Forta Corp.
Getting fibers properly mixed, especially into very low slump ba…
We’ve used fibers for more than 35 years to improve in-place properties but, contrary to popular belief, fibers are not all the same. Some have created problems—pains—for concrete producers and contractors. To explore the benefits and challenges of fiber-reinforced concrete, Concrete Construction is launching a column we’re calling Fiber Pains. I’ll listen to engineers, producers, and contractors describe their experiences and see if we can’t come up with some treatments to ease the pain.
To start, here are 10 sources of fiber pain that may sound familiar to you—or not. They’re based on conversations I’ve had with Dan Biddle at Forta Corp. of Grove City, Pa., arguably the original U.S. fiber manufacturer and a company that does much more than sell a product; Forta provides lifetime support, as do all of the other quality manufacturers. These pain points are in no particular order, and the importance of each will likely differ among the parties involved, depending on which end of the fiber use effects them the most. Tell me about your fiber pains at wpalmer@hanleywood.com or 773-494-4619. I welcome your insights.
Crack control. This may be the single most important aspect—after all, fibers are intended to control either or both plastic and hardened concrete cracking. If a particular fiber doesn’t reduce the cracking as anticipated, the pain arrives pretty quickly.
Chemistry. What the fiber is made of, its shape, even its color, can lend itself to a variety of pain sources.
Whether it’s steel, polypropylene, polyester, nylon, cellulose, or even natural, fiber makeup determines whether it has issues with corrosion, absorption, safety in use, adding air to the concrete. Each brings its own level of potential pain.
Lab performance versus user-friendly. Fibers can perform very well in laboratory testing but be extremely unfriendly in the field. Both aspects are important for a well-balanced experience.
Addition time and process. Different fibers have different protocols with regard to their optimum point addition to the concrete, and many fibers require specific time span requirements for that addition process. A fiber that must be sprinkled in or added in a “chicken feed” manner causes pain for the concrete producer. This is particularly critical for the high-speed, rapid mixing systems for paving applications, especially at high fiber dosages.
For instance, some plants require 40 pounds to 50 pounds of synthetic fiber to be added within a 20- to 30- second loading cycle, which is often impossible for a “trickle-in” fiber without causing balling or clumping problems.
Mixing time. Different than addition time, mixing time for a particular fiber to achieve uniform distribution varies widely by fiber type. Again, this is particularly critical in the short-cycle paving mixers, many of which have a mixing period of 60 seconds or less. That might not be nearly long enough for some fibers.
Uniform mixing without balling/clumping. A fiber’s tendency to ball and clump can obviously be a huge issue for many aspects of the placement, such as pumping and finishing. A uniform fiber distribution throughout the concrete is critical to performance and crack control, and fiber balls can be more than just a jobsite nuisance.
For every fiber ball found and thrown out of the slab, the ultimate fiber dosage is reduced across the board, and balls hidden within the slab present a future potential for soft spots and failure.
Pumping: line pressure. Pump pressures vary depending on fiber type, shape, and dosage. Fibers that require much higher pressures present obvious challenges on the project. A well-distributed fiber can actually reduce pressures to some degree, largely due to the more uniform nature of the resulting concrete mix caused by the fiber suspension of fine and coarse aggregates.
Pumping: grate flow. Fibers can complicate concrete flow through a conventional slatted pump grate, especially at elevated dosages. Making the fibers shorter or stiffer could help with grate flow, but post-crack performance will suffer considerably. Depending on fiber type and dosage, simply changing to a different grate shape can solve many pumping issues.
Mixing at high dosages. Many fiber types and shapes mix well at low dosages, from 1 pound per cubic yard up to 4 per cubic yard, yet struggle mightily to mix well at the higher dosages required on many extended-joint floors and joint-free mezzanines. A fiber’s inability to mix well at high dosages doesn’t make the need for the high-dosage performance go away.
Some fibers must resort to staying at the lower dosages, with a recommendation to substitute the missing dosage with chemical additives to help reduce shrinkage. Fibers and admixtures work in different ways – one mechanically, one chemically—and long-term durability and crack control may not be the same for both processes.
Surface finish. Many fibers produce whiskery or hairy surface finishes, primarily due to fiber shape, chemistry, color, length, and dosage.
For some applications, aesthetics aren’t important, and surface fibers will wear off given time and use. However, owners are becoming more sensitive to a fiber’s surface visibility for applications where cleanability (for food environments) or rideability (for robotic warehouses) are critical.
Naturally, a fiber’s finish can be improved by making them shorter and finer with lower dosages; but again, crack-controlling performance will suffer commensurately.
Recognize any of these? If so, let me know and we’ll tell your story in a future issue: wpalmer@hanleywood.com or 773-494-4619.
Pain points are adapted from a list compiled by Forta Corp.’s Dan Biddle.
