Tools you will need:
- Tape measure
- String line
- Long Spirit level
- Builders Square
- Claw hammer and club hammer
- Levelling screed
- Bolt Cutters
- Concrete Mixer
- Steel trowel and wood float
- Concrete edger and jointer
Materials to remember:
- Stone Aggregate
- Dry Cement or Ready-Mix concrete for small jobs
- Steel Reinforcing
- Bar Chairs
- Formwork Timber
- Stakes (50 x 25mm square). Length dependant on depth of concrete plus 300mm to go into ground
- Nails (50 x 2.8mm)
- Sheet of plywood or hardboard
- Sacking or polythene sheeting
- Remember to check with your local council or shire about any regulations for permits needed before you begin.
- If you are hand mixing your concrete, mix on a large sheet of plywood or hardboard, or use a heavy layer of polythene sheeting. NEVER MIX ON DIRT.
- Too much water in the mix will weaken the concrete and will cause shrinkage as it hardens. A guide to a workable mix, draw a shovel tip over the mix to leave a series of clear-cut ridges. If the mix is too dry, the ridges will crumble – if it's too wet, they will spread out and level.
- When the surface is level, tap the outside edge of the formwork with a hammer to settle the concrete – fill in any resulting hollows and level again.
STEP 1: Preparing the site
TIP: Take into consideration the size of the formwork you will be using and add this to your overall size of the area you are digging out.
If you are concreting over an old concrete base, make sure you break it up first; otherwise your new concrete will crack.
Measure and mark out the area you are concreting using stakes and stringline.
Remove the top soil from the area and level it out, moving soil from high place to fill in low spots.
Use a builders square to accurately set the corners.
If the ground is soft and loose, add a layer of stones or broken bricks and tightly compact.
STEP 2: Fixing the formwork
TIP: If required, check formwork is lower on one side to allow for rainwater run off. If near a structure, slope away from the building.
Space the stakes 500mm apart around the edge of the area. Using a stringline and spirit level ensure stakes are set at the desired height of the concrete area.
Nail form boards to stakes. To ensure the stakes don't loosen up during this step, place a heavy hammer or similar item to hold them in place.
To prevent concrete from cracking, steel reinforcement should be used. Cut reinforcement mesh 50mm smaller than actual area to stop rusting. Reinforcement mesh should sit off the ground on bar chairs.
STEP 3: Mixing the concrete
TIP: When you have finished using a mixer for the day, leave it running and add some water and aggregate to clean out the drum.
The use of a power mixer should be considered for any reasonable sized job and will save you time and effort.
While the mixer is running, add the aggregate to the drum. Then the sand and cement. Allow this to mix thoroughly before adding the water, usually 2 to 3 minutes.
Only add enough water to make a workable mix. Let this mix through for another minute.
Make sure you clean the drum with water after every mix.
STEP 4: Laying and Levelling
TIP: Concrete slabs should not be any bigger than 3 metre lengths without a joint to prevent cracking. A bituminous strip, plastic or rubber, the same depth as your concrete can be used to act as an expansion joint. Lay the strip whilst you are laying your concrete.
Pour concrete mix into the formwork. To spread evenly, use a rake or shovel.
Make sure that corners and edges are well filled in, and that all reinforcing is covered up.
To begin levelling the concrete, rest the tampering board at one end of the concrete on the formwork.
Using a sawing motion, move the board to and fro. This will allow the board remove excess concrete, and fill in areas where required. To compact the concrete, tap the board forward firmly.
STEP 5: Finishing Touches
There are several ways to finish the concrete slab.
- Use a wooden float, or steel trowel, for a smooth surface finish.
- For a non-slip surface, draw a stiff-bristled brush gently across the surface.
- To prevent the edges for chipping, and to get a professional looking finish, use an edging tool.
The concrete needs to cure (set) properly to prevent cracking and loosing strength. Cover the slab with damp sacking, keeping it moist regularly, or plastic sheeting for at least seven days.
When removing the formwork, tap gently from the top to loosen the bond and preventing damage to the concrete edges.