Slab Foundation: Laying The Perfect Slab

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Laying good foundations is vital. We even say ‘get the foundations right and the rest will fall into place’, when talking about any kind of project, not just in construction. That’s because we all know just how important solid foundations are. In this article, we will examine concrete slabs and slab foundations, to bring you an in-depth look at the various options available when pouring slabs, and to answer a few of the most common questions concerning concrete slabs.

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The basics of concrete slabs

A concrete slab is simply a horizontal element made of poured concrete, used within construction projects. Concrete slabs can be laid on foundations, or in some circumstances, directly onto subsoil, in order to form the ground floor of a building. For upper floors, thinner slabs can be strengthened with reinforcement steel, in order to form floors and ceilings.

There are a variety of different concrete slab types, and their use depends on the construction project in question, the site location and ground type, and whether the slab is at ground level or for an upper floor of a building. For the purposes of this article, we’ll look at slab-on-grade foundations.

What is a slab-on-grade foundation?

A slab-on-grade foundation is simply a concrete slab poured directly onto the ground, usually on a bed of gravel, which provides better drainage. The area of the slab has a trench around the outer edges, and the concrete is poured into this as well as across the surface area, to give an outer edge footing for the slab. Reinforcing rods may be used within the footing area to strengthen it, and a reinforcement mesh may be used in the top third of the concrete slab, in order to minimise cracking. Slab-on-grade is typically used when there is no danger of the ground below freezing, although insulation can sometimes be added, in order to use slab-on-grade in areas that are prone to frost.

What is a ground bearing slab?

You’d be forgiven for thinking that a ground bearing slab would be something completely different from a slab-on-grade, since there are two terms in use, but in fact, those two terms relate to the same type of concrete slab – where the footings and the slab itself are monolithic, that is, they are both formed in the same concrete pour.

Problems with slab-on-grade foundations

Slab-on-grade foundations can work well for many construction projects, but they are not suitable for construction projects where there is a high risk of ground movement, such as in areas that experience extensive freezing during winter, or in areas where the ground is liable to heave or other forms of movement.

Slab-on-grade foundations that are damaged by cracking could also compromise the structural integrity of the entire building, and are often difficult and expensive to repair. Other things that could damage a slab-on-grade foundation include tree roots, water ingress and subsidence.

Pouring the perfect concrete floor slab

To produce the best possible concrete floor slab, it’s important to prepare the ground area carefully first. The ground should be dug to the required depth and levelled carefully. The outer trench should also be dug at the same time. Any holes or low pockets should be meticulously filled with soil. Once level, the soil should be compacted using appropriate tools, and then small-grade gravel should be spread across the entire area, raking it to ensure even coverage.

Rebar rods should be used to strengthen the footings, and a wire mesh should be added to the entire slab area, carefully held in place and wired together. Finally, the concrete should be poured in one go, filling the footings and the slab area all at the same time. Concrete should never be poured on very cold days when the temperature is close to freezing, because there is a risk of ice forming in the wet concrete.

Working out your concrete slab foundation cost

There are several factors to take into account when working out the overall cost of laying a concrete slab:

  • the costs of clearing and levelling the ground, and of digging the outer foundation trench
  • the costs of the sub-slab gravel
  • the costs of the concrete reinforcement products (check out our handy calculator here)
  • the costs of the concrete itself
  • labour costs

When calculating how much concrete you will need for a basic slab foundation, you’ll need the length and width of the slab area, plus the required depth of the slab. Don’t forget that the trench, or footings, around the edge of the slab will be deeper, so you’ll need to calculate the concrete volume for this part separately from the main slab area. As a quick example, a slab area of 2m x 3m, with a 15cm depth would require 0.9m3 of concrete – that is, 2m x 3m x 0.15m.

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