What are Admixtures in Concrete?

Concrete is generally made from a combination of water, cement and aggregates. However, as we’ve frequently discussed in this blog, sometimes our concrete needs qualities that go beyond what these basic ingredients can achieve. In these cases, we sometimes include what are known as ‘admixtures’. 

In this article, we’re going to explore some of the more common types of admixtures in concrete. But first, we’ll explain what an admixture is – and what it isn’t. 

What is an admixture in concrete?

An admixture is an extra ingredient that is added when preparing a concrete mix. Usually in liquid or powder form, admixtures can have a significant impact on either the wet or hardened concrete and bring extra properties that make it more suitable to the different conditions under which it has to work.

An admixture should not be confused with an additive, which is something that is added during the manufacture of cement. Incidentally, the fibres that are added to create fibre reinforced concrete do not count as an admixture.

Types and uses of admixtures in concrete

Water Reducers

As the name suggests, this admixture is used to reduce the amount of water needed by about 5-10% while achieving the same level of workability. Usually, it does this by improving the way that the cement particles are distributed through the overall mix. Also known as plasticisers.

Benefits: Increased strength with good workability; greater resistance to water penetration; reduced risk of shrinkage and cracking.

Superplasticisers

Given that water reducers are also referred to as plasticisers, it follows that superplasticisers (also known as high-range water reducers) do the same, but to a greater degree. In fact, superplasticisers can allow you to use up to 30% less water in the mix without reducing the workability of your concrete.

Benefits: All the same benefits as with water reducers, but with even stronger concrete as a result. This makes it suitable for use in projects that need to have a high load-bearing capacity but also be able to self-compact.

Set accelerators

Set accelerators are used to shorten the setting time of fresh concrete. In ideal conditions, concrete should be poured in temperatures between 10ºC and 21ºC and be allowed to set and cure in its own time. However, conditions aren’t always perfect – sometimes you need to pour in cold weather or you’re up against a tight deadline and could do with your concrete setting faster than it normally would.

Benefits: Shorter setting time with increased strength; potentially lower labour costs through a shorter manufacturing process.

Set retarders

In contrast to accelerators, set retarders slow down the process of concrete setting without reducing its long-term strength or durability. That could prevent the concrete setting too soon if it needs to be transported further than usual to the pour site. In the same way that an accelerator can be used in cold weather, a retarder can be used in concrete being poured in hot weather.

Benefits: Good workability for longer; prevention of cold joints.

Air entrainers

Air entrainers introduce small air bubbles into a mix. Often, that’s something you’d probably want to avoid; however, if the concrete is being poured in cold conditions, it allows space for water molecules to expand when they freeze.  Air entrainers also provide improved workability. 

Benefits: Resistance to potential damage caused by a freeze-thaw cycle.

Do You Require Concrete?

Here at Stewart Brothers Concrete, we have decades of experience providing top-quality concrete for all kinds of commercial and domestic applications. Our expert team knows the perfect mix for just about any job, including when to use admixtures, and can deliver to you quickly across Kent and beyond from our depots in Ashford, Sittingbourne and Dover.

Get in touch today to find out more or place your next order.

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