1 Performance of an AASHTO Beam Bridge Prestressed2 with CFRP Tendons1

3 Nabil Grace1; Elin Jensen2; Vasant Matsagar3; and Prasadu Penjendra4

4 Abstract: Corrosion-induced deterioration of steel RC highway bridges is one of the major distress types that can render a structurally deficient5 bridge before reaching the design life. One feasible solution to the problem is to replace the conventional steel reinforcement with noncorrosive6 carbonfiberereinforced polymer (CFRP) reinforcements. However, theCFRP reinforcement as an internal reinforcement has not been explored in7 AASHTO-type prestressed concrete beam bridges. AASHTO-type beams have an I-type cross section with a bottom flange, and on integration of8 the deck slab, thefinal shape is a bulb-Tsection.Thispaper discusses the experimental investigationof a precast prestressedAASHTOcontrol beam9 and a bridge model. A 12.5-m-long one-third scale AASHTO-type control beam was experimentally investigated for its flexural behavior when10 reinforced and prestressed with CFRP. Subsequently, a one-third scale bridge model made of five such beamswas constructed, instrumented, and11 tested under both service and ultimate load conditions. As anticipated, the control beam and the bridgemodel failureswere initiated by rupturing of12 the prestressing CFRP tendons at the bottom layer. The observed flexural response of the bridge model was in close agreement with that of the13 control beam.As expected, the failuremodewas progressive, with extensive cracking of the bridgemodel, which gives significant warning prior to14 the ultimate collapse, overcoming issues related to the otherwise brittle behavior of the CFRP-reinforced structures. It is therefore highly15 recommended to provide the CFRP tendons in different layers along the depth of the beams to effectively address the issues related to brittle failure16 exhibited by CFRP reinforcements. DOI: 10.1061/(ASCE)BE.1943-5592.0000339. © 2013 American Society of Civil Engineers.

17 CE Database subject headings: Beams; Bridges; Fiber reinforced polymer; Prestressed concrete; Progressive collapse.

18 Author keywords: AASHTO beam; Bridge; CFRP; Ductility; Prestressed concrete; Progressive failure.

19 Introduction

20 In this paper, the flexural behavior of anAASHTO-type beambridge21 reinforced and prestressed with carbon fiberereinforced polymers22 (CFRPs) is presented. The high strength-to-weight ratio, superior23 fatigue resistance, ease of handling, low thermal expansion, and24 low relaxations are some of the key advantages of the composite25 materials compared with conventional steel reinforcement. These26 characteristics make the FRPs an emerging potential construction27 material in the bridge construction industry.28 The importance and use of AASHTO-type concrete beam bridges29 have gained momentum recently. For a wide range of practical spans,30 AASHTO beams are often selected because they have a simple cross31 section, have higher flexural capacities, and are less expensive to32 fabricate. In addition, the noncorrosive CFRP reinforcement provided

33in the bridges helps sustain the bridge for a longer lifespan with34minimummaintenance costs. Apparently, there has been very limited35research on the application of the FRP in AASHTO-type beams.36Achieving ductile failure of the FRP-reinforced structures in37flexure by various approaches 3has remained the focus of research in38recent years. Fam et al. (1997) concluded that the flexural behavior39of the beams prestressed with FRP tendons exhibited similar40stiffness as that of the beam prestressed with steel reinforcement.41Abdelrahman and Rizkalla (1999) confirmed that the flexural be-42havior of concrete beams prestressed with CFRP tendons under43repeated load was similar to that for the beams prestressed with steel44strands, if the steel is in the elastic stage before unloading. A full-45scale test of an overreinforced FRP prestressed double-tee beamwas46presented by Grace et al. (2003). The results provided from this full-47scale experimental study coupled with the design equations de-48veloped by Grace and Singh (2003) were directly used in the49construction of the United States’s first precast CFRP prestressed50concrete bridge called the Bridge Street Bridge in Michigan.51DolanandSwanson (2002) developed a strength design approach52to maximize the use of vertically distributed CFRP tendons located53at various depths. Naaman et al. (1993) experimentally and theo-54retically analyzed the partially prestressed concrete T-beams with55carbon fiber composite cable (CFCC) strands. Progressive failure56was achieved in the T-beams, and the postpeak load-deflection57behavior was characterized by an incremental steplike descent58corresponding to the rupture of CFCC strands.Morais andBurgoyne59(2003) proposed the step layering of the FRPs to have a progressive60failure and to improve ductility.61Attempts were made in developing design approaches for the62structures reinforced with FRP materials. In the unified design63approach for the design of CFRP prestressed concrete beams64proposed by Grace and Singh (2003), a strain compatibility-based65approach was suggested. This approach was validated by

