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Old 02-06-2011, 10:32 AM   #1
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concrete question


I am having a footing, stemwall, and slab poured to make my existing garage bigger.

Now that I have seen the contractor working, I feel that he may be trying to cut some corners because it is taking longer then he expected.

He is pouring the footing and stemwall together, then will strip the forms and pour the slab. The ground is sloped where the slab will be. The slab area will need about 6 inches of fill on one side and about 18 inches on the other, before 4 inches of concrete.

Can he use dirt to fill this up, or broken up concrete chunks?

It was to my knowledge that this should all be filled with gravel and compacted?

He has aready thrown some broken up concrete chunks in there so I am worried about the longevity of the slab.

Thanks for any advice on this.

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Old 02-06-2011, 10:38 AM   #2
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I have always seen compacted sand

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Old 02-06-2011, 11:42 AM   #3
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You should carefully review your agreement with the contractor. Normal practice is to place structural fill under the slab. Structural fill should be defined in your contract. General practice is to define structural fill as a mixture of sand and gravel (gravel in this case means rock fragments generally between 1/4 inch and 1 inch in diameter), with a maximum of 10 percent material passing the number 200 sieve, meaning no more than 10 percent by weight silt and clay. This is for commercial jobs, in a residential context the contract may specify filling with sand, gravel, or coarse fill.

You say the contractor is filling with "dirt", which is a poorly defined term. However, if your contract allows the contractor to fill with "dirt" and/or chunks of broken concrete, or miscellaneous fill material (undefined), you should expect to pay extra for telling the contractor that he must use better material like sand or gravel, since of course he is going to have to pay to truck this material in. If it were my slab, I would definitely stop the use of debris and "dirt", but plan to negotiate an extra charge if this was permitted in your contract.

By the way, the fill must be compacted prior to placing the slab. Compaction should be defined in the contract, if not you should plan to pay extra to the contractor to have him compact to a reasonable target, say 90% maximum Proctor density, or perhaps specify two passes with a minimum 5 ton sheepsfoot or smooth roller, or two passes with a handheld plate whacker.
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Old 02-06-2011, 12:32 PM   #4
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Quote:
Originally Posted by oregondiy View Post
I am having a footing, stemwall, and slab poured to make my existing garage bigger.

Now that I have seen the contractor working, I feel that he may be trying to cut some corners because it is taking longer then he expected.

He is pouring the footing and stemwall together, then will strip the forms and pour the slab.

I've seen a few attempts at this, but I've never actually seen one that I didn't feel was inferior to doing it in two steps. If you take a picture, it may be easier to comment on the type of construction.

The ground is sloped where the slab will be. The slab area will need about 6 inches of fill on one side and about 18 inches on the other, before 4 inches of concrete.

Can he use dirt to fill this up, or broken up concrete chunks?

More than likely, the soil/dirt has too much organic material in it, so probably NO. Concrete chunks are stable though, as long as the corret material is placed over/around them.

It was to my knowledge that this should all be filled with gravel and compacted?

Clear, #1 fractured limestone (or equivalent) is by far your best material option IMO. Anything containing fines will need adequate compaction or will be prone to settling.

He has aready thrown some broken up concrete chunks in there so I am worried about the longevity of the slab.

Thanks for any advice on this.
Again, the best choice IMO is clear fractured stone. Less than 1% of jobs we pour on use anything else, mostly because the cost/labor involved in compacting or settling material with fines far exceeds the minimal price difference per ton to use the correct material.

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Originally Posted by Daniel Holzman View Post
By the way, the fill must be compacted prior to placing the slab. Compaction should be defined in the contract, if not you should plan to pay extra to the contractor to have him compact to a reasonable target, say 90% maximum Proctor density, or perhaps specify two passes with a minimum 5 ton sheepsfoot or smooth roller, or two passes with a handheld plate whacker.
I would actually add to Dan's advice by saying I wouldn't allow anymore than 6" lift if using a plate compactor. Some are rated for 12" lifts, but they merely can't do 12" adequately. Another option with certain materials is to flood them with water to facilitate settling. I still like the fractured stone method, if locally available, as it is generally given a 95% Proctor value here w/o compaction.
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Old 02-06-2011, 12:35 PM   #5
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Things vary from region to region, depending on where you are in the country for soil preparation. That being said, after building in VA, KY, and TX, compacted sand is fine.
Basic explanation.
Purpose of adding gravel under a home slab is for drainage. Builders will add a layer of compacted gravel (not mixed with sand) if the house is built on an incline to allow for water to flow under the house without eroding the sand or soil.

