Message Board Thread - "Locate Rebar through concrete "

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Locate Rebar through concrete Gary Orlove 6/21/2000
I had a call yesterday from "Ray" who works for Cavanaugh Electric. He has to do a core cut through a 6" thick reinforced concrete floor and wanted to use his imager to help locate the rebar. Cutting through one rebar technically weakens the floor but according to various building codes derates that floor for load. Therefore careful location of rebar is essential. Ray proposed heating the underside surface with a torch. I explained that this causes "spalling" of the concrete and in cases where moisture may still be present may cause a localized steam explosion. Definitely the wrong way to go about this.

Two methods work for this. Rent a ground penetrating radar system - provides some detailed information but it is tough to locate the exact placement of the bar, particularly if there is more than one layer (doubtful in a 6" floor). Second, place a large amount of cable in the floor and use a welder to complete the induction coil. By doing so a circulating current is induced in the steel, which isn't the best electrical conductor so it heats up. Turn the heat off and watch for the heat to conduct to the surface. Works OK if you are perpendicular to the surface (no parallax). However, induce too much current and the concrete around the rebar spalls - floor gets weak for real.
Therefore, this is something to be used in small steps in induction heating. We tried this several times back in the early LN2 days. Worked fine. - Ron Lucier
 
Re:Locate Rebar through concrete Bill W 3/19/2004
While I like to suggest ways to use thermal imaging, there is a good magnetic technology for locating and characterizing rebar in concrete. Have a look at James Instrument's website for some good examples:
http://www.ndtjames.com/

Several years ago, I modified one of their rebar instruments for refractory thickness testing inside industrial furnaces. The steel shell was the "target" instead of rebar. Of course, thermal imaging while in operation provides great information, but in many processes, internal slag and process product coatings on the refractory hot side can (temporarily) insulate right over a seriuous refractory failure, giving a false illusion that all is well. The internal thickness testing was a supplemental procedure to complement thermography.

B.W. "Bill" West, P.E.
Rimkus Consulting Group, Inc.
Houston, Texas
bwwest@rimkus.com
 
Re:Locate Rebar through concrete Thermoimagen Roberto Cruz 3/19/2004
Wow, pleaser note that all threads are LIVE !!!

i want to congratulate to BIll, due no matter the date of the gary orlove - ron lucier post, he ..

my best regards BILL and ITC for keeping all the post from beginning.


 
Re:Locate Rebar through concrete IR Bill 3/21/2005
Not too late yet! (3/21/05) History should not be ignored lest we wish for it to be repeated...

Applications such as dense concrete should be approached cautiously by those having the best, most sensitive equipment, combined with extensive experience. The concrete's high thermal conductivity and heat capacity tends to "muddle" thermal differentials this technology needs to function. Surface moisture and shadowed radiant heat gain sources, etc.... can make a fool of one who forgets basic thermodynamics. Furthermore, induction or otherwise heating of rebar in concrete can be very damaging due to the differences in thermal expansion rates between steel and concrete.
 


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