The 4 C’s of fibers – Configuration, Chemistry, Contents, and Correct length
The most important criterion is the fiber configuration. Common sense would suggest that something deformed or irregular in shape will anchor much better than something straight and smooth. A thin, smooth finishing nail doesn’t hold like a heavy lag bolt. For the same reason, fibrillated network and macro fibers anchor better in concrete than smooth monofilament fibers. The fiber’s ability to anchor also determines its ability to contribute to short-term and long-term concrete durability. If the fiber’s objective on a project is simply to control plastic shrinkage cracking during the very early concrete stages, a monofilament configuration will be sufficient. For additional anchorage benefits, choose a fibrillated network or macro configuration to maximize the long-term durability results.
The chemical make-up of the fiber is important if the fiber is expected to hold up in the aggressive alkali environment of Portland cement concrete. For monofilament fibers, the buyer can choose nylon, which possess high strength and good resistance to alkali, or polypropylene, which combines strength with an excellent (inert) resistance to alkali and chemical attack. In addition, polypropylene is non-absorptive, which makes it an excellent choice for freeze-thaw environments and better anchoring power. For fibrillated or macro fibers, only polypropylene is suitable to the fibrillation or network manufacturing process.
Though it sounds obvious, using a sufficient quantity of fiber is an often overlooked factor. Even the best fiber in the world will fall short on performance if enough is not used to get the job done. After extensive FORTA® research, it became apparent that there is an optimum dosage level for a particular fiber type to achieve optimum results. For polypropylene or nylon monofilament fibers to reduce early shrinkage cracking, 1.0 lb. per cubic yard (0.6 kg per cubic meter) is sufficient. For fibrillated polypropylene fibers to act as a true temperature reinforcement, 1.5 to l .6 lbs. per cubic yard (0.9 to l .0 kg per cubic meters) is the standard. Even higher dosages of synthetic macro fibers can offer additional long-term benefits and performance.
Length is very similar to configuration with regards to fiber’s ability to anchor. If you try to break a short string held between your fingers, your fingers most likely slip off, while adding length to the string allows for better grip. Likewise, if a short fiber pulls out of the concrete before it breaks, the high tensile strength of the fiber has been wasted. Fiber length recommendations vary based on the configuration that is chosen. The optimum length for the monofilament fibers is typically in the ¾” (19 mm) range. For standard fibrillated fibers, lengths range from ¾” (19 mm) up to 1 ½” (38 mm). Even longer lengths, up to 2 ½” (60 mm), may be specified for macro fibers if the fiber bundles are pre-twisted during manufacturing.
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