Fibers have been used in construction materials for manycenturies. The last three decades have seen a growinginterest in the use of fibers in ready-mixed concrete, pre-cast concrete, and shotcrete. Fibers made from steel,plastic, glass, and natural materials (such as wood cellu-lose) are available in a variety of shapes, sizes, and thick-nesses; they may be round, flat, crimped, and deformedwith 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 concreteduring mixing. The main factors that control the perform-ance of the composite material are:

1. Physical properties of fibers and matrix2. Strength of bond between fibers and matrix

Although the basic governing principles are the same,there are several characteristic differences between con-ventional reinforcement and fiber systems:

1. Fibers are generally distributed throughout a givencross section whereas reinforcing bars or wires areplaced only where required

2. Most fibers are relatively short and closely spaced ascompared with continuous reinforcing bars or wires

3. It is generally not possible to achieve the same area ofreinforcement to area of concrete using fibers as com-pared to using a network of reinforcing bars or wires

Fibers are typically added to concrete in low volumedosages (often less than 1%), and have been shown to beeffective in reducing plastic shrinkage cracking.

Fibers typically do not significantly alter freeshrinkage of concrete, however at high enough dosagesthey can increase the resistance to cracking and decreasecrack width (Shah, Weiss, and Yang 1998).

ADVANTAGES AND DISADVANTAGES OF USING FIBERS

Fibers are generally distributed throughout the concretecross section. Therefore, many fibers are inefficientlylocated for resisting tensile stresses resulting from appliedloads. Depending on fabrication method, random orienta-tion of fibers may be either two-dimensional (2-D) orthree-dimensional (3-D). Typically, the spray-up fabrica-tion method has a 2-D random fiber orientation where asthe premix (or batch) fabrication method typically has a3-D random fiber orientation. Also, many fibers areobserved to extend across cracks at angles other than 90ºor may have less than the required embedment length fordevelopment of adequate bond. Therefore, only a smallpercentage of the fiber content may be efficient in resistingtensile or flexural stresses. “Efficiency factors” can be aslow as 0.4 for 2-D random orientation and 0.25 for 3-Drandom orientation. The efficiency factor depends on fiberlength and critical embedment length. From a conceptualpoint of view, reinforcing with fibers is not a highly effi-cient method of obtaining composite strength.

Fiber concretes are best suited for thin section shapeswhere correct placement of conventional reinforcementwould be extremely difficult. In addition, spraying of fiberconcrete accommodates the fabrication of irregularlyshaped products. A substantial weight savings can be real-ized using relatively thin fiber concrete sections having

CHAPTER 7

Fibers

Fig. 7-1. Steel, glass, synthetic and natural fibers with dif-ferent lengths and shapes can be used in concrete. (69965)

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HOMEPAGE

Steel fibers do not affect free shrinkage. Steel fibersdelay the fracture of restrained concrete during shrinkageand they improve stress relaxation by creep mechanisms(Altoubat and Lange 2001).

The durability of steel-fiber concrete is contingent onthe same factors as conventional concrete. Freeze-thawdurability is not diminished by the addition of steel fibersprovided the mix is adjusted to accommodate the fibers,the concrete is properly consolidated during placement,and is air-entrained. With properly designed and placedconcrete, little or no corrosion of the fibers occurs. Anysurface corrosion of fibers is cosmetic as opposed to astructural condition.

Steel fibers have a relatively high modulus of elas-ticity (Table 7-1). Their bond to the cement matrix can beenhanced by mechanical anchorage or surface roughnessand they are protected from corrosion by the alkalineenvironment in the cement matrix (ACI 544.1R-96).

Steel fibers are most commonly used in airport pave-ments and runway/taxi overlays. They are also used inbridge decks (Fig. 7-3), industrial floors, and highwaypavements. Structures exposed to high-velocity waterflow have been shown to last about three times longerthan conventional concrete alternatives. Steel fiber con-crete is also used for many precast concrete applicationsthat make use of the improved impact resistance or tough-ness imparted by the fibers. In utility boxes and septictanks, steel fibers replace conventional reinforcement.

