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0950-0618/03/$ - see front matter� 2003 Published by Elsevier Science Ltd.doi:10.1016/S0950-0618(03)00042-4

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2

3 North American design guidelines for concrete reinforcement and4 strengthening using FRP: principles, applications and unresolved issues

5

6 Antonio Nanni*7

8 Vernon and Maralee Jones Professor of Civil Engineering, Center for Infrastructure Engineering Studies, 1870 Miner Circle, ERL 224,9 University of Missouri–Rolla, Rolla, MO 65409, USA

1011 Received 24 April 2003; accepted 14 July 2003

1220

21 Abstract22

23 This paper reports on the North American state-of-the-art in the use of FRP composites in concrete structures. For new24 construction, FRP bars have been used as the internal reinforcement in concrete members to replace conventional steel rebars for25 a host of reasons. The principles for design and construction have been established and proposed to industry by the American26 Concrete Institute(ACI). For repair and upgrade, strengthening of concrete members with externally bonded FRP laminates or27 near surface mounted(NSM) bars has received remarkable attention. The design and construction principles for use in practice28 are being finalized by ACI. On the application side, FRP materials have been used in some multi-million dollar projects for29 strengthening parking garages, multi-purpose convention centers, office buildings and silos. The drivers for this technology are30 several, but perhaps the most relevant one is the ease of installation. In the repairyupgrade arena(as well as new construction),31 perhaps one of the most important unresolved question remains that of durability(including fire resistance). Further research and32 validation is necessary to increase confidence in FRP technology for concrete construction.33 � 2003 Published by Elsevier Science Ltd.3435 Keywords: Construction; Design; Externally bonded reinforcement; Fiber-reinforced polymer(FRP); Near-surface mounted reinforcement;36 Prestressed concrete; Reinforced concrete; Reinforcement; Repair; Strengthening37

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40 1. Design and construction of concrete reinforced41 with FRP bars

42 In this section, reference is made to key features in43 the recently published guidelines document by the44 American Concrete Institute(ACI) w1x, which provides45 recommendations for design and construction of FRP-46 reinforced concrete(RC) structures as an emerging47 technology. The ACI document only addresses non-48 prestressed FRP reinforcement. Only notations critical49 to the understanding of the section are defined and50 equations are expressed in US customary and SI units.

51 1.1. Design principles

52 1.1.1. General53 FRP materials are anisotropic and are characterized54 by high tensile strength with no yielding only in the697

698 *Tel.: q1-573-341-4553; fax:q1-573-341-6215.699 E-mail address: nanni@umr.edu(A. Nanni).

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direction of the reinforcing fibers. This anisotropic56behavior affects the shear strength and dowel action of57FRP bars, as well as their bond performance. Design58procedures should account for a lack of ductility in59concrete reinforced with FRP bars. Both strength and60working stress design approaches are considered accord-61ing to the provisions of ACI 318-95w2x (ACI 318 from62hereon). An FRP-RC member is designed based on its63required strength and then checked for serviceability64and ultimate state criteria(e.g. crack width, deflection,65fatigue and creep rupture endurance). In many instances,66serviceability criteria may control the design.67Design values: The design tensile strength that should68be used in all design equations is given as ,Uf sC ffu E fu

69where: f sdesign tensile strength of FRP consideringfu

70reductions for service environment;C senvironmentalE

71reduction factor given inTable 1 (column labeled72‘Int.’sNew constructionyinternal) for various fiber73types and exposure conditions; andsguaranteedUffu74tensile strength of an FRP bar defined as the mean

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3Table 1

4 Environmental-reduction factor C for various FRP systems and expo-E

5 sure conditions6

Exposure condition Carbon Glass Aramid78

Int.a Ext.b Int.a Ext.b Int.a Ext.b916 Interior exposure 1.0 0.95 0.8 0.75 0.9 0.8517 Exterior exposure 0.9 0.85 0.7 0.65 0.8 0.7518 Aggressive environment nys 0.85 nys 0.50 nys 0.701920 New constructionyinternal; Strengtheningyexternal; nys: Nota b

21 specified.

