On one of our projects we installed PVC conduit to feed some floor boxes. The iron workers add installed the rebar in the slab as usual. However at the areas of where a control joint was to be cut after the slab hardened, the iron workers had to have their rebar not attached if you will and it would slide through PVC that was greased so it could move. At the same locations of the sliding rebar and control joint, the electrical inspector had us install expansion couplings at these locations. At the time I was rushed and did not hesitate. I can't find any thing in the code for us to install a expansion fitting or coupling at a control joint. This is NOT an Expansion joint. Your all thoughts would be greatly appreciated.
Conduit in concrete slab
- Thread starter SKB
- Start date
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stickboy1375
Senior Member
- Location
- Litchfield, CT
The National Electrical Code Requirements
Section 352.44 of the National Electric Code? requires that
expansion fittings be installed for rigid nonmetallic conduit to
compensate for thermal expansion and contraction where the
length change is anticipated to be 0.25 in. (6.36mm) or greater,
in accordance with Table 352.44 of the NEC, in a straight run
between securely mounted items. Securely mounted items are
boxes, cabinets, elbows, or other conduit termination.
As a side note...
Use of Expansion Fittings Where Concrete Encased or Direct Buried
In normal circumstances, expansion fittings are not needed with
concrete encased nonmetallic conduit since the movement of
the conduit will coincide with the expansion and contraction of
the concrete. The only place where an expansion fitting would
be utilized is where the concrete itself has an expansion joint.
In direct burial situations, expansion fittings are not needed
because the ground has relatively constant temperatures.
In cold areas, buried lines must be below the frost line to prevent
buckling during freezing and thawing cycles. In warmer areas,
the depth of cover protects against temperature extremes.
Section 352.44 of the National Electric Code? requires that
expansion fittings be installed for rigid nonmetallic conduit to
compensate for thermal expansion and contraction where the
length change is anticipated to be 0.25 in. (6.36mm) or greater,
in accordance with Table 352.44 of the NEC, in a straight run
between securely mounted items. Securely mounted items are
boxes, cabinets, elbows, or other conduit termination.
As a side note...
Use of Expansion Fittings Where Concrete Encased or Direct Buried
In normal circumstances, expansion fittings are not needed with
concrete encased nonmetallic conduit since the movement of
the conduit will coincide with the expansion and contraction of
the concrete. The only place where an expansion fitting would
be utilized is where the concrete itself has an expansion joint.
In direct burial situations, expansion fittings are not needed
because the ground has relatively constant temperatures.
In cold areas, buried lines must be below the frost line to prevent
buckling during freezing and thawing cycles. In warmer areas,
the depth of cover protects against temperature extremes.
You are correct: the joint does not require an expansion fitting. In fact, the rebar didn't need any special consideration either... that is if in fact these location are the so-called control joints. Control joints are to "control" the locations at which the slab cracks... and it will crack. There is no other purpose. Rigid rebar is usually desired through control joint areas, such that the slab sections do not separate after it cracks!!!SKB said:
- Location
- Massachusetts
Smart, the 'greased re-bar in PVC' is a fairly common thing on large slabs, it makes the traditional control joint into an expansion joint, the slab can expand and contract sliding on the greased rebar (BTW the rebars are poured normally on one side of the joint and greased and sleeved on the other side of the joint.) The rebars make sure the elevation of the slabs remain constant regardless of the horizontal movement.
I would be willing to bet you ave walked over these if you have shopped in a big box store, all you would see is about a 1/2 wide gap filled with flexible silicone.
I would be willing to bet you ave walked over these if you have shopped in a big box store, all you would see is about a 1/2 wide gap filled with flexible silicone.
- Location
- Massachusetts
SKB said:
I disagree.
It is certainly not a traditional control joint where all we expect is a small crack.
You don't think they greased the re-bar inside plastic sleeves so it would not move did you?
We usually stay in the dirt per specs til we get to the floor box avoiding any issues with rebars etc.
peter d
Senior Member
- Location
- New England
iwire said:
Does this mean actually digging a trench and burying the pipe? Or will just laying the pipe on top of the dirt before the plastic and rebar suffice?
- Location
- Massachusetts
peter d said:
With S&S we would stay just flush with or below the dirt, and the excavators made any trenches we needed.
Looked the same at Lowe's last month.
Thank you for responding to my questions. I am aware of Article 352.44. This is for Expansion joints. I'm in a control joint area. I have an electrical inspection today and I'm going to bring this up to my inspector to look at this issue again. I will respond later to inform all of you what the out come will be. My thoughts are this. The NEC does not state anything at all about electrical installations in "control joints". I'll get back with you later and again thank you for your remarks.
Well the answer is we need to use the expansion coupling due to the verbage in the structual drawing notes that state "slippage". The actual installation of the expansion coupling needs to be free at both ends for it to work properly. This is still going to be completly encased in concrete and it will not move. I have gone to the Engineers now.
