Incredibly, my Pozible reached its target after a little over a week. The timing couldn’t have been better either; I was made redundant on the day I launched the project, so suddenly I had all this free time to make a shed. I pretty much started straight away.
I’m making more of a workshop than shed; it will be a place for making, not storing things, and I will design each part to enhance my making things capabilities. With this in mind, one of the first things I wanted was a nice, smooth, flat floor. During set up I’ll have some heavy machinery to move around and later when I’m making things it’s nice to have a flat floor to use as a reference surface.
The original pegged layout. I actually moved it out after this
The site slopes down to the front left corner, about 300mm fall over about 8 metres. Inconvenient, but manageable. Guidelines suggest 100mm depth of concrete for sheds, 150mm if you want to drive a car onto it. I started at the highest point, the back right corner next to the house, picked a reference point, then laid out formwork in the same plane as this reference. I had some massive oregon beams lying around so I’ve used those, with the plan of repurposing them into the build once the concrete is set. Once the formwork was established as a reference plane I then set about excavating and backfilling until the fill was 100mm below the formwork.
The final layout. The shirt on a stick was to stop me from backing the trailer into it
This has taken ages! Moving bricks and shoveling gravel is hard work. I lifted pavers and excavated a bit at the top and built up and filled in the bottom. The goal was 100mm coverage for most of the middle of the slab, with a 300-400mm trench around the outside, particularly under the long wall which would be load bearing. So I moved bricks, shoveled gravel and took a lot of measurements. This took about 2 weeks.
I imported a cubic metre of crushed granite to shape the underslab, then added another half cube of road base on top to harden it. A concreter came over and gave me some good advice “either compact it or leave it for a few weeks”. Leaving it for a few weeks appealed to my lazy side, but a friend had a compactor lying around so I put that to use. The heavy rain didn’t help but after a week of compacting and leveling I was happy.
Once I created the form for the underside of the slab I lined it with plastic. This is a ‘vapor barrier’ which controls moisture leaving the slab from below while it’s curing, and makes it stronger as a result. I also use this where the slab meets the wall of the house so there is a break point; if the far edge of the slab erodes or gets really wet, and the slab wants to slide downhill, it won’t take the wall of the house with it.
At this stage we essentially have a 7m x 4m bathtub in the front yard.
This is a biggish slab on a sloping site, so I wanted to make sure it was super strong. I had some welded steel mesh lying around so that went in, reinforcing the channel around the edge and about 40mm below the surface through the middle.
I also have all sorts of different chicken wire and fencing mesh, some dodgy bits and a couple of rolls. I wanted to use that but didn’t know much about how it would work, so did some reading.
Steel in concrete is a classic composite material. Concrete is very strong under compressive loads, terrible under tensile loads. Steel is pretty good under compression, but expensive to achieve that strength, and incredibly strong under tension. How does a slab experience a tensile load? Imagine an exaggerated case where the substrate for the whole front third of the slab washes away in the rain. The bit that is now suspended in the air will want to sag. This creates a tensile load in the top of the slab where it is trying to stretch, and compressive load under the slab where it is being squashed.
Not only is concrete bad at tension, it experiences brittle failure. Brittle failure is the difference between chocolate at room temperature and refrigerated chocolate. At room temperature, and higher, chocolate bends then slowly fails. When refrigerated it snaps. Brittle failure means cracks form and propagate quickly. Glass, ice, ceramics, anything that shatters undergoes brittle failure.
Steel prevents cracks propagating in concrete. If a situation arises that puts the concrete under tension steel acts to limit the stretch and so stops cracks forming.
This theory is applied to composites everywhere; one material that’s good with compression combined with another good at tension. Fibreglass is a common example. Incredibly, fibres made of glass can handle quite a lot of tension. The resin they are suspended in is better at compression. Carbon fibre is another, usually combined with a similar resin to that used in fibreglass. Chewing gum and hair also form a powerful composite material.
Knowing all of this I decided I couldn’t put too much steel into the concrete. so I’ve rolled out the mesh and folded it into complicated shapes to link the channel around the edge with the centre. This combination of concrete and steel is known as ferroconcrete, and is one of the strongest materials known to man. It’s so good that if Noah were making a new ark I would recommend he use ferroconcrete. Strong, easy to work with and made from simple components.
We poured on a Saturday, after a week of rain. Half a dozen friends came over to help and it went fairly well.
Team slab: Gavin, Ben, Donna, Evan, Dayne, Sean, Glen
The truck backed up to the slab but couldn’t pour to the far corner, so we barrowed the first half or so. Then once we could pour directly it all happened rather quickly. We stomped and smooshed the concrete into the corners and leveled it as best we could by eye. I ordered heaps too much so we also poured a dodgy extension to the driveway at the same time.
Keen to avoid spending $400 on a special concreting screed I bought some steel channel from the tip shop for $4 and made my own. Dayne said it was ‘okay’. I think the biggest mistake I made was not making the reference against the wall thick enough. I screwed a piece of angle steel on to use as a ledge while screeding, but the lip wasn’t thick enough and it was hard to use. I have no idea how to fix this without burying a piece of wood in the slab forever, but it’s probably not a problem I’ll ever have to deal with again.
Pouring and screeding took about 3 hours. The formwork (apart from against the wall) worked really well and the slab was obviously flat. At this stage the concrete is rough, with sand and aggregate poking up in places. To fix that one uses a bull float, back and forth for half an hour, maybe longer. This pushes the lumps down and brings up the mix of water and cement. This just makes the finish nicer.
The Zen of Bull Floating
After that we waited for the perfect moment to begin hand finishing. I waited, tested and waited. Then I got distracted doing something else and came back to find it much harder than I wanted. I did some hand polishing but it was utterly exhausting and hardly made a difference. At about this time I decided it was ‘good enough’ and called it finished.
I considered hiring a polisher and making the slab beautiful, but decided against it. This was partly targeted laziness and partly robust ethical consideration. Part of why I like recycled materials is the character they bring. Accept that reused materials will be imperfect and just enjoy their lack of uniformity. I think we get carried away seeking perfection and invest far more energy than we need to in making things. So why not just accept it when it works and admire its differences?
I popped the formwork off after a few days and pulled out the star pickets. The rain really helped with that. I’ve now started laying out the construction materials, so Slab Phase is officially over.