26Table 2

27 Creep rupture and fatigue stress limits in FRP reinforcement28

Fiber type Glass FRP Aramid FRP Carbon FRP2930 Creep rupture stress limit,Ff,s 0.20 ffu 0.30 ffu 0.55 ffu31

75

tensile strength of a sample of test specimens, ,Ufu,ave76 minus three times the standard deviations, ( Uf sfu

77 ).Uf y3su,ave

78 The design rupture strain should be determined simi-79 larly, whereas the design modulus of elasticity is the80 same as the average value reported by the manufacturer.81 Design parameters in compression are not addressed82 since the use of FRP rebars in this instance is83 discouraged.

84 1.1.2. Flexure85 Behavior and failure modes: If FRP reinforcement86 ruptures, failure of the member is sudden and catastroph-87 ic. However, there would be some limited warning of88 impending failure in the form of extensive cracking and89 large deflection due to the significant elongation that90 FRP reinforcement experiences before rupture. The con-91 crete crushing failure mode is marginally more desirable92 for flexural members reinforced with FRP bars since the93 member does exhibit some plastic behavior before fail-94 ure w3x. In conclusion, both failure modes(i.e. FRP95 rupture and concrete crushing) are acceptable in govern-96 ing the design of flexural members reinforced with FRP97 bars provided that strength and serviceability criteria are98 satisfied. To compensate for the lack of ductility, the99 suggested margin of safety against failure is, therefore,100 higher than that used in traditional steel-RC design.101 F factor: When concrete-crushing controls, a102 strength-reduction factor of 0.70 is adopted. Further-103 more, aF factor of 0.50 is recommended for FRP104 rupture-controlled failure. While a concrete crushing105 failure mode can be predicted based on calculations, the106 member as constructed may not fail accordingly. For107 example, if the concrete strength is higher than specified,108 the member can fail due to FRP rupture. For this reason109 and in order to establish a linear transition between the110 two values of F, a section controlled by concrete111 crushing is defined as a section in which the reinforce-

112

ment ratio,r , is greater than or equal to 1.4 times thef

113balanced reinforcement ratio,r ,(r G1.4r ) and afb f fb

114section controlled by FRP rupture is defined as one in115which r -r .f fb

116Minimum reinforcement: If a member is designed to117fail by FRP rupture,r -r , a minimum amount off fb

118reinforcement,A , should be provided to preventf,min

119failure upon concrete cracking(that is,FM GM wheren cr

120M is the cracking moment). The minimum reinforce-cr

121ment area is obtained by multiplying the existing ACI122318 limiting equation for steel by 1.8(i.e. 1.8s0.90y1230.50 which is theF ratio).124Crack width: For FRP-reinforced members, the crack125width, w, can be calculated from the expression shown126in ACI 318 with the addition of a corrective coefficient,127k , for the bond quality. Thek term is a coefficient thatb b

128accounts for the degree of bond between the FRP bar129and the surrounding concrete. For FRP bars having bond130behavior similar to steel bars,k is assumed equal tob

131one. Whenk is not known, a value of 1.2 is suggestedb

132for deformed FRP bars.133Creep rupture and fatigue: Values for safe sustained134and fatigue stress levels are given inTable 2. These135values are based on experimental results with an136imposed safety factor of 1y0.60.

1371.1.3. Shear138When using FRP as shear reinforcement, one needs139to recognize that: FRP has a relatively low modulus of140elasticity; FRP has a high tensile strength and no yield141point; tensile strength of the bent portion of an FRP bar142is significantly lower than the straight portion; and FRP143has low dowel resistance.144According to ACI 318, the nominal shear strength of145a steel-RC cross section,V , is the sum of the shearn

146resistance provided by concrete,V , and the steel shearc

147reinforcement,V . Similarly, the concrete shear capacitys

148V of flexural members using FRP as main reinforce-c,f

149ment can be derived fromV multiplied by the ratioc

150between the axial stiffness of the FRP reinforcement151(r E ) and that of steel reinforcement(r E ). Thef f s s

152equation forV is that shown inEq. (1) (noting Vc,f c,f

153cannot be larger thanV ).c

156r Ef fV s V (1)c,f cr Es s

157

158The ACI 318 method used to calculate the shear159contribution of steel stirrups,V , is applicable whens

160using FRP as shear reinforcement with the provision

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36Fig. 1. Top GFRP mat placement.

