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[SEAOC] Re: [SEAOC] Re: [SEAOC] Concret cracks

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I usually answer the question of "Did the earthquake cause these concrete
cracks ?" by evaluating other possible reasons for the cracks, such as:

1) are the soils expansive enough to cause the cracks? (what is the soil
expansion index, thickness and reinforcing of the slab, sources of soil
2) What about tree roots?  I've seen alot of slabs heave and crack near trees
with large, surface roots.
3) What is the age of the concrete slab?  How many seasonal cycles of
wet/dry, hot/cold have the concrete and soil been exposed to?
4) What is the orientation and width of the cracks, do they have vertical
offsets? Vertical offsets may indicate soil settlement, rather that
earthquake movement. (What about settlement caused by the earthquake?)  

I also look at paint lines, past patching of cracks, and any other evidence
for the age of the cracks.  If there are other, plausible reasons for the
cracks, then the liklihood  that they were caused exclusively by the
earthquake is lessened, in my estimation.

The bottom line, though, is that it is very difficult to determine
conclusively whether a particular slab crack was caused by earthquake or any
other cause.  Too bad we can't get pictures the day before the earthquake,
and compare them to pictures taken the day after the earthquake.

Steve Helfrich, GE