Making Patio Slabs

[Daza]

Due to the cost of patio slabs I'd like to try and make my own 800 slabs (600mmx600mm). My father has a vast knowledge of concrete floating and was a foundry moulder so there's no problem with actually making them but does anyone know how they are made in the industry - not the mix but the moulds. We could make individual moulds out of wood or fibreglass but surely slabs aren't produced like this on any large scale in the industry with each slab knocked out of an individual mould?.

[Rob Morley]

Why not? Big conveyor belt and a hopper and rollers, lots of moulds and racks and things. I expect the moulds sit several abreast on the conveyor, but it seems to me the biggest problem would be storing all of them while the concrete cures.

[Ian Stirling]

I've had good results with casting concrete at elevated temps. Make with boiling water, put it in an oven at 80C, and bake for an hour or so.

I bought a fair number of silicone sealant tubes at 99p each from Lidl.

Take sealant, and mix with damp alkaline powder. (a carbonate if you want bubbles) It then sets in around 3-5 minutes, depending on how much you mix it. Makes pretty good moulds. (in this case of a fish). I'd start out looking for some plastic trays 600*600*40mm or so. Then build an insulated box with shelves for them in. Make a dozen with boiling water, slide all the shelves into the box, shut, come back in an hour.

(mechanical properties of the concrete almost certainly will be worse than slow curing)

[Capitol]

Rubber moulds are available for paving slabs. Try E & M. Commercial slabs are hydraulically pressed, so the mould can be removed immediately AIUI, provided that bottom support is available. Nothing too difficult in producing slabs, if you want colour, buy it by the 50Kg bag. The top(bottom of mould) layer is normally a finer/stronger mix than the bulk. A mixer is really essential if you want consistent results, as is controlling the water content. Good luck.

Regards Capitol

[sPoNiX]

Wooden frames with decent polythene stapled to the bottom work fine. Make sure you place the mould on a really flat surface, otherwise the slabs you make won't be flat!

Grease the wooden frame before use so that the concrete doesn't stick.

You'll probably need to replace the polythene if you re-use the frames. Make sure the frame is screwed together (Not nailed) so that sides can be easily removed if a slab decides to stick in the mold.

sPoNiX

[Ian Stirling]

I find a sprayer with cooking oil in works well.

Making the sides slope out at 5 degrees is also perhaps a good idea.

When playing with this, I found a number of ways to easily alter the surface.

Make a strong mix, coloured differently from the rest, and splatter a few blobs onto the mould before pouring.

Do the same, but with a cookie-cutter, using it as a mould, fill with one colour, fill the rest up with the remaining mix, remove cutter, and vibrate a bit. Taking a dimpled plastic tray, smooth on a bit of one colour, scrape off to reveal some bare plastic, backfill with the other.

[R Taylor]

surprised that no one's mentioned a vibrating table, almost essential for knocking the bubbles out of the mix to ensure the slabs don't crack.

here's one similar to the one that some farmer I used to work for used when making not inconsiderable quantities of 2' square slabs.

bugger had me loading and delivering hundreds of them in a battered 7.5 ton cattle truck for too much of a long hot summer.

RT

[Ian Stirling]

If you get the right mix, it can be fairly strong, but still fairly pourable without voids.

Mixing the right quantities of PVA/accellerant/concrete/aggregate/sand/water and keeping consistent is even more important in this case.

Vibrating table is really, really nice, and makes the mix far less critical.

[Ian Middleton]

Casting concrete slabs years ago, we placed an orbital sander agains the mould when full to virbrate the bubbles out, works fine.