Slab thickness and stength

[Innovate1]

Saw on my plans the basement (non-garage) is called out as 3.5" thick with wire mess. The thickness surprised me. 3000 PSI. Foundation walls and footings also 3000 PSI.

For the garages (attached and detached) it is 4" thick with mesh. 3500 PSI.

For reference I looked at the plans for my current place. 4" in basement, 5" in garage. With mesh. All called out as 3500 PSI (but I have no idea if we really go that). Think the garage ended up as 4", or was supposed to be 4".

I read the concrete reference posts. Plan is 2" foam underneath. Have also read if it is spec'ed at 4" I probably will have more like 3.5, at least in spots. Should I be beefing this up? Don't plan to put anything terribly heavy on it.

 

[seanc_mt]

The garage concrete is one thing you can pretty much NEVER "redo". I did 5" with fiber mesh and a rebar on a 2 foot grid. Dont use that mesh **** get rebar.

 

[PWC Repair]

I have to agree with sean.....there's no "redo". I went 6" thick with rebar tied on 12-16" grid and set on 2.5" chairs. In the whole scope of the project it's really not much more to BEEF it up. You'll never regret overdoing it.

 

[GMCGarage]

Saw on my plans the basement (non-garage) is called out as 3.5" thick with wire mess. The thickness surprised me. 3000 PSI. Foundation walls and footings also 3000 PSI.

For the garages (attached and detached) it is 4" thick with mesh. 3500 PSI.

For reference I looked at the plans for my current place. 4" in basement, 5" in garage. With mesh. All called out as 3500 PSI (but I have no idea if we really go that). Think the garage ended up as 4", or was supposed to be 4".

I read the concrete reference posts. Plan is 2" foam underneath. Have also read if it is spec'ed at 4" I probably will have more like 3.5, at least in spots. Should I be beefing this up? Don't plan to put anything terribly heavy on it.

If the sub base is properly done, then 4" for the garage is plenty, unless planning on a lift, heavy equipment, etc. Make sure they have the mesh about 1.5" down from the top, that way it will help with crack control (thats all it is really doing)

If you go with rebar, then same thing about 1.5" down from the top. Also you want it spaced max 10ish inches so it can work on controlling the cracks. ACI min is usually 18" for rebar, so keep that in mind.

In the basement, same thing 3.5" will be fine.

Again, a slab on grade is only as good as the sub base under it. Its the sub base that takes the load.

 

[ConCretin]

I agree with GMC. While it's unusual to see 3.5" spec'd out for a slab it's fine for the application. Pay a little extra attention when you fine grade the base under the slab to avoid high spots. With that said, it's not very expensive to add a 1/2 inch or an inch if it gives you additional peace of mind.

I'd upgrade the concrete for your garage floor to 4000 psi. 3000 psi is fine from a structural standpoint but some extra cement will help create a strong durable surface.

 

[Milton Shaw]

The 3.5 inch comes from framing the pour with 2x4 lumber. That's the finish size of them. For a thicker you go to 2x6 lumber which is 5.5 inches. That is what I would recommend with rebar for the grid inside. You won't regret this is it is only probably a 1/3 more in price but double the strength when you ad a lift or heavy loads.

 

[ConCretin]

The 3.5 inch comes from framing the pour with 2x4 lumber. That's the finish size of them. For a thicker you go to 2x6 lumber which is 5.5 inches.

That might be the designers attempt but it sounds like a pain in the *** to me. You always want to set your forms to grade. A half inch of air under the form won't bother a thing compared to scratching out every little high spot.

 

[climb.on]

If the sub base is properly done, then 4" for the garage is plenty, unless planning on a lift, heavy equipment, etc. Make sure they have the mesh about 1.5" down from the top, that way it will help with crack control (thats all it is really doing)

If you go with rebar, then same thing about 1.5" down from the top. Also you want it spaced max 10ish inches so it can work on controlling the cracks. ACI min is usually 18" for rebar, so keep that in mind.

In the basement, same thing 3.5" will be fine.

Again, a slab on grade is only as good as the sub base under it. Its the sub base that takes the load.

And that is often a BIG "IF"....unfortunately.

 

[GMCGarage]

And that is often a BIG "IF"....unfortunately.

Agree, everyone focuses on thickness and 'rebar' in a slab. I have seen millions of square feet of un-reinfoced 4" slabs for warehouses poured, and not had any complaints. Thats the result of a good specification for the sub base installation.

 

[wssix99]

Great responses above.

Have also read if it is spec'ed at 4" I probably will have more like 3.5, at least in spots.

This could be the reason for your extra 1/2 inch; so you get a full 3" everywhere.

There's no need for more in a basement. Structurally, most of the "strength" (resistance to vertical loads) of these slabs comes from the soil and collective thickness of the slab and base layers underneath the slab. (The depth of the gravel base below acts similarly to the concrete for these forces, also.)

