concrete slab no rebar?

[ConCretin]

I'll stick with general questions. Should there be any different materials/techniques be used in prep for a fiber-only slab? Would it be preferable to go slightly thicker than 4" for a garage/shop slab?

The short answer is 'no'. An adequate base is essential regardless of reinforcing. Reinforcing whether it be fiber or rebar cannot make up for an inadequate base. A 4" slab is perfectly adequate for all but the heaviest uses if the base is adequate, A much thicker slab will fail if placed on an inadequate base.

Its important to understand what fiber is and isn't. From a structural standpoint, a fibermesh slab is the same as a bare slab with no reinforcing.

 

[wssix99]

Its important to understand what fiber is and isn't. From a structural standpoint, a fibermesh slab is the same as a bare slab with no reinforcing.

+1 fibermesh is only effective against plastic shrinkage (caused by the concrete curing too fast) https://fibermesh.com/slabs/residential-slabs/

I'm surprised that this would even be generally recommend for a coastal Pacific area. (I woudn't think this type of shrinkage would be an issue.) You might ask around locally for a second or third opinion as to if there's even any value there at all.

 

[Pluribus]

Thanks for input, folks. I am going to talk to a recommended concrete contractor based in this area. Previous info was from a GC/builder.

 

[GMCGarage]

we are pouring a 5,000 sq ft slab for a tent wedding venue

our concrete guy says the fiber mesh in the concrete is all you need and you don't need rebar or wore mesh in the slab. he says no one uses it any more

the concrete plant suggests still using rebar even with the fiber mesh. rebar is another $2,000

any concrete pros here that can weigh in?

If subgrade is prepared correctly, rebar is not needed. Those that do put it in, I doubt put it spaced to where its effective. Contractor is correct.

 

[GMCGarage]

They do in Texas. #3 @ 18" OCEW

What good is a #3@18???? What dictates this spacing?

 

[GMCGarage]

Here is a rule of thumb from CRSI. Note the spacing. Thinner slabs have 6" spacing. this helps control the cracks, if they develop.



2" - none
4" - one layer WWF 6x6-W1.4xW1.4
5" - one layer WWF 6x6-W2.1xW2.1
6" - one layer WWF 6x6-W2.9xW2.9
7" - two layers WWF 6x6-W2.9xW2.9
8" - two mats of #4@12"o.c. ea. way (top and bottom)
9" - two mats of #5@12"o.c. ea. way (top and bottom)
10" - two mats of #5@ 8 to 12" o.c. ea. way (top and bottom)

 

[ConCretin]

Here is a rule of thumb from CRSI. Note the spacing. Thinner slabs have 6" spacing. this helps control the cracks, if they develop.

2" - none
4" - one layer WWF 6x6-W1.4xW1.4
5" - one layer WWF 6x6-W2.1xW2.1
6" - one layer WWF 6x6-W2.9xW2.9
7" - two layers WWF 6x6-W2.9xW2.9
8" - two mats of #4@12"o.c. ea. way (top and bottom)
9" - two mats of #5@12"o.c. ea. way (top and bottom)
10" - two mats of #5@ 8 to 12" o.c. ea. way (top and bottom)

My only comment would be that 1.4 and 2.1 wire is pretty flexible and requires a lot of support. I'd alway suggest a more rigid 2.9 wire.

I did come across this in my wanderings.

http://resources.crsi.org/index.cfm...ne&fileID=29DEB831-DE7E-5203-850E51C60C95707B

It's worth noting that CRSI acknowledges that rebar and mesh won't prevent shrinkage or structural cracks. The closest they come is claiming that reinforcing will help a slab span localized soft spots in the sub grade, which I suppose it could. The primary value is clearly in holding cracks together.

Interestingly they talk about positioning reinforcing in the middle to upper part of the slab to help keep cracks tighter rather than in the bottom to increase the slabs tensile strength. Case closed?

 

[wssix99]

^ The CRSI document above is a pretty good, basic document. Thanks Doug!

What good is a #3@18???? What dictates this spacing?

If you dig into the formulas in the CRSI document, you'll see it's all about "As", which is the cross sectional area of steel in the slab.

