It's a mid-1990s house that appears to have been built quite
adequately. By looking from outside, it's a 4-to-6 inch slab,
likely, adequately reinforced. Again, the cracks do not look like ones
caused by lack of rebars.
Overlays do not appear an option - it will not repair the
subbase, right? Besides, what will we do with many existing doors?
As a repair, epoxying the slab appears enough. But if the subbase was
weakened, any repairs of the slab will not help, especially with
Steve Gordin SE
----- Original Message -----
Sent: Monday, December 29, 2003 4:17
Subject: RE: seismic cracking of
don't say how thick the slab is or whether it was refinforced with bar or
mesh. Is it possible that it is a very thin slab, with maybe too light a
Unless the slab was poured over an old foundation or some uneven ground
profile that was levelled up with dry sand - or some such explanation, your scenario
sounds most plausible.
the uneven slab is not an issue, then sealing it would probably work but if
you need a level floor, you will have to do something with it. How about
levelling it up with a good concrete and mesh.
Sharlie Huffman, P.Eng.
Bridge Seismic Engineer
4B - 940 Blanshard
BC V8W 3E6
I could not see rebars - it would require too much
demolition. I would assume they were there, first, because it was a
high-end house (for what it's worth), but mostly, because I've seen too
many cracked SOG's with and without rebars.
These cracks are different - approximately straight
and parallel, at, say, 48" or more, but without regular spacing and
correlation to the footprint layout (at an angle). They exist
throughout the house and patio, but not in the exposed garage slab (in
that, there are several definitely old cracks of different /"regular"
appearance, similar to cold joint distortion).
The single-level slab is divided into several "sub-slabs"
with elevations varying 12 inches max. and steps between them. All
steps are cracked horizontally.
All cracks of interest are definitely fresh; any
relevancy to columns, drains, conduits, etc. is
The best scenario I could come up with is the slab was
poured against the surfacing inclined shale that was disturbed by the
intensive seismic movements. That would be consistent with the
appearance of the cracks.
But if so - what (if anything) should be
----- Original Message -----
Sent: Monday, December 29, 2003 2:51
Subject: Re: seismic cracking of
My first guess would be that the cracks were linear because
they were occuring over reinforcing steel. But if you are getting
1/2 in. offsets at the cracks, there is likely no reinforcing
perpendicular to the cracks, and probably none in the other
direction either. And if that isn't a plug for slab on ground
reinforcement, I don't know what is. Fibers aren't going to do
anything to prevent faulting.
What is the spacing of the cracks?
Is it possible the cracks were already there (covered with tile)
but weren't visible until they opened wide enough to cause faulting?
Is there conduit or something in the slab that could cause
cracking? Is the cracking actually following some weakened plane
caused by columns or drains or some other embed?