What is a Raft Slab, and how does it work?

What is a Raft Slab, and how does it work?

A raft slab, a reinforced concrete slab placed on top of the ground and strengthened by integral concrete beams in each direction, is used to support new homes or extensions.

What is the thickness of a Raft Slab?

The slab portion of a slab footing or slab is usually 100mm (4 inches) thick.

Sometimes, the slab may be thickened up to 150mm (6 in) to make it stronger and more able to span.

The site’s soil conditions will affect the depth of the strip footings. The clays’ reactivity can be determined by soil testing.

What is the minimum thickness of a Raft Slab?

Residential buildings have a minimum thickness of 85mm (3.35inches) for waffle slab construction and 100mm (4inches) for rib raft floors slab construction.

How does a Raft Slab get built?

This is how to build a raft slab that will be used for your house.

Step 1 – Start Setting Out

A builder uses surveying equipment to determine the boundaries of the house.

Smart builders hire a surveyor to ensure the house is located in the right place and on the right site.

Step 2 – Edge beams

Edge beams are the footings that surround the exterior of the house. The ground is dug for the edge beams.

Unless soil conditions dictate otherwise, the width of the edge beams will be 300mm.

The soil’s reactivity determines the depth of the edge beam.

Step 3 – Steel reinforcement to raft slabs

The edge beams are reinforced with steel.

Concrete workers use plastic supports called “bar chairs” to keep the steel from getting too close to the ground.

The structural engineer will specify the amount of steel required based on the site’s reactivity and the depth of the edge beams.

Sometimes, the builder will dig the edge beams into the ground to ensure the footing is in hard soil. The structural engineer will then specify the depth at which the steel reinforcement should be suspended.

Step 4 – Edge beams filled

Concrete is used to fill the beams at the edges.

Steel starter bars are placed in the concrete at regular centres to connect the slab and the edge beams.

The slab must be poured before the strip footings can be poured.

It is not important where the starter bars are located, but how they are spaced is.

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Engineers will be concerned about the spacing and diameter of the starter bars. There must be enough bars to make the slab and beams function as one beam. If the slab and edge beam are being poured simultaneously, the starter bars do not need to be present.

Step 5 – Fill with sand

After the concrete edges have hardened, concreters will add the sand to the slab to adjust its height. I believe sand, crusher dust, and decomposed granite can all be acceptable.

Before the slab cures, the sand must be compacted to make it hard and firm.

Step 6 – Excavate strip footings

After the sand has been placed under the slab, concreters will remove it from the areas where the strip footings run.

The slab’s surface will allow for strip footings that are higher than the original ground. To get a sufficient depth of strip footing, concreters must dig through the sand to remove some of the old ground.

Step 7 – Apply a damp proof membrane

Concreters insert a black plastic sheet over the ground beneath the slab and into the footings.

Because the black plastic prevents moisture from entering the concrete, the edges must be taped and lapped.

Step 8 – Slab reinforcement

Concreters install steel trench mesh reinforcement in the strip footings and slab reinforcement mesh sheets everywhere within the slab.

Concreters use plastic bar chairs to ensure that the reinforcement does not touch the plastic.

The slab sides are made by concreters using formwork or concrete blocks.

Sometimes concrete blocks can be laid on top of the sand if the ground slopes. However, this makes it difficult to compact and place the sand correctly.

Step 9 – Pouring a raft slab

Concrete trucks and concrete pumps are used to pour concrete into the slab footings. Boom pumps are preferred as they are less likely to knock over the bar chairs.

Concrete concreters smoothen the concrete surface (called screeding) and allow it to dry (called curing).

Concrete that dries too fast can cause cracks in the surface. This is similar to mud cracks which occur when the concrete dries while it is still damp. Concrete workers use special chemicals known as ‘curing agents’ to ensure that all concrete cures simultaneously.