161

that the stress level in the FRP shear reinforcement,f ,fv

162 should be limited to control shear crack widths, maintain163 shear integrity of the concrete, and avoid failure at the164 bent portion of the FRP stirrup,f . The stress level infb

165 the FRP shear reinforcement at ultimate for use in166 design is given byf 0.002E Ff . An expression forfvs f fb

167 f is given in ACI 440.1R-01w1x.fb

168 1.1.4. Development length169 The development length of FRP reinforcement can be170 expressed as shown inEq. (2). This should be a171 conservative estimate of the development length of FRP172 bars controlled by pullout failure rather than concrete173 splitting.174175

176 d fb fuŸ s (2)bf 2700177178

179 d fb fuŸ s for SI units (3)bf 18.5180

181 Manufacturers can furnish alternative values of the182 required development length based on substantiated tests183 conducted in accordance with available testing proce-184 dures. Reinforcement should be deformed or surface-185 treated to enhance bond characteristics with concrete.

186 1.2. Application for internal FRP reinforcement

187 1.2.1. GFRP-reinforced concrete bridge decks188 The use of GFRP bars for reinforcing concrete bridge189 decks has captured some interest, particularly for the190 case of the replacement of the top steel matw4x. The191 idea is to eliminate one of the major causes of deterio-192 ration (i.e. the steel reinforcement embedded in the193 concrete region more exposed to chlorides) without194 significantly increasing cost of construction and without195 totally removing steel reinforcement(seeFig. 1).

196 1.3. Unresolved issues

197 As pointed out in ACI 440.1R-01w1x, future research198 is needed to provide information in areas that are a real199 or perceived disadvantage, are still unclear, or are in200 need of additional evidence to validate performance. A201 list of research topics is shown below.202 Materials: behavior of FRP-reinforced members under203 elevated temperatures; minimum concrete cover require-204 ment for fire resistance; fire rating; effect of transverse205 expansion of FRP bars on cracking and spalling of206 concrete cover; creep rupture behavior and endurance207 limits of FRP bars; end treatment requirements of saw-208 cut FRP bars; and strength and stiffness degradation of209 FRP bars in harsh environment.210 Flexureyaxial force: behavior of FRP-RC compres-211 sion members; behavior of flexural members with ten-

212

sion and compression FRP reinforcement; design and213analysis of non-rectangular sections; maximum crack214width and deflection prediction and control; minimum215member depth for deflection control; and long-term216deflection behavior.217Shear: concrete contribution to shear resistance; fail-218ure modes and reinforcement limits; and use of FRP219bars for punching shear reinforcement in two-way220systems.221Detailing: standardized classification of surface defor-222mation patterns; effect of surface characteristics of FRP223bars on bond behavior; lap splices requirements; and224minimum FRP reinforcement for temperature and225shrinkage cracking control.226Structural systemsyelements: behavior of FRP-rein-227forced concrete slabs on ground.228Test methods: bond characteristics and related bond-229dependent coefficients; creep rupture and endurance230limits; fatigue characteristics; coefficient of thermal231expansion; durability characterization with focus on232alkaline environment and determination of related envi-233ronmental reduction factors; strength of the bent portion;234shear strength; and compressive strength.

2352. Design and construction of FRP systems for236strengthening

237In this section, reference is made to the key issues of238a technical document under development at ACIw5x,239which provides guidance for the selection, design, and240installation of FRP systems for externally strengthening241concrete structures as an emerging technology. Infor-242mation on material properties, design, installation, qual-243ity control, and maintenance of FRP systems used as244external reinforcement is presented in this ACI docu-245ment. This information can be used to select an FRP246system for increasing the strength and stiffness of RC

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beams or the ductility of wrapped columns. Conditions248 are also identified where FRP strengthening is beneficial249 and where its use may be limited. The ACI document250 does not address masonry walls.