As we get into more exotic type of slabs, the steel area ratio (to the area of concrete) needs to be considered (instead of just the steel area alone) and also other factors like the strength of the concrete. ***


*** So, the CRSI formulas will work for "normal" things like 3" or 4" slabs, but start to fall down for some of our members who are installing thicker slabs like 6" or 8". (A 8" slab has the potential to develop 2X the shrinkage tension as a 4" slab, so it will need 2X the steel rebar to handle that stress in the same way.)

 

[Jeepster04]

It's a common misconception that rebar increases the load bearing capacity of a slab on grade. The reality is that a 4 or 5" slab isn't deep enough to take advantage of rebar's tensile strength and make the slab act like a beam and span between supports.

Firstly, by the time you allow an 1 1/2" of cover under the mat, it's essentially in the middle of the slab where there isn't any tension to resist. Secondly, even if this theory is correct, what happens around the perimeter where the slab cantilevers and the top is in tension.

The reality is that a slab on grade is not structural and simply bends and transfers the loads placed on it to the soils below. If those soils give, your slab is going to crack. You need to get into much thicker sections before you get a true structural slab.

For a slab on grade placed on a solid base constrained by perimeter walls, there is no absolute need for rebar. I have placed many hundreds of thousands of square feet of such slabs on engineered commercial and industrial projects over the last 30 years. Now a free floating mono slab is another matter. Rebar would be helpful here, not to prevent cracks but to hold everything together if the slab experiences some movement.

Rebar's only function in a slab on grade is to keep cracks from opening up. In fact rebar actually increases the likelihood of shrinkage cracks by restraining the slab from shrinking inward as it drys. Think of rebar as crack control not crack prevention.

Finally, to say that the absence of rebar somehow implies a contractor is inept is a ridiculous statement. There a many reasons including the owners specifications and budget that could determine whether rebar is installed.

I realize this flies in the face of conventional wisdom and our dearly held belief in overkill but we might as well pass on accurate information to those who come looking.

This may very well be the first time I've read a post where someone knew the purpose of steel in concrete....

 

[GMCGarage]

This may very well be the first time I've read a post where someone knew the purpose of steel in concrete....

I think alot of people know it, but are tired of trying to explain to others.

Put the time and effort into the subgrade is the best advice.

 

[GMCGarage]

My only comment would be that 1.4 and 2.1 wire is pretty flexible and requires a lot of support. I'd alway suggest a more rigid 2.9 wire.

I bet they were worried the bigger bars would telegraph thru in a thinner slab.

 

[TractorJeff]

I always enjoy reading these discussions on Concrete!
I am not an Expert but as others have stated I have opinions.
In the mid 1960's my Dad and older brothers poured a 12 by 16 slab with Mixer concrete using 2 x 4 frames.
In the early 2000's my younger brother and I broke it up along with the 10 by 12 that Dad and Grandpa Mixer poured in the late 1950's.
Big difference in the make up of the 2 slabs as far as strength and weight of chunks.
Failure was due to washout and settling.
In the mid 1970's our neighbor Truck poured a 12 by 50 driveway in two slabs which had wire mesh he brought home from work.
Those slabs still look good last I saw them a year ago!
I guess it all comes down to proper base compaction as the first 2 slabs are considered Farmer Concrete while the 3rd one was Mixer Truck and at least lawn roller compacted on a better base material.
Point is - a Wedding Slab on a good base with a consistent thickness probably does NOT need rebar nor wire mesh in it provided it is kept shaded and moistened while drying!

 

[WNYflyer]

The design thickness of a majority of concrete slabs on grade is based upon on the concrete alone carrying any tension stresses due to bending. Therefore the the thickness is based on limiting the tensile stress in the concrete itself to some allowable tension.

In reinforced concrete design theory the assumption is the concrete has ruptured/cracked on the tension side of the member and then and only then does any steel reinforcing become engaged and carry tension. Thus if the slab concrete is designed to never crack in tension due to bending the steel never gets engaged and does nothing in carrying bending stresses.

 

[ConCretin]

In reinforced concrete design theory the assumption is the concrete has ruptured/cracked on the tension side of the member and then and only then does any steel reinforcing become engaged and carry tension. Thus if the slab concrete is designed to never crack in tension due to bending the steel never gets engaged and does nothing in carrying bending stresses.

