Recommended Guide Specification for the Design

of Externally Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements

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REPORT 655

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NCHRP REPORT 655

Subscriber Categories

Bridges and Other Structures

Recommended Guide Specification for the Design

of Externally Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements

Abdul-Hamid ZureickBruce R. Ellingwood

GEORGIA INSTITUTE OF TECHNOLOGYAtlanta, GA

Andrzej S. NowakUNIVERSITY OF NEBRASKA-LINCOLN

Lincoln, NE

Dennis R. MertzUNIVERSITY OF DELAWARE

Newark, DE

Thanasis C. TriantafillouUNIVERSITY OF PATRAS

Patras, Greece

Research sponsored by the American Association of State Highway and Transportation Officials in cooperation with the Federal Highway Administration

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NCHRP REPORT 655

Project 10-73ISSN 0077-5614ISBN 978-0-309-15485-7Library of Congress Control Number 2010930888

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CRP STAFF FOR NCHRP REPORT 655

Christopher W. Jenks, Director, Cooperative Research ProgramsCrawford F. Jencks, Deputy Director, Cooperative Research ProgramsAmir N. Hanna, Senior Program OfficerEileen P. Delaney, Director of PublicationsMaria Sabin Crawford, Assistant Editor

NCHRP PROJECT 10-73 PANELField of Materials and ConstructionArea of Specifications, Procedures, and Practices

Paul V. Liles, Jr., Georgia DOT, Atlanta, GA (Chair)Tadeusz C. Alberski, New York State DOT, Albany, NY Jim Gutierrez, California DOT, Sacramento, CA Issam Harik, University of Kentucky, Lexington, KY Calvin E. Reed, Kansas DOT, Topeka, KS Andrew L. Thomas, Pennsylvania DOT, Harrisburg, PA Eric P. Munley, FHWA Liaison Stephen F. Maher, TRB Liaison

C O O P E R A T I V E R E S E A R C H P R O G R A M S

This report presents a recommended guide specification for the design of externallybonded Fiber-Reinforced Polymer (FRP) systems for the repair and strengthening of con-crete bridge elements. This guide specification addresses the design requirements for mem-bers subjected to different loading conditions (e.g., flexure, shear and torsion, and com-bined axial force and flexure). The guide specification is supplemented by design examplesto illustrate its use for different FRP strengthening applications. The guide specification ispresented in AASHTO LRFD format to facilitate use and incorporation into the AASHTOLRFD Bridge Design Specification. The material contained in the report should be of imme-diate interest to state engineers and others involved in the strengthening and repair of con-crete structures using FRP composites.

Use of externally bonded FRP systems for the repair and strengthening of reinforced andprestressed concrete bridge structures has become accepted practice by some state highwayagencies because of their technical and economic benefits. Such FRP systems are lightweight,exhibit high tensile strength, and are easy to install; these features facilitate handling and helpexpedite repair or construction, enhance long-term performance, and result in cost savings.In addition, research has shown that external bonding of FRP composites improves flexuralbehavior of concrete members and increases the capacity of concrete bents and columns.

In spite of their potential benefits, use of externally bonded FRP systems is hampered bythe lack of nationally accepted design specifications for bridges. Thus, research was neededto review available information and develop a recommended guide specification for suchrepair and strengthening systems.

Under NCHRP Project 10-73, Guide Specification for the Design of Externally BondedFRP Systems for Repair and Strengthening of Concrete Bridge Elements, Georgia Instituteof Technology conducted a review of relevant domestic and international information,identified and categorized the items necessary for developing a guide specification, anddeveloped a reliability-based guide specification that employs Load and Resistance FactorDesign (LRFD) methodology. The guide specification is accompanied by commentaries thatare necessary for explaining the background, applicability, and limitations of the respectiveprovisions. In addition, design examples are provided to illustrate use of the recommendedguide specification for different strengthening requirements.

The recommended guide specification will be particularly useful to highway agenciesbecause it will facilitate consideration of FRP systems among the options available for therepair and strengthening of concrete bridge elements and help select options that areexpected to yield economic and other benefits. The incorporation of the recommendedguide specification into the AASHTO LRFD Bridge Design Specifications will provide an easyaccess to the information needed for the design of externally bonded FRP systems for therepair and strengthening of concrete bridge elements.

F O R E W O R D

By Amir N. HannaStaff OfficerTransportation Research Board

C O N T E N T S

1 Summary

2 Chapter 1 Introduction and Research Approach

4 Chapter 2 Findings

12 Chapter 3 Interpretation, Appraisal, and Application

19 Chapter 4 Conclusions, Implementation, and Recommendations for Further Research

21 References

A-i Attachment A Recommended Guide Specification for the Design of Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements

B-i Attachment B Illustrative Examples

S U M M A R Y

Fiber-reinforced polymer (FRP) composites now are being used to strengthen or to upgradethe load-carrying capacity of a wide range of bridge structures. These materials must offer tech-nical and economical advantages in order to be successful in the highly competitive construc-tion marketplace. While FRP composites increasingly are being used in combination withtraditional construction materials for rehabilitation of existing structures, codes, and standardsfor structural condition assessment, evaluation and rehabilitation of bridge structures usingcomposite materials do not exist as of yet. Design information for most FRP composite materi-als has been developed mainly by the composites industry.

This report summarizes the research conducted in NCHRP Project 10-73 to develop a recom-mended guide specification for the design of externally bonded FRP composite systems for repairand strengthening of reinforced and prestressed concrete highway bridge elements. This infor-mation will facilitate the use of FRP materials in strengthening reinforced concrete and pre-stressed bridge elements by providing bridge engineers with a rational basis for such use. Theresearch produced a recommended Guide Specification for the Design of Bonded FRP Reinforce-ment Systems for Repair and Strengthening of Concrete Bridge Elements. This Guide Specificationis presented in a format resembling that of the AASHTO LRFD Bridge Design Specifications, 4thEdition (2007) in order to facilitate their consideration and adoption by the AASHTO. The rec-ommended Guide Specification is accompanied by commentaries that explain the background,applicability, and limitations of the provisions contained therein. Included also are step-by-stepcalculations in accordance with the recommended Guide Specification for six examples of com-monly used FRP strengthening applications. These examples would serve as a tutorial on how toapproach bridge strengthening projects in practice.

The report concludes with suggestions for implementing the guide specification and recom-mendations for further research to support future revisions and enhancements of the recom-mended guide specification.

Recommended Guide Specification for the Design of Externally Bonded FRP Systems for Repair and Strengthening of Concrete Bridge Elements

1

21.1 Background

Because of the technical and economic benefits achieved bythe use of externally bonded fiber-reinforced polymer (FRP)systems for the repair and strengthening of reinforced andprestressed concrete bridge structures, this method of reha-bilitation of bridge structures has become accepted practicein many state highway agencies. Such FRP systems are light-weight, exhibit high tensile strength, and are easy to install;these features facilitate handling and help expedite repair orconstruction, enhance long-term performance, and result incost savings. In addition, the external bonding of FRP com-posites improves flexural behavior of concrete members andincreases the capacity of concrete bents and columns.

