Long-Term Behavior of CFRPPrestressed Concrete Beams

Abass Braimah,Ph.D., P. Eng.

Post Doctoral FellowHalsall Associates LimitedOttawa, Ontario, Canada

Mark F. Green,Ph.D., P. Eng.ProfessorQueens UniversityKingston, Ontario, Canada

Khaled A. Soudki,Ph.D., P. Eng.

Associate ProfessorUniversity of Waterloo

Waterloo, Ontario, Canada

This paper presents an experimental programdesigned to examine the long-term behavior ofcarbon fiber reinforced polymer (CFRP)pretensioned concrete beams under sustainedload conditions. The test program consists of threebeams prestressed with CFRP rods and one beamprestressed with steel prestressing strands. Thepretensioned beams were subjected to a sustainedload of 53 kN (12 kips). Deflections andprestressing reinforcement strains at midspan weremonitored for a period of 651 days. An analyticalmodel was developed to predict the time-dependent behavior of the pretensioned beams.The experimental results show satisfactorybehavior of CFRP pretensioned beams comparedto the steel pretensioned beam. The ratio of long-term to instantaneous deflection at midspan wasobserved to increase with the level of prestressforce. The steel pretensioned beam exhibitedhigher long-term midspan deflections than thecompanion CFRP pretensioned beams.

Corrosion of steel reinforcement in highway bridgesand parking structures in North America is costingbillions of dollars annually in rehabilitation and replacement. In Canada alone, the rehabilitation cost of parking structures is estimated at 4 to 6 billion dollars, whereasin the United States, the cost of repair of highway bridges isestimated at 50 billion dollars.1 The cost of repair of structures damaged by reinforcement corrosion in Europe is estimated at 600 million dollars annually.2

Many methods have been proposed for the mitigation ofsteel reinforcement corrosion in concrete structures, but

98 PCI JOURNAL

r_.1

I__ZSMKl-J L-3

CFRP

none of these methods has been foundto totally eliminate the problem of corrosion. It has become evident that, toreduce rehabilitation and replacementcost of bridges and parking structures,a corrosion resistant material must befound to replace steel reinforcement.Fiber reinforced polymer (FRP), a corrosion resistant material, is a potentialreplacement for steel reinforcement inbridges and parking structures.

Much research has been performedin the past decade to examine the suitability of FRP in prestressing applications. Most of the research work hasconcentrated on the short-term behavior of FRP prestressed concrete structures and the results have shown satisfactory performance.3-5For the generalacceptance and wide-scale use of FRPin civil engineering applications, however, the long-term behavior and durability of FRP prestressed structuresmust be investigated.

Abdelrahman et al.,6 Rizkalla andTadros,7 Tsuji et aL,8 and Taerwe etal.9 have reported that many highwayand pedestrian bridges have been builtworldwide and are monitored continuously to study the long-term, fatigue,and general behavior of the FRP prestressed elements. These full-scalebridges have offered much insight intothe design and behavior of FRP pre

stressed structural elements, but thereis still the need for detailed laboratoryresearch into the long-term behaviorto complement these field tests.

liTERATURE REVIEWMaterial research has shown that

FRP reinforcement has different relaxation properties than steel reinforcement10 leading to the need to evaluatethe long-term behavior of FRP prestressed members. Early research results from long-term tests show thatFRP prestressed beams exhibit a similar behavior to those prestressed withsteel.1

Mathys and Taerwe2 tested threeseries of pretensioned concrete slabsusing steel wires and aramid fiber reinforced polymer (AFRP) rods as prestressing reinforcement. The first series of slabs were pretensioned withAFRP rods, while the other two serieswere pretensioned with steel wires.The slabs were subjected to a sustained load under constant room temperature of 200C (68F) and 60 percentrelative humidity.

The applied load caused cracking inall the slabs. The authors reported thatthe long-term deformations of theAFRP pretensioned slabs were higherthan those of the steel pretensioned

OBJECTIVESThe research program reported in

this paper was designed to study thesuitability of using CFRP (LeadlineTM)rods in pretensioned concrete beamsand to study the long-term behavior ofCFRP pretensioned beams under sustained loading conditions. The sustained load was chosen to enable thestudy of both cracked and uncrackedCFRP prestressed beams.

