Fiber reinforced concrete slabs on grade 5

[dmoench01]

Hello,

I'm sure this has been discussed to death but needed to ask your opinions about fiber. I work for a building supplier but have been a SE for 30 years and this topic keeps coming up. Will macrosynthetic fibers adequately replace regular rebar in slabs on grade?

I know it will it help for crack control but is there anything else I need to be concerned about? Namely, the building we are erecting onsite at our facility will have forklift traffic on it and the occasional semi backed into the building. The concrete guy wants to replace the #4 @ 18" o.c. that I've called out for this 6" slab with the synthetics per the report attached. I'm also concerned about the hairpins I have in the slab, will the synthetics adequately transfer that load?

Thanks,

Dwayne

 

[Lion06]

I will be interested to see other responses, but I would not use synthetic macrofibers for anything other than crack control. I certainly would not use it for transferring any real loading.

 

[KootK]

X2 for Lion06's answer...for now. There's research and non-North American experience suggesting otherwise. See comments by Brad805 and others here: Link

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

I was a member of ACI Committee 544 on Fibers in Concrete for most of the 80's during the initial research. During that time was when polymer fibers came about. Most of the original fibers were steel, in a variety of shapes and sizes.

The focus of the research and testing was to use fiber as an enhancement for the concrete mix. Its primary purpose was to control either "first crack" considerations and/or long term shrinkage cracking.

I have done research using fibers in concrete and asphalt....both for similar reasons.

The intent was never to replace rebar....it was and still is only a mix enhancement that increases the mechanical properties of a concrete mix, including compressive, shear and tensile strength....but never enough to delete rebar.

 

[EngineeringEric]

I would not allow it. From remembering some statistical models the fibers have a tendency to align in a unidirectional manner when going through a pump truck or nozzle. This would baisically only give you strength in one direction of the slab... may not be in the direction you want/need.

I also was taught, and practice, that fibers are for micro crack control. If you are a slab on grade, and it can support the load as plain concrete then your rebar is not needed. If you are a PEMB, and the slab is used to resist thrust, then you will still need a way to handle that load. it appears that #4 @ 18 is just the recommended shrinkage area (almost .002 Ac)...

 

[XR250]

I see it comes with a rock solid guarantee. Their liability is limited to the replacement cost of the fiber :>

 

[Brad805]

I would never consider synthetic being equivalent, but steel fibers are becoming more common. Finishers hate steel fibers. Now if your client is okay with the risk of an un-reinforced slab like I have seen Walmart do, then you might consider it.

 

[SlideRuleEra]

Ron - Y'all did a good job of educating us on fibers in the 80's. I started using them about 1985, in lieu of wire mesh. Even at the beginning it was made crystal clear that fibers were not for any other purpose.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dmoench01]

I appreciate the information guys. The concrete construction crew doing our slabs is sold on the fibers and the engineering group supporting them has showed them videos of this added fiber outperforming an 8" slab with 2 layers of #5 bars. I did tell them I've been hearing things like this for years but noticed its continued absence in new engineering concrete textbooks and general acceptance by any structural engineer I know in particular. I will stay on the safe side and keep specifying rebar.

 

[XR250]

I would love to see fiber outperform two layers of #5 rebar. I never specify it as, from what I have experienced, it does not work nearly as well as properly installed wire or rebar.

 

[msquared48]

Reinforcing or not, I still do not like fuzzy slabs.

Mike McCann, PE, SE (WA)

 

[Brad805]

Any links to a video? I too would be interested to see that. The fiber dosage must be very large. Not to surprising the supplier is keen on the idea.

 

[dmoench01]

I don't have the video, it is just one the construction crew saw at a conference, I guess.

 

[TehMightyEngineer]

Here's the only one I could find showing plain vs fiber but can't find a video comparing it to steel rebar.

https://www.youtube.com/watch?v=YpKgjUkuXdQ

Maine Professional and Structural Engineer
American Concrete Industries

http://www.americanconcrete.com

 

[Archie264]

>>>Reinforcing or not, I still do not like fuzzy slabs.<<<

And I wonder how it fares if moisture hits the whiskers?

 

[Ron]

Thanks, SRE! Ron Zollo at the University of Miami was the guru! He's still kickin' too!

Brad805....fiber dosages are generally about 1 percent by weight. For polypropylene fibers....that's a lot of fiber, but they stretch so you need more. For steel, that's reasonable and it was the starting point for all dosage rates.

 

[Trenno]

I'll just leave this here. Technical Guidance note from Istructe.

Includes a worked example for comparison too.

 

[dicksewerrat]

Is there an ASTM Standard for this?

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[BigH]

I've found this an interesting discussion - could not the slab be thought of similar to that of using fibre reinforced shotcrete for slope/rock support purposes? In these cases the fibre reinforced shotcrete is acting in tension - and has replaced the use of wire mesh in many cases.

 

[KootK]

Great doc Trenno, thank you.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.