In spite of their potential benefits, the use of externallybonded FRP systems is hampered by the lack of nationally ac-cepted design specifications for their use in the repair andstrengthening of concrete bridge elements. NCHRP Project10-73 was initiated to review available information and to de-velop a recommended guide specification for the design of ex-ternally bonded FRP systems. This specification will help high-way agencies consider FRP systems among the options for therepair and strengthening of concrete bridge elements and selectoptions that are expected to yield economic and other benefits.

1.2 Project Objective and Scope

The objective of this project was to develop a recommendedguide specification for the design of externally bonded FRPcomposite systems for their use in the repair and strengthen-ing of reinforced and prestressed concrete highway bridgeelements. FRP composite systems covered in this projectinclude thermoset polymers reinforced by carbon, glass, oraramid fibers. To achieve this project objective, the followingtasks were carried out:

Task 1. Information relevant to the design of FRP systemsused in repair and strengthening of concrete bridge elementswas collected and reviewed. This information was assembledfrom published and unpublished reports, contacts with trans-

portation agencies and industry organizations, and other do-mestic and foreign sources, including the American ConcreteInstitute Guide for the Design and Construction of Exter-nally Bonded FRP Systems for Strengthening Concrete Struc-tures (ACI 2002) and similar publications.

Task 2. Based on the information gathered in Task 1, theitems necessary for developing the guide specification wereidentified and categorized. These items addressed flexure,shear, axial loading, development length, detailing, and otherdesign considerations in a manner similar to that provided inthe AASHTO LRFD Bridge Design Specifications.

Task 3. Based on the information obtained in Tasks 1 and2, a tentative outline of the proposed guide specification anda work plan for developing the specification along with a com-mentary and design examples were prepared. The plan de-scribed the proposed approach for incorporating appropriateresistance factors and other design criteria in the specification.

Task 4. The plan for developing the guide specification wasexecuted. Based on the results of this work, the guide specifi-cation was developed.

Task 5. Using the specification developed in Task 4, a com-mentary and design examples to illustrate use of the specifi-cation were prepared.

Task 6. A final report that documents the entire research,including the recommended guide specification and commen-tary and the design examples was prepared.

1.3 Applicability of Results to Highway Practice

The research products resulting from this project providea technically sound and documented basis for using FRP re-inforcement in bridge rehabilitation and retrofit. The use ofFRP reinforcement will have a significant impact on the eco-nomics of bridge maintenance and rehabilitation at state andnational levels, and may permit them to upgrade the load-carrying capacity of bridge members through easy-to-installretrofits rather than replacement. The recommended guide

C H A P T E R 1

Introduction and Research Approach

specifications gives FRP manufacturers a consistent basis forreporting material properties while at the same time allowsbridge design and maintenance engineers to use such materialproperty data for conditions similar to those under which theseproperties are obtained. The guide specifications and com-mentary presented in Attachment A are formatted to facilitateconsideration and adoption by AASHTO.

1.4 Report Organization

This report consists of four chapters and two attachments.Chapter 1 describes the objective and outlines the various

tasks performed to accomplish the objective. Chapter 2 pres-ents the findings of this study, and Chapter 3 addresses theanalytical formulations and the experimental data that formedthe basis upon which the proposed Guide Specifications weredeveloped. Chapter 4 presents the conclusions and recom-mendations for further research. Attachment A presents rec-ommended guide specifications and commentaries for thedesign of externally bonded FRP reinforcement systems forthe repair and strengthening of concrete bridge elements. At-tachment B contains step-by-step illustrative examples thatserve as a tutorial on how to approach bridge strengtheningprojects in practice.

3

4The major research product of this project is a set of rec-ommended guide specications. The technical basis for theserecommendations is described in this chapter.

2.1 Review of Current Practice

This task consisted of a review of relevant available FRP en-gineering practice, specications, design guides, data, and re-search ndings from both national and international sources.A brief review concerning the development of externallybonded plates for strengthening reinforced concrete bridgestructures is provided below.

FRP composite materials provide effective and potentiallyeconomical solutions for rehabilitating and upgrading existingreinforced and prestressed concrete bridge structures that havesuffered deterioration. Whether a bridge has been damageddue to overload or material deterioration or requires strength-ening to resist increased future live loads due to trafc, wind orseismic forces, FRPs provide an efcient, cost-effective, andeasy-to-construct means to reinforce concrete members. FRPcomposites may be designed to act as exural, shear or con-nement reinforcement. They may be placed in situ with lessdisruption of bridge utilization and other functions than isusual when rehabilitation involves the addition of steel rein-forcement. (Meir and Betti 1997; Seible et al. 1997; Deaveret al. 2003; Hamelin et al. 2005; Triantallou 2007)

The concept of using externally bonded FRP reinforce-ment to strengthen concrete structures was developed as animprovement to the use of externally bonded steel plates.Strengthening with externally bonded steel plates commencedin 1964 in Durban, South Africa, to address the problem ofthe accidental omission of steel reinforcing bars of a base-ment beam in an apartment complex (Dussek 1980). This in-novative idea was developed further on a variety of construc-tion projects involving bridges, parking garage decks, andofce buildings in South Africa and Europe (Fleming andKing 1967; Parkinson 1978). A review of research and devel-

opment related to strengthening with externally bonded steelplate can be found in Eberline et al. (1988).

Realizing some difculties associated with the handling andconstruction of the relatively heavy weight steel plates usedfor strengthening purposes, the Swiss Federal Laboratoriesfor Materials Testing and Research Institute (EMPA) initi-ated an extensive research investigation in the early 1970s, theresult of which suggested that lightweight carbon ber rein-forced composite plates could be used in lieu of heavy steelplates for externally strengthening reinforced concrete struc-tures (Meier 1987; Kaiser 1989; Meier 1992; Meier et al.1993).The EMPA efforts led to the rst eld implementation of FRPrehabilitation for both bridge and building applications. TheIbach Bridge near Lucerne, Switzerland, and the City Hallof Gossau St. Gall in northeastern Switzerland both werestrengthened in 1991 by bonding pultruded carbon ber poly-mer plates to the exterior surfaces of the concrete structures.Details on some of these and other early applications are de-scribed by Meier et al. (1993).

The EMPA success in using carbon-ber reinforced poly-mer composites for externally bonded repair and strengthen-ing of reinforced concrete structures motivated researchers inNorth America, Europe, Asia, and Australia to further inves-tigate the use of externally bonded FRP materials in structuralrehabilitation. The results from these investigations led to anumber of recommended design guides and specications,including the following:

ACI 440.2R-02 Guide for the Design and Construction ofExternally Bonded FRP Systems for Strengthening ConcreteStructures (ACI 2002).

ISIS Canada Design Manuals, 2001, Strengthening Rein-forced Concrete Structures with Externally-Bonded Fiber-Reinforced Polymers, Winnipeg, Manitoba (ISIS 2001).

fib technical report bulletin 14, Externally bonded FRP re-inforcement for RC structures, published in Europe (b2001).

