Thickness of slab?

[Innovate1]

Building a detached garage (shop), 30 x 40. Will have a 2 car attached garage for regular vehicles. All floors are called out as 4" on the plans. Visited a friend who recently had a shop building built and he went with 6" floors and higher psi concrete. That seems like overkill but am thinking I should go to 5". I plan on parking a modest RV there and perhaps getting a bridgeport milling machine or something similar. Don't plan to do a lift but I suppose having the option for me or a future buyer would be good. Looking for advice on the floor for both garages...

 

[TommyK]

5 inches of 4000 psi is what I would do. 6 inches would require a small balance on a third truck which I would avoid for a variety of reasons unless absolutely necessary which isn't the case here IMO.

 

[Randy in Maine]

My 28x40 has 6" of 4000 psi crete and that was right about 21 cubic yards over 2" of foam insulation. Thermal mass of about 42 tons.

 

[Kaizen]

My 28x40 has 6" of 4000 psi crete and that was right about 21 cubic yards over 2" of foam insulation. Thermal mass of about 42 tons.

Same in my 30x36. That stuff is incredibly tough. Added only mesh in middle. Rebar on the thickened edge. I knew I wanted at least one two post lift. Difference in product was negligible. Wish I could have afforded pex and insulation.
I’d only do 4 in standard garage but might do 4k still. Plan on at least coating if you want them to last. ESP where daily drivers will be if salt is used


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[killer cougar]

Really going to depend on the soil conditions and sub-base prep. Most concrete driveways and such are only 4" and we'll do 6" for "heavy duty" (large military vehicles) and 14" around the hangars and runways.


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[brownbagg]

you talking the different of 3 yards, that cheap

 

[joes169]

Check out LLWillysfan's "Guide to FLoors", as it covers everything you're concerned with, very thoroughly, actually.

I'd lean towards 4" concrete for the attached garage, and 5" floor on the detached shop floor. I'm a firm believer in lighter rebar (#3'son 24-36" centers is most typical for us) and an extremely good gravel base and subsoil.

Keep in mind, jumping up an inch of concrete might only be 3-4 yards of additional concrete, but it's going to cost more than the concrete material. It requires extra labor to move the additional concrete,and may require more work to remove existing gravel/rock/stone if it's already backfilled.

 

[Innovate1]

I need to clarify things with some of the contractors. Some don't bid what's on the plans. One specifically said if it's not listed as included it is excluded. The plans call for nail strips in the foundation walls but they didn't include any but have a note that they are $3.50 a foot. The best bid I have and someone that is recommended includes nailers and states "Includes high range concrete in walls". Need to find out what high range is. I initially wanted to put foam under the floors but all the contractors didn't want to do it. I am not to floors yet but want to allow for the right amount. Foam is on plans just at the edges but I might just lay the foam myself. I need to fill some for the attached garage but the basement floor and detached garage needs some excavation so will be 2" more for insulation.

It's true that 1" more is only between 3 and 4 cu yds more so not a huge deal. Seems well worth it.

 

[NUTTSGT]

That extra inch will probably worth the small increase in the future and well worth it.

 

[pcmeiners]

"Really going to depend on the soil conditions and sub-base prep."

Agree

Rarely is the soil compacted properly, even if spec'd in a contract proper compaction is expensive and generally done less then proper ...therefore you go for added concrete, 5" -6" makes up for the soil condition.

Make sure you stipulate NO ADDED WATER to the crete supplier and to the driver. Contractors always want to add water.

 

[sqznby]

Rarely is the soil compacted properly, even if spec'd in a contract proper compaction is expensive and generally done less then proper ...therefore you go for added concrete, 5" -6" makes up for the soil condition.

Would this be the case as well on sandy soil? To add to the thickness?

 

[JoeMcGov]

Everything down below must hold up everything up above. That's the simple rule.

Go with 5" as mentioned. Endeavor to get the subgrade (dirt) compacted WELL.

 

[matt_i]

Personally I'd go with steel reinforcement, making sure the control joints got cut the next morning after the pour and a month-long wet-cure (or curing sealer) over adding 2" more. Which is 50% more. If you use a rotary laser level in the prep you can make sure you get a 4" minimum and not a 3-1/2" maximum....formed with a 2x4 on-edge.

