Mixing Concrete by Machine
There are many different types of mixers, from the portable mixers suitable for the small builder up to the very large static mixers suitable for very large sites where the concrete is being site mixed. Mixers can be powered by petrol or diesel, and the smaller portable ones by electricity.
Whenever a mixer is being used, all safety measures must be taken and the manufacturer’s instructions followed:
1. Set the mixer up according to the manufacturer’s instructions. It is always a good idea to use polythene to sheet down under and around the mixer, in order to minimize mess.
2. Ensure you have sufficient materials and small tools.
3. Start the mixer.
4. Add around half the water, using a bucket.
5. Add the pre-gauged materials or separate materials in the following order: half of the aggregate, then cement, then the other half of the aggregate.
6. Add more water as necessary, to achieve the required workability.
7. Allow the materials to mix for 3 minutes – any longer may cause segregation of the mix. Timing should be taken from when all of the materials are in the barrel of the mixer. All batches should be mixed for the same length of time.
8. Turn out the mix into a wheelbarrow – wetting the inside of the barrow will assist with tipping out the mix later.
Fig. 12 Electrically powered concrete mixer.