Concrete Construction Engineering Handbook, Second Edition · improve the mechanical properties of the concrete, both as a structure and a material, not as a replacement ... TA BLE

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. (0.25–0.90 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 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 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.


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 manufacturer’s 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)

Young’s Modulus,

psi × 106 (GPa)(5)

Ultimate Elongation (%)

(6)

Acrylic 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 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

Cement matrix — 1.5–2.5 0.4–1.0 (0.003–0.007)

1.5–6.5 (10–45) 0.02

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.



22.3 Mixture Proportioning

Mixing the fibers with the other mix constituents can be done by several methods. The methodselected—plant batching, ready-mixed concrete, or hand mixing in the laboratory—depends on thefacilities available and the job requirements. The most important factor is to ensure uniform dispersionof the fibers and to prevent segregation or balling of the fibers during mixing. Segregation or ballingduring 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 difficultto achieve a uniform mix. Although conventional mixing procedures can be used, it is advisable to usea 3/8-in. (9.7-mm) maximum aggregate size. The water requirement will vary from that of concretewithout fibers depending on the type of cement replacement cementitious pozzolans used and theirpercent by volume of the matrix. Table 22.2 and Table 22.3 give typical mixture proportions for normalweight fibrous reinforced concrete and fly-ash fibrous concrete mixes, respectively. A workable methodfor 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.

TABLE 22.2 Typical Proportions for Normal Weight Fiber-Reinforced Concrete

Material Range

Cement 550–950 lb/yd3

W/C ratio 0.4–0.6Percentage of sand to aggregate 50–100%Maximum aggregate 3/8 in.Air content 6–9%Fiber content 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 ConcreteInstitute, Farmington Hills, MI, 1996.

TABLE 22.3 Typical Fly-Ash Fibrous Concrete Mix

Material Quantity

Cement 490 lb/yd3

Fly ash 225 lb/yd3

W/C ratio 0.54Percentage of sand to aggregate 50%Maximum size of coarse aggregate 3/8 in.Steel fiber content (0.010 × 0.022 × 1.0 in.) 1.5% by volumeAir-entraining agent Manufacturer’s recommendationWater-reducing agent Manufacturer’s recommendationSlump 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 ConcreteInstitute, Farmington Hills, MI, 1996.



• Add the balance of the fiber to the previously charged constituents, and add the remainingcementitious 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 vibra-tion 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 LoadFiber-reinforced concrete in flexure essentially undergoes a trilinear deformation behavior as shown inFigure 22.1. Point A on the load-deflection diagram represents the first cracking load, which can betermed the first-crack strength (Mindess and Young, 1981). Normally, this is the same load level at whicha nonreinforced element cracks; hence, segment OA in the diagram would be the same and essentiallyhave the same slope for both plain and fiber-reinforced concrete. Once the matrix is cracked, the appliedload is transferred to the fibers that bridge and tie the crack to keep it from opening further. As the fibersdeform, additional narrow cracks develop, and continued cracking of the matrix takes place until themaximum load reaches point B of the load-deflection diagram. During this stage, debonding and pulloutof some of the fibers occur, but the yield strength in most of the fibers is not reached. In the fallingbranch, BC, of the load-deflection diagram, matrix cracking and fiber pullout continue. If the fibers arelong enough to maintain their bond with the surrounding gel, they may fail by yielding or by fractureof the fiber element, depending on their size and spacing.

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

(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 fibermatrix interfacial bond strength in terms of the critical fiber length, demonstrating that the strength of

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

LoadB

A

C

O Deflection

�cf

bf

d

v=

2σ



a composite increases continuously with the fiber length. This is of significance as it indicates that pulloutwork 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-basedcomposites 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 stress-intensity factor that controls fracture. The approach taken by Romualdi and Batson (1963) to increasethe tensile strength of the mortar was to increase the stress-intensity factor by decreasing the spacing ofthe fibers acting as crack arresters. Figure 22.2 relates the tensile cracking stress to the spacing of thefibers for various volumetric percentages. Figure 22.3 compares the theoretical and experimental valuesof the ratio of the first cracking load to the cracking strength of plain concrete (strength ratio). Bothdiagrams demonstrate that the strength ratio increases as the spacing of the fibers is reduced; that is, thetensile 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 thefibers, one expression from Romualdi and Batson (1963) gives:

(22.2)

where:

df = diameter of the fiber.ρ = fiber percent by volume of the matrix.

Another expression due to McKee (1969) gives:

(22.3)

FIGURE 22.2 Effect of steel-fiber spacing on the tensile cracking stress in fibrous concrete for ρ = 2.5, 5.9, and7.5%. (From Romualdi, J.P. and Batson, G.B., Proc. ASCE Eng. Mech. J., 89(EM3), 147–168, 1963.)

