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re: fire damaged concrete

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I agree with Harold but Steve made some good points too. I would do a visual review after cleaning and look for spalling and cracking, this is the main effect on concrete slabs-on-grade. But then again, concrete SOG all tend to crack some from curing, settlement, lack of expansion joints, etc. so you may have a difficult time diffentiating. I would not be too worried, but if it is a thickened/monolithic type wall footing you may want to look around bearing walls for slab cracking. But generally I think of SOG as a dirt cover.... Just seal the cracks like Harold said and be done with it (after your investgation).
 
However, I have done a few fire investgations of residential fires and masonry and concrete resist fire damage quite well. By that I mean no noticeable cracking or spalling is noticed. In an electrical panel explosion once on a project of mine it spalled off a chunk of the unreinforced CMU. But I would think heat would affect the slab the least as the hottest part of the fire would be in the ceiling structure not next to the slab, but this is just my opionion and I am no fire engineer.
 
But you did say this thing got hot enough for steel to deflect, and that can happen at around 575 deg F. That is the same approximate temp when concrete begins to lose its strength also. This is according to "Forensic Structural Engineering Handbook" by Ratay.  "Guide to Deterioration and Failure of Building Materials" by Heckroodt puts it this way:
The strength of concrete is not severely affected by temps below 250 c (482F), but drops 80% at 450 C (842 F), and to 50% at 650 C (1202 F). Not sure why these numbers are so much higher than Steve's source, but I am just the messenger.
 
I could scan a couple pages of this book if you would like.
 
HTH,
Andrew
 
According to Composition and Properties of Concrete (MGH 1968), concrete =
samples subjected to temperature "T" and crushed after cooling had the =
following compressive strength (relatively to that at 70F): 85% at =
T=3D200F, 70% at T=3D450F, 50% at T=3D700F.

Although technically unreinforced, SOGs may be subjected to considerable =
forces under the normal use conditions.  Besides, due to the concrete =
strength loss, the new post-installed anchors will not have the =
specified strength.  This means that the residual loss of strength due =
to the fire strongly justifies the post-fire replacement of at least the =
portions of SOG exposed to the fire.  =20

In my experience, the insurance companies did not have any problems =
covering such losses.

V. Steve Gordin, SE
Irvine CA
 
 
 
Andrew Kester, P.E.
Principal/Project Manager
ADK Structural Engineering, PLLC
1510 E. Colonial Ave., Suite 301
Orlando, FL 32803
M 407-921-1617
F 321-249-0349