Polypropylene burn out fibers

[MikeD55]

Would like to add some to castable cladding to be put over the bricks. Was wondering what type/length is best to allow steam to escape.

thanks

[david s]

They need to be really fine (finer than human hair). Mine are 19mm in length. I purchase them as part of a 2 pack system for concrete fibre reinforcing. The finer of the two fibres are designed to reduce slump and to prevent early shrinkage cracking, but also can be used for our purpose as burn out fibres as they melt at 160C. The thicker and longer fibres are used for concrete reinforcing. Add only the finer ones at 1% by volume. They are hard to disperse and this is vital for them to work properly. I’ve found adding them to the dry mix doesn’t work particularly well well with them clumping together like a fur ball. I Add them to the wet mix, pulling them apart in very small amounts then mixing for AT LEAST double the time you’d think for adequate dispersal. Because they’re so fine they are extremely hard to see, so inspection is difficult.
To check that you have the correct ones, take a pinch and place them in a preheated oven at 160C, which is their designed melting point.
If you want to strengthen that layer, use either AR glass fibres or stainless needles (melt extract fibres) neither of which will melt, but will provide strength at oven service temperatures.

[MikeD55]

Ok thank you. I ordered them on Amazon. 19mm although they didn’t mention a thickness. I’ll be using a basalt mesh for strength. 1% volume or weight???

thanks