Steel fibers are also widely used with shotcrete inthin-layer applications, especially rock-slope stabilizationand tunnel linings. Silica fume and accelerators haveenabled shotcrete to be placed in thicker layers. Silicafume also reduces the permeability of the shotcrete mate-rial (Morgan 1987). Steel-fiber shotcrete has been success-fully applied with fiber volumes up to 2%.

Slurry-infiltrated concrete (SIFCON) with fiber vol-umes up to 20% has been used since the late 1970s. Slurry-

the equivalent strength of thicker conventionally rein-forced concrete sections.

TYPES AND PROPERTIES OF FIBERSAND THEIR EFFECT ON CONCRETE

Steel Fibers

Steel fibers are short, discrete lengths of steel with anaspect ratio (ratio of length to diameter) from about 20 to100, and with any of several cross sections. Some steelfibers have hooked ends to improve resistance to pulloutfrom a cement-based matrix (Fig. 7-2).

ASTM A 820 classifies four different types based ontheir manufacture. Type I – Cold-drawn wire fibers are themost commercially available, manufactured from drawnsteel wire. Type II – Cut sheet fibers are manufactured asthe name implies: steel fibers are laterally sheared off steelsheets. Type III – Melt-extracted fibers are manufacturedwith a relatively complicated technique where a rotatingwheel is used to lift liquid metal from a molten metal sur-face by capillary action. The extracted molten metal is thenrapidly frozen into fibers and thrown off the wheel by cen-trifugal force. The resulting fibers have a crescent-shapedcross section. Type IV – Other fibers. For tolerances forlength, diameter, and aspect ratio, as well as minimum ten-sile strength and bending requirement, see ASTM A 820.

Steel-fiber volumes used in concrete typically rangefrom 0.25% to 2%. Volumes of more than 2% generallyreduce workability and fiber dispersion and require spe-cial mix design or concrete placement techniques.

The compressive strength of concrete is only slightlyaffected by the presence of fibers. The addition of 1.5% byvolume of steel fibers can increase the direct tensilestrength by up to 40% and the flexural strength up to 150%.

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Fig. 7-2. Steel fibers with hooked ends are collated intobundles to facilitate handling and mixing. During mixingthe bundles separate into individual fibers. (69992) Fig. 7-3. Bridge deck with steel fibers. (70007)

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Chapter 7 � Fibers

Modulus ofRelative density Diameter, µµm Tensile strength, MPa elasticity, MPa Strain at

Fiber type (specific gravity) (0.001 in.) (ksi) (ksi) failure, %

Steel 7.80 100-1000 500-2600 210,000 0.5-3.5(4-40) (70-380) (30,000)

Glass

E 2.54 8-15 2000-4000 72,000 3.0-4.8(0.3-0.6) (290-580) (10,400)

AR 2.70 12-20 1500-3700 80,000 2.5-3.6(0.5-0.8) (220-540) (11,600)

Synthetic

Acrylic 1.18 5-17 200-1000 17,000-19,000 28-50(0.2-0.7) (30-145) (2,500-2,800)

Aramid 1.44 10-12 2000-3100 62,000-120,000 2-3.5(0.4-0.47) (300-450) (9,000-17,000)

Carbon 1.90 8-0 1800-2600 230,000-380,000 0.5-1.5(0.3-0.35) (260-380) (33,400-55,100)

Nylon 1.14 23 1000 5,200 20(0.9) (140) (750)

Polyester 1.38 10-80 280-1200 10,000-18,000 10-50(0.4-3.0) (40-170) (1,500-2,500)

Polyethylene 0.96 25-1000 80-600 5,000 12-100(1-40) (11-85) (725)

Polypropylene 0.90 20-200 450-700 3,500-5,200 6-15(0.8-8) (65-100) (500-750)