1Dean, College of Engineering, and Director, Center for InnovativeMaterials Research (CIMR), Lawrence Technological Univ., Southfield,MI48075.

2Associate Dean, Graduate Studies and Research, College of Engineer-ing, and Associate Professor, Civil Engineering Dept., Lawrence Techno-logical Univ., Southfield, MI 48075 (corresponding author). E-mail:ejensen@ltu.edu

3Ph.D. Research Fellow, Center for Innovative Materials Research(CIMR), Civil Engineering Dept., Lawrence Technological Univ., South-field, MI 48075.

4Graduate Student, Civil Engineering Dept., Lawrence TechnologicalUniv., Southfield, MI 48075.2

Note. This manuscript was submitted on May 23, 2011; approved onNovember 15, 2011; published online on January 15, 2013. Discussionperiod open until July 1, 2013; separate discussions must be submitted forindividual papers. This paper is part of the Journal of Bridge Engineering,Vol. 18, No. 2, February 1, 2013. ©ASCE, ISSN 1084-0702/2013/2-1e12/$25.00.

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66 experimental results conducted on double-tee beam bridge models67 (Grace et al. 2003). Furthermore, a compression controlled failure68 mode was recommended as the design failure mode for the CFRP69 prestressed concrete beams. This recommendation was based on70 the better ductility characteristics of the overreinforced sections.71 ACI 440.1R-06 [American Concrete Institute (ACI) Committee72 440 2006] general guidelines suggest using a higher reserve strength73 of the FRP-reinforced members to compensate for the lack of74 ductility. Further, the failure mode involving concrete crushing has75 been preferred compared with rupture of the FRP reinforcement.76 Moreover, it concludes that the member would not exhibit ductility77 in any case as commonly observed in steel underreinforced mem-78 bers. Despite these ACI 440.1R-06 (ACI Committee 440 2006)79 guidelines to prefer overreinforced beams, for an underreinforced80 beam, a careful design of the vertical distribution of the longitudinal81 FRP tendons is essential to enforce a slow progressive failure of82 tendons, which simulates a ductile failure of the section reinforced83 with steel. In addition, strength equations and a strain compatibility84 approach of steel prestressed beams could be directly applied to85 calculate the flexural capacity of the underreinforced CFRP pre-86 stressed beams.87 The objective of this study is to investigate the flexural behavior88 of the AASHTO-type beam and a bridge model in demonstrating89 progressive failure. To assess the flexural behavior, the AASHTO-90 type control beam and a bridge model with CFRP tendons as in-91 ternal reinforcement were constructed, instrumented, and tested.92 The flexural behavior was investigated through the load-deflection93 responses, ultimate load-carrying capacities, modes of failures,94 load-strain responses, and energy ratios of the control beam and the95 bridge model.

96 Design Details

97 The issue of brittle behavior in CFRP-reinforced and prestressed98 concrete structures is common, and this could be overcome by99 achieving a progressive type of failure in deep beams. A mathe-

100 matical model based on the strain compatibility approach was de-101 veloped to test different levels of prestressing forces at various layers102 to achieve progressive failure of prestressing tendons. The model103 showed a pyramid-type prestressing pattern, with a reinforcement

104ratio of 0.35% as optimum given the model scale and laboratory105limitations. The unified design approach proposed by Grace and106Singh (2003) and ACI 440.4R-04 design guidelines (ACI Com-107mittee 440 2004) were also used in the development of the108laboratory-scale bridge model and control beam.

