Absco Sheds - Tip 3 - Formwork

First, measure and mark out the area to be concreted. Stretch stringline between pegs driven in the ground, and then remove the top soil until you've accurately marked out the shape of the area.

Use a builder's square to set the corners accurately. Then dig down to the depth you want the concrete. The prepared area should extend about 150mm beyond the edge of the finished slab so there is space for preparing the formwork.

Be sure the ground is firm, level and free from vegetation. If it is soft and loose, add a layer of stones or broken bricks and tightly compact into the surface with a sledgehammer. If laying immediately over an old concrete base, completely break up the old first and compact it or your new concrete will also crack.

Formwork is a kind of box that keeps the concrete in place while it hardens and provides an edge for levelling the poured concrete. Hammer in pegs at 1m intervals around the outside of the entire area. Place the formwork boards against the pegs using your stringline to accurately align them. Use a spirit level to ensure the tops will be flush with the finished level of your concrete.

If required, check that the formwork is slightly lower on one side so rainwater can run away. If your laying near a home, slope away from the house. Then nail the boards to the pegs making sure there are no gaps between boards, or boards and ground where concrete can seep through.
Finally, to help prevent your concrete cracking by earth movement, tree roots and usage, adding steel reinforcing, this is essential to increase strength. With bolt cutters cut your reinforcing mesh 50mm less than the distance between the formwork to protect it form rusting. Then place it inside the formwork supported off the ground on bar chairs suitable for the thickness of the concrete.

LUKE'S TIP: If you want curves in the formwork, use hardboard cut into strips as wide as the concrete is deep and support with frequent pegs around the curve.

On next weeks show we are going to show you how to pour the concrete. For further information visit www.absco.com.au.

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