8.0

6.0

4.0

2.0

0

MPa

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

1200

1000

800

600

400

200

0

Tens

ile C

rack

ing

Stre

ss (p

si)

Wire Spacing (in.)

7.0%5.0%

2.5%

s d f= 13 81 0

..

ρ

sV= 3ρ



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

(22.4)

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

FIGURE 22.4 Various shapes and sizes of steel fibers.

Indirect tensionBeam in bending

Theoretical

Experimental

Stre

ngth

Rat

io

3.0

2.5

2.0

1.5

1.0

0.50 0.2 0.4 0.6 0.8 1.0

Wire Spacing (in.)

0.010" × 0.022"× 1.0" Fiber

0.016" × 0.75"Deformed

0.016" × 1.0"Deformed

0.016" × 0.75"Round

0.014" × 1.0"Round

s d f= 13 8.�

ρ



22.4.4 Fiber Orientation: Fiber Efficiency Factor

The orientation of the fibers with respect to load determines the efficiency with which the randomlyoriented fibers can resist the tensile forces in their directions. This observation is synonymous with thecontribution of bent bars and vertical shear stirrups provided in beams to resist the inclined diagonaltension stress. If one assumes perfect randomness, the efficiency factor is 0.41l, but it can vary between0.33l and 0.65l close to the surface of the specimen, as trowling or leveling can modify the orientationof 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 typeof matrix, whether empirical data from laboratory experiments are used, or whether the design is basedon the bonded area of the fiber or the law of mixtures. An empirical expression for the composite flexuralstrength based on a composite-material approach is (Bentur and Mindess, 1990):

(22.5)

where:

σc = composite flexural strength.σm = ultimate strength of the matrix.Vf = volume fraction of the fibers adjusted for the effect of randomness.A, B = constants.�/d = 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 bySwamy et al. (1974) and were adopted by ACI Committee 544 (1993). These constants lead to the followingexpressions:

First crack composite flexural strength (psi):

(22.6)

where:

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

Ultimate composite flexural strength (psi):

(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 influ-enced by several factors:

σ σc m f fA V BVd

= −( )+1 �

σ f r m ff

f V Vd

= +0 843 425. �

σcu r m ff

f V Vd

= +0 97 494. �



• 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 modelssimilar in form to the elements being designed. As the fibers affect the performance of the end productin all material-resistance capacities such as in flexure, shear, direct tension, and impact, it is importantto 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 toact as reinforcement and assume the stress from the matrix when it cracks through the interface shearfriction interlock between the fiber and the matrix. This phenomenon is analogous to the shear frictioninterlock hypothesis presented in Nawy (1996) in his discussion on the mechanism of shear frictioninterlock. Deformed or crimped fibers have a greater influence than smooth and straight ones. Thepullout resistance in zone AB of Figure 22.1 is proportional to the interfacial surface area (ACI Committee544, 1993). The non-round fiber cross-sections and the smaller diameter round fibers induce a largerresistance per unit volume than the larger diameter fibers. This is also analogous to the crack-controlbehavior in traditionally reinforced structural members, where a larger number of smaller diameter barsthat are more closely spaced is more effective than a smaller number of large diameter bars for the samereinforcement volume percentage (Nawy and Blair, 1971). One reason for this is the larger surfaceinteraction area between the fibers and the surrounding matrix, resulting in a higher bond and shearfriction 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 toughnessare considerably enhanced as a function of the increase in the volume fractions and aspect ratios of thefibers used. Figure 22.5 shows the effect of an increase in volume fraction on the stress–strain relationshipof the fibrous concrete resulting from an increase in the fiber volume from 0 to 1.5% for concretes havinga compressive strength of 13,100 psi (90.3 MPa). Figure 22.6 and Figure 22.7 depict a similar trend with

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

15000

10000

5000

00 0.005 0.01 0.015 in./in.

Com

pres

sive

Stre

ngth

(MPa

)

Com

pres

sive

Stre

ngth

(psi)

100

80

60

40

20

mm/mm0.005 0.010 0.015

Unit Strain

Vf = 1.0%Vf = 0.5%

Vf = 0%



respect to both a volume fraction ratio up to 3% and an aspect ratio in the range of 47 to 100. Figure22.8 also demonstrates the influence of the increase in fiber content on the relative toughness of reinforcedconcrete members.

Toughness is a measure of the ability to absorb energy during deformation. It can be estimated fromthe area under the stress–strain or load-deformation diagrams. A toughness index (TI) expressionproposed by Hsu and Hsu (1994) follows:

(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-strengthconcretes within the limitations of the type, aspect ratio, and volume fractions of the steel fibers used inthose tests. In short, by increasing the volume fraction, both ductility and toughness have been shown toincrease significantly within the practical limits of workable volume content of fiber in a concrete mix.

