The Geology of Aggregates

About this course.

Construction aggregates are the granular mineral materials — crushed rock, natural gravels and sands — that make up the physical bulk of most of what we build. They are the largest input by volume into the built environment, and although each tonne is inexpensive compared with cement, steel or timber, the sheer quantities used make aggregates a materially and economically significant resource. In concrete they typically form the majority of the volume; in asphalt they are the structural skeleton bound together by bitumen; in road and rail construction they form the load-spreading base and sub-base layers; and in drainage they provide the free-draining, filtering medium around pipes and behind retaining walls. Understanding aggregates is therefore not a niche interest — it underpins almost every trade on a site.

It helps to start with the vocabulary. Aggregates are usually divided into coarse aggregate (the larger particles, produced by crushing rock or as natural gravel) and fine aggregate (sand, whether natural or manufactured by crushing). The division is made by particle size rather than by any fixed chemical difference, with sand being the fine fraction and gravels and crushed stone the coarse fraction. A further useful distinction is between "natural" aggregates won more or less as they are found — river and pit sands and gravels rounded by water and time — and "crushed" or "manufactured" aggregates produced by drilling, blasting and crushing solid rock into angular fragments. Each behaves differently in use, as later modules explain.