Hi everyone.

This is my first post and first time attempting to pre cast concrete. I'm based in the UK (unfortunately this means that I have much less variety of concrete materials available to me - we don't seem to have readymade countertop mix here) and by trade I am a furniture designer. My wife and I are having a wood burning stove fitted in our living room. Rather than the fire company laying a tiled slate hearth, I thought I'd have a go at casting my own out of concrete. Here's where I'm at:

- I've built a melamine faced board mould to make a hearth of overall dimensions 120cm x 85cm x 3.2cm (47" x 33" x 1 1/4") Please see image above.

- The hearth, once in position, will be sunk into a hole in the floorboards with only the top 1/2" showing.

- The most structural stress the hearth will experience will be moving it into position and so it only needs to be strong enough to withstand that.

- I will be placing steel wire mesh panel (see above, 1" square holes, 1/16" wire thickness) in the top quarter of the mould, so when the hearth is in place the reinforcement will be in the bottom quarter.

- My concrete mix is 3 parts sand, 2 parts 3/8" aggregate and 2 parts cement + small fibers to control the cracks, plasticiser and a charcoal pigment powder.

The issue I'm trying to solve at the moment is how to keep the mesh flat and at the correct level during pouring and knocking out the air bubbles.

I've tried suspending it from the edges of the mould but the middle dips down too far and some of the edges poke out the top. I would like to be able fill the mould 3/4 full, place the mesh in and then top up with the rest of the concrete. My worry is that during knocking all the bubbles out with a hammer and orbital sander, the mesh will either sink to the bottom or float to the top.

My thoughts now are to fill the mould 3/4 full, knock out the air bubbles, let it harden to a point where the mesh won't sink, place the mesh in and then top up with concrete.

What I'd like to know is:

1. approximately how long would it take for the concrete to get to the hardness where the mesh won't sink?

2. would I be able to prevent the 'top-up' concrete from hardening by continually mixing it while the rest hardens? Or would it be better to make it in 2 batches?

3. would this method affect the strength of the hearth and would a longer time between the 2 pours give mean less strength?

4. How would adding pigment to only the first batch (in order to create a deeper colour) affect the strength? I'm guessing there would be no bleed between the layers.

Any thoughts and advice would be greatly appreciated.

Many thanks

Ollie

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Just make two batches of concrete. Pour the first one, knock the bubble out and wait til it sets up....you can test with your fingers, it just needs to firm up a little bit....set the mesh in when its firm and pour your next batch over it. As far as the coloring, I wouldn't worry about strength loss. I would worry about strength loss if you use the "parts" method. Get a good recipe and do it by weight....not by volume.

I'm in the uk too.
I made a hearth stone about five years ago but never fitted it, it's still outside my shed in all the weather.
I used a melamine form and the mix I used was 1 part marble dust and 1part Portland cement, pigment and water and SBR mixed 1 to 1.
It was more of an experiment more than anything but it is really strong.

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