Concrete Construction Engineering Handbook Download

22-2. Concrete Construction Engineering Handbook. Part A. Fiber-Reinforced Concrete. 22.1 Historical Development. Fibers have been used to reinforce brittle ...

22 Fiber-Reinforced Composites Edward G. Nawy, D.Eng., P.E., C.Eng.* Part A. Fiber-Reinforced Concrete 22.1 Historical Development ....................................................22-2 22.2 General Characteristics .....................................................22-2 22.3 Mixture Proportioning .....................................................22-4 22.4 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 ..........................................22-8 Controlling Factors • Strength in Compression • Strength in Direct Tension • Flexural Strength • Shear Strength • Environmental Effects • Dynamic Loading Performance

22.6 Steel-Fiber-Reinforced Cement Composites .................22-14 General 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) Composites 22.8 Historical Development ..................................................22-18 22.9 Beams and Two-Way Slabs Reinforced with GFRP Bars...............................................................22-19 22.10 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 .................................................................22-16 22.12 Summary..........................................................................22-25 Acknowledgments......................................................................22-25 References ...................................................................................22-25

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

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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 Egyptian and 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 cement mortars. Research in the late 1950s and early 1960s by Romualdi and Batson (1963) and Romualdi and Mandel (1964) on closely spaced random ﬁbers, primarily steel ﬁbers, heralded the era of using the ﬁber composite concretes we know today. In addition, Shah and Rangan (1971), Swamy (1975), and several other researchers in the United States, United Kingdom, Japan, and Russia embarked on extensive investigations in this area, exploring other ﬁbers in addition to steel. By the 1960s, steel-ﬁber concrete began to be used in pavements, in particular. Other developments using bundled ﬁberglass as the main composite reinforcement 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 ﬁbers has been well established as a complementary reinforcement to increase cracking resistance, ﬂexural and shear 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 about 10 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 without the addition of continuous-bar reinforcing elements in the tensile zone of supported members such as beams or slabs. The developing microcracking and macrocracking, however, still cannot be arrested or slowed by the sole use of continuous reinforcement. The function of such reinforcement is to replace the function of the tensile zone of a section and assume the tension equilibrium force in the section. The addition of randomly spaced discontinuous ﬁber elements should aid in arresting the development or propagation of the microcracks that are known to generate at the early stages of loading history. Although ﬁbers have been used to reinforce brittle materials such as concrete since time immemorial, newly developed ﬁbers have been used extensively worldwide in the past three decades. Different types are commercially available, such as steel, glass, polypropylene, or graphite. They have proven that they can improve the mechanical properties of the concrete, both as a structure and a material, not as a replacement for 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 conventional 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 are made 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 0.016 in. (0.15 to 0.41 mm) in thickness and 0.01 to 0.035 in. (0.25–0.90 mm) in width and produced by shearing sheets or ﬂattening wire. The most common diameters of the round wires are in the range of 0.017 to 0.040 in. (0.45 to 1.0 mm) (ACI Committee 544, 1988, 1993, 1996). The wires are usually crimped or deformed or have small heads on them for better bond within the matrix, and some are crescent shaped in cross-section. The ﬁber content in a mixture where steel ﬁbers are used usually varies from .25 to 2% by volume— namely, from 33 to 265 lb/yd3 (20 to 165 kg/m3). A ﬁber content of 50 to 60 lb/yd3 is common in lightly loaded 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-resisting structures.

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Fiber-Reinforced Composites

TABLE 22.1 Typical Properties of Fibers Diameter, in. × 103 (mm) (2)

Speciﬁc Gravitya (3)

Tensile Strength, psi × 103 (GPa) (4)

Young's Modulus, psi × 106 (GPa) (5)

Ultimate Elongation (%) (6)

0.6–0.13 (0.02–0.35)

1.1

30–60 (0.2–0.4)

0.3 (2)

1.1

Asbestos

0.05–0.80 (0.0015–0.02)

3.2

80–140 (0.6–1.0)

12–20 (83–138)

1–2

Cotton

Type of Fiber (1) Acrylic

6–24 (0.2–0.6)

1.5

60–100 (0.4–0.7)

0.7 (4.8)

3–10

Glass

0.2–0.6 (0.005–0.15)

2.5

150–380 (1.0–2.6)

10–11.5 (70–80)

1.5–3.5

Graphite

0.3–0.36 (0.008–0.009)

1.9

190–380 (1.0–2.6)

34–60 (230–415)

0.5–1.0

Kevlar®

0.4 (0.010)

1.45

505–520 (3.5–3.6)

9.4 (65–133)

2.1–4.0

Nylon (high-tenacity)

0.6–16 (0.02–0.40)

1.1

110–120 (0.76–0.82)

0.6 (4.1)

16–20

Polyester (high-tenacity)

0.6–16 (0.02–0.40)

1.4

105–125 (0.72–0.86)

1.2 (8.3)

11–13

Polypropylene

0.6–16 (0.02–0.40)

0.95

80–110 (0.55–0.76)

0.5 (3.5)

15–25

Rayon (high-tenacity)

0.8–15 (0.02–0.38)

1.5

60–90 (0.4–0.6)

1.0 (6.9)

10–25

Rock wool (Scandinavian)

0.5–30 (0.01–0.8)

2.7

70–110 (0.5–0.76)

~0.6

0.5–0.7

Sisal

0.4–4 (0.01–0.10)

1.5

115 (0.8)

—

3.0

Steel

4–40 (0.1–1.0)

7.84

50–300 (03–2.0)

29.0 (200)

0.5–3.5

—

1.5–2.5

0.4–1.0 (0.003–0.007)

1.5–6.5 (10–45)

0.02

Cement matrix

a Density = Col. 3 × 62.4 lb/ft3 = Col. 3 × 103 kg/m3. Note: GPa × 0.145 = 106 psi. Source: Nawy, E.G., Fundamentals of High-Strength, High-Performance Concrete, Addison Wesley Longman, Reading, MA, 1996, p. 350.

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

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Concrete Construction Engineering Handbook

TABLE 22.2 Typical Proportions for Normal Weight Fiber-Reinforced Concrete Material Cement W/C ratio Percentage of sand to aggregate Maximum aggregate Air content Fiber content

Range 550–950 lb/yd3 0.4–0.6 50–100% 3/8 in. 6–9% 0.5–2.5% by volume of mix (steel, 1% = 132 lb/yd3; glass, 1% = 42 lb/yd3; nylon, 1% = 19 lb/yd3)

Note: 1 lb/yd3 = 0.5933 kg/m3; 1 in. = 2.54 cm. Source: ACI Committee 544, Fiber-Reinforced Concrete, ACI 544.1R, American Concrete Institute, Farmington Hills, MI, 1996.

TABLE 22.3 Typical Fly-Ash Fibrous Concrete Mix Material Cement Fly ash W/C ratio Percentage of sand to aggregate Maximum size of coarse aggregate Steel ﬁber content (0.010 × 0.022 × 1.0 in.) Air-entraining agent Water-reducing agent Slump

Quantity 490 lb/yd3 225 lb/yd3 0.54 50% 3/8 in. 1.5% by volume Manufacturer's recommendation Manufacturer's recommendation 5 to 6 in.

Note: 1 lb/yd3 = 0.5933 kg/m3; 1 in. = 2.54 cm. Source: ACI Committee 544, Fiber-Reinforced Concrete, ACI 544.1R, American Concrete Institute, Farmington Hills, MI, 1996.

22.3 Mixture Proportioning Mixing the ﬁbers with the other mix constituents can be done by several methods. The method selected—plant batching, ready-mixed concrete, or hand mixing in the laboratory—depends on the facilities available and the job requirements. The most important factor is to ensure uniform dispersion of the ﬁbers and to prevent segregation or balling of the ﬁbers during mixing. Segregation or balling during mixing is affected by many factors, which can be summarized as follows: • • • •

Aspect ratio (/df ), which is most important Volume percentage of the ﬁber Coarse aggregate size, gradation, and quantity Water/cementitious materials ratio and method of mixing

A maximum aspect ratio of /df and a steel ﬁber content in excess of 2% by volume make it difﬁcult to achieve a uniform mix. Although conventional mixing procedures can be used, it is advisable to use a 3/8-in. (9.7-mm) maximum aggregate size. The water requirement will vary from that of concrete without ﬁbers depending on the type of cement replacement cementitious pozzolans used and their percent by volume of the matrix. Table 22.2 and Table 22.3 give typical mixture proportions for normal weight ﬁbrous reinforced concrete and ﬂy-ash ﬁbrous concrete mixes, respectively. A workable method for mixing in a step-by-step chronological procedure can be summarized as follows: • Blend part of the ﬁber and aggregate before charging into the mixer. • Blend the ﬁne and coarse aggregate in the mixer, add more ﬁbers at mixing speed, then add cement and water simultaneously or add the cement immediately followed by water and additives.

