. MIXING CONCRETE
Small quantities of concrete will often be mixed by hand. The area selected for mixing must be flat, hard, and free from debris and dust. A concrete floor or patio area is ideal but the following factors must also be considered:
• Concrete will inevitably stain the mixing surface so an area should be chosen where this does not matter. Staining will still occur even if the area is hosed down immediately afterwards.
• The area should be large enough to use the mixing shovel and turn the materials easily, with sufficient space to move around. Preferably, the area should be close to the place where mixing materials are stored and/or where the mixed concrete is required to be placed.
• It is important not to mix for too long as this can cause the mix components to segregate. All batches should be mixed for the same amount of time. Sufficient mixing time should be allowed to ensure that all aggregates are coated with cement paste.
Bespoke large plastic mixing trays with raised edges are available from builders’ merchants but may be problematic
to store later or an unnecessary expense if they are only to be used once or infrequently.
The following tools and equipment will be required for hand-mixing concrete:
• a wheelbarrow for transporting materials;
• a water butt, such as a plastic or metal drum or barrel, or alternatively access to an outside tap or hose;
• builders’ plastic buckets, preferably three: one for water, one for cement and one for aggregates, since the latter are often damp. An alternative to a bucket is a gauge box;
• a builder’s shovel for mixing.
Measure out the correct proportions of materials and place the dry ingredients together on the mixing area. Using a shovel, turn the mix over into a single pile, ensuring good integration of the dry materials. Repeat the process by turning the mix back again. The main objective of this is an even distribution of the cement within the particles of aggregate. Repeat this process a third time, by which point the dry mix should be fully integrated.
Make a hole in the centre of the dry mix, ready to receive the water from a bucket. It is vital to ensure that the water does not escape, taking cement with it and thereby reducing the strength of the mix. The materials should be mixed with the water from the outside into the middle until all the water has gone. Turn all the materials over on to the top of the heap and continue turning the whole mix over until it is fully mixed and workable. Use a shovel to place the mixed concrete into a barrow. Make sure the inside of the barrow has been wet first to assist with tipping out the mix later.
Fig. 11 Mixing concrete by hand.