Concrete Confusion...

[USAFpj]

With the wealth of knowledge on this board, I wanted to ensure that I have a general idea of what I'm getting into.

I've signed on the dotted line to have a 30x5014 pole barn built up here in Upstate SC. The slab (currently) will be fiber concrete, 4in, and of 3000psi. The items that will be stored will be various WWII Jeeps, small jeep trailers, 4 post lift rated for 4in crete, and possibly an Oliver crawler and 58 Ford tractor. This slab is spec'd to have zero rebar or wire mesh. For $1556 more, the company will increase to 5in crete, but that, too, is without any rebar or wire mesh. Not only does this sound crazy$$, but it seems for $1556, you would get metal in your slab?

Our soils are very forgiving for slabs around here. Red mud, but sandy as well. I understand compaction is key with any slab, but what are your thoughts on the adequacy of the thickness and psi of this slab for my purposes?

 

[tcianci]

Your slab will be fine for your stated uses. As you mentioned, the lift is speced for the floor. If you're not going to be putting any wire or rebar in it, I would be inclined to use more control joints.

 

[hoho98925]

For a few hundred $ increase the PSI of the mix to 4000. Limit added water to what is specified in the design mix. Water = Ease of placing/finishing but weakens the concrete. Soft cut the slab same day or if not set enough, first thing next morning. I would cut into 15'x10' or even a 10'x10' grid for that size building. Cutting the slab is your cheapest insurance against visible cracks.
As far as the added concrete for your size building that amounts to:
4.62 CY around here A CY of concrete is roughly $100.00
100 x 5 CU = $500.00 + a couple of bucks a yard for placing.
Other then that they are trying to take advantage of you......IMO

 

[USAFpj]

Hoho, I see what you're saying, but won't the spec after 28 days end up at about 3500 anyway? I am interested in longevity as well. I hear both of you on the control cuts. 10x10 is what I was thinking as well.

 

[Brian80]

Im doing a 4" slab @4000 PSI, fiber mesh, we decided also to throw in wire mesh. But in actuality, its not needed. Also there will be a 2 post lift. The post locations will be poured at 6" x 3' radius

 

[hoho98925]

Hoho, I see what you're saying, but won't the spec after 28 days end up at about 3500 anyway? I am interested in longevity as well. I hear both of you on the control cuts. 10x10 is what I was thinking as well.

Yes- but at 28 days the 4000 psi will probably be at 5000 psi. IMO it is worth it. There is nothing worse then having a brand new cracked slab. On that same note, I would not put any load, vehicle or whatever on a slab for 30 days, at least not my own slab.

 

[tlmartin84]

There are several things that cause cracking, the biggest in a new slab are SHRINKAGE, and a poor SUBGRADE.

Make sure the subgrade is compacted well.

Use control joints 10 x 10 (Most use 15' but the more you can stand the better). Keep the slab wet for several days to limit evaporation and aid in curing, this will help reduce shrinkage. Fiberglass, metal shavings, and wire are all great things, but they really only control gapping after a crack has occurred.

If you want something structural go to rebar.....placed in the lower portion of the slab. This will aid in controlling tension from loads applied on the slab.

As mentioned above, the guy is ripping you off.....Most guys charge material, and then a set price to place and finish by the square foot (here it is right at $2.00). It doesn't take much time to place the additional inch or two as everything is worked off the top.

 

[Milton Shaw]

Most of the time a 4" slab is framed with 2x4 material so that 4" is more like 3.5". With wanting to store a crawler etc on it I would go to 5" and have rebar added to it. The cost now is nothing compared to having to do it over. The fiber does not really add a lot of strength to the slab and will have the fibers tips burned off that are sticking up when the slab dries from what I understand. I did a driveway 40 years ago and ended up having to do it over 30 years later because I cut some steps in doing it. Do it right once with 4000 psi 5" slab with rebar and have it ready for next 30 years.

