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Design recommendations for the use of FRP for reinforcement and strengthening of concrete structures Sami Rizkalla', Tarek Hassan2 and Nahla Hassan2

, North Carolina State University Raleigh, NC, USA 2 Ain Shams University, Cairo, Egypt

Summary The use of fibre reinforced polymer, FRP, as reinforcement for concrete structures has been growing rapidly in recent years. This paper summarizes the current state of knowledge of these materials and highlights the various FRP strengthening techniques that have been used for concrete and masonry structures. Material characteristics of FRP and fundamental design considerations are discussed. Selection of the appropriate materials and their corresponding

advantages and disadvantages are highlighted. Design philosophies for concrete members reinforced and/or strengthened with FRP are enumerated. Fundamental flexure, shear and bond behaviour of concrete members reinforced and/or strengthened with FRP according to the current ACI design guidelines are examined. The paper also reviews the durability aspects of FRP and describes selected field applications of these materials.

Key words: FRP; design; strengthening; reinforcement; concrete; near surface mounted

Prog. Struct Engng Mater. 2003; 5: 16-28 (001: 10.1 002/pse.139)

Introduction

In an aggressive environment, concrete may be vulnerable to chemical attacks, such as carbonation and chloride contamination which break down the alkaline barrier in the cement matrix. Consequently, the steel reinforcement in concrete structures becomes susceptible to corrosion. Such phenomena lead to delamination of the concrete at the reinforcement level, cracking and spalling of the concrete due to the volume increase of the steel reinfOrcement. In the United States, nearly one-third of the nation's 581000 bridges are considered structurally deficient or functionally obsolete[1]. A large number of these deficient bridges are reinforced or prestressed concrete structures, and are in urgent need of repair and strengthening. In the United Kingdom, over 10 000 concrete bridges need structural attention. In Europe, the cost of the repair of reinforced concrete structures because of corrosion of reinforcing bars is estimated to be over $600 million annuallY[2]. In Canada, it is estimated that the required repair costs for parking garages alone is in the range of $6 billion[3].

A possible solution to combat reinforcement corrosion for new construction is the use of

Published online 24 February 2003 Copyright © 2003 John Wiley & Sons. Ltd.

non-corrosive materials to replace conventional steel bars. High tensile strength, lightweight and corrosionresistant characteristics make FRP ideal for such applications. FRP also provides a practical technique for the repair and strengthening of concrete structures and bridges by using externally bonded sheets or prefabricated laminates. FRP tendons can also be used to strengthen old prestressed concrete girders[4].

Historical background

Development of FRP materials in various forms and configurations offers an alternative design approach for construction of new structures and rehabilitation of the existing civil infrastructure. The first use of FRP products was in reinforced concrete structures in the mid-1950s[5,6]. Since their early applicatiQn, many FRP materials with different types of fibres have been developed. FRP products can take the form of bars, cables, two- and three-dimensional grids, sheet materials and laminates as shown in Fig. 1. FRP products may achieve the same or better reinforcement objectives as commonly used metallic

Prog. Strua. Engng Mater. 2003; 5: 16-28

FRP FOR REINFORCEMENT AND STRENGTHENING 17

(a) (b)

(c) (d)

Fig. I FRP products for reinforced concrete construction: (a) grids, bars; (b) cages, grids; (c) sheets; and (d) cages

products such as steel reinforcing bars, prestressing tendons, and bonded plates.

Application and product development efforts in FRP composites are widespread to address many opportunities for reinforcing concrete members. In spite of earlier research on the use of FRP reinforcement in concrete, commercial applications of these materials were not recognized until the late 1970s. Research started in earnest to determine the feasibility of using composites versus the use of epoxy coated steel reinforcement. During the early 1980s, several pultrusion companies recognized the potential of the material and produced FRP reinforcing bars. In the late 1980s, the use ofFRP reinforcing bars increased considerably for several applications with special performance requirements. Typical applications included construction of seawalls, industrial roof decks, base pads for electrical and reactor equipment, and concrete floor slabs in aggressive chemical environments. In 1986, the world's first highway bridge, prestressed with glass-fibre-reinforced polymer, GFRP, was built in Germany. Since then, there have been bridges constructed throughout Europe and more recently in North America and Japan. During the 1990s, a major concern for the deterioration of ageing bridges in North America due to corrosion became more apparent[7]. The US and Canadian governments are currently investing significant funds focused on product evaluation and further development of FRP materials.

Copyright © 2003 John Wiley & Sons, Ltd.

