In order to hold together in a solid form, concrete must be mixed with a granular substance called an aggregate. Aggregates come in a variety of different shapes and sizes. Some are as finely ground as sand, others are coarse and rough like gravel. It is when these aggregates are mixed with cement and water that they form concrete. The aggregate substance accounts for between 60-75% of the total volume of the final product.
What are Aggregates?
Aggregates are regularly referred to as being inert compounds, or ‘filler’ in the concrete, but both of these imply that the aggregate substances are more or less interchangeable. In actual fact, we often want to manufacture concrete with slightly different properties. Aggregates allow us to achieve this and are, therefore, a crucial concept in construction. Both the solidity (hardness and flexibility) and durability of concrete when it hardens will depend upon the aggregate substance used.
How Aggregates are Made
Aggregates can come from a number of different sources, depending on the desired type. In the case of aggregates that are found naturally, such as gravel and sand, they can simply be taken from the bodies of water where they are naturally found. The other common type of aggregate is the crushed-type. Crushed aggregate is manufactured by breaking down large stones and boulders until they are the desired consistency. Some suppliers make use of old and discarded concrete, breaking it down and recycling it for use as an aggregate.
Once the raw materials have been sourced they are then washed and crushed to the desired consistency. At this point, they go through a quality control process to filter out all the pieces that don’t fall into the desired range of properties. This ensures more consistent performance as an aggregate. Washing them reduces impurities and, in doing so, is able to increase the concrete’s durability.
Types of Aggregate
Aggregates can broadly be divided into two separate categories: coarse and fine. The difference between them is the size of the particles of matter in the aggregate. In the case of coarse material, the aggregate resembles gravel, although it is usually slightly more finely ground than most gravel. On the other hand, the fine aggregate resembles sand.
The coarsest aggregate is crushed stone. This type of aggregate consists of a mixture of other aggregates and is used as a base in concretes that don’t require precise ratios of the aggregate material to the cement.
Gravel is sometimes used directly but is also often broken down slightly prior to use. Sometimes known as graded stone, this is another coarse type of aggregate.
The finest aggregate of all is sand, although as a material, sand itself is further subdivided into a number of different types.
Check with your supplier to find out what type of aggregate is best for your needs. Some suppliers are able to tailor their products to your needs. Check out this architectural precast aggregate from Kafka Granite as an example.
Choosing the right aggregate for a building project is important. Not only do you need the right aggregate from a safety and integrity perspective, it is also an area where you can reduce your costs.