22-5

Fiber-Reinforced Composites

Load B

Deflection

FIGURE 22.1 Schematic load–deﬂection relationship of ﬁber-reinforced concrete.

• Add the balance of the ﬁber to the previously charged constituents, and add the remaining cementitious materials and water. • Continue mixing as required by normal practice. • Place the ﬁbrous concrete in the forms. Use of ﬁbers requires more vibrating than required in nonﬁbrous concrete; although internal vibration is acceptable if carefully applied, external vibration of the formwork and the surface is preferable to prevent segregation of the ﬁbers.

22.4 Mechanics of Fiber Reinforcement 22.4.1 First Cracking Load Fiber-reinforced concrete in ﬂexure essentially undergoes a trilinear deformation behavior as shown in Figure 22.1. Point A on the load-deﬂection diagram represents the ﬁrst cracking load, which can be termed the first-crack strength (Mindess and Young, 1981). Normally, this is the same load level at which a nonreinforced element cracks; hence, segment OA in the diagram would be the same and essentially have the same slope for both plain and ﬁber-reinforced concrete. Once the matrix is cracked, the applied load is transferred to the ﬁbers that bridge and tie the crack to keep it from opening further. As the ﬁbers deform, additional narrow cracks develop, and continued cracking of the matrix takes place until the maximum load reaches point B of the load-deﬂection diagram. During this stage, debonding and pullout of some of the ﬁbers occur, but the yield strength in most of the ﬁbers is not reached. In the falling branch, BC, of the load-deﬂection diagram, matrix cracking and ﬁber pullout continue. If the ﬁbers are long enough to maintain their bond with the surrounding gel, they may fail by yielding or by fracture of the ﬁber element, depending on their size and spacing.

22.4.2 Critical Fiber Length: Length Factor If lc is the critical length of a ﬁber above which the ﬁber fractures instead of pulling out when the crack intersects the ﬁber at its midpoint, it can be approximated by (Mindess and Young, 1981): c =

df σf 2vb

(22.1)

where: df = ﬁber diameter. vb = interfacial bond strength. σf = ﬁber strength. Bentur and Mindess (1990) developed an expression to relate the average pullout work and the ﬁber matrix interfacial bond strength in terms of the critical ﬁber length, demonstrating that the strength of

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Concrete Construction Engineering Handbook

1200 8.0

6.0 800 600

4.0

7.0% 5.0%

400

MPa

Tensile Cracking Stress (psi)

1000

2.5% 2.0

200 0

0.2

0.4

0.6 0.8 1.0 Wire Spacing (in.)

1.2

1.4

1.6

FIGURE 22.2 Effect of steel-ﬁber spacing on the tensile cracking stress in ﬁbrous concrete for ρ = 2.5, 5.9, and 7.5%. (From Romualdi, J.P. and Batson, G.B., Proc. ASCE Eng. Mech. J., 89(EM3), 147–168, 1963.)

a composite increases continuously with the ﬁber length. This is of signiﬁcance as it indicates that pullout work may go through a maximum and then decreases as bond strength increases over a critical value. This loss of pullout work would be reduced to a typical range of = 10 mm in the cement-based composites discussed in Section 22.6. If a critical vb value of 1.0 MPa and a small-diameter ﬁber (e.g., df = 20 µm) are chosen, then an increase in bond may result in reduced toughness.

22.4.3 Critical Fiber Spacing: Space Factor The spacing of the ﬁbers considerably affects cracking development in the matrix. The closer the spacing, the higher the ﬁrst cracking load of the matrix. This is due to the fact that the ﬁbers reduce the stressintensity factor that controls fracture. The approach taken by Romualdi and Batson (1963) to increase the tensile strength of the mortar was to increase the stress-intensity factor by decreasing the spacing of the ﬁbers acting as crack arresters. Figure 22.2 relates the tensile cracking stress to the spacing of the ﬁbers for various volumetric percentages. Figure 22.3 compares the theoretical and experimental values of the ratio of the ﬁrst cracking load to the cracking strength of plain concrete (strength ratio). Both diagrams demonstrate that the strength ratio increases as the spacing of the ﬁbers is reduced; that is, the tensile strength of the concrete increases up to the practical workability and cost-effectiveness limits. Various shapes and sizes of steel ﬁbers are shown in Figure 22.4. Several expressions to deﬁne the spacing of the ﬁbers have been developed. If s is the spacing of the ﬁbers, one expression from Romualdi and Batson (1963) gives: 1.0 ρ

s = 13.8d f

(22.2)

where: df = diameter of the ﬁber. ρ = ﬁber percent by volume of the matrix. Another expression due to McKee (1969) gives: s=3

V ρ

(22.3)

22-7

Fiber-Reinforced Composites

3.0

2.5

Strength Ratio

Indirect tension Beam in bending 2.0 Theoretical 1.5 Experimental 1.0

0.5

0.2

0.4

0.6

0.8

1.0

Wire Spacing (in.)

FIGURE 22.3 Effect of ﬁber spacing on the strength ratio. Ratio equals ﬁrst cracking load of ﬁbrous concrete divided by strength of plain concrete. (From Romualdi, J.P. and Mandel, J.A., Proc. ACI J., 61(6), 657–671, 1964.)

0.010" × 0.022" × 1.0" Fiber

0.016" × 0.75" Deformed

0.016" × 0.75" Round

0.016" × 1.0" Deformed

0.014" × 1.0" Round

FIGURE 22.4 Various shapes and sizes of steel ﬁbers.

where V is the volume of one ﬁber element. An expression that also takes into account the length of the ﬁber gives (Mindess and Young, 1981): s = 13.8d f

(22.4)

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Concrete Construction Engineering Handbook

22.4.4 Fiber Orientation: Fiber Efficiency Factor The orientation of the ﬁbers with respect to load determines the efﬁciency with which the randomly oriented ﬁbers can resist the tensile forces in their directions. This observation is synonymous with the contribution of bent bars and vertical shear stirrups provided in beams to resist the inclined diagonal tension stress. If one assumes perfect randomness, the efﬁciency factor is 0.41l, but it can vary between 0.33l and 0.65l close to the surface of the specimen, as trowling or leveling can modify the orientation of the ﬁbers (Mindess and Young, 1981).

22.4.5 Static Flexural Strength Prediction: Beams with Fibers Only To predict ﬂexural strength, several methods could be applied depending on the type of ﬁber, the type of matrix, whether empirical data from laboratory experiments are used, or whether the design is based on the bonded area of the ﬁber or the law of mixtures. An empirical expression for the composite ﬂexural strength based on a composite-material approach is (Bentur and Mindess, 1990):

(

)

σc = A σm 1 − V f + BV f where: σc σm Vf A, B /d

= = = = =

(22.5)

composite ﬂexural strength. ultimate strength of the matrix. volume fraction of the ﬁbers adjusted for the effect of randomness. constants. aspect ratio of the ﬁber, where l is the length and d is the diameter of the ﬁber.

The constants A and B were obtained from 4 × 4 × 12-in. (100 × 100 × 305-mm) model beam tests by Swamy et al. (1974) and were adopted by ACI Committee 544 (1993). These constants lead to the following expressions: First crack composite flexural strength (psi): σ f = 0.843 f rVm + 425V f where: fr = Vm = Vf = /df =

(22.6)

stress in the matrix (modulus of rupture of the plane mortar or concrete) (lb/in.2). volume fraction of the matrix = 1 – Vf. volume fraction of the ﬁbers = 1 – Vm. ratio of length to diameter of the ﬁbers (i.e., the aspect ratio).

Ultimate composite flexural strength (psi): σcu = 0.97 f rVm + 494V f

(22.7)

22.5 Mechanical Properties of Fibrous Concrete Structural Elements 22.5.1 Controlling Factors From Section 22.4, it can be seen that the mechanical properties of ﬁber-reinforced concretes are inﬂuenced by several factors:

22-9

Fiber-Reinforced Composites

0.005

15000

0.010

0.015

mm/mm

80 10000 60 40

5000

Vf = 1.0% Vf = 0.5% Vf = 0%

0.005

0.01

0.015

Compressive Strength (MPa)

Compressive Strength (psi)

100

in./in.