I set out to pour a slab to rig 10,000# machines with a forklift onto it, its 6" @ 4ksi.

 

[ConCretin]

5" is stronger than 4" and 6" is stronger than 5" but no reasonable amount of concrete is strong enough to span an inadequate base. On the other hand, if you have a good base, 4" of concrete is plenty. As has been mentioned, it's fairly inexpensive to add an inch for peace of mind but it's really not necessary.

 

[johnnyradiant]

5" is stronger ....... necessary.

well said

 

[rayra]

4" slab on grade is typical here in desert southwest. I'd want something engineered much better anywhere there is real winter or groundwater or frost heaving.

 

[Retroman]

Any flat work we pour is 5" to 6" and we use 4500psi. extra sack of concrete is about $7.00 a yard and well worth the upgrade. I would go 5" with 4500 PSI over about 4" of 95% compacted type II with rebar or WWF. Options may differ with freeze thaw cycles.

 

[Innovate1]

The basement floor is called out as 3.5". It will have no vehicle traffic or other heavy loads. Is this ok or should this be 4"?

Already discussed garages. Thinking I will go with 4" for attached and 5" for detached. How important is this to be on plans or can I just tell the foundation and flat work contractors? My thinking is if it's on the plans there is less chance of confusion and it getting missed but I still need to point it out to the contractors. I have noticed that I have gotten questions on various things when the answers are on the plans.

The plan notes call out minimum PSI of:
basement slabs 2500
Foundation walls and footings 3000
Drives, walks, patios, steps, garage floors 3500
All concrete to be air entrained

I am thinking it would be wise to bump up the basement slab to 3000 and the drives and garage floors to 4000? That seems to be what most feedback has been on the floors.

 

[tlmartin84]

I would go with 6", and rebar on 2' centers. Less than 1,000 bucks for the bar and additional concrete.

I am sure everyone here who has had cracks in there floor, settlement issues or wanted to install a lift down the road and the concrete wasn't thick enough wish they had spent the money.

"The Bitterness of Poor Quality remains Long after the Sweetness of Low Price is Forgotten"

That's my .02 cents...

 

[tlmartin84]

Air entrainment is a waste of money on INTERIOR SLABS. Typically shoot for 7% on air entrainment on exterior slabs.

I would stick with 4" on the basement slab @ 3000 psi. (I doubt any plants would send out a mix less than that). 4000 psi on the others slabs. 3000 psi for walls and foundations.

Try to hold them to a 4" slump. Most want to slump it up to 5-6" so it "self levels".

Also it should be cured for a minimum of 3 days.

I am also to the point that I would hire a technician to test it. That's going to set you back 500.00 bucks, but after working with plants on a daily basis, I see a lot of incorrect mixes sent out. Most residential contractors don't know what they are even getting when it arrives.

 

[ConCretin]

The basement floor is called out as 3.5". It will have no vehicle traffic or other heavy loads. Is this ok or should this be 4"?

How important is this to be on plans or can I just tell the foundation and flat work contractors?

All concrete to be air entrained

I am thinking it would be wise to bump up the basement slab to 3000 and the drives and garage floors to 4000?

While 3.5" for your basement is probably fine, the cost to bump to it 4", which is more typical would be minimal. A lot of people are waking around on 2" rat slabs in their basements.

I would make sure your plans are as accurate as possible, especially if they are referenced in proposals or agreements

Be very careful with air entrained concrete for steel troweled interior floor slabs. If your finisher doesn't know what he's doing it can cause de-lamination. It's fine for foundations and broom finished exterior flatwork.

Since slabs on grade aren't structural, you don't need much compressive strength but higher strength mixes contain more cement, which will help with surface durability. I'd go with 4000 psi for hard working garage slabs.

Give my Guide to Floor Slabs in the link below for more detailed information.

 

[JamesW84]

If possible, level it off without adding fill. That will depend on your site, but that's what I did. Without adding fill, you will have (hopefully) virgin soil, which should be compacted.

I went with 5" of 5/8 clean gravel, 16 mil vapor barrier, 1/2 (#4) rebar on 24" centers (and chairs), and 5" of 4000 psi concrete. I think they actually brought 3500 psi concrete, but it was done while I wasn't here. I had footers and foundation walls poured so I have a stem wall of about 5" all around the perimeter.

