I have about a 1200 sq. ft. slab that needs pouring.  Slab will be about 4"-6" thick.  Will carry 15-20 tons on it at all times.  Weight is localized in certain spots rather than spread out.

GC says glass reinforced concrete is more than adequate.  I was under the impression that it needs rebar or wire mesh. 

any concrete should have rebar in it. no matter what load. especially when there is a load. 

Oh im no concrete expert but ive poured enough to know rebar is a must.

I'm no expert either but I always thought you needed some reinforcement...Whether it's steel mesh or rebar.  Especially when we're talking about load carrying capacity.

This is the first I've heard about glass reinforced crete.

Rebar and wire mesh. Make sure the rebar and mesh aren't just laid in the bottom. They need to be raised into the slab to do any good.

Can't just have one or the other? Gotta have both now?

I had the joy of spending $30,000.00 watching glass reinforced cement get ripped up off of wire mesh that was laying on dirt and then new cement put down at 6 inches with mesh raised to the middle. Don't turn your back on the cement guys...pay attention to what you are getting.

The fiber reinforced is very impressive, but that is a big slab not to have rebar in it.

Even if you don't need the rebar, I think I would spend the money to put it in.  It wont cost that much and could save you a big headache later.

It all depends on how the load is spread out.   What is the compaction of the soil under the slab?  Wire mesh at a minimum. To many variables to give a precise answer. I don't think we've used fiber concrete here in a few years.

Tell the GC to quit trying to half ass the job.

+1

I would think 6" at very minimum considering it's spot/point loads.

I just poured a slab at my new garage.

7" thick 4K psi #4 rebar 24" on center with commercial fiber power troweled over 4" of Gravel.

Not near as much weight as you will be putting on it.

I've been know to overkill as a GC though.

Compact the shit out of the sub grade, six inches of compacted gravel, #4 rebar at 1'x1' grid, 4,000psi concrete 6" depth.

Oh, and I am not an engineer, contractor, or any sort of expert.

Given your parameters, I wouldn't just use fiber reinforced concrete. If it was just a driveway or patio probably not a problem.

Soil is fairly compact.  We just ripped up a slab that was probably poured in the 50's.  And there's a concrete slab under that from the late 1800's.  

Rebar for sure.   Sounds like you're building a machine shop or production facility.

But the comment above about subgrade compaction is worth following up on rebar or not.

The subgrade is as important as the concrete if it's to last.

Your GC is cutting corners. I'd ask yourself what else he's likely to half ass.

Brewery expansion.  10bbl tanks weigh about 3500lbs full.  Each tank has about a 9 sq. ft. foot print.  Plus wine and spirit barrels stacked three high on racks.  

This is exactly what I was thinking all day.... :/

Depending on conditions, all of this and even some other factors apply. There is more to pouring crete than just adding rebar.

I'd ask the new contractor after I fired that one.

Is the load dynamic at all? (Does it move?)

Me thinks you need thicker concrete AND more reinforcement

I can pull the rebar plan for a multitude of structural slabs (drawn by guy's with fancy stamps that know this stuff) tomorrow if you would like.

Probably have 300+ sets of civil plans on my computer

Truck drives, bus parking, commercial parking, fire lane, equipment pads, etc... you name it I've probably got an example somewhere on my comp.

PM me if you want me to shoot some over.

@d_striker

Most of it is static.  Stainless tanks don't move once they get plumbed into glycol system.  They get emptied and then filled, over and over.  Each tank weighs approx 3500lbs full.  There will be about 4 of those in the middle of the slab.  

On the perimeter of the slab will be wine and spirit barrels on racks.  Three racks high.  So that's six 55gal barrels and racks weighing in at about 3500lbs. 



Then there will be utility shelving which will hold approx 2000lbs and misc pallets holding 2000lbs.  Oh yeah, and a smallish forklift that weighs approx 3000lbs will be parked in there and operating on the slab.

Thanks for the offer on plans but I think I just need to talk with the GC and tell him to quit fucking around and do it properly.  

This is exactly why there are engineers.  

Seriously, hire an engineer, it won't cost you much and the right design will last decades, not months.  This is where you're making a living, is it worth an extra grand or two?

I'm not trying to be self-serving either.  I just convinced a guy today that he doesn't need me.  You on the other hand...

Rebar isn't that expensive.  Insist on it.  Concrete guys can be lazy with this for some reason.  

I know...I really don't get it.  

I guess they figure once your slab cracks they are long gone.

^^^


Chairs, not bricks, OP

Good advice.  When you start getting into anything that resembles a point load, consult a pro.

Here's the thing, OP.  If your contractor is talked into placing regarding answer sticks it all on bricks or 2" chairs in a 4" slab, you might as well not use bar.

Just hire an engineer.  Costs less in the long run and should be done right.

Glass reinforced will let you down. Our company built a new warehouse a few years back and used that crap. It has been crack city. You can't beat steel in concrete and always pour thicker than they tell you to. I have never had regrets going thicker than the experts said I needed but have been disappointed many times taking their advice.

Get another estimate.

What's your minimum rebar cover?  2 to 3"?  Around here I've never seen less then 3" ( off the bottom ) except for one design on deck pan.

Why are chairs superior to bricks?

The only loads that are an issue would be the point loads from the shelves.  The barrels ( 3500/9= 388 lb/sf) really are not that much.  

The rebar does need to be off the mud, at least 2", but not in the middle of the slab.  The middle is called the neutral axis and does nothing to strengthen the slab.  the rebar needs to be in tension when loaded to be effective.  Most times with slabs, the rebar just keeps the concrete together after it cracks.

