Measure and cut two 700mm lengths and two 1470mm lengths of 90 x 35mm treated Pine to make the short and long sides of your formwork.
Clamp one 700mm length to the inside of one 1470mm length. Clamp a small offcut to one piece of timber and then use the second clamp to secure the other length of timber to the same offcut. Once securely clamped, use your impact driver to drive two screws through the long edge into the short edge. Repeat this process for the second long edge and then finally the second short edge. Measure each diagonal to check that the frame is square.
Clear the site and prepare the ground for the slab. Excavate an area that will fit the formwork within it by first laying the formwork on the ground and marking around the edge with the blade of your shovel. Check that it is level.
Place a layer of heavy-duty builder’s plastic at the bottom of the formwork to stop the soil from drawing moisture out of the concrete too quickly, which can cause cracks to appear. Cut it slightly longer and wider than the slab itself so you can tuck the overlap into the sides of the frame.
Use a hacksaw to cut the reinforcing mesh to size so that there is mesh over the slab’s entire surface area. Prop the mesh up on bar chairs so that the reinforcement sits in the middle of the slab.
With your wheelbarrow set up as close to the formwork as is practical, load 3-4 bags of concrete mix, and add the recommended amount of water found on the package.
Mix the concrete by digging your shovel into the dry mix and turning it over. Add more water until the mixture turns slushy, but not watery. The mix should flow slowly when the wheelbarrow is tilted. You will need three wheelbarrow loads of concrete for this project.
Tip the concrete inside the form, taking care not to spill any outside the frame. Spread it evenly once all the concrete from this wheelbarrow has been added. Mix and pour your other loads of concrete, keeping one bag spare.
Once all the concrete has been added, use the blade of your shovel to push the concrete down and pack it in at the edges and in the corners of the formwork so there are no gaps.
Use an 800mm length of timber to level the slab off by running it from one side of the slab to the other. Do this with each side of the timber resting on either side of the formwork, and gently move it back and forth so that sections of the slab that are too high are pushed along by the timber to fill in sections that are too low.
If there are gaps, mix small amounts from the spare bag of concrete until you have an even surface. Continue to screed the concrete back and forth until it is level and wipe off any excess concrete from the formwork.
Use a trowel to achieve a smooth finish. Let the slab set for half an hour so it’s not too wet and then smooth out the surface with large sweeping motions. Repeat with the trowel at a slightly higher angle until the desired smoothness is achieved.
Spray the slab down with a hose a few times a day for three days to avoid it drying too quickly. The concrete can crack if some areas of the slab dry out quicker than others.
Leave the formwork in place for at least 48 hours. Then to remove, unscrew one side of the box frame and then slide the rest of the frame off. You can remove any attached concrete with a bolster and use the timber in another project.