Natural

Wood cellulose 1.50 25-125 350-2000 10,000-40,000(1-5) (51-290) (1,500-5,800)

Sisal 280-600 13,000-25,000 3.5(40-85) (1,900-3,800)

Coconut 1.12-1.15 100-400 120-200 19,000-25,000 10-25(4-16) (17-29) (2,800-3,800)

Bamboo 1.50 50-400 350-500 33,000-40,000(2-16) (51-73) (4,800-5,800)

Jute 1.02-1.04 100-200 250-350 25,000-32,000 1.5-1.9(4-8) (36-51) (3,800-4,600)

Elephant grass 425 180 4,900 3.6(17) (26) (710)

Table 7-1. Properties of Selected Fiber Types

Adapted from PCA (1991) and ACI 544.1R-96.

infiltrated concrete can be used to produce a component orstructure with strength and ductility that far exceeds thatof conventionally mixed or sprayed fiber concrete.SIFCON is not inexpensive and needs fine-tuning, but itholds potential for applications exposed to severe condi-tions and requiring very high strength and toughness.These applications include impact and blast-resistantstructures, refractories, protective revetment, and taxiwayand pavement repairs (Fig. 7-4). Table 7-2 shows a SIFCONmix design.

Cement 1000 kg/m3 (1686 lb/yd3)

Water 330 kg/m3 (556 lb/yd3)

Siliceous Sand ≤ 0.7 mm (≤ 0.028 in.) 860 kg/m3 (1450 lb/yd3)

Silica Slurry 13 kg/m3 (1.3 lb/yd3)

High-Range Water Reducer 35 kg/m3 (3.7 lb/yd3)

Steel Fibers (about 10 Vol.-%) 800 kg/m3 (84 lb/yd3)

Table 7-2. SIFCON Mix Design.

Fiber modifications to improve long-term durabilityinvolve (1) specially formulated chemical coatings to helpcombat hydration-induced embrittlement, and (2) em-ployment of a dispersed microsilica slurry to adequatelyfill fiber voids, thereby reducing potential for calciumhydroxide infiltration.

A low-alkaline cement has been developed in Japanthat produces no calcium hydroxide during hydration.Accelerated tests with the cement in alkali-resistant-glassfiber-reinforced concrete samples have shown greaterlong-term durability than previously achieved.

Metakaolin can be used in glass-fiber-reinforced con-crete without significantly affecting flexural strength,strain, modulus of elasticity, and toughness. (Marikunte,Aldea, Shah 1997).

The single largest application of glass-fiber concretehas been the manufacture of exterior building façade panels(Fig. 7-5). Other applications are listed in PCA (1991).

Glass Fibers

The first research on glass fibers in the early 1960s usedconventional borosilicate glass (E-glass) (Table 7-1) andsoda-lime-silica glass fibers (A-glass). The test resultsshowed that alkali reactivity between the E-glass fibersand 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 stem-ming from infiltration of calcium hydroxide particles, by-products of cement hydration, into fiber bundles. Alkalireactivity and cement hydration are the basis for the fol-lowing two widely held theories explaining strength andductility loss, particularly in exterior glass fiber concrete:

• Alkali attack on glass-fiber surfaces reduces fiber ten-sile strength and, subsequently, lowers compressivestrength.

• Ongoing cement hydration causes calcium hydroxideparticle penetration of fiber bundles, therebyincreasing fiber-to-matrix bond strength and embrit-tlement; the latter lowers tensile strength by inhibit-ing fiber pullout.

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Fig. 7-5. (top) Glass-fiber-reinforced concrete panels are lightand strong enough to reduce this building’s structuralrequirements. (bottom) Spray-up fabrication made it easy tocreate their contoured profiles. (60671, 46228)

Fig. 7-4. Tightly bunched steel fibers are placed in a form,before cement slurry is poured into this application ofslurry-infiltrated steel-fiber concrete (SIFCON). (60672)

Synthetic Fibers

Synthetic fibers are man-made fibers resulting fromresearch and development in the petrochemical and textileindustries. Fiber types that are used in portland cementconcrete are: acrylic, aramid, carbon, nylon, polyester,polyethylene, and polypropylene. Table 7-1 summarizesthe range of physical properties of these fibers.

