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[SEAOC] Concrete cracks

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Yes, it was quiet for a little while anyway.  Concrete cracking is as good a
subject as any to wake up to.

Concerning cracks in slabs-on-grade and how to tell if they are Northridge
earthquake related:

1.  Cracks in slabs-on-grade are so common (unfortunately), that the only
time I worry about the exact cause is when there's insurance money involved.
 Sounds like your situation???

2.  It's very difficult to be sure.  Here's my "opinion":  Unless there is a
good indication of an "engineering" reason for the crack, my first assumption
would be that they were pre-existing with the possibility that they were
aggravated by the earthquake.  If they are concentrated diagonally in a
corner or have a pattern that appears to be related to diaphragm action, then
they might be judged to be earthquake related.  Or if the site has
liquefaction or settlement potential, and the cracks appear to related to the
location of footings, then that also might indicate an earthquake.  Also,
shrinkage cracks tend to have a certain "random" look to them more so than
earthquake cracks. 

3.  One usual telltale sign is that dirt in the cracks or discoloration
adjacent to the cracks would normally indicate old cracks.  Although cracks
from the Northridge earthquake would likely be filled in with dirt by now,
you still might get some hints such as paint stripes that go into a crack, if
the paint can be dated. (I don't mean by Strontium 90, rather if somebody in
maintenance can date the paint.)

Cracks in structural walls and columns are a lot easier, and can be related
to real engineering reasons.  I can't wait for more opinions.  

Thanks.  Carl Sramek  

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