Rebar in slab?

[stricht8]

I'm building a new garage which will have a 1200 sq ft slab. Some say to put rebar while others say fiber concrete is sufficient. It will be for residential use but I will likely be installing a lift so I'm planning on a 6 inch slab. Any thoughts on the rebar?

 

[NHBandit]

My 30x40 has fiber reinforced 4000psi concrete 4" thick but where the lift posts are we put in 8" deep 4'x4' sections reinforced with rebar. Not required by the lift manufacturer but a little overkill never hurts.

 

[PugetDude]

I used both.
It still cracked.

 

[Stuart in MN]

You may want to mention where you live, as construction techniques vary depending on state or country. When my garage was built they used wire mesh in the slab, which I think is common in the Minneapolis area.

 

[spike99250]

Contrary to belief, the fibers used in concrete are not meant to replace mesh or rebar.
They fibers help control small hairline or spiderweb cracks. You still need to put in mesh or rebar.
I personally would not waste the money on the fiber reinforcing. I would use a heavier gauge mesh and some rebar mats in the areas where the lift would be sitting.

 

[stricht8]

The garage is in Maine. Rebar where the lift will go is a good idea!

 

[Charles (in GA)]

Rebar. The slab will crack someday, but the rebar will keep it together. Beware of rebar in the lift baseplate area. You don't want to encounter it drilling for anchors. Figure out where the base plates will go, and surround the area, but not in it. If you go extra thick, you could put the rebar deep. Most lift installers prefer to drill all the way thru, to allow the anchors to be driven thru when the lift is removed. Otherwise you have to cut them off flush with the floor.

Charles

 

[brownbagg]

rebar in slabs are waste of money, ok piss off now, let me explain

rebar is only for load factors, the rebar counteracts the bending of the slab when load is applied, when the slab bends and there is no counteraction force, cracking and foundation failure is possible. so here the question, how much load are you planning on applying. remember the weight of the bldg is on the outside edge and the footing, a car doesnt really weight much when it comes to load factors. you got counteraction of the lift, but if installed correctly the lift force is a dead load apply down upon the slab.

now if you was apply the weight of a bulldozer or tractor trailer it be a different story

ok back to question, do you need rebar, no, but it doesn't hurt to put it in, its not going do anything till a load is apply but its there.

rebar has nothing to do with cracking, cracking is cause by either overloading, which you can do, and poor workmanship. Fiber doesn't control cracking, nether does rebar or mesh.

fiber and mesh control concrete shrinkage in the first 48 hours, then it does nothing. fiber is better than mesh due to the poor workmanship of those working with mesh.

you could place a slab with no rebar, no fiber, no mesh and it would be no different than one with all of the item. just remember you have to have one number five in footer, thats a code item. other than that none is needed.

now what would i do/ If i could afford it, all three, and number 5 O.C 12 inches each way. but I tend to over think stuff

 

[ConCretin]

As has been said, fiber and rebar serve different purposes. Fiber is intended to prevent early age shrinkage cracks while rebar serves to preserve the structural integrity of the slab by keeping cracks (including those in the bottom of control joints) from opening up. Rebar won't prevent cracks but fiber is useless once they occur.

If it were my slab, I'd opt for rebar, properly supported in the middle of the slab - something like #4's at 18" centers. I'd look to prevent random cracks with a solid sub base, controlling mix water and proper curing but I'd cut a couple control joints to make sure a random crack didn't ruin my day.

I'll disagree with my friend brownbagg on one point, the average residential slab section isn't deep enough for the rebar to act as a beam and support a load - the sub base ends up doing that job regardless of slab reinforcing. You could run #6 bars at 6" centers in a 4" slab placed on mud and it would crack if you drove a pick up onto it. The same slab poured on a solid base with no rebar wouldn't budge if you drove a semi over it.

 

[Bruce4310TX]

Contrary to belief, the fibers used in concrete are not meant to replace mesh or rebar.
They fibers help control small hairline or spiderweb cracks. You still need to put in mesh or rebar.
I personally would not waste the money on the fiber reinforcing. I would use a heavier gauge mesh and some rebar mats in the areas where the lift would be sitting.

This is correct dont waste a dime on fiber i just did a 42x40 about 48 yards 6 full beams and rebar thruout and Yes its going to crack but the bar should hold it together, fiber wont. Dont even think about putting a lift in without bar unless u put me on your insurance policy.....

 

[Zeke]

I've heard theory that rebar can weaken a slab. But at 6" you should be good. I agree that rebar helps distribute a load and holds a slab together and level when the crack DOES appear. Rebar on 4' centers would do that. Fat rebar on 12" OC is way overkill, IMHO. I've seen tennis courts built with less than that.