109Construction of AASHTO Beams

110Six prestressed AASHTO-type I-beams were constructed, each111502mmdeepwith top and bottom flangewidths of 203mm andweb112thickness of 95 mm. The horizontal shear transfer/composite action113between the beam and deck slab was ensured through 57-mm pro-114truding stirrups above the top of the beam (Fig. 1). Each AASHTO115beam reinforcing cage consisted of three longitudinal CFRP pre-116stressing tendons and eight longitudinal nonprestressing CFRP rods,117both having a diameter of 10 mm as flexural reinforcement dis-118tributed vertically along the depth of the beam in layers to achieve119a controlled progressive type of failure with successive rupturing of120the CFRP tendons. Steel stirrups, with a diameter of 10 mm spaced121at a center-to-center distance of 102 mm, were used as shear re-122inforcement. The steel stirrups were used in this investigation be-123cause the main focus was on the flexural behavior and not shear. An124arrangement to accommodate the transverse reinforcement for the125diaphragms was provided in the beam with the help of 13-mm-126diameter polyvinylchloride pipes at designated places. The material127properties of the CFRP tendons and steel stirrups used in the128construction of the beams are provided inTable 1.A rectangular end-129block with 532 mm length, 203 mm width, and 502 mm depth was130provided at each end of the beams to resist bursting stresses gen-131erated during the transfer of the prestressing forces. Moreover,132confinement in the end-block regions was provided by the rectan-133gular stirrups spaced at a reduced center-to-center spacing of 51mm.134The arrangement of the longitudinal reinforcement and stirrups135spacing is shown in Fig. 2.136On completion of constructing the formwork, a reinforcement137cage of corresponding materials was assembled and placed inside138the formwork. The CFRP prestressing tendons were passed139through the reinforcement cage at designated layers and positioned140between the two bulkheads. Calibrated load cells were mounted on141the prestressing tendons at the dead-end and connected to a data142acquisition system to monitor and record the level of pretensioning

Fig. 1. Control beam cross-section and reinforcement details

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143 forces. The pretensioning force was applied through a 305-mm144 center-hole hydraulic jack situated at the live end. Each prestressing145 CFRP tendon was stressed to an average jacking force of 90 kN146 (1,146 MPa) for achieving a total pretensioning force of 270 kN on147 each AASHTO beam. After the prestressing operation, the ready-

148mix concrete in accordance with the Michigan Department of149Transportation (MDOT) standard specifications for precast pre-150stressed beams was placed in the formwork. The average 28-day151compressive strength of the concrete in the AASHTO beams was15248 MPa.

153Construction of the Control Beam

154One of the six precast prestressed AASHTO beams was used as155a control beam for the bridge model. A 64-mm-thick and 502-mm-156wide deck slab was topped on this individual beam to simulate the157deck slab of the bridge model. For the deck slab reinforcement, four158longitudinal nonprestressing CFRP rods of 10 mm diameter and159transverse nonprestressing CFRP tendons of the same diameter160spaced at a center-to-center distance of 203 mm were used.

161Construction of the Bridge Model

162A three-dimensional (3D) view of the bridgemodel is shown in Fig. 3,163and its cross-section details atmidspan are shown in Fig. 4. The bridge164model was 12.5 m in length and 2.5 m in width, constructed of165five precast prestressed concrete AASHTO beams. The AASHTO

Table 1. Properties of CFRP Tendons and Steel Stirrups Used in ControlBeam and Bridge Models

Material characteristics

Material type

CFRP tendons[Diversified Composites

Inc. (DCI) 2000]Steel stirrups(Nawy 2003)

Nominal diameter, d (mm) 10 10Cross-sectional area (mm2) 71.3 71.3Ultimate tensile strength(MPa)

2,344 414

Elastic modulus (GPa) 157 200Ultimate tensile strain (%) 1.49 9.00Average measured breakingload (kN)

