Tuesday, 15 April 2014

Should be the consistency of thick brownie batter

So here's my recipe:
  • 1 part cement,
  • 2 parts sand, 
  • 3 parts gravel.
  • A little water
You will also need:
  • A wheelbarrow to mix in
  • A strong form to pour into
  • A pointy stick for rodding
Mummy may not want you to do this in the kitchen.

A plasterer was there doing the wall when I was pouring the first section of concrete. It's always a mixed blessing having someone standing over your shoulder giving you advice.

I'd already got the frame so that I thought it would hold the concrete. There's a good You tube video demonstrating the result of not properly keeping the two sides together.

The plasterer wandered over and told me that it was no good, and I'd have to push soil down the outside edges. I was just about to do that.

Later he came over when I was just about to start mixing. "You mix the cement and sand first, then add the gravel later," he told me. I was going to do that, but I thanked him politely.

"You weren't going to do that, were you?" He said next. I wasn't sure what to say. It's a fine line to tread between idiot and expert.

He also told me the stones in the gravel were too big, which other people have also said, but I've mostly been ignoring their advice. I've been using what gravel has been lying around, and not overly worrying about finely grading it. Certainly when I have used smaller gravel it's been easier to mix and pour, but I hope that it won't make much difference structurally.

Over the past couple of years I've been steadily producing gravel while getting a little half-decent top-soil for the garden as a by-product. It's not so much that the soil in the garden has a lot of stones in it, more a case that the stones in the garden have a little bit of soil mixed in with them. So it's been nice to find a use for them, and I've been loathe to pay money at the DIY store or pay trucks to bring a load to the house.

I've also been following various bits of advice, for example from Oli who suggested that washing the gravel was important.
This site has some useful ones, including this:

"Your cement should be the consistency of thick brownie batter. You want to have to scoop it into your mold, not pour it."

There's another good one about putting your concrete mould on top of a washing machine after you pour the concrete. Great idea as this will get the bubbles out, and the bubbles are air, and air will make the concrete crack. The reason it will make the concrete crack is that as the concrete heats up, everything in it will heat up, and that includes any air. As concrete heats up, it will expand at about 12 parts per million per degree. This is not a lot, although it adds up to one centimetre over a 25 metre length and a temperature rise of 40 degrees, which you'd get over the year. Air expands a great deal more as it gets hot, and any bubbles are going to be little pressure bombs adding forces in all directions, some of which are going to crack. I expect to see a few cracks forming in my earlier attempts at concrete, but the later ones should be OK.

Putting the poured concrete onto a washing machine is a great way of shaking all those bubbles out, unfortunately impossible for my 1.6 metres lengths, which I hope are bonding with the rubble below them.