which timber expands most when wet?

[geertswaans]

I need to make a mould soon for a concrete cast. Inside the cast there will be a long triangular groove/slot.
I though of making the mould of this groove from wood, having it soaked in water over night so it would expand slightly. Then pour the cast and hopefully(!) when the concrete has set, the wood will have dried out, and shrunk just enough so i can slide it out of the groove.
Do you think this would work? Would redwood work for that? What are your thoughts?
thanks

 

[Jacob]

Beech.
It'd need greasing up to prevent the concrete sticking to it so wetting it might not be good.

 

[Allylearm]

In my concreting days we used fish oil, it stunk like high heaven but the mould came out clean. I never made a mould to shrink, the plug was to be exact but you want it to shrink how do you regulate shrinkage. Your edge which is formed needs supporting until it is set and ready to be banged out so to speak and the cleaner the slippier the better.

Where is this triangle slot, you can make the mould upside down and the triangle can be formed on top of mould box so to speak, a drawing of what you are moulding would be good. I layed drip grooves like this with a piece of metal sitting in slots on gables going over open top so forming a drip groove. You just pulled it back and forth to form slit and left it there to form edge stength until dry.

 

[geertswaans]

this is what I need to cast.

Its a 45x30x4cm slab with two slots in it (20mm deep, 22mm wide approx).
If I were to make these dovetail moulds out of beech and wax it properly and perhaps also put a releasing agent on it, would that be enough to push it out after the concrete has set?
The reason I thought about soaking timbers for it is that tehy expend, so the concrete sets to their larger size. When the timber dries out, it shrinks again and i thought it would be easier to take it out.
Geert

 

[adidat]

Surely the timber could be titley wrapped in membrane with grease between then once the concerete has set they could just be tapped, out i would have thought.


Adidat

 

[CHJ]

Your inserts need to be made of a sandwich of three pieces with the centre piece being a metal strip that can be pulled out to release the two side tails.