Need advice on garage floor slab on grade
Need advice on garage floor slab on grade
(OP)
Hi -
I've been asked for advice on the "reinforcing" required for a residential garage. The largest loading it will see is a Chevy truck.
The garage floor will be a 4" slab on grade on top of a 4" crushed rock base. The garage will be heated - so between the slab and base will be a 2" R-10 rigid insulation board.
The owner would like to cut down on costs and wants to eliminate the rebar that was initially specified. He would like to use WWF or steel fiber reinforced concrete, or if possible use unreinforced concrete.
If he goes with the unreinforced option, according to ACI 360 a control joint spacing of 8' to 12' apart should be used for a 4" slab.
If the owner doesn't want a lot of joints in the floor, then my recommendation would be the WWF in the middle to upper third of the slab.
I haven't been able to find a lot of information on steel fiber reinforced concrete... would using the steel fiber be comparable to WWF as far the joint spacing is concerned?
Also, does anyone have information on the relative cost of steel fiber reinforced concrete compared to WWF?
I've been asked for advice on the "reinforcing" required for a residential garage. The largest loading it will see is a Chevy truck.
The garage floor will be a 4" slab on grade on top of a 4" crushed rock base. The garage will be heated - so between the slab and base will be a 2" R-10 rigid insulation board.
The owner would like to cut down on costs and wants to eliminate the rebar that was initially specified. He would like to use WWF or steel fiber reinforced concrete, or if possible use unreinforced concrete.
If he goes with the unreinforced option, according to ACI 360 a control joint spacing of 8' to 12' apart should be used for a 4" slab.
If the owner doesn't want a lot of joints in the floor, then my recommendation would be the WWF in the middle to upper third of the slab.
I haven't been able to find a lot of information on steel fiber reinforced concrete... would using the steel fiber be comparable to WWF as far the joint spacing is concerned?
Also, does anyone have information on the relative cost of steel fiber reinforced concrete compared to WWF?






RE: Need advice on garage floor slab on grade
Keep in mind that WWF is not for flexural reinforcement...it is intended to keep cracks that occur from thermal and drying shrinkage movement held closely together.
A 4-inch slab is a minimal structural section for even light vehicles. Combine that with a expected poor grade control and you'll have slab thickness that will likely vary between 3 and 4-1/2 inches.
Putting a compressible material such as insulation directly under a thin slab is asking for a flexural issue. That's probably why rebar was specified.
I find it interesting that owners will pay for an engineer's best judgment on a design, then try to subvert that judgment to save a few pennies. Foolish.
If you want an unreinforced slab, make the thickness 6 inches, use steel fiber (assuming you have a contractor who knows what he is doing with steel fiber enhanced concrete), and place the joints at 10 feet on center each way. Don't spread out the joints...you'll just get cracks between.
Don't even think about using polypropylene fibers...worthless.
RE: Need advice on garage floor slab on grade
Or bar reinforced with epoxy coating - but more cost.
RE: Need advice on garage floor slab on grade
Several times a week I do residential inspections, and rarely do I see CJs in a garage floor slab. But when I do, the cracking is minimal. Same for driveways. I regularly visit one particular neighborhood where I can make the rare statement as the contractor or concrete contractor really knew what they were doing. Probably joints at 6'-8' o.c. max, and I rarely find a crack in one of these driveways.
Therefore, I would go with Ron's advice, but the more joints your client can tolerate the greater the chances of cracks mostly occurring along the joints. I'd go as small as 6'-8' o.c. and keep the panels as close to square as possible with no odd angles or returns.
After a few months, he can have the joints all sealed and the floor coated with an epoxy paint that really looks nice. But this system requires maintenance where a bare slab you can just live with the joints.
And I agree with Ron, fibers may only reduce micro-cracking, not much else.
RE: Need advice on garage floor slab on grade
RE: Need advice on garage floor slab on grade
I allowed a 3000psi mix and he increased the slab thickness to 4-1/2". I thought that the normal 4000 psi mix would have shrunk more, causing more cracking. He pocketed the money saved on the cheaper mix and was happy to increase the thickness. He didn't like my #4@18", so I allowed two layers of flat WWF, one of the heftier gauges. No Joints. Haven't seen a single crack.