I had them put in 1/2" J bolts at locations I marked out before hand on their wall forms. I probably wouldn't do that again as they ended up different heights and distances from the edge. I ended up cutting some off, drilling and epoxying in threaded rod anchors.

If you're stick framing, look into Simpson HTT5 brackets for next to your overhead door framing if your engineer didn't mention it, but I bet he did or called for anchors embedded in the concrete.

 

[TheShrine]

R. O. I.!

 

[Innovate1]

Air entrainment... I read up and apparently for the interior slabs it isn't needed and can cause issues. A small part of the slab extends under the OHD and my understanding is this helps dealing with freezing temps and water. Is this small part something to worry about? For the basement slab it sounds like it clearly isn't needed.

For the foundation (footers and walls) is there any point of going with air entrainment?

 

[cvairwerks]

Really going to depend on the soil conditions and sub-base prep. Most concrete driveways and such are only 4" and we'll do 6" for "heavy duty" (large military vehicles) and 14" around the hangars and runways.

Man that's light for around the hangars unless it's for fling wings... We're getting some new ramp areas behind our hangars at work, and it's 24-48" thick.

 

[joes169]

Air entrainment... I read up and apparently for the interior slabs it isn't needed and can cause issues. A small part of the slab extends under the OHD and my understanding is this helps dealing with freezing temps and water. Is this small part something to worry about? For the basement slab it sounds like it clearly isn't needed.

For the foundation (footers and walls) is there any point of going with air entrainment?

I'd let the basement floor up to the contractor, let it be what they prefer to finish. It really depends on whether they finish by hand or by machine. You won't know the difference if they know what they're doing.


As for the garage floor(s), we typically use 1/2 air entrainment, which is about 4-5% air when tested, ideally. We do machine finish almost every floor, and find it's better to finish than no air or full air. The issue with full air is that it can delaminate and trap air "bubbles" at the surface, while no air can bleed excessively. The weather conditions have a lot to do with it, as well. If they're planning on hand finishing, I 'd let them use full air w/o objection.

When it comes to the small portion of floor outside of the OH door, I've never had an issue with 1/2 air, but have seen issues from other contractors that I assumed resulted from using no air entrainment. The failure to use low-chert stone in a garage floor can create issues at this area as well, in my experiences, though not nearly as bad as the lack of air.

The best solution for the issues of low-chert/regular stone and full air/partial air/no air at teh OH door are that I've found is to pour the stoop and garage together. Take the majority of the garage with regular stone on the first load, have low-chert stone with full air in the second truck (clean-up) for the OH area and the stoop. Around here, it's relatively common to do so, and the dispatchers at the ready-mix yards are accustomed to it.

On edit, I forgot about the footings and walls. No reason not to use air entrainment in either. You'd be hard pressed to find a negligible difference in strength between the two in cylinder breaks. As a matter of fact, unless you specifically make a point to NOT order air in these parts, you're getting full air as the default for everything.

 

[pplhze]

If you plan on installing a lift down the road you will need 6" pad. or run the risk of the lift cracking the concrete under a load.

 

[Innovate1]

Joes169, I see you are in WI so your weather is more severe than here although we do get down to around 0 F occasionally. I mentioned air entrained to a PE in construction who said it wasn't a big deal but just his opinion of course.

Just looked up Bendpack lift requirements. Don't have any current plans for lift but would be nice to have the option. for some reason their requirements for existing concrete are a LOT less than new. Their four post models get pretty big on 4-1/4, 3000 PSI with no reinforcing.

Someone here mentioned "surface hardened". They weren't sure anyone still did it but after pouring they put additional dry concrete on the surface and work it in for a harder surface. Sounds to me like something that may not be worth the labor vs just getting higher mix for whole loads but I don't know much about it.

 

[ConCretin]

I mentioned air entrained to a PE in construction who said it wasn't a big deal but just his opinion of course.

Someone here mentioned "surface hardened". They weren't sure anyone still did it but after pouring they put additional dry concrete on the surface and work it in for a harder surface. Sounds to me like something that may not be worth the labor vs just getting higher mix for whole loads but I don't know much about it.

