Chap7 - HOME PAGE CHAPTER 7 Fibers Fibers have been used in...

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Unformatted text preview: HOME PAGE CHAPTER 7 Fibers Fibers have been used in construction materials for many centuries. The last three decades have seen a growing interest in the use of fibers in ready-mixed concrete, precast concrete, and shotcrete. Fibers made from steel, plastic, glass, and natural materials (such as wood cellulose) are available in a variety of shapes, sizes, and thicknesses; they may be round, flat, crimped, and deformed with typical lengths of 6 mm to 150 mm (0.25 in. to 6 in.) and thicknesses ranging from 0.005 mm to 0.75 mm (0.0002 in. to 0.03 in.) (Fig. 7-1). They are added to concrete during mixing. The main factors that control the performance of the composite material are: 1. Physical properties of fibers and matrix 2. Strength of bond between fibers and matrix Although the basic governing principles are the same, there are several characteristic differences between conventional reinforcement and fiber systems: 1. Fibers are generally distributed throughout a given cross section whereas reinforcing bars or wires are placed only where required 2. Most fibers are relatively short and closely spaced as compared with continuous reinforcing bars or wires 3. It is generally not possible to achieve the same area of reinforcement to area of concrete using fibers as compared to using a network of reinforcing bars or wires Fibers are typically added to concrete in low volume dosages (often less than 1%), and have been shown to be effective in reducing plastic shrinkage cracking. Fibers typically do not significantly alter free shrinkage of concrete, however at high enough dosages they can increase the resistance to cracking and decrease crack width (Shah, Weiss, and Yang 1998). ADVANTAGES AND DISADVANTAGES OF USING FIBERS Fibers are generally distributed throughout the concrete cross section. Therefore, many fibers are inefficiently located for resisting tensile stresses resulting from applied loads. Depending on fabrication method, random orientation of fibers may be either two-dimensional (2-D) or three-dimensional (3-D). Typically, the spray-up fabrication method has a 2-D random fiber orientation where as the premix (or batch) fabrication method typically has a 3-D random fiber orientation. Also, many fibers are observed to extend across cracks at angles other than 90 or may have less than the required embedment length for development of adequate bond. Therefore, only a small percentage of the fiber content may be efficient in resisting tensile or flexural stresses. "Efficiency factors" can be as low as 0.4 for 2-D random orientation and 0.25 for 3-D random orientation. The efficiency factor depends on fiber length and critical embedment length. From a conceptual point of view, reinforcing with fibers is not a highly efficient method of obtaining composite strength. Fiber concretes are best suited for thin section shapes where correct placement of conventional reinforcement would be extremely difficult. In addition, spraying of fiber concrete accommodates the fabrication of irregularly shaped products. A substantial weight savings can be realized using relatively thin fiber concrete sections having 121 Fig. 7-1. Steel, glass, synthetic and natural fibers with different lengths and shapes can be used in concrete. (69965) Design and Control of Concrete Mixtures N EB001 Steel fibers do not affect free shrinkage. Steel fibers delay the fracture of restrained concrete during shrinkage and they improve stress relaxation by creep mechanisms (Altoubat and Lange 2001). The durability of steel-fiber concrete is contingent on the same factors as conventional concrete. Freeze-thaw durability is not diminished by the addition of steel fibers provided the mix is adjusted to accommodate the fibers, the concrete is properly consolidated during placement, and is air-entrained. With properly designed and placed concrete, little or no corrosion of the fibers occurs. Any surface corrosion of fibers is cosmetic as opposed to a structural condition. Steel