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Re: Coring concrete slab

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Tom Castle wrote:
>  Help!
>         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
> thoughts.
>         Thomas Castle, S.E.

Hilti has an alternative to the pachometer. It is very simple to use.
You just roll a wheel on the slab. The wheel is connected to a data
acqisition system running on a lap top computer. The reinforcing grid
shows on the screen with bar size and cover. When we tried this
equipment on an actual project some of the data could not be interpreted
as easily as we saw in the demonstration but nevertheless, results were
a lot better than what we would have got with a pachometer.

The only problem with this equipment might be its availability. Since it
is fairly new on the market, I don't think that too many testing labs
are equipped with it.

Bruno Côté