C H A P T E R 2

Findings

5 CNR-DT 200, Guide for the Design and Construction ofExternally Bonded FRP Systems for Strengthening Exist-ing Structures, Italian Advisory Committee on TechnicalRecommendations for Construction, Rome, Italy (CNR-DT 200 2006).

Japan Society of Civil Engineers (JSCE), Recommenda-tions for Upgrading of Concrete Structures with Use ofContinuous Fiber Sheets, (JSCE 2001).

The French Association of Civil Engineers, Title : Rpara-tion et renforcement des structures en bton au moyen desmatriaux composites (AFGC 2003).

Avis Technique 3/01-345, lement de structure renforcspar un procd collage de bres de carbone, EntripriseFREYSSINET, France (Freyssinet 2001).

German Provisional Regulations, Allgemeine Bauauf-sichtliche Zulassung, Nr. Z-36.12-65 vom 29, Deutsches In-stitut Fr Bautechnik, Berlin (German Provisional 2003).

Polish Standardization Proposal for Design Procedures ofFR Strengthening (Gorski and Krzywon 2007)

Caltrans Bridge Memo to Designers-MTD 20-4 (Caltrans2007)

GDOT Specication: Proposed Specications-PolymericComposite Materials for Rehabilitating Concrete Structures(Zureick 2002).

NCHRP Report 514: Bonded Repair and Retrofit of ConcreteStructures Using FRP CompositesRecommended Construc-tion Specifications and Process Control Manual (Mirmiranet al. 2004).

NCHRP Report 609: Recommended Construction Specifica-tions and Process Control Manual for Repair and Retrofit ofConcrete Structures Using Bonded FRP Composites (Mirmiranet al., 2008).

2.2 Development of Proposed Guide Specifications

The information gathered from national and interna-tional design guides as well as published and unpublishedresearch reports and archival journal papers germane to therepair and strengthening of concrete structures was assem-bled. The essential elements of all available design guideswere identified, selected, and categorized in a manner con-sistent with the AASHTO LRFD Bridge Design Specification,4th Edition (AASHTO 2007), yielding the following fivesections:

Section 1: General Provisions, Section 2: Material Requirements, Section 3: Members under Flexure, Section 4: Members under Shear, and Section 5: Members under Combined Axial Force and

Flexure.

Each section is further divided into subsections. The GuideSpecification (Attachment A) was also organized into theseve sections to facilitate its use by the professional bridge en-gineering community.

The recommendations contained in the Guide Specifica-tions utilize the load combination requirements found in theAASHTO LRFD Bridge Design Specifications. The resistancecriteria were developed using the same principles of struc-tural reliability analysis on which the AASHTO LRFD BridgeDesign Specification are based. Structural reliability analysistakes the uncertainties in concrete, steel, and FRP materialstrengths and stiffnesses into account using rational statisti-cal models of these key engineering parameters. The criteriafor checking safety and serviceability of structural membersand components that have been strengthened with externallybonded FRP reinforcement are based on a target reliabilityindex, , of 3.5 (the target value assumed in the developmentof the AASHTO LRFD Bridge Design Specification). The fac-tored resistance and factored loads used in these checks areconsistent with those found in customary engineering prac-tice to facilitate their use and to minimize the likelihood ofmisinterpretation.

2.3 Development of Reliability-Based Guide Specifications

2.3.1 Probability-Based Load and Resistance Factor Design

Design criteria for safety-related limit states in modernprobability-based codes based on the notions of load and re-sistance factor design (LRFD) are represented by the follow-ing relationship:

in which the required strength is determined from struc-tural analysis using factored loads, and the design strength(or factored resistance) is determined using nominal materialstrengths and dimensions and partial resistance factors. Theload and resistance factors account for uncertainties associ-ated with the inherent randomness of the load and resistancevariables, uncertainties arising from the use of approximatemodels to represent the mechanics of structural behavior,and consequences of failure, i.e., local vs. general or ductile vs.brittle. In the AASHTO LRFD Bridge Design Specification,these load and resistance factors are set in such a way thatstructural members, components, and systems are designed toachieve a target performance goal, which is expressed in termsof a reliability index, = 3.5 (at the inventory load level). Thedesign equation has the following form:

i i i n rQ R R = ( . )2 2

Required Strength Q Design Strength Rd d( ) ( ) (( . )2 1

6where

i = load modier, a factor relating to ductility, redun-dancy, and operational importance;

i = load factor;Qi = force effects;Rn = nominal resistance;Rr = factored resistance; and = Resistance factor dened in the AASHTO LRFD Bridge

Design Specifications.

2.3.2 Statistical Models for Structural Loads

Structural loads acting on bridges may be classied as perma-nent and transient in nature. Permanent loads remain on thebridge for an extended period of time and consist of the weightof the structure and permanently attached non-structuralcomponents. Transient loads include vehicular trafc, pedes-trian loads, and environmental loads such as those caused bywind, ice oes, earthquakes, etc. The relative importance ofthe different loads in bridge performance depend on nu-merous factors, including type of construction, length ofspan, and the nature of the environmental exposure at thebridge site. For short- and medium-span girder bridges, themost important load combination is live load and deadload. Environmental effects are significant for long-spanbridges. This project is focused on short- and medium-spanbridges with spans ranging from 30 ft to 200 ft. Only deadload, live load (LL), and dynamic load (IL) were consideredin the reliability analysis on which the recommendations arebased.

Dead Load. Dead load is the weight of structural compo-nents and nonstructural attachments permanently connectedto the bridge. The following are four components of deadload:

DL1 weight of factory made elements, DL2 weight of cast in place concrete, DL3 weight of trafc wearing surface, and DL4 weight of miscellaneous nonstructural components.

Statistical parameters for each component of dead loadhave been developed in previous research (Nowak 1999) andare summarized in Table 2.1 The bias factors, , dene theratio of the mean to nominal dead load, enabling the statis-tics of dead load in situ to be determined for a variety of sit-uations once the nominal value is identied from the designdocumentation. The coefcient of variation (COV), V, is de-ned as the ratio of standard deviation to mean value and isthe fundamental measure of uncertainty in structural relia-bility analysis.

Live Load. The live load is represented by the forces pro-duced by vehicles moving on the bridge. The statistical mod-els and parameters of live load effects (maximum momentsor shears) have been developed previously (Nowak 1999; Eomand Nowak 2001). In these models, the static (truck weight)and dynamic (impact) components of the live load, LL andIL, are considered separately. The statistical parameters of theload effect were estimated based on data obtained from atruck survey (Agarwal and Wolkowicz 1976). The ratio of themean maximum 75-year shear to AASHTO LRFD HL-93design shear, LL, varies from 1.28 to 1.22 depending on thespan length, while coefcient of variation, V, is 0.12 for allspans. In the case of two-lane bridges, it was found that themaximum 75-year live load effect was caused by two trucksside by side (Nowak 1999). Based on numerous eld tests(Kim and Nowak 1997; Eom and Nowak 2001), the meandynamic load factor has been assumed to equal 0.1 with acoefcient of variation of 0.8.

