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RE: Fibermesh questions

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Gail,
 
Thanks for taking the time for this nice post.  It is very informative.
 
Jim Persing, PE
-----Original Message-----
From: GSKWY(--nospam--at)aol.com [mailto:GSKWY(--nospam--at)aol.com]
Sent: Saturday, October 04, 2003 4:37 PM
To: Seaint(--nospam--at)seaint.org
Subject: Fibermesh questions

Although not a direct answer to the post, here are some comments on fibers.  The comments are in no particular order and there may be some bias since I do work for CRSI.

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There is a lot of confusion about the use of fibers; much of the confusion has actually been created by the fiber manufacturers.  There are three main fiber suppliers:  Synthetic Industries, Forta, and Nycon.  I think Forta was the first in the industry and someone left Forta to form Synthetic Industries but I may have that backwards.  Fibermesh is a trademark of Synthetic Industries and is their flagship product, as it were.  Somewhere along the line Synthetic Industries acquired Novacon, a steel fiber company, so now they sell both.   I am not sure where Nycon came from, it may be a textile company spinoff, but Nycon sells nylon fibers as opposed to Synthetic Industries and Forta which sell polypropylene fibers.  

So the first issue is that Nycon says nylon fibers are better than polypropylene; Forta and Synthetic Industries say polypropylene fibers are better.  

Fibers can also either be fibrillated or nonfibrillated.  Which is best depends on who you are talking to.  Polypropylene fibers can be either fibrillated or monofilament.  Fibrillated fibers are made from material that is extruded as sheets, then stretched and slit, producing interconnected fibers that open during concrete mixing.  Nylon fibers are always monofilament; monofilament fibers have less surface areas than fibrillated fibers, which may make them less effective for shrinkage control. Polypropylene  fibers are hydrophobic, however, which means that they repel water. Nylon is hydrophilic, meaning that it attracts water.  As a result, polypropylene fibers just have a mechanical bond while nylon has both a mechanical and chemical bond. Depending on who you listen to though, the hydrophobic nature of nylon may have a long term negative effect on the mechanical bond.

Fiber length and aspect (length to diameter) ratio also vary.  Most synthetic fibers are  fairly short  - 1/2 to 1 in.  About a year ago, Forta introduced a synthetic fiber called Ferro that they are marketing as a "structural fiber reinforcing".  The product is described as:
         "a blend of heavy-duty, nonfibrilating copolymer monofilament and finishable fibrillated polypropylene network bundles."   
Whatever that means.  The fibers are 2-1/4 in. long and are used at dosage rates of 4 - 30 lb/yd, versus the typical synthetic fiber dosage of 1.5 lb/yd.  The fibers are tinted grey to make them less visible; otherwise your concrete would look like it was full of cat hair.

W.R. Grace has weighed in with a synthetic (polymeric) product called STRUX 90/40 that they say is a replacement for steel fibers or steel mesh.  They also note that it "enhances safety during installation by eliminating the risk for potential injury caused by handling and placement difficulties commonly associated with steel fibers or welded wire fabrics."   The 90 is the aspect ratio, the 40 is the length in millimeters (1.55 in.)  The advertised dosage rate is up to 11 lbs/yd.  I don't think their marketing material says what the material is, it just says something like it is a "polymer blend."  My (uniformed) guess is that it may be part aramid because the elastic modulus is listed as 1378 ksi;   nylon, polyethylene and polypropylene have elastic moduli on the order of 500 to 750 ksi whereas aramid can have an elastic modulus of 9000 ksi.

Honeywell sells a nylon fiber.  How they got from thermostats to fibers is not clear but I imagine they will find that fibers are essential for both Homeland Security and rebuilding Iraq.

One of the comments you hear most often from people not involved in marketing fibers is that most synthetic fibers have very high tensile strength but a low modulus of elasticity.  Hence, their most positive contribution to concrete is during the early stages of strength development.  Their high aspect ratio allows them to control plastic shrinkage cracking and reduce segregation of the concrete, I guess because the aggregate can't settle as much.  They don't do much for hardened concrete though; in other words they don't do much for drying shrinkage cracking or temperature stresses.   One of the conditions of use in the Fibermesh ICBO report (ER-5675) is that for reinforced concrete, structural reinforcement and shrinkage and temperature reinforcement required in Section 1907.12 of the UBC be provided.  I don't see that a fiber like Strux 90/40 with a modulus of 1378 ksi can do  much in hardened concrete, either.  

As far as I can see, there is not a whole lot of strong research data supporting any manufacturer's position.  My understanding (which has mainly been acquired while standing in the hors d'oevre line during trade-show receptions) is that a manufacturer does a test slab,  takes a core and has someone do a petrographic exam.  The petrographer writes a report that says something like "the concrete does not seem to have any microcracking" and the manufacturer comes out with an advertising piece that says "our product prevents microcracking".

Some manufacturers also note that synthetic fibers "enhance the fire resistance of concrete", which is an impressive display of someone's creativity.  
Fire resistance (defined in ASTM E 176, Standard Terminology of Fire Standards) is a descriptive term that refers to the ability of a material or assembly to withstand fire or give protection from it.  Fire resistance of building elements is typically characterized by the ability to confine a fire and/or continue to perform a given structural function.

Synthetic fibers don't really do much for fire resistance,  what they do is melt when exposed to high temperatures.  The voids they leave create mini "relief chambers" that accommodate the expansion of gases at high temperatures and prevent the concrete from spalling as much.    I think they are required in some of the European codes for this reason but I don't think this is a structural issue.

Steel fibers have been shown to have various beneficial properties in slabs on ground however they drive the cost of the slab up significantly.  In order to make them economically attractive, some of the advertising has said that you can reduce the slab thickness.  I think this has lead to some problems.

Gail Kelley