FRP materials and test methods

FIBRES AND RESINS

Fibres are the basic load-bearing component of any FRP product. Fibres are often preassembled into various forms to facilitate the fabrication of composite products. Discontinuous fibres are often used also to produce low-cost composite products such as sheet molding compounds or non-woven mats. Fibre, filaments yarns and rovings generally represent parallel bundles of continuous filaments. The fibre bundles may be used directly in composite fabrication, such as by filament winding or pultrusion process to produce bars or structural shapes, or they may be further converted to other reinforced forms such as 'prepregs' (preimpregnated plates), fabrics and sheets.

Fibres used in modern composites can be broadly classified into three main categories[s,9]:

1. Polymeric fibres, including aramid fibres (i.e. Kevlar 29, Kevlar 49 and Kevlar 149 which is the highest tensile modulus aramid fibre);

2. Carbon fibres, including pan-based carbon and pitch-based carbon. Polyacrylonitrile (PAN) and cellulose are the common precursors from which pan-based carbon fibres are currently made. Petroleum and polyvinyl chloride are the common sources for the pitch used for carbon fibres. Pan-based carbon fibres have diameters of 5-7 11m while pitch-based carbon fibres have diameters of 10-12 11m.

Prog. Struct. Engng Mater. 2003; 5: 16-28

18

5000~----------------------------~

~ 4000 6 ~ 3000 c: !!! 'li) 2000 .J!1 'iii c: ~ 1000

Inorganic fibres S-Glass

Pan-based carbon Polymeric fibres

~norganic fibres E-Glass

~~~-________ Steel

2 3 4 Strain (%)

Fig. 2 Stress-strain relationship of FRP materials

3. Inorganic fibres including E-glass, S-glass and boron fibres_

5

Other specialty fibres such as optical fibres are currently being investigated for structural health monitoring applicationS(10,111_ Fig. 2 illustrates the strength and modulus of elasticity of various FRP materials.

Bars, prepregs or sheets are parallel filaments held by matrix material. The matrix could be thermoplastic resin or partially cured thermoset reSin[12,13]. The role of the resin in composites is vital. Resin selection controls the manufacturing process, upper use temperature, flammability characteristics, and corrosion resistance of the composite. Although loads are carried by the fibre composite, mechanical performance depends to a large extent on the resin modulus, failure strain and the bond between the resin and the fibre[9].

TEST METHODS FOR FRP The anisotropy and inhomogeneity of composite materials make the characterizations of their engineering properties a complex issue. The mechanical properties of advanced composite

/

materials systems are determined by special test methods. These test methods are mechanically simple in concept, but are extremely sensitive to specimen preparation and test execution procedures. Mechanical property characterization is used mainly in research, quality control and as data for design consideration. The mechanical properties include the elastic behaviour and strength of the material under tensile, shear or compression loading. Other properties such as fracture toughness, flexural strength and stiffness are also useful in characterizing the performance of a composite material. Finally thermomechanical and hygromechanical properties are of importance under changing temperature· and moisture environments.

There exist a host of test fixtures, specimen geometry and test procedures for the generation of

Copyright © 2003 John Wiley & Sons. Ltd.

CONCRETE CONSTRUCTION

mechanical property data for laminates, rods, bars, rigid plastics, fibre glass pipes and filament wound composites. Detailed reviews on test methods are reported elsewhere(14]. Only test methods that have been accepted by the engineering community are included in the ASTM standards[1s]. ACI Committee 440 recently completed Recommended Test methods for FRP Rods and SheetsU6], which is currently under review by the ACI Technical Activities Committee.

This document[16] includes descriptions of shortterm and long-term mechanical, thermomechanical, and durability testing techniques for FRP rods and sheets. It is anticipated that these test methods shall be considered, modified, and adopted by the ACI, and will be considered as the first draft for ASTM specifjcation. Currently, the characteristic of the tensile.response of the unidirectional lamina 0° and 90°, caR be determined by ASTM D3039[1s].

CompresSion testing is performed by subjecting the test specib:l.ento an increasing compressive load up to failure. To !'avoid buckling instability, relatively short gauge lengtlls are necessary which may lead to a decrease in the apparent compression strength. Several compression test methods have emerged during the past 20 years and much confusion exists as to their relative virtues. The methods may be grouped into distinct categories based on load introduction and specimen design (shear-loaded specimen test methods, ASTM D341O, and end-loaded specimen test methods, ASTM D6951)[1s].

The response of the material subjected to shear is commonly nonlinear, and full characterization requires the entire stress-strain curve. The tests may be grouped as in-plane tests and inter-laminar (out-ofplane) tests. The major difficulty in designing shear tests for composite material lies in attaining a uniform state of pure shear stress in the test section. Many shear test· methods exist and can be summarized as follows:

