Concrete mix ratio – how to mix concrete with aggregate mix and cement

Concrete consists of three things:

• Cement – the binder that hardens when mixed with water. We supply Portland cement in strength class CEM I 52.5.
• Aggregate – sand and stones that give the concrete strength and bulk. The easiest option is aggregate mix, a ready-made blend of building sand and stones.
• Water – which starts the chemical hardening process.

Aggregate mix 0-16 mm is the coarse mix with stones up to 16 mm and the obvious choice for foundations and larger casts. Aggregate mix 0-8 mm is finer and suits thinner casts, floors and smaller jobs. For very fine casting you can use building sand.

The mix ratio is given as parts cement to parts aggregate mix, measured by volume (e.g. buckets):

• 1:4 (1 part cement to 4 parts aggregate mix) – a strong concrete for most jobs, e.g. foundations, pad foundations, floors and retaining walls.
• 1:3 (1 part cement to 3 parts aggregate mix) – an even stronger, denser concrete for structures exposed to frost, de-icing salt or seawater.

Add water gradually. As a rule of thumb, use approx. ½ bucket of water per bucket of cement. The concrete should be plastic and hold its shape – not runny. Too much water makes the concrete weaker.

Example of a mix in the cement mixer (1:4): 1 bucket of cement + 4 buckets of aggregate mix + approx. ½ bucket of water.

For 1 m³ of finished concrete at a 1:4 ratio you need approx.:

• Aggregate mix: approx. 1,850 kg (approx. 1.15 m³)
• Cement: approx. 300 kg
• Water: approx. 150 litres

Note that you need more than 1 m³ of aggregate mix to make 1 m³ of concrete. This is because the cement and water fill the voids between the sand and stones.

To be sure of getting the right quantity, you can choose cement and aggregate mix combined. Here the cement and aggregate mix are measured out for e.g. 0.5-0.9 m³ of finished concrete and delivered in separate bulk bags.

Concrete is divided into exposure classes according to how harsh its environment is. The harsher the environment, the stronger and denser the concrete must be:

• Passive – indoors and dry, e.g. floors in heated rooms. Mix ratio 1:4.
• Moderate – outdoor structures not exposed to de-icing salt, e.g. foundations and retaining walls. Mix ratio 1:4.
• Aggressive – exposed to frost, moisture and de-icing salt, e.g. driveways, steps and ramps. Mix ratio 1:3.
• Extra aggressive – very harsh environments, e.g. structures by seawater. Mix ratio 1:3.

If in doubt, choose one class too high rather than too low.

1. Prepare the formwork: Build the formwork and place any reinforcement before you start mixing.
2. Add water to the mixer: Start with approx. half the water.
3. Add aggregate mix and cement: Add half the aggregate mix, then the cement and finally the rest of the aggregate mix.
4. Add the remaining water gradually: Let the mixer run for 3-5 minutes until the concrete is even and plastic.
5. Pour straight away: Pour the concrete into the formwork and vibrate it or tamp it with a batten to remove air pockets.
6. Level the surface: Screed the surface with a straight edge and finish it with a float if needed.

• Keep the concrete moist: Cover the concrete with plastic or water it lightly for the first 5-7 days so it does not dry too quickly and crack.
• Avoid frost: Do not cast in freezing weather, and protect the concrete from frost for the first days.
• Give it time: The concrete can usually be walked on after 1-2 days, but it only reaches full strength after approx. 28 days.

Calculate the volume of what you are casting: length × width × height in metres. A strip foundation 10 metres long, 30 cm wide and 40 cm deep needs 10 × 0.3 × 0.4 = 1.2 m³ of concrete. Add 5-10 % for waste.

If you are laying or rendering, you will also find ready-mixed mortar with us.

See more calculation examples in our guide How much gravel, sand and soil do I need?. We deliver cement and casting materials in bulk bags directly to your address – read more about delivery.