The primary objectives of the research program were to: Study, experimentally, the long-

term behavior of CFRP pretensioned beams;

Develop an analytical model forlong-term behavior of CFRP pretensioned beams;

Validate the analytical modelagainst experimental results;

Steel Tendons

HA

CFRP Tendons

ItIiIIIIIH:I-[IHB

cZEMK4

ILI-IIIIlIIIIIlIIILLI

A-A

+Ni,o j77

B-B

STEEL

Ma

MKI

MKIA

:

MK3

MK4

.z.

Description

8mm CFRP Rod

13mm Steel Strand

8mm Steel Rebar

Stem Steel Sunup

WWF 152x152 MWI8.7xMWI8.7

Fig. 1. Overall geometry and reinforcement layout of pretensioned beams.

Steel Anchor Plate

slabs. Mathys and Taerw&2 attributedthis behavior to the lower elastic modulus and higher relaxation characteristics of the AFRP rods.

The authors of this paper havefound only limited information on thelong-term behavior of beams prestressed with CFRP rods. This paperaims at increasing the knowledge inthis area to aid in the development ofappropriate design procedures.

March-April 2003 99

Compare the long-term behavior ofCFRP pretensioned beams to that ofsteel pretensioned beams.

EXPERIMENTAL PROGRAMThe experimental program con

sisted of long-term tests on four concrete T-beams pretensioned with either CFRP rods or conventional steelprestressing strands. Fig. I shows theoverall geometry and cross section ofthe beams.

Three of the pretensioned beamswere prestressed with 8 mm (0.31 in.)diameter CFRP rods and the fourthwith 13 mm (0.51 in.) diameter Grade1860 MPa (270 ksi) prestressing steelstrands. CFRP rods having a fiber volume ratio of 65 percent, spiral indentations, and a guaranteed strength of104 kN (23.4 kips) were used. Thespecified modulus of elasticity was

Fig. 2. Prestressingsetup.

147 GPa (21,300 ksi) and the matrixwas an epoxy resin.

The CFRP rods were arranged intwo layers of two rods each, while thetwo steel strands were in one layer(see Fig. 1). For each configuration,the centroid of the reinforcement layers was at the same depth. The prestress force was varied in the CFRPprestressed beams so as to study thelong-term behavior of both fully andpartially prestressed beams.

Partial prestressing was achieved bylowering the jacking stress. The firstCFRP pretensioned beam was prestressed to 50 percent of the manufacturer s guaranteed tensile strength ofthe rods, while the second wasstressed to 70 percent. In the thirdbeam, the rods in the top layer werestressed to 70 percent and those in thebottom layer were stressed to 50 percent of the manufacturers guaranteed

tensile strength. The beam pretensioned with conventional steel strandswas prestressed to 55 percent of theultimate tensile strength of the strands.

Table 1 summarizes the experimental program and also includes the totaljacking force per beam. The beam designation includes the prestressing reinforcement type (CFRP or steel) followed by a number representing theprestress level.

The pretensioned concrete beamswere fabricated at Pre-Con Ltd., a localprecast plant. The concrete was designed to have a 28-day compressivestrength of 40 MPa (5800 psi), a 3-day(at prestress release) compressivestrength of 30 MPa (4350 psi), and amaximum aggregate size of 16 mm (8in.). After casting, the beams weresteam-cured for about 3 days.

Fig. I shows the reinforcement details of the prestressed concretebeams. The prestressed beams werereinforced for shear using two-legged8 mm (0.31 in.) diameter steel stirrups,spaced at 100 mm (4 in.) throughoutthe length of the beam. The stirrupswere designed to ensure that flexuralfailure occurred before shear or bondfailure.

Prestressing of the concrete beamswas performed in a prestressing bedthat allowed two beams to be stressedand cast simultaneously. Fig. 2 showsa photograph of the prestressingsetup. The CFRP rods were tensionedusing conventional stressing techniques by connecting two CFRP rodsto a steel strand through a steel coupler (see Fig. 3).