fibers have a relatively high modulus of elasticity (Table 7-1). Their bond to the cement matrix can be enhanced by mechanical anchorage or surface roughness and they are protected from corrosion by the alkaline environment in the cement matrix (ACI 544.1R-96). Steel fibers are most commonly used in airport pavements and runway/taxi overlays. They are also used in bridge decks (Fig. 7-3), industrial floors, and highway pavements. Structures exposed to high-velocity water flow have been shown to last about three times longer than conventional concrete alternatives. Steel fiber concrete is also used for many precast concrete applications that make use of the improved impact resistance or toughness imparted by the fibers. In utility boxes and septic tanks, steel fibers replace conventional reinforcement. Steel fibers are also widely used with shotcrete in thin-layer applications, especially rock-slope stabilization and tunnel linings. Silica fume and accelerators have enabled shotcrete to be placed in thicker layers. Silica fume also reduces the permeability of the shotcrete material (Morgan 1987). Steel-fiber shotcrete has been successfully applied with fiber volumes up to 2%. Slurry-infiltrated concrete (SIFCON) with fiber volumes up to 20% has been used since the late 1970s. Slurry- the equivalent strength of thicker conventionally reinforced concrete sections. TYPES AND PROPERTIES OF FIBERS AND THEIR EFFECT ON CONCRETE Steel Fibers Steel fibers are short, discrete lengths of steel with an aspect ratio (ratio of length to diameter) from about 20 to 100, and with any of several cross sections. Some steel fibers have hooked ends to improve resistance to pullout from a cement-based matrix (Fig. 7-2). ASTM A 820 classifies four different types based on their manufacture. Type I Cold-drawn wire fibers are the most commercially available, manufactured from drawn steel wire. Type II Cut sheet fibers are manufactured as the name implies: steel fibers are laterally sheared off steel sheets. Type III Melt-extracted fibers are manufactured with a relatively complicated technique where a rotating wheel is used to lift liquid metal from a molten metal surface by capillary action. The extracted molten metal is then rapidly frozen into fibers and thrown off the wheel by centrifugal force. The resulting fibers have a crescent-shaped cross section. Type IV Other fibers. For tolerances for length, diameter, and aspect ratio, as well as minimum tensile strength and bending requirement, see ASTM A 820. Steel-fiber volumes used in concrete typically range from 0.25% to 2%. Volumes of more than 2% generally reduce workability and fiber dispersion and require special mix design or concrete placement techniques. The compressive strength of concrete is only slightly affected by the presence of fibers. The addition of 1.5% by volume of steel fibers can increase the direct tensile strength by up to 40% and the flexural strength up to 150%. Fig. 7-2. Steel fibers with hooked ends are collated into bundles to facilitate handling and mixing. During mixing the bundles separate into individual fibers. (69992) 122 Fig. 7-3. Bridge deck with steel fibers. (70007) Chapter 7 N Fibers Table 7-1. Properties of Selected Fiber Types Relative density (specific gravity) 7.80 Diameter, m (0.001 in.) 100-1000 (4-40) 8-15 (0.3-0.6) 12-20 (0.5-0.8) 5-17 (0.2-0.7) 10-12 (0.4-0.47) 8-0 (0.3-0.35) 23 (0.9) 10-80 (0.4-3.0) 25-1000 (1-40) 20-200 (0.8-8) 25-125 (1-5) Tensile strength, MPa (ksi) 500-2600 (70-380) 2000-4000 (290-580) 1500-3700 (220-540) 200-1000 (30-145) 2000-3100 (300-450) 1800-2600 (260-380) 1000 (140) 280-1200 (40-170) 80-600 (11-85) 450-700 (65-100) 350-2000 (51-290) 280-600 (40-85) 1.12-1.15 1.50 1.02-1.04 100-400 (4-16) 50-400 (2-16) 100-200 (4-8) 425 (17) 120-200 (17-29) 350-500 (51-73) 250-350 (36-51) 180 (26) Modulus of elasticity, MPa (ksi) 210,000 (30,000) 72,000 (10,400) 80,000 (11,600) 17,000-19,000 (2,500-2,800) 62,000-120,000 (9,000-17,000) 230,000-380,000 (33,400-55,100) 5,200 (750) 10,000-18,000 (1,500-2,500) 5,000 (725) 3,500-5,200 (500-750) 10,000-40,000 (1,500-5,800) 13,000-25,000 (1,900-3,800) 19,000-25,000 (2,800-3,800) 33,000-40,000 (4,800-5,800) 25,000-32,000 (3,800-4,600) 4,900 (710) 1.5-1.9 3.6 3.5 10-25 Strain at failure, % 0.5-3.5 Fiber type Steel Glass E AR Synthetic Acrylic Aramid Carbon Nylon Polyester Polyethylene Polypropylene Natural Wood cellulose Sisal Coconut Bamboo Jute Elephant grass 2.54 2.70 3.0-4.8 2.5-3.6 1.18 1.44 1.90 1.14 1.38 0.96 0.90 28-50 2-3.5 0.5-1.5 20 10-50 12-100 6-15 1.50 Adapted from PCA (1991) and ACI 544.1R-96. infiltrated concrete can be used to produce a component or structure with strength and ductility that far exceeds that of conventionally mixed or sprayed fiber concrete. SIFCON is not inexpensive and needs fine-tuning, but it holds potential for applications exposed to severe conditions and requiring very high strength and toughness. These applications include impact and blast-resistant structures, refractories, protective revetment, and taxiway and pavement repairs (Fig. 7-4). Table 7-2 shows a SIFCON mix design. 123 Table 7-2. SIFCON Mix Design. Cement Water Siliceous Sand 0.7 mm ( 0.028 in.) Silica Slurry High-Range Water Reducer Steel Fibers (about 10 Vol.-%) 1000 kg/m3 (1686 lb/yd3) 330 kg/m3 (556 lb/yd3) 860 kg/m3 (1450 lb/yd3) 13 kg/m3 (1.3 lb/yd3) 35 kg/m3 (3.7 lb/yd3) 800 kg/m3 (84 lb/yd3) Design and Control of Concrete Mixtures N EB001 Fiber modifications to improve long-term durability involve (1) specially formulated chemical coatings to help combat hydration-induced embrittlement, and (2) employment of a dispersed microsilica slurry to adequately fill fiber voids, thereby reducing potential for calcium hydroxide infiltration. A low-alkaline cement has been developed in Japan that produces no calcium hydroxide during hydration. Accelerated tests with the cement in alkali-resistant-glass fiber-reinforced concrete samples have shown greater long-term durability than previously achieved. Metakaolin can be used in glass-fiber-reinforced concrete without significantly affecting flexural strength, strain, modulus of elasticity, and toughness. (Marikunte, Aldea, Shah 1997). The single largest application of glass-fiber concrete has been the manufacture of exterior building faade panels (Fig. 7-5). Other applications are listed in PCA (1991). Fig. 7-4. Tightly bunched steel fibers are placed in a form, before cement slurry is poured into this application of slurry-infiltrated steel-fiber concrete (SIFCON). (60672) Glass Fibers The first research on glass fibers in the early 1960s used conventional borosilicate glass (E-glass) (Table 7-1) and soda-lime-silica glass fibers (A-glass). The test results showed that alkali reactivity between the E-glass fibers and the cement-paste reduced the strength of the concrete. Continued research resulted in alkali-resistant glass fibers (AR-glass) (Table 7-1), that improved long-term durability, but sources of other strength-loss trends were observed. One acknowledged source was fiber embrittlement stemming from infiltration of calcium hydroxide particles, byproducts of cement hydration, into fiber bundles. Alkali reactivity and cement hydration are the basis for the following two widely held theories explaining strength and ductility loss, particularly in exterior glass fiber concrete: Alkali attack on glass-fiber surfaces reduces fiber tensile strength and, subsequently, lowers compressive strength. Ongoing cement hydration causes calcium hydroxide particle penetration of fiber bundles, thereby increasing fiber-to-matrix bond strength and embrittlement; the latter lowers tensile strength by inhibiting fiber pullout. 