Synthetic fibers can reduce plastic shrinkage and sub-sidence cracking and may help concrete after it is frac-tured. Ultra-thin whitetopping often uses synthetic fibersfor potential containment properties to delay potholedevelopment. Problems associated with synthetic fibersinclude: (1) low fiber-to-matrix bonding; (2) inconclusiveperformance testing for low fiber-volume usage withpolypropylene, polyethylene, polyester and nylon; (3) alow modulus of elasticity for polypropylene and polyeth-ylene; and (4) the high cost of carbon and aramid fibers.

Polypropylene fibers (Fig. 7-6), the most popular ofthe synthetics, are chemically inert, hydrophobic, andlightweight. They are produced as continuous cylindricalmonofilaments that can be chopped to specified lengths orcut as films and tapes and formed into fine fibrils of rec-tangular cross section (Fig. 7-7).

Used at a rate of at least 0.1 percent by volume of con-crete, polypropylene fibers reduce plastic shrinkagecracking and subsidence cracking over steel reinforcement(Suprenant and Malisch 1999). The presence of polypropy-lene fibers in concrete may reduce settlement of aggregateparticles, thus reducing capillary bleed channels.Polypropylene fibers can help reduce spalling of high-strength, low-permeability concrete exposed to fire in amoist condition.

New developments show that monofilament fibersare able to fibrillate during mixing if produced with both,polypropylene and polyethylene resins. The two poly-

mers are incompatible and tend to separate when manip-ulated. Therefore, during the mixing process each fiberturns into a unit with several fibrils at its end. The fibrilsprovide better mechanical bonding than conventionalmonofilaments. The high number of fine fibrils alsoreduces plastic shrinkage cracking and may increase theductility and toughness of the concrete (Trottier andMahoney 2001).

Acrylic fibers have been found to be the most prom-ising replacement for asbestos fibers. They are used incement board and roof-shingle production, where fibervolumes of up to 3% can produce a composite withmechanical properties similar to that of an asbestos-cement composite. Acrylic-fiber concrete compositesexhibit high postcracking toughness and ductility.Although lower than that of asbestos-cement composites,acrylic-fiber-reinforced concrete’s flexural strength isample for many building applications.

Aramid fibers have high tensile strength and a hightensile modulus. Aramid fibers are two and a half times asstrong as E-glass fibers and five times as strong as steelfibers. A comparison of mechanical properties of differentaramid fibers is provided in PCA (1991). In addition toexcellent strength characteristics, aramid fibers also haveexcellent strength retention up to 160°C (320°F), dimen-sional stability up to 200°C (392°F), static and dynamicfatigue resistance, and creep resistance. Aramid strand isavailable in a wide range of diameters.

Carbon fibers were developed primarily for their highstrength and elastic modulus and stiffness properties forapplications within the aerospace industry. Comparedwith most other synthetic fibers, the manufacture ofcarbon fibers is expensive and this has limited commercialdevelopment. Carbon fibers have high tensile strengthand modulus of elasticity (Table 7-1). They are also inert to

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Chapter 7 � Fibers

Fig. 7-7. Polypropylene fibers are produced either as (left)fine fibrils with rectangular cross section or (right) cylin-drical monofilament. (69993)

Fig. 7-6. Polypropylene fibers. (69796)

ufacture of low-fiber-content concrete and occasionallyhave been used in thin-sheet concrete with high-fiber con-tent. For typical properties of natural fibers see Table 7-1.