Prior to prestressing, wedge-typeanchors supplied by the CFRP rodmanufacturer were preset at the endsof the CFRP rods (see Fig. 4). Thewedge-type anchors consisted of anouter stainless steel barrel with twoconical holes and a threaded surface,two two-piece wedges, and two thin-walled aluminum sleeves.

To preset the CFRP anchors, thealuminum sleeves were placed on therods and then the wedges and outersteel barrel were assembled onto therods. The assembled anchorage systemwas placed into a presetting steel boxtogether with a hydraulic jack. The hydraulic jack pushed the wedges intothe outer barrel.

Table 1. Summary of experimental program.Description Prestress level (percent) Jacking force (kN)4-8 CFRP 50 2084-80 CFRP 70

2914-8FRP

- 70/50- - - 250

2-130 Steel 55 202

[ BeamCFRP-70

LCFRP 70/50- Steel-55* The top CFRP rods are stressed to 70 percent and the bottom to 50 percent of ultimate strength.Note: 1.0 kN = 0.225 kips.

100 PCI JOURNAL

After presetting the anchors on tworods, each anchor was screwed intoone end of a steel coupler, while asteel strand was connected to the otherend with a conventional steel chuck.The steel strand was then anchored tothe dead end abutment of the prestressing bed. At the jacking end, a 30tonne (33 ton) hydraulic jack, placedbetween a steel chuck and jackingchair, was used to stress the prestressing strands.

Load cells were incorporated intothe prestressing setup at the dead endto monitor prestress levels in the prestressing rods, while electrical resistance strain gauges, attached to therods at predetermined locations, wereused to obtain a strain profile duringprestress release.

The concrete beams were cast together with twenty 305 x 152 mm (12x 6 in.) cylinders and six 152 x 152 x762 mm (6 x 6 x 30 in.) prisms. Theconcrete forms were stripped after thecuring period and the prestress forcewas gradually released by simultaneous sawcutting of wires of the steelstrands at both ends. During prestressrelease, the strain gauges attached tothe CFRP prestressing rods were usedto measure the transfer length of thebeam. The transfer length results arepublished in Reference 13.

nstrumentation and TestingThe prestressed beams were sub

jected to sustained loading at roomtemperature about two years after casting. The beams were simply supportedon concrete blocks and subjected tothird-point loading in a load frame(see Figs. 5 and 6).

The sustained load was applied bystressing two 13 mm (0.51 in.) diameter steel strands vertically between ahollow steel section and two steelchannels connected back to back. Thesteel channels were anchored to thelaboratory strong floor with anchorbolts. The steel strands were stressedby using two hydraulic jacks fed fromthe same hand pump.

Two steel helical spring coils wereincorporated on top of the hollow steelsection to maintain constant load,while load cells were incorporated underneath the steel channels to monitorthe loads applied to the beams.

Fig. 3. Prestressing setup showing steel coupler.

A total sustained load of about 53kN (12 kips) was applied to eachbeam. This load was chosen to inducecracking in all the pretensioned beamsexcept Beam CFRP-70. The hollowsteel section reacted on a spreaderloading beam that in turn applied concentrated loads at third points of theprestressed beams (see Fig. 5).

During sustained load testing, thevertical deflections of the beams atmidspan and under the load pointswere measured with mechanical dialgauges [with an accuracy of 0.01 mm(0.0004 in.)] mounted on immobilizedstands. The strain gauges on the pre

stressing rods, at midspan of thebeams, were monitored during the sustained load tests to determine prestresslosses with time.

DISCUSSION OFTEST RESULTS

During initial static loading to thesustained load level, the load-deflection responses at midspan and underthe load points were monitored. Time-dependent changes in deflection andprestressing reinforcement strains,under sustained loading, were monitored for a period of 651 days.

ie

Fig. 4. CFRP wedge-type anchor.

March-April 2003 101

A computer program, Time-Dependent Analysis of Concrete (TDAC),was developed and used to predict thetime-dependent behavior of the beams.The predicted and experimental resultsof the pretensioned beams were compared.