124 Fig. 7-5. (top) Glass-fiber-reinforced concrete panels are light and strong enough to reduce this building's structural requirements. (bottom) Spray-up fabrication made it easy to create their contoured profiles. (60671, 46228) Chapter 7 N Fibers Synthetic Fibers Synthetic fibers are man-made fibers resulting from research and development in the petrochemical and textile industries. Fiber types that are used in portland cement concrete are: acrylic, aramid, carbon, nylon, polyester, polyethylene, and polypropylene. Table 7-1 summarizes the range of physical properties of these fibers. Synthetic fibers can reduce plastic shrinkage and subsidence cracking and may help concrete after it is fractured. Ultra-thin whitetopping often uses synthetic fibers for potential containment properties to delay pothole development. Problems associated with synthetic fibers include: (1) low fiber-to-matrix bonding; (2) inconclusive performance testing for low fiber-volume usage with polypropylene, polyethylene, polyester and nylon; (3) a low modulus of elasticity for polypropylene and polyethylene; and (4) the high cost of carbon and aramid fibers. Polypropylene fibers (Fig. 7-6), the most popular of the synthetics, are chemically inert, hydrophobic, and lightweight. They are produced as continuous cylindrical monofilaments that can be chopped to specified lengths or cut as films and tapes and formed into fine fibrils of rectangular cross section (Fig. 7-7). Used at a rate of at least 0.1 percent by volume of concrete, polypropylene fibers reduce plastic shrinkage cracking and subsidence cracking over steel reinforcement (Suprenant and Malisch 1999). The presence of polypropylene fibers in concrete may reduce settlement of aggregate particles, thus reducing capillary bleed channels. Polypropylene fibers can help reduce spalling of highstrength, low-permeability concrete exposed to fire in a moist condition. New developments show that monofilament fibers are able to fibrillate during mixing if produced with both, polypropylene and polyethylene resins. The two poly- Fig. 7-7. Polypropylene fibers are produced either as (left) fine fibrils with rectangular cross section or (right) cylindrical monofilament. (69993) Fig. 7-6. Polypropylene fibers. (69796) 125 mers are incompatible and tend to separate when manipulated. Therefore, during the mixing process each fiber turns into a unit with several fibrils at its end. The fibrils provide better mechanical bonding than conventional monofilaments. The high number of fine fibrils also reduces plastic shrinkage cracking and may increase the ductility and toughness of the concrete (Trottier and Mahoney 2001). Acrylic fibers have been found to be the most promising replacement for asbestos fibers. They are used in cement board and roof-shingle production, where fiber volumes of up to 3% can produce a composite with mechanical properties similar to that of an asbestoscement composite. Acrylic-fiber concrete composites exhibit high postcracking toughness and ductility. Although lower than that of asbestos-cement composites, acrylic-fiber-reinforced concrete's flexural strength is ample for many building applications. Aramid fibers have high tensile strength and a high tensile modulus. Aramid fibers are two and a half times as strong as E-glass fibers and five times as strong as steel fibers. A comparison of mechanical properties of different aramid fibers is provided in PCA (1991). In addition to excellent strength characteristics, aramid fibers also have excellent strength retention up to 160C (320F), dimensional stability up to 200C (392F), static and dynamic fatigue resistance, and creep resistance. Aramid strand is available in a wide range of diameters. Carbon fibers were developed primarily for their high strength and elastic modulus and stiffness properties for applications within the aerospace industry. Compared with most other synthetic fibers, the manufacture of carbon fibers is expensive and this has limited commercial development. Carbon fibers have high tensile strength and modulus of elasticity (Table 7-1). They are also inert to Design and Control of Concrete Mixtures N EB001 ufacture of low-fiber-content concrete and occasionally have been used in thin-sheet concrete with high-fiber content. For typical properties of natural fibers see Table 7-1. Unprocessed Natural Fibers. In the late 1960s, research on the engineering properties of natural fibers, and concrete made with these fibers was undertaken; the result was these fibers can be used successfully to make thin sheets for walls and roofs. Products were made with portland cement and unprocessed natural fibers such as coconut