In a garage, there are additional forces at play due to the heavy rolling loads. Extra depth and extra insurance one has depth has a bigger payoff in that type of area.

All called out as 3500 PSI (but I have no idea if we really go that).

Higher PSI concrete generally has a higher cement ratio, which can make for a smoother finish. Sometimes the higher PSI is specified for it's finished properties, even though the compressive strength is not needed.

 

[Jlbc212]

As others have wrote proper preparation of the sub-base is very important. My 28 x 36 garage has a 4" thick, 4000 psi concrete floor with no mesh or rebar. I have a two post lift installed in the garage. The garage is four years old and there is not a single crack in the concrete other than at the control joints. The other key requirements for a good concrete floor, besides proper prep of the sub-base, are making sure the installer doesn't have too much water added into the mix while it's being poured and keeping the surface continually damp for at least two weeks after the surface is finished.

 

[n20junkie]

I choose 6” of 5000psi mix with 1/2” rebar on 24” centers. We cut the lines and had the top done with a trowel burnished finish. So far so good after 3 years and a few freee/thaw cycles before the shop was heated.

 

[Lynden]

There's good information here....

https://www.concretenetwork.com/concrete/slabs/

https://www.concretenetwork.com/concrete-subgrades-subbases/

 

[pcmeiners]

"If the sub base is properly done, then 4" for the garage is plenty, unless planning on a lift, heavy equipment, etc."
Agree 4" is OK if the base is prepared properly, only problem it is rarely done. Contractors always cut corners, bucket operators over dig, compaction is rarely done properly, curing is improper most of the time.

 

[GMCGarage]

"If the sub base is properly done, then 4" for the garage is plenty, unless planning on a lift, heavy equipment, etc."
Agree 4" is OK if the base is prepared properly, only problem it is rarely done. Contractors always cut corners, bucket operators over dig, compaction is rarely done properly, curing is improper most of the time.

So the consumer should just go ahead and pay for more product because the provider is not holding up his part of the contract??

 

[pcmeiners]

"So the consumer should just go ahead and pay for more product because the provider is not holding up his part of the contract?? "

Generally it has little to do with what is in the contract, as proper site prep is generally NOT in the contract.
It comes down to money for the most part. To do it correctly proper prep and ground compaction costs extra money, which the contractor does not want to mention as he want to bid low, nor does the contractee want to pay for it .... generally.

Proper prep?.....
Soil tested and compacted if needed for entire area of foundation/slab, basically any soil disturbed in recent history needs compaction( multiple decades). Proper depth for footing. ANY dirt disturbed by the dig is removed, no back filling, any fill to replaced removed dirt is sand or gravel compacted in thin layers. Area for slab is finely leveled, no rocks or high spots before final compaction. Then the concrete work begins Most of the time this does not happen.

 

[Jlbc212]

"So the consumer should just go ahead and pay for more product because the provider is not holding up his part of the contract?? "

Generally it has little to do with what is in the contract, as proper site prep is generally NOT in the contract.
It comes down to money for the most part. To do it correctly proper prep and ground compaction costs extra money, which the contractor does not want to mention as he want to bid low, nor does the contractee want to pay for it .... generally.

Proper prep?.....
Soil tested and compacted if needed for entire area of foundation/slab, basically any soil disturbed in recent history needs compaction( multiple decades). Proper depth for footing. ANY dirt disturbed by the dig is removed, no back filling, any fill to replaced removed dirt is sand or gravel compacted in thin layers. Area for slab is finely leveled, no rocks or high spots before final compaction. Then the concrete work begins Most of the time this does not happen.

This why it makes sense, if at all possible, to do it yourself. Just like automotive, plumbing, electrical, carpentry work you cannot always trust the "professional" to do it right.

 

[joes169]

The 3.5 inch comes from framing the pour with 2x4 lumber. That's the finish size of them. For a thicker you go to 2x6 lumber which is 5.5 inches.

Sorry, but this is one of my few pet peeves about concrete flatwork discussions.

First of all, there's usually little to no formwork involved in the pouring of a garage inside of a foundation, the foundation walls provide most of the forming. In my experiences, we form the doors based on whatever the door cut-outs are in relation to the top of floor elevation, most frequently 2x8's. But, again, that really means nothing in relation to the rest of the floor thickness.

Secondly, I can set 2x12 forms and pour a 4" floor, and subsequently pour a 12" floor with a 2x4 form.

I've yet to see any contractor that set's grade from the BOTTOM of a form board, it's all done form the top. The height of the form board has next to no relevance in the thickness to the slab........

 

[joes169]

I agree with GMC. While it's unusual to see 3.5" spec'd out for a slab it's fine for the application. Pay a little extra attention when you fine grade the base under the slab to avoid high spots. With that said, it's not very expensive to add a 1/2 inch or an inch if it gives you additional peace of mind.