Unit Strain

FIGURE 22.5 Inﬂuence of volume fraction of steel ﬁbers on stress–strain behavior for 13,000-psi concrete. (From Shah, S.P. and Rangan, B.V., Proc. ACI J., 68(2), 126–134, 1971.)

• • • • • •

Type of ﬁber (i.e., the ﬁber material and its shape) Aspect ratio /df (i.e., the ratio of ﬁber length to nominal diameter) Amount of ﬁber in percentage by volume (ρ) Spacing of the ﬁber (s) Strength of the concrete or mortar matrix Size, shape, and preparation of the specimen

Hence, it is important to conduct laboratory tests to failure on the mixtures using specimen models similar in form to the elements being designed. As the ﬁbers affect the performance of the end product in all material-resistance capacities such as in ﬂexure, shear, direct tension, and impact, it is important to evaluate the test specimen performance with regard to those parameters. The contribution of the ﬁber to tensile strength, as discussed in Section 22.3, is due to its ability to act as reinforcement and assume the stress from the matrix when it cracks through the interface shear friction interlock between the ﬁber and the matrix. This phenomenon is analogous to the shear friction interlock hypothesis presented in Nawy (1996) in his discussion on the mechanism of shear friction interlock. Deformed or crimped ﬁbers have a greater inﬂuence than smooth and straight ones. The pullout resistance in zone AB of Figure 22.1 is proportional to the interfacial surface area (ACI Committee 544, 1993). The non-round ﬁber cross-sections and the smaller diameter round ﬁbers induce a larger resistance per unit volume than the larger diameter ﬁbers. This is also analogous to the crack-control behavior in traditionally reinforced structural members, where a larger number of smaller diameter bars that are more closely spaced is more effective than a smaller number of large diameter bars for the same reinforcement volume percentage (Nawy and Blair, 1971). One reason for this is the larger surface interaction area between the ﬁbers and the surrounding matrix, resulting in a higher bond and shear friction resistance.

22.5.2 Strength in Compression The effect of the contribution of the ﬁbers to the compressive strength of the concrete seems to be minor, as seen in Figure 22.5 (Hsu and Hsu, 1994) for tests using steel ﬁbers; however, the ductility and toughness are considerably enhanced as a function of the increase in the volume fractions and aspect ratios of the ﬁbers used. Figure 22.5 shows the effect of an increase in volume fraction on the stress–strain relationship of the ﬁbrous concrete resulting from an increase in the ﬁber volume from 0 to 1.5% for concretes having a compressive strength of 13,100 psi (90.3 MPa). Figure 22.6 and Figure 22.7 depict a similar trend with

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Concrete Construction Engineering Handbook

10000

Smooth steel fibers /df = 83

8000 6000

Vf = 3%

MPa

Compressive Stress (psi)

Vf = 2%

4000

Vf = 1%

2000 Control 0

0.005

0.010 Axial Strain

0.015

0.020 in./in.

FIGURE 22.6 Inﬂuence of volume fraction of steel ﬁbers on stress–strain behavior for 9000 psi concrete. (From Fanella, D.A. and Naaman, A.E., ACI J., 82(4), 475–483, 1985.) 80 Smooth steel fibers Vf = 2%

10000

/df = 100

8000

6000 /df = 83 /df = 47

4000 2000 0

60 MPa

Compressive Stress (psi)

12000

Control 0.005

0.010 Axial Strain

0.015

0.020 in./in.

FIGURE 22.7 Inﬂuence of aspect ratio of steel ﬁbers on stress–strain behavior. (From Fanella, D.A. and Naaman, A.E., ACI J., 82(4), 475–483, 1985.)

respect to both a volume fraction ratio up to 3% and an aspect ratio in the range of 47 to 100. Figure 22.8 also demonstrates the inﬂuence of the increase in ﬁber content on the relative toughness of reinforced concrete members. Toughness is a measure of the ability to absorb energy during deformation. It can be estimated from the area under the stress–strain or load-deformation diagrams. A toughness index (TI) expression proposed by Hsu and Hsu (1994) follows: TI = 1.421RI + 1.035

(22.8)

where: RI = reinforcing index = Vf(/df ). Vf = volume fraction. /df = aspect ratio. Figure 22.9 illustrates the relationship of the toughness index to the reinforcing index of ﬁbrous high-strength concretes within the limitations of the type, aspect ratio, and volume fractions of the steel ﬁbers used in those tests. In short, by increasing the volume fraction, both ductility and toughness have been shown to increase signiﬁcantly within the practical limits of workable volume content of ﬁber in a concrete mix.

22-11

Fiber-Reinforced Composites

20 Constant rate of loading 0.01"/min area under the curve taken as a measure of toughness

Load

Toughness and Strength in Relation to Plain Concrete

Center line deflection Concrete: 1:2:3, 0.60 Maximum size = 3/8" 14-day moist curing

10 Fibers: 0.01" × 0.01" × 3/4" fy = 110,000 psi fs = 120,000 psi

Relative toughness

6 Results are average of 4 specimens

Relative strength

0.25

0.50

0.75

1.00

1.25

% Volume of Fibers

FIGURE 22.8 Relative toughness and strength vs. ﬁber volume ratio. (From Shah, S.P. and Rangan, B.V., Proc. ACI J., 68(2), 126–134, 1971.)

RI = Vf × /df TI = 1.421(RI) + 1.035

Toughness Index (TI)

2.5 2.0

1.5

1.0

0.5

0.1

0.2

0.3 0.4 0.5 Reinforcing Index (RI)

0.6

0.7

FIGURE 22.9 Toughness index vs. reinforcing index of ﬁbrous concrete. (From Shah, S.P. and Rangan, B.V., Proc. ACI J., 68(2), 126–134, 1971.)

22.5.3 Strength in Direct Tension The effect of different shapes of the ﬁber ﬁlaments on the tensile stress behavior of steel-ﬁber-reinforced mortars in direct tension is demonstrated in Figure 22.10. The descending portion of the plots show that the ﬁbers reinforced with better anchorage quality increase the tensile resistance of the ﬁber-reinforced concrete beyond the ﬁrst cracking load.

22-12

400

Straight Fibers

Hooked Fibers

/df = 66

/df = 75

Enlarged-end Fibers

3.0

/df = 67

300

2.0

200 1.0

100 0 0

MPa

Tensile Strength (psi)

Concrete Construction Engineering Handbook

0.004 0.006 0.008

0.004 0.006 0.008 0.010

FIGURE 22.10 Effect of the shape of steel ﬁbers on tensile stress in mortar specimens loaded in direct tension. (From Shah, S.P. and Rangan, B.V., Proc. ACI J., 68(2), 126–134, 1971.)

22.5.4 Flexural Strength Fibers seem to affect the magnitude of ﬂexural strength in concrete and mortar elements to a much greater extent than they affect the strength of comparable elements subjected to direct tension or compression (ACI Committee 544, 1993). Two stages of loading portray the behavior. The ﬁrst controlling stage is the first cracking load stage in the load-deﬂection diagram, and the second controlling stage is the ultimate load stage. Both the ﬁrst cracking load and the ultimate ﬂexural capacity are affected as a function of the product of the ﬁber volume concentration (ρ) and the aspect ratio (/df ). Fiber concentrations less than .5% of the volume of the matrix and with an aspect ratio less 50 seem to have a small effect on the ﬂexural strength, although they can still have a pronounced effect on the toughness of the concrete element, as seen in Figure 22.8. The ﬂexural strength of plain concrete beams containing steel ﬁbers was deﬁned in Equation 22.6 and Equation 22.7. For structural beams reinforced with both normal reinforcing bars and ﬁbers added to the matrix, a modiﬁcation of the standard expression for nominal moment strength, Mn = Asfy(d – a/2), must be made to account for the shear friction interaction of the ﬁbers in preventing the ﬂexural macrocracks from opening and propagating in the tensile zone of the concrete section, as seen in Figure 22.11 (Henager and Doherty, 1976). In this diagram, the area of concrete in the tensile zone is neglected and an additional equilibrium tensile force (Tfc) is added to the section. This moves the neutral axis down, leading to a higher nominal moment strength (Mn). The resulting expression for Mn becomes (Henager and Doherty, 1976):  h e a a M n = As f y  d −  + σt b (h − e ) + −    2 2 2 2 e = e ( fibers ) + 0.003  σt (psi) =

σ1 (MPa) =

c 0.003

(22.9)

(22.10)

1.12 d f ρ f Fbc

(22.11a)

0.00772l d f ρ f Fbe

(22.11b)

where: df ρf Fbe a b

= = = = = =

ﬁber length. ﬁber diameter. percent by volume of the ﬁbers. bond efﬁciency of the steel ﬁber depending on its characteristics (varies from 1.0 to 1.2). depth of the equivalent rectangular block. width of beam.

