I need guidance on ground prep for pole barn shop slab

[Fyrme]

Let me tell ya, between reading here til my eyes bleed, and getting mixed opinions from concrete contractors, I'm about to go find some lead to chew on. Not really, but I'm really getting aggravated with this. I've had bids all across the board from $3200-$6600.

I need help in what direction to go with my ground prep and slab. But first let me share the details.

I live in NE Oklahoma. Annual lows in the upper 20's. Annual highs in the mid 90's. Annual average humidity levels are pretty constant year round in the low 60% range. I live near a lake and have a very high compaction sandy soil. My shop pad is built using this soil and is easily at a 90% compaction, maybe even 100% in some areas. Like I said, the shop is a pole barn, so the slab will be floating independent of the shop walls. Walls and roof are insulated. The shop will not be a regulated climate. So hot in the summer, cold in the winter unless I'm out working, in which it will be heated rapidly.

I plan on having a power trowel finish and have decided to just put a good sealer down rather than an epoxy. This will be a working shop. Lots of dropping heavy stuff, lots of oil spills, occasional gas and solvent spills. Welding and cutting slag will be frequent, etc...

With all that in mind;

1. Do I need a vapor barrier under the slab?

I have had some locals tell me yes, and some no. The main reason I've
been told no, is because it will stress the concrete and give the surface
stress fractures all over it. Also with the cold temps we have this time of
year, it will take ALL day to finish it. (One contractor said he wouldn't pour
it, if I used a barrier) He suggested using water proofer in the mix.

2. 2' or 3' rebar on centers?

Most have said 1/2" on 2' centers, but I had one say 3' on center with 4000psi is more than sufficient.

3. What slump is too wet using 4000psi concrete?

I've had two argue a 7-8 wet mix won't weaken the slab and two tell me not to go over a 5-6 slump.

4. And finally, with the abuse the floor will be getting, do you agree that just a good sealer is the way to go vs an epoxy?

 

[theoldwizard1]

Not sure about the temp issue, but one benefit of a vapor barrier is not allowing the soil to remove water from the concrete.

What kinds of things are you going to be dropping on your concrete ? High strength, with fiber and mesh, not rebar, is more than enough.

 

[Fyrme]

Heavy things like tipping over a 6 ton jack stand, or removing something from under a vehicle while up on a lift and it accidentally falls to the floor, etc...
I poured my patio with with a broom finish using fiber mesh. I'm not sure I could get the glass finish I'm wanting if I go with fiber on this pour. Over time, it will start showing I'm afraid.

 

[Shootinok]

I'm currently building my shop. I went with a 5 1/2" slab reinforced with wire mesh. Nothing fancy there. I used 3500 psi concrete and poured about a 5-6" slump. Hard troweled. I let it cure a full 30 days then sealed with a solvent based clear commercial sealer.
I am very happy with the look. It squeegees out very nice and I don't feel like water gets slippery but will most likely be slick if I get something oily on it.

 

[Shootinok]

I did not use a vapor barrier.
One theory with a vapor barrier is it can assist with curing by limiting the amount of water leaving the bottom if the subgrade is sandy for example. It can also prevent moisture from migrating up through the slab later. Which could be very important for some floor finishes.
My thought in my shop is; it will not be conditioned, not be truly "sealed up" and it certainly will not have wood floors

I poured mine during hot temps then cured the slab by keeping it wet for two full days.

 

[Fyrme]

That looks good. I need to come see it in real life soon.
You said you let it cure for 30 days. Does that mean you kept it empty for that long? That would be tough for me to do!
As far as the ground wicking out the moisture, if dry, my sand soil would be a sponge. But I could easily soak it down first. Im only planning on just using a commercial sealer like you. I've heard a barrier is important when using an epoxy to prevent moisture from popping the bond. I'm not sure if a sealer is the same way or not.

 

[kj_mustang]

If you will ever air condition your shop, you will see the benefit of the vapor barrier to keep moisture levels down and not have items rust in the shop easily. If you will never have hvac in the shop, than there is not much benefit given your sandy soils. On the other hand, it doesn't cost much and you may change your mind in the future about hvac. Fractures in concrete aren't caused by vapor barriers.
WHY CONCRETE CRACKS

 

[RPH]

5.5 inch, 4500 psi concrete, and 6x6 metal. Smooth finish, plastic underneath. Acid stain with 4 different colors, this makes it very hard to detect a stain if one occurs. Ghostshield densifier and Ghostshield 8500 for surface protection. Oil, salt, and any other fluid resistant. Can be burnished if you want. Picture is with just the stain applied and the head engineer on the job inspecting!

