Fire in a concrete trough [Now with pics]

Frostex · 18th June 2012, 3:25 PM

So we have an old concrete horse trough. Approx 2.5m long, 70cm wide, 70cm high, 5cm thick concrete. It weighs approx 200-300kgs (Decided by 6 strong blokes lifting it).

I want to use this for a "fire pit". I have been doing research, and it seems the hottest parts of a standard backyard fire can get up to 500c-650. But the Flame itself is usually between 100c-200c. I also read that standard concrete can withstand up to 450c with no real worrys, but hotter than that is when degradation starts to occur. My understanding is it "sweats" out the moisture when hot, and when it cools it reabsorbs the moisture causing it to expand and crack. Can someone please confirm this?

It is dome shaped, as you'd expect a trough to be. If i put 5-10cm of sand in the bottom to stop the ulta hot coals resting on the concrete directly. What are the chances of things getting hot enough to cause the concrete to break??

Keep in mind this will only be used for 1-3 nights (4-8 hours at a time), before my mum turns it into a garden bed and it forever retires from housing fires.

Ninja edit: Technical answers only please as we are in the science forum. "my mum once heard of a bloke burning off motor oil in a concrete bowl for years and it never broke" type comments are not welcome thanks.

mrbean_phillip · 18th June 2012, 10:12 PM

Now why don't you get a few ceramic tiles to line it out with?

Anyway, I would think sand in itself should be ok as a thermal buffer -= but make for a pretty messy barbie.

ChiefBoss · 18th June 2012, 11:44 PM

If your dimensions are correct the trough will weigh ~522kg.

Also, you will have no issues with it as it is most likely steel reinforced and you are only using it a few times.

kthnxbye!

rickbishop · 4th July 2012, 2:08 AM

I would think that the sand lining would act as a thermal mass and get hotter because of the coals on top of it, thus accelerating the degradation.

If you do crack it, will it still be usable as a garden bed? Or will the fact that it no longer holds water be detrimental to this purpose?

Frostex · 4th July 2012, 9:32 AM

Depending how badly it cracks it should still be usable.

Im going to call up a firebrick place tommorow to see how much it would cost to get enough just to line the bottom of the trough.

kombiman · 4th July 2012, 9:41 AM

Quote:

Originally Posted by Frostex

Depending how badly it cracks it should still be usable.

Im going to call up a firebrick place tommorow to see how much it would cost to get enough just to line the bottom of the trough.

dont worry about fire bricks, they just hold thermal mass. If you can elevate the fire on a grate it will burn better too. If you're too concerned make a firepit!

Fishface · 4th July 2012, 10:00 AM

If there are no vent holes in the bottom this will mean that the fire will not burn very well.

Frostex · 4th July 2012, 2:30 PM

Quote:

Originally Posted by kombiman

dont worry about fire bricks, they just hold thermal mass. If you can elevate the fire on a grate it will burn better too. If you're too concerned make a firepit!

Yes finding a grate will be ideal. Not qute sure where to look though.

Quote:

Originally Posted by Fishface

If there are no vent holes in the bottom this will mean that the fire will not burn very well.

Vent holes will be drilled in it. Easily patchable if required for the garden bed later.

MyDixiesNormous · 4th July 2012, 2:53 PM

Just light the fire and see what happens. Short of lining it with a perlite mix and then fire bricks it will make no difference.

As long as the trough is reinforced it will only crack and not fall apart. It does however have the ability to explode. The moisture in the concrete will turn to steam and look for the easiest way out. I lit a fire on a concrete slab a number of years ago and after about an hour a piece of concrete the size of a rockmelon let go. I cant tell you how high it went as I was too busy trying to protect my noggin. The time between the explosion and landfall seemed like an eternity.

Just light small fires and slowly increase to fuel load after it has properly dried out. I would also keep it covered as an water will find it way into a small hair line crack and you will be in trouble.

digamma · 4th July 2012, 3:14 PM

It's crack and chip. No doubt about it. I've cracked concrete slabs that had braziers sitting on them. This trough won't stand up to fire sitting directly on/in it. Plus, if it has had water on or in it recently, then the concrete will be full of moisture and it will heat up and crack the thing; it might stay confined to surface chips, but if you plan to use it again to hold water, then don't put fire in it.

PabloEscobar · 5th July 2012, 11:27 AM

Is there any risk of air bubbles or other impurities in the cement expanding and violently shooting cement chips about the place.

Frostex · 5th July 2012, 11:36 AM

Quote:

Originally Posted by PabloEscobar

Is there any risk of air bubbles or other impurities in the cement expanding and violently shooting cement chips about the place.

Ive been googling my ass off on this and all signs point to no. Unless it has just been laid. The ONLY information i could find on exploding concrete was when it was still fresh (less than 2 weeks). Or if it was really wet recently and their was water in the microscopic cracks.

Otherwise its just cracks/degradation you have to worry about. The water deal can be avoided by making sure it is dry for a few weeks in advance and slowly building up the fire (over many hours) to dry out the concrete slowly (thus avoiding rapidly expanding steam).

Badger · 5th July 2012, 11:38 AM

Quote:

Originally Posted by PabloEscobar

Is there any risk of air bubbles or other impurities in the cement expanding and violently shooting cement chips about the place.

Water in the concrete turning to steam can shoot pieces, as mentioned by MyDixiesNormous above

mtma · 7th July 2012, 1:43 PM

I don't think he's going to be putting much water into it afterwards given that he's thinking of drilling ventilation holes into the bottom of it.

As long as the concrete is dry as mentioned I think you'll be fine once you put the ventilation holes into the bottom of it.

aXis · 9th July 2012, 11:51 AM

Pretty much 100% chance it will crack, either due to moisture or just thermal expansion.