164.6 44.1

Yield strain — 0.002

Fig. 2. Details of longitudinal flexural reinforcement

Fig. 3. 3D view of the bridge model

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166 beams of the bridge model were arranged at a center-to-center167 distance of 502 mm, joined with five equally spaced CFRP rein-168 forced transverse diaphragms, and topped with a 64 mm thick169 CFRP-reinforced deck slab. The AASHTO beams were placed170 side by side at a center-to-center distance of 502 mm. The CFRP171 rods of 2.5 m length were passed through the previously made172 transverse holes in the beams and epoxy grouted to brace the beams173 transversely (Fig. 4) to serve as the transverse reinforcement of the174 diaphragms. The vertical reinforcement of the diaphragms was175 attached to the transverse reinforcement, and formwork was176 provided around the reinforcement. Prior to the placement of the177 deck slab reinforcement, the formwork for the deck slab was at-178 tached to the beams and diaphragm formwork and supported as179 typically practiced in shored construction. The placing of concrete180 for the diaphragms and deck slab was carried out as mentioned181 earlier for the beams, achieving 28-day compressive strength of182 48 MPa. The completed bridge model is shown in Fig. 5.

183 Instrumentation and Test Setup

184 Both the control beam and the bridge model were simply supported185 with an effective span length of 12.2 m. The test program included186 an ultimate load test on the control beam, effective prestress force,187 ultimate load, and residual strength tests on the bridge model. Linear188 motion transducers were used to measure the deflections, and189 electrical resistance strain gauges were used to measure the strains at190 predetermined locations.

191Test Setup: Control Beam

192Flexural load was applied to the control beam through a load-193spreader, which was designed to distribute a two-point load sym-194metrically to achieve flexural failure. The test setup for the control195beam is shown in Fig. 6. The length between the two loading points196of the load-spreader was 1.2 m. The control beam was subjected197to several loading and unloading cycles to separate the elastic and198inelastic energies under the load-deflection curve; the ductility was199evaluated based on the energy concept (Grace et al. 1998). The200loading cycles applied on the control beam were 27, 53, 67, 80, 93,201107, and 120 kN and a final load cycle up to failure.

202Test Setup: Bridge Model

203The load applied to the bridge model was through a four-point load-204spreader. The length between two loading points of the load-spreader205was 1.2 m, and the width of each loading point was 508 mm, which206were obtained by scaling down (one-third) the axle distance of the207AASHTO HL-93 truckload. The bridge model was also subjected208to several loading and unloading cycles to separate the elastic and209inelastic energies. The loading cycles applied were 89, 178, 267, 356,210445, and 534 kN and a final load cycle up to failure. The first and211second loading cycles additionally helped in determining the effective212prestress forces in the bridge model.213The bridgemodel was further subjected to a residual strength test,214as only Beam B-3 failed partially during the ultimate test. A trans-215versely placed loading steel I-beam of 2.54 m length was used to

Fig. 4. Cross section of the bridge model at midspan

Fig. 5. Views of the bridge model

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216 reload the bridge model until its complete failure. Fig. 7 shows the217 ultimate load test setup and the residual strength test setup for the218 bridge model along with deflected shape of the bridge during ul-219 timate load test.

220 Results and Discussion

221 The flexural behavior of the control beam and the bridge model is222 discussed in this section in terms of the modes of failure; load-223 deflection responses; strains in the prestressing and nonprestress-224 ing tendons and the extreme concrete fibers; energy-based ductility225 indices; and the ultimate load-carrying capacities. To assess the226 performance of the CFRP tendons during progressive failure, the227 tension-controlled design approach was used.

228 Load-Deflection Response

229 The midspan deflections of the simply supported control beam and230 the bridge model were used to evaluate the load-deflection response.231 The load-deflection response of the control beam during the loading232 and unloading cycles is shown in Fig. 8. The load-deflection re-233 sponse is bilinear until reaching the ultimate load with two distinct234 slopes. The first slope (OA) is from the beginning of the loading until235 the cracking load, and the second slope (AB) is from cracking until236 the ultimate load. The change in slope corresponds to the reduction237 of flexural rigidity to 75% because of the developed flexural cracks238 in the tension zone of the concrete. The beam cracked in the second239 loading cycle at 20 kN, with a deflection of 10 mm. The ultimate240 load-carrying capacity of the control beam was 127 kN, with a de-241 flection of 215 mm.242 At the ultimate load, the prestressing CFRP tendons in the sev-243 enth layer (bottommost layer) ruptured. Because of this rupture,244 a sudden drop of load from 127 to 71 kN was observed, and then an245 increase in the applied load started again. The slope of the load-246 deflection curve (CD) at this stage was almost parallel to the slope247 of the curve from cracking load to the ultimate load (AB). The248 prestressing CFRP tendons in the sixth layer ruptured at a load of249 97 kN, with a deflection of 283 mm. This rupture caused another250 drop in the load from 97 to 63 kN. A further increase in the load251 resulted in a load-deflection response, with the slope (EF) almost

