How much reinf. in slabs on grade 15

[IJR]

There seems to be differing opinions on the min. amount of reinforcement and on how it should be placed.

It will be nice to learn more from experienced friends:

1. I understand that the reason behind such reinforcement is temperature and shrinkage, but definitely not the same amount as is specified in ACI318 for structural slabs. How much? How placed.

2. A friend argues that since vapor barriers are placed beneath there will be no shrinkage problems. I can not find logic in this argument since as I understand shrinkage and temperature effects are considered for the sake of limiting cracks and vapour barrier may cut down shrinkage but if no further cover is made above slab, wont temperature changes affect this unreinforced construction.

 

[JAE]

There is a wide range of opinions, formulae, and values in the engineering literature for reinforcing of slabs-on-grade. ACI 318 values for minimum steel (0.0018 x gross area) are not specifically intended for concrete on grade.

PCA provides the traditional subgrade drag formula which is:

As = FLw / (2 x fs)

where As is the amount of reinforcing in sq. in. per lin. foot slab width

F = coefficient of friction between base and slab (typically 1.5)
L = slab length between free ends, feet, in the direction of the steel
w = weight of the concrete slab, psf (usually 12.5 psf per inch of thickness)
fs = allowable steel stress, psi 24,000 psi for 60 grade steel

There are other formulae as well, all of which give differing amounts of steel reinforcing. I've seen many engineers use 0.0014 x gross area of concrete. Some use 0.0018.

Generally, the reinforcing is detailed in the upper portion of the slab, or at least near the center of the slab depth. There have been articles published which argue that the reinforcing should be in the lower portion of the slab due to the nature of the drag forces on the bottom surface and the theoretical tensile stresses that develop.

We have begun to use more fiber reinforcing in the slabs-on-grades due to the high cost of setting bars and the difficulty contractors seem to have in properly chairing welded wire fabric. I have never, ever, seen WWF end up where you want it to and we have ceased specifying it in slabs. The fiber reinforcing, to date, has given us very good quality slabs.

Your friend, I believe, is mistaken in thinking that a vapor barrier under the slab does anything to reduce shrinkage. Conversely, it may actually hurt the slab by promoting slab curl. The slab will begin drying and shrinking in the upper surface of the slab, while the lower layers of the slab cannot cure out as fast due to the vapor barrier retaining moisture in the concrete. The top layers shrink while the bottom layers do not....this causes the slab to curl up, especially at the corners. When any load is applied to the slab, the slab cracks.

Hope this helps.

 

[Ron]

JAE...you're on a roll!!

Would add the following to the excellent comments by JAE:

Pay attention to the mix design. Keep water-cement ratio near 0.50 and keep the slump to around 5 inches.

With fiber, my preference is steel fiber over polypropylene. All of the fibers will delay the shrinkage cracking, cause the cracks that form to be farther apart, but watch out.....as the spacing between cracks increases, the width of the crack also increases!!

Agree that WWF is waste of time and money. Some codes require it, but try to talk them out of it.

In general, you will also get better results without a vapor barrier. Make sure the subgrade is wet when you place the concrete. JAE's comments on curling are right on.

In summary....

Use good concrete
Control the flatness of the subgrade before placement
Use proper joint spacing
Consider fiber in lieu of WWF

 

[Qshake]

JAE, RON, You guys are all-right! I'll vote for you guys any day!

Qshake

 

[Ron]

Hey Q...where ya been, man??

Missed your comments in the mix!

 

[JAE]

Ron:
We've never specified steel fibers. Had some boys in some time back who were promoting them and it looked reasonable. Do you specify your own type/amount/etc.? or do you have the supplier make recommendations for a particular situation?

 

[IJR]

Thanks Ron,Jae,Qshake ,Kam for all.If you have more to comment on, please go on, theres lots for me(and may be us) to learn from you guys.

IJR

 

[Ron]

IJR...thanks, that's very kind of you. Keep in mind when we answer these questions, it isn't because we're smarter than the next guy, it's just that we've likely been burned and learned enough not to get burned twice!!! This is otherwise known as "experience". There is an adage that "good decisions come from wisdom, wisdom comes from experience, and experience comes from bad decisions"!!!

JAE...my experience with fiber goes back almost 20 years. My initial work was with short, ductile steel fibers and since has included most fiber types and materials. In my opinion and experience, the short, ductile deformed steel fibers perform quite well. I have used them in both portland cement concrete and asphaltic concrete (hot mixed asphalt). When properly distributed in the matrix, the results are excellent. Be careful of "balling" or accumulations of fiber that occur from improper introduction of the fiber into the mix, coupled with inadequate mixing. Polypropylene fibers provide some enhancement, but again in my opinion, not much.