• (± 45°) tension test described in ASTM D3518U5]; • V-notch shear test described in ASTM D5379[1s]; • rail shear test described in ASTM D4255us].

Flexural testing on rectangular prisms utilizes either three- or four-point loading, as described in ASTM D790-92[ls]. The flexure tests may be used to check the previously obtained tensile and compressive data.

DETERMINATION OF STIFFNESS AND STRENGTH

PROPERTIES

Recent application of FRP includes FRP tubes filled with concrete. The FRP tube provides the flexural shear strength and confinement of the inner concrete core, leading to significant increase of the concrete strength in compression. The system also exhibits significant ductility before failure. Owing to the heterogeneity of FRP, the properties in a composite

Prog. Struct Engng Mater. 2003; 5:16-28

material obviously vary from point to point. On the other hand, when the effective property of the composite is measured it is somehow averaged over the sample dimensions. Prediction models based on micromechanical analysis can be very useful in design if the models have been verified by comparing the predicted values with the experimental results. The four independent effective elastic constants of unidirectional continuous fibre-reinforced orthotropic lamina (longitudinal, transverse and shear moduli as well as the Poisson's ratio) are usually estimated by using a combination of mechanics of material models and semiempirical models[I7,18). The five effective strengths of an orthotropic lamina include the longitudinal tensile and compressive strength, transverse tensile and compressive strength as well as the shear strength, and may be estimated using micromechanics, but the accuracy of the estimates is typically not as good as the estimates of the elastic constants, because fracture, rather than elastic behaviour, is more sensitive to flaws and defects in the materiai[19).

Design philosophy

The use of FRP materials as reinforcement for concrete structures requires the development of design procedures that ensure adequate safety from catastrophic failure. Design recommendations for concrete members reinforced or strengthened with FRP are based on limit states design principles. Design of concrete members reinforced with FRP is based primarily on the required strength and then checked for serviceability criteria, fatigue endurance, and creep rupture endurance. In many instances, serviceability criteria, fatigue and creep rupture endurance limits may control the design.

With steel reinforcement, a confident level of safety is always provided by yielding of the steel reinforcement of section/subjected to flexural loads. Owing to the linear elastic behavior of FRP materials, flexural, shear and bond failures are unavoidably sudden and brittle. Current building codes and design specifications for FRP recognize the advantages and disadvantages of these materials and define analytical procedures which engineers can use for design[I6). The current design guidelines of ACI[2o,2tJ are introduced with conservative strength reduction factors to be compatible with the specific performance limitations of FRP materials.

The ultimate strength limit states for FRP (flexure, shear rupture, or debonding) are described below.

FLEXURAL FAILURES

Flexural design of reinforced/strengthened concrete members with FRP materials proceeds from basic equilibrium on the cross-section and constitutive



r ~ Eeu ~e hTJ ___ *' __________ .. Failure goveme~ by 11 N.A concrete crushmg f E

f fj<fu

r ~ Eeu ~ 11 ,,;. { --:2f<~At---- Balanced failure

Af EjU 1fo ~ Ec fc<fc

11-, ~Af F"~J",=by Af E .fj" ju

Fig. 3 Flexural failure modes in concrete structures reinforced with FRP

behaviour of the concrete and the FRP. The stress in the FRP reinforcement continues to increase with increasing strain until the FRP reinforcement ruptures.

Research studies[22,23) have shown that properly designed reinforced concrete members with FRP or strengthened steel reinforced members may fail in bending according to one of the following failure mode sequences: steel yielding followed by FRP rupture, or steel yielding followed by concrete crushing. For cases where the total area of steel and FRP reinforcement were relatively high, a third failure mode was observed to occur, in which the concrete crushed in a catastrophic manner. Different flexural failure modes are shown in Fig. 3. The nominal moment capacity Mn corresponding to these failures can be expressed by: steel yielding-FRP rupture

(1)

steel yielding-concrete crushing

(2)

concrete crushing-steel in the elastic range, as in eq. (2) with fy replaced by:

0.003(1 -~) c Es

d

where c is the depth of the neutral axis from the extreme compression fibre; b is the width of the crosssection; d is the depth from the extreme compression fibre to the centroid of the steel reinforcement; h is the depth from the extreme compression fibre to the externally bonded FRP reinforcement; 9 is the . distance from the centroid of the concrete stress blOCk to the FRP; PI is an empirical constant which depends

•

Prog. Struct. fngng Mater. 2003; 5: 16-28

20

on the concrete compressive strength; Ps and Pf are the reinforcement ratios of steel and FRP reinforcement, respectively, defined as As-steer/bd and AFRP Ibh; Es and Ef are the elastic moduli of the steel and FRP reinforcement, respectively; Gfu is the tensile failure strain of the FRP; fy is the yield strength of the steel reinforcement and f~ is the nominal compressive strength of the concrete at 28 days.

