Finishing the Slab

In my last post I took you as far as the installation of the foam and steel in the slab. All of that is quite standard for foundations construction but there are a few considerations to be made when building passive before the concrete gets poured; a conduit is needed for all cables from the outside to the inside so that no wall penetrations are made later on, the same for all plumbing, and then all little gaps need to be filled.

In the case of the latter, we used “squirty foam” (a foam insulation that expands and then sets rock hard) around the edges of many of the polystyrene blocks to cover up these gaps and prevent thermal bridges. You can see it on the photo below where it looks like a muddy slurry along the edge of the white block.

The guys from Base Construction arrived onsite super early and got working as soon as the first concrete mixer arrived to unload. It is so impressive watching these guys at work. They have to work at a high pace to get the concrete in place, all the air out, get it levelled and smooth, all while the concrete pours out at a rate of knots and the next mixer truck arrives to unload. It is a finely timed dance and the skill and expertise was evident. Here are some images of the process:

The pour took all morning and a dozen mixer trucks unloaded their contents – about 25m3 in total. The concrete is a darker colour than usual as we added a black oxide to give a darker colour when we polish the floor, which will be done much later. The fact we are polishing the floor placed a few other requirements on us, such as no fibres in the concrete and having to take extra care over the curing process to ensure it didn’t crack. To that end we had water ponding on the slabs for over a week, kept in place by a tube around the perimeter:

Keeping the water on top like this allows the concrete to remain cooler and not have such extremes of temperature, which result in natural cracks forming. This was especially important as the day of the pour was over 30 degrees C, and temperatures fluctuated quite a lot over the following days.

After about 10 days the boxing came off the edges and at that point the polythene damp proof membrane that comes out from under the floor was wrapped over the top of the insulation and onto the concrete. This means the entire floor is completely enclosed in a waterproof layer. Then a layer of Eterpan was added all around to protect the insulation against knocks, spades in the garden and other impacts. Finally a layer of Supercoat was added. It too is incredibly tough, after all we need the insulation to perform for the lifetime of the building (i.e. hundreds of years) so we need to be sure that the insulation is protected.