252parallel to the initial postcrack slope (AB). After reaching a load of253103 kN, failure of the nonprestressing CFRP rods in the bottom254layer and in the successive upper layers was experienced, which255exhibited a progressive type of failure, with an ultimate deflection256of 502 mm. The estimated ultimate load from the unified design257approach proposed byGrace andSingh (2003) was 131 kN, showing258a difference of about 3% between the predicted and experimental259results.260The load-deflection response of the bridge model during the261conducted loadingandunloadingcycles is shown inFig. 9. The failure262sequence of the bridge model included (1) rupture of the prestressing263CFRP tendons in the bottom layer of central BeamB-3, (2) local shear264failure of the deck slab around the load-spreader, (3) rupture of the265nonprestressing CFRP rods in the midspan diaphragm, and (4)266separation of loaded Beam B-3 from the deck slab and other dia-267phragms. The rupture of the prestressing CFRP tendons was ob-268served at 605 kN, and the experienced ultimate load was 650 kN,269with a deflection of 264 mm. At the separation of loaded Beam B-3270(shear delamination failure), an ultimate deflection of 342 mm was271experienced.272Although different postultimate failure modes were observed for273the control beam and the bridge model, the ultimate load-carrying274capacity of the bridge model was 5.08 times higher than that of the275control beam. This signifies the important role of the transverse dia-276phragms in load distribution among the beams in the bridge model.

277Strain in CFRP Reinforcement

278The load versus strain in the CFRP reinforcement of the control279beam is presented in Fig. 10. The strain responses up to the ultimate280load of all the nonprestressing and prestressing CFRP tendons were281bilinear, with a change in slope at the cracking load.

282Load-Strain Response of Prestressing CFRP Tendons283Two of the prestressing tendons were located at the bottom layer,284i.e., seventh layer, and the other prestressing tendon was located at285the sixth layer. The rupture strain observed in the prestressing ten-286dons in the seventh layer at the ultimate load was 1.57%, which is287a little over the guaranteed ultimate strain of 1.49% as specified by288the manufacturer (see Table 1). At the ultimate load of 127 kN, the289tensile strain in the prestressing tendon at the sixth layer was 1.34%.290Because of the rupture of the prestressing tendons in the bottom291layer, the load dropped suddenly from 127 to 71 kN. At a load of29297 kN, the prestressing tendon in the sixth layer ruptured, and the293recorded tensile strain was 1.51%, which was in close agreement294with the manufacturer’s specifications (see Table 1).

295Load-Strain Response of Nonprestressing CFRP Rods296The strain of the nonprestressing rods in the second layer was297compressive at the beginning of the loading because the position of298the neutral axis was below the level of the second layer of rein-299forcements. This reinforcement later experienced tensile strainwhile300increasing the load, which is attributed to an upward shifting of the301location of the neutral axis. The maximum tensile strain observed302was 0.45% at the ultimate load. At the ultimate load, the developed303tensile strain of the nonprestressing rods in the fourth and sixth304layers were 0.53 and 0.70%, respectively. These experienced strains305clearly indicate that the prestressing tendons only in the bottommost306layer ruptured at the ultimate load. Even after the rupture of the307prestressing tendons, the control beam could withstand the applied308load with the help of the nonprestressing CFRP rods.309The load-strain response of the CFRP reinforcement in the bridge310model is shown in Fig. 11. The prestressing tendon in the bottom

Fig. 6. Test setup for the control beam

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Fig. 7. Ultimate load and residual strength test setups for the bridge model

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311 layer of Beam B-3 experienced strain of 1.47%, including the initial312 prestress effects. However, because of the stiffening effect of the313 transverse diaphragms and loading points directly resting on the314 deck slab, the bridgemodel experienced local shear failure instead of315 controlled progressive failure. The maximum tensile strain experi-316 enced by the nonprestressing CFRP rods in the seventh layer of Beam317 B-3 was 1.38%, which is close to the guaranteed ultimate strain. The318 difference in strain of the prestressing tendons and nonprestressing319 rods in thefirst, second, sixth, and seventh layers at the beginning of the320 loading is a result of the effect of the prestressing force.

