Summary: Learn how to lay concrete outdoors, construct formwork, lay a sub-base, mix concrete, lay concrete and form a control joint.
Concrete is used for pathways, driveways and as a base for sheds. Concrete is laid in a timber frame called formwork. This not only contains the wet concrete mix but the top edges of the formwork provide level surfaces from which to compact the concrete.
The preparation required largely depends on the existing surface and what the concrete will be used for, but as an example we will look at the preparation needed for laying a concrete base for a shed. Try to avoid laying concrete in very cold weather to prevent frost damage.
The formwork will support the concrete before it hardens and is made up of timber planks positioned on their edge that go around the perimeter of the base. The top edge of the formwork is also the surface from which you level the concrete.
To keep the timber planks firmly in place, drive wooden stakes into the soil at the back of the timber.
If laying a circular concrete base or a curving concrete path the formwork should be constructed as follows:
Nail the timber edging to the stakes that you have marked out.
If you are laying concrete in warm weather, it may be a good idea to divide the formwork into sections to prevent the concrete drying too quickly. To do this, follow the same method as for concreting large areas below.
Start from one end of the form and gradually move the tamping beam across the surface of the concrete.
A control joint is a gap between sections of concrete fitted with strips of flexible board running across the width of the base.
The maximum area of concrete you can lay in one go before a control joint is required will vary depending on the thickness of the concrete and the width of the area. The narrower the concrete base (e.g. for paths) the closer the control joints should be.
As a rough guide for calculating how far apart control joints should be, multiply the thickness of the concrete by 30:
If the area is narrow, you may need to reduce these intervals. For example if the area is less than 2m wide, reduce the distance between control joints by 1m. If the area is less than 1m wide, you will need a control joint every 1.8m.
Place a filler strip inside the formwork at right angles to the sides.
Spread concrete into the next bay to support the filler strip and remove the batten and stakes.
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