That's very interesting WN. I know reinforcing is passive as opposed to say, post tensioning but I assumed that the rebar became engaged structurally as soon as the member came under load.

Is this just an assumption used in design or is it also true in a real world application? I've placed a lot of elevated concrete. Was all that concrete holding itself up there without any help from all the rebar we put in? No sarcasm intended. I'm actually very curious.

 

[My Old Tools]

We have expansive clay soils. We double the rebar.

 

[Culture]

Structural engineer here. My firm has designed millions of SF of slab, I was involving in one of the early studies of polymer fibers in concrete (at MIT) and I was on the ACI slab-on-grade committee. I have never, ever seen an unreinforced slab in commercial or industrial use. Plastic and steel fibers will not significantly reduce thermal or shrinkage cracking, and have ZERO structural strength. They do have some other great benefits (its complicated), but I would NEVER construct a large slab without steel reinforcing (rebar best, mesh ok). With a 5000 SF slab I would also install early entry saw-cut joints at 12-15 o.c.

I have no doubt that there are unreinfoced slabs used successfully in some areas based on local experience, but I think it is WAY to risky. At the end of the day, it comes down to risk vs. $$$.

 

[ConCretin]

I have no doubt that there are unreinfoced slabs used successfully in some areas based on local experience, but I think it is WAY to risky. At the end of the day, it comes down to risk vs. $$$.

Thanks for chiming in Culture. From your perspective, what is the risk side of the equation? What do you feel steel reinforcing will provide or prevent in a 5000 sf slab if an owner spends the extra $. Again, no sarcasm intended. It's an interesting topic (to me at least)

Btw. I'm in 100% agreement regarding fibermesh and control joints.

 

[matt_i]

Thanks for chiming in Culture. From your perspective, what is the risk side of the equation? What do you feel steel reinforcing will provide or prevent in a 5000 sf slab if an owner spends the extra $. Again, no sarcasm intended. It's an interesting topic (to me at least)

Here's my take, and I recognize you didn't ask me...the steel is a great insurance policy imo. For the rest of us, we might pour (have poured) 1-2 slabs in our lifetime, with a labor pool that's "variable".

If a reputable contractor sees you as a customer, interested in good prep, that contractor is thinking that you want a good job. Because it takes extra time and materials which lead to higher cost. You may bid them against others but they know that you are paving the way (pun intended) to aid them in doing a good job.

If a reputable contractor sees you wanting to play him or her against others in an all-out effort to beat out the lowest bid, then they are thinking that the quality is secondary, the customer wants it done quick and cheap. They will do what they can and let enough go to make a profit.

Land in a business relationship with a disreputable contractor and every corner in the book is being cut so they can get their money and be gone.

The kicker is this, how do you know which contractor from above you hired?

You find out 30 days after they are gone, and everafter.....

Because its concrete one risks the entire investment in labor and material, there is little to no remediation or repair, its just break it all out, pay to haul it off and start over. I'm going to guess the average pour for a 1000 sqft shop slab is 20-ish yards + $1500 labor to get to around $4-5k. (More for excavating and stem walls, etc).

Maybe the best tactic is to offer a $1000 tip for a good job after 1 year's assesement period, but personally I'd rather have it in the steel At very worst it will keep unintended cracks tightly knitted.

 

[R. Deschain]

rebar/ wire mesh is used all the time still. get another contractor

I second this opinion

 

[ConCretin]

I think alot of people know it, but are tired of trying to explain to others.

I'm starting to get your point GMC. Look guys, no-one is saying that rebar and mesh don't perform a valuable function in a slab on grade. Take a look at the Concrete Underground, I put something like 40 tons of rebar in my private residence, a significant portion of which is in slabs on grade.

This discussion is about what that value is and just as importantly the foolishness of selecting your contractor on the basis os whether or not he thinks rebar in a slab on grade is vital. Tell him what you want and pay the price.

The reality that no-one has refuted - or is likely to refute - is that steel reinforcing does not increase a slabs load bearing capacity or prevent shrinkage cracks. If you want the added insurance that your slab won't come undone if your base is **** or your finisher places your concrete at an 8 slump, have at it.