251 2.1. Design principles

252 2.1.1. General253 It is recommended that the increase in load-carrying254 capacity of a RC or prestressed concrete(PC) member255 strengthened with an FRP system be limited. The phi-256 losophy is that a loss of FRP reinforcement should leave257 a member with sufficient capacity to resist at least 1.2258 times the design dead load and 0.85 times the design259 live load. Design recommendations are based on limit-260 states-design principles. This approach sets acceptable261 levels of safety against the occurrence of both service-262 ability and ultimate limit states(i.e. deflections, crack-263 ing, failure, stress rupture, fatigue). In determining the264 ultimate strength of a member, all possible failure modes265 and resulting strains and stresses in each material should266 be assessed. For evaluating the serviceability of an267 element, engineering principles, such as modular ratios268 and transformed sections, can be used.269 FRP-strengthening systems should be designed in270 accordance with current ACI 318 strength and service-271 ability requirements. The strength-reduction factors272 required by ACI 318 should also be used. Additional273 reduction factors applied to the contribution of the FRP274 reinforcement are recommended to reflect the limited275 body of knowledge of FRP systems compared with steel276 RC and PC. For the design of FRP systems for the277 seismic retrofit of a structure, it may be appropriate to278 use established capacity design principles, which assume279 a structure should develop its full elastic capacity and280 require that members be capable of resisting the asso-281 ciated shear demands. The environmental-reduction fac-282 tor, C , to determine the FRP design strength and strainE

283 was given in Table 1 for different fiber types and284 exposure conditions(see column labeled ‘Ext.’s285 Strengtheningyexternal). A similarity with the corre-286 sponding values adopted for internal FRP reinforcement287 should be noted.

288 2.1.2. Flexure289 Failure modes: Guidance is given on the calculation290 of the flexural strengthening effect of adding longitudi-291 nal FRP reinforcement to the tension face of a rectan-292 gular RC member(concepts could be extended to cover293 T-sections and I-sections as well as PC). The nominal294 flexural capacity can be computed as per ACI 318. An295 additional reduction factor,c s0.85, is applied to thef

296 flexural-strength contribution of the FRP reinforcement297 to account for lower reliability of the FRP reinforcement.298 The strain level in the FRP reinforcement at the299 ultimate-limit state needs to be determined and limited

300

to an upper value equal to the productk ´ . The termm fu

301k is a factor no greater than 0.90 that is meant to limitm

302the strain in the FRP reinforcement to prevent debonding303or delamination. This term recognizes that laminates304with greater axial stiffness(i.e. the product elastic305modulus by cross sectional area) are more prone to306delamination. Thek term, that was primarily developedm

307to address interfacial debonding at intermediate cracks,308is only based on a general recognized trend and on the309experience of engineers practicing the design of bonded310FRP systems.311F factor: The strength-reduction factor depends on312the strain in the steel at ultimate,´ . F is set equal tos

3130.90 for ductile sections(´ G0.005) and 0.70 for brittles

314sections where the steel does not yield. A linear transi-315tion for the reduction factor between these two extremes316is then established.317Stress limits: To avoid plastic deformations, the exist-318ing steel reinforcement should be prevented from yield-319ing at service load levels. The stress in the steel at320service should be limited to 80% of the yield stress.321Similarly, to avoid failure of an FRP-reinforced member322due to creep rupture of the FRP, stress limits for these323conditions should be imposed on the FRP reinforcement.324Limits on sustained and fatigue stresses are those listed325in Table 2 and are identical to those for internal FRP326reinforcement.

3272.1.3. Shear328The nominal shear capacity of an FRP-strengthened329concrete member can be determined by adding the330contribution of the FRP reinforcing to the contributions331from the reinforcing steel and the concrete. An additional332reduction factorc , is applied to the contribution of thef

333FRP system. The additional reduction factor,c , shouldf

334be selected based on the known characteristics of the335application but should not exceed 0.85 for two- and336three-sided wrapping schemes and 0.95 for completely337wrapped elements.338The shear strength provided to a member by the FRP339system should be based on the fiber orientation and an340assumed crack patternw6x. It can be determined by341calculating the force resulting from the tensile stress in342the FRP along the assumed crack with an expression343similar to that of steel reinforcement. To compute the344tensile stress in the FRP shear reinforcement at ultimate,345it is necessary to calculate the effective strain,f , in thefe

346FRP.347FRP systems that do not enclose the entire section348(two and three-sided wraps) have been observed to349delaminate from the concrete before the loss of aggre-350gate interlock of the section. For this reason, bond351stresses should be analyzed to determine the usefulness352of these systems and the effective strain level that can353be achieved. The effective strain is calculated using a

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41Fig. 2. Typical stem cracking prior to strengthening.

