Garage Slab: Rebar, Mesh, Nothing?

[Markos]

Last weekend I retroactively pulled the old driveway asphalt out of my new garage. The earth is hard as a rock due to 2014 new asphalt prep with a grader and roller.

I need to decide if I want rebar, steel mesh, or nothing. Most everything that Inread suggests that rebar isn’t needed with a 4” slab. I talked to my concrete shop today and they recommend fiberglass rebar, which is more affordable (#3 with strength of #4) and much easier to handle.

I live in minor earthquake country. Last larger one was a 3.6 about 15 miles away. My main concern is cracking, not overall load. It is my understanding that rebar mostly helps with weight bearing capacity.

I was thinking of maybe putting rebar in the 3rd (e9) bay only. When my house project no longer owns me I may put in a four post or scissor. I only have 8’ of headroom.

If you suggest foam, I have zero desire to scrape another 2” off of my base. Temps are mild here and the ceiling will be insulated.

Main garage

E9 space:

 

[tferrer]

Sweet!

 

[Gransin]

I don't like cracks in concrete floors, so I'd put down some rebar, or anything to make it less likely to crack. The fibreglass rebar sounds like a good alternative if it's cheaper than steel.

I just did a floor in our new storage room, and even thou it wont see more loads than our bikes and things like that, I did put in rebar to prevent cracks down the road when things settle (even if the base underneath is compacted as good as can be).

 

[adawil2002]

When we did our garage floor, we did put down 2 inch styrofoam insulation, rebar mesh sheets, 1/2 inch pex tubing, rebar mesh again & dyed the concrete Terra Cotta Red. Then it was polished then cut in 12 foot squares to mitigate random cracking. It was lastly coated with a clear sealer & when that had cured I caulked the seams.

 

[Markos]

Any thoughts @autokunst or @rsporsche?

 

[boonies]

The fiberglass rebar is a good idea, anything that can help to prevent cracking. Much of that will have to do with what your soil conditions are like. expansive soils with seasonal movement will result in cracks, and the addition of rebar and even fibers directly mixed into the cement will help to reduce the occurrence.

also recommend plastic under the rebar to prevent or minimize moisture ingress.

 

[lip277]

I would put down a good vapor barrier and rebar.
In all three of the houses I have lived in here in Seattle - (including 6 garage/shop buildings) - the floors that were the most stable (and didn't have signs of moisture coming up) were the ones that had a good mechanical reinforcement (rebar) and vapor barrier.

The garage in my old house did not have a vapor barrier and there was a constant coating of 'fuzz' on the top surface of the concrete from where the moisture was bringing up salts and other things. Plus it kept the bottoms of whatever I put on the floor 'moist' - especially in the spring when it was wet out.

 

[autokunst]

Any thoughts @autokunst or @rsporsche?

As you noted, compacting the sub base is very important. That is your best first step. Personally, I would add rigid insulation, but it is not critical. But a vapor barrier is. As for "rebar", there are some semantics to clarify. As you note, what you are trying to reduce are cracks. Concrete always shrinks as it cures, so the reinforcing you are adding is reducing the effects of this shrinkage. All of the photos above from @Gransin and @adawil2002 are showing the use of WWF (welded wire fabric). I don't consider this "rebar", per se. Semantics. I would recommend using WWF throughout. This will reduce (probably not completely eliminate, but maybe) cracking during cure. It will also add some tensile strength for that future scissor lift. You are only going to pour this once, so invest a little in this step and be happy later.

There is an alternative - an additive of small metal or synthetic fibers that can be added to the concrete before it is poured. Think a bunch of metal bobby pins or 2" strands of synthetic fibers. These are mixed into the concrete and serve a similar purpose as the WWF. However, they present at the surface making any sort of finish a challenge. Our structural engineer likes the fiber strand additive (forta-ferro). But for aesthetic reasons, I prefer the WWF.