22-13

Fiber-Reinforced Composites

0.85fcʹ

εc = 0.003 a/2

e h

Neutral

axis εs (Fibers)

Tfc Trb

εs (Bars) σt (a)

(b)

(c)

FIGURE 22.11 Stress and strain distribution across depth of singly reinforced ﬁbrous concrete beams: (a) assumed stress distribution, (b) equivalent stress block distribution, and (c) strain distribution.

c = depth to the neutral axis. d = effective depth of the beam to the center of the main tensile bar reinforcement. e = distance from the extreme compression ﬁbers to the top of the tensile stress block of the ﬁbrous concrete. es = fy /Es of the bar reinforcement. ef = σf /Ec of the ﬁbers developed at pullout at a dynamic bond stress of 333 psi. σt = tensile yield stress in the ﬁber. Tfc = tensile yield of the ﬁbrous concrete in Figure 22.11 = σtb(h – e). Trb = tensile yield force of the bar reinforcement in Figure 22.11 = As fy.

22.5.5 Shear Strength A combination of vertical stirrups and randomly distributed ﬁbers in the matrix enhances the diagonal tension capacity of concrete beams. The degree of increase in the diagonal tension capacity is a function of the shear span/depth ratio of a beam. This ratio determines the mode of failure in normal beams that do not fall in the category of deep beams and brackets as detailed by Nawy (2008). Williamson (1978) found that when 1.66% by volume of straight steel ﬁbers are used instead of stirrups, the shear capacity increased by 45% over beams without stirrups. When steel ﬁbers with deformed ends were used at a volume ratio of 1.1%, the shear capacity increased by 45 to 67% and the beams failed by ﬂexure. Using crimped-end ﬁbers increased the shear capacity by almost 100%. In general, as the shear span/depth ratio (a/d) decreases and the ﬁber volume increases, the shear strength increases proportionally. Tests by Sharama (1986) resulted in the following expression for the average shear stress (νc) for beams in which steel ﬁbers were added (ACI Committee 544, 1993): 1

2 d4 νcf = f t′   3 a

(22.12)

where: ft′ = tensile splitting strength. d = effective depth of a beam. a = shear span, equal to the distance from the point of application of the load to the face of the support when concentrated loads are acting or equal to the clear beam span when distributed loads are acting.

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Concrete Construction Engineering Handbook

22.5.6 Environmental Effects 22.5.6.1 Freezing and Thawing The addition of ﬁbers to a matrix does not seem to result in an appreciable improvement in the freezing and thawing performance of concrete, as its resistance to such an environmental effect is controlled by permeability, void ratio, and freeze–thaw cycles. Fibers, however, tend to hold the scaling concrete pieces together, thereby reducing the extent of apparent scaling. 22.5.6.2 Shrinkage and Creep No appreciable improvement in the shrinkage and creep performance of concrete results from the addition of ﬁbers, but a slight decrease in shrinkage can result due to the need to add more paste mortar in the mixture when ﬁbers are used. Cracking due to drying shrinkage in restrained elements can be slightly improved, as the cracks are kept from generating because of the bridging effect of the randomly distributed ﬁbers.

22.5.7 Dynamic Loading Performance The cracking behavior of ﬁbrous concrete elements under dynamic loading seems to be three to ten times better that of plain concrete. Also, the total energy absorbed by the steel ﬁbrous concrete beams can be 40 to 100 times that for plain concrete beams, depending on the type, deformed shape, and percent volume of the ﬁbers (ACI Committee 544, 1993).

22.6 Steel-Fiber-Reinforced Cement Composites 22.6.1 General Characteristics Fiber-reinforced concretes are designed to contain a maximum 2% by volume of ﬁbers, using the same mixture design procedures and placement as nonﬁbrous concretes. Fiber-reinforced cement composites, on the other hand, could contain a volume fraction, namely, a ﬁber content by volume, as high as 8 to 25%. Consequently, neither the design of the mixture nor the constituent materials in the matrix can be similar to those of conventional ﬁbrous or nonﬁbrous concretes. Either cement only or cement with sand is used in the mixture, with no coarse aggregate, to achieve the high strength, ductility, and high performance expected from such composites. The 1980s saw the development of macrodefect-free (MDF) cements, which have a high Young's modulus and ﬂexural strengths up to almost 30,000 psi (~200 MPa), as well as densified small-particle (DSP) cements, which have a particle size less than 1/20 that of Portland cement (0.5 µm). The void content in any matrix can be reduced to a negligible percentage with the addition of pozzolans such as silica fume. With these developments as a background, the following are the types of cement-based composites being studied today: • • • • •

Slurry-inﬁltrated ﬁber concrete (SIFCON) and a composite for refractory use (SIFCA®) Densiﬁed small-particle (DSP) systems Compact reinforced composite (CRC) Carbon-ﬁber-reinforced cement-based composites Super-strength reactive powder concrete (RPC).

These cement-based composites can achieve a compressive strength in excess of 44,000 psi (300 MPa) in compression and an energy absorption capacity (i.e., ductility) that can be up to 1000 times that of plain concrete (Reinhardt and Naaman, 1992).

22.6.2 Slurry-Infiltrated Fiber Concrete Because of the high volume fraction of steel ﬁbers (8 to 25%), the mixture for a structural member is formulated by sprinkling the ﬁber into the formwork or over a substratum. Either the substratum is stacked with ﬁbers to a prescribed height or the form is completely or partially ﬁlled with the ﬁbers,

22-15

Fiber-Reinforced Composites

120 100

80 SIFCON

Conventional concrete

MPa

Compressive Strength (ksi)

0.1

0.2

0.3 in./in.

Strain

SIFCON - Vf = 11% Hooked 30 - Random (1) Matrix fcʹ = 13 ksi (2) Matrix fcʹ = 8 ksi (3) Matrix fcʹ = 5 ksi

(1)

(2)

200

150 MPa

Compressive Strength (ksi)

FIGURE 22.12 Stress–strain relationship of SIFCON with rupture strain in the range of 0.45 in./in. (From Naaman, A.E., in Proceedings of the International RILEM/ACI Workshop, Reinhardt, H.W. and Naaman, A.E., Eds., Chapman & Hall, New York, 1992, pp. 18–38.)

100

10 (3)

.000

.030

.060

.090 Strain

.120

.150 in./in.

FIGURE 22.13 Inﬂuence of matrix compressive strength on the stress–strain response of SIFCON in compression. (From Naaman, A.E., in Proceedings of the International RILEM/ACI Workshop, Reinhardt, H.W. and Naaman, A.E., Eds., Chapman & Hall, New York, 1992, pp. 18–38.)

depending on the requirement of the design. After the ﬁbers are placed, a low-viscosity cement slurry is poured or pumped into the ﬁber bed or into the formwork, inﬁltrating into the spaces between the ﬁbers. Typical cement/ﬂy-ash/sand proportions can vary from 90/10/0 to 30/20/50 by weight (Schneider, 1992). The water/cementitious ratio, W/(C + F), can range between 0.45 and 0.20 by weight, with a plasticizer content of 10 to 40 oz. per 100 lb of the total cementitious weight (C + F). Batch trials of the slurry mix have to be carefully made with regard to the W/(C + F) ratio to arrive at a workable slurry mix that can fully penetrate the depth of the ﬁbers. Figure 22.12 provides a stress–strain diagram for a SIFCON mixture (Naaman, 1992) with a compressive strength close to 18,000 psi but with a very large strain capability in the falling branch of the diagram. Figure 22.13 illustrates the inﬂuence of the matrix compressive strength on the stress–strain response of SIFCON in compression (Schneider, 1992). A ﬁber content (Vf ) of 11% resulted in total uniaxial strain in excess of 10%.

