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Coring Concrete Slab

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Thoughts in response to Tom Castle's post about finding existing rebar at
the 1000 holes in the existing slab.

1.  Pre-drilling as an alternative to a proper pacometer survey is not a
terrible idea but should be done using a small bit and perhaps a drill set
on rotation only (non-impact).  It will go a little slower but will minimize
damage and the operator should be able to detect a change in the material
(e.g. a conduit) before he electrocutes him (or her) self.

2.  Bottom mat bar locations can be determined by surveying the bottom of
the slab with a pacometer as well as the top.  Results will need to be
accurately transposed to the top surface via reference dimensions to an edge
or penetration, but it can be done with pretty good accuracy if necessary.
The question of course is whether it's worth the time and money in
comparison to alternatives.

3.  X-rays are not a cure-all.  Films need to be interpreted by qualified
technicians and it's easy to get confused as to which way is up when
tranposing the results into marks on the slab telling the contractor where
the bars are.

4.  We have in the past investigated alternative (e.g., ultrasonic) bar
finding equipment.  Last time we checked the use of high-intensity
ultrasound for bar finding was an unusual application but we talked to some
guys who thought they could do it.  If Tom is interested in pursuing this
option, he may respond by off list email or call me at 213-953-1770 and I
will dig up the names of the people we talked with.

Drew Norman, S.E.
Drew A. Norman and Associates