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Re: Fire Damaged Slab on Grade

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	Bill Polhemus wrote about chain drag:
		"The problem I have always had with this method is that it
seems like it would take "a while" to calibrate."

		That was certainly my experience. I've only had the
opportunity to test this way once. More than 10 years ago, I dragged a chain
over an entire parking deck built 20 years earlier. The GC told us the
joints between the precast Lin Tees were covered with tar paper about 8"
wide so the cast-in-place topping would not leak through. I could hear the
hollow sound at every joint. Sometimes, it continued about halfway across
the tee. The rest of the area had the ringing sound. As Daryl says, "The two
sounds [were] as different as tapping with your finger nails on a guitar vs.
a meat block." The composite action was required structurally and the owner
wanted to know how to ensure the deck would last at least another twenty
years. I rechecked a lot of locations just to make sure that what I had
recorded I could hear again. I got the same pattern of sounds in the same
places each time. We had some of the hollowest sounding locations (more than
4" from a tar papered joint) cored and tested for shear at the
precast-to-topping joint. IIRC, the shear was as high as ten times the
square root of f'c (down to about four times); much higher than two times
the code allows for shear capacity of monolithic concrete. We did not worry
any more about delamination.
		I'd say I was incompetent to eliminate false positives, and
I hope you can all laugh if I use that word. I have not had the opportunity
to try it again, but I doubt you all want me in your Heavyweight Aggregate
Concrete Delamination All Night Band!!
		BTW, every time I've looked at concrete after a fire, I
heard the same ringing sound when I hit the concrete with a hammer. I have
never seen the pink concrete that is supposed to occur when concrete has
gotten very hot. Often, the soot is not even burned off the ceiling, so I
know it did not get hot enough to hurt the concrete. I looked at an elevated
slab last summer that had been the floor of a garage holding gassed-up
vehicles. Green leaves on trees maybe 30' away were burnt off the branches.
There was essentially nothing left of 15-20 antique or otherwise prized
vehicles but the steel - no paint, no tires, no nothing. Everywhere I tapped
I got a nice ringing sound. So I cannot help much in telling you what to
look for, Jonathan, I've never seen damage.
	Jim Getaz
	Winchester, Virginia


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