22.6.3 DSP and CRC Cement Composites Densiﬁed small-particle (DSP) systems and compact reinforced composite (CRC) gain super high strength depending largely on the type of compact-density cements that are used for the cement-based composites and the proportioning used to considerably reduce or practically eliminate most of the voids in the paste. Figure 22.14 illustrates the fracture surface of a steel-ﬁber-reinforced concrete specimen.

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Concrete Construction Engineering Handbook

FIGURE 22.14 Fracture surface of steel-ﬁber-reinforced concrete. (Photograph courtesy of the American Concrete Institute, Farmington Hills, MI.)

22.6.4 Carbon-Fiber-Reinforced Cement-Based Composites Petroleum-pitch-based carbon ﬁbers have recently been developed for use as reinforcement for cementbased composites. Their diameters vary from 0.0004 to 0.0007 in. (10 to 18 µm), and their lengths vary from 1/8 to 1/2 in. (3 to 12 mm). Their tensile strength typically ranges from 60 to 110 ksi (400 to 750 MPa). They are incorporated in the cement-based composites in essentially the same manner as steel ﬁbers are in concrete, and they are uniformly distributed and randomly oriented. Because of the very small size of the carbon ﬁbers and their small diameter, a high ﬁber count is attained in the cementitious matrix at a typical volume fraction of 0.5 to 3% (Bayasi, 1992). The spacing between the ﬁbers is approximately 0.004 in. (0.1 mm) at a 3% ﬁber volume fraction. The function of the carbon ﬁbers function is similar to that of the steel ﬁbers in preventing microcracks from propagating and opening.

22.6.5 Super-Strength Reactive-Powder Concretes Super-strength reactive powder concrete (RPC) has a compressive strength ranging from 30,000 to 120,000 psi (200 to 800 MPa). The lower range is used today for the construction of structural elements. The higher ranges are used in nonstructural applications such as ﬂooring, safes, and storage compartments for nuclear waste. Concretes in the higher ranges are termed super-high-strength concretes and possess the very high ductility necessary for applications in structural systems. The principal characteristic of such concretes is the use of a powder concrete in which aggregates and traditional sand are replaced by ground quartz less than 300 µm in size (Richard and Cheyrezy, 1994). In this manner, the homogeneity of the mixture is greatly improved, and the distribution in the size of the particles is consequently reduced by almost two orders of magnitude. A major improvement in the properties of the hardened concrete is an increase in the Young's modulus value of the paste by almost a factor of three so its value can reach to 6 to 11 × 10–6 psi (55 to 75 GPa), thereby reducing the effects of incompatibility between the moduli of the paste and the quartz powder. Richard and Cheyrezy (1994) developed the following mechanical characteristics of RPC concrete: • Improved homogeneity resulting in a Young's modulus up to 11 × 10–6 psi (75 GPa) • Increase in dry compact density of the dry solids (although silica fume, with its small particle size of 0.1 to 0.5 µm and an optimum mix content of 25% cement by weight, gives excellent dry compact density, additional amounts of precipitated silica further improve the dry compact density)

22-17

Fiber-Reinforced Composites

TABLE 22.4 Mixture Composition and Concrete Mechanical Properties of Super-High-Strength Reactive Powder Concrete RPC 200 Concrete lb/yd3 (MPa)

RPC 800 Concrete lb/yd3 (MPa)

1614 (955) 1775 (1051) — 387 (229) 16.9 (10) 22.0 (13) 323 (191) 31.2 gal/yd3 (153 L/m3) 24–33 (170–230) 3.6–8.7 (25–60) 15,000–40,000 J/m3 7.8–8.7 × 103 (54–60 GPa)

1690 (1000) 845 (500) 659 (390) 389 (230) — 30.4 (18) 1065 (630) 36.7 gal/yd3 (180 L/m3) 71–99 (490–680) 6.5–14.8 (45–102) 1200–2000 J/m3 9.8–10.9 × 103 (65–75 GPa)

Mixture Constituents Portland cement, Type V Fine sand (150–400 µm) Ground quartz (4 µm) Silica fume (18 m3/g) Precipitated silica (35 m3/g) Superplasticizer (polyacrylate) Steel ﬁbers Total water Cylinder compressive strength Flexural strength Fracture energy Young's modulus

Note: 1 L/m3 = 0.2 gal/yd3 = 1.69 lb/yd3; 1 L (water) = 2.204 lb = 0.264 gal; 1000 psi = 6.895 MPa; 1 kg/m3 = 1.69 lb/yd3. Resistant type V cement was used in all the mixtures. Source: Richard, P. and Cheyrezy, M.H., in Proceedings, V.M. Malhotra Symposium on Concrete Technology: Past, Present and Future, Mehta, P.K., Ed., ACI SP-144, American Concrete Institute, Farmington Hills, MI, 1994, pp. 507–518.

• Increase in the density of concrete by maintaining the fresh concrete under pressure at the placement stage and during setting, which results in the removal of air bubbles, expulsion of excess water, and partial reduction of the plastic shrinkage during ﬁnal set • Improvement in microstructure by hot curing for 2 days at 194°F (90°C) to speed the activation of the pozzolanic reaction of the silica fume, resulting in a 30% gain in compressive strength • Increase in ductile behavior through the addition of an adequate volume fraction of steel microﬁbers. Table 22.4 gives the mix proportions for Type 200 and Type 800 RPC concretes. It also lists the major mechanical properties of these concretes.

22.7 Prestressed Concrete Prism Elements as the Main Composite Reinforcement in Concrete Beams Composite concrete that uses precast pretensioned prisms as its main tension reinforcement has shown promise for the effective control of cracking and deﬂection in structural concrete elements, particularly in the negative moment regions of reinforced concrete bridge decks. Most of the earlier studies on this subject were limited to experimental laboratory work (Chen and Nawy, 1994). Introducing highly precompressed prisms as the main reinforcement in the beam tension zone can increase ductility, control and delay the formation and propagation of cracking, reduce deﬂection, and improve the high-performance characteristics of environmentally exposed structural components, as reported by Chen and Nawy (1994). Their work involved several patterns of composite beams and a concrete strength of 14,000 psi (97 MPa), as shown in Figure 22.15. All evaluations were based on test-to-failure results. Fiberoptic Bragg grating sensors developed for this work were used to monitor strains, deformations, and crack widths in the 10-ft (3.3-m) simple, continuous beams that were tested. The relative mild steel and prestressed prism contents of the beams affected the behavior of these composite members (Chen and Nawy, 1994; Nawy and Chen, 1997). To account for this inﬂuence, a combined reinforcement index (ω) was used in the design of a prismreinforced structural concrete beam. This index value was obtained from the following expression: ω=

A ps f ps + Asmf y − As′ f y′ bd p f c′

(22.13)

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Concrete Construction Engineering Handbook

48.0"

60.0"

B #3 @ 8"

48.0"

4 #3 @ 4"

108.0"

6.0"

5 #3 @ 4"

#3 @ 6"

60.0"

108.0"

6.0"

228.0"

3#5

2 2" × 2" Prism 2 1#5 14.0"

4.0"

2#2 4

A - A (C-1)

2#2

10.313" 12.0" 4.0"

2#2

2#2 14.0"

8.0" 4.0"

10.75" 9.75"

× 4.0"

4#4

1 B - B (C-1, C-2, C-3)

2#2

4 A - A (C-3, C-4)

A - A (C-2) 14.0"

10.688" 12.0"

4.0"

3 2" × 2" Prism 14.0"

10.313" 12.0"

5.0"

8.0" 4.0"

12.0" 10.313"

14.0"

4.0"

2#5

B - B (C-4)

FIGURE 22.15 Prism composite reinforcement geometry. (From Nawy, E.G. and Chen, B., in Proceedings, Transportation Research Board. National Research Council, Washington, D.C., 1998.)

where Aps, As, and As′ are the areas of prestressing and nonprestressing reinforcement, fps is the design stress in the prestressing reinforcement at ultimate load, fy and fy′ are the yield strengths of deformed bars in tension and in compression, b is the width of the compression face of the member or ﬂange in the case of a T-section, and dp is the effective depth of the beam cross-section. Other systems of composites can also improve the performance of reinforced concrete beams through the use of two-layer systems, one of which is made out of normal- or high-strength concrete and the layer above or below is made of high-strength polymer concrete. In such cases, the beam cross-section is built in two layers, in a manner similar to that of the SIFCON two-layer system. Several investigations have demonstrated that the shear friction interaction of the interface between the two layers of different strength concretes, one of which is a polymer concrete, has a resistance to slip superior to the interlock between two layers made from concrete only (Nawy et al., 1992).

