Concrete Aggregates – Geological Considerations




Mixture Design

Fresh Concrete

Hardened Concrete

Dimensional Stability




Rocks and minerals

Aggregates are obtained from rocks, which, in turn, are composed of minerals (either a single mineral, or a mixture of minerals). A mineral is naturally occurring, inorganic, has an order internal arrangement of atoms, and has a definite composition (or range of compositions). For example, quartz, halite, gypsum, opal, feldspar, biotite (mica), etc.

Mineral properties:

  1. Colour
  2. Streak (on a porcelain plate)
  3. Lustre (metallic or non-metallic)
  4. Hardness (typically represented on Moh’s scale, where talc has a hardness of 1 and diamond a hardness of 10)
  5. Specific gravity
  6. Cleavage
  7. Fracture
  8. Crystal form
  9. Magnetism
  10. Tenacity (brittle or ductile)
  11. Diaphaneity (or transparency)
  12. Striations
  13. Reaction to acid (typically, HCl is used for this test)

An ore is a natural mineral that can be mined for a profit. For example, bauxite, iron ore etc.

Types of rocks

Rocks are classified as igneous, sedimentary, or metamorphic. 95% of the outer 10 miles of the earth’s crust is composed of igneous and metamorphic rocks, but 75% of the rocks exposed on the surface of the earth are sedimentary.

Igneous rocks are those which form as a result of cooling from the molten state. These are further classified as:

  1. Intrusive: when the molten matter cools slowly under the earth’s surface, and results in the formation of large rocks with typically large crystals, e.g., Granite, gabbro, pegmatite
  2. Extrusive: when the molten matter cools rapidly on the earth’s surface, resulting in the formation of rocks with smaller crystals, e.g., Basalt, andesite, rhyolite.
  3. Pyroclastics: these are formed due to the cementation of extremely fine ash deposits which cool very rapidly resulting in an amorphous rock, e.g., volcanic tuff, pumice, breccia.

Sedimentary rocks are deposited in a fluid medium due to lithification of weathered sediments. Lithification can occur as a result of cementation (common cements being iron oxide, calcite, or quartz), crystallization, or compaction (due to the application of high temperature and pressure). Shale, sandstone, and limestone make up 46, 32, and 22 % of all sedimentary rocks, respectively.

Metamorphic rocks are formed when pre-existing rocks are subjected to heat and pressure. Recrystallization often occurs, and the resulting rocks have typically large crystals with a well-defined cleavage. For example, marble, gneiss, schist, phyllite, slate, etc.

Engineering considerations of rocks

  1. . Fine-grained siliceous materials in igneous and sedimentary rocks can be susceptible to Alkali-silica reaction (see the link on Durability for more details). For example, amongst igneous rocks, rhyolite, andesite as well as the rocks containing volcanic glass are prone to ASR. The principal active siliceous ingredients are opal (SiO2.nH2O) and chalcedone, which is a fibrous variety of silica. Chert and greywacke among sedimentary rocks are also highly prone to ASR. Among metamorphic rocks, phyllite and argillite are susceptible to ASR because of the presence of strained quartz.
  2. Carbonate sedimentary rocks, limestone and dolomite, are susceptible to Alkali-Carbonate reaction when the minerals have a specific texture.
  3. . Very coarse grained rocks are undesirable because of poor abrasion resistance.
  4. Some igneous rocks may have soluble minerals like zeolite; also, in foundations, weathered igneous rocks should not be used. In the case of sedimentary rocks, the source of rock is important. Some rocks are less durable (e.g. stream gravels) and some are extremely porous (e.g. conglomerates are weak and porous), and not good for freezing conditions. Careful attention should also be paid to the presence of cavities and conduits within limestone and dolomite among sedimentary rocks, and marble among metamorphic rocks.
  5. Flat, flaky, and elongated pieces are obtained on crushing metamorphic rocks. Such rocks will pose a problem if used as concrete aggregates. Also, some metamorphic rocks show directional properties because of their foliation.
  6. As dimension stone, igneous rocks are the best because of their high resistance to weathering. Sedimentary rocks for use in dimension stone should be carefully chosen.

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