PVC Schedule 40 VS Hot dip galvinized Steel conduit

[nonsinusoidal]

I am currently evaluating the following two types of conduits to be utilized for passing through a concrete abutment support for a bridge crossing project. We have utilized hot dip galvanized steel conduits in the past. However, We have had situations where this type of conduit has been susceptible to corrosion issues. The conduits are to be used for a duct system which supports and protects underground electric cables.

1. Rigid PVC schedule 40 DB-60 Conduit manufactured as per ASTM F-512, NEMA TC6, and TC8.

2. Hot Dip Galvanized Steel conforming to ANSI C80.1.

Can anyone advice if RIGID PVC schedule 40 is acceptable to be used in place of the Hot Dip Galvanized Steel for the above mentioned application. It is my understanding that Hot Dip Galvanized is to be used for locations exposed to wet locations. The strength of steel is also superior to rigid PVC schedule 40 conduit when placed in concrete where the mechanical loads around are very high. Please advice.

 

[rbulsara]

PVC sch 40 should be adequate and is more common since many years. Reinforce the concrete if necessary.

In fact they both are suitable for direct burial as well. Also is most cases even thinner wall PVC Tyep EB are prevalent but for a road crossing sch 40 would be preferable.

The transportation authority may have their own standards though and you may want to stick with those.

Rafiq Bulsara

http://www.srengineersct.com

 

[xxjohnh]

PVC schedule 80 is considered the equivalent of hot dip galvanised for above grade applivation. Schedule 40 PVC is usually not threaded and is cheaper.Fibreglass is often used for long runs on bridges.

 

[7anoter4]

I agree with rbulsara and xxjohnh. I don't see any problem passing the conduit through wall-concrete or bricks made-PVC or GRS[Galvanized Rigid Steel conduit] .There are also galvanized steel conduit PVC coated[ see Ocal Blue or Calbond or Perma-Cote or other].
If the problem could be sealing there are sealing appurtenances and sealing putty [RTV or else] or if the problem is the fire stopping you may contact GE or Dow-Corning or other.

 

[davidbeach]

You don't say how you are filling the conduits, but if you are using one phase per duct you must, absolutely, not use steel. If the conduits aren't continuously encased in concrete, not clear from the description, you may find that fiberglass conduit works better than PVC as the fiberglass will have thermal expansion characteristics much closer to the bridge's than will PVC.

 

[nonsinusoidal]

I completely agree with all of your feedback. However, I should of said that the Hot Dip Galvanized Steel conduits through the abutment supports are installed as follows:

1. It is a requirement to cut off a rectangular piece of concrete from the abutment supports to install the duct system through the cavity created in it.

2. The duct system is then installed in the cavity within the abutment.

3. The steel reinforced rods are installed and formed around the duct system (composed of Hot Dip Galvanized Steel conduits) within the cavity.

4.Then, the concrete is pour per the specifications as mandated by the bridge owner.

The hot dip galvanized steel conduits only extend four feet on either side of the exits from the concrete abutment supports. The bridge crossing duct system under the bridge is built by using epoxy fiberglass conduits. The other portion of the duct system (is composed of PVC schedule 40 conduits)is concrete encased and buried underground. All of conduits for the complete conduit system will have three concentric underground electrical cables for distribution of power at medium voltage system level (Grounded wye system.)

Currently, the bridge crossing owners mandate to utilized hot dip galvanized steel conduits when penetrating abutments supports as described above. However, corrosion at the exit points at the abutment supports is becoming a problem. As a result I would like to build a case to utilized a different type of conduit to minimize the corrosion issue I explained. My other option, as I explained above, would be PVC SCHEDULE 40 or 80 UL listed product. Please advise as appropriate.

 

[dpc]

As mentioned already, you can obtain PVC-coated rigid galvanized steel conduit. This pretty much eliminates the corrosion where the conduit exits the concrete. We used to specify PVC-coated steel for embedded PVC conduits where they exited the concrete.

Either Sched 40 or Sched 80 PVC is a workable alternative.

 

[busbar]

For the truly insane, there is

http://www.stainlessconduit.com/conduit.htm.

 

[7anoter4]

From your post of 07/11/2009 13:38:00 I think I saw your problem. If it is as shown in the attached sketch indeed, this is a big problem.
First of all, if the intention is to concrete the entire duct you may use only one type of duct -PVC or GRS or GRP[or glass reinforced epoxy].But a continuous concrete beam fixed in the abutment could be a problem since if the abutment position would move in the Ground-usually it does- this concrete beam could crack. Then the beam has to be free in the abutment cavity-supported but not firmly hold.
On the other hand, if you don't intent to concrete all the conduits to form a continuous beam will be difficil to seal the connection point between different parts.