 

[homebuilt burner]

Just a note, I believe the Oliver crawler he is talking about is one of those little ones and they are so light I would believe a car would be more lbs/sq. ft.

I am thinking like am OC-2??

 

[Rookie2]

I would seal the concrete before you saw cut it, I did it backass wards and had to scrub and scrub all the concrete fines off before I could seal it.

 

[jrb2]

I can't believe that in 9 replies no one has said this yet. "You are crazy not to put some kind of steel in there!" Re-bar is probably not needed in your application, but I would put mesh in before I would pay to go to a thicker slab. Did you get a quote to add mesh? I think it would be well worth the extra cost.

 

[matt_i]

The conundrum is this. The finishers/placers care about themselves so they attempt to make it easy as they probably do this every day. Directionally at odds with what has proven to make a long lasting job.

Rebar is super easy to setup yourself. You buy chairs, you buy bar, you buy wire twisters. You need an angle grinder for a few cuts. You need a truck with a 2" receiver hitch for bends.

The placers/finishers won't like this as they can't drive the truck over all of this without smashing it to oblivion. I suggest using a pump truck as that has the added benefit of not needing to soup the mix.

The middle ground is to use the wire mesh and have them pull it up with rakes as they pour.

 

[Cyberbear]

Don't know the reason for no rebar, it's probably the cheapest part of the whole job, stay away from wire mesh, that's for sidewalks, etc. It all comes down to strength v. cost ratio, and be sure you get what you paid for by receiving a receipt showing the concrete mix you are getting, don't simply take their word, "mistakes" can be made.

 

[Slowgsr]

I'd do a 18"x18" grid of 10mm rebar, do it yourself - cheap insurance,

 

[joes169]

Most of the time a 4" slab is framed with 2x4 material so that 4" is more like 3.5".

I realize that it's a common misconception here, and other places, but a 3.5" form doesn't necessarily equate to a 3.5" slab. We pour 6" thick slabs ocassionally with 2x4's, it doesn't matter. What matters is the top of the form, not the bottom.......

To the OP, have you looked for other bids yet? I would forgo the "Fibermesh", it's a waste of money. Personally, here in WI, our minimum shed slab floors are 4" thick, 4000 psi, and have #3 rebar on 36" maximum centers each way. With a good base, this is sufficient for 90% of residential garage use........

 

[mixerfixer]

30 x 50 x 4"= 18.5 yards. 30 x 50 x 5"= 23.15 yards. You always want to add 10% to the total order to account for variations in grade. You can see the difference an inch over that square footage makes. With concrete averaging $110/ yard in my area for a 3000 psi mix. If you want to stick with 4" ask the question what the cost would be to step up to 3500 or 4000psi mix.

They are screwing you with $1550 to add 1" of concrete as the difference is only a hair more than 4 1/2 yards. In my area that would cost about $500 more for material. The labor is the same no matter the depth. Its a 1500 sq ft slab. $2 sq foot to prep and pour. Labor should be about $3000.

Sub base prep is critical as most cracks happen from settling. I would even consider deepening the slab to 6" only around the posts for the lift.

 

[Gotcha640]

No idea how residential works, but in the refineries and plants I work in, we buy by the truck. 0-10 yards, 1 truck, 10.1 yards, 2 trucks. We try to line up multiple jobs to use what we buy, but you just know they're doing driveways and municipal work with concrete that's been sold twice.

 

[brownbagg]

you dont need rebar, if you want it then its fine, but rebar is for load factor, rebar does nothing till a load is applied, you really dont have enough weight to justifly rebar.

i would spring for the five inch slab

 

[tlmartin84]

This is bad advice.........

you dont need rebar, if you want it then its fine, but rebar is for load factor, rebar does nothing till a load is applied, you really dont have enough weight to justifly rebar.

i would spring for the five inch slab

I can't believe that in 9 replies no one has said this yet. "You are crazy not to put some kind of steel in there!" Re-bar is probably not needed in your application, but I would put mesh in before I would pay to go to a thicker slab. Did you get a quote to add mesh? I think it would be well worth the extra cost.