FIGURE 22.6 Influence of volume fraction of steel fibers on stress–strain behavior for 9000 psi concrete. (FromFanella, D.A. and Naaman, A.E., ACI J., 82(4), 475–483, 1985.)

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.)

10000

8000

6000

4000

2000

0

Com

pres

sive

Stre

ss (p

si)

Control

Axial Strain0 0.005 0.010 0.015 0.020

in./in.

80

60

40

20

MPa

Smooth steel fibers�/df = 83

Vf = 3%

Vf = 2%

Vf = 1%

10000

12000

8000

6000

4000

2000

0

Com

pres

sive

Stre

ss (p

si)

Control

Axial Strain0.005 0.010 0.015 0.020

in./in.

60

80

40

20M

Pa

Smooth steel fibersVf = 2%�/df = 100

�/df = 83�/df = 47

TI RI= +1 421 1 035. .



22.5.3 Strength in Direct Tension

The effect of different shapes of the fiber filaments on the tensile stress behavior of steel-fiber-reinforcedmortars in direct tension is demonstrated in Figure 22.10. The descending portion of the plots show thatthe fibers reinforced with better anchorage quality increase the tensile resistance of the fiber-reinforcedconcrete beyond the first cracking load.

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

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.)

Constant rate of loading 0.01"/min area underthe curve taken as a measure of toughness

Center line deflection

Load

Relativetoughness

Relative strength

Results are averageof 4 specimens

% Volume of Fibers0

0

2

4

6

8

10

12

14

16

18

20

0.25 0.50 0.75 1.00 1.25

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

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

Toug

hnes

s and

Stre

ngth

in R

elat

ion

to P

lain

Con

cret

e

RI = Vf × �/df2.5

2.0

1.5

1.0

0.5

00 0.1 0.2 0.3 0.4 0.5 0.6 0.7

0.5

1.0

1.5

2.0

2.5

Reinforcing Index (RI)

Toug

hnes

s Ind

ex (T

I) TI = 1.421(RI) + 1.035



22.5.4 Flexural Strength

Fibers seem to affect the magnitude of flexural strength in concrete and mortar elements to a muchgreater extent than they affect the strength of comparable elements subjected to direct tension or com-pression (ACI Committee 544, 1993). Two stages of loading portray the behavior. The first controllingstage is the first cracking load stage in the load-deflection diagram, and the second controlling stage isthe ultimate load stage. Both the first cracking load and the ultimate flexural capacity are affected as afunction of the product of the fiber volume concentration (ρ) and the aspect ratio (�/df). Fiber concen-trations less than .5% of the volume of the matrix and with an aspect ratio less 50 seem to have a smalleffect on the flexural strength, although they can still have a pronounced effect on the toughness of theconcrete element, as seen in Figure 22.8. The flexural strength of plain concrete beams containing steelfibers was defined in Equation 22.6 and Equation 22.7. For structural beams reinforced with both normalreinforcing bars and fibers added to the matrix, a modification of the standard expression for nominalmoment strength, Mn = Asfy(d – a/2), must be made to account for the shear friction interaction of thefibers in preventing the flexural macrocracks from opening and propagating in the tensile zone of theconcrete section, as seen in Figure 22.11 (Henager and Doherty, 1976). In this diagram, the area ofconcrete in the tensile zone is neglected and an additional equilibrium tensile force (Tfc) is added to thesection. This moves the neutral axis down, leading to a higher nominal moment strength (Mn). Theresulting expression for Mn becomes (Henager and Doherty, 1976):

(22.9)

(22.10)

(22.11a)

(22.11b)

where:

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

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.)

Straight Fibers Hooked Fibers Enlarged-end Fibers400

300

200

100

00 00.004 0.006 0.008 0.006 0.0080.004 0 0.006 0.008 0.0100.004

1.0

2.0

3.0

MPa

Tens

ile S

treng

th (p

si)�/df = 66 �/df = 75 �/df = 67

M A f da

b h eh e a

n s y t= −

+ −( ) + −2 2 2 2

σ

ec= ( )+ e fibers 0 003 0 003. .

σρt f f bcd F

(.

psi) = 1 12�

σρ1

0 00772(

.MPa) = l

d Ff f be



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 diagonaltension capacity of concrete beams. The degree of increase in the diagonal tension capacity is a functionof the shear span/depth ratio of a beam. This ratio determines the mode of failure in normal beams thatdo 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 capacityincreased by 45% over beams without stirrups. When steel fibers with deformed ends were used at avolume ratio of 1.1%, the shear capacity increased by 45 to 67% and the beams failed by flexure. Usingcrimped-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 shearstrength increases proportionally. Tests by Sharama (1986) resulted in the following expression for theaverage shear stress (νc) for beams in which steel fibers were added (ACI Committee 544, 1993):

(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 distributedloads are acting.