Static Load-Deflection ResponseFig. 7 presents the load-deflection

response curves of the four beams during initial static loading and showsthat the precracking stiffness of theCFRP prestressed beams is independent of the amount of prestress force.

Furthermore, the precracking stiffnessof the steel prestressed beam isslightly higher than that of the CFRPprestressed beams. After cracking,however, a significant difference inthe slope of the load-deflection response is observed.

Beam CFRP-50 and Beam Steel-55were designed for about the sameamount of jacking force to enable direct comparison of the behavior ofCFRP and steel pretensioned beams.A comparison of Beam CFRP-50 andBeam Steel-55 shows that the crackingload is the same, but the post-crackingslope of the load-deflection curve ofBeam CFRP-50 appears slightlygreater than that of Beam Steel-55,suggesting a higher post-crackingstiffness of Beam CFRP-50. Thishigher stiffness is likely due to the difference in elastic modulus of the reinforcements and variability between thespecimens (e.g., slightly differentcracking patterns, concrete compressive strength, and concrete tensilestrength).

From the load-deflection responsecurves presented in Fig. 7, the steelprestressed beam, Beam Steel-55, exhibits the highest deflection [13.3 mm(0.52 in.)] while the uncracked BeamCFRP-70 exhibits the lowest deflection of 6.8 mm (0.27 in.). In general,the deflection of CFRP prestressedbeams is dependent on the level ofprestress force.

Beam CFRP-70, which was jackedto 70 percent of the manufacturersguaranteed strength, exhibited thelowest deflection while Beam CFRP50, jacked to 50 percent, showed thehighest midspan deflection. BeamCFRP-70/50 showed a midspan deflection of 9.7 mm (0.38 in.).

Time-DependentDeflection Behavior

Fig. 8 presents the time-dependentvertical midspan deflection of the prestressed beams. The general trend is ahigh rate of increase of deflection forthe early period after loading followedby a more gradual rate of increase ofdeflection.

This behavior was exhibited by allbeams, irrespective of the type of prestressing reinforcement or amount of

Side View

Section

Fig. 5. Schematic showing loading frame for long-term tests.

Fig. 6. Overview of loading frame for long-term tests.

102 PCI JOURNAL

jacking force. Fig. 8 shows periodicchanges in the rate of change of vertical deflection due to changes of ambient relative humidity and temperaturein the laboratory.

Table 2 presents the instantaneousand long-term midspan deflection, andthe ratio of long-term to instantaneousmidspan deflection of the beams. Theuncracked CFRP beam, Beam CFRP70, had the highest ratio of long-termto instantaneous deflection of 1,47,whereas Beam CFRP-50 had the lowest ratio of 1.15.

Beam Steel-55, which had a comparable jacking force to Beam CFRP-50and a comparable cracking load, suggesting a comparable level of prestresslosses at the time of sustained loading,had a higher ratio of long-term to instantaneous deflection of 1.26. Thisindicates that steel pretensioned beamsmay exhibit higher time-dependent deflection than a comparable beam pretensioned with CFRP reinforcement.More tests are required to confirm thisobservation.

The ratio of long-term to instantaneous midspan deflection of BeamCFRP-70/50 was 1.29. In general, theCFRP prestressed beams show a comparable or better performance in comparison to the steel prestressed beam.The ratio of long-term to instantaneous deflection of the CFRP beamsdepends on the level of prestress, withthe ratio increasing as the prestresslevel increases.

Time-Dependent PrestressingReinforcement Strain

Fig. 9 presents the time-dependentchange in the prestressing reinforcement strain of the pretensioned beams.The strains were measured with electrical resistance strain gauges attachedto the prestressing reinforcement atmidspan. The two bottom rods andone top rod were instrumented withstrain gauges in Beam CFRP-50 andBeam CFRP-70, while only one rodeach in the top and bottom were instrumented in Beam CFRP-70/50.

In the steel pretensioned beam, onewire in each steel strand was instrumented with strain gauges. The prestressing reinforcement strain readings presented in Fig. 9 are averages

of the instrumented bottom CFRProds and represent changes in thestrain from the instant of sustainedload application.

