Fibermesh

[jb777]

I have a customer who would like to use fibermesh concrete instead of rebar cage on an 8’ x 5’ x 3’ deep block footing supporting a heavy MRI unit (16,000 lbs) in a hospital building in California (no freeze-thaw issues). Foundation is only subject to compression loads of equipment, and there are some anchor bolts in the concrete. I don’t know enough about fibermesh to know if this would be OK or not. I’m inclined to say it’s not OK and rebar cage is better. Do you have someone there with experience who could comment? Thanks.

 

[JAE]

Fiber reinforcement is primarily there to help control shrinkage cracks (usually in slabs).
I'm not sure what good it would do in a large footing.
It certainly won't replace steel reinforcement for flexural stresses, shear, etc.

 

[hokie66]

I don't think plastic fibers do any good for strength. But this is the support for an MRI unit, which is probably the reason for avoiding steel. I imagine your size footing would be fine unreinforced.

 

[Tomfh]

Steel Fibres are now being used as flexural and shear reinforcement, ie strength design. We have done a job that used Helix Microbar fibres for flexural reinforcement of suspended slab on grade.

Our latest Australian concrete code AS3600-2018 includes a section on steel fibre design.

 

[r13]

I think these two papers are good read to further your understanding on 1) Fiber Reinforced Concrete, 2) Plastic-Based Rebar, and as step stones for further research.

[link]https://theconstructor.org/concrete/fiber-reinforced-concrete/150/[/url]
[link]https://www.concreteconstruction.net/how-to/materials/plastic-based-rebar-meets-special-demands_o[/url]

 

[Ron]

Fiber is not a reinforcement....it is a mix enhancement that improves the properties of concrete but does not provide adequate flexural reinforcement.

 

[hokie66]

Aside from all arguments about fibres, steel or plastic, the MRI manufacturer's recommendations must be followed as to the permissibility of magnetic materials within proximity of the machine. These machines cost millions, so you don't want your footing to be the tail wagging the dog.

 

[dik]

The ocncrete is improved by the polypropylene fibres effect on the bleed water. If you need real reinforcing use real fiberglass rebar.


Dik

 

[msquared48]

Without confining rebar, I would think any embedded anchors for the equipment would be compromised in their capacity.

Mike McCann, PE, SE (WA, HI)

 

[SWComposites]

Why not use fiberglass rebar?

 

[msquared48]

Used that for a residential foundation three years ago. Higher strength, but all the rebar must be shop formed and cut. No field modifications allowed.

Mike McCann, PE, SE (WA, HI)

 

[rapt]

Tomfh

I assume you mean that the steel fibre contributed to the strength rather than providing the full flexural strength!

 

[r13]

Short grained fiber vs long grained fiber makes difference.

 

[dik]

retired13: Maybe mostly for sales...


Dik

 

[Ron]

The aspect ratio of the fibers has some effect on the tendency to clump when mixing. Shorter fibers disperse a bit better than longer fibers for metal fibers. Polypropylene fibers are usually relatively short and have a different "first crack" affect than steel fiber. Steel fiber typically provides better property enhancement than poly fibers.

 

[dik]

Ron... if you ever try to stick a spade into a fibre mix, there is a huge difference. I've used steel fibres structurally, not polyiprooylene.


Dik

 

[Ron]

p_kefa....your reply was posted but it was also red flagged for deletion because promotion of products, books or services is not allowed on the site.

 

[dik]

I often spec specific items rather than generic stuff... 'with an app'd eq' or whatever.

Dik

 

[Ron]

@dik....I agree. Back in the early 80's I was a member of ACI Committee 544 on fiber. At that time it was mostly steel fiber with some poly, but now poly is dominant.

I did some work with research on short fibers (3/4"), long straight fibers and long deformed fibers. Also did research with steel fiber in hot mix asphalt (worked great but never caught on).

Moving any of them with a shovel is difficult, depending on dosage. At 1% or more, long fibers were almost impossible to penetrate.

 

[concretemasonry]

Has anyone considered the effects of fiber/fibre on freeze/thaw resistance of concrete?

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.