Concrete is made from cement, sand, gravel and
water. These ingredients are commonly combined in a 1:2:4
proportion to achieve maximum strength (1 part cement, 2 parts
sand, 4 parts gravel). You can buy the dry ingredients, ready
mixed in a bag or, for larger quantities you can order the
ingredients separately.
 Mixing
by hand:
The mixing area must be both clean and water tight.
Use the following procedure:
Spread the sand evenly over the mixing area. Spread
cement evenly over the sand and combine until the colour is
uniform. Spread the mixture out evenly and add the gravel on it
and mix it thoroughly again. All dry materials should be
thoroughly mixed before water is added.
If using bagged ready mixed concrete you should
still work the ingredients over until thoroughly combined.
Shape dry mix into a pile (like a volcano) and form
a hollow bowl in the centre. When mixing
concrete always use clean tap water that is free of salt and
contamination. Pour some of the water into the bowl, gradually
mixing in the dry mixture until all the water is absorbed. Re-form
the pile and bowl, add and mix more water.
Add a little water at a time until you
have achieved a firm consistency. The amount of water used to mix
the ingredients is by far the most important factor in determining
the final strength of the concrete: Use the least amount of water
that will still give you a workable mix.
A
workable mix should be smooth and plastic (not wet and runny or
dry and crumbly). If the mix is too wet, add small amounts of sand
and gravel (in the proper proportions) until the mix is workable.
If the mix is too stiff, add small amounts of water and cement
until the mix is workable. Note the amounts of materials added for
future batches.
Mixing by Machine:
Add approximately 2/3 of the required water. Add the coarse
aggregate followed by the sand and then the
cement. Blend together until a uniform colour is achieved. Add
rest of water slowly and sparingly until a workable mix is
achieved. Run the mixer for 5 or 6 minutes after all the materials
are in the drum.
Even a firm mixture of concrete will flow into shape when tapped
or vibrated because the water moves to the top. For this reason
the best way to remove air bubbles from the form work side that
will be visible when the job is finished is to tap the board
firmly with a hammer. Always protect new concrete from rain until it has hardened.
Never lay concrete on extremely hot days because the evaporation
during mixing and curing will give a poor result. Curing involves
keeping the concrete damp so that the chemical reaction that
causes the concrete to harden will continue for as long as
necessary. When the slab
has hardened, lightly spray with water for the first day and/or
cover with wet hessian bags so it can cure slowly. |
