With a coarse aggregate grind, fibers disappear, though there may be few fiber ends that are not visible to the naked eye.
David Olsen Photography With a coarse aggregate grind, fibers disappear, though there may be few fiber ends that are not visible to the naked eye.

Question: We are placing an interior floor that is to be colored and polished. We would like to use fibers in the mix to eliminate cracking but the architect is afraid that the fibers will show in the surface after polishing or will be “fuzzy.” Is that something to worry about?

Answer: We asked Dan Biddle, vice president of sales at Forta Corp., about this. “As is typical in the synthetic fiber business,” he says, “the answer to this question is a resounding….it depends! For microfibers, it depends on the type to some degree — for low-dose monofilaments, there should be little fiber evidence. Microfibrillated fibers can sometimes fuzz, depending on the level of polish. For low- to high-dose macros, it depends on the level of grind and polish. Two of the three polish depths are generally fine, while a middle grind can leave “worm” imprints.

Here are the three polishing levels to consider:

High-polish only: This level would represent a polishing or burnishing of just the very top skin layer of the concrete with non-aggressive resin-bond attachments to add sheen and light-reflectivity to the floor. This process would involve removing only a very thin amount of the floor skin, less than 1/64 inch. With the proper cream-inducing finishing practice (that is, using a roller bug), fibers that may have been at or near the surface would have been covered with paste and therefore not revealed by the minimum grind. There would be very little evidence, if any, of fiber appearance on the surface.

Salt-and-pepper grind: This level would be a grind into a deeper paste depth to expose some of the fine aggregates (sand) used in the mix for a salt-and-pepper reveal. This depth is about 1/32 to 1/16 inch for most applications, and digs into the surface paste area where the fibers are hiding. Because of the finishing screed process that lays surface fibers over primarily into a horizontal plane, the fibers revealed appear as irregular “worms” in the surface, similar to a textured stucco look. Though surface fibers are visible, the pattern is quite uniform and appealing to many floor owners.

Coarse-aggregate grind: This level requires grinding through the skin and paste layers and into a portion of the coarse aggregates near the surface. This level is one of the most aesthetically stunning and pleasing of all polished concrete, and one of the most popular decorative concrete looks as a result. In this grind depth for an aggregate reveal, typically in the range of 1/8 inch, evidence of horizontal or otherwise situated fibers disappear, though a microscopic inspection might show a few fiber ends that are not visible to the naked eye.

The actual grind or polish depths mentioned are somewhat arbitrary and subject to each project’s mix design, paste content, and finishing methods. The correct depths will be those that result in the appearance that the client prefers: surface polish only, sand exposure, or aggregate reveal. In any case, the value of preproject finishing and grinding trials is immeasurable.

The information in this Problem Clinic is excerpted from the Forta Corp. Technical Report: Fiber-Reinforced Decorative Concrete. The full report is available at http://info.forta-ferro.com/literature.