321 Ductility of Control Beam Bridge Model

322 Because the CFRP materials have a linear stress-strain relationship323 up to rupture, the conventional approach to evaluate the ductility of

324the steel-reinforced beams is inapplicable. Hence, the energy-based325approach proposedbyGrace et al. (1998) is used to quantify ductility326of the CFRP-reinforced and prestressed structures. The ductility327index determined from the ratio of absorbed inelastic energy to the328total energy as

Einelastic

Etotal¼ Einelastic

Eelastic þ Einelastic þ Einelastic;additional

329where Einelastic 5 inelastic energy absorbed; Einelastic,additional 5330additional inelastic energy absorbed after the ultimate load; and331Eelastic 5 elastic energy absorbed.332For the control beam, the estimated inelastic energy was 3,758333kN-mm, the elastic energy was 14,535 kN-mm, and the additional

Fig. 8. Load-deflection response and progressive failure of the control beam

Fig. 9. Load-deflection response of middle Beam B-3 for different loading/unloading cycles

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334 inelastic energy was 20,559 kN-mm, as shown in Fig. 12. The total335 energy developed in the control beam was 38,852 kN-mm, which336 resulted in an energy ratio of 62.6%. It is observed that the337 progressive failure of the control beam caused significant inelas-338 tic energy to be absorbed. Although theoretically, the control beam339 falls into the brittle failure category, it still provided a sufficient340 and significant warning in terms of excessive cracking as in the341 case of well-known conventional prestressed steel-reinforced con-342 crete beams and had a significantly large deflection of 502 mm at343 failure.344 Similar to the control beam, the energy ratio is calculated based345 on the load-deflection curve of loaded Beam B-3 of the bridge346 model, as shown in Fig. 13. The inelastic energy experienced by the347 bridge model prior to the ultimate load was 31,543 kN-mm, and the348 elastic energy was 70,129 kN-mm. The additional inelastic energy

349after the postpeak response was 32,668 kN-mm. The total energy350absorbed by the bridge model was 134,340 kN-mm, which resulted351in an energy ratio of 48%.352Because all the CFRP tendons in the bridge model were not353completely ruptured, as Beam B-3 only failed partially during the354ultimate load test, a significant amount of additional inelastic energy355of the remaining beamswas available in the bridgemodel. To release356this additional inelastic energy, a residual strength test was con-357ducted on the bridge model, and the obtained additional inelastic358energy was added to the energy ratio calculated from the ultimate359load test.360The energy ratio of the bridgemodel developed from this residual361strength test is presented in Fig. 14. The elastic energy for the re-362sidual strength test was 71,339 kN-mm. This energy was in close363agreement with the elastic energy observed in the ultimate load test,

Fig. 10. Prestressed and nonprestressed CFRP reinforcement strain responses in the control beam

Fig. 11. Load-strain response of CFRP reinforcement in Beam B-3 of the bridge model

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364 whichwas 70,129 kN-mm.Because no load cycleswere conducted in365 the residual strength test, there was no inelastic energy measured in366 the bridge system. However, successive rupture of the prestressing367 CFRP tendons after the ultimate load created additional inelas-368 tic energy. The measured additional residual inelastic energy after369 postpeak response was 18,945 kN-mm. Thus, total energy experi-370 enced by the bridge model was 153,214 kN-mm, and the summation371 of inelastic energy, additional inelastic energy from the ultimate load372 test and the additional residual inelastic energy from the residual373 strength test, was 83,156 kN-mm. The calculated energy ratio for the374 bridge model from the ultimate load and residual strength tests was375 54.3%.376 Although the energy ratios of the control beam (62.6%) and the377 bridge model (54.3%) are different, the elastic energy ratio of the378 bridge model was similar to the energy ratio of the control beam.379 Hence, the postpeak behavior governed the energy ratios of the380 control beam and the bridge model, i.e., the ductility of the two381 models during failure.