Unprocessed Natural Fibers. In the late 1960s, researchon the engineering properties of natural fibers, and con-crete made with these fibers was undertaken; the resultwas these fibers can be used successfully to make thinsheets for walls and roofs. Products were made with port-land cement and unprocessed natural fibers such ascoconut coir, sisal, bamboo, jute, wood, and vegetablefibers. Although the concretes made with unprocessednatural fibers show good mechanical properties, theyhave some deficiencies in durability. Many of the naturalfibers are highly susceptible to volume changes due tovariations in fiber moisture content. Fiber volumetricchanges that accompany variations in fiber moisture con-tent can drastically affect the bond strength between thefiber and cement matrix.

Wood Fibers (Processed Natural Fibers). The proper-ties of wood cellulose fibers are greatly influenced by themethod by which the fibers are extracted and the refiningprocesses involved. The process by which wood isreduced to a fibrous mass is called pulping. The kraftprocess is the one most commonly used for producingwood cellulose fibers. This process involves cookingwood chips in a solution of sodium hydroxide, sodiumcarbonate, and sodium sulfide. Wood cellulose fibers haverelatively good mechanical properties compared to manymanmade fibers such as polypropylene, polyethylene,polyester, and acrylic. Delignified cellulose fibers (ligninremoved) can be produced with a tensile strength of up toapproximately 2000 MPa (290 ksi) for selected grades ofwood and pulping processes. Fiber tensile strength ofapproximately 500 MPa (73 ksi) can be routinely achievedusing a chemical pulping process and the more common,less expensive grades of wood.

MULTIPLE FIBER SYSTEMS

For a multiple fiber system, two or more fibers areblended into one system. The hybrid-fiber concrete com-bines macro- and microsteel fibers. A common macrofiberblended with a newly developed microfiber, which is lessthan 10 mm (0.4 in.) long and less than 100 micrometer(0.004 in.) in diameter, leads to a closer fiber-to-fiberspacing, which reduces microcracking and increases ten-sile strength. The intended applications include thinrepairs and patching (Banthia and Bindiganavile 2001). Ablend of steel and polypropylene fibers has also been usedfor some applications. This system is supposed to com-bine the toughness and impact-resistance of steel fiberconcrete with the reduced plastic cracking of polypropy-lene 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 fib-

most chemicals. Carbon fiber is typically produced instrands that may contain up to 12,000 individual fila-ments. The strands are commonly prespread prior toincorporation in concrete to facilitate cement matrix pene-tration and to maximize fiber effectiveness.

Nylon fibers exist in various types in the marketplacefor use in apparel, home furnishing, industrial, and textileapplications. Only two types of nylon fiber are currentlymarketed for use in concrete, nylon 6 and nylon 66. Nylonfibers are spun from nylon polymer and transformedthrough extrusion, stretching, and heating to form an ori-ented, crystalline, fiber structure. For concrete applica-tions, high tenacity (high tensile strength) heat and lightstable yarn is spun and subsequently cut into shorterlength. Nylon fibers exhibit good tenacity, toughness, andelastic recovery. Nylon is hydrophilic, with moistureretention of 4.5 percent, which increases the waterdemand of concrete. However, this does not affect con-crete hydration or workability at low prescribed contentsranging from 0.1 to 0.2 percent by volume, but should beconsidered at higher fiber volume contents. This compar-atively small dosage has potentially greater reinforcingvalue than low volumes of polypropylene or polyesterfiber. Nylon is relatively inert and resistant to a widevariety of organic and inorganic materials includingstrong alkalis.

Synthetic fibers are also used in stucco and mortar.For this use the fibers are shorter than synthetic fibersused in concrete. Usually small amounts of 13-mm (1⁄2-in.)long alkali-resistant fibers are added to base coat plastermixtures. They can be used in small line stucco andmortar pumps and spray guns. They should be addedto the mix in accordance with manufacturer’s recom-mendation.

For further details about chemical and physicalproperties of synthetic fibers and properties of syntheticfiber concrete, see ACI 544.1R-96. ASTM C 1116 classifiesSteel, Glass, and Synthetic Fiber Concrete or Shotcrete.

