Usage of galvanized melt extract fibers

[Joda81]

Hi there, I'm in the progress of gathering material for my WFO build and the stainless steel melt extract (SS needles) is a pain to find.
However I got 42mm equivalent but galvanized instead of stainless steel. As I understand these are not good for the homebrew casting of the inner dome since it contains zinc with melting point 419°C. Can I use them in the outer layers?

[david s]

The zinc coating certainly wouldn’t melt in the outer rendered layer, but I wouldn’t be using them on a thin layer over a compound curve. Being stiff and 42 mm long they will be hard to trowel on and you’ll have quite a lot sticking out from the surface. I’ve tried quite a few different fibres and currently use AR glass fibres which are 25mm long and around 0.3mm thick. Being short and somewhat flexible they are easy to work with and finish up well.

[Joda81]

Thank you for the reply!
Yes you are right they would be too long to use, have realized this.
Do you mean you use the AR Glass fibers in the inner homebrew layer aswell, instad of SS needles? Any idea of alternative to SS needles?

Would polypropene fibers also be a good option in the outer layer or is it a risk that they will melt there?

[david s]

You could use the AR glass fibres for the inner layer as they have a melting point of around 900C. They are much easier to work with than the ss melt extract fibres.
There’s no way the outer layer will see temps of 160 C, so the fine polypropylene fibres can be used there as well, but they won’t improve strength as they’re too fine. They do help to hold the wet mix together and assist in reducing shrinkage cracks on early setting though. The AR glass fibres will enhance strength. I use both.

[Joda81]

Appreaciate your support :-)

I will try to get hold of AR glass fibers instead then. Sound actually like a better option.

So to summarize:
Inner layer: Homebrew with AR glass fibers (for enhancing strength) and polypropylene fibres (for creating microchannels after melting)
Outer layer: Rendering with AR glass fibers (for enhancing strength) and possibly polypropene fibres for easier application and less cracks

Any indication on how much AR glass fibrers to use in a 80cm inner dome around 5 cm thick?

[david s]

For every 10 L of wet castable, add around 150g of AR glass fibres. Those fibres are used extensively by the concrete countertop manufacturers, so try their suppliers.

[Joda81]

Ok so if I use "3.1.1.1. Homebrew Cast Oven Calculator" found here in the forum I get 77,2 liters wet castable for my 80 cm inner dome. 150g x 77,2 will result in approx 1,2kg of glass fibrers.
Is this somewhat right?

Volume of dome	56,8	litres
Volume of gallery (half cylinder) entrance	13,4	litres
Volume of Oven	70,2	litres
10% wastage allowance	77,2	litres
Total DRY volume of loose material required (3:1:1:1)	110,3	litres
Volume of (3 parts) builders/fine sand required	55,1	litres
Volume of (1 parts) powdered fireclay required	18,4	litres
Volume of (1 parts) portland cement required	18,4	litres
Volume of (1 parts) hydrated lime required	18,4	litres
Total DRY volume of loose material when mixed	82,7	litres
Approx. volume of water required	24,5	litres
Approx. total volume of material mix after water addition	77,2	litres

[david s]

That’s correct, plus some more for the outer shell and how much more depends on how thick you make it.

[Joda81]

Excellent! Appreciate your help! Thanks