After still more trials and tribulations – all involving misdeeds or failures to act by my contractor – work on our safe-house is underway again. David (our mason) was out here with yet another of his sons (this time his oldest, also David). They worked hard all day, as always. Today they started on the “forms” – the wooden structures that will hold the liquid concrete in place when they pour the roof. Today they largely completed the part of the form that will shape the interior ceiling. Tomorrow they will put a few more supports for this in place, then construct the forms that will shape the overhang of the roof, and its edges.
The weight of the concrete roof (which I've calculated will be about 23 tons) means that the supports for the form have to be very, very strong. At the same time, though, the form is a temporary structure, and it must be easy to take apart. Those constraints make the form-work an interesting engineering challenge. One simple device David used will make the disassembly much easier – he's using special nails with two “heads” on them. One head is where it usually is, on the end of the nail opposite the pointy end. The other head is about a quarter inch from the first head, down the nail's shaft. When the nail is hammered in, it stops at this extra head – and the usual head is then exposed (with a bit of the nail's shaft), ready for easy removal with a hammer claw. A simple, but very effective, invention.
When David has finished this interior form tomorrow, I'm going to do some rough calculations just to make sure the loading on those 4x4 posts is acceptable. David seems very sure of himself, but to my eye this just doesn't look strong enough to hold up such a ginormous block of concrete. I expect my calculations will prove David right, but I will feel better if I can prove it to myself… Can you imagine the mess it would make if that form collapsed, letting 23 tons of liquid concrete slosh out? Ach, I don't even want to think about it! And even worse – what if someone was up on the roof when this happened? No, it's better in this case to be paranoid, and to triple-check every step of it...
There was one big surprise for me when I first saw the form this afternoon: the form is even with the tops of the cement blocks on the east side (at the right in both this photo and the one at the top of this post), about about 2 inches below the blocks on the west side (at the left in the photo at the top of the post). David explained that it was important for the form to be perfectly level, so that the top of the roof is also level (except for the slight drainage slope that will be built in) and the roof is of the correct thickness. Well, that makes sense. But…something else was bugging me. I knew darned well that the slab was level to within a fraction of an inch – I watched them do this, and double-check it. So why were the blocks slightly higher on one side than the other?David was unable to explain this to me, largely, I think, because of the language barrier (his English isn't up to a sophisticated discussion, and my Spanish is non-existent if you're not talking about food). My current theory is that there was some kind of systematic error in the way David placed the string that formed his level reference as he was laying the block. I expressed some concern about how this might affect us passing the inspection (this inspection will occur just before we pour the roof). David understood that concern well, and assured me that such small errors are a commonplace occurrence in block wall construction. I sure hope he's right…
