9 Steps to Build a Concrete Slab

Summary

– Step 1: Prepare the slab site

– Step 2: Dig the ground

– Step 3: Lay the hedgehog

– Step 4: Make a formwork

– Step 5: Set up the expansion joints

– Step 6: Lay out the reinforcement

– Step 7: Calculate the amount of concrete needed

– Step 8: Case 1: Mix the concrete manually

– Step 8: Case 2: Mix the concrete with a concrete mixer

– Step 9: Pour the concrete slab

A concrete slab is often needed for garden landscaping, such as creating a deck or installing a large above-ground pool.

Making a concrete slab outside is not very complicated but requires organization and good ground preparation. Indeed, before pouring the concrete, it is necessary to dig the ground, install a bed of rubble and sand, and set up a formwork.

Here are the materials and steps required to build an outdoor concrete slab.

Materials needed to make a concrete slab.

Tarp

Concrete mixer

Calculator

Gardener’s line

Manual tamper

Geotextile felt

Expansion joints

Tape measure

Grinder

Mortar

Spirit level

Shovel

Wooden stakes

Wooden boards

Rake

Aluminum ruler 2 m

Sand

Jigsaw

Smoother

Welded wire mesh

1. Prepare the site of the slab

If necessary, start by leveling the ground and then mark out your slab:

– Place wooden stakes at the 4 corners.

– Connect the stakes with a rope and stakes planted at each corner.

2. Dig the ground

Prepare your slab by digging the ground:

– with a shovel for a small area: 10 m² maximum;

– with a mini-shovel for a large surface: this equipment is available for rent.

To know the appropriate depth, add:

– the thickness of the gravel layer: from 8 to 10 cm for a slab reinforced with reinforcement;

– the thickness of the reinforcement;

– the concrete layer: 15 cm for a concrete slab reinforced with reinforcement (recommended).

Dig the ground to the required depth.

3. Lay the hedgehog

The layer of gravel/gravel placed at the bottom of the slab is called a hedgehog. It provides strength and stability to the slab.

For a slab reinforced with reinforcement (the most recommended):

– Place a bed of rubble 10-12 cm thick.

– Spread the rubble well over the entire surface of the slab with a rake.

– Place a mixture of sand and gravel about 3 to 5 cm thick on top.

– Tamp it all down with a manual tamper or a tamping machine (available for rent) depending on the surface area concerned.

4. Make a formwork

The formwork makes it possible to delimit the slab and contain the concrete until it is scorched. It is essential for an outdoor slab.

– Equip yourself with boards equal in width to the depth of the slab and 27 mm thick so that they can withstand the pressure generated by the concrete.

– Cut the boards with a jigsaw to the size of the perimeter of the slab.

– Place the boards in the recess, making sure there are no gaps at the corners.

– Reinforce the formwork by driving stakes behind the boards.

– Check the level of the formwork with a spirit level.

Good to know: in the case of an uncovered terrace, the formwork must be slightly sloped to allow rainwater to run off: 1 cm/m maximum.

– Finally, place a polyethylene film at the bottom of the formwork:

◦ It will prevent rising dampness.

◦ It must be large enough to cover the inner face of the formwork boards.

◦ If you must lay it in several strips, overlap them 15 to 20 cm.

5. Set up the expansion joints

If your slab has a large area, plan to install expansion joints to prevent the concrete from cracking over time.

– Divide the slab into sections of about 15 m².

– Grid the slab area, spacing the expansion joints 1.80 m apart: use your 2 m long mason’s rule so that it will slide more easily.

– Ensure that the joints are maintained by placing small concrete blocks at their base.

– Allow hardening before pouring the slab.

6. Lay out the reinforcement

Reinforcement is highly recommended, especially for large slabs.

– The reinforcement must be raised: place small wedges on which you will place the mesh.

– Cut the mesh to size with a grinder:

◦ A margin of 3 cm must remain between the formwork and the lattice.

◦ If expansion joints are present, cut the mesh into as many pieces as necessary, keeping the margin.

– If more than one mat needs to be laid to cover the slab space adequately, bind them together with binding wire.

Caution: Do not bind the reinforcement of the different sections delimited by the expansion joints.

7. Calculate the quantity of concrete required

Start by calculating the surface area of your concrete slab:

Length in m × width in m = Area in m²

Then multiply this result by the height of the slab: 0.15 m for a reinforced concrete slab.

Area in m² × (Depth in m) = Overall volume of concrete to be poured in m3

Then calculate the amount of concrete needed, knowing that a reinforced slab (reinforced by a reinforcement) requires a concrete dosed at 350 kg/m3, i.e.:

– 350 kg of cement: 10 bags of 35 kg;

– 630 kg (or 420 liters) of fine sand;

– 700 kg (or 840 liters) of gravel;

– 175 liters of water.

Multiply these quantities by the total volume of concrete needed for your slab.

Tip: For ease of use, use ready-mixed concrete. This way, you avoid renting a concrete mixer! It is also possible to have ready-mix concrete delivered by truck.

8. Case 1: Mix the concrete manually

– For a large quantity of concrete, spread a tarp on the ground, near the construction site and your equipment.

– For a small quantity, use a clean wheelbarrow.

– Pour the materials onto a tarp or into a container with a bucket in the following quantities:

◦ 1 volume of cement;

◦ 2 of sand;

◦ 3 of gravel.

– Mix dry with a trowel or shovel.

– Form a crater in the center of the mixture and pour in water gradually.

– Knead the mixture and adjust the amount of water until a smooth paste is obtained.

8.2 Case 2: Mix the concrete with a concrete mixer

– Place the mixer on a stable, level surface and make sure you have your equipment nearby. Equip yourself with suitable clothing and protective gloves.

– Turn on the mixer.

– Tilt the tank back 20°.

– Pour in a bit of water first, then add the ready-mix concrete with a shovel.

If you are not using ready-mix concrete, apply the 1 – 2 – 3 – ½ rule: 1 volume of cement + 2 of sand + 3 of gravel + ½ of water.

– Let the concrete mixer work until a smooth consistency is achieved.

 

Read more: How Do You Spray Concrete With a Concrete Mixer

 

9. Pour the concrete slab

– Pour the prepared concrete into a wheelbarrow and pour it into the center of the slab.

– Spread it with a rake.

– Level the concrete with a large 2 m ruler by sliding it over the form boards and expansion joints: always pull the ruler towards you, backing and shearing.

– Continue pouring and leveling the concrete as you go.

– Once the concrete is poured and leveled, smooth it with a float in circular movements.

– Then, let it dry for at least 24 hours before removing the formwork and walking on your slab.

– You will need to wait 1 to 2 months before you can lay a covering.