Research studies[22-24) on concrete members reinforced or strengthened with FRP materials indicated a sudden and brittle failure mode when the FRP reinforcement ruptured in tension. A more progressive and less catastrophic failure with a higher deformability factor was observed when the failure was governed by crushing of the concrete after yielding of the internal steel reinforcement. The use of high-strength concrete allows for better use of the high-strength properties of the FRP materials and increases the stiffness of the cracked section, but the brittleness of high-strength concrete compared with normal-strength concrete can reduce the overall deformability of the flexural member.

Conservative strength reduction factors should be adopted to provide a higher reserve strength in concrete members reinforced or strengthened with FRP. The Japanese recommendations for design of flexural members using FRP suggest a strength reduction factor for FRP materials equal to 1/1.3[25). Other researchers in Canada[26) suggest a value of 0.75, determined on the basis of probabilistic concepts. The American Concrete Institute[2o) suggests a strength reduction factor of 0.5 for rupturecontrolled failures and a value of 0.7 for concrete crushing failures. It should be mentioned that direct comparisons of these numbers should not be made because these strength reduction factors correspond to different load factors in the respective countries.

SHEAR FAILURES

Shear failures are typically brittle and should be avoided as a failure mode for concrete members reinforced or strengthened with FRP[5). Shear failure modes of members with FRP as shear reinforcement can be classified into two types: shear-tension failure mode (controlled by the rupture of FRP shear reinforcement) and shear-compression failure mode (controlled by the crushing of the concrete web). The first mode of failure is more brittle, and the latter results in larger deflections. Experimental results have shown that the modes of failure depend on the shear reinforcement index Pfv Ef, where Pfv is the ratio of FRP shear reinforcement to the effective crosssectional area and Ef is the modulus of elasticity of the FRP stirrups. As the value of pfvEf increases, the shear capacity in shear-tension increases and the mode of failure changes from shear-tension to shearcompression. The current design assumption that

- Copyright © 2003 John Wiley & Sons, Ltd.


i- wrapped (a)

i- Continuous sheets (b)

111111111111""""1. f i- 9()o (c)

: U : 1f ii- Bonded to sides

Fig. 4 Various shear strengthening configurations with FRP: (a) bonded surface configuration; (b) FRP reinforcement distribution; and (c) fibre orientation

concrete and reinforcement capa<;ities can be added to estimate the shear capacity of the member is accurate when shear cracks are adequately controlled. Therefore, the tensile strain in the FRP shear reinforcement should be limited to ensure that the design approach is applicable. The Canadian Highway Bridge Design Code[27) limits the tensile strain in FRP shear reinforcement to 0.2%. A pan-European collaborative research programme EUROCRETE conducted a series of research and demonstration projects, which limit the value of the shear strain in FRP reinforcement to 0.25%[28).

Members with FRP longitudinal reinforcement and steel stirrups did not experience unusual shear behaviour(29). Special attention should be devoted to the reduced dowel contribution of the FRP reinforcement out of plane in the presence of shear cracks[30). External shear reinforcement in the form of bonded FRP overwrap has been applied to beams with insufficient shear strength. This procedure can provide sufficient shear resistance, depending on the amount and the configuration of the externally bonded FRP reinforcement, and allow full development of the flexural capacity of the beam(21).

For concrete members strengthened in shear using externally bonded FRP sheets, loss of aggregate interlock of the concrete has been observed to occur at fibre strains less than the ultimate fibre strain. To preclude this mode of failure, the maximum strain recommended for design by ACI Committee 440 is limited to 0.2%(21). The same approach was applied in the Canadian and the European Standards.

Recently, many shear-strengthening configurations have been proposed to increase the shear capacity of reinforced concrete beams through FRP retrofit. Multiple options exist for shear strengthening, as shown in Fig. 4, including laminate bonding to the

Prog. Struct fngng Mater. 2003; 5: 16-28

sides of the beam, V-jacketing around the bottom, and total wrapping of the beam.

The shear-resisting system can be in the form of continuous sheet or laminates with spacing. Fibres can be oriented either perpendicular to the axis of the beam or perpendicular to the potential shear cracks, or a combination of both orientations. The contribution of shear strength provided to a member by the FRP system is based on the fibre orientation and an assumed crack pattern and can be expressed as:

Vf = Afvffe(sintX) + costX)df

Sf (3)

