22-1

22Fiber-Reinforced

Composites

Edward G. Nawy, D.Eng., P.E., C.Eng.*

Part A. Fiber-Reinforced Concrete22.1 Historical Development ....................................................22-222.2 General Characteristics .....................................................22-222.3 Mixture Proportioning .....................................................22-422.4 Mechanics of Fiber Reinforcement ..................................22-5

First Cracking Load Critical Fiber Length: Length Factor Critical Fiber Spacing: Space Factor Fiber Orientation: Fiber Efficiency Factor Static Flexural Strength Prediction: Beams with Fibers Only

22.5 Mechanical Properties of Fibrous Concrete Structural Elements ..........................................22-8Controlling Factors Strength in Compression Strength in Direct Tension Flexural Strength Shear Strength Environmental Effects Dynamic Loading Performance

22.6 Steel-Fiber-Reinforced Cement Composites .................22-14General Characteristics Slurry-Infiltrated Fiber Concrete DSP and CRC Cement Composites Carbon-Fiber-Reinforced Cement-Based Composites Super-Strength Reactive-Powder Concretes

22.7 Prestressed Concrete Prism Elements as the Main Composite Reinforcement in Concrete Beams .............22-17

Part B. Fiber-Reinforced Plastic (FRP) Composites22.8 Historical Development ..................................................22-1822.9 Beams and Two-Way Slabs Reinforced

with GFRP Bars...............................................................22-1922.10 Carbon Fibers and Composite Reinforcement .............22-20

Carbon Fibers Hybrid GFRP and CFRP Reinforcement for Bridges and Other Structural Systems Use as Internal Prestressing Reinforcement Use as External Reinforcement

22.11 Fire Resistance .................................................................22-1622.12 Summary..........................................................................22-25Acknowledgments......................................................................22-25References ...................................................................................22-25

* Distinguished Professor, Civil Engineering, Rutgers University, The State University of New Jersey, Piscataway, NewJersey, and ACI honorary member; expert in concrete structures, materials, and forensic engineering.

2008 by Taylor & Francis Group, LLC

22-2 Concrete Construction Engineering Handbook

Part A. Fiber-Reinforced Concrete

22.1 Historical Development

Fibers have been used to reinforce brittle materials from time immemorial, dating back to the Egyptianand Babylonian eras, if not earlier. Straws were used to reinforce sun-baked bricks and mud-hut walls,horse hair was used to reinforce plaster, and asbestos fibers have been used to reinforce Portland cementmortars. Research in the late 1950s and early 1960s by Romualdi and Batson (1963) and Romualdi andMandel (1964) on closely spaced random fibers, primarily steel fibers, heralded the era of using the fibercomposite concretes we know today. In addition, Shah and Rangan (1971), Swamy (1975), and severalother researchers in the United States, United Kingdom, Japan, and Russia embarked on extensive inves-tigations in this area, exploring other fibers in addition to steel. By the 1960s, steel-fiber concrete beganto be used in pavements, in particular. Other developments using bundled fiberglass as the main compositereinforcement in concrete beams and slabs were introduced by Nawy et al. (1971) and Nawy and Neuwerth(1977), as discussed in Section 22.8 of this chapter. From the 1970s to the present, the use of steel fibershas been well established as a complementary reinforcement to increase cracking resistance, flexural andshear strength, and impact resistance of reinforced concrete elements both in situ cast and precast.

22.2 General Characteristics

Concrete is weak in tension. Microcracks begin to generate in the matrix of a structural element at about10 to 15% of the ultimate load, propagating into macrocracks at 25 to 30% of the ultimate load.Consequently, plain concrete members cannot be expected to sustain large transverse loading withoutthe addition of continuous-bar reinforcing elements in the tensile zone of supported members such asbeams or slabs. The developing microcracking and macrocracking, however, still cannot be arrested orslowed by the sole use of continuous reinforcement. The function of such reinforcement is to replace thefunction of the tensile zone of a section and assume the tension equilibrium force in the section. Theaddition of randomly spaced discontinuous fiber elements should aid in arresting the development orpropagation of the microcracks that are known to generate at the early stages of loading history. Althoughfibers have been used to reinforce brittle materials such as concrete since time immemorial, newlydeveloped fibers have been used extensively worldwide in the past three decades. Different types arecommercially available, such as steel, glass, polypropylene, or graphite. They have proven that they canimprove the mechanical properties of the concrete, both as a structure and a material, not as a replacementfor continuous-bar reinforcement when it is needed but in addition to it.

