The K-Zone: A porch too far: How not to pour a concrete foundation slab

Just how difficult can it be...

...to pour a ton or two of concrete into a hole? Constructing a concrete foundation slab for a small building is described in any number of DIY books and Web sites, and appears to be perfectly straightforward. I've been involved in this DIY lark now for more years than I care to remember, but until now I've not had occasion to try out this particular procedure. But it shouldn't be that tricky, right?

Here's one I made earlier

OK, so you need a concrete slab to support a building, and its top needs to be at or around ground level. This is the textbook procedure...

Dig a big hole. Tamp down the soil at the bottom of the hole until it's solid
Within the hole, construct a wooden frame (`formwork') whose inner dimensions are the required size of the slab. Stake it into the ground. This will serve as a mould for the concrete
Pour concrete into the formwork, until it heaps up just above the level of the top of the formwork
Stir and tamp down the wet concrete to bring any air pockets to the surface
When the concrete starts to harden, `strike off' the top of the mound by dragging a board across the formwork to give a perfectly flat, level surface
When the concrete is firm enough to press on, finish the surface smooth with a float
Round the edges with an arrissing tool
Have a cup of tea. Pat yourself on the back and brag to your mates about how clever you are
Wait a few days
Prise the formwork off the slab. Admire the results

Well, I've got a spade, some wood, a hammer and nails; I can put my hand on some teabags, and there are at least a few people still willing to listen to my DIY boasts, so it looks like I'm sorted. Or not. Here's what really happened.

Dig a big hole

According to the textbooks, you need to excavate an area about a foot larger than the intended size of the slab. Now, it's easy to underestimate the amount of soil you need to dig out. By the time I had dug to the size of the slab, I had already filled two Hippobags with soil, and was ready to start on a third. The prospect of digging out a further cubic yard of soil didn't fill me with enthusiasm. What's more, I was putting the slab in the middle of an area of paving, and I didn't fancy taking up and relaying more of the paving than was absolutely essential.
There is at least one good reason, I discovered, for digging a rather bigger hole than the size of the slab: if you do this, you can fasten the formwork timbers to the stakes from the outside. If you have a small hole, you have to fasten them from the inside of the formwork. The problem is that if you drive screws into the timber to construct the formwork, you're going to have a heck of a job unscrewing them when their heads are under a foot of set concrete. Ideally you should use nails, anyway, not screws. If you use nails, then even if you have to prise apart for formwork because it won't dismantle cleanly, it's a lot easier if you've nailed it rather than screwing it together. If you're working from inside the formwork, you can't easily use nails, because you've got nothing to brace the wood you're nailing against.

Build the formwork

Wood is surprisingly bendy with a couple of tons of conrete pushing against it. According to the experts, you need to construct the formwork from 2"x4" timbers, supported by stakes every 3' or so. This seems a very expensive way to build something that you're probably going to have to throw away after use. I built my formwork from a bunch of 1"x6" floorboards because I happened to have them laying around, and didn't want to spend any more money. This was a mistake. When poured, the weight of the concrete caused the boards to bow outwards. I might have got away with it, I suppose, if I had used more stakes to support the formwork. But I didn't. So my slab now has slightly rounded sides. It's not the end of the world, because it won't be visible when construction has finished. But it's a bit naff, all the same.

Pour the concrete

Concrete is a fluid. This is particularly apparent with ready-mixed concerete delivered in a mixer truck. In order to make the concrete flow smoothly, the mixer operators keep it slightly wetter than you would probably use if you mixed it yourself with a shovel in a wheelbarrow.
Now, because concrete is fluid, it tends to ooze out underneath the formwork if the formwork doesn't sit right on the bottom of the hole, all along its length. It's actually quite difficult to dig a hole and make the bottom absolutely flat, so you can't altogether stop this oozing. But the less that oozes, the better. Even though the ooze is below ground level (at least in my case), it's still a problem, because you end up with concrete on both sides of the formwork. And this, in turn, makes it difficult to prise out the formwork when the concrete has set. When I did come to remove the formwork, I had to prise the stakes off the boards because (see above) with a crowbar I had screwed them on from the inside. The presence of concrete on both sides of the stakes make this much more difficult than it ought to have been.

Stir and tamp down

This part, at least, went according to plan!

Strike off the surplus concrete

According to the textbooks, the concrete should be poured until it is heaped up just a bit higher than the formwork. This allows it to be struck off flush with the formwork before it sets. Well, the reality is that if your concrete is delivered wet, it won't heap. Wet, low-density material of low viscosity will rise to the top of the formwork, while denser material will settle. The wet material won't heap up. At best you'll get it flush with the top of the formwork. This will leave the level of the denser, firm concrete a little below the top of the formwork. This makes levelling up a real chore. You can't just drag a board across the formwork, because there's nothing there. You have to use a shorter piece of wood and drag the setting concrete around inside the formwork, checking with a spirit level that it's level and even. Of course, if you've leveled up the top of the formwork properly, you'll have that as a guide to the levelness of the concrete itself. But it's still a long, tiresome job.

On the positive side, having the concerete delivered ready mixed by a mixer truck turned out to be cheaper than buying it in bags from Wickes and mixing it myself, as well as a whole lot quicker.

Finish the surface

At this stage, I should have had a perfectly even, flat, relatively smooth, concrete slab, which just needed finishing with a float to make it smooth. But I didn't. The problem is that the low-viscosity layer that separates out on top of the wet concrete mix will will level itself out, but that doesn't mean that the underlying, more viscous material, will be level. In fact, the most solid material will tend to heap up where it was poured, and this won't be apparent until the top layer has dried out or been absorbed. The problem is that, as the concrete was poured out of the mixer truck, it looked even. The unevenness did not become apparent until later. I had been able to get the slab surface more-or-less level by dragging the setting concrete around, but as it dried the surface become uneven, although still substantiall level. What I should have done was used a shovel and thoroughly mixed up the wet concrete before it started to set. As it happens, I was able to get a reasonably smooth, even surface with a float, but it was a chore because I was moving volumes of hardening concrete around, rather than just smoothing the top. Had the slab been any bigger, it would have been almost impossible.

Round the edges with an arrissing tool

An arrising tool is like a float, but with a curved, right-angle bend at one side. The principle of operation is that the bent part is pushed between the slab and the formwork, and the whole thing pulled along the edge of the slab to round it off. A round edge not only looks nicer, but is less prone to pieces splintering off. But could I find an arrissing tool? Never 'eard of it, guv'nor. I tried constructing one out of a piece of bisket tin lid, but it wasn't an overwhelming success.

Prise the formwork off

As you might imagine from the foregoing tale of incompetence, this turned out to be something of a chore. The oozing of the concrete meant that, even when I got the formwork off, the stakes had to be smashed out of the ground with a club hammer. Not a particularly decorous end to this part of project.

What I learned

Well...

The excavation really does need to be substantially bigger than the slab will be
The formwork really does have to be constructed from 2" timber, or at least supported by a substantial number of stakes
Concrete can ooze substantially under even a half-inch gap between the formwork and the soil
If the concrete is too wet, it won't heap above the formwork, and that makes levelling really horrible. Despite appearances, concrete isn't really self-levelling

Approximate costs

Concrete, delivered	£150
Timber (if you have to buy it)	£50
Waste removal (skip/Hippobag)	£100

Total	£300