I used WWF, but only in and around my lift bay. The rest of the slab is unreinforced. Even though I hated them, the concrete guys I used did a good job and I ended up with no cracking. I have 2" of rigid insulation under the slab save for the area where the lift is supported. This allowed me to pour a 4" slab throughout but it went to 6" thick (simply by omitting the 2" of insulation) by the lift.

Watching with interest.

 

[adawil2002]

With my slab, we did the 2 layers of WWF, one to hold the Pex tubes & the second so I could put lifts anywhere. The Terra Cotta dye also has a reinforcing effect like the fibers @autokunst mentioned. My slab is 6 inches thick & pitched toward the garage doors.

Over the years road salt on our Winter cars has eaten through the top coat finish & etched the concrete.

 

[autokunst]

Over the years road salt on our Winter cars has eaten through the top coat finish & etched the concrete.

Ours too - right at the tire patches of our daily drivers.

 

[mulberryworks]

I agree with everything autokunst said. Concrete will shrink. Rebar, WWF, or fibers work to reduce the effects and control joints attempt to influence where cracking is likely to happen.
Two post lifts often spec a min thickness of 4" of concrete so I'm leery of trying to hit that mark and coming up a bit short and so will be making my floor the same 6" thickness under the lift location as autokunst suggests. It seems like cheap insurance.

A vapor barrier is definitely a must. My '80's garage floor doesn't have one it seems and the lack of gutters meant there was a large water load in the soil next to the garage after rains that would result in a moist slab. I've added gutters and piping to carry the water away from the house which has made a huge difference but a vapor barrier will be installed for my new bay floor to prevent any future issues.
A cold floor can cause water from warmer moist air to condense on the surface during the winter as a friend in Bothell has shared with me. But you have living space above your garages so that's much less likely to be an issue for you. I think you'll be ok skipping the insulation under the slab.

 

[dave v. in nc]

Two thumbs up on moisture barrier...guy that built our house put three layers in the basement...The dryness is wonderful, and there is never any musty smell in thirty years...

 

[Norm!]

As a City Engineer responsible for building five fire stations for very picky firefighters, I learned how to give them slabs they could polish in their spare time and not worry about unsightly cracks spoiling their spotless stations.
As stated above, on a stable base of non expansive soil (very low clay content) concrete cracks because of shrinkage. Soil with clay in it can swell with moisture and cause cracking and offsets. If your soil has clay you should use base rock. An unreinforced four inch slab over ten feet in any direction will crack unless it has rebar in it which can extend the the size to maybe sixteen feet or twenty feet with a 6 inch slab. Rebars also keeps any cracks small and doesn’t allow the cracks to offset vertically. Wire mesh or fiber will not help with shrinkage near as as much as rebar, but is does help a little and it increases the load capability a little. You could still see small cracks in a slab over ten feet with either of these measures.
The way to avoid cracks without rebar is to pour smaller slabs in a checkerboard or create deep joints by either troweling, using plastic zip strips or saw cutting within 24 hours. The joints need to be 1/3 the slab thickness. Also keep the slab moist or covered for as long as possible. Concrete hardens by a chemical reaction, not drying. Drying reduces it’s strength and increases shrinkage because it takes away the water component of the reaction. To control hairline cracking in large slabs with rebar, slip dowels are used at the slab joints to allow for shrinkage cracks to occur at the joint while still controlling vertical displacement. At our fire stations we went for overkill and poured twenty foot checkerboard six inch slabs with slip dowels at the joints. We used rebar and to appease the fire fighters also added fiber. No cracks!