This is correct:

Don't know the reason for no rebar, it's probably the cheapest part of the whole job, stay away from wire mesh, that's for sidewalks, etc. It all comes down to strength v. cost ratio, and be sure you get what you paid for by receiving a receipt showing the concrete mix you are getting, don't simply take their word, "mistakes" can be made.

'

Rebar is inexpensive, when you are buying the mesh it isn't much more for the rebar. As I said earlier rebar will not only offer stabilization if a crack occurs if in the lower portion of the slab it will add strength.

If you use mesh, get it in the sheets and not the roll.... and use a heavier grade. Again the sole purpose of mesh in a slab is to keep it from spreading, we do a lot of demolition work and in the older slabs that had wire we find that near joints and cracks it has rusted into after a 20 years or so.

Minimum 4" slab (thicker if you can afford it), with rebar. Do it once, do it right, and have no regrets.

 

[tlmartin84]

FWIW.....

I'd rather have a 3000 psi slab with rebar, than a 4000 psi slab with mesh, or fiberglass.

 

[USAFpj]

Wow- some great details to hash out.

 

[KaiserJeep]

Just went down this road myself, and here's what we did...

Newly poured slab in a pole barn, 5" thick - 3000 psi mix - no fiber. Wire mesh laid on top of the plastic, and tied each sheet together every foot or so. The heavy gauge stuff, not the fence wire you get at HD or Lowes. (My concrete finisher guy said it really does nothing in a slab this size, and really has no effect on the strength/durability of the crete at all. So I probably wasted a few hundred bucks there.)

But, I did have a bunch of 28' x 3/4" rebar that needed a new home. It ended up in the bottom of the slab, and extending out in the aprons at both doors. Should be enough to easily handle any load I put in there. Overkill? Sure, but the rebar was free!

Slab was poured just as dry as it came off the trucks, only added a little water to the last load. We didn't want to water down/weaken the mix, and I had several young guys to drag and screed. The control cuts were made about 10 hours after the last burn with the power trowel - 12' x 12' pattern. The floor was kept wet for 4 days, and then 2 coats of acrylic cure and seal applied.

Six weeks later, the toys are now in the barn, and Momma is happy the basement is not cluttered any more!

 

[Badattitude]

This is bad advice..........

I disagree

brownie knows his stuff better than most when it comes to CC

 

[tlmartin84]

Encouraging someone "not to" put steel in any concrete is bad advice. Is it necessary, no will it improve it, yes. If you have any subgrade issues at all down the road, you will be glad you had the rebar....it keeps the slab from spreading and offsetting.

Again, it is a drop in the bucket compared to the cost of the rest of the slab.

When it comes to concrete, I will put my "stuff" up against anyones.

 

[allenb12]

I have been building with concrete for the last 45 years. Everything from pored in place jails to waste water treatment plants. The last project I was on, a pumping station, contained over 8,000 cubic yards. IMO the last person you want advice from is a concrete finisher. They only want their life easy and will advise anything that accomplishes that. 3,000psi is fine. Wire mesh is a waste of money. Nobody can pull it up properly. Use 3/8" reinforcing steel 16" on center each way. 4" is ok but I would use 5". I would thicken the perimeter a couple of inches and add a piece of reinforcing steel of the bottom of the thickened edge. Water to cement ratio is the key. You can use a plasticizer to make the mix easier to work with.

 

[KaiserJeep]

....IMO the last person you want advice from is a concrete finisher. They only want their life easy and will advise anything that accomplishes that. .......Wire mesh is a waste of money. Nobody can pull it up properly. ......