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

b

h d

e ca

a/2

C

0.85fc

Tfc

Trb

σt

εc = 0.003

εs (Fibers)

εs (Bars)

Neutral

axis

(a) (b) (c)

ʹ

νcf tfd

a= ′

2

3

1

4



22.5.6 Environmental Effects22.5.6.1 Freezing and Thawing

The addition of fibers to a matrix does not seem to result in an appreciable improvement in the freezingand thawing performance of concrete, as its resistance to such an environmental effect is controlled bypermeability, void ratio, and freeze–thaw cycles. Fibers, however, tend to hold the scaling concrete piecestogether, 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 theaddition of fibers, but a slight decrease in shrinkage can result due to the need to add more paste mortarin the mixture when fibers are used. Cracking due to drying shrinkage in restrained elements can beslightly improved, as the cracks are kept from generating because of the bridging effect of the randomlydistributed fibers.

22.5.7 Dynamic Loading PerformanceThe cracking behavior of fibrous concrete elements under dynamic loading seems to be three to ten timesbetter that of plain concrete. Also, the total energy absorbed by the steel fibrous concrete beams can be40 to 100 times that for plain concrete beams, depending on the type, deformed shape, and percentvolume of the fibers (ACI Committee 544, 1993).

22.6 Steel-Fiber-Reinforced Cement Composites

22.6.1 General CharacteristicsFiber-reinforced concretes are designed to contain a maximum 2% by volume of fibers, using the samemixture 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 to25%. Consequently, neither the design of the mixture nor the constituent materials in the matrix can besimilar to those of conventional fibrous or nonfibrous concretes. Either cement only or cement with sandis used in the mixture, with no coarse aggregate, to achieve the high strength, ductility, and highperformance 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 Portlandcement (0.5 µm). The void content in any matrix can be reduced to a negligible percentage with theaddition of pozzolans such as silica fume. With these developments as a background, the following arethe 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) incompression and an energy absorption capacity (i.e., ductility) that can be up to 1000 times that of plainconcrete (Reinhardt and Naaman, 1992).

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



depending on the requirement of the design. After the fibers are placed, a low-viscosity cement slurry ispoured 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 plasticizercontent of 10 to 40 oz. per 100 lb of the total cementitious weight (C + F). Batch trials of the slurry mixhave to be carefully made with regard to the W/(C + F) ratio to arrive at a workable slurry mix that canfully 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 capabilityin the falling branch of the diagram. Figure 22.13 illustrates the influence of the matrix compressivestrength 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 CompositesDensified small-particle (DSP) systems and compact reinforced composite (CRC) gain super highstrength depending largely on the type of compact-density cements that are used for the cement-basedcomposites and the proportioning used to considerably reduce or practically eliminate most of the voidsin the paste. Figure 22.14 illustrates the fracture surface of a steel-fiber-reinforced concrete specimen.

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.)

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.)

120

100

80

60

40

20

MPa

0.20.10 0.3Strain

20

15

10

5

0

Com

pres

sive

Stre

ngth

(ksi)

in./in.

SIFCON

Conventionalconcrete

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

(1)

(2)

(3)

Strain.000 .030 .060 .090 .120 .150

in./in.

5

10

15

20

Com

pres

sive

Stre

ngth

(ksi)

200

150

100

50M

Pa

ʹ

ʹʹ



22.6.4 Carbon-Fiber-Reinforced Cement-Based Composites

Petroleum-pitch-based carbon fibers have recently been developed for use as reinforcement for cement-based composites. Their diameters vary from 0.0004 to 0.0007 in. (10 to 18 µm), and their lengths varyfrom 1/8 to 1/2 in. (3 to 12 mm). Their tensile strength typically ranges from 60 to 110 ksi (400 to 750MPa). They are incorporated in the cement-based composites in essentially the same manner as steelfibers are in concrete, and they are uniformly distributed and randomly oriented. Because of the verysmall size of the carbon fibers and their small diameter, a high fiber count is attained in the cementitiousmatrix at a typical volume fraction of 0.5 to 3% (Bayasi, 1992). The spacing between the fibers isapproximately 0.004 in. (0.1 mm) at a 3% fiber volume fraction. The function of the carbon fibersfunction 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 120,000psi (200 to 800 MPa). The lower range is used today for the construction of structural elements. The higherranges are used in nonstructural applications such as flooring, safes, and storage compartments for nuclearwaste. Concretes in the higher ranges are termed super-high-strength concretes and possess the very highductility necessary for applications in structural systems. The principal characteristic of such concretes isthe use of a powder concrete in which aggregates and traditional sand are replaced by ground quartz lessthan 300 µm in size (Richard and Cheyrezy, 1994). In this manner, the homogeneity of the mixture is greatlyimproved, and the distribution in the size of the particles is consequently reduced by almost two orders ofmagnitude. A major improvement in the properties of the hardened concrete is an increase in the Young’smodulus value of the paste by almost a factor of three so its value can reach to 6 to 11 × 10–6 psi (55 to 75GPa), 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 drycompact density, additional amounts of precipitated silica further improve the dry compactdensity)