Fig. 9 shows that prestressing reinforcement strains in the crackedbeams (Beam CFRP-50, Beam Steel-55, and Beam CFRP-70150) progressively decreased with time, while thestrain in the uncracked beam (BeamCFRP-70) remained, more or less, unchanged. The cracked pretensionedbeams had higher compressive

pretensioned beams.

stresses in the top of the beam due to areduced area of concrete in compression, which led to increased creepstrains and deformations. BeamCFRP-50 exhibited the highest prestressing reinforcement strain changeunder the sustained load, while BeamCFRP-70 exhibited the lowest change(almost zero) in strain.

A comparison of the time-dependent change in prestressing strain inBeam CFRP-50 and Beam Steel-55shows that the instantaneous change in

60

50

40

;300

20

10

00 2 4 6 8 10 12 14

Vertical Deflection [mm] CFRP-50 CFRP-70 Steel-55 CFRP-70/50

Fig. 7. Comparison of midspan deflection of

1816

0 100 200 300 400 500 600 700Time [Days]

[ZFRP..50 CFRP-70 Steel-S 5 CFRP-50170 I

Fig. 8. Comparison of time-dependent vertical midspan deflection of beams.

March-April 2003 103

0.0020.00180.00160.00140.0012

0.0010.0008

0.00040.0002

strain in the prestressing reinforcement was higher for Beam CFRP-50.This difference was due to the fact thatthe eccentricity of the prestressing reinforcement in Beam Steel-55 wassmaller than the lower rods in BeamCFRP-50 (see Fig. 1). Also, it is notcertain if the strain in Beam Steel-55is representative of the average strainin the steel strand as only one wire inthe seven-wire strand was instrumented with strain gauges.

Comparison of Experimental andAnalytical Results

The computer program TDAC wasdeveloped and used to model the time-dependent behavior of the prestressedbeams. Since the pretensioned beamswere monitored only under the sustained loading, the analytical resultsare limited to this period. Both theACI Committee 209 recommendations and CEB Model Code5 wereused in the time-dependent model to

calculate the creep coefficient andshrinkage strains.

The input data for the computer program consisted of the cross-sectionalproperties and reinforcement scheduleof the beams (see Fig. 1), the 28-daycompressive strength (f), and thejacking stress of the prestressing reinforcement (f) (see Table 3). The compressive strength of concrete was notdetermined at 28 days for the prestressed beams. Therefore, the 28-daycompressive strength of concrete wasdetermined by extrapolation using theACT Committee 20914 or CEB ModelCode15 provisions.

The extrapolated compressivestrengths of concrete are presented inTable 3. The relaxation of CFRP rodshas been reported by Yagi et al.6 to benegligible. For time-dependent analysis, relaxation of CFRP reinforcementwas, therefore, assumed to be zero,while the relaxation of prestressingsteel was obtained from various equations (ACT Committee 209 and CEBModel Code5).

Table 4 summarizes the comparisonof analytical and measured results.The predicted time-dependent changein deflection by using the CEB ModelCode is slightly higher than the predicted deflection by using the ACTrecommendations. The ratio of predicted to measured deflection forBeam CFRP-50 is 2.0 when the CEBCode is used to calculate creep andshrinkage and 1.68 when the ACTCommittee 209 recommendations areemployed (see Table 4). The changesin strain at the final age of 651 daysand ratio of the predicted to experimentally measured prestressing reinforcement strain change are also presented in Table 4.

The predicted results from the computer program, TDAC, in most casescorrelate with the experimental results,with the ACT Committee 209 recommendations giving a better correlationwith experimental results than theCEB Model Code. Table 4 shows thatthe average ratio of predicted to experimental values for the vertical midspandeflection is 1.42 (range 1.12 to 2.00)for the CEB Model Code and 1.28(range 0.98 to 1.68) for the ACT recommendations.

These ratios were obtained from the

0.0006

0 100 200 300 400 500 600 700Time [DaysJ

CFRP-50 CFRP-70 Steel-55 CFRP-50/70

Fig. 9. Comparison of time-dependent prestressing reinforcement strain.