Combination of Dead and Live Loads. This load combi-nation consists of the three components of dead load, staticlive load, and dynamic load:

The mean, Q, and variance, 2Q, of Q are:

where

DL1 and DL1 = mean and standard deviation of thedead load due to factory made (precast)elements,

DL2 and DL2 = mean and standard deviation of the deadload due to cast in place concrete,

DL3 and DL3 = mean and standard deviation of the deadload due to miscellaneous nonstructuralcomponents, and

LL+IL and LL+IL = mean and standard deviation of the liveload with impact.

Q DL DL DL LL IL2

12

22

32 2 2 5= + + + + ( . )

Q DL DL DL LL IL= + + + +1 2 3 2 4( . )

Q DL DL DL LL IL= + + + +1 2 3 2 3( . )

*A 35-in. thick wearing surface is assumed.

Dead Load Component

Bias Factor Coefficient of Variation, V

DL 1 1.03 0.08

DL 2 1.05 0.1

DL 3 1.00* 0.25

DL 4 1.05 0.10

Table 2.1. Statistical parameters of dead loadcomponents.

7The mean value, LL_ P+IL, of the combination of live load,LL, and dynamic load, IL, per girder is calculated as:

where

LL = the nominal HL-93 live load,1.1 = the mean dynamic impact,

GDF = the bias factor for the girder distribution factor,and

LL_P = the live load analysis factor, which is assumed toequal 1.

The coefcient of variation, VLL_ P , and standard deviation,LL_ P, of the static part of the live load are:

where VLL is the coefcient of variation of the live load, andVP is the coefcient of variation of the live load analysis fac-tor equal to 0.12, and LL_P is the mean value of the static partof the live load.

The standard deviation, LL_P+IL, and the coefcient of vari-ation, VLL_P+IL, for the mean maximum load combination oflive load and dynamic load are:

2.3.3 Resistance Model

The resistance, R, is modeled as the product of fourcomponents:

where

M = the variation of the material parameters,F = variable reecting the uncertainties in fabrication,

R R MFPn= ( . )2 11

VLL P ILLL P IL

LL P L_

_

_

( . )++

+

=

12 10

LL P IL LL P IL_ _ ( . )+ = +2 2 2 9

LL P LL P LL PV_ _ _ ( . )= i 2 8

V V VLL P LL P_ ( . )= +2 2 2 7

LL P IL LL GDF LL PLL_ _. ( . )+ = 1 1 2 6i i i i

P = the analysis factor (theoretical model error), andRn = nominal resistance.

The mean value of resistance, R, is:

whereM, F, and P are the mean values for M, F, and P,

respectively.As with the load models, it is convenient to express the re-

sistance, R, in terms of the nominal resistance, Rn, and a biasfactor, R. The bias factor, R, and the coefcient of variation,VR, of the resistance, R, are:

WhereM, F, and P are the bias factors, and VM, VF, and VP are

the coefcients of variation of M, F, and P, respectively.

2.3.3.1 Material Strengths

Statistical parameters reecting the variability due to ma-terial and fabrication uncertainties are those of concrete, re-inforcing steel rebars, and FRP reinforcement. These statis-tical data should be representative of values that would beexpected in a structure, and should reect uncertainties dueto inherent variability, modeling and prediction, and meas-urement. There are extensive databases that describe the prob-abilistic models obtained from previous probability-basedcode studies (e.g., Galambos, et al. 1982; MacGregor, et al.1983; Bartlett and MacGregor 1996). These data are summa-rized in Table 2.2 for concrete and grade 60 reinforcement.The mean compressive strength of concrete reects the dif-ference between standard-cured and in situ conditions, andincludes an allowance for aging.

For FRP reinforcement, the strength depends on the engi-neering characteristics of the bers, matrix and adhesive sys-tems and on the workmanship in fabrication and installation.In general, FRP composites used for strengthening reinforcedconcrete structures are made of aramid, carbon or glass bers

V V V VR m F P= ( ) + ( ) + ( )2 2 2 2 14( . ) R M F P= ( . )2 13

R n M F PR= ( . )2 12

Material Property Mean/Nominal COV CDF Rebar yield strengthtension

1.12 0.10 Lognormal

Concretecompressive strength

fc = 4000 psi 1.00 0.18 Normal fc = 6000 psi 1.20 0.15 Normal

Table 2.2. Statistical parameters of concrete and reinforcing steel properties.

8with a thermoset resin matrix to bind them together. Zureickand Kahn (2001) classied these systems in two groups:

Shop-manufactured composites. Pre-manufactured com-posites in the form of plates, shells, or other shapes that arebonded in the eld to the surface of the concrete memberusing structural adhesives. These composites are manufac-tured by a variety of techniques such as the pultrusion, lament winding, and resin transfer molding.

Field-manufactured composites. Fibers in the form of towsor fabrics are impregnated in the eld and placed on thesurface of the structure requiring strengthening. Methodsof impregnation have been done manually (hand lay-up),by a portable impregnator machine, or infusion under vac-

uum. The composite is bonded to the concrete and thenleft to cure under ambient or elevated temperature.

An advantage of the shop-manufactured composites overthe eld-manufactured composites is the ability to control thequality and uniformity in the composite reinforcing systems.Conversely, eld-manufactured composites are better able toconform to non-uniform concrete surfaces. Figures 2.1 and 2.2illustrate the scatter in material data for eld-manufacturedand shop-manufactured composites, respectively.

In this project, four single-layered and multilayered uni-directional carbon ber-reinforcement systems evaluated forthe strengthening of bridge pier caps in Georgia (Deaver et al.2003) were examined.

Figure 2.1. Load-strain relation for shop-manufacturedcomposite system.

Figure 2.2. Load-strain relation for a field-manufacturedcomposite system (not to failure).

9The strength of FRP in tension is described by a two-parameter Weibull distribution, dened by:

where u and are parameters of the distribution that can berelated to the sample mean and sample coefcient of varia-tion, as described subsequently.

The parameters u and can be estimated from the samplemean, x, and sample coefficient of variation, V (Zureick,Bennett, and Ellingwood 2006). As an approximation:

Statistical data for four systems of FRP reinforcement aresummarized in Table 2.3.

2.3.3.2 Modeling (or Analysis) Error

In addition to the uncertainties in resistance that arise fromthe uncertainties in material strength and fabrication, the sta-tistics of resistance must include the effect of modeling un-certainties. The equations dening the limit states of interestinvariably are based on idealizations of structural behavior.For example, in the Bernoulli-Navier hypothesis regardingbeam behavior, strain hardening is neglected in steel rein-forcement, structural materials are assumed to be homoge-neous, etc. These factors are reected in the mean and coef-cient of variation in the parameter, P, in Equation 2.11. Thesestatistical parameters describe the bias and variability that arenot explained by the analytical model used to predict resist-ance. The mean and COV of P are determined by calculatingthe mean and coefcient of variation in the ratio of test-to-calculated strengths where the calculated strengths are deter-mined from material strengths determined from companionspecimen tests and known specimen geometry. When thestructural mechanics of a limit state is well-understood andthe design equation captures this understanding (beams inexure usually fall into this category), P normally is close to1.0 and VP is approximately 0.05. Conversely, when the me-

=1 2

2 17.

