BDCS Notes - Geological Classification of Aggregates

Geological Classification of Aggregates

All natural aggregates result from breakdown of large rock masses. There are 3 kinds:

1. Igneous (rocks from volcanic action)
2. Metamorphic (exposed heat and pressure)
3. Sedimentary (exposed to weathering)

Igneous Rocks

Extrusive Igneous – Rocks formed by cooling at the surface

Intrusive Igneous – Rocks formed by cooling underground (in plutons)

Extrusive Igneous – Finer grain size and potentially include air voids and other inclusions.

Intrusive Igneous – Coarser grain size and fewer flaws.

Coarse grains = larger than 2mm

Fine grains = less than 0.2mm

Classification – based on silica content, specific gravity, color, and the presence of free quartz.

Sedimentary Rocks

Coalesce from deposits of disintegrated existing rocks or inorganic remains of marine animals. Wind, water, glaciers, or direct chemical precipitation transport and deposit layers of material that become sedimentary rocks, resulting in a stratified structure.

Bonds – formed by natural cementing of particles.

Classification – based on predominant mineral present:

1. Calcareous (limestone, chalk, etc.)

2. Siliceous (chert, sandstone, etc.)

3. Argillaceous (shale, etc.)

Metamorphic Rocks

Form from igneous or sedimentary rocks drawn back into earth’s crust and exposed to heat and pressure. Have crystalline structure, with grain sizes ranging from fine to coarse.

All 3 rocks are used successfully in civil engineering applications. The best possible prediction of aggregate suitability for a given application is based on historical performance in a similar design.