Use at least 6" concrete with rebar or 8" non-reinforced.  Spacing on the control joints is also critical to control where the concrete cracks, cause it will crack.  The cuts need to be a minimum of 25% of the slab thickness, so 2" deep for 6" slab.  generally 12' to 15' spacing between cuts.

For several reasons. You don't want to alter the design strength of the concrete, also, water can enter and over time corrode the wire and rebar.  I'm sure the engineers can point out other reasons.

sounds like a residential gc, they will use fibers.


how many commercial jobs use fiber? 0


how many commercial jobs use steel?  all of them.


it's a no brainer. residential guys are cheaper,  but do not think the same.   let the guy above give you plans. tell your gc, "this is what i want"

A slab that big I would go with post tension and have piers dug as well. What is this going to be used for?

3 guarantees with concrete: It's gonna get hard, it's gonna turn white, and it's gonna crack.

Rebar and/or mesh will not stop your slab from cracking.

Cuts control the cracking, or at least they are supposed to with the proper spacing and layout. Hard troweled floors we cut same day with early entry blades, outside broomed creteis cut the next morning unless we do a morning pour and it's hot. Then we'll saw the same afternoon.

The problem with 6x6 mesh is that while the guys may pull it up, everybody has to walk in the crete to do the pour and they are stepping the mesh down the whole time, unless your pour crew consists of ballerinas walking on their tip toes.  Rebar 1-2 ft OC may get stepped on occasionally, but nowhere near as much as mesh. That and 3/8 or 1/2 in bar is far better than what, 6ga? wire mesh.

I use rebar in pretty much everything.

Many of our commercial jobs have fiber specified for our decks, slabs and walks. Specified by the engineers, not the contractors. Sometimes we use mesh or bar AND fiber.

Fiber does NOT replace rebar. Fiber is to help stop cracks during the curing process only.

Even on dobies or chairs mesh will flex when the finishers step on it.  Rebar is more likely to stay up in the mud.

The slab is only as good as what's under it.  If you have crap soil under your slab it will fail.  If you have 2 feet of highly compacted gravel on top of shit soil - your slab is still going to crack.

Found this note in a concrete journal

“The minimum amount and spacing of reinforcement to be used in structural floors, roof slabs, and walls for
control of temperature and shrinkage cracking is given in ACI 318 or in ACI 350. The minimum-reinforcement
percentage, which is between 0.18 and 0.20%, does not normally control cracks to within generally acceptable
design limits. To control cracks to a more acceptable level, the percentage requirement needs to exceed about
0.60%”.

.6% rebar is a lot of rebar.  For your 6" slab ACI minimum would be roughly a #4 bar 16"o.c.  The 0.6% would be #5 on 8.5" centers, roughly.


Fibermesh does ok for controlling hairline cracks, not so much controlling larger cracks.


Without knowing the subgrade you have it's hard to say much about rebar.  I'd ask for 4,000 psi mud, #4 on 12" centers e.w., a dry mix with a water reducing admixture.  ( A low water / cement mix will be much less prone to shrinkage cracking - but it's a bitch to place and finish, so they add super-plasticizers to the mix so it's workable without adding water.   It's not cheap)   Keep it wet or keep a good cure on it and keep it out of the sun if at all possible.

Do not let the contractor add water on site if you've spec'd a mix design.

If you or the boss hates any cracks then you could add fibermesh to that mix.  It'll probably still crack.

Around here the code officials want the 3" clear on bar poured directly against soil.  Bar in the middle of the slab is fine, but you don't make 3" clear on a mat of 4 bar e.w.


Pretty much a shot in the dark without knowing what you're pouring on though.


Also - call the local batch plant.  Tell them you want a mix design for a low water/cement ratio mix, 4,000 psi f'c, with a plasticizer.   They probably have something on file that has been tested by a testing agency.  Give the mix design to your contractor and tell him thats what you want for mud.

6" thick 4000 psi

#4 bar wired into a 12" x 12" grid and raised into the slab with chairs or half cement bricks.

Don't over complicate it. Wire mesh is junk, don't even bother with it. Base prep is more important than the slab most of the time.

It's going to crack, it always does, have them saw cut it the next day to make the cracks neat looking.

Why on earth did you waste that much money on a garage floor? You could have done a 6-8" base, fiber reinforced concrete at 4" and no grid and it would have held up just the same with car/truck use.

Now some poor guy is going to have to jackhammer that thing out in 70 years when it needs to be replaced.

You need to hire a civil engineer to design your slab.

Bricks (concrete ones, not clay) win in the end for us, they don't get crushed when you step on them so the bar stays where it's supposed to.

I have never had an inspector tell us we needed to use nylon instead of the bricks when I asked (I always ask to make sure it will pass the rebar inspection)

You need a new contractor

Not certain on the math, but with a 4000 psi minimum mix design, when fully cured, it should be able to support the 21,000 lbs he need it to support on 5.25 square inches of slab.

I assume the racks have a much larger foot print than that however.

Concrete is really good at taking compression.

You need to hire a structural engineer, it will be money well spent.

This..

I have never done anything with concrete but as man i would tend to agree with the above

Code is 2" to a formed edge and 3" to an earth edge.

You have to worry about a couple things - punching shear (high rack loads on small footprint)  and bending stress in the slab.  Basically a slab is a plate on an elastic foundation.  When you put a localized load on it - like a wheel load or a rack support the slab and subgrade flex which puts one face or the other of the slab in tension.

Just poured my own patio at my house. 40x12 and I used a bunch of rebar. Haven't poured much concrete but I've always used and been told to use rebar or mesh. My patio was a little more than 4 inches thick with 5 inches of gravel underneath. I would hope for that kind of weight it would be 6" thick atleast and definitely have rebar or mesh!