This type of polyester exhibits physical and chemical

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This type of polyester exhibits physical and chemical char- acteristics that depend on manufacturing techniques. Select- ed fiber properties are shown in Table 4.1 . One of several techniques involves the production of highly crystalline pel- lets, which are converted to filaments in a melt extraction process and subsequently stretched approximately 400 per- cent before cutting to desired length. All thermoplastics are temperature sensitive. At tempera- tures above normal concrete service temperatures, fiber characteristics are altered. Temperatures above 536 F (280 C) cause molecular breakdown [4.20]. Polyester fibers are somewhat hydrophobic (do not absorb much water) and have been shown not to affect the hydration of the portland cement concrete [4.24]. Bonding of polyester fibers within the cement matrix is mechanical. 544.1R-41 FIBER REINFORCED CONCRETE
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There is no consensus on the long-term durability of poly- ester fibers in portland cement concrete. 4.2.6— Polyethylene Polyethylene has been produced for use as concrete rein- forcement [4.25] in monofilament form with wart-like sur- face deformations along the length of the fiber. These deformations are intended to improve the mechanical bond- ing in cement paste and mortar. Selected fiber properties are shown in Table 4.1 . It has been reported that polyethylene fibers could be eas- ily dispersed in concrete mixtures in volume percentages of up to 4 percent using conventional mixing techniques [4.26] Polyethylene in pulp form has also been applied in con- crete mixtures. In this application the pulp, a fine irregular form of fiber, acts to retain cement fines by acting as filter fibers [4.27-4.29] and its use is intended as an alternate to the use of asbestos fibers. 4.2.7— Polypropylene Monofilament form fibers are produced in an extrusion process in which the material is hot drawn through a die of circular cross section, generating a number of continuous fil- aments at one time called a tow. Fibrillated polypropylene fibers are the product of an ex- trusion process where the die is rectangular. The resulting film sheets of polypropylene are slit longitudinally into equal width tapes. To achieve a lattice pattern, the tape is mechan- ically distressed or fibrillated with a patterned pin wheel or split film technique to produce the main and cross fibril net- works. In some cases, the fibrillated tape is twisted prior to cutting to enhance the opening of the bundle. Fibers thus produced are termed collated, fibrillated polypropylene and are cut to desired lengths [4.14, 4.30]. Selected properties of polypropylene fibers are shown in Table 4.1 . Polypropylene is hydrophobic, meaning it does not absorb water. Polypropylene fibers are not expected to bond chemically in a concrete matrix, but bonding has been shown to occur by mechanical interaction [4.31]. Polypropy- lene fibers are produced from homopolymer polypropylene resin. The melting point and elastic modulus, which are low relative to many other fiber types, may be limitations in cer- tain processes such as autoclaving [4.32]. However, refrac-
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