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Foundation - Concrete Slabs

Concrete slabs

Concrete slabs are an important part of the modern home. Most homes have foundations that have ether concrete strip footings and pads or a concrete slab will form the foundation and floor of the new home. When a house is designed, an engineer will take into account the type soil, the load, and slope of the land to determine the strength of the slab required. The general class type of soils when foundations are considered is as follows:

Class and Foundation (ground type)

• A - Mostly made up of sand and rock and has very little change when wet.
• S - Slightly reactive clay in the soil with only slight movement due to moisture.
• M - Moderately reactive clay with moderate movement due to moisture.
• H - Highly reactive clay site with High ground movement.
• E - Extremely reactive site with extreme movement due to moisture.
• P - Soft soils, land slip, mine subsidence, etc. Collapsing soils due to moisture and soil structure changes.

The main types of slab construction are footing slabs, stiffened raft, Waffle raft, pier and slab/pile and slab. The construction of a slab does also depend on the type of house construction used such as double brick, timber clad, and brick veneer.

Stiffened Raft

Stiffened raft is a slab that is poured on to the ground with concrete edge and internal beams. The whole slab is poured at the same time.

Waffle Raft

Waffle raft is a concrete slab that made up of close grid stiffening ribs. These ribs are made by using waffle polystyrene void forms which are laid out in a grid pattern and locked together so they do not move during the concrete pour. The waffle grid must be laid down on a level surface and locked together. This type of slab is ideal for very reactive clay sites as the whole slab sits on top of the ground and is not embedded into it.

Pier and Slab

Pier and slab is a slab system that uses a pier grid into the soil and then the slab will be poured on top. The holes are drilled down into the soil to the engineers specifications and then filled with concrete. Then the slab is poured on top of the piers connecting the piers and the slab. The size of the piers and the depth depends on the soil type and the load of the house; these will be detailed by a structural engineer.

Slab Materials

House slabs are not just made up of concrete they have a system of materials that make up the slab. The concrete slab or foundations must be able to cope with the soil type and the load of the house that will be constructed on top of it.

The House Slab Process

Firstly a site is leveled to suite the slab and house construction with the outer form work put into place. Then the drainage plumbing is put into place under the slab site. After this a vapour barrier or a damp proofing membrane is installed. This barrier is plastic sheeting that is placed under the slab this prevents moisture from the ground rising and damaging the steel reinforcement, floor coverings, and walls. When installing the plastic membrane the plastic sheets should over lap a minimum of 200mm and then be taped together with waterproof tape. Next the steel reinforcing is put into place as per the engineers specifications with the whole grid of steel reinforcement sitting on plastic chairs so the concrete can encase the steel when poured. Then the concrete is poured and spread on top it then smoothed and flattened with the use of concrete floats and mechanical trowels.

For steep sloping sites post or columns are often used: see Column sub floors. However a stepped concrete floor/slab can also be used but will need concrete piers.