RE: Need advice on garage floor slab on grade
In regards to WWF, it all gets pushed to the very bottom as the workers walk on it, so it doesn't provide much shrinkage and temp reinf. on the top surface where you need it, which is why a lot of contractors prefer fiber for residential S.O.G.
There are many types of foam products that I can guarantee you have way more capacity than the ground you are putting it on. They use that stuff for fill for interstate highways to give you an idea. The typical residential garage floor load is a 40psf LL which translates into about .27 lbs per square inch, not including dead loads.... or in other words not much. You will not be "squishing" the foam and getting instability cracks.
Follow up with a good spec for water/cement ratio and finishing techniques and it won't crack.
RE: Need advice on garage floor slab on grade
I do know they make high compressive strength insulation.
ht
I have never used it. Have considered using it on a few jobs but the they fell through. I don't know how much it costs or how available it actually is. It might be cheaper to just place reinforcing in the slab.
RE: Need advice on garage floor slab on grade
RE: Need advice on garage floor slab on grade
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Need advice on garage floor slab on grade
I don't quite agree with your .27 psi calculation. You are forgetting the 3000# point load requirement over a 4.5" x 4.5" area. This will give you a load much greater than .27psi.
RE: Need advice on garage floor slab on grade
For the 3000# point load applied over a 4.5"x4.5" area...assuming I have a 4" slab and the load spreads out at 45° in each direction, that would give me a 12.5"x12.5" area. The pressure would then be 3000/(12.5)^2 = 19 psi. So I would need to make sure that my foam board insulation is good for 20 psi minimum, correct?
RE: Need advice on garage floor slab on grade
I have gone through the argument that you could never get a floor jack right on the edge of a slab (against the wall)based on the configuration of most vehicles, but the code doesn't discuss this and doesn't specifically identify the load as a car jack.
And last but not least, the load distribution could probably be taken as a 1.5 to 1. See ACI appendix D. Chapter 22 for plain concrete might help to see if you could stretch your area out a bit on the load distribution based on the capacity of the concrete.
RE: Need advice on garage floor slab on grade
RE: Need advice on garage floor slab on grade
RE: Need advice on garage floor slab on grade
It sounds to me like the steel fiber would essentially perform the same task that WWF would. Any idea on the relative cost?
RE: Need advice on garage floor slab on grade
4" thick should be OK for light loading, and I would use reinforcing steel located 1-1/2" clear from the top along with sawcuts that were 1-1/2" dp. Sawcuts should be 1/4". Fill the sawcuts with a suitable flowable filler. I prefer rebar; use a sufficient size to permit spacing at 16" o/c (easier to step through) and use proper accessories for chairing (no clay bricks).
Steel fibres are OK, but staining issues as well as difficulty in working with the concrete (ever try to stick a spade into steel fibre concrete... you're in for a real treat!).
I've used rigid insulation numerous times, but not with a 4" slab. It works really well, but have to use a DOW Hi-Load extruded polystyrene foam or equivalent. Works really well if frost is an issue.
Do it properly and it's well worth the added money.
Dik
RE: Need advice on garage floor slab on grade
Cap, you should call a couple of finishers. I don't know how common steel fibers would be for residential work. I have used them in industrial applications, and I think they are great for that, but I know finishers do not like them. I think we are talking pennies difference.
I have watched Walmart put down a 4" unreinforced slab on an expansive clay. Rebar is a serviceability requirement. You have offered good advice that corresponds to generally accepted practices. I think it is time to move on. Your client is penny pinching on the wrong thing. Tell him to buy a couple of cheaper fixtures :)
Brad
RE: Need advice on garage floor slab on grade
RE: Need advice on garage floor slab on grade
Brad
RE: Need advice on garage floor slab on grade
Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask
RE: Need advice on garage floor slab on grade
RE: Need advice on garage floor slab on grade
FWIW I think miecz's garage slab sounds like a good idea if you can find a concrete sub to do that correctly and not step all over the WWF and push it down into the soil. That's why I think when I do my dream house garage I may try no joints and #4 @ 18" o.c., maybe 5" thick, so the guys can step on or around the bars...
But if you want to use fibers only for micro crack control, the only thing I have seen work in the field is lots of CJs...