I'd upgrade the concrete for your garage floor to 4000 psi. 3000 psi is fine from a structural standpoint but some extra cement will help create a strong durable surface.

This is excellent information, IMO, from another contractor that actually warranties concrete flatwork (although I don't think Doug does many residential garage and basement floors).

It's extremely rare for us to pour any floor with less than a 4000 psi mix for a number of reasons. First, it finishes better with less effort. Second, in a garage, it will hold up much better to freeze-thaw conditions, de-icing chlorides and such, and will wear much better. Third, it set's faster/better and allows the contractor to complete the job in less time, meaning the little additional cost for additional cement/fly ash/water-reducer/etc.... is negligible when accounting for labor savings......

 

[3rdgendslmech]

I had a friend who owns his own concrete buisness and he's been around for 30+ years.
I did all the prep work, base, vapor barrier and wire mesh. The base was FLAT! No more than 1/4 difference from one end to the other. Got up early the next day before concrete arrived, his crew showed up early too, all we did was drive some pins in the ground and mark 4 inches up and we were good to go.

 

[GMCGarage]

"So the consumer should just go ahead and pay for more product because the provider is not holding up his part of the contract?? "

Generally it has little to do with what is in the contract, as proper site prep is generally NOT in the contract.
It comes down to money for the most part. To do it correctly proper prep and ground compaction costs extra money, which the contractor does not want to mention as he want to bid low, nor does the contractee want to pay for it .... generally.

Proper prep?.....
Soil tested and compacted if needed for entire area of foundation/slab, basically any soil disturbed in recent history needs compaction( multiple decades). Proper depth for footing. ANY dirt disturbed by the dig is removed, no back filling, any fill to replaced removed dirt is sand or gravel compacted in thin layers. Area for slab is finely leveled, no rocks or high spots before final compaction. Then the concrete work begins Most of the time this does not happen.

This why it makes sense, if at all possible, to do it yourself. Just like automotive, plumbing, electrical, carpentry work you cannot always trust the "professional" to do it right.

All the better reason to have a proper specification for your project, not just '3.5" concrete slab with mesh' called out.

You do get what you pay for.

 

[pcmeiners]

"This why it makes sense, if at all possible, to do it yourself. Just like automotive, plumbing, electrical, carpentry work you cannot always trust the "professional" to do it right. "
Damn right, but I rarely trust the "professionals"


"All the better reason to have a proper specification for your project, not just '3.5" concrete slab with mesh' called out. "

Personally I am an advocate of hiring an architect for builds, not only do they run up plans but spec out the work. "3.5" concrete slab with mesh" would be the typical but pathetic line in a contract around my area, which gives the contractor the right to do just about anything at any level of quality.

 

[pogrelis97]

I have 4in of concrete in my shop (6in where the lift is) and regularly drive my 9,000lb skid loader, dump trucks, ect in all the bays and don't have any cracks. shop is 8 years old. IMO anything more than 4in is overkill for a garage unless your putting a lift in.

 

[JoeMcGov]

Agree, everyone focuses on thickness and 'rebar' in a slab. I have seen millions of square feet of un-reinfoced 4" slabs for warehouses poured, and not had any complaints. Thats the result of a good specification for the sub base installation.

Yes Sir ^^^. And I've poured a lot of that. And billions of square of concrete paving...unreinforced. In both cases the subgrade is usually compacted to 98% of its scientific maximum compactness. With a few examples where 100% was required. Pretty much, for the common person, you would say that the subgrade is "hard as steel." ABSOLUTELY imperative that the subgrade be properly prepared.

 

[theoldwizard1]

The 3.5 inch comes from framing the pour with 2x4 lumber.

What Milton is saying is that "4 inch" spec is somewhere between 3.5" and 4.0", closer to 3.5".

My concrete guy years ago said, "Mesh does not add much strength but it helps keeping the big chucks from moving around when they crack."

 

[finn]

I have 4in of concrete in my shop (6in where the lift is) and regularly drive my 9,000lb skid loader, dump trucks, ect in all the bays and don't have any cracks. shop is 8 years old. IMO anything more than 4in is overkill for a garage unless your putting a lift in.

My 48’x75’ shop was originally built by a logging contractor who regularly parked quad axle logging trucks, bulldozers, skidders, and tree harvesters inside.

I recently drilled for a lift, and, much to my surprise, found out the slab was only a little over 4” thick.

There are a few minor curing cracks that probably have been avoided by adding saw cuts , but no real structural cracks. There are also a few thermal cracks over the pex tubing. The slab is totally functional, albeit a little ugly where the steel crawler tractor and harvester tracks have chewed up the concrete surface.

Thick slabs for a residential garage are grossly overrated, and not cost effective.