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Not long after the Oakland Hills fire in the early 90's I was talking with a homeowner. I ignorantly said that at least he can still rebuild on the same foundation. He said that at about 2,000°F the rebar in the foundation gets hot enough to expand, leaving voids in the foundation when it cools, and at over 2,500°F all the water gets cooked out of the foundation, leaving you with a pile of concrete dust that looks like a foundation. In either case the foundation has to be replaced. The extreme temperatures created from the burning eucalyptus trees were supposedly responsible for those temperatures.

My question is twofold:

  1. Are the temperatures cited more or less correct?
  2. Are these temperatures encountered in a non-wildfire environment where the only fuel is the structure's flammable materials?

It seems like rebar would get hot enough to expand well below 2,000°F.

  • 1
    So that's what they said, eh? The foundations needed to be replaced because after the Oakland Hills fire (1991 btw), almost every home was replaced with a very much larger home. That's the way housing finance worked back then and it was much to their financial advantage to do so, to the point of being foolish not to. You just can't put a 3000sf McMansion on the foundation of a 1200sf cottage, fire damage or not. Hence new foundation. – Harper - Reinstate Monica Oct 2 '18 at 22:39
  • @Harper, the Oakland Hills fire was hot enough that the foundations needed to be replaced regardless of what replaced the previous homes. – Jasper Oct 3 '18 at 3:17
  • @Harper, yeah, it was 1991. Not sure what made me think '80s, as I remember it was shortly after the '89 quake. I have corrected my error. – BillDOe Oct 3 '18 at 18:02
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The information is incorrect, the temperature at which concrete becomes unsafe for re-use is significantly lower, 570 degrees F actually. The 'telltale" sign is if concrete that was not charred from nearby combustibles turns a pinkish hue. That color change is due to chemical changes in the iron-containing compounds in the aggregates used in making concrete reacting with the Portland cement that is binding them together. Once it turns pink, it is unusable. The fracture mechanics are permanently changed and it will crumble under stress. That has nothing to do with the rebar inside.

Even IF it is not noticeable damaged, you may not be able to re-use it, or want to. After a fire where the rest of the house is completely destroyed or had to be demolished, when you go to try to re-use a foundation, the Building Permit code requirements will be "reset" to the current codes, you are not longer "grandfathered" into the code requirements from when it was originally built. But since it already exists, the inspectors cannot see HOW it was made. So they will require an Engineering Report from a licensed Civil PE certifying that the foundation meets or exceeds the current building Code requirements. To get that report from anyone other than a scam artist, the PE will require core testing, wherein a company comes out and drills cores from your foundation and takes them to a testing lab to test the strength. In addition, they may require a survey / map of the foundation for rebar locations (done with a metal detector). By the time you get done with all of that, it can be so expensive that tearing out the old foundation and replacing it will be less expensive.

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    Yes, the rebar has nothing to do with it. When the heat drives out the water of hydration from the Portland/ ordinary concrete it is unusable ( 570 F sounds good). It does not take much temperature to destroy concrete. – blacksmith37 Oct 2 '18 at 23:44
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    The other cost associated with salvaging the old foundation is that you are chained to the design of the old house, and cannot modernize the house for better saleability. TheMcMansioning of the Oakland Hills firezone being a rather extreme example of that. – Harper - Reinstate Monica Oct 3 '18 at 2:18
  • @J.Raefield, 570 sounds a lot more realistic. Thanks for the good answer. (I'd normally ask for references, but nothing you've said sounds unreasonable and was corroborated by one other comment.) – BillDOe Oct 3 '18 at 18:06
  • I agree that it's not the rebar but the concrete. I have seen 2 plants burn one was a plywood mill the 10"+ slabs had huge voids blown out like bombs went off craters several feet apart, a berry sorting building had 6" slabs and this place had craters not as wide but thousands of 6" wide and less craters. In both cases some wire inside the conduit melted but not the rigid pipe, the pipe got hot and sagged but was intact. + – Ed Beal Oct 3 '18 at 18:15

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