From paths to driveways, patios to parking areas, laying a concrete slab forms the basis for many of the more significant home improvement projects. While laying concrete is within the scope of the DIY enthusiast, a long-lasting and attractive end product requires proper attention to preparation and sound construction practices.
To lay a concrete slab you’ll need a range of tools. Essential among these are a string line, a straight edge, a builder’s level showing vertical and horizontal levels, a beam for screeding, a large tri-square and concrete finishing tools. Ensure your safety with protective gloves and goggles and by isolating all electrical equipment through a transformer.
- Begin laying your concrete slab by removing all vegetation and topsoil from your site. Clear an area slightly larger than the proposed concrete slab to allow room for the boxing that will be used to support the poured concrete.
- Dig the slab bed down to a level that will accommodate the thickness of your concrete plus any sub-base material. Concrete can be laid directly on to prepared ground, but a crushed stone sub-base will provide extra stability and has the advantage of allowing rain and ground water to drain from beneath the slab. Standard minimum concrete thicknesses are: 75mm for paths and 100mm for patios and driveways.
- Use a plate compactor or garden roller to thoroughly compact the base of the slab bed.
- Lay construction lines using pegs and string to guide the placement of the wooden boxing into which the concrete will be poured.
- Set out the boxing and anchor with wooden stakes spaced close enough together to prevent excessive flexing when the boxing is filled with concrete. Boxing should be the same depth as the proposed concrete slab and should have sufficient cross-fall bordering it to ensure rainwater runoff.
- Lay your sub-base material within the boxing and use a compactor to create a surface that is firmly packed and smooth, without dips or bulges.
- If you are laying a reinforced concrete slab place your reinforcing bars on top of the compacted sub-base.
Pouring Your Concrete
- Pour your concrete. Tap the sides of the boxing regularly during this process to release air pockets. Use a rake to spread the concrete so that it sits about 2cms above the required finished slab depth.
- Use a screeding beam and spirit level to refine the surface down to the required level. Hold the beam across the concrete and move in a sawing motion to create a slightly rippled, semi-smooth finish.
- Once the “bleed water” that screeding draws to the surface has disappeared use a trowel or hand float to smooth the surface of the concrete. For a non-slip finish drag a broom across the concrete after trowelling.
- Cut control joints to prevent uncontrolled cracking. Control joints can be made by cutting channels in the concrete with a concrete saw within a day of setting. They should be 25% of the depth of the concrete and should be placed no further than 4 metres apart.
- Allow the concrete to cure by covering with plastic sheeting and leaving undisturbed for five to seven days.
By following the steps above you’ll lay a concrete slab that will add value to your property and provide you with years of service. Good luck!