Aggregates are used in concrete for very specific purposes. The use of coarse and fine aggregates in concrete provides significant economic benefits for the final cost of concrete in place. Aggregates typically make up about 60 to 75 percent of the volume of a concrete mixture, and as they are the least expensive of the materials used in concrete, the economic impact is measurable.
In addition, the use of aggregates provides volume stability to the hardened concrete. The shrinkage potential of a cement paste is quite high when compared to the aggregates. Controlling shrinkage of the concrete material is important since shrinkage and cracking potential increase together. Higher shrinkage potential means more cracking when the concrete is restrained from movement by contact with the base material beneath a slab-on-grade, steel reinforcement within structural members, or contact with adjoining concrete members in a structure.
It is commonly accepted that water demand and cement content in a concrete mixture increases as the maximum coarse aggregate size decreases. The required volume of paste in a concrete mixture must increase, due to the increased surface area of smaller aggregate sizes, to coat all of the aggregate particles. With this increase in paste quantity there is a reduction of volume of the aggregates per unit of concrete produced, thus the shrinkage of the mixture increases. Again, an increase in shrinkage potential combined with restraint of the concrete section may add substantially to the cracking potential of a concrete section.
In short, the aggregates are used to improve economy, but more importantly do contribute significantly to the final properties of any concrete mixture.
Aggregates for Use in Concrete CD, CD047