3.2.1 Sources of Aggregate
Aggregate may be produced from:
- crushed and screened quarry products
- natural sands and gravels
- manufactured aggregates
- recycled materials.
Igneous rocks are the most common source of processed quarry aggregate, and may include basalt, dolerite, andesite, granite, porphyry, rhyolite, diorite, etc.
Metamorphic rocks such as hornfels, schists, gneisses, and quartzites are also used as asphalt aggregates.
Sedimentary rocks and low grade metamorphic rocks encompass a large range of sources of various composition and properties. Use of sedimentary aggregates in asphalt is dependent on material of adequate hardness and absence of planes of weakness as well as factors of availability and particular application.
Natural sands and gravels may be crushed and screened, washed and screened, or obtained as untreated bank run or pit sand.
Manufactured aggregates may be either the by-products of an industrial process, such as industrial slag, calcined bauxite, or products specially manufactured for use as aggregates.
Recycled materials for use in the manufacture of asphalt include reclaimed asphalt pavement (RAP), recycled concrete, crushed glass, etc.
A general guide to suitability of various source rock types for asphalt is shown in Table 3.1.
|Rock type||Mechanical strength||Durability||Chemical stability||Surface characteristics||Hardness, toughness||Surface texture||Crushed shape|
|Serpentine||Fair||Fair||Good||Fair to poor||Good||Good||Good|
A general outline of the desirable properties of aggregates for use in asphalt is provided below. A more detailed guide to Australasian practice in the selection and testing of source rock for pavement construction materials is provided in the Guide to Pavement Technology Part 4J: Aggregate and Source Rock (Austroads 2008a).