Part B. Fiber-Reinforced Plastic (FRP) Composites 22.8 Historical Development Use of nonmetallic ﬁbers, particularly ﬁberglass elements, bundled into continuous reinforcing elements has been considered since the 1950s for prestressing reinforcement (ACI Committee 440, 1996; Nawy, 1996; Rubinsky and Rubinsky, 1954). Advances were made in polymer development by using polymerimpregnated bundled ﬁberglass ﬁbers as rods for anchorages in tunneling. In the mid-1960s, Nawy et al. (ACI Committee 440, 1996; Nawy and Neuwerth, 1977; Nawy et al., 1971) conducted extensive work on the use of bundled and resin-impregnated glass ﬁbers formed into deformed bars as the main

Fiber-Reinforced Composites

22-19

reinforcement in structural elements. Except for cases where magnetic ﬁelds in supporting structures had to be avoided, commercial application of bundled and resin-impregnated reinforcement in structural concrete elements was not recognized until the late 1970s. It is important to state at this juncture that the term plastic could be misleading; hence, there is general consensus at this time to deﬁne FRP as fiberreinforced polymer composites. In the 1980s, an increased interest in and use of such glass-ﬁber-reinforced polymer (GFRP) reinforcing bars was developing. This was particularly overdue and important for reinforced concrete surrounding or supporting magnetic resonance imaging (MRI) medical equipment. Such equipment includes sensitive magnets and cannot tolerate the presence of any steel reinforcement. Also, where environmental and chemical attacks are present, GFRP reinforcement is more durable and efﬁcient as concrete reinforcement. As stated in ACI Committee 440 (1996), composite rebars have more recently been used in the construction of seawalls, industrial roof decks, base pads for electrical and reactor equipment, and concrete ﬂoor slabs in aggressive chemical environments. In 1986, Germany built the world's ﬁrst highway bridge using composite reinforcement (ACI Committee 440, 1996). In the United States, signiﬁcant funds have been expended on product evaluation and further development, and at least nine major companies have been actively marketing this product since the early 1990s. Additionally, considerable progress has been made in using glass ﬁber ﬁlaments as a supplement to concrete matrices to improve the mechanical properties of concrete, but not as a replacement for the main bar reinforcement in supporting structural components. Glass ﬁbers that are alkali resistant are also gaining wide use. These normally contain zirconium (ZrO2) to minimize or eliminate the alkaline corrosive attack on glass present in the cement paste. Synthetic ﬁbers made from nylon or polypropylene, both loose and woven into geotextile form, have recently begun to be utilized due to the availability of information on their mechanical performance in the matrix and a better understanding of their structural contribution to crack resistance. Although the use of other types of nonmetallic ﬁbers has been explored, interest has grown primarily in the use of carbon ﬁbers as the main reinforcement apart from its use in cement-based composites. It is safe to state now that the science of ﬁbrous concrete and composites has advanced to an extent that justiﬁes its extended use in the years to come.

22.9 Beams and Two-Way Slabs Reinforced with GFRP Bars In the late 1960s and early 1970s, Nawy and his team at Rutgers University (Nawy and Neuwerth, 1977; Nawy et al., 1971) researched the use of glass-ﬁber-reinforced plastic bars as a substitute for mild steel reinforcement. Those investigations involved testing to failure a total of 30 beams and 12 two-way slabs. The slabs had an average thickness of 2-1/2 in. and an overall dimension of 7 × 7 ft. The slab panels had 5.5 × 5.5-ft effective spans and were fully restrained along all four boundaries. The GFRP reinforcement was spaced at 3 to 8 in. In the various slabs the reinforcement area varied from 0.196 in.2/ft to 0.074 in.2/ ft in each direction, giving reinforcement percentages of 0.769% and 0.290%, respectively. The beams were either simply supported or continuous over two spans. The centerline span was 9 ft, 11 in. The reinforcement percentages in the beams ranged from 1.045 to 0.696%. These original tests and analyses indicated that both the ﬁberglass-reinforced slabs and the beams behaved similarly in cracking, deﬂections, and ultimate load to steel-reinforced beams. The large number of well-distributed cracks in the GFRP-reinforced beams and slabs indicated that a good mechanical bond developed between the GFRP bar and the surrounding concrete. The research also demonstrated that the equations for ﬂexure accurately predicted the ﬂexural behavior of GFRP-reinforced members with the same accuracy as for the mild steel reinforced beams. A typical stress–strain diagram of the reinforcement is shown in Figure 22.16. This research led to investigations by Larralde et al., Satoh et al., Goodspeed et al., Ehsani et al., Zia et al., Bank and Xi, Porter et al., Faza and GangaRao, and Nanni (1993). A summary of their work and publications as well as details of the original Nawy (1971) work are given in the ACI Committee 440 report (1996). Table 22.5 provides a relative comparison of the mechanical properties of GFRP and steel reinforcement.

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Concrete Construction Engineering Handbook

(1 ksi = 6.895 MPa) 1200

120

900

600

Fiberglass bundled filaments Externally resin coated

0.118" diameter (nominal) Fu = 154.8 ksi Fy = 145.0 ksi (0.123% offset) Ef = 7.3 ×103 ksi

.003

.006

.009

.012 .015 Strain ("/")

.018

400

Stress (MPa)

Stress (ksi)

150

200

.021

.024

(a) 900 100

(1 ksi = 6.895 MPa) 600

400 Fiberglass twisted filaments Polymer resin impregnated (Bar resin content = 40%)

200

0.25" diameter (nominal) Fu = 105.5 ksi Fy = 96.5 ksi (0.12% offset) Ef = 3.8 ×103 ksi

0.004

0.008

0.012

0.016

0.020 Strain ("/")

0.024

0.028

Stress (MPa)

Stress (ksi)

0.032

(b)

FIGURE 22.16 Typical stress–strain relationship of ﬁberglass composite bar reinforcement: (a) Coated ﬁlaments. (From Nawy, E.G. and Neuwerth, G.E., Proc. ASCE J. Struct. Div., 103(ST2), 421–440, 1977.) (b) Impregnated ﬁlaments. (From Nawy, E.G. et al., Proc. ASCE J. Struct. Div., 97(ST9), 2203–2215, 1971.)

22.10 Carbon Fibers and Composite Reinforcement 22.10.1 Carbon Fibers Essentially, the two types of carbon ﬁbers are high-modulus Type I and high-strength Type II. The fundamental difference between their properties is the result of the differences in their microstructures, which depend on the arrangement of the hexagonal graphine-layer networks in the graphite (ACI Committee 440, 1996). To attain a modulus of 30 × 106 psi (200 GPa), the graphine layers of highmodulus Type I carbon ﬁbers are aligned approximately parallel to the axis of the ﬁbers. Examples are

22-21

Fiber-Reinforced Composites

TABLE 22.5 Comparison of Mechanical Properties of GFRP and Steel Reinforcement Steel Rebar

Prestressing Steel Tendon

GFRP Bar

GFRP Tendon

CFRP Tendon

Tensile strength MPa ksi

483–690 70–100

1379–1862 200–270

517–1207 75–175

1379–1724 200–250

1665–2068 240–300

Yield strength MPa ksi

276–414 40–80

— —

200 29

414–552 6–8

48–62 7–9

152–165 22–24

276–414 40–80

— —

310–482 45–70

— —

11.7 6.5 7.9

9.9 5.5 1.5–2.0

9.9 5.5 2.4

0 0 1.7

Property

Tensile modulus GPa ksi × 10–3 Compressive strength MPa ksi

Coefficient of thermal expansion 11.7 (× 10–6)/°C (× 10–6)/ºF 6.5 Specific gravity 7.9

Note: All strengths are in the longitudinal direction. Source: ACI Committee 440, State-of-the-Art Report on Fiber-Reinforced Plastic Reinforcement for Concrete Structures, ACI 440R, American Concrete Institute, Farmington Hills, MI, 1996.