coir, sisal, bamboo, jute, wood, and vegetable fibers. Although the concretes made with unprocessed natural fibers show good mechanical properties, they have some deficiencies in durability. Many of the natural fibers are highly susceptible to volume changes due to variations in fiber moisture content. Fiber volumetric changes that accompany variations in fiber moisture content can drastically affect the bond strength between the fiber and cement matrix. Wood Fibers (Processed Natural Fibers). The properties of wood cellulose fibers are greatly influenced by the method by which the fibers are extracted and the refining processes involved. The process by which wood is reduced to a fibrous mass is called pulping. The kraft process is the one most commonly used for producing wood cellulose fibers. This process involves cooking wood chips in a solution of sodium hydroxide, sodium carbonate, and sodium sulfide. Wood cellulose fibers have relatively good mechanical properties compared to many manmade fibers such as polypropylene, polyethylene, polyester, and acrylic. Delignified cellulose fibers (lignin removed) can be produced with a tensile strength of up to approximately 2000 MPa (290 ksi) for selected grades of wood and pulping processes. Fiber tensile strength of approximately 500 MPa (73 ksi) can be routinely achieved using a chemical pulping process and the more common, less expensive grades of wood. most chemicals. Carbon fiber is typically produced in strands that may contain up to 12,000 individual filaments. The strands are commonly prespread prior to incorporation in concrete to facilitate cement matrix penetration and to maximize fiber effectiveness. Nylon fibers exist in various types in the marketplace for use in apparel, home furnishing, industrial, and textile applications. Only two types of nylon fiber are currently marketed for use in concrete, nylon 6 and nylon 66. Nylon fibers are spun from nylon polymer and transformed through extrusion, stretching, and heating to form an oriented, crystalline, fiber structure. For concrete applications, high tenacity (high tensile strength) heat and light stable yarn is spun and subsequently cut into shorter length. Nylon fibers exhibit good tenacity, toughness, and elastic recovery. Nylon is hydrophilic, with moisture retention of 4.5 percent, which increases the water demand of concrete. However, this does not affect concrete hydration or workability at low prescribed contents ranging from 0.1 to 0.2 percent by volume, but should be considered at higher fiber volume contents. This comparatively small dosage has potentially greater reinforcing value than low volumes of polypropylene or polyester fiber. Nylon is relatively inert and resistant to a wide variety of organic and inorganic materials including strong alkalis. Synthetic fibers are also used in stucco and mortar. For this use the fibers are shorter than synthetic fibers used in concrete. Usually small amounts of 13-mm (1/2-in.) long alkali-resistant fibers are added to base coat plaster mixtures. They can be used in small line stucco and mortar pumps and spray guns. They should be added to the mix in accordance with manufacturer's recommendation. For further details about chemical and physical properties of synthetic fibers and properties of synthetic fiber concrete, see ACI 544.1R-96. ASTM C 1116 classifies Steel, Glass, and Synthetic Fiber Concrete or Shotcrete. The technology of interground fiber cement takes advantage of the fact that some synthetic fibers are not destroyed or pulverized in the cement finishing mill. The fibers are mixed with dry cement during grinding where they are uniformly distributed; the surface of the fibers is roughened during grinding, which offers a better mechanical bond to the cement paste (Vondran 1995). MULTIPLE FIBER SYSTEMS For a multiple fiber system, two or more fibers are blended into one system. The hybrid-fiber concrete combines macro- and microsteel fibers. A common macrofiber blended with a newly developed microfiber, which is less than 10 mm (0.4 in.) long and less than 100 micrometer (0.004 in.) in diameter, leads to a closer