Exploding concrete!?

[KoreaScouter]

Okay, here's the scene. Klondike derby, out in the depths of a wooded military training area. One of my ASMs is going to demonstrate survival fire building techniques he learned in survival school. Throughout the area are these fairly large concrete slabs that used to have buildings on them. Buildings are gone, slabs still there; all of them about 30 years old and 3 feet thick. One about 10 feet from our troop Baker tent. After the instruction, he throws a little fuel wood on it and shows how to dry wet boots with a fire. Then it's on to s'mores and stories.

 

After a couple hours, the Scouts head off to bed, and I'm tidying up the Baker with another ASM, keeping an eye on the embers. Suddenly, an explosion from where the fire is. We both look over to see embers flying five feet in the air, and spreading out over the whole concrete slab. After moving a couple camp chairs further away, we check the spot where the fire was, and there's a crater in the concrete about a foot-and-a-half across and three inches deep at the center. Spalled concrete fragments are laying in a ten-foot radius.

 

My ASM remembers hearing years ago that concrete exposed to heat may do this, because of air pockets that get expanded. I had never heard of it or seen it, and have seen many fires on concrete, usually at vehicle accidents.

 

The sobering thing is that if this had happened an hour earlier, nine people would have gone to the emergency room. We did our morning formation on the slab, and explained to our Scouts what happened.

 

The irony here is that we consciously decided to do this on the slab, so that we wouldn't create an unsightly fire ring on the ground in this wooded area. We could have LNT'd the fire ring, but there's nothing I could do to repair the bomb crater.

 

I'll never build a fire of any size on concrete again. Any of you knew about this tendency with concrete?

 

KS

[Twocubdad]

I'd guess the explosion related to the size and thickness of the slab. A normal, 4-inch slab (like for a house foundation, sidewalk or driveway) would have probably just cracked. A fire in the center of the slab would have created a lot of thermal expansion in a highly concentrated area. Because of the mass of the rest of the slab prevented that area from moving, the pressure built up and the only place it had to go was up and out -- and in a fairly violent manner. If there was a lot of moisture in the concrete, that could have been a factor, too.

 

Folks don't realize the force with which concrete can move. Our CO has a brick wall which runs the length of the cemetery, several hundred feet long. But because it was built without expansion joints, in one section of the wall the bricks have been crushed end for end by the movement of the wall.

 

Question for you: even without the explosion, wouldn't the burn marks left on the concrete have been a problem to for LNT?

[KoreaScouter]

Actually, we got the surrounding area of the concrete cleaned up okay...used some bleach from our dishwashing kit...my ASM who did the fire instruction took care of that; I'm not sure how much manual labor it took for him.

 

Thanks for the info on the concrete.

 

KS

[eisely]

Thanx for the info. Learned something new today.

[rlculver415]

My husband tells a story of when his troop built a fire on concrete under the local high school bleachers, and the consequent explosion. Fortunately, nobody was injured then either. They had a lot of cleaning up to do though. That was about 33 years ago.

 

 

[Stan Riddle]

KS, I was a volunteer firefighter several years ago. We had a call to go to an old abandoned building which caught fire. It had a front porch which was concrete. As we were attacking the fire from the front door, I had a chunk of concrete hit my face shield. It had blown up from this slab. Thankfully, the shield stopped it.

[Mike Long]

Cinderblocks will do that to.

 

Keep all fire away from man-made stone.

 

[red feather]

First, yes,thankfully no one was unjured. I agree with the thermal expansion and possible moisture in an old slab being the cause of the crater. Have seen stream bed rocks do the same thing because of moisture in them, but, this is the first I have heard of concrete. Guess this is why there is a seperate category of brick to be used in fireplaces.

 

Thanks for the heads up.

 

yis

[KoreaScouter]

One of my other ASMs was telling his friends about this over a card game last night, and one of his friends finished the story for him, having experience with the same phenomena.

 

The strange thing for me, is that this is a not unknown risk, and is also one that I have never heard of. I've been a "first responder" for 26 years, and have been to countless traffic accidents, with cars on fire, fuel on fire, cargo on fire, on concrete. I've been warned in training sessions about nearly everything, including not breathing fumes from burning vehicle upholstery, but I've NEVER heard anything about exploding concrete until this happened to us two nights ago.

 

KS

[Bob White]

I believe the actual culprit with concrete is the same reason we warn scouts not to use sedimentary rocks to ring a campfire. Like concrete, this rock type is usually formed in water or is porous. When concrete or rocks have even minute water droplets trapped in its structure and it is heated to a high enough temperature the trapped water vaporizes and the resulting trapped pressure explodes the rock.

 

BW

[ASM7]

Years ago when I worked construction, we were installing large pipes that were used to pump water to the top of a 40 story building. Existing concrete floors had to be cut for the pipes to pass through. Concrete was chipped out until we reached the reinforcing still and the steel under flooring. These had to cut with a blow torch. We learned real fast that the concrete explodes when super heated and its very violent. From my engineering classes in the study of plain concrete (boring), I learned that this is from trapped air. No matter how well the concrete is vibrated, there is still trapped air and it will explode. The water content is cured out of the concrete, but it is porous.

Scouts need to be made aware of the dangers of building fires on concrete and it should be included in the next edition of the Scout Handbook. Bob, do you know how to get this done?

 

Doug

 

[SagerScout]

Here in Central Texas, there are very few areas where fires can be build on the bedrocks. They blow up as described. If any of you who come from igneous rock land come down here remember you heard it here first.

 

Several inches of sand under the fire might have prevented it, if you really wanted to demonstrate the survival fire thing (assuming you don't use liquid chemical accelerants). But a metal trashcan lid full of dirt would have really done the job.

 

[acco40]

Check out the following: www.hse.gov.uk/research/otopdf/2001/oto01074.pdf

 

From the above: When concrete is exposed to fire, the build-up of internal water pressure under steep temperature gardients generates high local stresses, which may cause spalling, potentially explosively."

[packsaddle]

You can find expert information on concrete composition and behavior at the Geotechnical/Structures Laboratory, one of the research labs for the US Army. They design fortifications, runways, bridges, missile silos, etc. and test them under battlefield conditions. A link below:

http://www.wes.army.mil/SL/gsl.html

I can give you a name and telephone number if you want.

[red feather]

With winter camping due again, this thread might be useful.

 

yis