( . )V

u V x= + ( )[ ]1 3 8 2 16( . )

F x e xx

u( ) = 1 0 2 15

, ( . )

chanics is not well understood and the design equation isbased on approximations of behavior (as with reinforced con-crete beams in shear), P typically is greater than 1.0 (becausethe approximate equations normally are selected to be con-servative for design purposes) and VP may range from 0.15 to0.20 or more, representing a substantial contribution to VR inEquation 2.14.

2.3.3.3 Resistance

A summary of the resistance statistics for typical reinforcedconcrete bridge girders without externally bonded FRP rein-forcement is presented in Table 2.4, where the components ofthe statistics of the parameters in Equation 2.11 are also pre-sented. These statistics were determined from previously pub-lished assessments of statistics in resistance of reinforced con-crete structures (MacGregor et al. 1983; Bartlett and MacGregor1996; Nowak 1999).

2.4 Reliability Basis for ProposedResistance Criteria

2.4.1 Selection of Representative Structural Members

Representative bridge members were analyzed for purposesof developing reliability-based resistance factors. These are

1. Members under exure: Non-prestressed rectangular sections having overall di-

mensions of 12 in. 24 in. with a wide range of rein-forcement ratios.

AASHTO Type III prestressed girder.

Shop-Manufactured

Field-Manufactured

System 1 System 2 System 3 System 4 Sample Size 30 16 25 15

COV 4.3% 24.2 18.2 12.2 Bias 1.06 1.48 1.32 1.19

Table 2.3. Statistical data for various FRP reinforcingsystems in tension.

Type of Structure FM P R FM VFM P VP R VR

Reinforced Concrete Moment 1.12 0.12 1.02 0.06 1.14 0.13 Shear with steel 1.13 0.12 1.08 0.10 1.20 0.16 Shear without steel 1.17 0.14 1.20 0.10 1.40 0.17

Prestressed Concrete Moment 1.04 0.05 1.01 0.06 1.05 0.08 Shear with steel 1.07 0.10 1.08 0.10 1.15 0.14

Table 2.4. Statistical parameters of resistance.

10

2. Members under shear: Reinforced concrete rectangular sections, having the

dimensions of 12 in. 24 in. and 14 in. 36 in. that arerepresentative of a wide range of bridge girders.

3. Members under axial and under combined bending andaxial loading: A reinforced concrete circular section having a diameter

of 36 in. A reinforced concrete square section having the dimen-

sions of 24 in. 24 in.

2.4.2 Reliability Analysis Procedure

The starting point for developing probability-based designcriteria is a description of the limit states of concern by a math-ematical model, based on principles of structural mechanicsand supported by experimental data. This model, denoted thelimit state function, is given by:

where X = (X1, X2, . . . Xm) = vector of resistance and load vari-ables that, in general, are random. The failure event is de-ned, by convention, such that the limit state is reached whenG(X)

11

fs = stress in the steel compression reinforcement at de-velopment of nominal exural resistance (ksi),

c = depth of the concrete compression zone (in.),ds = distance from extreme compression surface to the

centroid of nonprestressed tension reinforcement(in.),

ds = distance from extreme compression ber to thecentroid of compression reinforcement.

h = depth of section (in.),Tfrp = tension force in the FRP reinforcement (kips),frp = resistance factor determined from reliability analysis,

andk2 = multiplier for locating resultant of the compression

force in the concrete.

For shear:

Where

Vc = the nominal shear strength provided by the concretein accordance with Article 5.8.3.3 of the AASHTOLRFD Bridge Design Specifications,

Vs = the nominal shear strength provided by the transversesteel reinforcement in accordance with Article 5.8.3.3of the AASHTO LRFD Bridge Design Specifications,

Vp = component of the effective prestressing force in thedirection of applied shear as specied in Article 5.8.3.3of the AASHTO LRFD Bridge Design Specifications,

Vfrp = the nominal shear strength provided by the exter-nally bonded FRP reinforcement,

= 0.9, andfrp = resistance factor determined from reliability analysis.For axial compression:

where

= 0.85 for spiral reinforcement and 0.80 for tie rein-forcement;

= resistance factor specied in Article 5.5.4.2 of theAASHTO Bridge Design Specifications, 4th Edition;

Ag = gross area of section (in.2);Ast = total area of longitudinal reinforcement (in.2);fy = specied yield strength of reinforcement (ksi); and

f cc = compressive strength of the conned concrete deter-mined according to Article 5.3.2.2.

The starting point for the factored resistance (or designstrength) in all cases was the equations that are found in Sec-tion 5 of the AASHTO Bridge Design Specifications or ACIStandard 318-05 (ACI 2005). The contribution of the FRP re-inforcement to factored resistance is added to the existing

P f A A f Ar cc g st y st= ( ) +[ ] 0 85 2 22. ( . )

V V V V Vr c s p frp frp= + +( ) + ( . )2 21

term(s) for factored resistance. This format accomplishes twoobjectives: (1) In situations where only light FRP reinforce-ment is required, the design equation yields a factored resist-ance that is essentially the same as the factored resistance ofthe original structural member without FRP reinforcementand (2) Assigning a separate partial factor (frp) to the FRPcontribution facilitates achieving the reliability objectivessummarized in Section 2.4.3 throughout a range of materialstrengths, beam geometries, and spans.

The reliability achieved in LRFD depends on both thenominal resistance and the resistance factor. As a manufac-tured product, the variability in strength of FRP reinforce-ment (Equation 2.15 and Table 2.3) must be reected in thefactored resistance. It has been customary in many structuralengineering applications to identify the nominal strengthwith the 0.10-fractile (10 percent exclusion limit) of thestrength distribution in the end use condition. Accordingly,the resistance criteria in these Guide Specifications are basedon the assumption that the nominal ultimate tensile strengthof the FRP is dened by the 10th percentile value of the two-parameter Weibull distribution as follows:

in which the parameters are estimated from Equations 2.16and 2.17. To ensure the level of structural performance envi-sioned in the reliability analysis, the nominal strength stipu-lated in the construction documents should be the 10th per-centile value of strength.

The frp factors found in Sections 3, 4 and 5 of the GuideSpecifications were determined by iteration. A series of typicalstructural members (as identied in Section 2.4.1 above)were designed using the dead and live load requirements inthe AASHTO LRFD Bridge Design Specifications and a rangeof tentative frp factors (rounded to the nearest 0.05), and thereliability indices for these structural members were deter-mined using the procedure in Section 2.4.2. Following thecompletion of this reliability assessment, the resistance fac-tors proposed for the Guide Specifications were those that pro-vided the closest t to the target reliability index = 3.5. Thisapproach was also used in developing the AASHTO LRFDCode (Nowak 1999).