Kevlar® 49 by DuPont and Twaron® 1055 by Akzo Nobel. Ultra-high-modulus-ﬁber Kevlar® 149 and Twaron® 2000 are also available. Table 22.6 lists the minimum average strength values of Kevlar® and Twaron® reinforcing ﬁbers. Additionally, hybrid composites made from carbon–glass–polyester are available with a strength of up to 115,000 psi (790 MPa), a modulus of 18 × 106 psi (124 MPa), and a density in the range of 0.060 to 0.069 lb/in3. It is important to state that, because of the low ductility of carbon ﬁber in comparison with steel, it is unlikely that it would be used as composite main bar reinforcement. Economically, it would be cost prohibitive; however, it can be used and is being used as prestressing reinforcement and as fabric reinforcement because of its high strength and high modulus, as seen in Table 22.6.

22.10.2 Hybrid GFRP and CFRP Reinforcement for Bridges and Other Structural Systems Fiber-reinforced plastics have become widely popular in Japan, where it was originally initiated, as well as in the United States and elsewhere. They have been utilized in such transportation structures as bridge decks and in column encasements in earthquake retroﬁt construction, particularly hybrid GFRP bars. Figure 22.17 shows essentially negligible deﬂection at service load, even up to the ultimate load, with the TABLE 22.6 Properties of Kevlar® and Twaron® Reinforcing Fibers Property Tensile strength, psi (MPa) Modulus, psi (MPa) Elongation at break (%) Density, lb/in3 (g/cm3)

Kevlar® 49

Twaron® 1055

525,000 (3,620) 18 × 106 (124,000) 2.9 0.052 (1.44)

522,000 (3600) 18.4 × 106 (127,000) 2.5 0.52 (1.45)

Source: ACI Committee 440, State-of-the-Art Report on Fiber-Reinforced Plastic Reinforcement for Concrete Structures, ACI 440R, American Concrete Institute, Farmington Hills, MI, 1996.

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Concrete Construction Engineering Handbook

40 35

Load (kips)

30 25 20

Ultimate Design Load

Service Load

10 ACI Design Load

Box 1. Exp. Box 2. Exp. Box 1. Theor.

5 0 0

200

400

600

800

1000

Deflection (milli-in)

0.05174 K 0.05174 K

0.00138 K

1.4976 K 3.101 K 2.9014 K 2.9014 K 3.101 K 1.4976 K

1.40835 K 3.13282 K 2.95883 K 2.95883 K 3.13282 K 1.40835 K

2 –.0

FIGURE 22.17 Deﬂection–load relationship of hybrid GFRP concrete box culvert analysis and laboratory testing. (From Nawy, E.G., Concrete: The Sustainable Infrastructure Material for the 21st Century, Circular E-C103, Transportation Research Board, Washington, D.C., 2006, pp. 1–24. Courtesy of Dr. A. Nanni.)

22-23

Fiber-Reinforced Composites

53rd Avenue Bridge Bettendorf, Iowa

FIGURE 22.18 Composite technology evaluation. (From Nawy, E.G., Concrete: The Sustainable Infrastructure Material for the 21st Century, Circular E-C103, Transportation Research Board, Washington, D.C., 2006, pp. 1–24. Courtesy of Dr. A. Nanni.)

Level 1 Beams and Joists

Parking Garage

FIGURE 22.19 Upgrade of telecom building using hybrid GFRP-reinforced concrete in beams and joists. (From Nawy, E.G., Concrete: The Sustainable Infrastructure Material for the 21st Century, Circular E-C103, Transportation Research Board, Washington, D.C., 2006, pp. 1–24. Courtesy of Dr. A. Nanni.)

reserve deﬂection control capacity being almost twice that at the theoretical ultimate load. Figure 22.18 shows the deck of the hybrid GFRP-reinforced bridge deck in Bettendorf, Iowa, as an example. Figure 22.19 shows the use of this reinforcing system in beams in the super structure of a typical parking garage. Laminates of carbon-ﬁber-reinforced polymer (CFRP) can also be effectively used for mounting or wrapping concrete elements, such as damaged beam surfaces. Also, CFRP resin-impregnated strands can be spirally wound onto the surface of an existing concrete element, such as a bridge pier or structural building column. Due to the conﬁnement imposed on the member, shear capacity and ductility are improved. CFRP plates are mounted on deteriorated surfaces using high bonding epoxies (Nawy, 2001). Table 22.7 lists the common properties of strengthening laminates. Such innovations can eliminate problems with durability and reinforcement corrosion that often plague bridge structures, garages, and deteriorated beam and slab elements in buildings.

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TABLE 22.7 Properties of Strengthening Laminates Strengthening System Property Type of ﬁbers Fiber orientation

III

CFRP Unidirectional

GFRP Unidirectional

CFRP Unidirectional

2937 (426) 230 (33.4)

758 (110) 62 (9.0)

413 (60) 21 (3.0)

GFRP Bidirectional (x) (y) 482 (70) 310 (45) 14 (2.1) 11 (1.6)

1.2 5 (0.02)

1.2 13 (0.05)

2.0 10 (0.04)

3.0 13 (0.05)

1.4 13 (0.05)

Tensile strength, MPa (ksi) Modulus of elasticity, GPa (× 103 ksi) Failure strain (%) Thickness, mm (× 10–1), in.

2399 (348) 149 (21.7)

Sources: Nawy, E.G., High-Performance Concrete, John Wiley & Sons, New York, 2001, p. 440; Grace, N.F. et al., Strengthening reinforced concrete beams using ﬁber-reinforced polymer (FRP) laminates, ACI Struct. J., 96(5), 865–874, 1999.

22.10.3 Use as Internal Prestressing Reinforcement Fiber-reinforced-plastic carbon tendons made with ﬁber elements 0.125 to 0.157 in. (3 to 4 mm) in diameter are used for prestressing. Their ultimate strength is comparable to prestressing strands, ranging between 270,000 and 300,000 psi (1866 and 2070 MPa).

22.10.4 Use as External Reinforcement Unidirectional FRP sheets made of carbon (CFRP) or glass ﬁber (GFRP) bonded with polymer matrix (epoxy, polyester, vinyl, or ester) are being used to provide protection against corrosion, and they eliminate the need for joints because of the unlimited length of the composite sheets (ACI Committee 440, 1996). They are useful in increasing the ﬂexural and shear strength of concrete members when these composite plates are epoxy bonded to the exterior facing of the elements. They are also of particular use in the retroﬁt of deteriorating concrete structures and in retroﬁtting columns in seismic zones. Several techniques for wrapping concrete elements with CFRP sheets have been developed (ACI Committee 440, 1996; Nanni, 1993). CFRP resin-impregnated strands can be spirally wound onto the surface of an existing concrete element such as a bridge pier or a structural building column. In this manner, due to the conﬁnement imposed on the member, shear capacity and ductility are improved. Nanni's (1993) work on the effect of wrapping conventional concrete demonstrated that signiﬁcant enhancement can be achieved in the strength and ductility of the wrapped concrete element.

22.11 Fire Resistance The resistance of ﬁber-reinforced polymer composites is relatively lower than that of other systems, as degradation of the polymer resin content under heat and ultraviolet light can lead to some long-term durability problems. The carbon and glass ﬁbers and the fabrics used in the FRP can withstand normal ﬁre exposure and are durable under ultraviolet light, but the weak link is the organic polymers used to prepare the ﬁberglass or carbon used as reinforcing elements through impregnation or wrapping. One way to address this deﬁciency is to substitute an inorganic resin for the organic polymer (Foden et al., 1996). An inorganic resin can be an alkali aluminosilicate that can set at moderate temperatures and be able to withstand up to 1000°C. The system is highly impermeable so it can protect the carbon ﬁlaments from oxidation. Tests conducted on carbon, silicon carbide, and glass composites under tension, bending, shear, and fatigue loading indicated that the mechanical properties of the nonorganic composites used are comparable to those of organic polymer composites while having the advantage of relatively higher ﬁre resistance (Foden et al., 1996).

Fiber-Reinforced Composites

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22.12 Summary The concretes described in this chapter have demonstrated that the strength, ductility, and performance of concretes and cement-based composites have and will continue to achieve higher plateaus. A new era in construction materials technology has commenced that promises to have a revolutionary impact on constructed systems in the 21st century. Considerable work must be done to enhance the practicability of these materials and make them cost effective. It is only with simplicity and practicability in application and the achievement of a cost-effective competitive end product that these developments in the science of materials technology can gain universal acceptance and large-scale application.

