In May 2003, following several days of heavy rain in the Atlanta area, a 110-foot-tall, 3-foot-diameter oak tree fell onto the home of one of the authors, Jerry Holland, cutting the house nearly in half, just missing his wife, and damaging a small portion of a 69-foot-long concrete walkway. Since the insurance company agreed only to pay to replace that portion of the walkway that was damaged, which would have looked unattractive, Holland decided to replace the entire walkway. He took this opportunity to design and build a unique jointless, crack-free walkway.
Holland had been evaluating the new high-volume synthetic fibers (HVSF) in relation to mixability, workability, and finishability, plus short-term and long-term performance in terms of curling, linear shrinkage, and cracking. He decided to make himself a guinea pig and replace the walkway himself, with the help of some friends. To give it an exceptionally good test, he decided to construct it with no joints and no steel reinforcing.
The result has been an easily constructed, 69-foot-long, 3-foot-wide, 3½-inch-thick walkway with no joints, a 90-degree turn at one end with an unreinforced re-entrant corner, and an 80-degree curve at the other end. After more than three and a half years of shrinkage and substantial thermal contraction due to a drop in temperature of about 90° F, there are no visible cracks—even at the re-entrant corner.
Traditional synthetic fibers (such as Forta-Fiber, Fibermesh, or Microfiber) have been used at dosages of 0.5 to 1.6 lbs/yd3 for almost 30 years—this is low-volume synthetic fiber (LVSF). At these dosages, the benefits are primarily in the concrete plastic state but not significantly in the hardened state. Holland had thought for many years that there would be significant benefits to using a much higher amount of fiber in the mix; however, at higher dosages the mix could not be properly mixed, much less have sufficient workability and finishability. But more recent developments in concrete admixtures, aggregate proportioning, and research with polymer fiber geometry (moving beyond the traditional monofilament or fibrillated fibers) made him realize that using HVSF in production concrete (rather than in “lab-crete”) was possible. Although he had 30 years of design, materials, and construction experience with steel fibers and almost as much with LVSF, he felt that HVSF concrete would act in a substantially different manner than either of those materials. His theory was that adding enough of the right amount and type of fiber would fundamentally change the concrete's inherent properties and behavior. For this to be viable in production concrete, though, the problems of adequate mixability, placeability, workability, and finishability would have to be overcome.
Proportioning and mixing
Thomas Concrete of Georgia Inc. provided the concrete, and Forta Ferro fibers were furnished by the Forta Corp. Although Holland had been designing mixes for more than 35 years and proportioning special combined aggregate for more than 20 years, he wanted to use normal production ready-mixed concrete with as little fine-tuning as possible. Therefore, no special aggregate blending was used (although he did analyze the mix design to ensure there would be no potential issues of excessive shrinkage or curling), and a low amount of a normal production midrange water-reducer was added at the plant. The concrete mix included 592 lbs/yd3 of portland cement, which did not provide the additional workability/finishability that an addition of fly ash or slag would have, but did keep the water-cement ratio below what is recommended for the Atlanta area's freeze/thaw conditions. To reduce the workability benefits that entrained air can provide, the air content was kept to the lowest percentage required to provide proper freeze/thaw protection (about 4%). The fibers were added at the plant at a dosage rate of 7½ lbs/yd3. Holland and his friends observed the mixing action of the HVSF at the plant, noting that it was quite good, thus requiring no adjustments to the original mix design. They then followed the truck to the house.