 

[rsporsche]

Any thoughts @autokunst or @rsporsche?

quick thought - concrete is excellent at compression, lousy in dealing with tension ... that's where steel (or some other tensile material) comes in. whether its rebar or WWF (mesh) - that will handle the tension. a slab will deflect (when a load is placed on it) - as it is loaded the slab will deflect in the middle causing compression on the top layer and tension on the bottom layer. if it deflects too much, it will crack. the other variable is thickness - a thicker slab that doesn't span far needs less steel. the basic problem is thicker concrete costs more than adding some steel ... and WWF is the most cost effective. concrete does shrink as noted, the more water it has in it, the more likely it is going to shrink / crack while drying.

what the slab sits on is the determinant. as @autokunst notes, a well compacted base is important. otherwise it becomes necessary for the slab to be a structural slab, necessary to have rebar to allow support mid slab. i believe in setting a slab directly on a vapor barrier over a sand base ... on top of the compacted base. while @Norm! has some excellent points for heavy equipment on wider spans, this is a bit of over-design for a residential garage that looks to be 12' wide -

 

[rsporsche]

now that i have re-read Markos' original post ... i would definitely use WWF on the entire garage - just be sure to make sure it is at least 1 1/2" above the bottom of the slab - doesn't do much good if its under the concrete. i would also add some rebar across the slab where the lift will be. you will probably end up with a scissor lift since you don't have a lot of headroom - don't forget the garage door is going to eat into that height. i personally wouldn't worry about insulating the slab - you can always put a blanket / rug on top of the slab when you have to lie on a cold floor ... don't decrease height or go to excavation expense. BTW, i have zero experience with fiberglass rebar - so no comment ... seems reasonable. maybe you need carbon fiber ... a thin sheet - excellent with tension (JUST KIDDING)

 

[Norm!]

My discussion was not focused on wheel load. It was about the shrinkage component. We added more rebar than typical and a deeper base to handle the wheel load. For a twelve foot wide four inch thick 20 foot long slab, I would make one deep joint in the middle and use fiber or WWF. If you don’t want a deep joint, you definitely need to use rebar. If you get any cracks they will be small hairline cracks.

 

[Markos]

quick thought - concrete is excellent at compression, lousy in dealing with tension ... that's where steel (or some other tensile material) comes in. whether its rebar or WWF (mesh) - that will handle the tension. a slab will deflect (when a load is placed on it) - as it is loaded the slab will deflect in the middle causing compression on the top layer and tension on the bottom layer. if it deflects too much, it will crack. the other variable is thickness - a thicker slab that doesn't span far needs less steel. the basic problem is thicker concrete costs more than adding some steel ... and WWF is the most cost effective. concrete does shrink as noted, the more water it has in it, the more likely it is going to shrink / crack while drying.

what the slab sits on is the determinant. as @autokunst notes, a well compacted base is important. otherwise it becomes necessary for the slab to be a structural slab, necessary to have rebar to allow support mid slab. i believe in setting a slab directly on a vapor barrier over a sand base ... on top of the compacted base. while @Norm! has some excellent points for heavy equipment on wider spans, this is a bit of over-design for a residential garage that looks to be 12' wide -

Thanks @rsporsche! I’m on board with the mesh. I can also throw fiberglass rebar in for good measure also if it helps (maybe just in the e9 bay). The “rebar” is only $8 for a 20’ stick.

My main garage is 23x24, and the e9 garage is 11x24.

 

[rsporsche]

you might want to place bars 12" apart (across the slab) for the area that the lifts will be located. a scissor lift moves, but keeps the load distributed over a wider area than the 'point' loads of a 2 or 4 post lift

 

[Markos]

Honestly I would love to do the foam. The ground is really hard glacial till and it is difficult to scrap off another 2 inches without seriously disturbing the soil. You can’t stick your shovel more than an inch into the ground without hitting golf ball to baseball sized “river rock” (glacier rock).

However, I do have to put foam under my entry slab because it is a conditioned space. That’s only 11x7 though.

 

[tferrer]

Honestly I would love to do the foam. The ground is really hard glacial till and it is difficult to scrap off another 2 inches without seriously disturbing the soil. You can’t stick your shovel more than an inch into the ground without hitting golf ball to baseball sized “river rock” (glacier rock).

However, I do have to put foam under my entry slab because it is a conditioned space. That’s only 11x7 though.

Can you lay down a thin plastic moisture barrier?