There’s a non-profit recycling center in town that I’ve helped out from time to time over the years. Recently they canned a worthless employee and had one of their buildings burn down mysteriously the next night about 1 AM.
The building was a post frame – Posts and trusses 8′ O.C. They want to go back in and replace the building more or less exactly as it was.
The building had concrete slab/floor. How can you tell if the concrete was damaged by the fire? How hot does it have to get before it’s damaged? Are there any visible signs I should look for?
The building itself is a total loss. There were bales of cardboard ans plastic stored in it waiting for shipment, so there was plenty of fuel and the fire was a hot one. Almost every stick of wood in the place is burned through.
This is complicated by the fact that the director of the center decided to go water skiing over the weekend with his Grandkids, and ended up breaking his artificial hip. Why a 68 year old would want to water ski is beyond me. But as a result of that I may end up inheriting this project.
Any thoughts that might help me out would be appreciated.
If You Can’t Be Good, Be Bad With Me
Replies
Boss-
In all my years working construction I haven't seen a fire damaged concrete floor. If you've ever seen a bathtub after a fire you know the smoke will penetrate the smallest crack imaginable.
Most times if the surface is spalled it is old freeze damaged. The most I ever saw came from a steel beam that fell.
This being a recycle center I suspect the use not to be serious i.e. multi family or heavy commercial. I feel your ok.
If in doubt the city inspectors and or insurance people will have required it's demo.
Good luck
J.ust A G.uy W.ith A H.ammer
It takes a heck of a lot of heat to damage concrete.
Surface damage will show as spalling and its significance depends on the depth.
Damage due to thermal movement will show up as major cracking so check at the junction of walls and floors on the top of the slab and at mid-span or thereabouts on the underside. Basically you're looking for evidence that the slab has sagged
Any crack less than the thickness of a pencil line is reckoned as insignificant, over that get a structural engineer to examine the structure.
IanDG
I'm sorry to hear about the fire, and the broken hip.
The insurance company may have someone who knows. Some adjusters have the training for construction, others don't.
Since you are non-profit, you may be able to get a civil engineer (with concrete experience) to come at out and look at it for free as a community service. If you're in a populated area the city may have some expertise on staff, as well. If testing is needed your insurance company should pay for it.
In your new building you may want some more secure construction, fire resistance, and perhaps sprinklers at least in the cardboard and plastic area. Insurance may require it, and give you better rates with some basic protection in place.
Thanks for all the replies so far.
As for having an engineer look at it, I doubt that's gonna happen. I don't think we have an engineer in the entire county. There's no building inspector or any other kind of professional around.
They had insurance of some sort, but I figure they'll say it DOESN'T need to be replaced just so they don't have to pay for it.
I was hoping there was a reasonably simple way to tell from a visual inspection if the slab was bad. Doesn't sound like that's the case. Bumpersticker: Don't be sexist. Broads hate that.
If ya hit it with a hammer, and it breaks like glass, y'all got a problem.....lol
Bob
Boss,
From the sounds of it, I'd say if it looks good, it probably is good.
While all of the cautionary advice you havew received here is no doubt well-intended and well-founded, I doubt if the structural competence of the foundation was compromised.
Here's why I say this;
I started my working career as a firefighter, and had to take college courses in building construction to become a fire officer. These courses were interesting to me, and becoming a builder was a natural evolution.
So I've spent a fair amount of time studying what happens when buildings burn, both in the classroom and in the buildings as they are burning.
The interior temperature of a building on fire varies widely as a result of the particular conditions, including the type of construction, how the fire developed and progressed, the physical shape of the building, the contents, and how high you are off the floor.
The last point is key.
While temperatures at the ceiling can be tremendous, temperatures at the floor in the same room can be quite tolerable. A thousand-degree difference across a vertical distance of eight feet is perfectly reasonable to assume if the place is really rocking when you show up.
I don't know if this foundation is reinforced concrete or not -- from what I read here, it seems some places in the country don't put rebar in their foundations.
Anyway, as I know you already know, reinforced concrete gets its flexural or tensile strength from the rebar and the compressive strength from the concrete. Acting together,it is a composite material that acts neither like plain old regular concrete or steel. [OK gang, it's an oversimplification, just go with me for a minute here <G>].
The point is, steel doesn't lose a dangerous amount of its strength in a fire until it hits about 900 degrees F, so if we're talking about a foundation below grade and a fire above a slab, it is highly unlikely that the steel ever saw a temperature approaching half that. Even if it did, the total loss of strength is not permanent.
