# Duct bank questions



## william1978 (Sep 21, 2008)

Dnkldorf said:


> Anyone care to share a supplier of pipe spacers? I found one supplier on the web, but have no experience in building them.
> 
> Any info you can share would be appreciated.
> 
> Do you vibrate the concrete when pouring, if racks exceed one row high?


 You can vibrate the concrete, but you will need to be carefull so that the conduit doesn't float up to the top of the concrete.

The spacers just snap together, but I wrap some tie wire around the spacers so the don't pop apart.


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## JayH (Nov 13, 2009)

http://www.wescodirect.com/direct/images/products/pdf/019_Elec08.pdf

Here are a few options. You should be able to order these through your wholesale house. 

We do vibrate our concrete when pouring. Just drop the vibrator in every five feet or so and pull it right back out. 

I've been on a few military jobs that the concrete hardness and slump was specified. If it's not specified then just pour enough for the concrete to not flow away and be wet enough for easy working of the concrete.


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## 10492 (Jan 4, 2010)

Is that a 4' deep trench?
Who makes the spacers?

Don't you rebar it down, before pouring, to avoid conduit float?


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## JayH (Nov 13, 2009)

That's going to be some awfully wet concrete if your duct bank is floating in it.


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## william1978 (Sep 21, 2008)

Dnkldorf said:


> Is that a 4' deep trench?
> Who makes the spacers?
> 
> Don't you rebar it down, before pouring, to avoid conduit float?


The ditch was about 3' deep.

I don't remember who makes those spacers it has been about 3 years ago.

Yes, we stake them down to help with the conduits floating to the top.


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## 10492 (Jan 4, 2010)

I see....I think....

You nail the bottom row into the dirt with rebar, and then the rest of the spacers tie to the bottom row?


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## william1978 (Sep 21, 2008)

Dnkldorf said:


> I see....I think....
> 
> You nail the bottom row into the dirt with rebar, and then the rest of the spacers tie to the bottom row?


 I find some rebar that is left over that has a 90 bent on it and stake the whole rack down.


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## varmit (Apr 19, 2009)

Spacers: If memory serves me correctly, most of the PVC manufacturers also sell spacers.

If you don't stake the PVC down, it will float like a balloon. My method for staking was to drive a piece of rebar into each side of the ditch, near the top of the duct bank, then tie a piece of rebar across the top. If you have more than 3 tiers of pipe, you probably need to install more than one rebar "strap". If you want the pipe to be strait, so it is easier to pull the wire, stake it down every 6 to 8 feet depending on the number of conduit in the cross section.

Another consideration is thermal expansion. Four inch PVC will expand in length about 1" to 1 1/2" per 100 feet with a 30 degree temperature change.
Say you install the conduit on a day when it is 50 deg. F and sunny, you plan on pouring the next morning, over night the temperature drops to 20 degrees, if the run length is 300 feet, the pipe will now be 3 to 6 inches shorter than when it was installed. 

A job that I worked MANY years ago had huge duct banks with 30 to 80 conduit from 1 1/2 to 6 inch. There, to avoid the expansion / contraction problem, the concrete crew would start pouring when the conduit was installed out to about 80 feet from the starting manhole. The concrete would almost be splashing on us when the run was finished at the next man hole. A lot of these runs would take several days to install, but the concrete was always poured to within 20 feet of the end of that days installation. It was stupid hard work, but I was young and poor then. ( I am still poor, just older.)


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## manchestersparky (Mar 25, 2007)

Something I always did when doing duct banks.
I ordered my concrete with pea gravels instead of the standard # 6 stone.
There was no difference in cost and it flowed better between the racked conduits.


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## LGLS (Nov 10, 2007)

william1978 said:


> The ditch was about 3' deep.
> 
> I don't remember who makes those spacers it has been about 3 years ago.
> 
> Yes, we stake them down to help with the conduits floating to the top.


Yes, April 7, 2007 If I recall...


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## MDShunk (Jan 7, 2007)

manchestersparky said:


> Something I always did when doing duct banks.
> I ordered my concrete with pea gravels instead of the standard # 6 stone.
> There was no difference in cost and it flowed better between the racked conduits.


I always order concrete that way, no matter what I'm pouring. It works and finishes so much easier, with a lot less air pockets. Flows so much better. Generator pads, housekeeping pads, concrete encasement, whatever. You're absolutely right. No difference in price.... just gotta tell the batch plant when you call to order it.


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## nitro71 (Sep 17, 2009)

Carlon makes spacers. Tie wire your pipe to the spacers. Set a stake every now and then and tie wire off to it so your pipe doesn't float. I recommend vibrating as you don't want gaps in your concrete. Be careful if you are using EB as it's very thin and could get punctured by the vibrator.


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## jw0445 (Oct 9, 2009)

EB has a tendency to egg also.


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## william1978 (Sep 21, 2008)

jw0445 said:


> EB has a tendency to egg also.


 What is a EB?


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## 10492 (Jan 4, 2010)

william1978 said:


> What is a EB?


 
What he said.


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## nitro71 (Sep 17, 2009)

EB is a thin PVC that you can use for concrete encasement. Doesn't hot box and cracks easily. 

http://www.carlon.com/Master%20Catalog/P&C_Duct_Brochure.pdf


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## william1978 (Sep 21, 2008)

nitro71 said:


> EB is a thin PVC that you can use for concrete encasement. Doesn't hot box and cracks easily.
> 
> http://www.carlon.com/Master Catalog/P&C_Duct_Brochure.pdf


 Ok. Around here we call that schedule 20.


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## Sparky480 (Aug 26, 2007)

william1978 said:


> You can vibrate the concrete, but you will need to be carefull so that the conduit doesn't float up to the top of the concrete.
> 
> The spacers just snap together, but I wrap some tie wire around the spacers so the don't pop apart.


Ive used those same spacers. I always tye wire them all together and than to a peice of re bar. Id love to work in trenches like that we get nothing but roots/ rocks up here in NY


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## adh11676 (May 28, 2010)

*Conduit Retaining Device*

Here's an interesting product that may be useful in this application. I think the width of the trench would have to be consistently the same size to be effective.

www.sta-down.com


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