Ready to start the slab. need advice

[aarcuda]

Im in Arkansas. no permits or inspections required. I have brought in 22 dump truck loads of shale to flatten out my land (its on the top of a hill so the edge of my property drops off). The shop will be 30x50.

We spread the dirt using a dozer and Ive been compacying it for the past month or so by driving over it. Had a couple of good rains and it seems pretty compact.

So now I'm ready for footings and the slab. Im going to cut a footing and drop some rebar into it. My concrete guy (a friend too) said that I dont need a rebar grid across the slab and that I just need wire mesh on top of plastic (visqueen? sp?).

Will wire mesh be enough> how much is rebar if I wanted to put that down? If I wanted rebar, what is the grid? 24" or something else?

the slab with will poured at the same time as the footing.

 

[bmwpower]

I see no mention on compacted stone. Are you going to lay down stone before putting the vapor barrier down?

 

[aarcuda]

no, I wasn't told that compacted stone is required. Is it? what is the purpose of it?
The vapor barrier was going right on top of the compacted shale. and the slab on top of that.

 

[twostory]

I just poured a 12 x 12 patio yesterday. I recommend you use rebar, since the 6 x 6 inch grid stuff is impossible work with. Since you walk on it, you press it down to the ground, so you have to pull it up into the concrete as you pour and work the concrete. Note: I did support the grid with broken up bricks, but is still gets pressed down.

Rebar is easy to place on support chair. It stays 1.5 inches off the ground where you want it. You walk between the rebar and never press it down.

As for my garage slab (26 x 44 ft) I am placing #4 rebar in a 18 inch grid. Fully supported everywhere with metal chair about 1.5 inches high. Under the concrete slab is 4 inches of #57 stone, with a 6 mil plastic sheet between the concrete and rock. I am pouring a 5 inch thick slab.

That 4 inches of stone acts as a "capillary break" that makes it very hard for water to move up through. Thus it keeps your slab dry. The 6 mil plastic just stops the rock or dirt from absorbing moisture out of the wet concrete, and causing the concrete to dry out too fast.

Placing a 24 inch grid will work, but more rebar is always a good thing. I would put 2 or 3 rows of rebar in your footer. The footer really needs the rebar for strength, the weight of the building is pushing down of it, versus a slab that just has a car on it.

Also if you ever plan on putting an epoxy floor down, do not use any concrete sealers on your slab after the pour. If you do, you will have to mechanically remove this sealer before you can paint the floor.

 

[DIGGER_DAVE]

I just poured a 12 x 12 patio yesterday. I recommend you use rebar, since the 6 x 6 inch grid stuff is impossible work with. Since you walk on it, you press it down to the ground, so you have to pull it up into the concrete as you pour and work the concrete. Note: I did support the grid with broken up bricks, but is still gets pressed down.

Rebar is easy to place on support chair. It stays 1.5 inches off the ground where you want it. You walk between the rebar and never press it down.

As for my garage slab (26 x 44 ft) I am placing #4 rebar in a 18 inch grid. Fully supported everywhere with metal chair about 1.5 inches high. Under the concrete slab is 4 inches of #57 stone, with a 6 mil plastic sheet between the concrete and rock. I am pouring a 5 inch thick slab.

That 4 inches of stone acts as a "capillary break" that makes it very hard for water to move up through. Thus it keeps your slab dry. The 6 mil plastic just stops the rock or dirt from absorbing moisture out of the wet concrete, and causing the concrete to dry out too fast.

Placing a 24 inch grid will work, but more rebar is always a good thing. I would put 2 or 3 rows of rebar in your footer. The footer really needs the rebar for strength, the weight of the building is pushing down of it, versus a slab that just has a car on it.

Also if you ever plan on putting an epoxy floor down, do not use any concrete sealers on your slab after the pour. If you do, you will have to mechanically remove this sealer before you can paint the floor.

twostory says it all.

I (the contractor, not experienced in concrete installs) used only the 6 x 6 wire when my slab was poured.
Because of the fairly wide range of temperatures up here; the slab started to crack and heave within a year.

