Mixing concrete by hand is very labour intensive and only really suitable for small quantities. Mixing concrete with a mixer is much easier and quicker, allowing larger quantities to be produced in a relatively short time. You can readily hire a suitable size mixer from your local hire shop – both electric and petrol models are available.
Remember that concrete can cause alkali burns so wear protective gear and avoid skin and eye contact. Never put hands or tools inside the mixer while it is switched on.
Setting up the Mixer
The mixer should be stood on a firm level area so that it cannot tip over during use. Ideally, set it on a paved area with a suitable sheet of ply or similar to protect the surface. Next to this, you will need your materials – sand, ballast and cement. You’ll also need a water supply – keep this well away from the electricity supply if you’re using an electric mixer, and be sure that the mains supply is protected by an RCD. Mixing large quantities of concrete is a two man job. One will be mixing while the other is borrowing, pouring and working the concrete.
Measuring Sand, Cement and Aggregate
Check the total volume that the mixer can mix in one go and work out the proportions of your materials to suit. Use a bucket to measure the materials – using a shovel is not particularly accurate as the amount of each that a shovel can hold will vary. It’s easy to find that the amount of cement on each shovel full is considerably less than a shovel full of ballast.
Mixing the Concrete
Set up the mixer so that the drum is at a 45o angle. Start it up and begin adding the materials by dropping them from the bucket into the drum. Don’t put your hands inside the drum and keep all tools away from it. Start by adding half of the sand and half of the ballast. Add a little water and allow the machine to mix this for a minute or so.
Now add the total amount of cement required followed by the rest of the sand and ballast, and allow it to mix for a couple more minutes. Adding a little water at a time, keep mixing until the required consistency is achieved. This should be an even mix that has the right ‘slump’. It should neither be slopping around in the drum nor crumbling as it turns. If it’s too dry, there will be no flow and the concrete will not fill the area properly. If it is too wet, the mortar will flow out of the mix. The concrete should be sufficiently workable so that all the voids can be filled without air pockets being left.
A useful guide is to achieving the correct slump is to watch the mix as it travels around the drum and note the point at which it drops. If the mix reaches the top before dropping, it’s too dry. If the mix doesn’t reach the side as it turns, it’s too wet. A good consistency will travel halfway up the side of the drum before dropping.
Pouring the Concrete
When the mix is ready, position your barrow in front of the mixer and hold it steady while the other person tilts the drum slowly to load the barrow. Barrow the concrete to the work area and pour it in. While you’re working in this area, the other person can mix the next batch. The concrete should be worked into the area with a shovel, making sure that it completely fills the area leaving no voids or gaps. Continue the mixing and pouring process until the work is complete and up to the required level.
Cleaning the Mixer
Do not allow the mixer to dry before cleaning it. When all the mixing is finished or you’re stopping for a well earned break, clean the mixer and all your equipment thoroughly. Dried concrete is hard to remove – particularly from a mixer drum. Put a small amount of sand into the drum and plenty of water – leave it to run for a few minutes then empty. Follow this with water to clear the remainder then switch off the mixer. Finally, if needed, you can use a stiff brush to clean the blades. Brush off remaining concrete from shovels and buckets etc and then hose them down leaving them clean and ready for use again later.