You are all asking yourself why I don't go deeper to get out of the concrete. We are where we can. Some places you just can't. This is construction you know?!
You are all asking yourself why I don't go deeper to get out of the concrete. We are where we can. Some places you just can't. This is construction you know?!
iwire said:
Bob,
I understand the purpose. I'm just saying it is unnecessary for expansion fittings at "control joints".
What was described in the OP is called a "construction joint", where adjacent slabs are not of a homogenous pour. Some slab designs use load transfer slip-dowel devices. One end of the dowel (rebar) must be free to permit the concrete to contract and move unrestrained from that end of the dowel. Usually this is accomplished by use of a sleeve in which the free end of the dowel device rests (never heard of anyone using grease in the sleeve). The other end is embedded in concrete of the adjoining slab.
The reason for such is, on changing from a water saturated to a dry condition, average concrete will contract approximately 2/3 of an inch per 100 linear feet. This change is also approximately of the same magnitude that a concrete slab undergoes with a 100?F temperature decrease.
You'd win that bet, though it wouldn't be me betting against you. I've also stepped over these joints prior to the concrete being poured and Laser Screed'ed.
- Location
- Massachusetts
Smart we will have to remain in disagreement.
Any place that has so much movement that sleeved and greased rebar is used IMO (and apparently the AHJs) needs a way for the pipe to do the same.
Any place that has so much movement that sleeved and greased rebar is used IMO (and apparently the AHJs) needs a way for the pipe to do the same.
tallgirl
Senior Member
- Location
- Glendale, WI
- Occupation
- Controls Systems firmware engineer
Smart $ said:
Technically, it's to allow the joint to move restrained in two axises.
You have it wrong... I agree with you. But as usual, you misinterpret what I write.iwire said:
If you go back to my first post in this thread, you will see that I wrote,
The joints at issue are called neither "control joints" nor "expansion joints". What they are called is "construction joints".
tallgirl
Senior Member
- Location
- Glendale, WI
- Occupation
- Controls Systems firmware engineer
Smart $ said:
Are you sure they only ever contract away from each other?
(Sorry -- I just had to. If you can get me on saying the joint doesn't move, I can get you only saying the slabs only ever contract ...)
First, I didn't say they only contract. Slabs do expand and contract on temperature change... but compared to contraction from curing and subsequent dehydration, expansion during that interim period is usually overly negated. It would take an extreme temperature swing to be of any consquence.tallgirl said:
- Location
- Massachusetts
Smart $ said:
You really want to pin yourself down to that? I would not.
I am a little confused. You appear to agree that the slab can both expand and contract sliding on the greased re-bars but you don't see the need for a sliding section of conduit?
I find that odd. Beyond what the AHJ thinks it may well be a requiment in the specs.
But hey what the heck do I know, I am not smart.
Last edited:
Sure, I'll pin myself to that. Let me elaborate a little though...iwire said:
The statement was referring only to the "curing" contraction contrasting thermal expansion, concurrently. Meaning, it would take a temperature swing of, say freezing to tropical temperatures for thermal expansion to more than offset "curing" contraction.
Furthermore, the statement had no correlation whatsoever to any electrical issue.
In post #13, I wrote,iwire said:
What part of that did you not understand?
Regardless, let me spell my position out in no uncertain terms:
Is the joint under consideration an expansion joint? Technically, no.
By the NEC, is an expansion fitting required at such a joint? Technically, no.
Should an expansion fitting be installed at this type of joint even though it is not required by the NEC? Yes.
Should the NEC require an expansion fitting at this type of joint? Yes.
By the NEC, is an expansion fitting required at such a joint? Technically, no.
Should an expansion fitting be installed at this type of joint even though it is not required by the NEC? Yes.
Should the NEC require an expansion fitting at this type of joint? Yes.
Sorry I created such as stir up hear. Calm down. After all this and that the engineers are re thinking the control joints and will probably make them go away. Unbelievable.
My thoughts again on this particular installation would be to install a listed raceway for this application. Notice I didn't say "expansion coupling". That is listed for Expansion joints not control joints.
I tried to attach a PDF from a leading manufacture of PVC products in regards to the installation requirements of the expansion coupling, but it was larger than what the forum can handle. It clearly is not the right listed raceway for this application.
Thanks and IPA or bust!
My thoughts again on this particular installation would be to install a listed raceway for this application. Notice I didn't say "expansion coupling". That is listed for Expansion joints not control joints.
I tried to attach a PDF from a leading manufacture of PVC products in regards to the installation requirements of the expansion coupling, but it was larger than what the forum can handle. It clearly is not the right listed raceway for this application.
Thanks and IPA or bust!
tallgirl
Senior Member
- Location
- Glendale, WI
- Occupation
- Controls Systems firmware engineer
Smart $ said:
We get them where I live. The annual temperature variation during my time living here has been over 110F, which is about double "freezing to tropical".
Welcome to Texas. Now please go home.
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