Concrete is the most commonly used building material in the world and has been around for a couple of thousand years at least. If you are a keen DIY-er, it is more than likely that you have mixed and used concrete yourself. It is seen as an easy-to-use construction material for everything from walls to driveways, but what is concrete?
What is Concrete?
Concrete is defined as a composite construction material, being made up of several different components. Modern concrete is most often made using a mix of water, Portland Cement, sand and aggregate in the form of small stones or crushed rocks. Additives to increase or decrease setting times or to help reduce the damage from frost are often also added to the mix.
Without any one of the main four components, concrete is not really concrete. Portland Cement may be partially replaced by another cementitious material such as Fly Ash (a by-product of coal-fired electricity generation), various types of sand can be used, aggregates can be a variety of materials and sizes, but the four components remain essentially the same. The one element which doesn’t change is water, although the amount used does.
What is Concrete Used For?
Concrete can be used in a huge number of different situations and, depending on the mix and the additives, can result in dozens of different finishes. You may find concrete holding fence posts in place, forming floors and walls of houses, acting as a solid foundation where no natural foundation occurs and even being used as a kitchen work surface.
The intended use for the concrete heavily influences how it is mixed. Different concrete mixes are used for different projects, but the most common mix for most DIY projects will probably be 1:2:3 or 1 part cement, 2 parts sand and 3 parts aggregate.
Components of Concrete
As mentioned earlier, the basic components of a concrete mix do not change. Concrete always contains Water, Cement (or cementinious material), Sand and Aggregate, but might also contain a number of other additives, either chemical additives or mineral additives.
Cement – Portland Cement is the key to making concrete as it is the cement reacting chemically with the water which binds the mix together (this process is called hydration). There are other types of cement available, but Portland Cement is by far the most common, particularly for a DIY project. Cement is made from a combination of calcium, silicon and aluminium oxides. Cement is highly caustic and wet cement can quickly cause burns to skin if not washed off quickly.
Sand – Sand acts as a fine aggregate in concrete and usually forms the bulk of the mix along with the course aggregate. The sand used for mixing concrete will often, if not always, be referred to as Sharp Sand or Builders Sand. In certain circumstances, some of the sand will be replaced by a decorative additive such as crushed glass.
Course Aggregate – Traditionally this would be natural gravel or crushed stone, but increasingly recycled aggregates from construction or demolition are being used, at least partially. The course aggregate forms the bulk of most concrete mixes and makes the resulting concrete much stronger than it would ever be without it. Aggregates form the bulk, whilst the cement fills all the gaps and binds it together.
Water – Without water, concrete would just be a pile of stones and dust. Water is the essential element which causes the cement to react and solidify. The amount of water you add to the mix will greatly effect how you work with the concrete and the final result. Add more water to the mix for a more free-flowing concrete (ideal for filling formwork), include less water for a stronger, more durable final product.
There are many different additives (or admixtures) for concrete, each with a specific required outcome. When adding anything extra to a concrete mix, you must ensure that the additive does not upset the mix ratio.