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bond-reduction coefficient,k , so that́ sk ´ F0.004v fe v fu

355 (for U-wraps or bonding to two sides).356 The bond-reduction coefficient is a function of the357 concrete strength, the type of wrapping scheme used,358 and the stiffness of the laminatew6x. The methodology359 for determiningk has been validated for members inv

360 regions of high shear and low moment, such as simply361 supported beams. The methodology has not been con-362 firmed for shear strengthening in areas subjected to363 simultaneous high shear and moment loads, such as364 continuous beams. In such situations, conservative val-365 ues fork are recommended.v

366 2.1.4. Compression members367 The axial compressive strength of a non-slender mem-368 ber confined with an FRP jacket is calculated using the369 conventional expressions of ACI 318 substituting for370 f9 the factored confined concrete strength . Thec f9c f cc

371 additional reduction factor is set toc s0.95. Verticalf

372 displacement, section dilation, cracking, and strain lim-373 itations in the FRP jacket can also limit the amount of374 additional compression strength that can be achieved375 with an FRP jacket. If the member is subjected to376 combined compression and shear, the effective strain in377 the FRP jacket should be limited based on the criteria378 given by´ s0.004F0.75́ .fe fu

379 At load levels near ultimate, damage to the concrete380 in the form of significant cracking in the radial direction381 occurs. The FRP jacket contains the damage and main-382 tains the structural integrity of the column. At service383 load levels, this type of damage should be avoided so384 that the FRP jacket will only act during overloads that385 are temporary in nature. To avoid radial cracking under386 service loads, the transverse strain in the concrete should387 remain below its cracking strain at service load levels.388 This corresponds to limiting the stress in the concrete389 to 0.65 . In addition, the stress in the steel shouldf9c390 remain below 0.60f to avoid plastic deformation undery

391 sustained or cyclic loads. By maintaining the specified392 stress in the concrete at service, the stress in the FRP393 jacket will be negligible. Because FRP jackets provide394 passive confinement, service load stresses in the FRP395 jacket should never exceed the creep-rupture stress limit.

396 2.1.5. Near surface mounted (NSM) reinforcement397 Although not directly addressed by ACI 440w5x, the398 use of NSM FRP bars is a promising technology for399 increasing flexural and shear strength of deficient RC400 and PC membersw7x. The advantages of NSM FRP bars401 compared with external FRP laminates are the possibility402 of anchoring the reinforcement into adjacent RC mem-403 bers, and minimal surface preparation work and instal-404 lation time. For installation, a groove is cut in the405 desired direction into the concrete surface, the groove is406 then filled halfway with adhesive paste, and the FRP407 bar is placed in the groove and lightly pressed. This

408

forces the paste to flow around the bar and fill com-409pletely between the bar and the sides of the groove.410Finally, the groove is filled with more paste and the411surface is leveled. As this technology emerges, the412structural behavior of RC and PC elements strengthened413with NSM FRP bars needs to be fully characterized.

4142.2. Applications of strengthening

4152.2.1. Strengthening with CFRP externally bonded416laminates417A 90 000 m (1 million foot ) parking garage in2 2

418Pittsburgh, Pennsylvania, was strengthened in order to419address concerns regarding distress in the double tee420beams supporting each elevated floorw8x. The short421term parking garage at Pittsburgh International Airport422is a three-story precast concrete structure. The two423elevated levels(levels 2 and 3) consist of 2.8-m(9.2-424foot) wide by 18.5-m(60.7-foot) long precastyprestres-425sed double tees. The double tee units are generally 760426mm (2.5 fppt) in total depth and are dapped at the ends427to bear on ledger beams. Cracking at the re-entrant428corners of the dapped ends of the double tees was429observed and initially gave cause for concern. Cracking430was observed in nearly every double tee in the garage,431and though the crack angle varied widely, most cracks432were inclined between 08 (horizontal) and 458 (seeFig.4332). As part of a condition survey, four of the double tee434beams were selected to be load tested in order to assess435their structural adequacy. During load testing, flexural-436shear cracking developed at a location approximately4371.5 m (4.9 foot) from the dapped ends. As loading438increased, the flexural-shear cracking became so severe439that the beams were deemed to have failed at roughly44075% of their design capacity.441In assessing various strengthening alternatives, exter-442nally bonded CFRP reinforcement was determined to be443the most cost effective, the least disruptive to the444operation of the parking garage, and virtually unnotice-

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46 Fig. 3. Installed FRP U-wraps(0y908).

