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RE: Conduit in concrete slabs

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Most of our work is multi level PT concrete with some metal deck w/ cip over.  Typically, conduit is embedded in our exposed pt slabs but not in the metal deck as the metal deck normally has a drop ceiling that allows surface mounting (preferable from my point of view).  I have noted that conduit frequentlycreates a sort of "fuse" in the slabs such that cracks initiate and propagate where the concrete section has been reduced by the conduit. In general, my preference is to surface mount conduit where possible in all forms of slabs.  This is not usually possible with an exposed soffit as in a parking structure.  We don't differentiate in our specs between the different slab construction types.
 
As far as conduit materials, our specs call for rigid EMT in the PT slabs.  Most GC's want to use (usually do use) either rigid PVC or similar, or flexible "SMRF" tube.  There is also plastic coated flexible metal conduit which I have seen used, albeit infrequently.  The coating is on the order of 3/32" and the conduit is usually preloaded.  >From my field experiences, the rigid EMT is the hardest for the installers to place without causing all sorts of nightmares with the PT and rebar, really experienced installers do a great job but they are few and far between. As such, I prefer the use of something other than EMT.  The rigid PVC option seems to be a pretty decent choice, usually flexible enough to snake around PT and reinf without disturbing them and so far have not lost any runs due to joint failure or crushing.  The blue SMRF tube is my least favorite, but is fairly common especially in D/B work.  Problems  I have had are: too much conduit between j-boxes so that conduit takes a looping path (definitely leads to cracking), conduit separating from box so that it fills with paste, conduit not adequately supported peaking out bottom of slab.  The plastic coated, flexible metal conduit seems like a pretty good product but again, haven't seen it used much.  I try to discourage the use of SMRF tube for these reasons. 
 
Interesting to note that while our specs specifically limit conduit to EMT, the electrical subs almost never bid the work that way.  Since we are not usually privy to the specifics of a bid, it comes down to either an early preconstruction meeting or noting non-compliance during field observation and them attempting to get the conduit removed/changed.
 
Nick Blackburn
 
 
-----Original Message-----
From: Paul Feather [mailto:pfeather(--nospam--at)SE-Solutions.net]
Sent: Wednesday, July 24, 2002 11:37 PM
To: SEAOC List
Subject: Conduit in concrete slabs

Hello All,
 
I am looking for consensus on the current practice regarding the permitting of conduit in elevated concrete slabs.  We are reviewing our specifications.
 
I am considering three categories, conventional, post-tensioned, and concrete over metal deck.  Also, do you differentiate between rigid conduit and the currently popular plastic flex conduit.
 
All input is appreciated.  We are accused of being out-of date and too restrictive (what else is new).  I will be happy to repost a summary once it is compiled.
 
Thanks,