Garage Slab Question

[HawkinsHollow]

I am pouring a slab for a new garage over an existing, very poorly executed asphalt garage slab. The existing slab is stable but drops in grade 13" from one corner to the next. They planned it this way for drainage and it worked for the equally poorly executed post and beam carport that was there. However, this is not very conducive to throwing up a stick built garage. Therefore, I need to pour a new level slab. I had to bring in 6 tons of gravel to level the slab. My main question is would you use a plastic vapor barrier between the gravel and concrete if the entire slab is sitting on asphalt? Seems a bit overkill to me. And seems there would be much better adhesion between gravel and concrete if one was not used. What says ye ole wise slab masters?



I have attached a picture. I have a 36" tall by 6" thick turn down slab on one side of it. Gonna be a big pour, my generous calcs come out to about 9yards, I ordered 10 to be on the safe side. It is my first big pour, but I have a buddy who worked in concrete coming to run the show and a few other minions to help. Any advice you slab masters can give a guy for a pour like this??

As a side question. It has rained on this gravel leveling course the last 2 days, and will probably rain on it again today. So if I DO use a vapor barrier, what happens to the moisture trapped in the gravel when I pour Monday. Good, bad or indifferent?

Thanks for your help!

 

[dusty3030]

I wouldn't in a garage unless the garage will be air conditioned.

 

[HawkinsHollow]

I wouldn't in a garage unless the garage will be air conditioned.

I do plan on putting ac in it, it is going to be more of a shop than a garage. It get hot down here in the summer. Can you elaborate? I guess we would be worried about condensation then?

 

[dusty3030]

If the slab is able pull water out of the ground then yes you would likely have a problem with the floor "sweating" with an AC system running.

 

[dstig1]

It will help keep the moisture down in the shop later on, but you have to make sure that it is right under the slab, not under the gravel or you will trap water between the slab and plastic. It sounds like that is your plan anyways, so go for it. Plastic is cheap.

 

[MickeyDBC]

Vapor barriers also help keep water from migrating through the concrete which can eventually damage the rebar that can cause all kinds of problems down the road. We have an older farmhouse with either no vapor barrier or a failed one and it will definitely limit the houses life span. You can actually see where the rebar is in the garage floor by small cracks and discoloration caused by water migration.

 

[4570Man]

My first advice would be to not attempt the floor yourself. It’s hard work and hard to get right especially in the heat. And I would use a vapor barrier.

 

[deezler]

Yes, use the vapor barrier. Any moisture in the gravel underneath can find its way out and equalize over time (assuming you properly manage the drainage around the building once built). You really don't want damp concrete inside a conditioned building. "adhesion" of the concrete to the ground isn't a concern - the slab isn't going anywhere. Did you only hand-tamp the gravel for compaction? You should probably rent a plate compactor or jumping foot type and go over it all a few times.

And where is your rebar? I would use a 2x2' grid of #4 rebar supported on chairs to be as close to the middle of the slab thickness as you can achieve. All concrete cracks (you'll probably want to cut control joints in a ~10x10' grid to tell it exactly where to crack) but if you don't have any rebar, the cracked pieces can start to move around over time - not good.

Do you have all the tools you'll need? rakes, floats, trowels, knee pads, etc. Don't attempt this job until you have all the tools needed, and have reviewed the plan with your full team. It's going to be very hard work that you need to do FAST. And realize that doing it yourself means the finish could come out kinda lumpy and rough. Or, rent a power-trowel to help on that front.

EDIT: didn't notice the thread post dates, did you already pour it? Good luck.....

 

[CoyPatton]

I do plan on putting ac in it, it is going to be more of a shop than a garage. It get hot down here in the summer. Can you elaborate? I guess we would be worried about condensation then?

You should be worried about condensation! Use a vapor barrier!
I have a major concern about the base for your slab however. Asphalt is typically not put down on a solid base in most residential areas. I fear your asphalt base with develop sink spots and then your concrete slab will not have support and will follow the sink.
Just my opinion, but we have all seen asphalt roads that have been there for decades develop sink spots follow a wet season or a freeze/thaw cycle.

 

[Egon]

Use vapour barrier.