Casting a concrete hearth

Today’s experimental building project is an attempt to make a solid concrete hearth for our tiny woodstove. I like the idea of a big chunk of thermal mass to capture some of the stove heat and reradiate it for a few hours after the fire goes out. Also the simple gray block is more of the look we are going for as opposed to a tiled hearth.

I’ve built about half a dozen small structures with slab concrete foundations and poured a zillion footings but I’ve never cast anything in a mold where the bottom surface ends up being the top, so this is brand new to me.

The idea of going out and buying a sheet of brand new melamine just to make a concrete block seemed like kind of an environmental crime to me so I just decided to use some scrap plywood and smoothed the bottom and corners with epoxy, covered it with paste wax and rubbed in some olive oil. Obviously a seat of the pants sort of operation.

Because this will be sitting up on legs with no other support I also built a reinforcing grid out of rebar and tie wire.

After that we mixed up a fairly dry mix of two bags of high psi concrete to fill the 26x26x2 inch mold, scooped it in, did an initial screed on the surface, submerged the rebar just slightly, and then tried something that is either going to turn out to be a brilliant or a totally stupid idea, which is using a sawzall with the blade off to vibrate the form to get the air bubbles out before screeding again. We won’t know how well it worked until we take the form off in a couple days.

After that I waited an hour for the concrete to set and then pushed four small dowels in where the screws for the legs are going to be, because it’s always easier to cast a hole than to drill one.

An hour later I hit it with the trowel just for practice even though this will be the bottom surface. I’ll post how it turned out this weekend!

– Brian

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