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How to Lay Concrete

Date posted: 8 January 2015

It is possible to pour smaller quantities of concrete simply using everyday items from your garden shed, although for larger projects you may wish to hire a few pieces of specialist concreting equipment such as a mechanical concrete mixer or a compactor plate. You will also need to be relatively strong because the mix is fairly heavy. Other than a little planning and some physical effort, however, pouring concrete can prove to be quite straightforward.

Key steps in the process:

Prepare the ground

The area in which you plan to pour the concrete will need to be cleared first by removing any debris such as rocks, stones and weeds or even old concrete. All that should remain is raw, exposed earth.

Prepare the subbase

A subbase consists of any material on which the concrete will rest. It lies on top of the soil underneath, which is called the subgrade. This subgrade needs to be well compacted (flattened and stable) before the subbase is added. Sometimes, on small projects, the subbase is simply the compacted earth - but usually stones are used as the subbase. Again these need to be level and as compacted as possible to provide a stable foundation for the concrete. You can choose cheaper, larger, open grade stone as your subgrade and because this does not contain the smaller stones it will give water a means of passing through. The downside to open grade stone is that it does not compact as effectively as the more expensive, finer grade stone. The subbase should be about 10-20cm thick before it is compacted. You can compact it using a simple hand tamper or hire a plate compactor - well worth it if you are pouring over a larger area.

Create a form / formwork

A form is usually simply a wooden perimeter built around the pouring site to contain the concrete until it has hardened. It is held together with special nails or screws. If it is well-built it will help you achieve more professional results. If you need a square or rectangular form ensure the corners form precise 90 degree angles. With a tape measure check both diagonals of the square or rectangular - these should be the sameand if they are not you need to start again. The wood should also have a slight slope because if level the water will build up in the middle of your concrete area. Try to create a slight slope of about quarter of an inch (0.63 cm) for every foot (30.48 cm).

Decide whether to add rebar or wire mesh

You may want to consider adding rebar or wire mesh to your form – both have their advantages. This step is optional on many projects, and may be overkill – but it will offer extra stability for your structure so it is well worth the extra effort if the concrete is going to be load bearing, such as on a house driveway. Choose rebar if you require improved structural integrity, particularly on higher load-bearing surfaces. Use wire mesh if you are more keen to guard against small cracks spreading, and want to gain stability across two axes, but are not so concerned about structural integrity.

Mix the concrete

Buy your concrete ingredients – Portland cement, sand, and coarse aggregate (gravel) in a ratio of 1:2:4. You will also need a supply of water to add in order to bind all the dry ingredients together.

You can mix concrete in a wheelbarrow or, for larger projects, you may want to consider hiring a concrete mixer. Always add water very conservatively – you do not want to add too much or you will end up with weaker concrete. Drier mixes are more resistant to cracks and deterioration. Mix thoroughly until you achieve a nice, smooth consistency.

Pour the concrete

The next step is to pour your well-mixed concrete into the form. Spread it out over the entire area using shovels and rakes. At this stage you will benefit from having some extra people on board to help spread out the mix as quickly as possible.

Screed the top surface

To flatten out the wet concrete you need to screed it, which can be achieved using a wooden plank. Move it gently back and forth over the surface, preferably immediately over the forms, to flatten the surface, working from top to bottom of the area.

Float the surface

Immediately after screeding, move on to floating the surface before the concrete starts to dry. This compacts the concrete. Use a large floating device (bull float) to force down aggregate and bring the cream (i.e. the gravel-free concrete) up to the surface. You achieve this by shifting the float away from you whilst keeping the tailing edge a little bit elevated, and then moving the float back towards you, again keeping the leading edge a little bit elevated. Then pass a magnesium hand float over the surface using long, sweeping movements.

Add traction

If you want to prevent the concrete from becoming too slippery when it rains, add some traction by sweeping a broom or similar tool over the surface. This will leave marks on the top that will offer some grip.

Cure / seal the concrete to help protect it

Finally, prevent the concrete from drying out too quickly, which can cause cracks, discolouration and other deterioration. You can do this by physically covering it with a moisture-retaining barrier – either in the form of a layer of plastic sheeting, or by adding a chemical curing compound.

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