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How much reinf. in slabs on grade

How much reinf. in slabs on grade

How much reinf. in slabs on grade

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.

RE: How much reinf. in slabs on grade

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.

RE: How much reinf. in slabs on grade

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

RE: How much reinf. in slabs on grade

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


RE: How much reinf. in slabs on grade

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

Missed your comments in the mix!

RE: How much reinf. in slabs on grade

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?

RE: How much reinf. in slabs on grade

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.


RE: How much reinf. in slabs on grade

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.

RE: How much reinf. in slabs on grade

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.

RE: How much reinf. in slabs on grade

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.

RE: How much reinf. in slabs on grade

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.  

RE: How much reinf. in slabs on grade

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?

RE: How much reinf. in slabs on grade

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....

RE: How much reinf. in slabs on grade

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.

RE: How much reinf. in slabs on grade

Thanks a bunch, Ron.

RE: How much reinf. in slabs on grade

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

RE: How much reinf. in slabs on grade

Thanks Riz, I will try that.

RE: How much reinf. in slabs on grade

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.

RE: How much reinf. in slabs on grade

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).


RE: How much reinf. in slabs on grade

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.


RE: How much reinf. in slabs on grade

Keeping in mind that we're talking about slabs on grade here, I think if the subgrade is poorly prepared or the slab is exposed to extreme loads then we should be looking at a structural slab.  However, for slabs on grade, the main concern is shrinkage and temperature cracking.  I recently constructed a workshop and RV storage area 36'x48' and used fibre (poly not steel).  Since this was my personal building I was particular about the outcome.  It is located in an area with moderately severe winters and has been in place for two of them.  So far not one crack!

I'm ashamed to admit that on clients projects I haven't pushed hard enough for use of fibres.  As was earlier mentioned, I've also never seen wwf show up where the drawings indicate.  Could be because the worker hooks the fabric, pulls it approximately into place then turns around and steps on it.  If the mud is fairly high slump, which for flatwork it usually is, then he probably is pushing the fabric back down.


RE: How much reinf. in slabs on grade

I'd like to add my own concerns. I have never heard of fibres being used here in concrete and I was discussing a slab for heavy plant with a client. I mentioned that WWF never ends up in the right place and he jumped on that ans said "well let's leave it out". Obviously it is a cost saving but I am concerned that any problems with the slab would be attributed to the lack of reinforcement because it is religiously put in here on every project. Any thoughts as to the risk of leaving it out and using plain concrete with saw cut joints at 5m centres both ways.

Carl Bauer

RE: How much reinf. in slabs on grade

Carl...you should not have a problem leaving the WWF out.  Concentrate on good subgrade prep (flat as possible to minimize thickness variations in the slab...remember, you can make the top flat, it's the bottom that causes cracking problems!), good concrete mix design with as large an aggregate as practicable, and keep both the cement content and the water as low as you can and still achieve durability.  Strength will not likely be an issue if you accommodate durability.

If you want to use fiber, I suggest steel fiber.  Polypropylene is OK, but in my opinion, is not as effective as steel fiber.


RE: How much reinf. in slabs on grade

Thanks Ron et al for the info, just getting to grips with this site. Alot of info to look at.

Ron can see that you are not a great fan of fibres particularly synthetic.

Reading the threads there seems to be alot of generalisation about fibres - there are of course many different types all held with varying claims by the manufacturers.

1. The monofilament polyprpolene fibres that are commonly used in concrete to reduce plastic shrinkage, increase impact resistance etc etc are "non structural" and am in agreement with the general consnsus that they do nothing that a good quality concrete should do in relation to cracking etc. Although nowadays they are being used in enhancing the fire resistance of concrete.( they melt and let the water in the concrete - now steam- escape through the capillaries so reducing spalling)which is probably worth paying for in u/g structures.

2. Second type of fibre we can term is structural. As far as I am aware this is about the "post-crack performance" of concrete - so how the fibre performs after the concrete cracks. Esentially if you do a flexural test on a beam the first crack will be the same whatever the fibre reinforcement - the flexural strength is dependant on the concrete matrix not the reinforcement.

There are a number of tests that can compare the strength of fibre after cracking - known as toughness.

As for the argument for/against steel or polypropylene fibres with regard to toughness there are poly fibres on the market nowadays that can equal steel. eg

7-8kg polypropylene fibre = 40kg steel hooked end fibre.

There are obvious advantages of using poly in that they don't rust, lighter, cheaper etc etc. Whether this better than steel is upto the designer.

The issue for me is that if I take out the steel mesh what is the replacement for steel fibres.

Interested in MERBAS.What are you using the monofilament fibres in precast for?


RE: How much reinf. in slabs on grade

Isa001...I would disagree that the flexural properties of concrete are not enhanced by fiber addition, even before first crack.  The properties are significantly enhanced by fiber...that is one of the main reasons it was developed and initially used.

As for the amount of fiber to use to replace wire mesh...in steel fibers, about 1 percent by volume is usually used.  

As for the polypropylene fibers, you'll have to check with the supplier for "equivalence".

One clarification....I am not against the use of fibers.  I am against the use of fibers (or anything else) as a means to compensate for poor construction practices.

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