382Strain in Extreme Concrete Fiber

383The longitudinal compressive strain of the extreme concrete fiber384was monitored through the readings of the strain gauges installed385at the top of the deck slab at midspan. The concrete strains were386expected to be less than 0.003, as the control beam and the bridge387model failures were governed by rupture of the prestressing CFRP388tendons. The compressive strain in the control beam is shown in389Fig. 15, and the maximum strain observed was 0.002 at the ultimate390load and a strain of 0.0025 at complete collapse. The compressive391strain in the middle three beams of the bridge model is shown in392Fig. 16, and themaximum strain observedwas 0.0017 at the ultimate393load.

394Failure Mode of the Bridge Model

395For the bridge model, a flexural tension failure of central Beam396B-3 in the form of rupture of the prestressing CFRP tendons was

Fig. 12. Energy ratio of the control beam

Fig. 13. Elastic and inelastic energies of the bridge model

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397 experienced. It was followed by the punching shear of the deck slab at398 the centrally loaded beam, Beam B-3, and then failure of the trans-399 verse diaphragms. The diaphragms at the midspan and quarter-spans400 completely failed by crushing of the concrete and either rupture of the401 transverse CFRP rods or its debonding from the beams, i.e., failure of402 the epoxy grout. Although the bridge model was designed to achieve403 a flexural type of failure through rupturing of all the prestressing404 CFRP tendons, only the prestressing tendons in the bottom layer of405 loaded Beam B-3 were ruptured at the ultimate loading condition.406 Thiswas a result of the higher stiffness contribution of the diaphragms407 through the arching/tension-stiffening action and use of a smaller408 loading area on the deck slab by the load-spreader. The ruptured409 prestressing CFRP tendons were expected to initiate progressive type410 of failure in the other tendons before complete structural collapse as411 experienced in the control beam failure.

412Effective Prestress Force

413A decompression test was conducted on the bridge model to de-414termine the effective prestressing force level. The bridge model was415loaded until the initiation of the first crack at the bottom surface of the416beams at the midspan section. Strain gauges were attached at both417sides of the first crack, and the bridge model was loaded again. The418strain induced and load applied were monitored and recorded as419shown in the load-strain curve (Fig. 17). The decompression load420observed from the plot is 31.1 kN,which is used to derive the effective421prestressing force level and the prestress losses of 6.2%. The prestress422loss estimated in conventional stress-relieved tendon is approximately42315% (Naaman 2004). This result suggests that the use of CFRP424tendons coupled with its developed anchorage systems proved to be425an efficient system in producing less prestress losses compared with426the conventional steel-reinforced/prestressed members.

Fig. 14. Elastic and inelastic energies of the bridge model after residual strength test

Fig. 15. Midspan compressive strain response under applied load in the control beam

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427 Conclusions

428 The conclusions arrived at from this investigation are as follows:429 1. The failure mode of the control beam was a sequential pro-430 gressive rupture of the prestressing CFRP tendons in the431 bottom layers, followed by the rupture of nonprestressing432 CFRP rods. The sequence of failure of the bridge model433 consisted of rupture of the prestressing tendons, local punch-434 ing shear failure of the deck slab, rupture of the CFRP rods in435 the transverse diaphragm, and separation of the loaded beam.436 2. The failure modes of the control beam and the bridge model437 were governed by the rupture of the prestressing CFRP438 tendons.439 3. The controlled progressive failure yielded a significant amount440 of inelastic energy released during postpeak response and

441resulted in a higher energy ratio of the control beam. After442the ultimate load test and residual strength test, the bridge443model experienced an energy ratio of 54.3%, which was less444than the energy ratio (62.6%) of the control beam.4454. Although theoretically, the energy ratios of the control beam446and the bridge model fall in the category of brittle failures,447significant and substantial warning was experienced by ex-448tensive cracking before failure. The combined rupture of the449prestressed and nonprestressing CFRP tendons yielded large450deflection up to failure, whereas the concrete in the compres-451sion zone did not fail.4525. The effective prestress loss measured from the experiment in453the tested bridge model was 6.2%. The use of the CFRP454tendons coupled with their specialized anchorage systems455proved to be an efficient system resulting in less prestress