But if you'd rather prepare a proper base and spend that $2k on epoxy or some new tools, do so with the confidence that your slab is likely to be just fine.

 

[RustyJunk]

If it's only for weddings why not just lay pavers, for what their getting for concrete these days it may even be cheaper and it sure will look a lot nicer.

 

[wssix99]

The design thickness of a majority of concrete slabs on grade is based upon on the concrete alone carrying any tension stresses due to bending. Therefore the the thickness is based on limiting the tensile stress in the concrete itself to some allowable tension.

This depends on the dynamics side of the equation and is more true for roadways where traffic is traveling at speed. For driveways and garages, where traffic is slow and the forces are closer to the static condition, (and thicknesses are less) other factors (like spreading bearing forces on the base) may control the design.

In reinforced concrete design theory the assumption is the concrete has ruptured/cracked on the tension side of the member and then and only then does any steel reinforcing become engaged and carry tension. Thus if the slab concrete is designed to never crack in tension due to bending the steel never gets engaged and does nothing in carrying bending stresses.

A slab is also continuously supported, unlike a beam, which is simply supported. So, the ground pressure helps restrain the physical bending and a thin slab (with a proper base underneath it!) can take pressures that a beam would never be able to.

 

[wssix99]

I have never, ever seen an unreinforced slab in commercial or industrial use.

This is because they could be exposed to hard wheels/tires. If I recall correctly, ACI warns specifically against using this type of equipment (forklifts, pallet jacks, etc.) on un-reinforced slabs.

I have no doubt that there are unreinfoced slabs used successfully in some areas based on local experience, but I think it is WAY to risky. At the end of the day, it comes down to risk vs. $$$.

+1

I'll add that the investment in reinforcement is an insurance policy, BUT it won't make up for foolish control joint, rebar spacing, or placement practices. (We have a number of threads from members who reinforced their slabs and saw cracking from asymmetrical rebar placement, late saw cutting, improper cut placement, distant cut spacing, etc.)

 

[pickmh]

you just go to the store, then buy one.

 

[matt_i]

This is because they could be exposed to hard wheels/tires. If I recall correctly, ACI warns specifically against using this type of equipment (forklifts, pallet jacks, etc.) on un-reinforced slabs.

Definitely something to strongly consider as those pieces of equipment already carry heavy loads, which lead to highly loaded areas under the tires/wheels. Also in both of those there is no "sprung" suspension, sure cushion tires and maybe pneumatic tires for a rough terrain forklift, but a pallet jack has none of that. Very small contact area especially in the PJ. Plus, any and all impact loads get transferred almost directly to the concrete....

I also read some material which recommends against using a sand base for a slab intended for forklift traffic, something about allowing too much local deflection under the higher wheel loading.

 

[Culture]

Thanks for chiming in Culture. From your perspective, what is the risk side of the equation? What do you feel steel reinforcing will provide or prevent in a 5000 sf slab if an owner spends the extra $. Again, no sarcasm intended. It's an interesting topic (to me at least)

The natural state of concrete is cracked. Concrete will cracking from drying shrinkage and thermal contraction. It may also crack from other factors, including structural loads and base movement (soil movement). Reinforcing steel does not stop cracking from shrinkage and thermal movement. What is does is holds together the crack that do form, keeping them from getting large. This in turn allows loads to be transferred across the crack due to aggregate interlock. Therefore, with rebar, you will end up with smaller cracks and no vertical displacement across the cracks. Image the lawsuit when someone trips on a displaced crack :-(.

Weirdly, as someone has pointed out, steel does nothing significant until the concrete cracks. It might as well not be there until that time. But the concrete will crack in a 5000 sf slab.

Unfortunately, reinforcing in normal slabs does not provide any structural strength (this comes from the concrete tensile strength as someone up-thread noted). However, if you do exceed the concrete tensile strength, it will hold the crack together and keep the concrete from crumbling.

So the risk is your slab falling apart. The extent of the risk depends on many factors including mix design, sub-base design, in-service loads, soil conditions, climate, etc. It is very complicated.

