The role of aggregates and sands in the construction industry.

The rapid growth of South Africa’s urban areas has put immense pressure on existing construction material resources, as there is increasing demand for land use for infrastructure, housing, recreation and industrial activity. The quality of aggregates is a critical component in determining the use and life of infrastructures. It is therefore important that the aggregates supplied are affordable, durable and secured for long term supply. The report focuses on the relationship between aggregate and sand sales and the impact on the construction industry.


Furthermore, the report establishes the link between Outcome Six of Government priority areas, which identifies infrastructure network as one of the key areas in unlocking the economic growth of the country. The infrastructure build programme had played a key role in South Afica’s sustained economic growth since the 2008/09 economic downturn, with R1-trillion spent on economic infrastructure. The report also seeks to avail relevant informationto decision makers when carrying out large magnitude infrastructure network projects as identified by the National Development Plan, focusing on measures that will contribute to South Africa’s economic transformation.

Aggregates are granular raw materials, including gravel, crushed stone and recycled concrete that are used in residential construction and commercial construction. Public funded infrastructure projects consume most aggregates, usually for use in road construction. Aggregates are vital to the construction industry which maintains and enhances the country’s built environment and transportation infrastructure. The quality of aggregates is a critical component in determining the use and life of infrastructures. It is therefore important that the aggregates supplied are affordable, durable and secured for long term supply.

The rapid growth of South Africa’s urban areas has put immense pressure on existing construction material resources as there is increasing demand for land use for infrastructure, housing, recreation and industrial building activity. The widespread use of aggregates result not only from its general availability and low value but also from the fundamental role they play in developing and sustaining modern society and economy. Traditionally, sources of aggregate were found close to their demand locality due to the high cost of transportation resulting in quarries located on the outskirts of cities and towns. However, in numerous instances, these deposits have been depleted and new sources further away had to be found resulting in greater transportation costs. Urban development itself has, in recent times been responsible for the rapid depletion of readily available deposits. Hard rock aggregate quarries are often located on the slopes of hills and mountains, usually resulting in the defacing of areas of natural scenic beauty. Growing environmental awareness increasingly influences the exploitation of such resources in localities of this nature. 

Before any project of a large magnitude can be undertaken, especially those identified by the Presidential Infrastructure Coordinating Commission (PICC), it is essential that an investigation of quality aggregate availabiltyis conducted. This will ensure thatdecision makers are aware of the position and spatial extent of potential construction and building materials during land-use planning.

South Africa has various types of rocks which are characterised as aggregates which are widely distributed in abundance in a variety of geological environments. Coarse aggregate are derived from a wide variety of parent bedrock materials. The three main groups of coarse aggregates are:

Igneous rocks – Andesite, basalt, dolerite, felsite, gabbro, granite, granodiorite, norite, rhyolite and syenite.

Metamorphic rocks – Granite-gneiss, granulite, hornfels, quartzite and slate.

Sedimentary rocks – Quartzite, sandstone, greywacke, shale and tillite.

A good concrete aggregate must be clean, chemically inert, and durable and roughly cubic in shape after crushing and of a size grade suitable to make concrete of desired physical qualities. Natural sand consists of loose grains which are commonly the result of the chemical weathering and/or physical breakdown of rocks. The range of particle sizes is dependent on the original texture of the source rock and the state and degree of weathering. Natural sands include alluvial/eluvial sands, aeolian/windblown sands and marine/beach sands. Sand accumulates in rivers, on beaches, as dunes and in valleys between mountains. Manufactured sand is produced by the mechanical crushing or milling of rock and gravel. Mine-dump sand being a waste product in the mining industry can also be classified as manufactured sand. 

