Slab Thickness 4" or 6" ?

[JDGREEN4ME]

We are having a slab poured for a garage and most contractors are bidding 6". There is one who is bidding 4" 3500# rated with fiber mesh and he states we don't need 6". I have seen this guys construction work and we really like it but I am concerned about the concrete. Should I be?

 

[AMR]

4" is plenty, be sure the base is good - compacted, free draining, etc use a relatively tight mix, ie low water to cement ration, slump in the 3 to 3.5 range tops, and control joints are formed either with preformed joint material or sawcut within the first 24 hours and be sure that the curing conditions are good, at least 50 F, I suggest keeping it wet for 7 days, typically burline and a soaker hose and it will last forever

 

[Dargo]

Personally, I'd never consider 4" except for sidewalks and porches or other foot traffic areas. I've posted several pics here of my concrete work as it was being done. So far I've used right at 2000 yards of it. I've positively never regretted going stronger. My driveway is 6" of 4500 psi concrete with 1/4" wire matting and 1/2" rebar. Both barns are at least 8" thick with lots of steel and several high stress areas (like the front edge under the entrance and under internal walls) 12" thick. Again, all using 4500.

Remember, you can't make it thicker later if you realize it's too thin. You get to tear it all out and do it over, which isn't very cost effective. Also, based on actual experience, I don't consider the fiber mix worth a hoot. The one area I was talked into using it was terrible. The finish looked like it needed a shave and it didn't hold up at all. I ripped out the 6" pad of fiber crap in no time just using a 8 lb sledge and my smaller tractor. A section of the steel mat reinforced 4500 mix had to be re-done and it was a nightmare to rip out. It took several days with a 90 pound jackhammer and demolition saw to cut the steel along with an excavator. No comparison. Do it right or you'll almost certainly regret it.

 

[AlanB]

I will echo what Dargo said, when it is done, it is awful hard to go back.

I went 6" with rebar and remesh in mine and 4500 psi concrete.

At least then, if (when) it breaks, I can say I did everything reasonable.

Everyone that told me to go 4" only had to say it was cheaper, that was the sole reason ever given too me.

For me, there are some things I don't want to scrimp on, and that is one because it is SOOOOOO hard to fix.

I scrimped on my doors, upsets me to no end and has been a tremendous source of aggravation for me, but I will be making some changes to them here shortly to make them better and it is "fairly" easy to do.

Good luck

 

[FallbrookFarmer]

Echo the previous posters,
It will only be fractionally more expensive to add two more inches, but if you are set on 4 inches, I would put some rebar into it, actually, I would put rebar in the 6 in also, but then I tend to overbuild.

 

[herd]

We are having a slab poured for a garage and most contractors are bidding 6". There is one who is bidding 4" 3500# rated with fiber mesh and he states we don't need 6". I have seen this guys construction work and we really like it but I am concerned about the concrete. Should I be?

I also have poured many, many yards of concrete in my life; I used to pay to have it done but in the last five years, I do my own forming and finishing. From my experiences, the following holds true:

-Agree to forego the fibre, it's like Ethanol, overly-hyped and a contrived product.
-Use wire panels, Lowe's carries them, I think 5'x7'. You can also use rolled wire, it's cheaper but a bit aggravating
-four inches is okay in the middle under very good preparation but I would never pour anything without a good, rebarred footer. I just build a 26x11 carport, poured a 12'x12'x12' footer with rebar, used wire in the middle at 4+'' firmly packed and on top of good subsoil, not topsoil.
-take the $5/yard that you saved on not using fiber and use it to pour 4000# concrete, it's well worth it.
-If you used a keyway for expansion, be sure it gets packed well, some fininshers don't do this. Also, I like to do expansions joints every 12', no longer.
-Be sure to tamp the form walls with a sledge while the concrete is being finished and still loose. This will leave you with a smooth outside wall, not ''honeycombed.'
-If you pour on grade, be sure to dig the outside edges down below grade, that way, you won't have washout/slab erosion.
-Verify the ''fall'' of your forms before you pour. The reason I started doing my own is because on my last paid pour, I wound up with a ''reverse fall'' and back toward the house. Not a good thing!

Also, you may know this but a 4'' slab is usually 3.5'' and a 6'' slab is usually 5.5''; that's because of the finished dimensions of a 2x4 or 2x6.

 

[MossRoad]

My dad used to say something to the effect that the nice thing about concrete is if you goof it up you get to look at it for eternity.

 

[JDGREEN4ME]

I think 6" sounds like the way to go...we are also planning on having a drain added and it seems the thicker concrete would help in that regard also...maybe I am just thinking too much!

 

[eepete]

I'm just about ready to move into a Morton building (pole barn). "standard" was a 4" fiberfill. I've never seen fiber fill work. We re-did some 8" aprons at the fire station with just fiberfill, in 3 years it's cracked and having problems.

So I went with nominally 5.5 - 6.5". Morton swapped the fiber fill for wire mesh (even trade), and then I put in #3 (3/8" rebar) on a 3' grid and extra pieces at the door where I went down another inch or two on my own. Then the inspector said "floor drains or slope", with all the rebar down we went slope, so I got an extra 2" at the far end sloping down to the 5.5" by the doors. Relief cuts were done the next morning.

The reason I have 5.5 to 6.5 is when they were putting in the 4" of gravel base, they said that they didn't have to compact it- it was "self compacting". Been there before. So I said "Then I guess you won't mind if I rent a compactor and have at it then". All they could say at that point was OK. So I ended up with an extra .5 to 1" of concrete. In the contract, I specified a minimum of 5". This was 4000# concrete.

I also put mesh and rebar in the aprons, using 1/2" where it was right up against the building. I used 4 pieces of 3/8" to tie the apron to the building, and there was a 4.5" metal keyway there with about 1.5" open under it. I've heard opposing views on if this is a good idea, but I've seen to many aprons with their strings to the building untied (they moved). There's a good chance these aprons will crack, but with all the rebar and mesh they will be fine.

Finally, instead of paying extra for a small pad by the door, I framed up a little 3' x 3' pad, 3.5" deep, with rebar tie-in to the building and a 3" gravel base. They always have a bit more concrete than they need, so I had them dump it there. I'll leave it to you if I'm a cheapskate or just overly frugal

Pete

 

[MikeD74T]

As a union cement finisher for many years I agree with Herd. Plastic fiber is a joke & any extra money spent on concrete should go into portland cement not chemicals. Don't use any contractor that uses extra slump to avoid moving the truck or to accomodate several chutes to flow to the far reaches of the slab. Drive the truck onto the slab or wheel the mix if more chutes than the truck carries are needed. Backing the truck over the wire in a straight line won't hurt the wire. The wire needs to be held up by the chute tender while the concrete is poured thru it.
I've only poured a couple slabs that needed an honest 6". One was a foundry that moved large bins of castings by forklift. We poured 6 bag mix with 1 1/2" stone and no reinforcing. The fill in the building was 18" of stonedust pounded down to where the concrete trucks left no tracks deeper than the tire tread. The other was a paint shop that specialized in sign/logo painting on trucks, heavy equipment, cranes, etc. That was 4000lb mix with wire. I visited these slabs two years later & they showed no cracks. The fill under them is 24" of well compacted gravel.

If you pour 6" perimeter & 4" middle on a good layer of compacted well drained base you'll be happy.
One thing many miss is pouring on expandable material, something that expands when it gets wet and/or freezes. Concrete has great compressive strength but poor tensile strength & will fail when uplifted. Don't scrimp on the base. A weak concrete will last longer on a good base than a good concrete on a bad base. MikeD74T