fiber-to-fiber spacing, which reduces microcracking and increases tensile strength. The intended applications include thin repairs and patching (Banthia and Bindiganavile 2001). A blend of steel and polypropylene fibers has also been used for some applications. This system is supposed to combine the toughness and impact-resistance of steel fiber concrete with the reduced plastic cracking of polypropylene fiber concrete. For a project in the Chicago area (Wojtysiak and others 2001), a blend of 30 kg/m3 (50 lb/yd3) of steel fibers and 0.9 kg/m3 (11/2 lb/yd3) of fib126 Natural Fibers Natural fibers were used as a form of reinforcement long before the advent of conventional reinforced concrete. Mud bricks reinforced with straw and mortars reinforced with horsehair are just a few examples of how natural fibers were used long ago as a form of reinforcement. Many natural reinforcing materials can be obtained at low levels of cost and energy using locally available manpower and technical know-how. Such fibers are used in the man- Chapter 7 N Fibers rillated polypropylene fibers were used for slabs on grade. The concrete with blended fibers had a lower slump compared to plain concrete but seemed to have enhanced elastic and post-elastic strength. Morgan, D. R., "Evaluation of Silica Fume Shotcrete," Proceedings, CANMET/CSCE International Workshop on Silica Fume in Concrete, Montreal, May 1987. Monfore, G. E., A Review of Fiber Reinforcement of Portland Cement Paste, Mortar and Concrete, Research Department Bulletin RX226, Portland Cement Association, http://, 1968, 7 pages. PCA, Fiber Reinforced Concrete, SP039, Portland Cement Association, 1991, 54 pages. PCA, "Steel Fiber Reinforced Concrete," Concrete Technology Today, PL931 Portland Cement Association, http://, March 1993, pages 1 to 4. Panarese, William C., "Fiber: Good for the Concrete Diet?," Civil Engineering, American Society of Civil Engineers, New York, May 1992, pages 44 to 47. Shah, S. P.; Weiss, W. J.; and Yang, W., "Shrinkage Cracking Can it be prevented?," Concrete International, American Concrete Institute, Farmington Hills, Michigan, April 1998, pages 51 to 55. Suprenant, Bruce A., and Malisch, Ward R., "The fiber factor," Concrete Construction, Addison, Illinois, October 1999, pages 43 to 46. Trottier, Jean-Francois, and Mahoney, Michael, "Innovative Synthetic Fibers," Concrete International, American Concrete Institute, Farmington Hills, Michigan, June 2001, pages 23 to 28 Vondran, Gary L., "Interground Fiber Cement in the Year 2000," Emerging Technologies Symposium on Cements for the 21st Century, SP206, Portland Cement Association, March 1995, pages 116 to 134. Wojtysiak, R.; Borden, K. K.; and Harrison P., Evaluation of Fiber Reinforced Concrete for the Chicago Area A Case Study, 2001. REFERENCES ACI Committee 544, State-of-the-Art Report on Fiber Reinforced Concrete, ACI 544.1R-96, American Concrete Institute, Farmington Hills, Michigan, 1997. Altoubat, Salah A., and Lange, David A., "Creep, Shrinkage, and Cracking of Restrained Concrete at Early Age," ACI Materials Journal, American Concrete Institute, Farmington Hills, Michigan, July-August 2001, pages 323 to 331. Banthia, Nemkumar, and Bindiganavile, Vivek, "Repairing with Hybrid-Fiber-Reinforced Concrete," Concrete International, American Concrete Institute, Farmington Hills, Michigan, June 2001, pages 29 to 32. Bijen, J., "Durability of Some Glass Fiber Reinforced Cement Composites," ACI Journal, American Concrete Institute, Farmington Hills, Michigan, July-August 1983, pages 305 to 311. Hanna, Amir N., Steel Fiber Reinforced Concrete Properties and Resurfacing Applications, Research and Development Bulletin RD049, Portland Cement Association,, 1977, 18 pages. Johnston, Colin D., Fiber Reinforced Cement and Concretes, LT249, Gordon & Breach, Amsterdam, 2000, 368 pages. Marikunte, S.; Aldea, C.; and Shah, S., "Durability of Glass Fiber Reinforced Cement Composites: Effect of Silica Fume and Metakaolin," Advanced Cement Based Materials, Volume 5, Numbers 3/4, April/May 1997, pages 100 to 108. 127 ...
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