Concrete slab moulds and making your own slabs

[Stones]

I have acquired a number of (plastic) concrete slab moulds (riven face) with the intention of casting my own slabs.

 

I plan to trial different aggregate types (coarse gravel / quarry dust), and adding colourant to see what comes out best, using a basic 1:2:3 mix.  

 

I'm aware I will need to use a release agent of some sort in the moulds. I know vegetable oil has been suggested here before (rather than the significantly more expensive release agents you can buy).  Anything else anyone would advise using?

[Bonner]

From work experience, silicone release spray is best. It is expensive but it goes a long way, a fine mist will stop anything sticking. Anything else may need a thicker film and could leave a residue on the moulded surface.

[SteamyTea]

This

[saveasteading]

I seem to remember that diesel works as a shutter oil, but was frowned upon for more significant structures. Perhaps more a retarder than a release agent. Go on, risk  one of your slabs and let us know.

[Stones]

Trial slabs.  Concrete 3:2:1 mix by hand in barrow.

 

Slab 1, Veg oil used as release agent

 

Clearly I didn't get it all fully mixed.

 

Slab 2, veg oil used as release agent.

 

 

Slab 3, silicone spray used as release agent.

 

 

This slab was made with quarry dust as the aggregate, whereas 1 and 2 used Type 1 MOT. It was also a wetter mix which I think is why it broke.

 

Lessons learned:

 

Veg oil as release agent works, as does silicone spray.

 

Concrete mix shouldn't be overly wet.

 

72 hours in mould to cure may be better than the 48 I gave them.

 

Use a concrete mixer to ensure consistency of mix.

 

I'm reasonably happy with first slabs cast. My neighbour is kindly lending me his mixer, and I'm hopefully getting a loan of a vibrating table to speed up production.