Aggregate

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Kathy Feick

Sand, gravel and crushed stone are all considered to be aggregates. More simply, aggregates include grains or fragments of rock. They are a non-renewable resource which we benefit from daily, but rarely think about. The aggregate industry is Ontario’s largest natural resource industry, affecting almost every aspect of our daily lives.

different sizes of aggregate materials beside each other in piles

Uses:

Aggregate is used in foundation, concrete blocks, brick, mortar, shingles, steel, glass and asphalt, just to name a few. It is used to build and maintain:

  • Urban, suburban and rural infrastructures including commercial and residential buildings
  • Highways, bridges, sidewalks and parking lots
  • Factories, power generation facilities
  • Water storage, filtration and delivery systems
  • Wastewater collection and treatment systems

Facts:

  • The annual production of aggregate worldwide totals about 16.5 billion tons (15 million metric tons, which is valued at more than $70 billion)
  • In Ontario alone we use 170 million tons per year (from 1992-2003). Aggregates are used in greater quantities than any other natural resource
  • Over 440 tons of aggregate (22 truck loads) are used in the construction of every house
  • A small school uses approximately 13,000 tons of aggregate (650 truck loads)
  • An office tower uses over 16,000 tons (800 truck loads)
  • The construction of one kilometre of a six-lane expressway requires 51,800 tons of aggregate
  • (2,590 truck loads).

Availability:

Aggregate is purchased in bulk, for a fairly low price. Transporting far distances can increase its cost substantially, so it is often important to use aggregate that is located close to where it is needed. In some cases, little supply is available, the aggregate which is available does not meet the requirements for use, or there is a land conflict which interferes with the mining process. This often results in high prices for building and maintaining infrastructures as the material needs to be brought in from a distance.

aggregate mining operations

Mining:

Aggregate materials are either used in their natural state or after crushing, washing, and sizing. The materials can be used loosely, or sand, gravel, and crushed stone can be combined with a binding media to form concrete, mortar or asphalt.

The mining process begins with the removal of the overburden to reveal the materials underneath. Soil and partially weathered rocks can be pushed aside with bulldozers, and then removed with conventional loaders and haul trucks.

Sand and gravel

Sand and gravel deposits are the product of erosion of bedrock and the subsequent transport, abrasion and deposition of these materials. Water and glacial ice are the principal agents which affect the distribution of deposits of sand and gravel. For this reason, aggregate materials are widely distributed and abundant near past rivers and streams, in alluvial basins, and in previously glaciated areas.

Sand and gravel are mined from open pits, or dredged from underwater depots. Sand and gravel which are located above the water table can be mined with conventional earth-moving equipment (bull dozers, front loaders, back hoes, and scraper graders). Where the materials lie under the water table it is easiest to pump the water from the pit before moving the materials. If pumping out the water is not possible, wet-mining techniques may be used (draglines, clamshells, bucket and loader, or hydraulic dredges).

Bedrock

Bedrock is the source material for crushed stone. Not all bedrock is equally useful as aggregate. Some rocks are too soft and absorptive; others too chemically reactive to be useful. If the material is suitable for mining, then in most cases the stone must be drilled and blasted to break it apart. To do this, holes are drilled into the rock and partially filled with explosives. The top portion of the hole is filled with non-explosive materials which act as a plug. The explosives are then detonated. The total blast usually lasts only a fraction of a second, and consists of many smaller individual blasts, separated by delays a few thousandths of a second apart. It the rock is still too large, secondary breaking may be required and is usually accomplished by hydraulic hammers, drop balls, or other mechanical devices. The blasted material can then be removed in the same ways that sand and gravel are removed.

aggregate sorting machinery and other parts of operations
  • Aggregate Sorting image source: http://www.cee-environmental.com/en/company/info/id/623

Processing:

Aggregate processing commonly consists of transporting the materials to a plant, crushing, screening, washing, stockpiling and loading. The first step is to move the material from the mine to a primary crusher. The crushed material then moves via conveyer to a surge pile. A gate in a tunnel at the bottom of the surge pile releases the sand, gravel, or crushed stone at a constant rate onto a conveyer belt, and then to a secondary crusher and screening system, where it is further crushed and sorted by size. Rock that is too large is sent back through the crusher and screening process. The material may be washed before sale if required. Upon sale, the aggregate is loaded into trucks, railcars, or barges for transport to the final destination.

References:

Langer, W. H., Brew, L. J., & Sachs, J. S. (2004). Aggregate and the environment.