 

[73RR]

1. Do I need a vapor barrier under the slab?

I would. Very helpful keeping moisture from migrating up.


2. 2' or 3' rebar on centers?

#4 @ 24" both directions.


3. What slump is too wet using 4000psi concrete?

Don't base it just on slump until you know if there are any admixtures (plasticizers, water reducers)
Base your mix on a water/cement ratio of 'x' then add the Type F or whatever is offered.
You can get by with an initial w/c ratio of .35 to .45 and have great concrete but .35 with 60oz/cy of type F will make exceptional concrete and give you an 8" slump.


4. And finally, with the abuse the floor will be getting, do you agree that just a good sealer is the way to go vs an epoxy?

Look at L&M Chemical 'Hard Seal'

 

[theoldwizard1]

1) One theory with a vapor barrier is it can assist with curing by limiting the amount of water leaving the bottom if the subgrade is sandy for example.

2)It can also prevent moisture from migrating up through the slab later. Which could be very important for some floor finishes.

Yes and Yes !

I poured mine during hot temps then cured the slab by keeping it wet for two full days.

I still remember when I was a "little shaver" (about 5 or 6), watching my Dad watering the garage slab. At the time I thought it was very strange to put water on concrete and it was not even hot out !

 

[rburke65]

So if you believe the folks that tell you not to use a V Barrier and find out later you should have used a V Barrier...now you are screwed. USE the vapor barrier. You can't go back. Forget that guy that says he won't do the floor IF you have a vapor barrier down. He just wants to get in and get out. And, in my opinion, if using sand as a base it will be forever wicking moister and that is the best reason for a vapor barrier.

 

[dw1]

So if you believe the folks that tell you not to use a V Barrier and find out later you should have used a V Barrier...now you are screwed. USE the vapor barrier. You can't go back. Forget that guy that says he won't do the floor IF you have a vapor barrier down. He just wants to get in and get out. And, in my opinion, if using sand as a base it will be forever wicking moister and that is the best reason for a vapor barrier.

I put a vapor barrier down, you can get a roll of 6 mil plastic at HD or Lowes for about $100

 

[joes169]

1. Do I need a vapor barrier under the slab?

I have had some locals tell me yes, and some no. The main reason I've
been told no, is because it will stress the concrete and give the surface
stress fractures all over it. Also with the cold temps we have this time of
year, it will take ALL day to finish it. (One contractor said he wouldn't pour
it, if I used a barrier) He suggested using water proofer in the mix.

Do you mind if the floor sweats periodically through out the year? If so, install a vapor barrier, it cost's all of $.04 a square foot, plus a tiny bit of time to install it. Forget the contractor who won't pour over it, find someone with more knowledge & experience. We pour EVERY garage floor, even small attached parking garages, with a vapor barrier, it's not that hard to compensate for a professional.


2. 2' or 3' rebar on centers?

Most have said 1/2" on 2' centers, but I had one say 3' on center with 4000psi is more than sufficient.

90% of what I pour is #3 rebar on 36" centers. I have 10's of thousands of yards of concrete under my name in the last 18 years, and I've yet to see a failure that could be blamed on lack of re-enforcement on our work. Make sure the base is extremely hard beore pouring, as that's a far bigger cause of failure than re-enforcement.

3. What slump is too wet using 4000psi concrete?

I've had two argue a 7-8 wet mix won't weaken the slab and two tell me not to go over a 5-6 slump.

A 6" slump is already pushing the limits IMO, especially if you have a pitch to the slab or floor drains. There's plenty of ways to make the concrete a higher slump w/o adding water, like water-reducers/super plasticizers/etc. A good contractor will be well versed in this area.


4. And finally, with the abuse the floor will be getting, do you agree that just a good sealer is the way to go vs an epoxy?

There's a ton of good sealers out now-a-days, and your ready mix supplier should be able to tell you what they have available for your needs. Personally, for what you're talking about, I like solvent based silanes or siloxanes.