therefore, maxi-
mizing the amount of aggregate, to a certain extent, improves the quality and
economy of the mix.
In asphalt concrete, aggregates constitute over 80% of the volume and
92% to 96% of the mass. The asphalt cement acts as a binder to hold the
aggregates together, but does not have enough strength to lock the aggre-
gate particles into position. As a result, the strength and stability of asphalt

FIGURE 5.2 Compacted aggregate base before placing the hot-mix asphalt or
portland cement concrete layer of a paved road.