Friday, 3 October 2014

How to make pourable self-levelling concrete


Today we're going to take a break from the beauty of nature to delve into some construction notes, mostly for our own future reference. Getting a level floor is incredibly useful, but hard to achieve with a standard stiff concrete mixture. This is an experimental liquid mix devised by Andy, my Viking associate, which forms a level floor even on an uneven surface.

It's only designed to be strong under compression, so is good for floors but not much else. We've not fully tested it under load, so this article may be edited in the future to add success or improvement notes.

The proportions are as follows: 3 parts cement, 9 parts fine sand, ~5 parts water.

All measurements are by volume and use grey cement and very fine sand such as this from Bostik, The grain size of your sand is incredibly important to this particular mix - if you use standard coarse building sand, the mix will separate upon pouring and won't set properly.

Start with 3 parts water - you're after a very liquid mix, but depending on factors such as the consistency of your sand and how wet it is, you may need less than five parts: add the remaining water slowly. Use a power stirrer drill attachment to mix and pour immediately after stirring - within 30 seconds is ideal.

You want one of these attached to a power drill to stir your concrete.
You also want to remember to clean it after use
It's advisable to test the mix for your own ingredients in advance by setting up a box on a slant, pouring in a small amount of concrete and making sure that it both levels and sets hard.

We created a stiffer concrete surround to enclose the floor area we were pouring, but if you're pouring into an area that's already vaguely watertight, you can simply reinforce any small potential leakage spots with duct tape, for example.

Note that the setting time is on the order of days, not hours, for this stuff - it took around five for a full set.

Cat-proofing: not necessarily possible under real-world conditions
If you have pets or small children running about the place, you'll want to cover the area you've poured if possible - we used a combination of wood and cardboard. As evidenced by the photo above, this won't necessarily work on cats, who will apparently move the cardboard in the middle of the night, make paw prints, and then push the cardboard back into place.

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