cement under arch

[TrentonTerry]

I have a really simple and cheap setup. I use cement blocks (8 inch high by 1 foot with the two big holes in them) with iron bars along the top to hold my hotel pans up if one of the bricks break. I have a door on one end, the other end is made up into a chimney of the same blocks.

Last year this worked great on the driveway as it was a gravel driveway. This year we poured a cement pad and I put in a grate on top of the cement pad and then built my arches up around this (I use 2 arches so i can boil on 6 hotel pans).

So the first boil about 2 hours into it, the cement pad started 'blowing up' chunks of concrete into the pans and well made a big mess and now there is a crater in the cement pad. Not to mention the reluctance of my wife to come near the sugarshack now after the explosions the prior day (I was more sad about the almost finished syrup). I talked to a cement person and he said since the concrete is on damp ground, it will always be absorbing moisture and I will always have this problem if the concrete gets too hot (the trapped water vapour cant escape fast enough and hence the explosions). His suggestion was to put a foot of sand below the grate and then i will be fine.

I can't be the first person that has this happen to them, but I can't seem to find any talk about this (or maybe I don't use the right terms). Are there alternatives to using a foot of sand? Sand is cheap and local but now I have to use like 10 more blocks for my arch to make the firebox high enough to hold the sand, the grate and then 10 or so inches for wood and then the bottom of my hotel pans.

I attached last years photos of prior arches so you can get an idea of my very basic setup.

[RileySugarbush]

Oh you are not the first! My first block arch was on an old slab. Several hours in and Ka-Blooie! Scared the crap out of me.

Moisture or not, concrete can fail like that. It's called spalling and happens a small area of concrete or rock gets hotter and expands while the rest of the rock does not.

http://www.interfire.org/features/spalling.asp

I prevented it from happening any more by throwing down some cheap thing patio blocks under the grate. They are thin enough that they heat through and done explode. The high temperature does weaken them, but it's better than scaring the whole neighborhood (or spilling syrup).

[Cake O' Maple]

It's my first year doing it, but I've boiled hard 3 successive days for 10-12 hours each day, and so far, so good.

I put down a layer of 1" ceramic blanket inside my firebrick "arch." I'm using a 110 BTU and a 220 BTU burner inside the arch.

Here is a pic with the arch partially built so you can see the setup. I just butted the brick up to the blanket.

If you've got a grate over the blanket, under your fire, I can't see why that wouldn't work, but I'm not all that familiar with the insides of a fire-burning arch...

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