50

51Fig. 4. Completed installation of CFRP laminates.

445

able once the installation was complete. A load test of446 several tees after the installation of the FRP reinforce-447 ment demonstrated that the retrofitted tees could support448 loads over 100% of the design load. Externally bonded449 CFRP laminates were used as shear reinforcement at 0y450 908 combination on the stem of each double tee and as451 flexural reinforcement at 08 on each stem’s soffit (see452 Fig. 3). The amount of FRP laminates provided was453 based on the level of deficiency of the member(i.e. one454 or two vertical ‘U’ wraps per stem; two horizontal plies455 per side of stem; and two horizontal plies on the bottom456 of each stem). The demand on the level 3 double tees457 is greater due to a thicker asphalt overlay and exposure458 to snow loading. The deficiencies in the double tees459 that needed to be addressed were both flexural and shear460 deficiencies at the ends of each tee.461 The precast double tees have a nominal concrete462 strength of 31 MPa(4500 psi). The FRP material used463 utilizes a high strength carbon fiber fabric with the464 following characteristics: thickness,t s0.165 mmf

465 (0.0065 in); elastic modulus,E s227 GPa(33 Msi);f

466 and strain at rupture,́ s0.017 mmymm (inyin). Thef

467 ends of the double tees were reinforced continuously468 with the FRP reinforcement(as opposed to reinforcing469 with strips of reinforcement spaced apart). The depth of470 the FRP reinforcement,d , varied based on variations inf

471 the effective depth of the section,d .p472 In order to accommodate any horizontal component473 of force and to provide additional anchorage for the474 vertical FRP reinforcement, horizontal plies of FRP475 reinforcement were added to the sides of the stems as476 well. In addition to the FRP shear reinforcement, two477 100-mm(4-inch) wide CFRP strips were placed longi-478 tudinally on the soffit of the double tee stems. This479 reinforcement was used to limit the flexural strain level480 on the stem soffit to 0.005 mmymm (inyin) under a481 positive bending moment of 493 kN-m(363 foot-kip).482 All of the existing cracks wider than 0.50-mm(0.02483 inch) in the areas where the FRP was to be installed

484

required epoxy injection. The surface to which the FRP485was being installed also had to be profiled by water486blasting.487In addition to buildings, a number of applications488have included bridges. For example, three bridges in489Boone County, Missouri were selected for strengthening490with CFRP laminates in both shear and flexurew9x. The491objective of the rehabilitation program was to remove492the load posting that had been imposed on each of the493bridges. The three bridges were all simply supported494spans, ranging from 5.92 m(19.43 foot) to 11.84 m495(38.84 foot), and composed of precast RC channel496sections. The RC channel sections had a total depth497ranging from 457 mm(18 inches) or 610 mm (24498inches).499The existing capacity of the bridges was determined500using basic principals of RC theory. Based on material501properties and geometry, the existing factored moment502and shear capacities for a single RC channel section of503one of the bridges were:fM s201 kN-m (148 kip-n

504foot) andfV s159.2 kN(35.8 kip). Design loads weren

505based on an HS20 truck load as given by AASHTO506design guidelinesw10x for precast multi-girder bridge507decks. The moment and shear demand imposed by an508HS20 for a single RC channel section of the reported509bridge were:M s240 kN-m (177 kip-foot) and V su u

510191.3 kN(43.0 kip).511The FRP material selected for this application had512the same characteristics and installation process of that513described in the previous parking garage application.514Flexural strengthening was attained by applying strips515of CFRP laminate to the soffit of the channel legs with516the fibers oriented in the longitudinal direction(along517the length of the bridge) (seeFig. 4). For the reported518bridge, a 127-mm(5-inch) wide single ply strip is519applied over the entire length of the span developing a520new flexural capacityfM s247 kN-m(182 kip-foot).n

521Shear strengthening the bridges was achieved by

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56 Fig. 5. Horizontal and vertical grooves.

60

61Fig. 6. FRP bar installation.