where Afv is the area of the FRP shear reinforcement; ffe is the effective stress in the FRP, taken as the smaller of the design tensile strength ffu or the stress corresponding to 0.002 Ef ; Ef is the modulus of elasticity of FRP; df is the depth of the FRP shear reinforcement; Sf is the spacing of the FRP shear reinforcement and tX is the angle of inclination of the FRP shear reinforcement[21]. The nominal shear capacity of concrete members reinforced or strengthened with FRP in shear can be determined by adding the full contribution of the FRP reinforcement to the contributions from the reinforcing steel and the concrete.

DEBONDING FAILURES

Debonding failures are very common in concrete members retrofitted with externally bonded FRP systems. These types of failures are often brittle, occur with little or no visible warning, and take place at load levels significantly lower than the flexural or shear strength of the retrofitted system. For FRP sheets/strips in bending, this mode of failure initiates with a horizontal crack atone of the edges of the bonded FRP sheets/strips below the internal steel reinforcement level and/propagates toward mid-span, leading eventually to a complete separation of the FRP with the adjacent concrete cover layer. Delamination from existing shear cracks is characterized by differential vertical displacements at crack tips in retrofitted concrete beams. Such a phenomenon causes a stress intensity in the bond region, which initiates fracture and results in delamination propagation under increased loading. Debonding failures are characterized by propagation of the failure process parallel to the plane of the laminate, while other failures such as flexural or shear failures propagate perpendicular to this plane. Debonding includes failure of the concrete layer between the FRP and the flexural steel followed by delamination or peeling of the FRP from the concrete. Three identified debonding failure mechanisms categorized by many researchers are illustrated in Fig. 5.



Fig. 5 Delamination failure modes

Theoretical work on the bond strength and delamination models can be simplified into three main categories:

• empirical models, based directly on regression of test data[31,321. Various relationships were proposed relating the bond length of the FRP sheets/strips to the average bond str~ngth. These models underestimated the bond strength and led to large scatter[31 ,32];

• Mechanics of materials approach in which the elastic stress field at the termination point of the FRP materials was analysed[33,34]. The interfacial shear stresses between the FRP sheets/strips and the adhesive were calculated by considering the equilibrium of an infinitesimal portion of the FRP. These types of solution assume linear elastic behaviour of both concrete and adhesive. Consequently, such type of analysis is limited to regions of low damage, such as inflection points where the normal stresses are generally low[33,34I;

• Fracture mechanics approach[35-37]. Quantitative studies of FRP delamination through fracture mechanics concepts offer great potential in understanding the role of relative materials and design properties on the overall failure process through delamination. Interfacial fracture mechanics can be used to characterize crack tip stresses, crack propagation, and crack path evaluation. However, the use of interfacial fracture mechanics with laminated concrete structures has been limited.

Many experimental programs, reporting debonding failures in theJiterature do not have provisions for detecting the exact origin, propagation, and secondary mechanisms involved with the debonding process. Thus, an improved experimental technique for monitoring debonding failure is needed. ACI Committee 440 design guidelines recommend a maximum elongation limit for the design of FRP sheets/strips to avoid peeling of the FRP reinforcement based on experimental results conducted by other researchers. The document also includes some detailed provisions dealing with delamination failures.

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22

FRP strengthening techniques

EXTERNALLY BONDED FRP SYSTEMS

The most imperative characteristic of externally bonded FRP systems in repair/strengthening applications is the speed and ease of installation. FRP may be bonded to the tension side of concrete beams, girders and slabs to provide additional flexural strength, and/ or on the sides of beams and girders to provide additional shear strength. For seismic zones, FRP may also be used to wrap columns to enhance the ductility due to the induced confinement of the concrete. FRP material selection should be based on strength, stiffness and durability required for a specific application. Resins are selected on the basis of the environment to which the FRP will be exposed, as well as the method by which the FRP is manufactured. FRP plate bonding technology was first investigated at the Swiss Federal Laboratory for Materials Testing and Research (EMPA)[38]. FRP composites have been used in other areas such as the aerospace industry for many years and their superior properties compared with other conventional structural materials are well known[39].