Acknowledgments This chapter is based on material taken with permission from Fundamentals of High-Strength, HighPerformance Concrete, by E.G. Nawy (Addison Wesley Longman, 1996); High-Performance Concrete, by E.G. Nawy (John Wiley & Sons, 2001); Reinforced Concrete: A Fundamental Approach, 6th ed., by E.G. Nawy (Prentice Hall, 2008); Prestressed Concrete: A Fundamental Approach, 5th ed., by E.G. Nawy (Prentice Hall, 2006); and from various committee reports and standards of the American Concrete Institute, Farmington Hills, MI.

References ACI Committee 440. 1996. State-of-the-Art on Fiber Reinforced Plastic Reinforcement for Concrete Structures, ACI 440R. American Concrete Institute, Farmington Hills, MI. ACI Committee 544. 1988. Design Considerations for Steel Fiber Reinforced Concrete, ACI 544.4R. American Concrete Institute, Farmington Hills, MI. ACI Committee 544. 1989. Measurement of Properties of Fiber Reinforced Concrete, ACI 544.2R. American Concrete Institute, Farmington Hills, MI. ACI Committee 544. 1993. Guide for Specifying, Proportioning, Mixing, Placing, and Finishing Steel Fiber Reinforced Concrete, ACI 544.3R. American Concrete Institute, Farmington Hills, MI. ACI Committee 544. 1996. Fiber Reinforced Concrete, ACI 544.1R. American Concrete Institute, Farmington Hills, MI. Bayasi, M.Z. 1992. Application of carbon ﬁber reinforced mortar in composite slab construction. In Proceedings of the International RILEM/ACI Workshop, Reinhardt, H.W. and Naaman, A.E., Eds., pp. 507–517. Chapman & Hall, New York. Bentur, A. and Mindess, S. 1990. Fiber Reinforced Cementitious Deposits. Elsevier, London. Chen, B. and Nawy, E.G. 1994. Structural behavior evaluation of high strength concrete beams reinforced with prestressed prisms using ﬁber optic sensors. ACI Struct. J., 91(6), 708–718. Di Ludovico, M., Nanni, A., Prota, A., and Cosenza, E. 2005. Repair of bridge girders with composites: experimental and analytical validation. ACI Struct. J., 102(5), 639–648. Fanella, D.A. and Naaman, A.E. 1985. Stress–strain properties of ﬁber reinforced concrete in compression. ACI J., 82(4), 475–483. Foden, A., Lyon, R., and Balaguru, P. 1996. A high temperature inorganic resin for use in ﬁber reinforced composites, paper presented at First International NSF Conference on Composites in Infrastructures, January 15–17, Tucson, AZ. Grace, N.K., Abdel-Sayed, G., Soliman, A.K., and Saleh, K.R. 1999. Strengthening reinforced concrete beams using ﬁber reinforced polymer (FRP) laminates, ACI Struct. J., 96(5), 865–874. Henager, C.H. and Doherty, T.J. 1976. Analysis of ﬁbrous reinforced concrete beams. J. Struct. Div. ASCE, 102, 177–188. Hsu, L.S. and Hsu, T.C.T. 1994. Stress–strain behavior of steel-ﬁber high-strength concrete under compression. Proc. ACI Struct. J., 91(4), 448–457.

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Lopez, A. and Nanni, A. 2006. Composite technology evaluation. Concrete Int. Design Construct., 28(1), 74—80 . McKee, D.C. 1969. The Properties of Expansive Cement Mortar Reinforced with Random Wire Fibres, Ph.D. thesis, University of Illinois, Urbana. Naaman, A.E. 1992. SIFCON: tailored properties for structural performance. In Proceedings of the International RILEM/ACI Workshop, Reinhardt, H.W. and Naaman, A.E., Eds., pp. 18–38. Chapman & Hall, New York. Nanni, A. 1993. Flexural behavior and design of RC members using FRP reinforcement. ASCE J. Struct. Eng., 119(11), 3344–3359. Nawy, E.G. 1996. Fundamentals of High-Strength, High-Performance Concrete. Addison Wesley Longman, London. Nawy, E. G. 2001. Fundamentals of High-Performance Concrete. John Wiley & Sons, New York. Nawy, E, G. 2006a. Concrete: The Sustainable Infrastructure Material for the 21st Century, Circular E-C103. Transportation Research Board, Washington, D.C. Nawy, E.G. 2006b. Prestressed Concrete: A Fundamental Approach, 5th ed. Prentice Hall, Upper Saddle River, NJ. Nawy, E.G. 2008. Reinforced Concrete: A Fundamental Approach, 6th ed. Prentice Hall, Upper Saddle River, NJ, 934 pp. Nawy, E.G. and Blair, K. 1971. Further studies on ﬂexural crack control in structural slab systems. In Proceedings of the International Symposium on Cracking, Deflection, and Ultimate Load of Concrete Slab Systems, Nawy, E.G., Ed., ACI SP-30, pp. 1-30–1-42. American Concrete Institute, Farmington Hills, MI. Nawy, E.G. and Chen, B. 1998. Fiber optic sensing of the behavior of prestressed prism-reinforced continuous composite concrete beams for bridge deck application. In Proceedings, Transportation Research Board. National Research Council, Washington, D.C. Nawy, E.G. and Neuwerth, G.E. 1977. Fiber glass reinforced concrete slabs and beams. J. Struct. Div. ASCE, 103, 421–440. Nawy, E.G., Neuwerth, G.E., and Phillips, C.J. 1971. Behavior of ﬁber glass reinforced concrete beams. J. Struct. Div. ASCE, 97, 2203–2215. Nawy, E.G., Ukadike, M.M., and Balaguru, P.N. 1992. Investigation of concrete PMC composite. J. Struct. Div. ASCE, 108, 1049–1063. Parretti, R. and A. Nanni. 2004. Strengthening of RC members using near-surface mounted FRP composites: design overview. Adv. Struct. Eng. Int. J., 7(6), 469–483. Reinhardt, H.W. and Naaman, A.E., Eds. 1992. High performance ﬁber reinforced cement composite. In Proceedings of the International RILEM/ACI Workshop, p. 565. Chapman & Hall, New York. Richard, P. and Cheyrezy, M.H. 1994. Reactive powder concretes with high ductility and 200–800 MPa compressive strength. In Proceedings, V.M. Malhotra Symposium on Concrete Technology: Past, Present, and Future, Mehta, P.K., Ed., ACI SP-144, pp. 507–518. American Concrete Institute, Farmington Hills, MI. Romualdi, J.P. and Batson, G.B. 1963. Mechanics of crack arrest in concrete. Proc. ASCE Eng. Mech. J., 89(EM3), 147–168. Romualdi, J.P. and Mandel, J.A. 1964. Tensile strength of concrete affected by uniformly distributed closely spaced short lengths of wire reinforcement. J. ACI, 61(6), 657–671. Rubinsky, I.A. and Rubinsky A. 1954. An investigation into the use of ﬁber-glass for prestressed concrete. Mag. Concr. Res., Vol. 6. Schneider, B. 1992. Development of SIFCON through application. In Proceedings of the International RILEM/ACI Workshop, Reinhardt, H.W. and Naaman, A.E., Eds., pp. 177–194. Chapman & Hall, New York. Shah, S.P. 1983. Fiber reinforced concrete. In Handbook of Structural Concrete, Kong, F.K. et al., Eds., pp. 6-1–6-14. McGraw-Hill, New York. Shah, S.P. and Rangan, B.V. 1971. Fiber reinforced concrete properties. ACI J. Proc., 68(2), 126–135.

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Sharama, A.K. 1986. Shear strength of steel ﬁber reinforced concrete beam. ACI J. Proc., 83(4), 624–628. Swamy, R.N. 1975. Fiber reinforcement of cement and concrete. J. Mater. Struct., 8(45), 235–254. Swamy, R.N., Mangat, P.S., and Rao, C.V. 1974. The mechanics of ﬁber reinforcement of cement matrices. In Fiber Reinforced Concrete, ACI SP-44, pp. 1–28. American Concrete Institute, Farmington Hills, MI. Williamson, G.R. 1978. Steel ﬁbers as web reinforcement in reinforced concrete. Proc. U.S. Army Service Conf., 3, 363–377.

(a)

(b)

(c)

Bonded concrete overlays extend the life of bridge decks. (a) Bay Bridge in Maryland receives LMC overlay; (b) LMC overlay is placed in Virginia; (c) silica fume overlay is placed in Virginia.

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