522

applying strips of CFRP laminate(with fiber perpendic-523 ular to the direction of the bridge) to the sides of the524 web on each channel e.g.(seeFig. 4). Due to the way525 sections are joined side-by-side, the CFRP strips could526 only be applied to one side of each channel leg. Each527 strip would extend from the channel web across the528 soffit of the leg, covering the longitudinal FRP strip.529 Shear strengthening for all three bridges consists of 610530 mm (24 inches) wide single-ply CFRP strips spaced at531 864 mm(34 inches) center-to-center developing a shear532 capacityfV s213.9 kN(48.1 kip).n

533 2.2.2. Strengthening with NSM carbon FRP bars534 Six cement silos, located in the Boston, Massachu-535 setts, were upgraded with the NSM FRP bar technology536 w11x. The silos are part of a larger complex composed537 of structures built in different periods. In 1962, a cluster538 of four silos (coded 1, 2, 3 and 4) was constructed;539 each silo is 45.7 m(150 foot) tall, with a diameter540 equal to 6.7 m(22 foot). In 1979, other four silos541 (coded 5, 6, 7 and 8) were added, each standing 39.6542 m (130 foot) tall and 13.4 m(44 foot) in diameter. The543 repair was carried out on silos 1, 2, 3, 4, 7 and 8. They544 are characterized by raft foundations and a hopper545 independently supported by a ring beam and column546 system. Wall thickness is equal to 203 mm(8 inches)547 for the first four silos and 254 mm(10 inches) for the548 other two; for all of them, a single layer of vertical and549 horizontal steel reinforcement is placed close to the550 outside surface of the walls.551 Initial signals of distress were observed in 1994,552 alerted by cracks and leakage of material through the553 silo walls. Inspections and analysis evidenced several554 problems. In particular, field measurements of reinforc-555 ing bar spacing showed inconsistency with construction556 drawings and revealed that the actual spacing of the557 vertical and horizontal steel bars was larger than speci-558 fied in many regions. This resulted in significant levels

559

of under-reinforcement. After an accurate economic560evaluation and considering the peculiar cluster orienta-561tion of the silos, with common intersecting walls and562partial access around their perimeter, the owner decided563to repair the structures and, among different upgrade564techniques, selected the use of NSM CFRP bars.565The design required that an 8-mm(5y16-inch) diam-566eter CFRP bar be used in the vertical direction along567the entire height of the silo. The spacing of these bars568is such that one bar is located midway between each of569the existing vertical steel bars(i.e. 460 mm(18 inch)570o.c. spacing). In the horizontal(hoop) direction, an 11-571mm (7y16-inch) diameter CFRP bar was used along the572height of the silo starting from the level of the interior573ring beam and continuing to the top of the silo. The574spacing of these bars is also such that one bar is located575midway between each of the existing horizontal steel576bars(i.e. 200 mm(8 inches) o.c. spacing). The speci-577fications for the CFRP bar were as follows: minimum578elastic moduluss148 GPa(21.5 Msi), and minimum579strain at failures0.0105 mmymm (inyin). The CFRP580bar surface was roughened for improved bond with581concrete by use of a peel ply during fabrication or by582means of sand blasting after fabrication.583Installation of the bars began with the grooving584operation (see Fig. 5). Customized grooving tools585allowed technicians to cut the appropriate grooves in586one pass. Where the groove intersected the common587wall locations, a hole was drilled that was tangent to588the curve of the silo to the depth determined by pull589out test results. A two-component, high-viscosity, epoxy590adhesive was injected into the deeper, vertical grooves.591The bars, some 46 m(150 foot) long, could be handled592in single pieces due to their lightweight properties. The593vertical bars were then inserted into the groove and594embedded in the adhesive(seeFig. 6). A second layer595of adhesive was applied on top of the FRP bar. Upon596completion of the vertical installation, the circumferen-597tial bars were installed. Adhesive was installed in the

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dowel holes in the two common wall intersections and599 in the horizontal grooves. Starting at the first dowel600 hole, a single length bar was inserted into the dowel601 hole and then placed into the horizontal groove around602 the circumference until it met the other common wall603 intersection. The remaining length was inserted into the604 second dowel hole. The second layer of adhesive on top605 of the horizontal bar was installed, and the material was606 then finished to create a surface appearance that could607 be easily hidden for aesthetics.

