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Coring concrete slab

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	I am working on a seismic retrofit of a group of residential
timber framed buildings over a mild reinforced concrete parking flat
slabs.  We are adding a lot (over 1,000) hold down rods through the
deck.  Our plans show drilling through the 12 inch slab and placing a
square plate washer on the underside of the slab.  The project is
located in the City of Los Angeles.  I mention this fact because I
believe this precludes the option of using epoxy anchors into the slab
due to the low allowable values in the City and the relatively high
loads (as much as 25,000# tension).  I specified on the plans to verify
by nondestructive means the rebar location before drilling.  I did not
want to cut any rebar because the slab appeared to need all it could
get, specially with the new loads that could be imposed on it.  

	The contractor "saved" the client $40,000.00 by deleting the
pacometer tests to find the rebar.  He used a roto hammer to drill the
holes and stopped if rebar was hit and moved the hole.  That was fun and
it worked for the first couple hundred holes.  Then came a portion of
the project located next to the electrical feed panels.  When the roto
hamer hit them, the lights went out before the drilling could be
stopped.  They tried the pacometer at this point but said that it would
not detect the bottom bars or the bottom conduit.  

	Their current thought now is to X-ray the slab at $120.00 per
location at night (because no workers are allowed within a certain
radius while the X-ray is working).  I have asked repeatedly if there
was another method of nondestructive testing that could be employed.
They say no.  I can't help but think in this day that someone has
developed some device that could be used for this purpose that does not
require the expense and inconvenience of the X-ray.  Has any one seen
any thing?  It's really the contractor's problem, but it would make the
job go alot easier if we could figure something out.  Thank you for any

	Thomas Castle, S.E.