Laying concrete foundations
Laying concrete voids

Do Concrete Foundations Need a Void?


Ground swell and subsidence is a huge problem for homeowners and builders alike. You make the perfect structure only to have it come apart at the seams a few years later. And if it’s not the ground swelling, then it could be the ground subsiding. In the United States, ground swell causes $2billion worth of damage every year - so this is clearly a significant struggle. 

Now, usually, a foundation for a building is something like a big slab of poured concrete – so you’d imagine that this shouldn’t be very affected by a bit of ground movement.  But you’d be wrong.

Let’s look at how concrete foundations work.

How concrete foundations work

Generally, the aim for a foundation is to provide a stable, flat platform on which to build the rest of a building. One way to do this is by ‘Piling’ - which is the process of driving metal or concrete columns into the floor until you hit bedrock. You know that the bedrock is unlikely to move, so anything you put on top of those piles is going to be stable, regardless of what happens to the soil in between.

Piling, however, is very material and labour intensive, so subsequently very expensive. The most common alternative is to create a poured concrete slab foundation. In essence, you dig a square-ish hole, shore up the sides with wooden planks to make a mould, and fill it with concrete. 

Once set, you’ll have a relatively stable, fairly flat platform. But the actual stability and flatness of this platform depends on a variety of factors.

Concrete has incredible compressive strength, but it’s very brittle and prone to chipping and cracking – so it’s usually reinforced with metal grids that act as a sort of ‘skeleton’ for the concrete to stick to. The concrete ‘sticks’ to itself, but the metal grid provides enough rigidity that it’s not going to crack from its own weight.

Assuming the ground has been prepared properly and the concrete has been poured properly, you’ll have a flat surface – but if the concrete slab isn’t deep enough then any movement underneath it affects everything on top.

But here’s where ground movement becomes a problem.

Even though concrete is reinforced by the metal bars, it’s still entirely dependent on what’s below it to remain flat.

Let’s visualise the differences:

Ground floor construction diagram

When you start with a flat slab, all is well. That is, until the ground moves.

The ground can move either upwards (heave) or downwards (subsidence). But with either of these movements you get either a force pushing up against the slab, or you get a hollow where the slab is no longer supported.

Ground substance diagram

Either way, the slab cracks. Now, there’s more leeway with subsidence because the slab will be able to support itself, up to a point - but eventually it’s still going to crack.

The solution, then, is to provide footing. This is a similar concept to piling insofar as it’s using a narrower, deeper support. This means that there’s less chance of the small changes in the ground causing significant damage.

Footing flat diagram

The problem here, however, is that you still have the opportunity for more concentrated changes in ground conditions to cause instability. Moreover, this instability can cause further damage:

Footing problems diagram

The solution, then, is to include voids underneath the slab and in between the footing. These voids provide enough leeway between the ground and the slab so that swelling or subsiding won’t put pressure on the slab itself.

This looks something like this:

Void diagram
What causes ground movement?

The question now is, what actually causes ground movement? Well, primarily, it’s changes in humidity.

Things such as general ground movement, due to earthquakes or large impacts, can displace ground, but these are comparatively rare. The most common way for ground to change is through change in moisture. The actually damaging aspects of ground movement, the swelling and subsidence, happens when soil expands or contracts at different rates.

In theory, if the soil were entirely one material, then it would absorb and release moisture at exactly the same rate, and would therefore not bulge or buckle in any parts. In reality, however, it’s very rare, if not impossible, to find a completely homogeneous bit of ground to put a foundation on. 

Most soils are composed of varying minerals; things such as clay minerals and bentonite absorb moisture at different rates, and therefore swell at different rates. It’s this that causes problems.

Ground may look solid in midsummer, but a few days of heavy rain can change that dramatically.

Do concrete foundations need a void? And how deep should it be?

So, to answer the original question, it depends on the size of the foundation. It’s usually a good idea to include a void in all foundation work. Yes, the foundation for a small shed may not need the extra effort of a void formed – but for any more permanent structure, it’s worth going to the effort of making sure that the structure is built on a firm foundation.

The actual depth that a void needs to cover depends wildly on the environment in which it needs to be. The actual depth of the void needed is established using a soil ‘Plasticity Index’ – this is a measure of how much the ground swells or contracts due to moisture and is dependent on the geology of the environment. Usually, a void will need to be between 50mm and 150mm deep.

Need a void former?

Now the question is, how do you form a void? The concrete slab needs to be supported until it has set enough to be able to hold its own weight. For that, you need void formers – and we sell some of the best on the market.

See, most void formers are made from inorganic, non-biodegradable material such as polystyrene. And when you put a 150mm slab of polystyrene in the ground and pour concrete over it, you’re essentially just making a deeper concrete slab rather than a void. Any movement moves the polystyrene which, again, cracks the concrete slab. So you often need to excavate 40-75mm deeper with polystyrene formers than you would with cardboard void formers.

If you want to save time and money digging, as well as muck away, check out our Clayboard page to find out about a sustainable void former that’ll make your job easier and result in a better build.

26 April 2021

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