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



• Increase in the density of concrete by maintaining the fresh concrete under pressure at theplacement stage and during setting, which results in the removal of air bubbles, expulsion of excesswater, 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 activationof 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 majormechanical 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 shownpromise for the effective control of cracking and deflection in structural concrete elements, particularly inthe negative moment regions of reinforced concrete bridge decks. Most of the earlier studies on this subjectwere limited to experimental laboratory work (Chen and Nawy, 1994). Introducing highly precompressedprisms as the main reinforcement in the beam tension zone can increase ductility, control and delay theformation and propagation of cracking, reduce deflection, and improve the high-performance character-istics of environmentally exposed structural components, as reported by Chen and Nawy (1994). Theirwork involved several patterns of composite beams and a concrete strength of 14,000 psi (97 MPa), asshown in Figure 22.15. All evaluations were based on test-to-failure results. Fiberoptic Bragg grating sensorsdeveloped 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 thebeams 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 prism-reinforced structural concrete beam. This index value was obtained from the following expression:

(22.13)

TABLE 22.4 Mixture Composition and Concrete Mechanical Properties of Super-High-Strength Reactive Powder Concrete

Mixture ConstituentsRPC 200 Concrete

lb/yd3 (MPa)RPC 800 Concrete

lb/yd3 (MPa)

Portland cement, Type V 1614 (955) 1690 (1000)Fine sand (150–400 µm) 1775 (1051) 845 (500)Ground quartz (4 µm) — 659 (390)Silica fume (18 m3/g) 387 (229) 389 (230)Precipitated silica (35 m3/g) 16.9 (10) —Superplasticizer (polyacrylate) 22.0 (13) 30.4 (18)Steel fibers 323 (191) 1065 (630)Total water 31.2 gal/yd3 (153 L/m3) 36.7 gal/yd3 (180 L/m3)Cylinder compressive strength 24–33 (170–230) 71–99 (490–680)Flexural strength 3.6–8.7 (25–60) 6.5–14.8 (45–102)Fracture energy 15,000–40,000 J/m3 1200–2000 J/m3

Young’s modulus 7.8–8.7 × 103 (54–60 GPa) 9.8–10.9 × 103 (65–75 GPa)

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.

ω =+ − ′ ′

′A f A mf A f

bd fps ps s y s y

p c



where Aps, As, and As′ are the areas of prestressing and nonprestressing reinforcement, fps is the designstress in the prestressing reinforcement at ultimate load, fy and fy′ are the yield strengths of deformedbars in tension and in compression, b is the width of the compression face of the member or flange inthe 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 throughthe use of two-layer systems, one of which is made out of normal- or high-strength concrete and thelayer above or below is made of high-strength polymer concrete. In such cases, the beam cross-sectionis built in two layers, in a manner similar to that of the SIFCON two-layer system. Several investigationshave demonstrated that the shear friction interaction of the interface between the two layers of differentstrength concretes, one of which is a polymer concrete, has a resistance to slip superior to the interlockbetween 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 elementshas 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 polymer-impregnated bundled fiberglass fibers as rods for anchorages in tunneling. In the mid-1960s, Nawy etal. (ACI Committee 440, 1996; Nawy and Neuwerth, 1977; Nawy et al., 1971) conducted extensive workon the use of bundled and resin-impregnated glass fibers formed into deformed bars as the main

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

60.0"

6.0"

14.0"3 # 5

48.0" 48.0"

108.0"108.0"228.0"

60.0"

6.0"

PP BB

B B

A

A

3

14

2 2

2

1

23 2" × 2" Prism2 2" × 2" Prism

4 4 4

#3 @ 8" 4 #3 @ 4" #3 @ 6" 5 #3 @ 4"

2

12.0

"

4.0"4.0"

4.0"

14.0"

4.0"

5.0"

14.0"14.0"

10.3

13"

12.0

"

8.0"

4.0"

4.0"

10.6

88"

10.7

5"9.