Fig. 16. Concrete compressive strain response at the midspan in longitudinal direction

Fig. 17. Load-strain response to estimate the decompression load

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456 losses compared with the conventional steel prestressed457 members.458 6. The theoretical ultimate load capacity of the control beam459 based on the unified design approach (Grace and Singh 2003)460 was 131 kN, whereas the experimentally obtained ultimate461 loadwas 127 kN.Hence, the unified design approach proposed462 byGrace and Singh (2003) is recommended for the design and463 analysis of the multilayered CFRP-reinforced and prestressed464 AASHTO beams.465 7. The cracking load, ultimate load, deflection at ultimate load,466 and elastic energy of the bridge model were in agreement with467 the control beam. This suggests that testing a control beam for468 any large-scale bridge model will help in predicting the469 behavior of the bridge model.

470 Acknowledgments

471 This investigation was supported by the U.S. Department of Trans-472 portation (US-DOT) (Contract No. DTOS 59-06-G-0030), awarded473 to N. Grace. The support and guidance ofMr. Carl Swerdloff, Senior474 Policy Analyst (US-DOT), and Mr. Benjamin Tang (Federal High-475 way Administration) are truly appreciated.

476 References

477 Abdelrahman, A. A., and Rizkalla, S. H. (1999). “Deflection control of478 concrete beams pretensioned by CFRP reinforcements.” J. Compos.479 Constr., 3(2), 55e62.480 American Concrete Institute (ACI) Committee 440. (2004). “Prestressing con-481 crete structures with FRP tendons.” ACI 440.4R-04, Farmington Hills, MI.

482American Concrete Institute (ACI) Committee 440. (2006). “Guide for the483design and construction of concrete reinforced with FRP bars.” ACI484440.1R-06, Farmington Hills, MI.485Diversified Composites Inc. (DCI). (2000). DCI tendons—Reinforcing486bar product specifications, Diversified Composites Inc., Erlanger,487KY.488Dolan, C. W., and Swanson, D. (2002). “Development of flexural capacity489of a FRP prestressed beam with vertically distributed tendons.” Com-490posites, 33(1), 1e6.491Fam, A. Z., Rizkalla, S. H., and Tadros, G. (1997). “Behavior of CFRP for492prestressing and shear reinforcements of concrete highway bridges.”ACI493Struct. J., 94(1), 77e86.494Grace, N. F., Enomoto, T., Abdel-Sayed, G., Yagi, K., and Collavino, L.495(2003). “Experimental study and analysis of a full-scale CFRP/CFCC496double-tee bridge beam.” PCI J., 48(4), 120e139.497Grace, N. F., and Singh, S. B. (2003). “Design approach for carbon fiber-498reinforced polymer prestressed concrete bridge beams.” ACI Struct. J.,499100(3), 365e376.500Grace, N. F., Soliman, A. K., Abdel-Sayed, G., and Saleh, K. R. (1998).501“Behavior and ductility of simple and continuous FRP reinforced502beams.” J. Compos. Constr., 2(4), 186e194.503Morais, M. M., and Burgoyne, C. J. (2003). “Experimental investigation504of the ductility of beams prestressed with FRP.” 6th Int. Conf. on Fiber505Reinforced Polymers for Reinforced Concrete Structures, Singapore,5061013e1022. 4507Naaman, A. E. (2004). Prestressed concrete analysis and design, 2nd Ed.,508Techno Press, Ann Arbor, MI.509Naaman, A. E., Tan, K. H., Jeong, S. M., and Alkhairi, F. M. (1993).510“Partially prestressed beams with carbon fiber composite tendons:511Preliminary tests evaluation.” Int. Symposium on Fiber-Reinforced-512Plastic Reinforcement for Concrete Structures, 441e464. 5513Nawy, E. G. (2003). Prestressed concrete: Fundamental approach, 4th Ed.,514Prentice Hall, Saddle River, NJ.

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