Externally bonded FRP systems come in a variety of forms, including wet lay-up systems and precured systems. Wet lay-up FRP systems consist of dry unidirectional or multidirectional fibre sheets or fabrics impregnated with a saturating resin on-site. Precured FRP systems consist of a wide variety of composite shapes manufactured off-site. Typically, adhesive along with the primer and putty are normally used to bond the precured shapes to the concrete surface. The primer is used to penetrate the surface of the concrete, providing an improved adhesive bond for the saturating resin or adhesive. The putty is used to fill small surface voids in the substrate and to provide a smooth surface to which the FRP can bond. Precured FRP systems include unidirectional laminates, multi-directional grid and precured shells.

Since 1982, externally bonded FRP sheets/strips have been successfully applied to reinforced concrete

Stage I

I

If I

I I

I '\FRP 8tri p I

! Stage II

fig. 6 Delamination of externally bonded FRP reinforcement



beams[40]. Researchers worldwide suggest that FRP sheets / strips could replace externally bonded steel plates with overall cost savings emanating from the simplicity of the strengthening method. Externally bonded FRP sheets and strips are currently the most commonly used techniques for strengthening bridges and concrete structures. In spite of the significant research being reported on their structural mechanism and performance, there are still heightened concerns regarding possible premature failure due to debonding, especially in zones of combined high flexural and shear stresses. Delamination failure of externally bonded FRP sheets is illustrated in Fig. 6. In addition, externally bonded FRP reinforcement is relatively unprotected against wear and impact loads. The structural performance of the externally bonded FRP could also be greatly affected by harsh environmental conditions.

NEAR-SURfACE-MOUNTED FRP SYSTEMS

Use of near-surface-mounted (NSM) FRP rods and strips can preclude delamination-type failures, frequently observed by using externally bonded reinforcement[41-43]. FRP bars or strips can be inserted in specially constructed grooves within the concrete cover layer, and adhered to the concrete with epoxy adhesives as shown in Fig. 7. The NSM technique becomes particularly attractive for flexural strengthening in the negative moment regions of slabs and decks, where external reinforcement would be subjected to mechanical and environmental damage and would require protective cover, which could interfere with the presence of floor finishes. NSM steel bars have been used in Europe since 1947[44]. Tests on concrete beams reinforced with steel bars and others reinforced with steel bars grouted into diamond-sawn grooves showed identical behaviour for both sets of specimens[44]. Flexural or shear strengthening with NSM FRP strips showed a greater anchoring capacity compared with externally bonded FRP strips[41-43,45]. The feasibility of NSM FRP bars and strips have been investigated experimentally by many researCherS[41-43,45-47]. Test results showed that the efficiency of NSM FRP strips, defined as the ratio of the percentage increase in capacity to construction cost, was three times that of the externally bonded strips [42].

A general methodology to evaluate the development length of NSM FRP bars of different configurations was investigated by the authors. The model is based on equilibrium and displacement compatibility procedures using finite element analysis, and accounts for distinct characteristics of concrete, epoxy and FRP materials. Fig. 8 shows a schematic representation of the principal tensile stresses around a NSM FRP bar. The development length is highly dependent on the dimensions of the

Prog. Strua. Engng Mater. 2003; 5: 16--28

Fig. 7 Strengthening procedures with NSM FRP bars

Fig. 8 Typical tensile stress distribution around a NSM FRP bar

/

strips, concrete properties, adhesive properties, internal steel reinforcement ratio, reinforcement configuration, type of loading, and groove width[42].

Two different types of debonding failures can occur for NSM FRP bars. The first mode of failure is due to splitting of the epoxy cover as a result of high tensile stresses at the FRP-epoxy interface, and is termed 'epoxy split failure'. Increasing the thickness of the epoxy cover reduces the induced tensile stresses significantly. Furthermore, using adhesives of high tensile strength delays epoxy split failure. Epoxy split failure usually forms with longitudinal cracking through the epoxy cover. The second mode of failure is due to cracking of the concrete surrounding the epoxy adhesive and is termed 'concrete split failure'. This mode of failure takes place when the tensile stresses at the concrete-epoxy interface reach the



tensile strength of the concrete. Widening the groove minimizes the induced tensile stresses at the concrete-epoxy interface and increases the debonding loads of NSM bars[43].

Analytical modelling for NSM FRP strips is based on the combined shear-bending model for externally bonded FRP plates. The model is modified to account for the doubly bonded area of NSM strips. The model accounts also for the continuous reduction in flexural stiffness due to cracking of the concrete. Debonding of NSM FRP strips occurs as a result of the high shear stress concentration at the cut-off point. For simply supported beams subjected to a concentrated load P, at mid-span, the shear stress at the strip cut-off point r can be expressed in terms of the effective moment of inertia, Jeff, and the thickness of the FRP strip, tf , as follows:

r = !!. [npl0Y w + nPY] 2 2Jeff 2Ieff

(4)

where:

2 2Ga w =--tatfEf

(5)