75"

12.0

"

8.0"

4.0"

12.0

"

10.3

13"

10.3

13"

2 # 2

2 # 23

1

14.0"2 # 2

2 # 2

2 # 54 # 4

2 # 2

1 # 5

A - A (C-3, C-4)A - A (C-2)

B - B (C-4)B - B (C-1, C-2, C-3)

A - A (C-1)

×



reinforcement in structural elements. Except for cases where magnetic fields in supporting structureshad to be avoided, commercial application of bundled and resin-impregnated reinforcement in structuralconcrete elements was not recognized until the late 1970s. It is important to state at this juncture thatthe term plastic could be misleading; hence, there is general consensus at this time to define FRP as fiber-reinforced polymer composites.

In the 1980s, an increased interest in and use of such glass-fiber-reinforced polymer (GFRP) reinforc-ing bars was developing. This was particularly overdue and important for reinforced concrete surround-ing or supporting magnetic resonance imaging (MRI) medical equipment. Such equipment includessensitive magnets and cannot tolerate the presence of any steel reinforcement. Also, where environmentaland chemical attacks are present, GFRP reinforcement is more durable and efficient as concrete rein-forcement. As stated in ACI Committee 440 (1996), composite rebars have more recently been used inthe construction of seawalls, industrial roof decks, base pads for electrical and reactor equipment, andconcrete floor slabs in aggressive chemical environments. In 1986, Germany built the world’s first highwaybridge using composite reinforcement (ACI Committee 440, 1996).

In the United States, significant funds have been expended on product evaluation and further devel-opment, and at least nine major companies have been actively marketing this product since the early1990s. Additionally, considerable progress has been made in using glass fiber filaments as a supplementto concrete matrices to improve the mechanical properties of concrete, but not as a replacement for themain bar reinforcement in supporting structural components. Glass fibers that are alkali resistant arealso gaining wide use. These normally contain zirconium (ZrO2) to minimize or eliminate the alkalinecorrosive attack on glass present in the cement paste.

Synthetic fibers made from nylon or polypropylene, both loose and woven into geotextile form, haverecently begun to be utilized due to the availability of information on their mechanical performance inthe matrix and a better understanding of their structural contribution to crack resistance. Although theuse of other types of nonmetallic fibers has been explored, interest has grown primarily in the use ofcarbon fibers as the main reinforcement apart from its use in cement-based composites. It is safe to statenow that the science of fibrous concrete and composites has advanced to an extent that justifies itsextended 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 steelreinforcement. 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 had5.5 × 5.5-ft effective spans and were fully restrained along all four boundaries. The GFRP reinforcementwas 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 beamswere either simply supported or continuous over two spans. The centerline span was 9 ft, 11 in. Thereinforcement 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 beamsbehaved similarly in cracking, deflections, and ultimate load to steel-reinforced beams. The large numberof well-distributed cracks in the GFRP-reinforced beams and slabs indicated that a good mechanicalbond developed between the GFRP bar and the surrounding concrete. The research also demonstratedthat the equations for flexure accurately predicted the flexural behavior of GFRP-reinforced memberswith the same accuracy as for the mild steel reinforced beams. A typical stress–strain diagram of thereinforcement is shown in Figure 22.16. This research led to investigations by Larralde et al., Satoh etal., 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) workare given in the ACI Committee 440 report (1996). Table 22.5 provides a relative comparison of themechanical properties of GFRP and steel reinforcement.



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. Thefundamental 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 (ACICommittee 440, 1996). To attain a modulus of 30 × 106 psi (200 GPa), the graphine layers of high-modulus Type I carbon fibers are aligned approximately parallel to the axis of the fibers. Examples are

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.)

150

120

90

60

30

0.003 .006 .009 .012 .015 .018 .021 .024

Strain (“/”)

(a)

Stre

ss (k

s i)

(1 ksi = 6.895 MPa)1200

900

600

400

200

0

Stre

ss (M

Pa)

Fiberglass bundled filamentsExternally resin coated

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

100

80

60

40

20

00.004 0.008 0.012 0.016 0.020 0.024 0.028 0.032

Strain (“/”)

(b)

Stre

ss (k

si)

(1 ksi = 6.895 MPa)

900

600

400

200

0

Stre

ss (M

Pa)

Fiberglass twisted filamentsPolymer resin impregnated(Bar resin content = 40%)0.25" diameter (nominal)Fu = 105.5 ksiFy = 96.5 ksi (0.12% offset)Ef = 3.8 ×103 ksi



Kevlar® 49 by DuPont and Twaron® 1055 by Akzo Nobel. Ultra-high-modulus-fiber Kevlar® 149 andTwaron® 2000 are also available. Table 22.6 lists the minimum average strength values of Kevlar® andTwaron® reinforcing fibers. Additionally, hybrid composites made from carbon–glass–polyester are avail-able with a strength of up to 115,000 psi (790 MPa), a modulus of 18 × 106 psi (124 MPa), and a densityin the range of 0.060 to 0.069 lb/in3. It is important to state that, because of the low ductility of carbonfiber 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 prestressingreinforcement and as fabric reinforcement because of its high strength and high modulus, as seen inTable 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 wellas in the United States and elsewhere. They have been utilized in such transportation structures as bridgedecks 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 22.5 Comparison of Mechanical Properties of GFRP and Steel Reinforcement