5

4

00 100 200 300 400

Time [Days]500 600 700

H Experimental ACI CEB I

Fig. 10. Experimental and theoretical vertical midspan deflection of CFRP-70/50.

104 PCI JOURNAL

deflection readings at 651 days afterapplication of sustained load. The experimental results fluctuated with thechanges in ambient relative humidityand temperature; therefore, there areinstances where the correlation between the analytical and experimentalresults is much better and would resultin lower ratios.

Fig. 10 shows the measured time-dependent deflection of Beam CFRP50/70 compared with the analytical results. If fluctuations in the experimentalcurve due to temperature and humidityare removed, then the general trend ofthe experimental curve compares adequately with the analytical results.

The predicted time-dependentchange in vertical midspan deflectionof Beam CFRP-70 is within 15 and 2percent of the experimentally measured change in midspan deflectionwhen the CEB Model Code and ACTCommittee 209 recommendations, respectively, are used in the analyticalmodel. This result is better than thoseobtained for the other beams becauseBeam CFRP-70 remained uncracked,thus minimizing the dependence ofvertical deflection on the extrapolatedtensile strength of concrete.

Table 4 shows that Beam CFRP-50exhibited the highest ratio of predicted to experimental vertical deflections. A sensitivity analysis was carried out to evaluate the effect of inputparameters (compressive strength ofconcrete, jacking stress of prestressing reinforcement, and applied sustained load) on the vertical deflection.Table 5 presents the results of the sensitivity analysis.

The 28-day compressive strength ofconcrete was increased to evaluate itseffect on the time-dependent verticaldeflection under sustained load. A 25percent increase in concrete strength,from 40 to 50 MPa (5800 to 7250 psi),resulted in a 9.5 percent decrease invertical deflection.

The jacking stress in the prestressing reinforcements was increased inrelation to the experimentally measured stress during prestressing (seeTable 3). A 10 percent increase in thejacking stress led to a 1 percent reduction in the time-dependent vertical deflection. The applied sustained loadwas also reduced by about 8.6 percent

[from 52.5 to 48 kN (11.8 kips to10.8 kips)], and this reduction led to adecrease of time-dependent verticaldeflection by 11.2 percent.

The sensitivity analysis of time-dependent change in vertical deflectionhas shown that the jacking stress inthe prestressing reinforcement has anegligible effect on the predicted vertical deflection. Even though the compressive strength of concrete is shownto affect the time-dependent change invertical deflection, it is not likely the

cause of the departure of the predicteddeflection from the experimentallymeasured value.

Even with a concrete compressivestrength of 50 MPa (7250 psi), whichis greater than the experimentally determined strength at 46 days of 43MPa (6250 psi), the time-dependentchange in vertical deflection is 2.49mm (0.098 in.), which is still greaterthan the experimental time-dependentchange of 1.64 mm (0.065 in.).

During the static load to the sus

Table 2. Deflection results of pretensioned beams.Applied load instantaneous Long-term

.. (kN) deflection (mm) deflection (mm) RatioBeam (2) (3) (4) (4)1)3)

CFRP-50- 52.5

..

12.) 14.0 1.15CFRP-70 52:5 iLL. 6.8 10.0 1.47

CFRP-50/70 53.8 9.7 12.5 1.29Steel-55 53.4 13.3 16.7 1.26

Note: 1.OkN =0.225 kips: 1.0 mm =0.0394 in.

Table 3. Input data for computer program.Concrete strength Age at Applied

Time (t) (MPa) loading loadBeam (days) f(t) (MPa) (days) (kN)

CFRP-50 46 43 40 Top = 1035 753 52.5Bottom = 1165

CFRP-70 39 37 Top = 1540 747 52.5Bottom = 1565

Steel-55 28 :. 36 36 1080 726 53.4CFRP-70/50 34 36 33 Top = 1535 742 53.8

Bottom = 1325Note: 1.0 MPu 145 psi; 1.0 kN = 0.225 kips.

