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How to make
a concrete slab
how to pour a concrete slab
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How to prepare the ground, pour and finish concrete

Example: A small shed floor: Part one
A step-by-step example of how to make a concrete slab floor for a 1800mm x 1800mm (6ft x 6ft) garden shed.

The steps for this task are:
bullet Making up the formwork (the frame that borders the concrete and stops it spreading).
bullet Positioning the formwork on prepared level ground.
bullet Fixing the formwork in place with pegs.
bullet Digging a trench around the perimeter of the project.
bullet Mixing and pouring the concrete and placing a piece of reinforcing steel rod.
bullet Screeding and finishing the concrete.
bullet Dismantling the formwork.

preparing to make a concrete slab

Materials you will need:
bullet Approximately 12m (40ft) of 100x25 (1x4) or 100x50 (2x4) for the formwork, brace and pegs.
bullet Three 40kg (94lb) bags of Portland cement.
bullet 0.2 cubic meters (7 cubic ft) sand.
bullet 0.3 cubic meters (11 cubic ft) gravel 20mm (3/4") diameter and smaller.
bullet 10 meters (30 ft) #3 rebar (which is steel rod that is 10mm (3/8") thick).
bullet Handful of 75mm (3") nails.

Did you know? Cement is sold in bags of varying weights throughout the world. Common bag weights are: USA 94lb; Canada 87.5lb; UK 112lb; Australasia and parts of Europe 50kg and 40kg. There are also smaller bags sold as well.

Tools you will need: spade or small shovel; concrete finishing trowel; hammer; level; measuring tape; pencil; carpenters square; wheelbarrow; bucket.


ensuring the concrete floor will be square

Step 1. Picture above.
Make up the formwork out of 100x20 (1x4) or 100x50 (2x4) lumber. The lumber should be straight but a low quality grade will do since it will eventually be discarded. The formwork is the frame that borders the concrete and stops it from spreading.

Tip: the formwork in this example DOES NOT have to be made up in place. It can be made up anywhere on even ground and carried in place later.

Cut two pieces of formwork lumber exactly 1800mm (6ft) long and cut another two pieces approximately 2400mm (8ft) long. Nail the four pieces together so that they make up a square with inside measurements of 1800mm x 1800mm (6ft x 6ft).

bracing the concrete floor frame

Step 2. Picture above.
Ensure that the formwork frame is square. This can be done by making the diagonals equal length. Simply measure diagonally from corner to corner (see Step 1. picture) and then measure the other opposing two corners. The formwork frame is square when the distance between the opposing diagonals are equal.

When the formwork frame is square, nail a piece of lumber on top of the formwork frame from corner to corner, to make sure the formwork frame remains square until pegged in place.

The formwork is now ready to be picked up and carried into position, but first make sure that the ground has been prepared, that is, made level, with all vegetation and any soft ground removed.

determine where the concrete floor will go

Step 3. Picture above.
Lay the formwork in place. Ensure that it is level and then secure it by hammering pointed 50x50 (2x2) or 50x25 (1x2) pegs at 900mm (3ft) intervals against the outside of the formwork.

Nail the pegs to the formwork but not all the way in, as the nail will need to be pulled out later.

peg the boxing for the concrete floor

Step 4. Picture above.
Dig a trench around the perimeter of the floor area about 100mm (4") deep and about the width of the spade wide or about 200mm (8"). This is like a 'footing' to give the perimeter of the concrete floor more depth and strength, as that is the part of the floor that will be supporting the shed walls.

dig footing for concrete floor

Step 5. Picture above.
Saw off the tops of the pegs so that they are even with the top of the formwork. The area is now ready for concrete.

mixing cement, gravel and sand to make concrete

Step 6. Picture above.
Lightly hose (dampen) the pour area with water prior to pouring the concrete. Work as quickly as possible when placing concrete in formwork. For instructions on how to mix concrete click here.

Pouring and finishing the concrete slab: Part Two Continue

Related topics
What is concrete?
How to mix concrete
Reinforcing steel in concrete
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