Tamping number 1 stone?

[Greg Woloshyn]

Hi everyone, I'm almost ready to pour the new slab for my concrete floor for my shop. I will be bringing in 2 yards of #1 crushed stone for a base and wondering if I need to tamp it down or can I just rake it out and pour over it without tamping? Tamping would be a hassle since I'd have to rent one, but I want the job done right. I have about 6-8 inches of stone, footers in every corner, and reinforcing the slab with a ton of rebar.

[Mitchell Andrus]

When I pour I don't tamp #1. I want the concrete to flow into the stone a bit. I would however tamp the base beneath the stone if it's been disturbed. Specs for concrete pours, including Gov't specs, can be found on-line no doubt. I used to set up above ground oil storage tanks for Uncle and they don't let anything go by. That was 25 years ago and may have changed though.

[Brian Effinger]

I'll take the opposite position - tamp it. Tamping stone locks it all in and creates a firm and solid base for the concrete. If you don't tamp the stone could settle, creating a void under the slab, especially with the reinforcing. In reality, it probably won't affect the strength of the slab over time, but you might as well do it correctly.

So guys, how was my grammar & spelling? ;)

[Scott T Smith]

I'll take the opposite position - tamp it. Tamping stone locks it all in and creates a firm and solid base for the concrete. If you don't tamp the stone could settle, creating a void under the slab, especially with the reinforcing. In reality, it probably won't affect the strength of the slab over time, but you might as well do it correctly.

So guys, how was my grammar & spelling? ;)


Brian, you done good.... :D

I've done it both ways, and prefer to tamp for the reasons that Brian stated. Mitchell is spot-on with his comments about compacting the sub base. Make sure that you've removed all vestiges of topsoil, roots, etc, and that any soil that was disturbed was well compacted.

My last big pour was 156 yards a year and a half ago, and the only crack in the slab today is on the outside where a sidewalk section was not well compacted, and in one spot where the expansion joint did not connect directly with one of my poles (pole barn slab). It is an elevated slab on compacted backfill, and routinely experiences heavy loads (12,000 psf or greater) from forklifts, etc on it too.

[Greg Woloshyn]

Compacting the sub base is a good idea however I already have 6-8 inches of stone already there (I broke out the old floor because the garage was sinking). I installed footers, so the garage is sitting on those. It would be a real pain to take out the stone that's in there just to tamp the sub base. I suppose I will get a tamper and compact the stone anyhow.

[Brian Effinger]

If you didn't disturb the sub-grade too much, then it should be ok. You could always go over the stone a few more times to attempt to tamp the stone & sub-grade down together.

[David G Baker]

You might want to consider doing everything you can to stabilize everything underneath the new slab because you wrote "(I broke out the old floor because the garage was sinking)" if you don't you may have a repeat of the sinking garage. Did you find the cause of the sinking?

[Greg Woloshyn]

The reason the garage was heaving and sinking is because there were no footings installed or anything below the frost line to prevent it from shifting. It was simply a garage built on top of a concrete slab with some stone underneath.

In order to prevent this from happening again, before I took out the floor I dug out 4 feet down in each corner and in the middle of each wall and installed 48" sonatubes for footers. The weight of the garage is now sitting on these and not so much the floor. Also, like I said earlier I am re-barring the heck out of it which should help alot.

[Brian Effinger]

In order to prevent this from happening again, before I took out the floor I dug out 4 feet down in each corner and in the middle of each wall and installed 48" sonatubes for footers. The weight of the garage is now sitting on these and not so much the floor. Also, like I said earlier I am re-barring the heck out of it which should help alot.
I have a couple questions for you. Was the existing slab thicker at the edges? Also, do you plan on leaving a gap between the framed wall and the concrete slab?

I ask, because while the structure will be somewhat frost protected, the new slab will not be, and may still heave which could cause problems with the framing.

[Glen Butler]

Wow different terminology where I am from. I have done concrete professionally for years, and do not follow what you are talking about with the #1 stone. Residental projects out here are basically poured on two different mediums. Road base and gravel. Road base is inorganic material of varying sizes; 3/8 minus rock with sand the slightest amount of clay to make it tamp rock hard. There is straight pea gravel which is a 3/8 minus rock, but is typically just used in concrete. Gravel which is 3/4" - 1" rock and you can specificy how coarse you want it. Gravel is by nature self compacting, but you can help it lock together with a plate compactor. A jumping jack is too forceful.

