# Concrete Firepit



## Mazman (May 5, 2014)

I'm looking for suggestions on what type of concrete I should use to pour a firepit. Obviously it will be taking some pretty intense heat. The cement landscaping blocks I have used in the past always crack from the heat. Looking for a permanent, non-cracking solution. Thanks for your help.


----------



## gma2rjc (Nov 21, 2008)

Good question. I have wondered about this also.


----------



## Tscarborough (Mar 31, 2006)

Either fire-brick or castable refractory anywhere the flames may touch.


----------



## Canarywood1 (May 5, 2012)

Tscarborough said:


> Either fire-brick or castable refractory anywhere the flames may touch.


 

+1 to what Tscarborough said, fire is the worst enemy of concrete.


----------



## stadry (Jun 20, 2009)

take a tip from fireplace masons - they use firebrick :thumbsup:

irc


----------



## ront02769 (Nov 28, 2008)

Worked as a bake master at a clambake place for a year or so and regularly did bakes for 600-900 people. Our cooking location was an open steel structure with a vented tin roof. Floor was firebrick set in sand. Fires regularly got to 1200- degrees, zero cracks in the brick, no worries. So, as others have suggested, use what the pros use! Ron


----------



## jomama45 (Nov 13, 2008)

One more vote for fire brick lined concrete. Here's one we did a few years ago inside a patio............


----------



## Mazman (May 5, 2014)

Thanks for all the great advice. Jomama45...beautiful job! I may steal your design! What did you use to adhere/grout the brick. That must be a special heat resistant mortar, no?


----------



## jomama45 (Nov 13, 2008)

We set the bottom firebrick over Type M mortar typically, and use a hydraulic set refractory mortar for the joints between the firebrick. Here's 2 different options for the refractory:

http://alsey.com/products/residential-mortar-outdoor/

http://www.heatstoprefractorymortar.com/mortar50.html


----------



## chairmanme0w (Apr 21, 2014)

I'm not an expert, but just want to quickly add that using regular concrete is not a good idea. Reason is, any air that may get trapped during the pour will get super heated and build pressure until it literally makes the concrete explode.

There was a new camp site near my house that temporarily closed due to multiple fire pits exploding when they first opened.


----------



## Tscarborough (Mar 31, 2006)

It is not entrapped air that causes explosive spalling, it is the aggregate.


----------



## chairmanme0w (Apr 21, 2014)

Tscarborough said:


> It is not entrapped air that causes explosive spalling, it is the aggregate.


Have to respectfully disagree here. The aggregate is inert, contains no moisture, and almost no voids.

It's the voids and moisture trapped during the pour that expands under heat to create pressure until.... kaboom.


----------



## Tscarborough (Mar 31, 2006)

You can disagree all you want, but do a little reading and get back to me. Read up on "differential thermal expansion co-coefficients at high temperatures".


----------



## Tscarborough (Mar 31, 2006)

Here is a good report:

http://www.hse.gov.uk/research/otopdf/2001/oto01074.pdf


Note that they expected the cause of explosive spalling to be moisture content (not in voids, but free moisture in the cement matrix) but the test results indicate thermal differential between the aggregate and the matrix as the cause.


----------



## chairmanme0w (Apr 21, 2014)

Tscarborough said:


> Here is a good report:
> 
> http://www.hse.gov.uk/research/otopdf/2001/oto01074.pdf
> 
> ...


The very first sentence in that report cites the cause is WATER, not cement, not aggregate. I skimmed through the rest of the report to find specific references to explosive aggregate, but I didn't find it.

Also, the differential is not due to a thermal gradient as you mentioned, but the difference in the rate of expansion between the areas of varying relative humidity (which is affected by water).

Throw a sealed container of dry cement in a fire, will it explode? No.
Throw a sealed container of dry aggregate in a fire, will it explode? No.
Throw a sealed container of water into a fire, will it explode? Yes.

I didn't mention water in my first comment, because that will eventually evaporate out if left to cure long enough, but just like air, once water goes through the phase change it will start to expand according to the Gas Laws.

Now that I'm thinking more about water, I think it should always be a consideration since most fire pits are exposed to rain.

We're getting off track, but we all agree that it's not a good idea to use concrete to build a fire pit.


----------



## Tscarborough (Mar 31, 2006)

They cite that as the reason going into the study, read down to the charts where the experiments showed which beams failed explosively; it was the ones that used limestone aggregate, regardless of water content.


----------



## chairmanme0w (Apr 21, 2014)

Tscarborough said:


> They cite that as the reason going into the study, read down to the charts where the experiments showed which beams failed explosively; it was the ones that used limestone aggregate, regardless of water content.


You're implying that limestone aggregate caused the explosions. You're twisting words on the report.

Nowhere in the report does it doesn't cite limestone as the reason it exploded. Limestone based cement is more susceptible to explosive failure, in comparison to the other two materials. moisture content is still the underlying cause.


----------



## Tscarborough (Mar 31, 2006)

I am not implying it, since the only beams that exploded were the ones with limestone aggregate, no matter what the water content, it is a direct cause of the beam exploding.

All aggregate will not explosively spall, only those with a high differential thermal co-efficient of expansion from the cement matrix, like limestone. Basalt for example, will not, nor will grog, as a rule.


----------



## Tscarborough (Mar 31, 2006)

We are wandering away from the point though. Concrete is bad in a firepit, no matter what aggregate, because the cement itself begins to disintegrate (return to it's constituent parts to be exact) beginning at 500 degrees F.


----------