The applicability of the proposed Guide Specification is lim-ited to structural members, components, and systems thatcan be shown to have a minimum strength prior to the appli-cation of externally bonded FRP reinforcement. This limithas been imposed to avoid a situation where the behavior ofthe strengthened member depends unduly on the perfor-mance of the FRP reinforcement. If the eld application ofthe reinforcement is decient or if the strengthened bridge isloaded accidentally beyond the level of the enhanced factoredresistance, a sudden and potentially catastrophic failure of thestrengthened component is likely to occur.

x u0 101

0 1054 2 23. . ( . )= ( )

12

3.1 General

This chapter presents the analytical formulations and theexperimental data that form the basis upon which the pro-posed Guide Specification is based.

In recent years, there have been numerous publicationsaimed at demonstrating the effectiveness and benefit of exter-nally bonded reinforcement for strengthening reinforced con-crete structural members. However, the vast majority of testsdescribed in these publications reported FRP material prop-erties without sufficient information to enable independentverification of how the data were obtained. In this project onlyexperimental test data, with sufficient documentation to per-mit their direct comparison with results determined from atechnically sound structural analysis were considered in thereliability assessment on which the resistance factors in theproposed Guide Specification is based. This approach was fol-lowed throughout the entire project and resulted in small datasets.

Experimental data for various limit states governing thebehavior of steel-reinforced concrete members with externallybonded FRP reinforcement are summarized in the followingsections.

3.2 Flexural Strengthening

Experimental and analytical investigations of the behaviorof reinforced concrete beams and slabs in flexure have shownthat FRP strengthened reinforced concrete members mayexhibit, in most cases, one of the following failure modes(Pelvris et al. 1995; Triantafillou 1998; Triantafillou andAntonopoulos 2000):

1. Crushing of concrete in compression (before or afteryielding of the tension steel).

2. FRP reinforcement debonding at flexural crack locations.3. FRP reinforcement end peeling.

3.2.1 Crushing of Concrete in Compression

Tests conducted on reinforced concrete members strength-ened with externally bonded FRP composites yielded resultsthat were consistent with prior tests of non-FRP reinforcedbeams when crushing of concrete in compression occurredfirst. The experimental data reported by Saadatmanesh and Ehsani (1991); Spadea et al. (1998); Ross et al. (1999);and Almusallam and Al-Salloum (2001) were examined withinthe context of customary assumptions associated with theanalyses of reinforced concrete flexural members in accor-dance with the AASHTO LRFD Bridge Design Specifications.This examination yielded an average experimental to com-puted flexural strength ratio of 1.13 and a coefficient of varia-tion of 11%. The experimental to calculated ratios are shownin Figure 3.1.

3.2.2 Debonding of FRP Reinforcement at Flexural Crack Locations

The limit state of FRP plate debonding in steel-reinforcedconcrete members that also have been reinforced externallywith FRP plates occurs when the strain at the concrete/FRPplate interface reaches a limiting value on the order of one-half the ultimate tension strain of the composite materialsdetermined from a standardized direct tension test (e.g.,ASTM D3039), as illustrated in Figure 3.2. Laboratory testresults (Meier and Kaiser1991; Saadatmanesh and Ehsani1991; Arduini and Nanni 1997; Spadea et al. 1998; Swamy,et al. 1987; Zureick et al. 2002) have indicated that this limitstate is reached when the strain in the FRP reinforcement isbetween 0.003 and 0.008, as shown in Figure 3.2. In thepresent analysis, which focuses on flexural strengthening,the strain at the limit state of FRP plate debonding frp is setequal to 0.005; lesser values may be appropriate for otherlimit states involving more brittle failure modes. When the

C H A P T E R 3

Interpretation, Appraisal, and Application

13

strain in the FRP system is as low as that shown in the afore-mentioned experiments, the maximum compressive strainin the concrete compression zone invariably is below 0.003.This differs from the customary assumption made in flex-ural analysis of underreinforced concrete beams that thestress in the compression zone can be modeled by a uniformstress equal to 0.85f c, and a more realistic stress distribution,such as that in Figure 3.3, is necessary for calculating themember flexural strength from the linear distribution ofstrain. For the development of the Guide Specification in thisproject, a nonlinear concrete model (Desayi and Krishnan,1964; Todeschini, et al, (1964) was adopted. The stress-

strain relationship for such a model is defined by the follow-ing equations:

where c is the concrete strain, f c is the compression strengthof the concrete, and 0 is the strain, corresponding to themaximum stress, computed from:

0 1 71 3 2=

. ( . )f

Ec

c

ff

cc c

c

=

( )( )+ ( )

2 0 9

13 10

02

.( . )

Figure 3.1. Ratios of test to computed flexural strength for crushing of concrete in compression.

Figure 3.2. Ratios of test to computed flexural strength for the limit state of debondingof FRP reinforcement.

14

where Ec is the modulus of elasticity for normal weight con-crete. The compressive force in the concrete is obtained byintegrating Equation 3.1. Alternatively, for a constant-widthcompression zone, the compressive force in the concrete canbe approximated by an equivalent rectangular stress blockhaving a depth c and an average stress of 2(0.9f c), in which2 is defined as:

Thus the compression force in the concrete is:

C bc fc c= ( )2 0 9 3 4. ( . )

2

0

2

0

1

3 3=

+

ln

( . )

c

c

The center of gravity of the compression zone is k2c fromthe compression outer edge of the concrete section, where k2is given in the form:

The flexural strength of a rectangular beam or slab exter-nally reinforced with an FRP reinforcement system can bedetermined from the conditions for equilibrium of forces andcompatibility of strains on the cross section, as illustrated inFigure 3.4.

3.2.3 FRP Reinforcement End Peeling

The reinforcement end of an externally bonded reinforce-ment system, when subjected to combined shear and flexure,may separate in the form of debonding in three different modes:(1) critical diagonal crack debonding with concrete cover sepa-ration (Yao and Teng 2007) or without concrete cover separa-tion (Oehlers and Seracino 2004); (2) concrete cover separation(Teng et al. 2002); and (3) plate end interfacial debonding(Teng et al. 2002).

Critical diagonal crack debonding may occur where theFRP end is located in a zone of high shear force and theamount of steel shear reinforcement is limited. In such acase a major diagonal shear crack forms and intersects theFRP and then propagates toward the end of the FRP rein-forcement. This failure mode is suppressed if the shearstrength of the strengthened member remains higher thanthe flexural strength.

k

c c

c

20 0

1

1

2

=

arctan

0

23 5

( . )

Figure 3.4. Strain and force diagrams for a reinforced concreterectangular section.

fc =

2(0.9f'c)(/0)

1+(/0)2 0.9f'

c

0 alt

Stre

ss, f

c

Strain,

Figure 3.3. Stress-strain relationshipfor concrete.

15

In beams with heavy steel shear reinforcement, multiplediagonal cracks of smaller widths dominate the behavior suchthat concrete cover separation will become the controllingdebonding failure mode. Failure of the concrete cover is initi-ated by a crack near the FRP end due to the stress concentra-tion. The crack then propagates to and then along the level ofthe longitudinal steel tension reinforcement. This mode offailure has been observed in tests on beams with externallybonded steel plates (Jones et al. 1988; Oehlers and Moran1990) and FRP reinforcement (Malek et al. 1998; Lopez andNaaman 2003; Yao and Teng 2007). Plate-end interfacialdebonding is also initiated by high interfacial shear and nor-mal stresses near the end of the FRP that exceed the strength ofthe weakest element, generally the concrete. Debonding in thiscase propagates from the end of the FRP towards the middle,near the FRP-concrete interface. This failure mode is onlylikely to occur when the FRP is significantly narrower than thebeam section.