Concrete fiber composites are concrete elements made from a mixture comprised of hydraulic cements,fine and coarse aggregates, pozzolanic cementitious materials, admixtures commonly used with conven-tional concrete, and a dispersion of discontinuous, small fibers made from steel, glass, organic polymers,or graphites. The fibers could also be vegetable fibers such as sisal or jute. Generally, if the fibers aremade from steel, the fiber length varies from 0.5 to 2.5 in. (12.7 to 63.5 mm). They can be round,produced by cutting or chopping wire, or they can be flat, typically having cross-sections 0.006 to 0.016in. (0.15 to 0.41 mm) in thickness and 0.01 to 0.035 in. (0.250.90 mm) in width and produced byshearing sheets or flattening wire. The most common diameters of the round wires are in the range of0.017 to 0.040 in. (0.45 to 1.0 mm) (ACI Committee 544, 1988, 1993, 1996). The wires are usually crimpedor deformed or have small heads on them for better bond within the matrix, and some are crescentshaped in cross-section.

The fiber content in a mixture where steel fibers are used usually varies from .25 to 2% by volumenamely, from 33 to 265 lb/yd3 (20 to 165 kg/m3). A fiber content of 50 to 60 lb/yd3 is common in lightlyloaded slabs on grade, precast elements, and composite steel deck topping. The upper end of the range,more difficult to apply, is used for security applications such as vaults, safes, and impact-resistingstructures.

2008 by Taylor & Francis Group, LLC

Fiber-Reinforced Composites 22-3

The introduction of fiber additions to concrete in the early 1900s was aimed primarily at enhancingthe tensile strength of concrete. As is well known, the tensile strength is 8 to 14% of the compressivestrength of normal concretes with resulting cracking at low stress levels. Such a weakness is partiallyovercome by the addition of reinforcing bars, which can be either steel or fiberglass, as main continuousreinforcement in beams and one-way and two-way structural slabs or slabs on grade (Nawy and Neuw-erth, 1977; Nawy et al., 1971). As indicated earlier, the continuous reinforcing elements cannot stop thedevelopment of microcracks. Fibers, on the other hand, are discontinuous and randomly distributed inthe matrix, in both the tensile and compressive zones of a structural element. They are able to add tothe stiffness and crack-control performance by preventing the microcracks from propagating and wid-ening and also by increasing ductility due to their energy-absorption capacity. Common applications offiber-reinforced concrete include overlays in bridge decks, industrial floors, shotcrete applications, high-way and airport pavements, thin-shell structures, seismic- and explosion-resisting structures, super flatsurface slabs on grade in warehouses, and for the reduction of expansion joints. Table 22.1 describes thegeometry and mechanical properties of various types of fibers that can be used as randomly dispersedfilaments in a concrete matrix. Because of the wide range of properties for each type of fiber, the designershould be guided by the manufacturers data on each particular product and experience with it before afiber type is selected.

TABLE 22.1 Typical Properties of Fibers

Type of Fiber(1)

Diameter, in. 103 (mm)

(2)

Specific Gravitya

(3)

Tensile Strength,psi 103 (GPa)

(4)

Youngs Modulus,

psi 106 (GPa)(5)

Ultimate Elongation (%)

(6)

Acrylic 0.60.13 (0.020.35)

1.1 3060 (0.20.4) 0.3 (2) 1.1

Asbestos 0.050.80 (0.00150.02)

3.2 80140 (0.61.0) 1220 (83138) 12

Cotton 624 (0.20.6) 1.5 60100 (0.40.7) 0.7 (4.8) 310

Glass 0.20.6 (0.0050.15)

2.5 150380 (1.02.6) 1011.5 (7080) 1.53.5

Graphite 0.30.36 (0.0080.009)

1.9 190380 (1.02.6) 3460 (230415) 0.51.0

Kevlar 0.4 (0.010) 1.45 505520 (3.53.6) 9.4 (65133) 2.14.0

Nylon (high-tenacity) 0.616 (0.020.40) 1.1 110120 (0.760.82)

0.6 (4.1) 1620

Polyester (high-tenacity) 0.616 (0.020.40) 1.4 105125 (0.720.86)

1.2 (8.3) 1113

Polypropylene 0.616 (0.020.40) 0.95 80110 (0.550.76)

0.5 (3.5) 1525

Rayon (high-tenacity) 0.815 (0.020.38) 1.5 6090 (0.40.6) 1.0 (6.9) 1025

Rock wool (Scandinavian) 0.530 (0.010.8) 2.7 70110 (0.50.76) ~0.6 0.50.7

Sisal 0.44 (0.010.10) 1.5 115 (0.8) 3.0

Steel 440 (0.11.0) 7.84 50300 (032.0) 29.0 (200) 0.53.5

Cement matrix 1.52.5 0.41.0 (0.0030.007)

1.56.5 (1045) 0.02

a Density = Col. 3 62.4 lb/ft3 = Col. 3 103 k