Concrete reacts differently on sand and dirt vs gravel. Road base sucks up water, gravel repels it. So when finishing concrete on road base it is important to lay it quickly and get initial shaping done as it will take its first set quicker from water loss. On gravel you will have more time to get initial shaping done, but when doing final finish the water will bleed to the top. This will cause the conrete to flash and you will need to work quicker at the end.

Simply reaching below frost level in the corners is not going to solve the heaving because the concrete that is not below frost level will still be able to lift. In fact I can see you causing even more problem by having some areas below frost level and some not. You will be causing some strange forces by allowing some of the concrete to lift with frost and some to not. You have to get below frost level everywhere to keep the building from heaving, unless you plan on maintaining a void under the structure which is not below frost level. In that case you are creating a post and beam structure with the concrete which gets into some even more complicated engineering. FYI, here in Utah frost level is 30". Check with local codes to see if you need more than that. If you get frost you would do well to pour a solid footing 30" down around the whole perimeter of your structure.

[Greg Woloshyn]

I will not be installing expansion joints and the floor was around 6" thick around the perimeter and roughly 4" thick in the center. I will say again, i'm installing a ton of rebar to reinforce the slab and also tying it in with the footers I've installed. Also, around the structure I'm building a retaining wall 3 feet out to hide my footers and to provide more frost line protection. Here are some pictures.

http://i872.photobucket.com/albums/ab284/gwoloshyn/Picture032.jpg

http://i872.photobucket.com/albums/ab284/gwoloshyn/Picture025.jpg

http://i872.photobucket.com/albums/ab284/gwoloshyn/Picture026.jpg

http://i872.photobucket.com/albums/ab284/gwoloshyn/Picture034.jpg

[Glen Butler]

That is an amazing accomplishment. You even got the concrete off the anchors attaching the framing.

Clearly your interior concrete pad will not be monolithic with the sonotubes so the concrete will still be able to move properly. I am sorry to say that you still have a problem on your hands though. The new concrete will be poured so that it flows under, seals off and supports the bottom plate. You need to this to support the structure properly or you will have created a post and beam structure with no beams. It is vital that you protect the new concrete pad from frost or it will still heave. Considering the work you have done to this point it would not be too much more to dig around the perimeter so you can pour a monolithic footing along with the pad. A grid work of #4 bar at 12" on center is enough for a suspended slab. With what you are doing you could probably go 16 or 24" centers.

[Greg Woloshyn]

Thanks for the advice. Unfortunately, the extra concrete I would need for a footing all the way around is out of my budget at this point. I already have my forms in place so the concrete will flow underneath the bottom plate and help support the structure. Then, I will build my retaining wall to hide my sonatubes and provide some more frost protection. I plan to run re-bar 16" on center throughout.

[Glen Butler]

Is owens corning 1" blue foam out of the budget? It can work wonders for frost. You could cut strips to go around the outside of the concrete, even trench a little so the foam goes into the ground 24".

[Greg Woloshyn]

I suppose I could do that once the concrete cures, but the problem is my stone will end up falling away as I dig the trench for that foam. My forms are already in place and I really want to avoid taking them out, they are already backfilled some too for extra strength. How big of a problem am I looking at given these three circumstances: my garage is being supported by footers and not resting on the slab so much, I am using rebar in the concrete which makes it that much stronger, and I am installing a retaining wall around the garage (however this doesn't do much for the slab since it's just below grade and way above frost level).

[Glen Butler]

What is #1 stone and how thick of a layer is it in. If the structure is mostly resting on the sonotubes and you have only 4" of gravel in there I doubt that enough rock will fall out to undermine the conrete enough to cause a problem. Even if the conrete is cantilevering above the earth around the perimeter it will surely hold the load that the walls are going to place on it. You are more likely to cause problems by not being below frost or not protecting from frost. Once conrete is in you can always test a small section and see what the result is to decide if you want to continue.

[Greg Woloshyn]

There's actually going to be 7-10 inches of stone underneath the slab. #1 stone is gravel 1/2-3/4" big used for drainage underneath slabs. I've decided I'm going to tack some expansion joint material underneath the bottom plate all around, as well as around each sonatube before I pour. This way, my slab will float, and reinforced with re-bar should be hard to crack.