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Old 06-09-2008, 06:33 PM   #1
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Concrete pad questions


We will be pouring a concrete pad that measures 10'x4' with a 1' depth.
We plan on reinforcing it with 4 rebars to span the length (so that's about 9ft of rebar each).
Should we also be placing rebar in the other direction (spanning the width)?
Also, how high should we be placing the rebars?
And for a slab this long, should we be putting expansion joints as well?

Thanks in advance.

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Old 06-09-2008, 07:26 PM   #2
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First off, why so thick? That is unusually thick unless you're the owner of a Sherman tank. 4-5" is the normal thickness for patios. Driveways run 5-6". Interior basement slabs are typically 4". All slabs should sit on a bed of gravel equal to the slab's thickness. Even commercial slabs are rarely poured 12" thick...That is a lot of weight and in most cases is unnecessary.
Ok, now that I have that off my chest...

How you reinforce it depends on what it is being used for. If it is a driveway or parking space, you need bars at about 24" on center each way. For a driveway, 6" welded wire mesh is every bit as good.

For a patio or basement slab, there are no reinforcing requirements in the code, provided they're placed on undisturbed soil that won't settle too much. If it were mine, I'd still go for rebar at 24" centers.

If you must pour it 12" thick...And I hope you have a reason for doing that...I'd suggest putting the rebar right in the middle. If it is a structural slab on fill material, you should have an engineered design, but in that case I'd opt for putting it in the top of the bottom third.

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Old 06-09-2008, 07:27 PM   #3
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Expansion joints would probably be a good idea. I'd just do one at 5', half of the 10' dimension.
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Old 06-09-2008, 07:59 PM   #4
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I forgot to mention that it is to be used for the installation of 3 post office cluster boxes.
The thickness is specified by the manufacturer of the box (8-12 inches is the specification actually but our local PO requires a 12in slab).
On the installation instructions, it only specifies for rebars that span the length of the slab and not the other way (which is why I am wondering about that).
It also doesn't say anything about expansion joints but I've seen similar structures around the area and the concrete has since cracked hence the question.

Here's the website where I downloaded the specs for the slab,
http://auth-florence.com/downloads/t...structions.pdf
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Old 06-09-2008, 08:11 PM   #5
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Ahhh, now that makes sense! Be sure to put a few inches of gravel underneath it.

The way I see it, rebar is cheap insurance. The more you have, the less likely that the inevitable tiny cracks will become undesireable big cracks.
I'd run it both ways, and I might even consider doing a top mat and a bottom mat of rebar. Be sure to have 2" of concrete cover on all bars.
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Old 06-09-2008, 08:27 PM   #6
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Sorry for being obtuse but what exactly would a top and bottom mat of rebar look like?

Also, how do I ensure coverage of 2" of concrete in the bottom (i.e. how do I suspend the rebar mat so that it doesn't touch the gravel bottom or do I?)?

We're going to be putting at least 8" of compacted gravel underneath by the way. I'm really not sure how deep as we have yet to assess the ground but I'd say anywhere between 8" to 2'.

ETA: Thank you by the way for the answers.
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Old 06-09-2008, 08:37 PM   #7
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No problem.

The best way to suspend the rebar is to use "chairs", which are made of wire and are available at most places that sell rebar. If you can't get them, small pieces of concrete blocks are typically allowable. I don't recommend "stabbing" rebar into the ground to tie horizontal bars to, as this creates a rust channel that will eventually lead to corrosion within the slab to some extent.

With a slab this thick, I think two mats are a good idea. For an extra $12 worth of bar, you have twice the reinforcement. Imagine your rebar layout 2" from the bottom, and then the exact same "mat" of rebar a few inches higher on top of the first mat. You can use stacked chairs or short little pieces of rebar to elevate the upper mat. It will look kind of like a cage.
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Old 06-10-2008, 07:50 AM   #8
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you do it EXACTLY as specified & don't change a thing AT ALL !!!!!!

DO NOT LET THE QUALITY OF YOUR WORK, REPUTATION, KNOWLEDGE ENTER YOUR THOUGHTS - THIS IS AN USPS JOB TO THEIR SPECIFICATIONS,,, you do have an obligation to inform an owner if you expect/notice/are knowledgeable, in your professional opinion, of concerns that may result in a less-than-anticipated final product but the contract CLEARLY specifies the manner in which that shall be done.

if it were my job, i would've submitted a rqst for clarification quoting aci specs,,, all usps contracts i've seen're so low we don't bother anymore but there's a bunch of guys who specialize in these lettings.

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Old 06-10-2008, 08:53 AM   #9
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If you've been given reinforcement specs for this slab, I agree that you should follow them to the letter (no pun intended).
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Old 06-10-2008, 02:09 PM   #10
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Thank you for your perspectives. I just called the PO and the manufacturer today and they ok'd any additional reinforcements.

On another note, we just found out that the ground which we will pour this slab is pretty much sand, what kind of gravel do we need as a base for the concrete pad, if any?

I'm confused with this, some say pit gravel, others say that we need higher quality type of gravel.

Any thoughts?
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Old 06-10-2008, 02:19 PM   #11
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Assuming there's not already a specification, I'd go with 3/4" clean gravel. Compact it as much as possible. I'm not a fan of AB3/road base for this situation because it doesn't promote drainage as well as gravel does.
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Old 06-10-2008, 02:31 PM   #12
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Why is gravel on top of sand preferred (as opposed to just sand alone)?

In the manufacturer spec, it said, 8" of compact gravel. Which I am all ready to follow but the sand and gravel guys that I've talked to say that I don't need gravel anymore because the area where we're going to pour is all sand (hence the confusion)

Also, we plan on soaking the sand for 3-4 days prior to pouring of concrete.

Is that an ok plan?
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Old 06-10-2008, 03:54 PM   #13
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Nope. Follow the spec that says 8" of gravel. That way you don't end up getting sued for not following the spec. Here, we don't pour on anything but gravel. Never sand. I'm sure that varies regionally, but sand can wash out easier than gravel.
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Old 06-10-2008, 05:42 PM   #14
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Quote:
Originally Posted by thekctermite View Post
I'm sure that varies regionally, but sand can wash out easier than gravel.
That's what i thought as well. But yeah, I'd follow manufacturer's specs.
Which leads me to my next question....


Earlier you were surprised about the 12" depth of concrete. Well, I called a ready mix company today and they were quite surprised as well. He said that among he's never poured anything that deep and he has poured a lot of slabs for the same type of project I am doing. He says he's never seen anything poured deeper than 6" (they're one of the only 2 companies that do ready-mix here).

But like I said, I'd like to follow specs. PO specs=12", manufacturer specs=8" MINIMUM.

My question is, what are the possible negative implications of a 4'x10'x1' slab? Will I have a problem with it being too heavy and sinking? Because everyone seems to be suggesting that 1' depth is too deep.

Thank you again.
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Old 06-10-2008, 05:52 PM   #15
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12" is surprising to me, too,,, look at it this way - you bid the work according to job specs so do it,,, you don't want to be introduced to some little pissant who'd only joy in life'll be harassing you.

learned a long, LONG time ago that i couldn't get into trouble if i did it as the boss told me to do it,,, hell, these people can't even deliver o'nite mail - why do you think fedex boxes're outside post offices ???

if it sinks, you'll get paid to raise it OR replace it welcome to public works ( not an intended pun, either ! )

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