Agreed! My mesh was already in place when he showed up for the pour. If I had asked his advice, which sounds like yours also, I could've save a few hundred bucks. He was impressed with the industrial sized rebar, which actually came from a water treatment plant and intake construction site. Extra stuff, they were going to bury on site.

 

[wssix99]

Wow- some great details to hash out.

Well, you'll also need to put on your rubber boots in order to wade through some of the BS, also.

Our soils are very forgiving for slabs around here. Red mud, but sandy as well. I understand compaction is key with any slab, but what are your thoughts on the adequacy of the thickness and psi of this slab for my purposes?

To your original question - you are fine. If you calculate the actual pressure your vehicles exert on your slab, (weight of vehicle / number of tires / size of tire contact patch) you'll see that the pressure they exert on your slab is measured in tens of psi. The slab will be plenty strong. As you point out, the key to the real strength is the base underneath. You want a good solid, compacted base that can also support those tens of psi.

Unless you are using industrial trucks (forklifts) that put a lot of weight on hard wheels with small contact patches or are moving vehicles at high speeds, you don't need to worry about "strength" of the pad, itself. (Those situations really drive up the stresses beyond what is normal and put you in to a situation where an industrial floor is needed.)


The biggest challenge in this type of situation is cracking. The cement in the concrete shrinks at it cures, which causes the slab to crack. Rebar/steel is just one of many ways to control this cracking. (it adds nothing to the "strength")

Will your contractor give you a warranty against cracking and repair the slab if it does? If so, I would not deviate from the specs that they are offering you. Things that sound smart and intuitive can work against you with concrete. For example, increasing the psi of the mix adds more cement to the slab. This increases the shrinkage stresses, which can overwhelm the countermeasures your contractor put in for crack control (mesh, rebar, fiber, control joints, etc.) - and there goes your warranty. Increasing the thickness can have the same effect.

 

[sberry]

You can simply toss some rods in at the time of pour, dump the mud and float them in.

 

[theoldwizard1]

I have been told the main reason for mesh is to prevent the slab from moving WHEN it cracks !

 

[myredracer]

I used to work with a structural engineer. He always said it's much better to go with a 5" slab and always use rebar. Fiberglass in the mix is not a great idea - rebar is superior and the fibers tend to stick up (as noted above). Forget mesh in a slab that size and for that use. Proper compaction is important. Concrete placers are often reckless when walking around and can knock the rebar off the stands or push them down and the rebar will not be located where it's supposed to be. Plastic stands tend to fall over too easily - small broken pieces of concrete pavers work well. Placers can also tear a lot of holes in a poly vapor barrier - I've used the heavy stuff and 2 layers of it as a precaution.

I believe the standard longest reach for a concrete chute is 30' so as long as the truck can drive on both sides won't be a problem.

I would use a color hardener - get's broadcast onto the surface just before finishing and makes for a nice hard surface. Our 2K sq. ft. garage/shop has french grey color and looks nice.

Concrete always shrinks, no matter what. "Zip strips" work great - a plastic strip that gets pressed into the concrete while still wet. When the concrete cracks (and it will) all you see is a hairline crack instead of wide saw cut.

If you do end up using rebar and plan to drill the slab for anchors for the lift, make a drawing where the bar is you won't drill into them.

 

[North Run Grader]

Rebar and used concrete papers as chairs. As much as you can afford. When we poured the slab at my old job, we used 4" of recycled foam board, and leftover 5/8" rebar on 12" centers. All over top a super compacted base. 4" lifts of 3/4" crushed gravel, wetted and compacted till you couldn't cut it with a sharp spade. We ended up with 1 hairline crack because we ended up putting in some sump afterwards and didn't get the trench equally compacted. Much swearing was involved when that crack showed up that first winter. I think we averaged 5" of concrete. At the time, we had dye added to the truck as it was being mixed for maybe $25 a load. Be sure to see samples beforehand, we wanted a light red, ended up with a dusty rose colour instead. Still better then basic gray for the money.