 

[nehog]

I used rebar. My contractor did not have a very high opinion WRT fiber. Three (or is it four?) years later, no cracks.

 

[p_mori7]

As previously stated, the BASE under the slab is crucial...get the base right, and it will prevent many other problems from appearing later on.

My slab is 5", and yes I have rebar...and wire mesh.

No fiber...I didn't want to chance having a fuzzy floor.

My slab is 2 years old now...and still looks GREAT.

 

[brownbagg]

I did not get into the compaction of the sub grade because the art of pouring a crack free slab, i could write volume on, There are many variable, and one thing that its just one item that would create cracking, when there are hundreds, yes the above mention that a slab is not thick enough to act like a beam, which he his correct, this is also the same principal that a four inch slab is not thick enough for rebar, as per load factor, while maybe and i mean maybe a eight inch slab could benefit from rebar.

The compaction and material , also vapor barrier has huge effect on concrete cracking, for these reason, i don't like using gravel aggregate or insulation under a slab. but that just my presence. a settlement of a 1/4 inch would be enough to create cracking. the only aggregate i would use would be a limestone base, crush concrete moves around took much to be considered as a base material, although it is approved by highway departments.

another myth about poly fiber is the fuzzy concrete comment, those that believe this has never work much with fiber. the fiber will stand up but couple days of walking on the slab will break them off. for those that are miss, a propane torch will remove them in microseconds.

another subject that people refuse to comment on to control concrete cracking which is 90% of the problem is workmanship. poor workmanship without controlling the water cement ration will crack a slab in minute. the theory that all concrete cracks was invented by sloppy workmanship that refuse to take responsibility for quality work. all concrete doesn't crack but all poor workmanship does make concrete crack.

so on cracking you have subsurface failure, curling, load factors, shrinkage, poor workmanship, quality of mix design., you could include weather,time, temperature , humidity, vapor barrier, curing. many more variables

 

[Justanoldguy]

I used both.
It still cracked.

Rebar is not put in to stop cracking.
A popular myth for sure.

 

[stricht8]

Does vapor barrier prevent or promote cracking?

I did not get into the compaction of the sub grade because the art of pouring a crack free slab, i could write volume on, There are many variable, and one thing that its just one item that would create cracking, when there are hundreds, yes the above mention that a slab is not thick enough to act like a beam, which he his correct, this is also the same principal that a four inch slab is not thick enough for rebar, as per load factor, while maybe and i mean maybe a eight inch slab could benefit from rebar.

The compaction and material , also vapor barrier has huge effect on concrete cracking, for these reason, i don't like using gravel aggregate or insulation under a slab. but that just my presence. a settlement of a 1/4 inch would be enough to create cracking. the only aggregate i would use would be a limestone base, crush concrete moves around took much to be considered as a base material, although it is approved by highway departments.

another myth about poly fiber is the fuzzy concrete comment, those that believe this has never work much with fiber. the fiber will stand up but couple days of walking on the slab will break them off. for those that are miss, a propane torch will remove them in microseconds.

another subject that people refuse to comment on to control concrete cracking which is 90% of the problem is workmanship. poor workmanship without controlling the water cement ration will crack a slab in minute. the theory that all concrete cracks was invented by sloppy workmanship that refuse to take responsibility for quality work. all concrete doesn't crack but all poor workmanship does make concrete crack.

so on cracking you have subsurface failure, curling, load factors, shrinkage, poor workmanship, quality of mix design., you could include weather,time, temperature , humidity, vapor barrier, curing. many more variables

 

[stricht8]

I would use a heavier gauge mesh and some rebar mats in the areas where the lift would be sitting.

What are rebar mats?

 

[ConCretin]

Does vapor barrier prevent or promote cracking?

There IS a school of thought that a vapor barrier promotes shrinkage cracks. If I remember correctly, it's rooted in an old study done by an ACI committee that subsequently recommended against vapor barriers for this reason.

I think most would probably disagree today based on current mix designs that do a better job of controlling water. At the very least, most would agree that the value of a vapor barrier outweighs the risk of shrinkage cracks.

 

[ConCretin]

What are rebar mats?

Mat just refers to rebar that has been arranged in a grid with bars run in both directions and secured with wire or clips.

 

[ConCretin]

another subject that people refuse to comment on to control concrete cracking which is 90% of the problem is workmanship. poor workmanship without controlling the water cement ration will crack a slab in minute. the theory that all concrete cracks was invented by sloppy workmanship that refuse to take responsibility for quality work. all concrete doesn't crack but all poor workmanship does make concrete crack.

