Need advice on garage floor slab on grade

[Cap07]

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?

 

[Ron]

While steel fiber is a good enhancement to the properties of concrete, it does not replacement reinforcement. Yes, some code officials will allow substitution for WWF.

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.

 

[JAE]

If you are in a cold climate - where chlorides are used to de-ice streets, then the steel fibers might not be a good idea. Unreinforced slab would then the be choice.
Or bar reinforced with epoxy coating - but more cost.

 

[a2mfk]

To pile on with Ron, ask the client whether he wants joints or cracks, because he's getting one or the other. Technically, the joints will "encourage" the cracks to occur along the joints, so he is getting cracks no matter what, but you get the gist. I've had homeowners point out cracks along CJs as a problem, and I just say that the slab did what we hoped it would do..

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.

 

[FinnB]

Be careful with steel fibres. When the concrete sets the fibres will stick out of the top of the slab. The fibres need to be grinded down. You should get a good smooth finish but there is a bit of quality control needed to ensure no portion fibre is left sticking up out of the slab.

 

[miecz]

I had my garage floor poured in New England about ten years ago. Plan dimensions were about 20'x24'. The contractor and I compromised as follows:
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.

 

[bigmig]

There are a bunch of synthetic fiber additives that are not steel that do an excellent job. Ask your concrete plant to send you some info on a couple. a2mfk hit the nail on the head. You will have cracks or joints, so go ahead tell your owner to use joints, as this is your responsibility as the design professional. It will also save you some foolish future conversations. Remember, you know better, the owner might not. It is your job to be the consultant.

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.

 

[SteelPE]

I would be a bit worried about the rigid insulation under the slab as well. It seems like you are asking for problems if you use it.

I do know they make high compressive strength insulation.

http://foamular.com/foam/products/foamular-1000.aspx

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.

 

[FinnB]

Where I work rigid insulation under floor slabs is the standard method for all ground bearing slabs in houses or for any habitable rooms. For me I think a 4" slab is a bit thin and I would be more comfortable with 5". Joints for a 5" deep slab with WWF I would place at 15' spacings.

 

[msquared48]

Have to agree with Ron here, but I am not big on steel fiber addition to toughen the slab due to the rust spots seel later in the slab due to wet vehicles and cleaning. Can become unsightly in my opinion.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[SteelPE]

bigmig

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.

 

[Cap07]

Thanks for the input guys - your comments have been very helpful!

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?

 

[bigmig]

SteelPE is right. For some reason I thought that had come out of the new code, but it is in there. Keep in mind that your most conservative point load will happen when it is right on the edge of a control joint. In other words you won't have the 1 to 1 distribution if the slab has cracked at the joint, or if you are on the edge of your garage slab. So if you jacked your car at the intersection of joints, your area would be something like 72.25 in^2.

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.

 

[Cap07]

I didn't think about the loading being on a joint...that's a good point bigmig. I'll check out the ACI references you gave. Thanks!

 

[hokie66]

The variety of advice here is typical when engineers are asked about a slab on ground. Of all the answers given, I would agree most with miecz, if it were my garage. For somebody else's garage, I don't give advice, other than...CURING...keep it wet.

 

[Cap07]

I found the following on steel fiber reinforcement in the ACI 360R (paragraph 10.3.2.1): Random crack control - Steel fibers are commonly used for random crack control. As in the case with conventional reinforcement, the fibers do not prevent cracking, but serve to hold cracks tight such that the slab performs as intended during its service life. The degree of random crack control by the fibers is directly related to the fiber type and quantity.

It sounds to me like the steel fiber would essentially perform the same task that WWF would. Any idea on the relative cost?

 

[dik]

A well consolidated base and sub-base will go along way to minimising cracking along with a uniform thickness of concrete slab. Base material should be 'flat'.

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

 

[Brad805]

Odd, the Texan likes water for curing.

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

 

[hokie66]

Who are you calling a "Texan"?

 

[Brad805]

Are you not from Texas? My bad. I recall you mentioned Texas in one discussion.

Brad