Personally I'd just line the entire thing with either paving tiles or bricks. These will still most likely crack, but they wont be structural and the concrete behind them will be protected.

18th June 2012, 3:25 PM	#1
Frostex Member Join Date: Aug 2007 Location: Melbourne Posts: 618	Fire in a concrete trough [Now with pics] So we have an old concrete horse trough. Approx 2.5m long, 70cm wide, 70cm high, 5cm thick concrete. It weighs approx 200-300kgs (Decided by 6 strong blokes lifting it). I want to use this for a "fire pit". I have been doing research, and it seems the hottest parts of a standard backyard fire can get up to 500c-650. But the Flame itself is usually between 100c-200c. I also read that standard concrete can withstand up to 450c with no real worrys, but hotter than that is when degradation starts to occur. My understanding is it "sweats" out the moisture when hot, and when it cools it reabsorbs the moisture causing it to expand and crack. Can someone please confirm this? It is dome shaped, as you'd expect a trough to be. If i put 5-10cm of sand in the bottom to stop the ulta hot coals resting on the concrete directly. What are the chances of things getting hot enough to cause the concrete to break?? Keep in mind this will only be used for 1-3 nights (4-8 hours at a time), before my mum turns it into a garden bed and it forever retires from housing fires. Ninja edit: Technical answers only please as we are in the science forum. "my mum once heard of a bloke burning off motor oil in a concrete bowl for years and it never broke" type comments are not welcome thanks. Last edited by Frostex; 24th July 2012 at 1:35 PM.

18th June 2012, 10:12 PM	#2
mrbean_phillip Member Join Date: Jun 2002 Location: Doreen (3754) Vic Posts: 2,056	Now why don't you get a few ceramic tiles to line it out with? Anyway, I would think sand in itself should be ok as a thermal buffer -= but make for a pretty messy barbie. __________________ HP Blackbird -> Dual-Quad Xeon ; Macho-Mini -> mini-ITX Xeon Server ; i-Prodigy -> Watercooled mini ITX ; iDuplex -> Prodigy SLI iRacing Button Box

4th July 2012, 2:08 AM	#4
rickbishop Member Join Date: Jun 2001 Location: Caboolture, QLD Posts: 4,784	I would think that the sand lining would act as a thermal mass and get hotter because of the coals on top of it, thus accelerating the degradation. If you do crack it, will it still be usable as a garden bed? Or will the fact that it no longer holds water be detrimental to this purpose? __________________ Goose1981 is a true gold soul. He will always be welcome at mine for beer and a bed. Buy my stuff! http://www.nuclearlx.com

4th July 2012, 2:53 PM	#9
MyDixiesNormous Member Join Date: Jan 2003 Location: Melb Posts: 2,257	Just light the fire and see what happens. Short of lining it with a perlite mix and then fire bricks it will make no difference. As long as the trough is reinforced it will only crack and not fall apart. It does however have the ability to explode. The moisture in the concrete will turn to steam and look for the easiest way out. I lit a fire on a concrete slab a number of years ago and after about an hour a piece of concrete the size of a rockmelon let go. I cant tell you how high it went as I was too busy trying to protect my noggin. The time between the explosion and landfall seemed like an eternity. Just light small fires and slowly increase to fuel load after it has properly dried out. I would also keep it covered as an water will find it way into a small hair line crack and you will be in trouble. __________________ Reward offered for details of Vic reg XZH195 pm me.

4th July 2012, 3:14 PM	#10
digamma Member Join Date: Mar 2002 Location: Brisbane, Southside. Posts: 2,428	It's crack and chip. No doubt about it. I've cracked concrete slabs that had braziers sitting on them. This trough won't stand up to fire sitting directly on/in it. Plus, if it has had water on or in it recently, then the concrete will be full of moisture and it will heat up and crack the thing; it might stay confined to surface chips, but if you plan to use it again to hold water, then don't put fire in it. __________________ The ultimate weakness of violence is that it is a descending spiral. Returning violence with violence only multiplies violence, adding deeper darkness to a night already devoid of stars.

18th June 2012, 11:44 PM	#3
ChiefBoss Member Join Date: Mar 2012 Location: Melbourne Posts: 52	If your dimensions are correct the trough will weigh ~522kg. Also, you will have no issues with it as it is most likely steel reinforced and you are only using it a few times. kthnxbye!

4th July 2012, 9:32 AM	#5
Frostex Member Join Date: Aug 2007 Location: Melbourne Posts: 618	Depending how badly it cracks it should still be usable. Im going to call up a firebrick place tommorow to see how much it would cost to get enough just to line the bottom of the trough.

4th July 2012, 10:00 AM	#7
Fishface Member Join Date: Mar 2004 Location: Adelaide Posts: 1,593	If there are no vent holes in the bottom this will mean that the fire will not burn very well.

5th July 2012, 11:27 AM	#11
PabloEscobar Member Join Date: Jan 2008 Posts: 2,817	Is there any risk of air bubbles or other impurities in the cement expanding and violently shooting cement chips about the place.

7th July 2012, 1:43 PM	#14
mtma Member Join Date: Aug 2009 Posts: 2,605	I don't think he's going to be putting much water into it afterwards given that he's thinking of drilling ventilation holes into the bottom of it. As long as the concrete is dry as mentioned I think you'll be fine once you put the ventilation holes into the bottom of it.

9th July 2012, 11:51 AM	#15
aXis Member Join Date: Jun 2001 Location: Kalgoorlie, WA Posts: 3,314	Pretty much 100% chance it will crack, either due to moisture or just thermal expansion. Personally I'd just line the entire thing with either paving tiles or bricks. These will still most likely crack, but they wont be structural and the concrete behind them will be protected. __________________ Former OCAU Extreme Cooling Club H2O Member Intel Core Duo E6600 Noctua NH-U12F