Plastic fibres can improve some of the properties of concrete, but do not act as reinforcement. The marketing of these products as reinforcement is fraudulent.
this subject has been discussed several times before, you can find much debate on the subject by doing a search - see the google search bar at the top of the page. Here are a couple of threads discussing the topic:
I just read an article today at home about the use of steel fibers to replace reinforcing steel in a precast concrete tunnel liner. I will try to bring the article to work tomorrow. Although their research and testing showed they could replace all of the reinforcing, they chose to replace only a significant percentage of the steel. Stay tuned.
tunnel liners are generally loaded in compression. concrete is incredibly strong in compression, hence the need for light reinforcement. Finishing and curing under controlled conditions reduces the amount of cracking.
In contrast, slabs on grade may be subjected to high bending and shear stress which may require reinforcement and certainly requires attention to the subgrade to provide adequate resistance to the stress. Placement and curing are not done in the controlled conditions like precast elements, so measures to control cracking should be used.
I agree about tunnel liners and compression. However, there are at least two sides to every story. There may be as many success stories for fibers in tension as there are horror stories. I suggest getting information from fiber proponents as well as from opponents. Then make an intelligent decision based on the intended application.
See attached fiber story I mentioned in my previous post.
I agree, I have nothing against fiber "reinforcement" when used properly. I have used fibers in non-reinforced slabs and both WWF and rebar in reinforced slabs with success on all accounts.
LMG66,
My comment about the fraudulent marketing of plastic fibres does not apply in the case of steel fibres. Steel fibres, as referenced in PEinc's article, have many benefits, some only now being utilised fully, as in the Spanish tunnel liners.
I would be very careful using steel fibers in an exposed concrete surface such as a slab. If the concrete is not finished properly, the fibers can protrude from the concrete and make the surface very prickly. Someone can get hurt.
In the 1980's I was a member of ACI Committee 544 on "Fiber Reinforced Concrete". I always objected to the characterization of fibers as being "reinforcement" in the classical sense and felt that it led to confusion. It does.
The arguments back then in the formative years of the ACI recommended practice were three-fold. There were the ductile steel fiber producers (U.S. Steel/Mitchell Fibercon), the stiff steel fiber producers (Dramix, etc.), and the glass/poly fiber producers. Glass dropped out rather quickly for general commercial use. More specialized.
Each touted its own benefits. For the poly fibers, it was the number of fibers (many, many thousands). For the ductile steel, it was strain compatibility and finishing. For the stiff steel it was strength and bond.
As hokie66 noted, fibers enhance some of the properties of concrete, but do not provide mass tensile reinforcement as would rebar or wire mesh. For instance, most of my experience (though I've used each type)has been with ductile steel fibers in concrete and asphalt, and I have seen the compressive strength and modulus of rupture of the steel fiber modified concrete increase by as much as 50 percent. About a 10 to 20 percent increase was common for both properties at the recommended "dosage" rates. Increases in strength for poly fiber concrete are gained, but not as much. These strength increases may reduce the required steel in the same manner that increasing the concrete strength by any means would do, but they don't replace the tension steel.
Crack propagation properties are changed quite a bit by fiber enhancement. Steel fibers tend to reduce cracking. Poly fibers tend to increase the space between cracks but make the cracks wider.
You can replace wire mesh in a slab on grade with fiber if you choose...since as a practical matter, you don't really need either. (OK>>>here come the arguments!). DO NOT EVER replace tension steel in a flexural member with fiber. That's patently dangerous.
Ron,
To follow up on your last paragraph, I don't specify wire mesh for slabs on grade or pavements. I do specify plastic fiber. I have demoed a few slabs and observed the construction of a few other. The wire mesh is always at the bottom of the slab, where it does little good.
The most important factor in preventing drying shrinkage cracking is timely sawing of properly spaced contraction joints. I believe the plastic fiber is enough help in preventing cracking to justify its cost and will give you a little more leeway in timing of sawcutting.
I don't understand why wire mesh is used in structural applications. I would never trust the workers to get the mesh right, whereas rebar is easier to place, inspect, and will remain in place during a pour. Plus, workers seem to take rebar installation more seriously than wire mesh in general.
jgailla...I agree that wire mesh is essentially useless. One problem that is prevalent with poly fibers is that they are sold to contractors as the cure for all ills, so they take liberties with concrete placement that they should not take, assuming the fiber will save them.
Even if you use fiber, specify the timing and location of sawcutting and make the contractor adhere to it. Late sawcutting won't be mitigated by fiber.
Heard about your new business venture...good luck.