In summary, provided that shear failure (in the form of adiagonal shear crack in the concrete) is suppressed (throughshear strengthening, if needed), stress concentrations near theFRP reinforcement end may result in debonding through theconcrete layer near the level of the longitudinal steel (or, rarely,near the FRP-concrete interface). A wide range of predictivemodels that include numerical, fracture mechanics, data-fitting, and strength of material-based methods have beendeveloped to address this complex mode of failure (Yao 2004).However, the equations presented in the proposed Guide Spec-ification are based on the approximate analysis of Roberts(1989), because of its simplicity for design purposes. Figure 3.5shows evaluations of test results conducted on reinforced con-

crete beams reinforced with externally bonded FRP reinforce-ment (Yao and Teng 2007) and with externally bonded steelplates (Oehlers and Moran 1990; Swamy et al. 1987). Theresults of the peeling stress, fpeel, predicted by Roberts formulawere normalized with respect to the interface shear transfer

strength of , in which f c, is in ksi. For the vast

majority of tests, the prediction is on the safe side.

3.3 Shear Strengthening

Shear strengthening of reinforced concrete members usingFRP reinforcement may be provided by bonding the externalreinforcement (typically in the form of sheets) with the prin-cipal fiber in the direction (insofar as practically possible) ofmaximum principal tensile stresses to maximize the effective-ness of FRP reinforcement. For the most common case inwhich the applied loads acting on a structural member areperpendicular to the member axis (e.g., beams under gravityloads or columns under seismic forces), the maximum prin-cipal stress trajectories in the shear-critical zones form anangle with the member axis which may be taken roughly equalto 45o. However, it is normally more practical to attach theexternal FRP reinforcement with the principal fiber directionperpendicular to the member axis.

Experimental and analytical investigations of the behaviorof reinforced concrete members strengthened in shear haverevealed the following failure modes:

1. Steel yielding followed by FRP debonding.2. Steel yielding followed by FRP fracture.

int .= 0 065 fc

Figure 3.5. Normalized calculated peeling stress for externally bonded FRP reinforcement and for steel plates.

16

3. FRP debonding before steel yielding.4. Diagonal concrete crushing.

Depending on the amount of usable steel shear reinforce-ment in the structural element, FRP debonding can occureither before or after steel yielding. The third failure mode ishighly unlikely to occur if proper detailing is provided.

Diagonal concrete crushing in the direction perpendicularto the tension field can be suppressed by limiting the totalamount of steel and FRP reinforcement. Fracture of the FRPreinforcement is highly unlikely to occur because the strainwhen FRP debonds is substantially lower than that correspond-ing to the FRP fracture strength.

3.3.1 Reinforcing Schemes

Typical FRP strengthening schemes for beams and columnsare summarized in the following paragraphs.

Side bonding is the least effective FRP shear reinforcementscheme due to premature debonding under shear loading andshould be avoided if possible (Figure 3.6). Side bonding doesnot allow for the development of the shear-resisting mecha-nism based on a parallel chord truss model that was first pro-posed by Ritter (1899), due to the lack of tensile diagonals.

U-jacketing is the most common externally bonded shearstrengthening method for reinforced concrete beams and gird-ers (Figure 3.7). This system is prone to premature debond-ing of the FRP, which may reduce its effectiveness. However,the system is quite popular in practice due to its simplicity.

Jacketing combined with anchorage aims to increase theeffectiveness of FRP by anchoring the fibers, preferably in thecompression zone (Figure 3.8). Properly designed anchors mayresult in the fibers reaching their tensile capacity, permittingthe jacket to behave as if it were completely wrapped.

Complete wrapping ensures maximum straining of thefibers and is therefore the most desired reinforcing method, ifpractically possible (Figure 3.9).

Because of the lack of well-documented shear tests in whichparameters relevant to the development of these guide specifi-cations have been reported, only data related to U-jacketswere examined. A total of 24 reinforced concrete test speci-mens having sufficient information necessary to calculatetheir nominal shear strengths were selected from the work ofDeniaud and Cheng (2001), Deniaud and Cheng (2003),Taerwe et al. (1997), Norris et al. (1997), Leung et al. (2007),and Pellegrino and Modena (2006). The average value of theratio of the experimental shear strength to that computed valueusing the equation proposed in the Guide Specification is 1.13with a coefficient of variation of 28%. The data scatter is showngraphically in Figure 3.10.

3.4 Axially Loaded Members

3.4.1 Axially Loaded Compression Members

The most commonly used method of strengthening orupgrading the load-carrying capacity of reinforced concretecolumns with FRP reinforcement is to wrap the reinforcement

Figure 3.6. Side bonding.

Figure 3.7. U-jacketing.

Figure 3.8. Jacketing with anchorages.

Figure 3.9. Complete wrapping reinforcement.

17

around the section circumference, thus providing confinementthat increases both the axial strength and ductility of the col-umn. A review of available work related to strengthening ofaxially and eccentrically loaded columns with FRP reinforce-ment can be found in Teng et al. (2002). The axial compres-sion strength of a column can be determined directly fromArticle 5.7.4.4 of the AASHTO LRFD Bridge Design Specifica-tions, in which the compression strength of unconfined con-crete is replaced by the compression strength of the confinedconcrete. A review of design guidelines for FRP reinforce-ment confining reinforced concrete columns of non-circularcross sections is found in Rocca et al. (2006).

In the recommended guide specification, lateral confine-ment pressure for reinforced concrete columns are based onthe Canadian guidelines (ISIS 2001). The confinement modelis simple enough for adoption in design and yields results thatare consistent with the limited test data.

3.4.2 Strengthening Under Axial Loadingand Flexure

The vast majority of research conducted on strengtheningof axially loaded members has been limited to studying theeffect of concrete confinement on the axial concentric com-pression strength of short reinforced concrete columns andpiers, especially for seismic retrofitting that is necessitated bythe inadequacy of transverse reinforcement. A comprehen-sive review of work conducted prior to 2001 was publishedby Triantafillou (2001). An examination of the 60 papers citedby Triantafillou (2001) shows clearly that there had been no

systematic studies that address (1) FRP strengthening ofreinforced concrete members under concentric tension and(2) FRP strengthening of reinforced concrete members sub-jected to combined axial loading and bending. The latterissue was recognized by researchers at the Laboratoire Cen-tral des Ponts et Chausses in Paris, France, who examinedtwo groups of 2.5 m columns of two different concretestrengths that had been externally strengthened with carbonfiber reinforcement and subjected to combined axial com-pression and bending (Quiertant et al. 2004; Quiertant andToutlemonde, 2005).