Table 4. Comparison of analytical and experimental results.Deflection (mm) Strain x l0

CEB ACT Exp. (2) (3) CEB ACI Exp. (5) (6)Beam (2) (3W) (4) (41 (4) (5) (6) (7) (7) (7)

CFRP-50 3.36 2.75 1.64 2.00 1.68 245 199 171 1.10CFRP-70 3.60J3.12 3.17 1.14 0.98 260 251 37.2

Steel-55 . 2.994 3.22 2.65 1.12 1.22 77 185 313

CFRP-70/50 3.78 3.25 2.65 1.43 1.23 245 242 175 1.40 1.38Average 1.42 1.28 1.40 1.27

Note: t.0 mm = 0.0394 in.

Table 5. Sensitivity analysis of vertical deflection.Concrete Vertical Jacking Vertical Applied Vertical Relative Verticalstrength deflection stress deflection load deflection humidity deflection(MPa) (mm) (percent) (mm)

(kN) (mm) (percent) (mm)40

2.75 0 2.75 52.5 2.75 L 40 2.7545 2.61

- +5 2.74 50 - 2.56- 60 2.40

50 2.49 +10 2.72 48 2.44 90 . 1.87Note: 1.OMPa= 145 psi; 1.0 mm=0.0394 in.; 1.OkN=0.225 kips.

March-April 2003 105

tamed load level, the load cells used tomonitor the applied load had an accuracy of 0.17 kN (0.04 kips). The accuracy of load cells is 0.3 percent of thetotal applied load of 52.5 kN(11.8 kips). It is, therefore, inconceivable that the departure observed fromthe experimental time-dependentchange is due to experimental error inmonitoring the applied load.

The sensitivity analysis has revealedthat an increase in the relative humidity from 40 to 90 percent resulted in adecrease in the predicted vertical deflection of about 30 percent. The effect of relative humidity is significantand contributes to the observed departures between measured and predictedvertical deflections. For the analyticalpredictions reported in this paper, arelative humidity of 40 percent wasassumed. The observed outside relative humidity during the test period,however, varied between 40 and 90percent.

The only other reason for the departure of predicted to the experimentalvertical deflection is the accuracy ofthe deflection measurement. BeamCFRP-50 was located on the outsideof the beam row, and the bases of thedial gauges were inadvertently moveda number of times during the long-term test. Even though efforts were always made to obtain continuity of thedeflection results, the error introducedis not quantifiable. This is the mostlikely reason for the greater departureof predicted to the experimental deflections in Beam CFRP-50.

CONCLUSIONSThe experimental results of a test

program designed to investigate thelong-term behavior of CFRP prestressed concrete beams has been presented and compared with the resultsfrom an analytical time-dependentcomputer model.

Based on the results of this investigation, the following conclusions canbe drawn:

1. The CFRP prestressed beamsshow a comparable or better long-termperformance in comparison to steelprestressed beams.

2. The ratio of long-term to instantaneous deflection was found to increasewith the level of prestress force.

3. The strains in the prestressing reinforcement decreased with time whenthe concrete section was cracked andremained unchanged when the sectionwas uncracked.

4. The average ratio of predicted toexperimental deflection was 1.28when the ACT recommendations wereused in analytical modeling and 1.42when the CEB Model Code was used.

5. For design purposes, establishedmethods for predicting the time-dependent behavior of steel prestressedconcrete can be used for predicting thebehavior of CFRP prestressed beamsas long as:

Modifications are made to accountfor the lower modulus of theCFRP, and

The relaxation of the tendons is assumed to be zero.

RECOMMENDATIONSThe relaxation properties of FRP re

inforcements are not very well documented. A relaxation of zero was usedfor the CFRP rods in the analyticalmodel for time-dependent analysis. Itis recommended that more testing onthe relaxation properties of FRP reinforcement be conducted and the effectof concrete creep and shrinkage on therelaxation of various FRP reinforcement be established.

The analytical investigation of thetime-dependent behavior of CFRPpretensioned beams did not include aparametric study using TDAC. It isrecommended that such a study becarried out to establish the effects ofenvironmental conditions and concreteproperties on the time-dependent behavior of FRP prestressed beams.