Ef n=-

Ec (6)

Ef is the elastic modulus of the FRP strip, Ec is elastic modulus of concrete, Ga is the shear modulus of the adhesive, fa is the thickness of the adhesive, 10 is the unbonded length of the strip and Y is the distance from the strip to the neutral axis of the transformed section.

Premature debonding of NSM FRP strips is governed by the shear strength of the concrete. OtJ;1.er


24

components of the system such as the epoxy adhesive and the FRP strips have superior strength and adhesion properties compared with concrete. Knowing the compressive and tensile strength of concrete, the Mohr-Coulomb line, which is tangential to both Mohr's circles for pure tension and pure compression, can be represented, and the maximum critical shear stress for the pure shear circle can be expressed as:

I~ let 'max = I~ + let (7)

where Ie is the compressive strength of concrete after 28 days and let is the tensile strength of concrete.

Debonding loads for NSM FRP strips can be determined for simply supported beams loaded with a concentrated load at mid-span by equating the shear strength proposed in eq. (7) to the shear stress given in eq. (4), Other loading cases (e.g. simply supported beams subjected to a uniform load, simply supported beams subjected to two concentrated loads) are reported in[42].

In general, strengthening limits for concrete members retrofitted with FRP should be specified, such that a loss of FRP reinforcement should leave the concrete member with sufficient capacity to resist at least unfactored dead and live loads.

Field applications

UNITED STATES

The United States has had a long and continuous interest in fibre-based reinforcement for concrete structures since the 1930s. However, actual development and research activities into the use of these materials for retrofitting concrete structures started in the late 1980s. FRP materials have quickly moved from the state-of-the-art to mainstream technology.

/

EUROPE

Research on the use of FRP in concrete structures began in Europe in the 1960s[5,6]. In the field of strengthening with FRP, pioneering work took place

Fig.9 Taylor Bridge, Headingley, Manitoba, Canada



in the 1980s in Switzerland and resulted in successful practical applications[4o]. One of the first field applications of FRP strengthening in Europe was performed in 1991 on the Ibach Bridge, a concrete box girder in Lucerne, Switzerland. GFRP was used to retrofit the Kattenbusch Roadway Bridge in Germany to reduce the steel stresses in the tendon couplers[48]. A pan-European collaborative research programme EUROCRETE was established in 1993 and ended in 1997. The program was aimed at developing FRP reinforcement for concrete, and included partners from the United Kingdom, Switzerland, France, Norway and The Netherlands. NSM carbon FRP strips were used to rehabilitate the Tobel Bridge in Southern Germany in 1999. The bridge consisted of prestressed, precast T-girders with a cast-in-place concrete deck. One of the girders was damaged by a truck, which crushed the web of that girder. CFRP strips of 20 x 1.2 mm were inserted inside 23-mm grooVeS[45].

JAPAN

Together with Europe, Japan developed the first FRP application for construction in the early 1980s. A sudden increase in the use of FRP was attained after the 1995 Hyogoken Nanbu earthquake. As of 1997, the Japanese led in FRP reinforcement usage with 1000 demonstration/ commercial projects and FRP design provisions in their standard specifications of the Japan Society of Civil Engineers[25]

CANADA

The use of FRP for repair and strengthening of concrete structures began in earnest in the late 1980s. A significant international research breakthrough was achieved in 1998 by the opening of the Taylor Bridge in Headingley, Manitoba. The two-lane, 165.1-m-long structure has 4 of it's 40 precast girders reinforced with CFRP stirrups, as shown in Fig. 9. These girders were prestressed with CFRP cables and bars. GFRP bars were used to reinforce portions of the barrier walls. The deck slab was reinforced by CFRP bars, similar to the reinforcement used for prestressing the girders. The bridge boasts a complex embedded fibre



Fig. 10 Shear strengthening of AASHTO girders with externally bonded FRP sheets

optic structural sensing system that will allow engineers to compare the long-term behaviour of the materials. The sensors are not only immune to electromagnetic interference, they also have long-term stability in advanced materials. In 1999 a trial application of CFRP sheets as a first step in upgrading the shear capacity of the Maryland Street bridge in Winnipeg, Manitoba, was conducted as shown in Fig. 10.

Durability of FRP

Although FRP composites perform extremely well in practice, there are heightened concerns related to their durability in the field as related to civil infrastructure applications. In these cases, FRP composites are exposed to harsh environmental conditions, ranging from wide temperature fluctuations and humidity levels to rain and snow.

There are limited theoretical and experimental studies on the durability oftli.e bond between FRP and concrete. Investigations by different researchers are focusing on the durability of externally bonded FRP reinforcement. No literature is currently available on the durability of NSM FRP reinforcement. The authors expect that the performance of NSM FRP reinforcement will be superior under severe environmental conditions as the reinforcement is protected inside the concrete. However, the durability of bonding adhesives needs to be investigated. Further studies are still needed to establish accurate reduction factors to be used in bond strength models for design purposes.