The technology of interground fiber cement takesadvantage of the fact that some synthetic fibers are notdestroyed or pulverized in the cement finishing mill. Thefibers are mixed with dry cement during grinding wherethey are uniformly distributed; the surface of the fibers isroughened during grinding, which offers a better mechan-ical bond to the cement paste (Vondran 1995).

Natural Fibers

Natural fibers were used as a form of reinforcement longbefore the advent of conventional reinforced concrete.Mud bricks reinforced with straw and mortars reinforcedwith horsehair are just a few examples of how naturalfibers were used long ago as a form of reinforcement.Many natural reinforcing materials can be obtained at lowlevels of cost and energy using locally available manpowerand technical know-how. Such fibers are used in the man-

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Design and Control of Concrete Mixtures � EB001

rillated polypropylene fibers were used for slabs on grade.The concrete with blended fibers had a lower slump com-pared to plain concrete but seemed to have enhancedelastic and post-elastic strength.

REFERENCES

ACI Committee 544, State-of-the-Art Report on FiberReinforced Concrete, ACI 544.1R-96, American ConcreteInstitute, Farmington Hills, Michigan, 1997.

Altoubat, Salah A., and Lange, David A., “Creep,Shrinkage, and Cracking of Restrained Concrete at EarlyAge,” ACI Materials Journal, American Concrete Institute,Farmington Hills, Michigan, July-August 2001, pages 323to 331.

Banthia, Nemkumar, and Bindiganavile, Vivek, “Repairingwith Hybrid-Fiber-Reinforced Concrete,” Concrete Inter-national, American Concrete Institute, Farmington Hills,Michigan, June 2001, pages 29 to 32.

Bijen, J., “Durability of Some Glass Fiber ReinforcedCement Composites,” ACI Journal, American ConcreteInstitute, Farmington Hills, Michigan, July-August 1983,pages 305 to 311.

Hanna, Amir N., Steel Fiber Reinforced ConcreteProperties and Resurfacing Applications, Research and De-velopment Bulletin RD049, Portland Cement Association,http://www.portcement.org/pdf_files/RD049.pdf,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 GlassFiber Reinforced Cement Composites: Effect of SilicaFume and Metakaolin,” Advanced Cement Based Materials,Volume 5, Numbers 3/4, April/May 1997, pages 100to 108.

Morgan, D. R., “Evaluation of Silica Fume Shotcrete,”Proceedings, CANMET/CSCE International Workshop onSilica Fume in Concrete, Montreal, May 1987.

Monfore, G. E., A Review of Fiber Reinforcement of PortlandCement Paste, Mortar and Concrete, Research DepartmentBulletin RX226, Portland Cement Association, http://www.portcement.org/pdf_files/RX226.pdf, 1968, 7 pages.

PCA, Fiber Reinforced Concrete, SP039, Portland CementAssociation, 1991, 54 pages.

PCA, “Steel Fiber Reinforced Concrete,” Concrete Tech-nology Today, PL931 Portland Cement Association, http://www.portcement.org/pdf_files/PL931.pdf, March 1993,pages 1 to 4.

Panarese, William C., “Fiber: Good for the ConcreteDiet?,” Civil Engineering, American Society of CivilEngineers, New York, May 1992, pages 44 to 47.

Shah, S. P.; Weiss, W. J.; and Yang, W., “ShrinkageCracking – 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 fiberfactor,” Concrete Construction, Addison, Illinois, October1999, pages 43 to 46.

Trottier, Jean-Francois, and Mahoney, Michael, “InnovativeSynthetic Fibers,” Concrete International, American Con-crete Institute, Farmington Hills, Michigan, June 2001,pages 23 to 28

Vondran, Gary L., “Interground Fiber Cement in the Year2000,” Emerging Technologies Symposium on Cements for the21st Century, SP206, Portland Cement Association, March1995, pages 116 to 134.

Wojtysiak, R.; Borden, K. K.; and Harrison P., Evaluation ofFiber Reinforced Concrete for the Chicago Area – A Case Study,2001.

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