cracking in hearth slab

[camooch]

After running the oven for a few days, today up to 400C, some hairline cracks developed at the edges of the hearth slab at the middle of each side. So, four small cracks, one on each side. Has anyone had this happen?

It seems to be due to expansion of the centre of the slab directly beneath the oven relative to the cooler edges. These cracks are not a worry but probably could have been prevented by placing additional reinforcement along each edge of the slab.

The oven is sitting at 300C at the moment and the underside of the slab shows 60C. Is this high? Are we getting too much heat leakage through the 50mm CaSi and 40mm perlite insulation?

Stuart

[Lburou]

Only time is going to reveal the significance of the cracks...

In my view, the significance of your cracks is likely very small. The cracks could be serious if; 1) You didn't use a good recipe for the concrete, 2) Didn't cure the concrete properly, or, 3) didn't use proper steel reinforcement. The plans are designed to support more weight than your oven, so, you can make several variances to the plans and still have adequate support.

[david s]

Re: cracking in hearth slab

60C is pretty high. Sounds like a wet oven to me. A bit too late to back off with the fire now. Keep firing it will fire better and the bottom of the slab will be cooler as you drive out the moisture. The water under the floor is about the hardest to eliminate.