Pouring concrete through lattice
Concrete foundation slabs

What is a Void Former Used For?

Void formers are an often-overlooked element in creating foundation slabs. The traditional assumption is that you either build on post and beam foundations, or you lay a really thick slab to help counteract any ground movement. Both methods, in theory, ‘work’ – many Victorian buildings have stood the test of time – but they’re not especially efficient ways of foundation-building and they come with their own drawbacks.

Let’s look at some of these drawbacks quickly and then look at how void formers, and especially compressible void formers, help to solve them.

Post and beam foundations are prone to damp

Most Victorian and Edwardian houses were built by excavating soil for basements, installing pillar-type concrete or brick blocks underneath the level of the basement, and building the rest of the structure bearing on top of these. Of course, there are lots of very beautiful old houses still around today – but the basements all tend to suffer from damp.

The main problem with post foundations on their own is that movement in a particular corner affects the stability of the whole structure — there’s nothing to tie all the foundations together so they end up being quite like stilts.

Thick foundation slabs are prone to more cracking

The other common modern alternative to pillar foundations is to pour a much thicker foundation slab. This has the effect of bulking out the mass in order to help counteract the forces involved with ground movement – primarily those of ground swell and heave.

But the bigger they are, the harder they fall.

The main issue with larger slabs is that they’re still prone to cracking because they’re still having to withstand outside stress. What’s more, when a thick foundation slab cracks, you have a much tougher job on your hands to fix it because you now have to excavate even more.

Void formers form voids

The absolute best way to build a foundation is to pour a foundation slab with a void – and the best way to make a void is with a void former. Essentially, void formers are there to support the liquid concrete for long enough for it to set, and then the void formers can be rendered ‘inert’ through either removing them or with another method so that what’s left is, well, a void.

The best other method, in our humble opinion, is with our Clayboard sheets. These are honeycomb card core boards that are able to hold up a huge amount of weight when dry. You excavate the soil, lay them with our Voidpack system, and then pour the slab!

Once the slab is dry, you then wet the Clayboard through the Voidpack pipes and it decomposes – great for the environment and great for your foundations!

These are classed as “compressible” void formers because you don’t need to remove them after the foundation’s poured. Wetting the honeycomb core starts a normal wet-paper decomposition process and stops them holding any weight, so ground swell simply moves into the void where Clayboard used to be.

If you need foundations that’ll last, let’s talk.

7 August 2023