Fibre reinforced concrete slab with uplift 1

[eqcivil84]

I am designing a slab on grade with fibre reinforcement. The portal frames attached to the slab result in some uplift which needs to be taken by the concrete mass. I have a perimeter bar in the bottom of the thickened slab edge and starter bars (attached to the perimeter bar and going out into the top 1/3 of the slab) near the frames, but they do not provide enough capacity to resist the uplift.

Can I rely on the tensile capacity of the fibre reinforced concrete - and use full section, elastic capacity analysis? If so what are "typical" allowable stresses - and is there research to back this up?

 

[Ron]

No. You cannot rely on the tensile strength of the fiber enhanced concrete. Keep in mind that the fiber only enhances some of the properties of the concrete, such as a slight increase in compressive strength, a moderate increase in the modulus of rupture, and a reasonable resistance to first crack propagation; which occurs when the tensile strength of concrete is relatively low.

Fiber enhancement, whether polypropylene, glass, or steel, does not replace rebar.

To resist uplift, make sure the slab is tied together with rebar or welded wire fabric.

 

[msquared48]

NNNNNNNNNNNNNNOOOOOOOOOOOOOOOOOOOO!!!!!!!!!!!!!!!!!!!

Mike McCann
MMC Engineering

 

[Ron]

Well said, Michael!!

 

[msquared48]

I am a man of few words.

Mike McCann
MMC Engineering

 

[eqcivil84]

Thanks guys. That is what I thought.

 

[BAretired]

Is there any real advantage, then, to going to the additional cost of using fibre reinforcement?

BA

 

[Ron]

BAretired...in certain applications, yes. Mostly, no.

Using steel fibers to enhance the flexural properties of concrete works well. I've used it successfully in pavement applications. Polypropylene fibers are overused and are often used to overcome poor construction practices....they don't.

Still no valid substitute for proper mix design, proper mix selection for the application, proper placement and proper curing. Some repair applications and some specialty applications are good for fiber enhancement, but it's a waste of money for common applications of concrete.

 

[WillieWonka]

Allow me to clarify. The problem with most polypropylene, nylon and most plastic fibers used in slabs is that the modulus is wrong. To use a high tech term they are "stretchy". Like trying to reinforce concrete with rubber bands. When loads are applied they stretch enough to allow the concrete to break before they resist the load. They provide a moderate amount of crack reduction when the concrete is new and relatively weak, but after that those fibers are not much help. In addition, they are often used like pixie dust, sprinkled in the concrete in relatively small amounts (but with best intentions and happy thoughts).
Glass fibers or steel fibers that DO contribute to strength are not stretchy, but enough of them must be used to make a real difference. This is difficult in a slab, but simpler in a sprayed up or cast panel or shape.
This is what allows the production of GFRC panels and shapes.

http://www.strombergarchitectural.com/materials/gfrc

where glass fibers do take the place of steel reinforcing. In GFRC the fiber content is 5% or more by weight.

 

[miecz]

WillieWonka-

Nice explanation.

BAretired

This question came up a few times before you joined the forum. A search on "Fiber Reinforcement" brings up 20 threads. One that I participated in was thread507-172007.

 

[HeavyCivil]

Sorry to hijack but I'll slip this in:

At what point does synthetic fiber reinforcing have a significant detrimental effect on slab appearance? Will 4lbs/yd be pretty ugly in low traffic areas? Also- how does one quantify the proportion of fibers in a mix to the degree of "shrink resistance"? E.G.: In an unreinforced slab with joints spaced at S'= 2xD" is there any rule of thumb for increasing that spacing (itself based on rule of thumb..)?

 

[Ron]

1. Don't increase the joint spacing, no matter what the supplier says.
2. Synthetic fibers present at the surface after finishing can be burned off with a roofing torch with just a light pass over the surface.

 

[eqcivil84]

I have been looking into these fibres a bit more now and found some by Buckeye called UltraFibre500 (ultrafiber500.com). Does anyone know anything about these? Are they really all they claim to be? They look to out perform all the synthetic and steel fibres.
Any information would be greatly received.

 

[Ron]

UltraFibre500 .....an absolute crock of "crap". There is no way a cellulose based product has the same or similar properties to steel fiber or even polypropylene fibers. The is a LEEDS crock.

 

[eqcivil84]

Ron - is this your opinion or do you have references to back it up? The research available on the product looks reliable.

 

[BAretired]

What research? There is nothing of substance so far as I have seen. I agree with Ron. Products of this sort should be regarded with suspicion until they are able to prove otherwise.

BA

 

[msquared48]

What I mainly see in the Leeds project list is non-structural applications for the GFRC - Architectural applications. What the OP initially asked here was a structural application. I see no proof on the Leeds site of such to warrant the use as the OP needs.

GFRC is acceptible for aiding with crack control, and that is all as far as I am concerned... Not increasing the structural load carrying capacity of any concrete member in tension.

I stand by my original recommendation here too. NO.

Mike McCann
MMC Engineering

 

[Ron]

eqciv84...I have reviewed their evaluation report, which by the way, does not contain the data from their testing. I also reviewed the referenced acceptance criteria for the ICC evaluation report.

The evaluation report makes a statement that the company does not make in its literature, specifically that these "fibers" are considered an admixture, not a reinforcement. Further, it states that the fibers do not replace reinforcing steel for flexure or for temperature shrinkage control. Additionally, their claims of tensile strength of cellulose fibers are based on a dry test, whereas when the fibers are used, they are in a wet condition. For comparison, take a strip of cardboard and try to pull it apart in the dry condition and then do the same after wetting it to saturation. Big difference.

The principle they are trying to promote with this fiber is that it works at the microscopic level and will reduce plastic shrinkage cracking. They discuss the interruption of microcracks in the cement matrix, which has some validity, if the strength of the fibers is maintained through the curing process of the concrete. I suspect these do not maintain their strength. I believe the way this material works for plastic shrinkage is that it absorbs water (up to 80% of its weight) and thus prevents surface evaporation which leads to plastic shrinkage cracking. That probably works to some degree; however, plastic shrinkage cracking is a specific phenomenon that doesn't occur all that often except on large flat placements on windy days with low humidity.

Granted, if you reduce plastic shrinkage cracking, you will reduce overall cracking as it is one of the propagation methods of concrete "random" cracking.

In the acceptance criteria of the ICC for this material, it only requires that the material not cause a reduction in the properties of the concrete as compared to a control specimen. For instance, if the compressive strength of the UF500 specimen is at least the same as the control specimen, it passes their criteria. I suspect this is why they do not offer the hard data...it only meets and doesn't exceed the control specimens.

For steel fiber, the enhancement is clear and measurable. Similarly, but less so, polypropylene fibers provide some enhancement.

I find their data and claims to be misleading and geared more toward sales than technical performance.

If you want to put fiber in concrete, use one that actually enhances the properties of the concrete. Forget trying to use it for replacing any steel reinforcement necessary in the concrete. That just doesn't work.