For most slab on grade concrete applications, 0.75 to 1.25 percent by volume is appropriate for steel fibers. Most manufacturers will recommend about 1.0 to 1.5 percent by volume. This is fine, but sometimes more fiber than you need (more cost!).

I typically specify the type and quantity, with some reliance on the manufacturer, but usually not the ready-mix supplier.

 

[JAE]

Thanks, Ron.....just wanted to hear from someone who has used them before. The reps who were in our office claimed that some of there new steel fibers were shaped differently to avoid the balling in the mix.

How do you determine the volume? Usually the poly fibers are specified by weight (i.e. 1.5 lbs./cy). Do you just specify 1.5% of 1 cy per cy of concrete? I would think you'd specify it by weight as an easier way to quantify it.

 

[Ron]

JAE...
The difficulty in specifying by weight is that the unit weight of the materials can be significantly different. For example, you might have two different steel fibers with a 100 percent difference in cross section, but the bond surface areas are comparable and the unit stresses on the fibers are not critical for either fiber. To specify by weight would give you twice as many fibers of the lighter one or half as many of the heavier, depending on which you specified.

For the desired effect of fibers, near homogeneous distribution is more important than the unit stress attenuation of the fibers, therefore, the more actual fibers we have in the mix, typically the better the response.

1.5 lbs. of poly fibers probably gives you a fiber count higher than 50 lbs. of steel fiber. In this instance, the unit stress attenuation IS important because of the elasticity of the poly fibers, as well as their reduced unit stress bond and strength as compared to steel.

 

[JAE]

Ron:
OK, that makes sense. But how does the steel fiber supplier measure volume of fibers? If they come in a bag, all mixed together randomly, you have a larger volume than if they came in a container where they are somehow stacked with less air space between. This would seem to say that different fiber profiles would produce different densities in a bag....or doesn't this really happen. Do they sell the fibers by volume? I'm just trying to get a handle on how you would verify that they placed the required amount in the mix. We check concrete mix designs by the weight tickets.

 

[JAE]

By the way...we were just writing a spec for concrete and found a paragraph that specified steel fibers by weight (50 lbs./cy). This was Masterspec. Are they off base here?

 

[Ron]

Interestingly enough, the fibers are sold by weight, also. The 50 lb/cy spec. is about right for a 1% volume. The volume measurement is bulk by the box. More later....

 

[Ron]

Back again...OK, the 50 lb/cy is about right for general dosage rates for crack control, but does not equate to 1% by volume (I pulled out my trusty slide rule to be sure!). It is about 1.3% by weight. 1% by volume is necessary to start replacement of steel, per unit area, in thin slabs (...I'm not in favor of using short fibers to replace rebar for bending in structural members other than slabs on grade).

The 50 lb/cy dose rate is about 0.40% by absolute solid volume. 1% by volume would yield ==> (Weight = 0.27cf(1% of 1cy)*7.7(specific gravity of steel)*62.4(unit weight of water)=129.7 lb/cy

My apologies for confusing or misleading on the dosage. The typical of the fibers I have used has been one 50lb box of fiber per cubic yard of concrete, for slabs on grade. According to the manufacturer I am most familiar with, this is a low to mid range dosage, and is used for light traffic slabs with static loading.

 

[JAE]

Thanks a bunch, Ron.

 

[Riz]

IJR,
Can u try locating Concrete Ground Floor: Their design, construction and finish. By R. Colin Deacon , Cement and Concrete Association Publication (UK).
Riz

 

[IJR]

Thanks Riz, I will try that.

 

[you]

Hi Ron
Would think that the 1% by weight might not apply for steel fibers in concrete. Have very limitted experience with concrete and fiber reinforced concrete. Is there a general rule for weight per cubic yard for non metalic fiber? Have you encountered the 'bones' steel fiber?
ken ballard, p.e.
you@surfbest.net

 

[Ron]

Ken,
One percent by weight would be about 35 to 40 lbs per cubic yard. For polypropylene, that's a lot of fiber. For steel, not much.

Am not familiar with the "bones" fiber. Have experience with most of the steel fibers (deformed, round, hooked, straight, ductile, non-ductile, etc.). For those, you want around 0.75 to 1 percent by volume (0.2 to 0.27 cubic feet).

Ron

 

[sc]

I'm still not convinced about steel fibres as I have seen them used in footpaths and ground slabs. They appear to work fine in controlling minor shrinkage cracking, but also appear to add nothing to structural strength. We used them at the urging of the sales people as a demonstration site for them and hence installed them at their directions. The sales people no longer talk of this demonstration site.

Don't know if I am being unfair based on limitted experience, but I would not recommend steel fibres to anyone.


sc