Most hard rock material used to produce coarse aggregate is sourced from open pit quarries and waste dumps. Quarrying usually requires drilling and blasting, after which the rock is extracted by means of bulldozers and draglines. The broken rock is transported to a processing facility for scalping, the process which involves the removing of unwanted materials, such as clay. The scalped material is then crushed to obtain the desired fragment size. The resulting material is then screened to obtain aggregates of the desired grade (Fig.1). In some instances, blending is done to meet customer’s specification. Natural sand is dug from the ground often using hydraulic excavators. The quality and final use of the sand usually determines the amount of processing necessary. The sand undergoes complementary processing including washing and scrubbing, primarily to make them cleaner before being stockpiled to go on to their end use. It is now a common practice to further beneficiate the aggregate into: 

  • Ready-mix concrete
  • Asphalt
  • Bricks and paving material

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Compiled By: Refiloe Motsie – Refiloe.motsie@dmr.gov.za Eulendah Malematja – eulendah.malematja@dmr.gov.za Issued by and obtainable free of charge from The Director: Mineral Economics, Trevenna Campus, 70 Meintjies Street, Arcadia, Pretoria 0001, Private Bag X59, Arcadia 0001 Telephone (012) 444-3531, Telefax (012) 341-4134 Website: http://www.dmr.gov.za

Aggregate materials, and their role in the manufacturing of concrete

Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and cement, are an essential ingredient in concrete.

For a good concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete.

Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories–fine and coarse. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch sieve. Coarse aggregates are any particles greater than 0.19 inch, but generally range between 3/8 and 1.5 inches in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder.

Natural gravel and sand are usually dug or dredged from a pit, river, lake, or seabed. Crushed aggregate is produced by crushing quarry rock, boulders, cobbles, or large-size gravel. Recycled concrete is a viable source of aggregate and has been satisfactorily used in granular sub-bases, soil-cement, and in new concrete.

After harvesting, the aggregate is processed:  crushed, screened, and washed to obtain proper cleanliness and gradation. If necessary, a benefaction process such as jigging or heavy media separation can be used to upgrade the quality. Once processed, the aggregates are handled and stored to minimize segregation and degradation and prevent contamination.

Aggregates strongly influence concrete’s freshly mixed and hardened properties, mixture proportions, and economy. Consequently, selection of aggregates is an important process. Although some variation in aggregate properties is expected, characteristics that are considered include:

  • grading
  • durability
  • particle shape and surface texture
  • abrasion and skid resistance
  • unit weights and voids
  • absorption and surface moisture


Grading refers to the determination of the particle-size distribution for aggregate. Grading limits and maximum aggregate size are specified because these properties affect the amount of aggregate used as well as cement and water requirements, workability, pumpability, and durability of concrete. In general, if the water-cement ratio is chosen correctly, a wide range in grading can be used without a major effect on strength. When gap-graded aggregate are specified, certain particle sizes of aggregate are omitted from the size continuum. Gap-graded aggregate are used to obtain uniform textures in exposed aggregate concrete. Close control of mix proportions is necessary to avoid segregation.

Shape and Size Matter

Particle shape and surface texture influence the properties of freshly mixed concrete more than the properties of hardened concrete. Rough-textured, angular, and elongated particles require more water to produce workable concrete than smooth, rounded compact aggregate. Consequently, the cement content must also be increased to maintain the water-cement ratio. Generally, flat and elongated particles are avoided or are limited to about 15 percent by weight of the total aggregate. Unit-weight measures the volume that graded aggregate and the voids between them will occupy in concrete. 

The void content between particles affects the amount of cement paste required for the mix. Angular aggregates increase the void content. Larger sizes of well-graded aggregate and improved grading decrease the void content. Absorption and surface moisture of aggregate are measured when selecting aggregate because the internal structure of aggregate is made up of solid material and voids that may or may not contain water. The amount of water in the concrete mixture must be adjusted to include the moisture conditions of the aggregate.
 
Abrasion and skid resistance of an aggregate are essential when the aggregate is to be used in concrete constantly subject to abrasion as in heavy-duty floors or pavements. Different minerals in the aggregate wear and polish at different rates. Harder aggregate can be selected in highly abrasive conditions to minimize wear.