Walls - Concrete mix ratio?

homestyal

homestyal

Registered
I`ve made a mould (200mm x 50mm x 25mm internal) out of sturdy rubber/latex type material ready for the mixture to be added.

Although I could make from resin, I`d prefer to make from a concrete mix but I`m not too sure of what mix ratio to use.

I have it in my head to use 2pts Sand, 1pt or 2pts fine gravel (Alpine grit) and 1pt cement.

Any comments/suggestions would be appreciated.
20150104_122752[1].jpg
 
"A concrete mixture ratio of 1 part cement, 3 parts sand, and 3 parts aggregate will produce a concrete mix of approximately 3000 psi."

Taken from this site;

http://www.everything-about-concrete.com/concrete-mixing-ratios.html
 
I would be careful to make sure that the gravel is salt free. When I made some cast bridges I used crushed stone as this had sharp edhes and is stronger. If you use dusty or very fine gravel you you need to make the mix stronger perhaps 1:2:3 cement, sand, gravel/stone. Make sure that you use sharp sand and not not builders sand.
 
With the fine texture that you want to produce you may fine it a hepl to add som e plasticiser to the mix.
 
One more point would be to use smaller aggregate. I would say nothing larger than Pea gravel. It will flow better into your forms.
 
If in doubt, chuck in some more cement.


The main issues are:

Tap the moulds/forms sufficient times for all of the air bubbles to be released - again, you can't overdo this activity at this level.

Don't overdo the water. If you can get it to the point that it seems a bit stiff on the trowel/shovel, but when you start to work it with the float it goes like jelly i.e. you press down one end and it starts to rise the other, then you've got the water ratio about right.

Cement is a chemical reaction, it needs water to take place, but too much water in the mix leads to cracking.

You can always put some light steel in the moulds as reinforcement. Concrete has high compressive strength, but is extremely weak in tension. Concrete and steel have similar coefficients of expansion, that's why reinforced concrete works so well :D :D :D :D :D
 
Rhini Chugger comment on reinforcement is spot on, I would however recommend something that does not Rust for smaller mouldings.

Also suggest that you forget the Gravel. A 3:1 Mix of Silica Fine Sand to 1 of Cement would probably ensure that you get a Good Fine Rendering from the Moulds. If You Wanted something a little coarser, then as Funand says use Sharp Sand. The Plasticiser is also a good suggestion as well, if you do not wish to buy a Large amount, Fairy Liquid just a few drops will work well. Finally you could also add some Waterproof PVA to the mix. This will add strength and longevity to your castings. But I would suggest some kind of release agent in the Moulds if you add the PVA.

Pics please of what you make!
JonD
 
I would suggest that you look at using dry packaged acrylic cement render with a rod along for reinfocing. The packaged stuff that you mix with water is semi flexible and is cement based but I used it to cast some garden gnomes and its lasted maybe 10 years. http://www.dunlopdiy.com/Renders/r_acrylic_render.html
 
You must log in or register to reply here.
Back
Top Bottom