608 2.3. Unresolved issues

609 Several of the unresolved issues relative to internal610 reinforcement also pertain to the case of strengthening,611 for example, durability(including fire) and test methods,612 so that repetitions are not necessary. Further research613 into the mechanics of bond of FRP reinforcement is of614 critical importance. New experimental evidence and615 analytical tools should yield more accurate methods for616 predicting delamination at the interface and in the617 concrete. Furthermore, developments will likely account618 for the stiffness of the laminate, the stiffness of the619 member to which the laminate is bonded, and the620 influence of the adhesive thickness and properties. The621 interim recommendations to limit the strain in the FRP622 to prevent delamination need to be revisited and con-623 firmed. In addition to delamination, it is critical to better624 understand other failure phenomena associated to exter-625 nal reinforcement such as peeling of the concrete cover.626 A list of critical issues follows:627 Bond: effect of concrete surface profile, effect of628 adhesive thickness and properties, cover delamination,629 methods to improve anchorage, effect of distributed or630 damaged regions(holes, voids, etc.), detailing.631 Seismic upgrade: develop special provisions for seis-632 mic rehabilitation of concrete structures using FRP.633 Inspection and quality control: develop protocols and634 test methods for quality assurance.635 NSM reinforcement: efforts need to be made to allow636 for other forms of FRP strengthening.

637 3. Conclusions

638 Even with some unresolved issues that should become639 a priority for future research, it can be concluded that640 the availability of design and construction guides devel-641 oped by ACI for the use of FRP internal and external642 reinforcement for new and existing structures should

643

allow the construction industry in North America to take644full advantage of this emerging technology.645Applications for new construction where internal FRP646reinforcement is used are slowly developing. It is expect-647ed that the use of glass FRP bars will dominate in this648market. Applications for strengthening of existing struc-649tures with externally bonded laminates and NSM bars650has already captured a significant market share with651several multi-million dollar projects.

652Acknowledgments

653The author, who has drawn freely from the cited ACI654documents and his personal notes at technical meetings,655wishes to acknowledge the direct and indirect contribu-656tions of the members of ACI 440 Committee and all657individuals involved in the preparation of its documents.

658References

659w1x American Concrete Institute—Committee 440. Guide for the660design and construction of concrete reinforced with FRP rebars,661ACI 440.1R-01. Farmington Hills, MI, USA: ACI, 2001.662w2x American Concrete Institute—Committee 318. Building code663requirements for structural concrete and commentary, ACI 318-66495yR-95. Farmington Hills, MI, USA: ACI, 1995.665w3x Nanni A. Flexural behavior and design of reinforced concrete666using FRP rods. J Struct Eng 1993;119(11):3344–59.667w4x Bradberry, TE. Fiber-reinforced-plastic bar reinforced concrete668bridge decks, 80th Annual Transportation Research Board, Jan.6699–13, 2001, Washington, DC, CD�01-3247.670w5x American Concrete Institute—Committee 440. Guide for the671design and construction of externally bonded FRP systems for672strengthening of concrete structures, Farmington Hills, MI,673USA: ACI, 2001,(in print).674w6x Khalifa A, Gold WJ, Nanni A, Abdel Aziz MI. Contribution675of externally bonded FRP to shear capacity of flexural mem-676bers. ASCE-J Compos Constr 1998;2(4):195–203.677w7x De Lorenzis L, Nanni A. Characterization of FRP rods for678near surface reinforcement. ASCE-J Compos Constr6792001;5(2):114–21.680w8x Gold, WJ, Blaszak, GJ, Mettemeyer, M, Nanni, A, Wuerthele,681MD., Strengthening dapped ends of precast double tees with682externally bonded FRP reinforcement, ASCE Structures Con-683gress 2000, Philadelphia, PA, CD�40492-045-003.684w9x Schiebel, S, Parretti R, Nanni A, Huck M. Strengthening and685load testing of three bridges in Boone County, MO. ASCE686Pract Period Struct Des Constr, 2002(in print).687w10x American Association of State Highway and Transportation688Officials. Standard specifications for highway bridges, 16th689ed., Washington, D.C., 1996:760.690w11x International Concrete Repair Institute. Cement silo repair and691upgrade. Concr Repair Bull 2001;14(5):16–9 (2000 Award of692Excellence Winner). 693