Well a theory that has served me well in the past has had to be thrown right out the window. The slab is finally down and in direct contradiction to an earlier post embarrassingly titled “the slab stage always looks so tiny” it actually looks surprisingly big.
I think I need to make an adjustment to the theory for waffle pod slabs like the one we have. Because a waffle pod sits above the ground rather than in it you can see the whole thing and my gosh there’s a lot of it. Our slab has 375mm waffle pods under a 130mm slab so all up our floor level is half a meter above the ground next to it. With a normal slab in the ground you only see the top part. Apart from making the slab look much bigger than I expected there are a couple of other nice surprises that has come along with it. Firstly the slab is a great height for a seat and you are spoilt for choice of where to sit. Secondly, even through it is only half a meter up when you are standing on top it feels like the view is so much more satisfying. We would have loved to had a two storey home for the views but we are now thinking the much more economical waffle pod will suit us nicely. We still have to add a little bit of fill and concrete paths around the perimeter of the house meaning we will loose some of our height. With 340mm left there will be two comfortable steps up into the house. If we then add some decking at the same level as the house floor we still have much of our outdoor seating taken care of.
When I got to the block at 7:00am it was already a hive of activity with the guys adding the last of the form work and the concrete pump being set up. Some where around 8:00am the first concrete truck arrived, one of seven. With around seven cubic meters of concrete per truck this meant there was just short of 50 cubic meters worth in our house slab. From a carbon emission point of view this is probably not ideal and when I am feeling courageous I might even do the sums. I do however hope that all that concrete can earn its keep with the house having been designed to maximise its use as thermal mass soaking up the sun during the day in winter and the coolth of the night in summer.
The concrete was coming up from Adelaide so by the time it arrived it had been on the road for around 45 minutes. The topping mix used for the polished floor has a much higher percentage of cement in the mix, this makes it harder but as a consequence it also sets faster. The first five minutes of a truck being on site was spent adding water to the mix to get it back to the right consistency.
The slab was poured over the next three or so hours in two main parts. The main beams around the edge of the slab and between the waffle were poured with a standard mix before the harder topping mix on top of that. One of the slightly unusual things about this slab is the extruded foam insulation around the edge. Extruded foam (XPS) is different to the more common expanded foam (EPS). XPS is stronger, more impact resistant and is impervious to water. This will help keep the heat from the hydronic floor system inside as much as possible. Tho concrete was poured up the the bottom level of the foam and troweled level. The foam was then set on top of this inside the form work and the remainder poured around it. As no one on site had used the XPS before it was anyones guess as to what would happen when the formwork was taken away. Thank fully it had stuck and didn’t fly off with the wind.
The concrete was finished off with several passes with the hand trowel and ‘helicopter’, progressively winding up the blades to get a smoother and smoother finish. If we had wanted to have a burnished finish they would have continued to wind the blade up to run on a steeper angle. Eventually the baldes are so steep the edges are effectively burning the concrete giving it the high polish burnished concrete is known for. The slab finished the day with a coat of curing agent from the company who will be grinding the floor for the polished concrete finish. By this stage the chemical reactions in the slab had made it quite warm and it was cooking nicely. We’ve been told that if it cracks it should be in the first 72 hours. Touch wood… 30 hours in its looking good and with the mild weather yesterday, today and for the rest of the week it should help the slab cure a bit more slowly.