WET-DRY EXPOSURE

Wet-dry cyclic exposure has a significant effect on the bond length, shear stress distribution and differential strain between FRP and concrete. Several research


programs [49] have focused on the behaviour of concrete beams strengthened with GFRP and CFRP sheets and subjected to wet-dry cycles.

Chajes et al.[49] subjected concrete beams, externally strengthened with GFRP and CFRP sheets, to a series of wet-dry cycles. A wet-dry cycle was completed by immersing the beams into a 3.5-4% salt solution for 18 h, followed by 8 h drying at room temperature. Test results showed that GFRP strengthened beams lost 36% of the unexposed strength, while CFRP strengthened beams lost 19% after 100 wet-dry cycles[49].

The shear modulus of the adhesive at the FRP-concrete interface was found to be a critical parameter for obtaining durable FRP retrofitted concrete members. On exposure to a 20DC dry environment, an 80% decrease in the shear modulus of the adhesive was observed compared with test specimens subjected to normal exposure conditions at room temperature[50].

Adhesives are generally sensitive to water. Deterioration of bonded joints is characterized by absorption of water by the adhesive and by moisture diffusion to the adhered interface. Voids can be created in the adhesive layer and at the interface. Presence of voids implies less area of contact. Water can also replace the adhesive by capillary transmission, and this weakens the bond between the externally bonded FRP reinforcement and concrete[51].

FREEZE-THAW EXPOSURE

The influence of freeze-thaw cycles on the structural performance of concrete beams strengthened with FRP sheets and strips was studied by several researCherS[52-55]. No detrimental effect on the overall structural performance of the beams after 300 cycles from -25 to +25DC was observed. Test results showed that freeze-thaw action did not degrade the bond of FRP-strengthened beams.


26

Fig. I I Failure modes of the repaired masonry wall

THERMAL EXPOSURE

High temperatures showed a detrimental effect on the bond characteristics of FRP rebars[56]. Test results showed a reduction of 80-90% in the bond strength of FRP rebars with different surface treatment at temperatures ranging from 20 to 250°057]. In comparison, steel rebars showed a reduction of 38%.

Repair of Masonry Walls

A large number of existing masonry walls are not designed with sufficient seismic resistance. Previous research has shown that using FRP for retrofit is a feasible solution to increase seismic strength and ductility of masonry walls[58]. Most of the reported research has focused on strengthening undamaged masonry walls in the out-of-plane direction. The performance of severely damaged and repaired clay brick masonry walls using FRP sheets was recently investigated[58,59]. Test results indicated that the strength of the repaired walls was restored and exceeded the original wall strength by 11 and 38 percent in the push and pull directions respectivelY[59]. The displacement capacity of the repaired walls was more than twice that of the original walls. Failure was due to vertical splitting of the masonry as well as delamination and outward local buckling of the FRP sheets, as shown in Fig. 11. It should also be noted that the capacity of the repaired walls was always limited by the capacity of the concrete footing. Further research is needed to improve the repair technique at the joint between the wall and footing.

Conclusions

FRP reinforcing bars, strips and sheets are relatively new products, and require extensive testing before

Cppyright © 2003 John Wiley & Sons, Ltd.


Delamination of the FRP sheets

they can be recommended for widespread application in concrete construction or repair of existing structures. The promise of FRP materials lies in their high-strength, lightweight, non-corrosive, nonconducting, and non-magnetic properties. In addition, FRP manufacturing offers a unique opportunity for the development of shapes and forms that would be difficult or impossible with conventional steel materials. Although the fibres and resins used in FRP systems are relatively expensive compared with traditional strengthening materials, labour and equipment costs to install FRP systems are often lower. FRP systems can also be used in areas with limited access where traditional techniques would be impractical. However, it is crucial to ensure that FRP strengthening systems are durable and capable of providing the design strength over the life of the structure. Certain environmental conditions, such as temperature, humidity, wet-dry cycles, freeze-thaw cycles, and ultraviolet exposure could have an adverse effect on the FRP strengthening systems, and should be investigated extensively. For the research results to be universally acceptable, it is imperative to standardize the test methods for evaluating basic mechanical properties of the FRP materials. Standardization plays a key role in streamlining and effectively categorizing FRP reinforcements for inclusion in design specifications and standard codes.

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