3.5 Seismic Retrofitting with Externally Bonded FRP

Seismic retrofitting of existing reinforced concrete struc-tural elements may be necessitated by the following:

(a) The inadequacy of transverse reinforcement, which maylead to brittle shear failure. This mechanism is associated withinclined cracking (diagonal tension), cover concrete spalling,and rupture or opening of the transverse reinforcement. Theshear capacity of sub-standard elements (columns, shear walls,piers, exterior joints, etc.) can be enhanced by providing exter-nally bonded FRPs with the fibers mainly in the hoop direction,in (preferably) closed-type jackets (Figure 3.11a,b).

(b) Poor confinement in flexural plastic hinge regions (col-umn ends), where flexural cracking may be followed by cover-concrete crushing and spalling, buckling of the longitudinalreinforcement, or compressive crushing of the concrete. Aductile flexural plastic hinging at the column ends can be

Figure 3.10. Scatter of computed strength of reinforced concrete beams withU-jacket FRP reinforcement.

18

achieved through added confinement in the form of FRPjackets with the fibers placed along the column perimeter(Figure 3.11c). This local confinement prevents spalling anddelays crushing of concrete; also it delays or even eliminatesbuckling of longitudinal steel reinforcing bars.

(c) Poor detailing in lap splices at the lower ends of columns.The flexural strength of RC columns can only be developed

and maintained when debonding of the reinforcement lapsplice is prevented. Such debonding occurs once verticalcracks develop in the cover concrete and progresses withincreased dilation and cover spalling. The associated rapidflexural strength degradation can be prevented or limited withincreased lap confinement, again with fibers along the columnperimeter (Figure 3.11d).

(a) (b) (c) (d)

Figure 3.11. Seismic strengthening examples: (a) shear strengtheningof RC column, (b) strengthening of beam-column joint, (c) localconfinement in flexural plastic hinge regions, and (d) local confinement at lap splices.

19

4.1 Conclusions

This research developed a set of Recommended Guide Speci-fication for the Design of Externally Bonded FRP ReinforcementSystems for Repair and Strengthening of Concrete Bridge Ele-ments. The provisions contained in these specifications utilizethe load combination requirements found in the AASHTOLRFD Bridge Design Specifications 4th Edition (2007) and em-ploy the LRFD methodology. The load and resistance factorshave been developed from structural reliability theory basedon current probabilistic/statistical models of loads and struc-tural performance and are designed to achieve the reliabilitylevels that are already embedded in current AASHTO designand rating guidelines.

The recommended Guide Specification is accompanied bycommentaries that are necessary for explaining the back-ground, applicability, and limitations of the provisions con-tained therein. The guide specifications and commentary arepresented in a format resembling that of the AASHTO LRFDBridge Design Specifications in order to facilitate their consid-eration and adoption by AASHTO.

The Guide Specification consists of five sections. Section 1addresses the scope, general requirements, and design basisof the proposed specifications. Section 2 defines the require-ments for polymeric composite material systems intended foruse for repair and strengthening of concrete bridge elements.The material provisions contained in Section 2 give a consis-tent basis for reporting material properties and the condi-tions under which these properties are obtained. Sections 3,4, and 5 contain the recommended design provisions based onthe limit states governing the response of a steel-reinforcedconcrete member subjected to flexure, shear, and combinedaxial loading and flexure, respectively.

In addition, step-by-step calculations in accordance withthe Recommended Guide Specification are presented as exam-ples of common FRP strengthening applications. These ex-amples cover the five sections of the proposed Guide Specifi-

cation and would serve as a tutorial of how to approach bridgestrengthening projects in practice.

4.2 Implementation

The successful implementation of the Guide Specificationdeveloped in this research will require in-depth training forbridge design engineers to become fully acquainted with thefundamental principles, assumptions, limitations, and inves-tigative procedures associated with the behavior and designof steel-reinforced concrete structural members reinforcedwith externally bonded FRP reinforcement. The training ma-terials must emphasize the underlying basis for, and the de-tails of, all relevant provisions of the Guide Specification. It isalso recommended that the proposed Guide Specification beused for trial design by independent designers to identifypotential improvements.

4.3 Recommendations for Further Research

Based on the findings and limitations of this research, fur-ther research on FRP strengthening of reinforced and pre-stressed concrete structural members is recommended. Thefollowing topics are proposed:

FRP Material Requirements. Material requirements in theproposed guidelines stipulate that the FRP reinforcementbe conditioned in four distinct environments and for a du-ration of 1,000 hours and then tested to ensure that theproperty of interest retains 85% of its original value. Theserequirements have been derived from the work of Steckelet al. (1999a, 1999b), and Hawkins et al. (1999), under thesponsorship of the California Department of Transporta-tion (Caltrans), which called for property measurementsafter exposure intervals of 1,000 hours, 3,000 hours and10,000 hours to allow estimates of degradation over the

C H A P T E R 4

Conclusions, Implementation, andRecommendations for Further Research

projected service life. It is suggested that research be con-ducted to (1) examine the reasonableness of test durationsof up to 10,000 hours and (2) establish a standard practiceof how to analyze and report the data resulting from anylong-term tests.

Effect of Temperature on the Response of ReinforcedConcrete Structural Members Externally Reinforced withFRP Reinforcement. The proposed guide specifications re-quire that the characteristic value of the glass transition tem-perature of the composite system and for the adhesive (whenused) determined in accordance with ASTM D4065 shall beat least 40F higher than the maximum design temperature,TMaxDesign, defined in Section 3.12.2.2 of the AASHTO LRFDBridge Design Specification. Research is needed to examinethe validity of the requirement proposed in the Guidelines.

Experimental Studies Addressing FRP Reinforcement EndPeeling. It is suggested that the FRP reinforcement end peel-ing design equation be further validated under static and fa-tigue loading conditions and with exposure to various envi-ronmental conditions in order to establish the validity of the

limiting peel stress stipulated in the guidelines. Shear Strengthening of Reinforced and Prestressed Con-

crete Girders with FRP Reinforcement. Data concerning

0 065. fc

shear strengthening of concrete members are limited. Re-search on shear strengthening of bridge members withFRP reinforcement is needed. Such research should include(1) Complete wrapping of structural members, (2) Theshear behavior of deep reinforced concrete beams fullywrapped with FRP reinforcement, and (3) Design guide-lines and specifications related to the shear behavior of shal-low and deep reinforced concrete beams reinforced withFRP reinforcement combined with mechanical anchorages.It is recommended that these studies include specificationsfor anchorage systems and design guidelines for anchor-ages for use in conjunction with the FRP reinforcement(Schuman and Karbhari 2004; Monti et al. 2004a, 2004b).

Experimental Behavior of Confined Rectangular ColumnsUnder Axial Loading and Combined Axial Loading andFlexure. Tests on eccentrically loaded compression slen-der and non-slender columns confined with FRP rein-forcement are needed to address (1) the most appropriateconfinement model for design purposes, (2) the effective-ness of the FRP reinforcement with respect to the crosssection aspect ratio, and (3) experimental behavior of con-fined reinforced concrete columns under axial tension andflexure.

20

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