The slab was replaced last year with PLENTY of re-bar in it this time!
(by pro's!)

 

[Qtrmeg]

Broken shale is hardpack, but you didn't compact it by driving on it.

I don't see why mesh isn't good enough if you have the right base.

What's the right base? Let's start with a properly drained building site with the correct aggregate and depth for your soil type, compacted properly. You don't run over a foot of cheap dirt with heavy stuff, you install layers of material and compact as you go. The material is determined by the soil you are covering, and the depth of the layer is determined by the size of your compactor.

This isn't a rebar or mesh question, this is about proper site prep.

Aarcuda, shale bits pack wickit nice, but it's cheap money to have someone come in with a vibratory compactor now. I'd do it. I'd also make damn sure you control water above and below ground.

You say you filled a drop, if the drop is on the right what do you see when you look to the left? A hill? Don't fight water, control it. You may be screwed, shale doesn't only pack well it drains well and you may have set this up so water will always be running under your slab. That's a major NoNo. Talk this over with your site guys.

As far as plastic under the slab fine, but dust down a few yards of screened sand so it is worth something when you are done.

 

[aarcuda]

after each truckload of shale (and there were a lot of them), the guy spread it out with a dozer. Ran over each truckload flattening it out.

My property is the top of a hill. its 130 feet across my property (width) where the shop is going. if you split that you have the 75 feet or so from the front of where the slab will be to the street is flat with a slight slope to the street (in otherwords, it drains toward the street).

from the spot where the slab will go to the back of my property, it slopes off down the backside of the hill. thats where I had it filled in to flatten it off. I also the raised the area up about 6" from the rest of the property so as far as drainage goes, everything is away from the building.

the soil underneath is a very tough mixture of rock and dirt (A LOT of rock). This property is on a ridge so there are rocks everywhere (doesnt make for a very nice lawn either) but its hard.

One idea is to dig out some areas near the backside of where the slab will be (where the shale fill is the deepest) and go down to the rock and put some concrete block piers in (stack them up to the top of the shale and then fille the holes back in). that way, even if the shale settles, the concrete block will be there to support the slab.

If I were to do that along the back 50' of the slab, what would be good spacing? every 15' (three piers)?

 

[twostory]

One idea is to dig out some areas near the backside of where the slab will be (where the shale fill is the deepest) and go down to the rock and put some concrete block piers in (stack them up to the top of the shale and then fille the holes back in). that way, even if the shale settles, the concrete block will be there to support the slab.

If I were to do that along the back 50' of the slab, what would be good spacing? every 15' (three piers)?

You need to get an engineer to look at this. Since your fill is on a slope, and you think piers maybe required, you are "in over your head." Please take no offense, I am a DIY kinda guy also, but I know when to get a professional, and you need a professional now.

Your fill is on top of a hill, on a slope. What is to stop it form moving/sliding around after you pour a slab? Anytime fill is brought in, you have to be careful that it is compacted properly and expect some settlement.

 

[Qtrmeg]

Cuda, forget pilings, just go with the floating slab like you intended. I would compact it though. I brought one in to do a driveway that is 1/3 mile long and it just cost about $150. The guy fit the job in so I just paid for the move and the time he rolled around. Just tell him what you filled, how deep and where the shop is going, and to compact the s*** out of it. You really want to do this.

It sounds like we're just back to the rebar or not question. That really depends on how well you compact that quarry you are sitting on more than anything. I'd be inclined to just tie the mesh into the footing rebar, that transition is the biggest area of concern.

I'd also worry about pouring when it is too hot and/or the mix being too wet. Concrete is strongest when it is poured dry and cures slow. I had a guy pour a slab last year and while the slop splashed out I asked "What's the slump on that mess, 3 feet?". I was beyond pissed, but it was too late.

I know you said this is your friend doing the crete but be there, you have to live with this. The wetter the mix is the easier it is for the install, the dryer it is the stronger it is for you. You'll also want to talk with him about control joints, how many etc.