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Old 02-06-2011, 12:44 PM   #6
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The footing and wall together is what is refered to as a mono pour. It makes a small job go quicker and it is actually more solid as it is one piece as opposed to having a cold joint between the wall and footing.
The old concrete can go in as fill as long as there are no voids around it. Sand and water can fill all voids
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Old 02-06-2011, 12:54 PM   #7
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The footing and wall together is what is refered to as a mono pour. It makes a small job go quicker and it is actually more solid as it is one piece as opposed to having a cold joint between the wall and footing.
The old concrete can go in as fill as long as there are no voids around it. Sand and water can fill all voids
I certainly understand that, but it's not really any issue on a garage wall that has equally balanced fill. I'd still like to see a picture or drawing of the wall/footing, because that's where it gets complicated seeing as the footing needs to be substantially wider than the wall on each side. Not saying it can't be done, just never seen a picture of it done successfully.
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Old 02-06-2011, 06:49 PM   #8
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I certainly understand that, but it's not really any issue on a garage wall that has equally balanced fill. I'd still like to see a picture or drawing of the wall/footing, because that's where it gets complicated seeing as the footing needs to be substantially wider than the wall on each side. Not saying it can't be done, just never seen a picture of it done successfully.

It easy. Done it many times. Set up your footing good and solid. Place 1x4 sleepers every four feet with a diagonal piece on the corners. Lay out your perimiter and inner wall with its 3/4 of set for the form ply. Attach a 2x4 kicker to the 1x4s in and out to secure the forms.
We usually use quick strip so it is just ties and wailer bars. Top 2x4, brace and straighten, pour or pump in a thick mix
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Old 02-06-2011, 08:04 PM   #9
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Thanks everyone for your replies.

The footing is 12" wide by 6" thick with a 6" stem wall. I don't have a picture of it and it is dark right now.

Every contractor I got a bid from said they would pour the footing and stemwall together in one step.

In this area(Portland, OR) I have never seen anything but gravel used under concrete, I don't think we have fractured limestone around here. (Not to say this a bad material to use or anything)

I hope that he is successfull with his pour tomorrow.
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Old 02-07-2011, 04:10 PM   #10
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Originally Posted by masterofall View Post
It easy. Done it many times. Set up your footing good and solid. Place 1x4 sleepers every four feet with a diagonal piece on the corners. Lay out your perimiter and inner wall with its 3/4 of set for the form ply. Attach a 2x4 kicker to the 1x4s in and out to secure the forms.
We usually use quick strip so it is just ties and wailer bars. Top 2x4, brace and straighten, pour or pump in a thick mix
I certainly understand the process, just fail to see how it's any stronger if you can't consolidate the concrete inside the forms w/o it pumping up out of the footing. I also don't see how it could possibly be economical compared to manufactured wall panels that set and strip extremely fast.

Quote:
Originally Posted by oregondiy View Post
Thanks everyone for your replies.

The footing is 12" wide by 6" thick with a 6" stem wall. I don't have a picture of it and it is dark right now.

Every contractor I got a bid from said they would pour the footing and stemwall together in one step.

In this area(Portland, OR) I have never seen anything but gravel used under concrete, I don't think we have fractured limestone around here. (Not to say this a bad material to use or anything)

I hope that he is successfull with his pour tomorrow.
It doesn't necessarily need to be fractured stone, any stone with out fines in it will be plenty stable, as long as they stripped any and all topsoil out of the garage first.

The footing sounds small compared to our code requirements here, which would be an absolute min. 8" by 14" footing for a 6" wall. Hopefully they're forming a floor ledge into the top of the wall as well. Good luck.
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Old 02-07-2011, 04:42 PM   #11
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[quote=jomama45;586537]I certainly understand the process, just fail to see how it's any stronger if you can't consolidate the concrete inside the forms w/o it pumping up out of the footing. I also don't see how it could possibly be economical compared to manufactured wall panels that set and strip extremely fast.



If you centre an 8" wall on a 16" footing with a 2x4 on each side holding the forms in place there is only a 3/4" gap from sitting on the 1x4 cleats for the concrete to come out of. We will do this with 2' - 4' walls for frost equirements. Two people can start an average foundation on Monday pump on Wednesday and come back to strip and clean up on Friday.
Not quite the same as the 18' tall retaining wall on the side of a mountain, multi pour fountations I've spent 4 months on
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Old 02-07-2011, 05:02 PM   #12
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If you centre an 8" wall on a 16" footing with a 2x4 on each side holding the forms in place there is only a 3/4" gap from sitting on the 1x4 cleats for the concrete to come out of. We will do this with 2' - 4' walls for frost equirements. Two people can start an average foundation on Monday pump on Wednesday and come back to strip and clean up on Friday.
Not quite the same as the 18' tall retaining wall on the side of a mountain, multi pour fountations I've spent 4 months on
If we're talking about approx. 24' by 24' addition on a garage with 72' of wall and footing, here's how I would estimate an actual job here. And our frost depth is 48", so they'd be 4' walls minimum.

Set-up and pour footings on Mon. morning with inspection scheduled for 9 AM.

Set up wall forms within an hour or two after pouring footing and pour wall between 3-4 PM.

Strip & load forms by 10 AM Tues. morning & call for backfill inspection.

Two guys at 12 hours a piece max., with additional OH figured in for forms & consumables, which are very few (break-away ties mostly).

And to be honest, there's plenty of competitors in this market that could cut that 24 man hours down to 16.

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