Concrete is made through the bringing together of several ingredients. In its simplest form, concrete is a mixture of aggregates, water and cement. Different materials and different ratios create different strengths and types of concrete. These distinct types of concrete are used for a variety of construction purposes.
Coarse aggregate is often known as “ballast”. When you add cement to aggregate its particles bind together, creating a strong mixture. Water and other additives are then added to make the mixture suitable for pouring.
What is ballast?
Ballast is a mixture of sharp sand, small stones or gravel. As well as being a key component of concrete, it’s also used for a variety of landscaping purposes such as kerb edges and shed bases.
Concrete ballast or aggregates make up the bulk of the cement mix. The amount, particle size and how they are distributed will determine the strength of the concrete. To create a decorative finish different materials may be added to the mix. These might include shattered glass, river stones and quartzite.
Understanding concrete strength
The compressive strength of concrete is measured in megapascals (MPa) or pounds per square inch (psi). This gives an indication of how well a material will resist loads due to compression. Concrete with a MPa over 35 is usually regarded as commercial grade and is used in all kinds of structural projects such as roadworks and foundations. Bridges and motorways will usually require a concrete strength of 40MPa or above. Lower strength concrete has a strength of about 14MPa and is usually a runny mix used for non-structural purposes such as backfill or road bases.
Getting the right strength for your project will largely be dependent on the amount of water that’s added to the mix. Using less water will make the concrete stronger, excess water will make the mixture less workable. With more water there’s also the risk of the sand and aggregate particles separating, causing microscopic pores to develop in the concrete. This will make the concrete weaker than it should be.
How does ballast fit in?
As a general rule, the smaller the ratio of cement to ballast in your mix the stronger the resulting concrete will be. For instance, for high-strength concrete, you would mix 1 parts cement to 5 parts ballast. This will create a strong, water-tight concrete suitable for concrete panels or building slabs. Increasing the ratio to 1 part cement to 6 parts ballast creates a mix that’s suitable for driveways, patios and paths. Increasing the ratio further to 1 part cement to 8 parts ballast gives you foundation-strength concrete.
In MPa terms, to create a cubic metre of concrete with 20MPa strength, mix 300kg of cement, 600kg of sand, with 1200kg of aggregate. These amounts can easily be scaled up or down depending on the quantity you need. The right amounts of cement, ballast and sand mix can be expressed in simple to understand ratios. The ratio of the mix above would be 1:2:4, or a cement to ballast ratio of 1:6.
For low-strength concrete with a 10MPa strength, the ideal ratio would be 1:3:6. For stronger, high-strength 30MPa concrete you would need a ratio around 1:0.75:1.5.
Once you understand this three part ratio system and how it works in practice it’s much easier to calculate the amount of ballast you might need in your concrete mix. Scale down the quantities required for smaller batches but retain the ratio differentials in your components. Small quantities can be mixed in a bucket or drum and should usually be used within 45 minutes to ensure high-levels of workability.
To give concrete extra strength it’s also possible to add extra components such as steel bars.
Getting it right
Getting the right mixture in your concrete can sometimes seem complicated. At Goodmix Concrete we’ve accrued over a decade of experience. We understand what it takes to create the perfect concrete mix for the job. We can help you to complete a DIY task, from a garage base to a driveway. Alternatively, we are a supplier of choice for commercial construction sites and housebuilders – delivering on time and to an exact specification agreed in advance.
If you have a project in mind that requires concrete, no matter how big or small, don’t hesitate to contact us for advice.