How to lay concrete
This is a step by step guide for preparing the ground work for laying a concrete base.
How to prepare the ground for laying concrete
- Preparation is key, so ensure you have enough time to plan and don’t rush the preparation process. Typically this takes around 2 hours
- Mark out the area where you are going to cast your slab, using wooden pegs and string to mark out the area where the concrete is going to be poured.
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You must plan to begin laying as soon as the concrete is discharged as the setting and stiffening begin immediately after discharge
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You will need to dig the area to accommodate the sub base (100mm) and damp-proof membrane (dpm) and include the thickness of the concrete
Pouring the Concrete
- You should plan to use ready mix concrete within two hours; otherwise, if it stiffens too much you may not be able to use or lay it
- If concrete dries out too quickly it can crack, this is more evident in warmer, drier weather. To help prevent rapid drying you can lay polythene sheeting over your concrete. Do this once the concrete has stiffened enough not to be disturbed by the act of laying the sheeting
- Specialist spray can be applied, or curing membranes can also be used. Be sure to seek specialist advice before using. Full hardening and strength development, sufficient to allow use of the laid concrete will take between seven to ten days, depending on weather conditions
Tools
- Spirit level
- Builder’s square
- Lump hammer
- Garden spade and shovel
- Heavy roller
or wacker plate - Strong wheelbarrow
- Pegs and string
Materials
- Ready mix concrete
- Timber planks
- Hardcore
- Fibreboard filler strips
for expansion joints - Polysheet
- Wood preservative
(if required)
Calculating the volume required
As a guide, multiply length x width x depth of the space you need to fill. This will calculate a cubic size (or volume), of concrete required. You can use our calculator tool.
Note: you may also need to consider additional features of the job e.g. slope, drainage, accessibility etc. If you’re unsure it may be best to contact us to seek further advice from our technical team.
Health and safety considerations
Suitable clothing including impervious gloves, long sleeves, eye protection and suitable impervious boots should always be worn when handling concrete. We strongly recommend that you visit our health and safety page, and take the time to read our concrete health and safety datasheet for more information.
Preparing the concrete base
The preparation needed will depend on the existing surface and what the concrete will be used for. Measure and mark out the area you need to concrete, using a string line and wooden pegs, ensuring that the corners have a 90 degree angle. As a rule; your base must be 75mm larger that the finished slab to allow for formwork. Clear the area of stones and plants and excavate to a minimum depth of 100mm (deeper for soft ground). The ground should be levelled and well compacted using a small garden roller or a lump hammer to provide a base for your concrete. A layer of hardcore of a minimum of 100mm is needed for a sub-base and should be well compacted.
Setting up a concrete framework
The formwork is made up of timber planks 25mm thick that go around the edge of the area you want to concrete. This will support the concrete as it hardens and also is used to form a level. When building your formwork it is important to remember to allow for run off, allowing water to drain from the surface and to knock some of the timber planks in deeper than others (use a spirit level to check this). As a guide, an area of concrete 2m wide should have a drop of 25mm to prevent standing water. If you are laying a circular concrete base, you could use a rope or hosepipe to mark the outline of the curve you want to follow. The pegs and string will need to be closer together to maintain the curve and the timber will need to be cut to about half its depth on the side that will form the inside of the curve.
Tips for pouring your concrete into the project area
Spread the concrete between the forms 10-15mm higher than the finished surface and compact it down using a tamping tool. Choose a piece of wood with a good straight edge ensuring it’s longer than your form. With one person at each end, raise it above the form and bring it down. Ideally this process should be repeated twice. In order to level the concrete use a sawing motion with the tamping tool across the surface whilst moving forward. Ensure that the concrete gets into the corners of your forms and that there are no low spots. If there are then fill these in and repeat the tamping process.
What are the finishing options for the concrete surface?
Depending on the use there are a number of options for finishing the surface of the concrete. Leaving it as a tamped surface will provide a low-slip surface suitable for drives and paths. To achieve a smooth surface suitable for house floors, ponds etc you can use a float which is a flat piece of wood or metal that you can draw across the surface of the concrete. A float or a shovel can be used to create fish scale effects by using circular movements across the surface of the concrete. Using a broom head you can create a brushed effect which indents the surface without pulling it apart.
Setting and curing
Cover the concrete with polythene sheeting weighted down at the edges to prevent it from drying out too quickly and leave for ideally seven days in summer and ten in winter to allow it to cure before removing the covering. Allow an additional day before removing the formwork to ensure that it is set.
What about damp proofing?
- Use polythene sheeting thicker than 0.010” (known as 1,000 gauge)
- Spread the sheeting over the sub-base, ensuring the edges are turned up at the wall to form a tray
- Where there are joints; ensure they are overlapped and secured with waterproof tape. Please contact your local sales office, when you need watertight concrete or jointing materials.
