Thursday, September 7, 2017

Fire and Stone

If you need to build a fire for warmth, cooking, or morale, then you should be looking at being in control of that fire. Fire rings made of steel are common at campgrounds and the old standard for a campfire was a circle of (dry*) stones, both of which keep the fire from spreading to grass and nearby weeds. (I've dealt with a few grass fires; they take some skill and a lot of time to kill, so they are best avoided. Look at the West Coast any summer to see what a grass or brush fire can turn into.) A steel ring also provides a handy stand for supporting a grill or rotisserie to make cooking easier.

In the last year or so I have seen a bunch of “ideas” for building fire pits, most of which use common cement blocks and/or landscape blocks. Since we humans have enjoyed sitting around a fire since before recorded history began, I can understand the desire to have a “safe” fire on your patio or in the backyard. If the electricity goes out, having a way to cook all of the food in your freezer before it spoils would be a good thing. The problem I see with the plans being shared on various social media is the use of concrete to contain the fire.

Standard concrete is a combination of cement and aggregate. The aggregate is sand and small stones, which provide the strength (in compression**) of the concrete. The cement is the glue that holds the aggregate together. Cement is made by grinding limestone into small pieces and then heating them to about 2700° F long enough to drive off all of the water and carbon. When mixed with water, the cement turns back into limestone (admittedly, that's an oversimplification, but it's close to what actually happens), binding the aggregate together tightly.

As you can imagine, the transformation of limestone to cement is reversible if you add heat and evaporate the water. What happens when you build a fireplace or fire-pit out of concrete and then build a fire in it? Depending on the heat of the fire, it will start to degrade or fall apart.
  • Up to 212°F, concrete is safe and isn't damaged, so boiling your water isn't a problem.
  • At about 570°F, the cement starts to lose water and shrink, but the aggregate is going to be expanding and causing stress inside the concrete. The concrete will take on a pink color when it cools.
  • Between 850° and 1050°F, the hardened cement starts to decompose back to dry cement, leaving the aggregate unsupported.
  • Around 900°F, the cement starts to rapidly absorb CO2, which creates carbonic acid when mixed with water. This causes widening of the pores in the surface of the concrete, which exposes more surface area to damage from chemicals in the smoke.
  • When the temps get up to 1,100°F, any quartz in the aggregate explosively boils off into vapor. This will create small voids within the concrete, turning it into heavy styrofoam. The concrete will turn a light gray in color.
For reference: wood, kerosene, coal, and other organic materials have flame temps between 3,000-4,000°F. That's more than enough to destroy concrete in a matter of hours, assuming the concrete doesn't explode first.

The general rule of thumb after a house fire is to treat any concrete that is pink or gray as damaged and unsafe. If the goal is to contain your fire you don't want to use damaged material, so I suggest avoiding the use of concrete.

If you want to make your own fire pit for cooking, signaling, morale, or warmth, use firebrick (silica sand that is fused into blocks) or ceramic (fired clay) materials to be safe. Yes, they're more expensive, but they'll last a lot longer and are a lot less likely to hurt people by exploding. Better yet, use a ring of steel like a section of a barrel or a truck tire rim - I've seen tractor tire rims used to make really large fire-rings that a dozen people can sit around in comfort, but that seems to be a bit wasteful to me. To each his own, I guess.

*I specify dry stones because if you use stones pulled out of a lake or stream, they will likely have water trapped inside them. As the stones heat up from the fire, the water inside will boil off to steam, expanding 1500 times the original volume and turning the stone into a bomb.

** Concrete is very good at holding up heavy weights (compression), but poor at being pulled apart (tension) or twisted (torsion). Steel has the opposite strengths, which is why we use steel bars or rods to reinforce concrete structures.

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