Air entrained concrete provides additional protection in freeze/thaw conditions caused by moisture in concrete. Interior concrete doesn't need this protection for obvious reasons. In addition, if your finisher doesn't know what he is doing or you end up placing in hot or windy conditions, you'll have surface de-lamination. Best to leave the air entrainment for exterior, broom finished concrete.

What you refer to as "surface hardened" is just the opposite. Adding dry cement to the surface will give you a weak chalky floor. Do not consider this under any circumstances. There are products called dry shake hardeners that are broadcast over fresh concrete to harden the floor but this is not the same as using cement.

 

[Innovate1]

Not sure of the exact process on the surface hardening. Was told something was spread on the surface and worked in to the top layer. It wasn't just spread on the surface. I don't intend to try to do something like this as it doesn't seem worth the trouble for my use and something that isn't standard.

 

[Innovate1]

Some people think the air entrained flows better and can be substituted for more water, with water weakening the mix and being the poorer choice. No idea if this is really valid or not.

 

[bad_idea]

I built a 30x40 last year. The cost to upgrade from 4" to 6" thick slab was only a thousand or so. Might as well go to 6". Lets be honest with ourselves, the concrete guys will shoot for 5" but there will be thinner spots. Most lifts want 5" thick concrete, with 6" you will be fine if they get it close.

 

[ConCretin]

Some people think the air entrained flows better and can be substituted for more water, with water weakening the mix and being the poorer choice. No idea if this is really valid or not.

I doubt very many people could tell the difference between air entrained and non air entrained concrete by looking at it. Air entrainment is achieved with a chemical admixture that creates millions of microscopic air bubbles. It takes special testing equipment to measure. Concrete naturally has a percent or two natural air, which is typically bumped to 6% or so with the admixture. More or less water doesn't have much to do with it.

The risk of using air entrained concrete for steel troweled interior slabs is that it slows down the release of bleed water. If the surface is sealed with steel blades before bleeding stops, bleed water will pool under the concrete surface creating voids and what is called de-lamination of the surface.

 

[joes169]

Just about finished with one now, 19 yards of straight 5.5 bag mix (no ash), mid range water reducer, .75% non-chloride accelerator, half air-entrainment, 1" washed stone. We use this mix on 90% of basement, garage, shop, pole barn, grade beam slabs and simply adjust accelerants to the weather conditions. Today it's a perfect 70 degrees with 42% humidity at the moment, and the floor is finishing just as well as I could want or expect.

I'd be somewhat careful on what level you try to create specifications for the concrete floor. There's a lot to this, and theres never one exact answer that works best all of the time. I'd simply discuss these details with your concrete sub a few weeks before pouring. He may just have zero interest in doing a job where a homeowner specs out the concrete.

 

[dutchgray]

Not sure of the exact process on the surface hardening. Was told something was spread on the surface and worked in to the top layer. It wasn't just spread on the surface. I don't intend to try to do something like this as it doesn't seem worth the trouble for my use and something that isn't standard.

Had it done on a job I worked on, was a coloured product as well, was meant to make the concrete lighter, spread it out and power floated it in, except they were supposed to apply it and float it in two layers, but they did it all at once and it ended up very patchy, it was a harder surface but it still chips if you drop something heavy on it. If you want a harder floor its better to have more cement in the mix and if your weather is suitable go for pure OPC and not a blended mix.
I would go 6" personally in a garage with steel in it and make sure the ground is well compacted and of even firmness, we also never put concrete on dirt, always at least a 6" layer of stone, but usually 12" and put plastic under it to keep the water in so it goes off better.

 

[mcbane]

Concrete is cheap but you need to figure in the cost of rebar for crack control and decide what total cost you are ok with. The ACI 0.18 % steel ratio is an absolute minimum. To avoid big cracks you want a ratio closer to 0.6%. The saying that “all concrete cracks” is a bit of a cop out. The real issue is how big the cracks are. With reasonable sub grade and sub base prep and sufficient reinforcement the cracks are very, very small.

If you don’t have large concentrated loads 4” should be fine and will be visibly crack free if properly reinforced. If you anticipate pallet racking in a seismically active area or possibly bottle jacks under heavy equipment, you will need a few more inches.



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