Concrete Slab Questions

[Yookdew]

Hey all,

longtime lurker here but making my first post.

I am in the process of building a detached garage, 40' ft wide and 30' deep. The footers and block foundation are complete. I am in the East Tennessee area so we do get some freeze and thaw.

My next step is getting the slab poured. Before I start contacting concrete contractors, I wanted to get some opinion. Right now, I am planning plastic sheeting moisture barrier over gravel base, 4" slab using 4000 psi fiber reinforced concrete. My main question is whether or not I need rebar also.

My friend poured a similar size slab approximately 2 years ago with no rebar and it has held up fine so far.

The garage will mainly be used for working on my jeep and other vehicles. All vehicles are normal passenger vehicles with the heaviest being a 1/2 ton truck. I will also be parking my boat in the garage. The combined weight of the boat and trailer is approximately 6500 lbs. I do plan to install a 2 post lift in the future. The areas where the lift posts will be mounted, I will dig out a 3' X 3' area to make the concrete 6" to 8" thick.

For those with concrete knowledge, what advice do you have for my situation?

Thanks!

 

[AC-WC]

There is no way I would do a slab without rebar or mesh of some sort. Your plan for gravel (must be compacted), vapor barrier and 4" is very standard for a garage without a lift. With a lift I would make sure it is at least 6" with rebar/mesh. Since you will have a single pour you get more strength for the lift. You must be sure no cut lines are within 8" of your lift columns. Also be sure there is no rebar/mesh where the bolt holes go. If you don't you may ruin several bits.

 

[Rusted Nut]

Rebar in a slab on grade (assuming good compaction) only holds the slab together if it cracks, which it will unless you have good control joints. 6x6 mesh is probably fine for your use, but definitely add rebar at your lift posts, at least per manufacturers recommendations.

 

[wssix99]

My main question is whether or not I need rebar also.

The best advice I can share with you is to pick out a lift and read the instructions. Don't get your "knowledge" from the internet. Your lift manufactures have real engineers and have tested their products to failure and can give the specs you need.

You will not often see a lift pad specification (for new construction) calling for rebar. Just like in sidewalks, it is not needed. Rebar in slabs (with limited exceptions) does not add strength - it is only used for crack control for shrinkage cracking that occurs with every pour. (Proper saw cut joints are a more cost effective way of controlling this cracking.)

I do plan to install a 2 post lift in the future. The areas where the lift posts will be mounted, I will dig out a 3' X 3' area to make the concrete 6" to 8" thick.

You should not do this. Thickened areas of the concrete induce differential stresses in the slab when it shrinks and will lead to cracking where you don't want it. For this reason, you will NOT see such a method called for in any lift instructions. Its a really bad idea, even though it may be an intuitive thing to do. Lifts do not need footers. (The vertical pressure where the posts sit is no more than the pressure the slab sees when a car is parked on it.)

As you look at lift instructions and slab specs, you may come across some instructions for thickened sections with rebar in them. This is for retrofit applications where one cuts out a section of a substandard slab. These specs are NOT for new construction. Contiguous new construction slabs are meant to be consistent across their entire section and flat.

 

[wssix99]

Rebar in a slab on grade (assuming good compaction) only holds the slab together if it cracks, which it will unless you have good control joints. 6x6 mesh is probably fine for your use, but definitely add rebar at your lift posts, at least per manufacturers recommendations.

This actually causes cracking. (Unless the rebar is placed consistent across the entire slab.) Manufacturers never recommend rebar special for posts on a new construction slab. (They do on a repair slab, but that rebar is not for the posts, its part of the system for the slab.)

 

[kbeefy]

Just make sure the substat and base is compacted properly.
4" of 3000psi reinforced is what most lift manufactuers spec.

 

[rd65]

Just make sure the substat and base is compacted properly.
4" of 3000psi reinforced is what most lift manufactuers spec.

If this is correct then I can add a lift in my building. I thought you had to be 5"+ for a two post lift. Hmmmm.....

 

[kbeefy]

Every lift has it's own requirements....

 

[kbeefy]

Derek Weaver, Forward and BendPac requires 4 1/4, Eagle says 4".

12k and up is 6"

GJ thread

 

[kbs2244]

forget the fiber
go with the rebar

 

[dougf]

I would use rebar and be very, very particular about the compaction of the soil and gravel. Absolutely no organic material and ensure it's compacted in proper lifts with a suitable compactor for the substrate used. Too many concrete guys will disregard any of these instructions to cut costs and get the $$$ and split, ensure you are present throughout the process.

Get the slab specs in writing and ensure you include your need for a slope, drains, ledge or slope for garage doors so water doesnt intrude, the PSI, the particular finish you want (I chose broom finish for my shop and love the no-slip finish), the proper slump (don't let the finishers add water and ruin the strength), and control joints.

 

[ConCretin]

Check out my Guide to Floor Slabs by clicking the link below.

 

[Chaznsc]

Rebar in a slab on grade (assuming good compaction) only holds the slab together if it cracks, which it will unless you have good control joints. 6x6 mesh is probably fine for your use, but definitely add rebar at your lift posts, at least per manufacturers recommendations.

Actually, wire mesh holds a cracked concrete together, but it must be chaired to be effective. If I was considering mesh, I’d go fiber. Rebar adds tensile strength, which concrete has very little of.

 

[BombShelter]

I was a lowly flatwork salesman for a few years, we have fairly harsh conditions up here, the owner was 2nd generation old-school. We never used mesh or fiber and I can still drive around town 15 years later and our work still looks top-notch.

We always went 5"+ (4500 PSI Granite Mix) on driveways/garage slabs and used rebar only. The slabs, we replaced, with a high failure rate had either mesh or fiber, the mesh will rust through (right at the crack) and won't support anything. The slab cracks and settles into the void, personally I'd never use mesh on any of my personal projects. Fiber seems to have it's own issues but they might have been addressed over the last few years. It tends to wick up water and you get a ton of little surface cracks, it's pretty easy to tell it was part of the mix after a few years.

If you want to spend a little more money, look into CETCO Voltex, it's a bentonite membrane, with vapor barrior, that is laid on the ground right before the concrete is poured, no water will ever move up through the concrete with it installed.

 

[jonesg]

There is no way I would do a slab without rebar or mesh of some sort. Your plan for gravel (must be compacted), vapor barrier and 4" is very standard for a garage without a lift. With a lift I would make sure it is at least 6" with rebar/mesh. Since you will have a single pour you get more strength for the lift. You must be sure no cut lines are within 8" of your lift columns. Also be sure there is no rebar/mesh where the bolt holes go. If you don't you may ruin several bits.

have you never seen a diablo rebar demon bit, it has carbide teeth. not expensive.
we did my garage floor this summer, I used rebar everywhere without regard for where I'll install the lift , I know I can drill any rebar that gets in the way.

 

[billconner]

It all depends on the base. A 4" floating slab on a great base and robust vapor barrier will be fine with no reinforcement.

To the OP, look at steel fiber reinforcement.

 

[Yookdew]

Thanks for all the replies and info. I will keep this in mind as I talk to the concrete folks and come up with a final plan.

My base is solid, virgin ground on a hard shale. When I was using rebar as grade stakes for the foundation, the ground is so hard I has to drill with a masonry bit to get the stakes in. You can see in the background in the attached pics what's been excavated to get the land ready to build the garage.