Follow these simple step-by-step instructions to lay a small concrete slab in your garden using Cement Australia Concrete Mix.
Tools you will need:
Materials you will need:
For larger jobs it is worthwhile hiring a concrete mixer and a plate compactor to get the job done properly.
Although you can use any product from the Cement Australia concrete range, we recommend Cement Australia's Concrete Mix.
For increased strength & durability we recommend using Cement Australia's Extra Strength.
Before you begin any project, please ensure you protect yourself with the following:
Measure, mark using a Carpenters square, cut squarely and nail the formwork together so that the internal measurement is the required dimensions for the slab.
With square or rectangular slabs, ensure that the formwork is true to square by measuring the diagonals between the opposite corners - these must be equal for the shape to be correctly square.
The height of the timber used for your formwork should equal the thickness of your concrete slab to enable you to screed the finished surface level with the top of the formwork.
Place the formwork in situ and use a spade to mark around the outside edge of the formwork. This will mark the area you will need to dig out to the required depth.
Move the formwork out of the way and dig out the area for the slab to the required dimensions. Allow an extra 5 cm of depth for a bed of crusher dust or road base under the slab.
Place the formwork back in position and ensure that it is level using a spirit level. You might need to dig a trench to ensure that the formwork is set at the correct height. Backfill against the formwork so that it stays in place, and hammer in timber pegs at 100 cm intervals on the outside of the formwork to hold it firmly in position whilst you pour your slab.
Ensure the pegs are level with or below the top of the formwork so that it is possible to screed the surface when the concrete is poured.
Place a bed of road base or crusher dust 6-7cm thick, rake it to a level surface and tamp it down firmly all over to a thickness of 5cm. This provides a stable surface for the slab.
The secret to a crack-free slab is a firm, well drained base so for larger areas consider hiring a plate compactor to do the base preparation work properly.
If reinforcing mesh is required, cut it to fit inside the formwork allowing 5 cm clearance between the formwork and end of the bars so the reinforcing will be completely covered in concrete. Use plastic bar chairs placed closely together to support the reinforcing mesh and hold it in the centre of the slab. The reinforcement must be at least 5cm above the compacted base and around 4 cm below the top of the slab.
Do not allow the reinforcement to sag between the supports or sit directly on the compacted road base; it is important that it is fully encased by the finished concrete.
In a non-porous vessel, such as a wheelbarrow, add the Concrete Mix no more than 2 bags at a time. Add water (1.75 litres of clean water per bag of Concrete Mix) to the mixing vessel. Empty the Concrete Mix blend slowly into the water and mix thoroughly. Only if absolutely needed, add small amounts of additional water (in increments of ~100ml) to achieve a workable mix. Too much water ruins good concrete.
Ensure you check the back of the packaging for correct water quantity. Using Extra Strength Concrete Mix will have a different water mix ratio.
Guide to getting the consistency of Concrete right
Prior to placing the concrete, soak the compacted base with water to minimise moisture loss. Start by placing the concrete against one edge of the formwork and add further batches working away from the edge, spreading with a shovel to ensure that all of the edges and corners of the formwork are completely filled. Fill all the way to the top of the formwork, using your shovel to tamp the product into position to get any air pockets out of the concrete. Level the concrete with a screed held against the top of the formwork and by working from side to side and moving from one end of the formwork across to the other. It is necessary to repeat this process to ensure that the surface is flat. Fill in any low spots as you work the surface.
Be ready to keep on mixing concrete from when you start until the formwork is completely full. This is not a job where you can take a long lunch break half way through!
Water (bleed water) will appear on the surface and then gradually evaporate. Do not work the surface at all while the bleed water is present. When it has evaporated and the surface has stiffened a little, use a float to flatten the concrete and remove any ridges from the initial screeding. If you require a very smooth surface finish use a steel trowel. However, this may be too slippery for outdoor concrete - particularly for pathways so alternatively broom the concrete or use a wooden float to produce a textured, non-slip surface.
Work the concrete in a circular motion angling the trowel towards you and slightly off the surface to prevent it from digging into the concrete.
Use an edging trowel around the entire perimeter of the concrete which will also compact the edges of the concrete. For larger slabs, use a jointing trowel to provide control joints at 150cm intervals, by creating a straight groove around 2cm deep in the slab. The inclusion of control joints will control cracking.
As it dries concrete will shrink slightly and adding control joints creates a straight line break rather than allowing random cracking to form.
Moisten the surface and edges of the slab using a hose on a gentle spray twice daily for 7 days. Curing provides a continuous supply of moisture to ensure a better surface quality and a stronger slab. Curing must start the same day – as soon as you have finished the surface.
Formwork may be carefully removed after 24 hours. You can walk on the surface after 3 days but avoid any heavy loads for at least 7 days.
If using Extra Strength Concrete Mix, formwork can be removed in half the time.