Property Steel Rebar Prestressing Steel Tendon GFRP Bar GFRP Tendon CFRP Tendon

Tensile strengthMPa 483–690 1379–1862 517–1207 1379–1724 1665–2068ksi 70–100 200–270 75–175 200–250 240–300

Yield strengthMPa 276–414 — — — —ksi 40–80 — — — —

Tensile modulusGPa 200 200 414–552 48–62 152–165ksi × 10–3 29 29 6–8 7–9 22–24Compressive strengthMPa 276–414 — 310–482 — —ksi 40–80 — 45–70 — —

Coefficient of thermal expansion(× 10–6)/°C 11.7 11.7 9.9 9.9 0(× 10–6)/ºF 6.5 6.5 5.5 5.5 0Specific gravity 7.9 7.9 1.5–2.0 2.4 1.7

Note: All strengths are in the longitudinal direction.

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

TABLE 22.6 Properties of Kevlar® and Twaron® Reinforcing Fibers

Property Kevlar® 49 Twaron® 1055

Tensile strength, psi (MPa) 525,000 (3,620) 522,000 (3600)Modulus, psi (MPa) 18 × 106 (124,000) 18.4 × 106 (127,000)Elongation at break (%) 2.9 2.5Density, lb/in3 (g/cm3) 0.052 (1.44) 0.52 (1.45)

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



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, Transpor-tation Research Board, Washington, D.C., 2006, pp. 1–24. Courtesy of Dr. A. Nanni.)

40

35

30

25

20

15

10

5

00 200 400 600 800 1000

Ultimate Design Load

Deflection (milli-in)

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

Load

(kip

s)

Service Load

ACI Design Load

0.00138 K

1.40

835

K

1.40

835

K

1.49

76 K

1.49

76 K

0.05174 K0.05174 K

2.90

14 K

2.90

14 K

3.10

1 K

3.10

1 K

3.13

282

K

3.13

282

K

2.95

883

K2.

9588

3 K

–.02986

K

Z

XY



reserve deflection control capacity being almost twice that at the theoretical ultimate load. Figure 22.18shows the deck of the hybrid GFRP-reinforced bridge deck in Bettendorf, Iowa, as an example. Figure22.19 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 orwrapping concrete elements, such as damaged beam surfaces. Also, CFRP resin-impregnated strands canbe spirally wound onto the surface of an existing concrete element, such as a bridge pier or structuralbuilding column. Due to the confinement imposed on the member, shear capacity and ductility areimproved. 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 eliminateproblems with durability and reinforcement corrosion that often plague bridge structures, garages, anddeteriorated beam and slab elements in buildings.

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

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

53rd Avenue BridgeBettendorf, Iowa

Level 1 Beams and Joists

Parking Garage



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) indiameter are used for prestressing. Their ultimate strength is comparable to prestressing strands, rangingbetween 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 theyeliminate the need for joints because of the unlimited length of the composite sheets (ACI Committee440, 1996). They are useful in increasing the flexural and shear strength of concrete members when thesecomposite plates are epoxy bonded to the exterior facing of the elements. They are also of particular usein the retrofit of deteriorating concrete structures and in retrofitting columns in seismic zones. Severaltechniques 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 existingconcrete element such as a bridge pier or a structural building column. In this manner, due to theconfinement imposed on the member, shear capacity and ductility are improved. Nanni’s (1993) workon the effect of wrapping conventional concrete demonstrated that significant enhancement can beachieved 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, asdegradation of the polymer resin content under heat and ultraviolet light can lead to some long-termdurability problems. The carbon and glass fibers and the fabrics used in the FRP can withstand normalfire exposure and are durable under ultraviolet light, but the weak link is the organic polymers used toprepare the fiberglass or carbon used as reinforcing elements through impregnation or wrapping. Oneway 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 beable to withstand up to 1000°C. The system is highly impermeable so it can protect the carbon filamentsfrom 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 usedare comparable to those of organic polymer composites while having the advantage of relatively higherfire resistance (Foden et al., 1996).

