Aggregates in Concrete Applications

Aggregates used in building and construction are mineral-based materials needed for mixing with cement, bitumen, lime, gypsum or other adhesive to form concrete or mortar. Each aggregate added to a mix contributes volume, stability, resistance to wear or erosion, or other desired physical properties to the finished product. Examples of aggregates are typical things available in the environment, such as sand, rocks, boulders, gravel, etc. These materials bind together with other ingredients to create asphalt and concrete, which are also considered to be aggregates.

General uses of Aggregate

The aggregate industry began to boom along with the growth in civil engineering projects, accelerated by the introduction, in the early 1900s, of the automobile that required paved roads. Even the most average of structures (eg a small school) requires approximately 15k tons of aggregates in the concrete build. Almost every construction project uses aggregates, regardless of whether the job is a residential, commercial or industrial project.

Aggregate and Concrete

When approaching this topic, one easy way of simplifying the concept is to imagine the steps involved in filling an empty box and apply this methodology to concrete:

  1. Start with larger items that take up the majority of the space.
  2. Then add smaller pieces to fill the remaining gaps.
  3. Finally, fill all the ‘holes’ with fine material.
  4. Lastly, add liquid to saturate all the materials and fill the box completely.

Usually, the standard items required to make up concrete in order of size are stone, sand, cement and water. The first three make up the body of the mix at approximately 60%-80% of the volume, but these percentages increase when the actual volume changes in its mass. Aggregates provide compressive strength, bulk and stability to any mix and the choice in the variety available is dependent on the work that needs to complete for the project.

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 a deterioration in the concrete.

Coarse and Fine Aggregates

An aggregate is generally defined as either coarse or fine and may also be natural, manufactured or recycled (refer to section ‘Types of Aggregates’).

Course Gravel Aggregates

A.   Coarse Aggregate

This is any material larger than 9.5mm. In Southern Africa 19mm and 13,5mm are the most common size used for the desired final product.

Sizes of aggregate in this category are:

Fine gravel: 4mm – 8mm
Medium gravel: 8mm – 16mm
Coarse gravel: 16mm – 64mm
Cobbles: 64mm – 256mm
Boulders: > 256mm

Fine Aggregates

A.   Fine Aggregate

Sizing here is anything less than 9.5mm, for example sand, silt and clay, which fills the gaps left by the coarse aggregate:

Stone Dust: 0.5mm – 5mm
Coarse Sand: 0.5mm – 2mm
Medium Sand: 0.25mm – 0.5mm
Fine Sand: 0.06mm – 0.25mm
Silt: 0.002mm – 0.06mm
Clay: < 0.002m

Types of Aggregates

The type of aggregate used in the concrete mix will affect the concrete in almost every way, from colour to durability, strength and so on. The concrete used for industrial floors, for example, uses quartz and granite because these aggregates have a high resistance to scratching/abrasion and can handle heavy industrial traffic.

GRAVEL – This is generally coarse, as it is formed from rocks that are unconnected to each other. It is a naural, hard-wearing material found in streams, riverbeds and other geological formations, although it can also be man-made by crushing large rocks to pieces. Gravel is a common aggregate used to make concrete and asphalt.

SAND – this is usually a fine aggregate made from stone which is quarried, crushed and ground to produce a variety of sizes to fit required specifications. This is best used as a component of concrete, as well as for manufacturing bricks, in landscaping, in water filtration projects and for agricultural usage.

GRANITE – Granite is a naturally-occurring aggregate and is considered to be one of the best materials for construction, crushed into different sizes for various applications. It is formed by the slow crystallisation of magma (extremely hot liquid and semi-liquid rock) below the earth’s surface.

BALLAST – This is the coarsest kind of aggregate available, typically made of crushed stone. Sometimes it also consists of material like clay or sand and is best used for concrete mixes that will be used to fill heavy-duty areas and pathways.

RECYCLED CONCRETE – This is basically crushed construction materials from building sites that is recycled (cleaned and processed), then re-purposed for use as a coarse aggregate. Crushed concrete is best to fill trenches, as the layer of concrete used to seal the earth and create the ground floor of a building and as granular sub-base for deep areas. 

VERMICULITE AGGREGATE – Vermiculite is a mineral which expands significantly when heated and is used as insulating concrete, for example in roof decks.

GLASS – Glass aggregate is graded by colour and size, ranging from 150mm rocks, to gravel-sized pieces, to a fine talc-like powder. Using polishing and grinding techniques, the glass is exposed for use in architectural and design effects on bare surfaces.

SLAG – Slag is a manufactured aggregate and contains no organic impurities, as the chemical composition is completely uniform and is used in civil projects and roadworks.

Grading of Concrete Mix

Depending on requirements, the concrete mix for any product is carefully calculated using different-sized aggregates. Cement is the most expensive part of any mix and if larger materials are used, then more cement is required, which increases the base cost. The converse applies when a mixture of aggregate with a lower proportion of cement. BLEND’s concrete batching plants can accommodate up to 52mm for coarse aggregate requirements and additional hoppers can be accommodated for specific units.

MMI’s BLEND Plant solutions can cater for a wide choice of applications, whether it be stabilisation cement, bridge mix, laboratory specified mix, colour cement, backfill, cold asphalt or standard concrete. Contact Blend Plants for more information on your specific needs.