The concrete itself should likewise be relatively intact, and likely capable of providing bearing competence and stability for what sounds like a pretty light building.
As an aside, the concerns expressed here would be totally appropriate if we were talking about a bond beam, a lintel, or a bridge. In this case, the concrete has an entirely different job to do and the rules change.
But in this case, I think I'd give the place a good cleaning, check it over, and if it looks good, just put the building back up.
DRC
Thanks for the thoughts.
The building is roughly 20 years old, and the guy who built it is dead. So I have no idea if there's rebar in the slab or not. The slab has been there for at least 5 years that I know of. It may or may not have been poured when the building was built.
I hadn't thought much about the heat rising, but that's a good point. Seems like a slab sitting on the ground would be a good heat sink and would absorb and dissipate a lot of heat.
The slab doesn't really have to hold any of the building up, since it's a post frame. The posts are set in the ground, and the concrete poured in later. If the slab is left there, we'll have to cut the concrete back from the posts and then patch it back in.Bumpersticker: If you can read this, I've lost my trailer.
Boss,
I hope that somewhere along the line I will be as much help to you as you already have been to me over these past few years.
If the slab isn't holding up anything but itself, I wouldn't give it another thought. If you find out later that it's been compromised (doubtful), just pour over it.
DRC
Boss,
Dave Crosby summed it up nicely. It is unlikely that there was any damage to the slab. If there was, you would be kicking it off with your boot. We use 2" of concrete cover for fire protection in cut-and-cover tunnels. Our biggest concern with concrete in a fire is within a bored tube using a precast segmental liner. A commuter train car burns incredibly hot (we burned one to be sure and it burned twice as hot as the industry expected - that's fun to know), so the temperatures are many times that of a typical house fire....that's not a mistake, it's rustic
I can't have read your original post carefully enough -- I thought you were talking about a suspended slab.A slab on grade is mainly under compression and for fire to affect its structural strength it would involve significant spalling of the concrete which will be obvious.As to the heat affecting the rebar, a slab on grade, if it has rebar at all, will have it cast in well away from the effects of anything short of a plasma torch!
IanDG
Edited 8/20/2003 3:01:46 PM ET by IanDG
Hey Boss,
While in the Navy I took firefighting twice, 7 years apart, in the same concrete building. this building had 3 to 10 fuel oil fires a week lasting form 10 mins to 1 hour, depending on how "engulfed" they wanted it and how efficient we firefighters were.
They did have to replace the steel walkways over the floor though.
SamT
Yo all -
I stopped by the building on my way home tonight to see if I could tell anything about the condition of the slab. But there's too much debris on it to get a clue yet.
Sounds like there's probably nothing to worry about though. I appreciate the input from so many of you.You've got the brain of a four-year-old boy, and I bet he was glad to get rid of it [Groucho Marx]
It's highly unlikely that the slab would have been able to sustain any damage as a result of the fire.
The only time that I can remember having to replace a foundation after a fire was a townhouse project that was under construction in the winter and a bunch of 12 year olds decided to play with matches in the protective straw over the footings and against the foundation walls.
You have to remember that the concrete was still green with moisture and the combustionable material was directly against the foundation wall.
There were large pizza sized pieces that had popped off the wall but the floors were fine.
We hoe rammed the walls and saved the footings and basement floors.
Gabe
Sounds like you probably don't need to know if there's steel in the slab, but if you want to know a metal detector should do the trick.Any jackass can kick down a barn, but it takes a carpenter to build one.
An afterthought:
If you end up doubting if the slab is strong enough to handle the point loads of post and beam construction right back on the existing slab, a grade beam around the perimeter made out of one course of ICF with a liberal dose of rebar would probably take care of any problems.
DRC
Boss-
Definitely have an engineer look at it and take cores for testing. We had six houses "mysteriously" burn down in Las Vegas, and the slabs had to be torn out. The heat of the fire actually crystalizes the sand in the concrete, essentially turning it into glass. Given the cardboard and other items in the building, I'd say the fire got very hot, especially down low- probably wiped out the slab.
Bob
"Why a 68 year old would want to water ski is beyond me."
BECAUSE, just because!
REF; Banana George
http://www.bananageorge.com/
Boss, my limited understanding of fire damage to concrete is that the damage will be to the embedded steel. It takes surprisingly little amount of fire/heat to change the steel composition. May not be a problem for your floor (does it even have rebar?), but I suggest you find an expert, e.g. a fire inspector.
Regards,
Tim Ruttan