Filling Barn Stalls with Concrete

[Isidoro]

Hi All,

I'm beginning a minor restoration/conversion of a horse barn into a car workshop.

The barn is 24' x 36' and is built on concrete footings with a slab "driveway" running down the middle connecting 2 overhead doors. There are 4 horse stalls with dirt floors, each measuring 11' x 11'6". The first part of the project is to fill in the horse stalls with concrete to bring the stalls level to the existing slab/footings. Essentially pouring four 11' x 11'6" slabs using the existing footers as forms. (diagram attached)

The existing slab will support a scissor lift. The new slabs will serve as a garage floor. There is the possibility that I will want to add another 2 overhead doors use the shop as a garage - so cars may end up parked on the new slabs.

I am in Albuquerque NM, zone 5. Pretty arid, mild winters (I don't blow out my sprinkler system), monsoons, but little rain/snow. I Technically the soil is classified as loam, but it seems like expansionary clay to me (house is on a post tension slab). I plan on heating the shop, but it is not heated currently. I have spoken with a few concrete contractors and I'm getting different opinions about how this needs to be done. Going into it I was assuming 4" of 3000psi concrete poured directly on compacted soil.

I'm hoping there are some concrete experts who can give me some straight answers to the following questions. I suspect some local contractors are telling me I can do without "upgrades" so they can make a lower bid.

Can the concrete be poured right up against the existing footers/slab, or does it need isolation joints or bond breaks?
Is fiber enhanced concrete sufficient, or does it need welded wire or rebar?
Should I use 3000psi, 3500 psi, or 4000 psi concrete? The cost difference is nominal.
Do I need air entrained concrete?
Should I pour a 6" slab or is 4" fine?
Would using 4000 psi or a 6" slab allow me to skip crack control joints being cut into each slab?
Does all loose dirt need to be removed or can it just be compacted down?
Does gravel need to be installed under the concrete?
Does a vapor barrier need to be installed? Should it run up the side of the existing concrete footings/slab?

Sorry for the wall of text and so many questions. Any input appreciated. Thanks!

 

[Firebrick43]

Horse stalls are going to have organic material. It needs removed with out a doubt.

Vapor barriers should always be installed, even in dry areas to allow the slab to cure evenly and not crack from curling while curing.

As far as bare soils, other than gravel/sand mix soils, yes. The rule of thumb has always been 3 times the slabs thickness to be bomb proof, but 2 times in a world apart from 2 times. It needs to be clean crushed stone and compacted in lifts to be truly effective.

It can be poured directly up to another slab and pinned to it. If pinned I would do 10' rebar as the pins and across on a 12" grid.

I always use 4000 psi concrete, no reason not to, use some super plasticizers as well to keep the concrete men from adding to much water and weaking the mix. There is no reason for air entrainment if its inside.

As far as reinforcement, fiber is not that. Its to help prevent microcracking and will not keep a slab together when large cracks happen. A well compacted subbase that is thick enough will keep the salb from having voids and if done in crushed stone with a good vapor barrier will allow the slab to contract and shrink without cracking. Fill stone like pea gravel does not as it causes the slab to be uneven thickness all over where people walk. In that case reinforcement is probably desired. A mix of rebar and woven wire is desirable as they really perform different functions, and the rebar on chairs can keep the wire in proper place. Its a fools errand to have wire and then have the concrete guys pull it up with hooks as they stand on it. Every piece of concrete I have removed with welded wire had a larger portion of it on the bottom instead of where it should be, 1/2 to 1/3 down from the top. Being as small as you are doing however I don't think both are necessary.

Your pours are perfect size without needing cuts.

 

[Isidoro]

Thanks for the input!

When you mention the gravel is it triple the thickness is bomb proof, but double is a world apart from just 4" of stone?

The barn is unheated and it's 50/50 to remain so. So although the pour is inside, don't I still need air entrainment?

Pinning is a lot more work. Is that a must do in your mind for a slab of this size?

Are you suggesting 10 ft lengths of rebar pinned to all 4 sides in a 12 inch grid? Or just two sides? All 4 sides and the rebar would overlap itself by ~8 ft. I assume that chairs are strong enough for the concrete guys to walk on without the wire getting smooshed.

If you had to do just rebar, or just wire, which would it be?

Any thought on the 4" vs 6" slab depth?

I guess if the decision had to be made today it would be:

4" of 4000psi, 5-7% air entrained, 1.5 lb fiber/yd, plasticized concrete poured over 8" of compacted 11/4 washed top size stone and 6mil vapor barrier. Then either 1/2" isolation joints on all 4 sides or pinning. Leaning towards #3 rebar 12" grid, chaired on concrete brick. Loose/organic material removed before stone.