TABLE 22.7 Properties of Strengthening Laminates

Strengthening System

Property I II III IV V

Type of fibers CFRP CFRP GFRP GFRP CFRPFiber orientation Unidirectional Unidirectional Unidirectional Bidirectional Unidirectional

(x) (y)Tensile strength, MPa (ksi) 2937 (426) 758 (110) 413 (60) 482 (70) 310 (45) 2399 (348)Modulus of elasticity,

GPa (× 103 ksi)230 (33.4) 62 (9.0) 21 (3.0) 14 (2.1) 11 (1.6) 149 (21.7)

Failure strain (%) 1.2 1.2 2.0 3.0 1.4Thickness, mm (× 10–1), in. 5 (0.02) 13 (0.05) 10 (0.04) 13 (0.05) 13 (0.05)

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



22.12 Summary

The concretes described in this chapter have demonstrated that the strength, ductility, and performanceof concretes and cement-based composites have and will continue to achieve higher plateaus. A new erain construction materials technology has commenced that promises to have a revolutionary impact onconstructed systems in the 21st century. Considerable work must be done to enhance the practicabilityof these materials and make them cost effective. It is only with simplicity and practicability in applicationand the achievement of a cost-effective competitive end product that these developments in the scienceof 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, High-Performance Concrete, by E.G. Nawy (Addison Wesley Longman, 1996); High-Performance Concrete, byE.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 (PrenticeHall, 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 Struc-tures, ACI 440R. American Concrete Institute, Farmington Hills, MI.

ACI Committee 544. 1988. Design Considerations for Steel Fiber Reinforced Concrete, ACI 544.4R. AmericanConcrete Institute, Farmington Hills, MI.

ACI Committee 544. 1989. Measurement of Properties of Fiber Reinforced Concrete, ACI 544.2R. AmericanConcrete Institute, Farmington Hills, MI.

ACI Committee 544. 1993. Guide for Specifying, Proportioning, Mixing, Placing, and Finishing Steel FiberReinforced Concrete, ACI 544.3R. American Concrete Institute, Farmington Hills, MI.

ACI Committee 544. 1996. Fiber Reinforced Concrete, ACI 544.1R. American Concrete Institute, Farm-ington Hills, MI.

Bayasi, M.Z. 1992. Application of carbon fiber reinforced mortar in composite slab construction. InProceedings 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 Infrastruc-tures, January 15–17, Tucson, AZ.

Grace, N.K., Abdel-Sayed, G., Soliman, A.K., and Saleh, K.R. 1999. Strengthening reinforced concretebeams 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 com-pression. Proc. ACI Struct. J., 91(4), 448–457.



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 Inter-national 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 ConcreteSlab Systems, Nawy, E.G., Ed., ACI SP-30, pp. 1-30–1-42. American Concrete Institute, FarmingtonHills, MI.

Nawy, E.G. and Chen, B. 1998. Fiber optic sensing of the behavior of prestressed prism-reinforcedcontinuous composite concrete beams for bridge deck application. In Proceedings, TransportationResearch 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 com-posites: 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. InProceedings 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 MPacompressive 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 distributedclosely 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 InternationalRILEM/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.



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 ServiceConf., 3, 363–377.


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

(a)

(b)

(c)


Table of ContentsChapter 22: Fiber-Reinforced CompositesPart A. Fiber-Reinforced Concrete22.1 Historical Development22.2 General Characteristics22.3 Mixture Proportioning22.4 Mechanics of Fiber Reinforcement22.4.1 First Cracking Load22.4.2 Critical Fiber Length: Length Factor22.4.3 Critical Fiber Spacing: Space Factor22.4.4 Fiber Orientation: Fiber Efficiency Factor22.4.5 Static Flexural Strength Prediction: Beams with Fibers Only

22.5 Mechanical Properties of Fibrous Concrete Structural Elements22.5.1 Controlling Factors22.5.2 Strength in Compression22.5.3 Strength in Direct Tension22.5.4 Flexural Strength22.5.5 Shear Strength22.5.6 Environmental Effects22.5.6.1 Freezing and Thawing22.5.6.2 Shrinkage and Creep

22.5.7 Dynamic Loading Performance

22.6 Steel-Fiber-Reinforced Cement Composites22.6.1 General Characteristics22.6.2 Slurry-Infiltrated Fiber Concrete22.6.3 DSP and CRC Cement Composites22.6.4 Carbon-Fiber-Reinforced Cement-Based Composites22.6.5 Super-Strength Reactive-Powder Concretes

22.7 Prestressed Concrete Prism Elements as the Main Composite Reinforcement in Concrete BeamsPart B. Fiber-Reinforced Plastic (FRP) Composites22.8 Historical Development22.9 Beams and Two-Way Slabs Reinforced with GFRP Bars22.10 Carbon Fibers and Composite Reinforcement22.10.1 Carbon Fibers22.10.2 Hybrid GFRP and CFRP Reinforcement for Bridges and Other Structural Systems22.10.3 Use as Internal Prestressing Reinforcement22.10.4 Use as External Reinforcement

22.11 Fire Resistance22.12 SummaryAcknowledgmentsReferences

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