Sorry for the triple post but ..."Amen Brother"

 

[HAY YOU]

Now's not the time to get cheap. Because that's really what it comes down to. Put mesh in & rebar around where the anchor bolts go. I would use J-bolts & use plywood tempt plates to hold the anchor bolts in place while you pour. That's how they do it on the big jobs.

 

[wssix99]

I'm building a new garage which will have a 1200 sq ft slab. Some say to put rebar while others say fiber concrete is sufficient. It will be for residential use but I will likely be installing a lift so I'm planning on a 6 inch slab. Any thoughts on the rebar?

As mentioned above, rebar/wire mesh help control the major shrinkage cracking and fibers help control other smaller cracking and is generally used for desired finish qualities.

You can also use rebar to help counteract cracking from bending of the slab. Traffic, lifts, etc. all cause the slab to deflect/bend a little bit, but typically not enough to cause cracking in a residential application. As pointed out above, the quality of the base is critical. (For the garage I'm building, my base is sketchy and on top of several feet of layered fill. So, I've chosen to over-reinforce for bending.)

rebar is only for load factors, the rebar counteracts the bending of the slab when load is applied, when the slab bends and there is no counteraction force, cracking and foundation failure is possible.

On a suspended slab or in a parking garage, this would be true. For a slab on grade with the steel reinforcing in the middle, the reinforcing acts as crack control and does nothing for bending resistance. The extreme bending forces are on the outside of the slab and the base helps to counteract these forces.

rebar has nothing to do with cracking, cracking is cause by either overloading, which you can do, and poor workmanship.

Rebar is not put in to stop cracking.
A popular myth for sure.

Reinforcing does not stop cracking, but it can control the width of them and how frequently/close together they occur. The American Concrete Institute has a publication "ACI 360R Design of Slabs-on-Ground" (they update it periodically and have a dash and then year number after the 360R for the year it was released) that talks about all of the strategies that can be used to control cracking, along with guidelines on how much reinforcing to use to counteract cracking due to shrinkage and bending.

I've heard theory that rebar can weaken a slab.

It can if there is too much steel or not enough cover on top of the rebar. For a 5-6" slab reinforced in the middle, we'd go broke on the cost of steel before we'd hit that limit.

As previously stated, the BASE under the slab is crucial...get the base right, and it will prevent many other problems from appearing later on.

Great point.

Does vapor barrier prevent or promote cracking?

No, although the water content of the mix can. There are also additives that make the concrete flow better with lesser amounts of water, which can also help. I recall that the ACI document above discusses how the positioning of the vapor barrier can cause curling, but I understand that's more of a concern for large commercial floors.

Now's not the time to get cheap. Because that's really what it comes down to. Put mesh in & rebar around where the anchor bolts go.

I'd add that one would want the reinforcing consistent across the entire slab, or at least from expansion joint to expansion joint. If one area is reinforced differently from another, some odd shrinkage cracking could pop up. (Probably right around the areas where the reinforcing changes.) Some people pour pads, isolated by expansion joints and pinned or keyed in to the main slab. In that case, the pad that is isolated could be reinforced differently.

I would use J-bolts & use plywood tempt plates to hold the anchor bolts in place while you pour. That's how they do it on the big jobs.

This is something to be careful with and I'd suggest consulting the lift manufacturer on. Some of the lift bolts resist pull-out forces and the depth of the J bolts and the legs/diameter of the "J" would need to be sized/calculated along with the strength of the concrete to make sure they would not "punch out" of the slab. Wedge anchors work by putting the surrounding concrete under compression, where its strong. J bolts (when resisting punch-out) put the concrete under tension, where it is the weakest.

 

[wssix99]

The big thing steel reinforcing does for you is allow you to space your control joints further apart with confidence or buy you extra insurance that the cracking will occur within the control joints you do have. The ACI document above has guidelines and recommendations on control joint spacing for various sizes of slab and quantities of reinforcement.

So, I'd start there in designing your slab. Figure out where you want the control joints and how far you want them spaced. From that point, advisable amounts of reinforcing can be looked up.

It also doesn't matter what size of rebar and how its spaced so much as the cross sectional area of steel in the slab. So, if you go with a thicker slab, you'll need more steel to counteract the same shrinkage stresses vs. what you would need for a thinner slab. (So, extra slab thickness costs more in concrete and steel.)

In addition to my garage's base/bending issue, I also needed to space my control joints out widely to keep them away from my lift base plates, (I also chose a wider lift) as per the manufacturer's instructions. This also governed the rebar calculation along with two columns penetrating the slab and a radiant system. These last two made the choices even more difficult. The columns also forced where the control joints would go and the radiant system impacted the slab width and thickness of reinforcing bar. Fortunately, all this was worked out before the house was built and I was able to move the columns around a bit and specify the radiant strategy to help.