Concrete Slabs

Namely for a garage. Unheated non-insulated (at this stage), above grade.
How important is it to have 2" foam board underneath the 5" slab?
How important is it to have a plastic vapor barrier underneath the 5" slab?
Bids are coming in; some show it with some show it without.

The biggest concern I have is that I don't want the floor to show wet whenever the humidity is extremely high.
Thanks.

 

Very important, especially with what is under the slab for soil base. Min of 6" of compacted sand really makes a nice job. If you have clay underneath, that is a PITA for sure. Also make sure they use adequate steel reinforcement i.e. rebar or 4" steel screen.

 

This is the project that was started about 4 years ago.
You can see that everything was excavated and pit run hauled in for fill, then sand and compacted. It is about 10" high at back part and about 3' high on this end.
Plus the 4 winters of snow helped with compacting and settling.

That is why I'm wondering if the foam underneath the slab is necessary?

Originally sized for a 30' by 40' garage.

 

Dave, if you can afford to absorb the extra cost, the foam is certainly worth it. Keeping the frost away from the underside goes along way to prevent
uneven shifting of the pad which could result in cracking if there is not ample reinforcement in the pour. Could you probably skip it? Sure but it is a little extra insurance.

 

Yooperdave,
You are looking good, having excavated and imported great fill materials that has had 4 + years to compact. Your question involves moisture not compaction. You have the compaction handled. But, all concrete is going to weep moisture from the ground no matter what material is the base. At a minimum, 4 to 6 mil poly vapor barrier under the slab. Foam will help if you ever want to heat the building. It will prevent the frost from coming up from under the slab. The reason some guys don't want to put it under neath is because it takes the concrete longer to cure because the moisture can't get away by draining out of the bottom.
If I were bidding your project this is what I would propose. Around the perimeter of the building a 12" x 12" turn down grade footing with 4 - 1/2" rebar in a boxed pattern for compression strength of the walls. Tied into that rebar pattern would be 1/2" rebar out into the slab that would crisscross every 24". 4" slab with air entrained. ( that give the concrete some flexibility, We would cut control joints at a 10'x10' pattern or if you don't like cut control joint we could tool them in at a slightly higher cost. 6 mil poly under slab. Broom finish would be standard. Two add alternates would be given. #1 would be 2" foam to replace 6 mil poly and #2 would be to go to 6" thick floor. In case you have heavy equipment on that slab.
Marty

 

Foam isn't necessary for unheated garages but if there is a chance you will heat it, it's a good idea. It may help some with sweating also. I certainly would if i was doing the same project.

It will do a little but not alot to keep the frost from penetrating below the slab (for an unheated garage) especially if there is no perimeter footing insulation.

As said foam (in this case) isn't there do to poorly compacted soils. Allowing your subgrade to go through several freeze-thaw cycles and using a pit run was a good idea.

Resteel spacing for floor slabs should be tightened up to 16" to get full benefits of the added tensile (not compressive) strength.

I generally recommend going to a min. 5" slab with steel reinforcement. Easier to get 2" separation to either surface of the concrete. I hate wire mesh as it is very hard to keep the mesh where it is supposed to be during construction.

Air entrainment (6 to 7%) and a low slump (max. 4" ) will give you good results for stronger and more durable concrete.

Stay away from a steel trowel surface (too slippery when wet). Mag floating is good. Broom finish on interior concrete is a bit rough for my liking.

My 2 cents...which is probably only worth 1.5 cents.

 

Yup.
Yup, but I think only one drain.
Nope. Plans for wood stove in the work shop.

 

I have one "over the phone" quote for around $5.50 sq.ft. (until he rechecks the price of concrete).
They propose an 18X12 footing with a 5" slab with (what I thought is a unique idea) a 5.5" by 5.5" molded curb...instead of block....for the walls.
I don't remember what re-enforcing will be in the footing, and I'm not sure is there will be re-enforcing in the floor, but it is a reputable company so I'm sure there must be in the floor also-at least something. And as far as the poly goes, I would make sure that it gets included. I do remember him saying that the foam acts also acts as a vapor barrier so there would be no need for both of them.
I have no plans to heat the garage, but I do intend to heat the workshop in the garage; maybe just foam underneath that portion.

 

I will be choosing a 5" slab as there is a remote chance a tractor may be in the distant future.
I'll be sure to ask about the air entrainment also.

 

Yes, the foam with taped joints will act as a vapor barrier. The integral curb is a nice idea, especially for areas that might get wet.

 

He did say that the joints on the foam would be taped. Not sure if I liked the idea of the built in curb at first, but it sure does sound better all the time. The slab would be above grade even at the "shallow end" so I don't think (hope not) there would ever be an issue with it getting wet.

 

Have you checked your building codes? We can't build on a slab that big here in New York. I think 20x30 is the biggest you can have here.

 

Wow. They regulate a slab size?? I wonder what that reason is?

 

The slab will have a footing all the way around it. If it were a garage with living quarters above, I think then the footings would have to be four feet below grade.

 

As you know, from that thickened edge slab design description, you won't have frost free footings.

I'm a big fan of frost free footings even on garages. It helps differential settlement and heaving, after freeze thaw cycles.

There are ways to help your layout act like a frost free by incorporating a buried "curtain" of board foam around the building perimeter.

 

Yooperdave and schlot,
we are pretty much on the same page. The reason for the curb is for the separation of the siding to the grade on the exterior. Most inspectors today really watch the finish grade on the exterior. so that 5 to 7" is critical.
The interior finish on the concrete is a matter of personal preference. I like the 18" x 12" footing. 24" oc crisscross is minimun. 16" is better.
Regarding not allowing a structural slab, which in essence this is..... is puzzling. They have used structural slabs in Alaska for years.
Price sounds fair to me.

 

If there is ANY chance you will want to insulate the building in the future, I would put the foam down.

 

You're right on, floating a slab like this can be done even better with today's designs.