 

[Culture]

If you really want an education, go to:

http://gen.lib.rus.ec/

Search for and download:

Design Slabs on Grade, ACI 360R-92

This is an old version (1992) but will tell you everything you need to know.

 

[428PI]

Well, good grief. If the rebar would keep a crack from getting wider with age that is fully worth it. You should see my garage, my shop, my shed for cracks that keep getting wider without any rebar.

 

[ConCretin]

The natural state of concrete is cracked. Concrete will cracking from drying shrinkage and thermal contraction. It may also crack from other factors, including structural loads and base movement (soil movement). Reinforcing steel does not stop cracking from shrinkage and thermal movement. What is does is holds together the crack that do form, keeping them from getting large. This in turn allows loads to be transferred across the crack due to aggregate interlock. Therefore, with rebar, you will end up with smaller cracks and no vertical displacement across the cracks. Image the lawsuit when someone trips on a displaced crack :-(.

Weirdly, as someone has pointed out, steel does nothing significant until the concrete cracks. It might as well not be there until that time. But the concrete will crack in a 5000 sf slab.

Unfortunately, reinforcing in normal slabs does not provide any structural strength (this comes from the concrete tensile strength as someone up-thread noted). However, if you do exceed the concrete tensile strength, it will hold the crack together and keep the concrete from crumbling.

So the risk is your slab falling apart. The extent of the risk depends on many factors including mix design, sub-base design, in-service loads, soil conditions, climate, etc. It is very complicated.

I was going to let this thread sink into the abyss of endless internet debates but since you were kind enough to respond to my question Culture, i’ll add a final thought. Your well written response remakes the case i’ve been arguing about the use of steel reinforcing in slabs on grade. It holds cracks together.

As an engineer, I can see why you would spec steel reinforcing. It’s relatively short money for the peace of mind it provides. Personally I wouldn’t place a slab without it, especially a slab that is going to get any kind of heavy use.

Some designers have a different view. We’ve placed many hundreds of thousands of square feet of slabs on grade in commercial and institutional buildings over a 30 year span. The base supports the loads and timely, properly spaced control joints relieve the tension from drying shrinkage and keep the joints tight. Granted, these slabs are lightly loaded but they perform just fine.

I think the best way to think about reinforcing is as relatively cheap insurance. You may never need it, but you’ll be glad you have it if you do.

 

[ManCave]

Concrete is done. We are on 20 feet of sand here and sand is rated for 3000 per square foot bearing capacity so it's a really good base for a slab on grade.

We put in #3 rebar and also used the fiber mesh in the mix. Control joints have all been cut. It should be good for a long time I hope. Thanks for all of the excellent advice! I'm glad I stuck to my guns on the rebar.

 

[SALIV8]

It's a common misconception that rebar increases the load bearing capacity of a slab on grade. The reality is that a 4 or 5" slab isn't deep enough to take advantage of rebar's tensile strength and make the slab act like a beam and span between supports.

Firstly, by the time you allow an 1 1/2" of cover under the mat, it's essentially in the middle of the slab where there isn't any tension to resist. Secondly, even if this theory is correct, what happens around the perimeter where the slab cantilevers and the top is in tension.

The reality is that a slab on grade is not structural and simply bends and transfers the loads placed on it to the soils below. If those soils give, your slab is going to crack. You need to get into much thicker sections before you get a true structural slab.

For a slab on grade placed on a solid base constrained by perimeter walls, there is no absolute need for rebar. I have placed many hundreds of thousands of square feet of such slabs on engineered commercial and industrial projects over the last 30 years. Now a free floating mono slab is another matter. Rebar would be helpful here, not to prevent cracks but to hold everything together if the slab experiences some movement.

Rebar's only function in a slab on grade is to keep cracks from opening up. In fact rebar actually increases the likelihood of shrinkage cracks by restraining the slab from shrinking inward as it drys. Think of rebar as crack control not crack prevention.

Finally, to say that the absence of rebar somehow implies a contractor is inept is a ridiculous statement. There a many reasons including the owners specifications and budget that could determine whether rebar is installed.

I realize this flies in the face of conventional wisdom and our dearly held belief in overkill but we might as well pass on accurate information to those who come looking.

I always enjoy your educated, professional responses to concrete questions.