ACKNOWLEDGMENTThe authors are members of the In

telligent Sensing for Innovative Structures Network (ISIS Canada) and wishto acknowledge the support of theNetworks of Excellence Program ofthe Government of Canada and theNatural Sciences and Engineering Research Council (NSERC). The supportof Mitsubishi Chemical Corporationand Pre-Con Limited is greatly appreciated. Many thanks are owed toQueens University for their generoussupport during this research project.

The authors would also like to thankthe PCI JOURNAL reviewers for theirinsightful and constructive comments.

106 PCI JOURNAL

REFERENCES

1. Bedard, C., Composite Reinforcing Bars: Assessing TheirUse in Construction, Concrete international, V. 14, No. 1,January 1992, pp. 55-59.

2. Hassan, T., and Rizkalla, S., Flexural Strengthening of Prestressed Bridge Slabs with FRP Systems, PCI JOURNAL, V.47, No. 1, January-February 2002, pp. 76-93.

3. McKay, K. S., and Erki, M. A., Flexural Behaviour of Concrete Beams Pretensioned with Aramid Fibre Reinforced Plastic Tendons, Canadian Journal of Civil Engineering, V. 20,No. 4, August 1993, pp. 688-695.

4. Fam, A. Z., Rizkalla, S. H., and Tadros, G., Behaviour ofCFRP for Prestressing and Shear Reinforcement of ConcreteHighway Bridges, ACI Structural Journal, V. 94, No. 1, January-February 1997, pp. 77-86.

5. Naaman, A. E., et al., Partially Prestressed Beams with Carbon Fibre Composite Strands: Preliminary Tests Evaluation,Fiber-Reinforced-Plastic-Reinforcement for Concrete Structures, International Symposium (Editors: A. Nanni and C. W.Dolan), ACI Publication SP-138, 1993.

6. Abdeirabman, A. A., Tadros, G., and Rizkalla, S. H., TestModel for the First Canadian Smart Highway Bridge, ACIStructural Journal, V. 92, No. 4, July-August 1995, pp. 451-458.

7. Rizkalla, S. H., and Tadros, G., A Smart Highway Bridge inCanada, Concrete International, V. 16, No. 6, June 1994, pp.42-44.

8. Tsuji, Y., Kanda, M., and Tamura, T., Application of FRPMaterials to Prestressed Concrete Bridges and Other Structuresin Japan, PCI JOURNAL, V. 38, No. 4, July-August 1993,pp. 50-58.

9. Taerwe, L. R., Lambotte, H., and Miesseler, H. J., Loading

Tests on Concrete Beams Prestressed with Glass Fiber Tendons, PCI JOURNAL, V. 37, No. 4, July-August 1992, pp.84-97.

10. Erki, M. A., and Rizkalla, S. H., FRP Reinforcement for Concrete Structures, Concrete International, V. 15, No. 6, June1993, pp. 48-53.

11. Currier, J. B., and Dolan, C. W., Deformation of PrestressedConcrete Beams with FRP Tendons, Draft Final Report forU.S. Army Corps of Engineers, Waterways Experiment Station, Vicksburg, MS, 1995, p. 114.

12. Mathys, S., and Taerwe, L., Long-Term Behaviour of Concrete Slabs Pretensioned with AFRP or Prestressing Steel,Durability of Fibre Reinforced Polymer (FRP) Composites forConstruction (Editors: B. Benmokrane and H. Rahman), 1998,pp. 95-106.

13. Soudki, K. A., Green, M. F., and Lee, C., Transfer Length ofCarbon Fiber Rods in Precast Pretensioned Concrete Beams,PCI JOURNAL, V. 42, No. 5, September-October 1997,pp. 78-87.

14. ACT Committee 209, Prediction of Creep, Shrinkage, andTemperature Effects in Concrete Structures (ACI 209R-82),American Concrete Institute, Farmington Hills, MI, SP 76-10,1982.

15. CEB, CEB-FIP Model Code 1990, Comit Euro-Internationaldu Bton, Thomas Telford Services Ltd., London, England,1993.

16. Yagi, K., Hoshijima, T., Ando, T., and Tanaka, T., The Durability Tests of Carbon Fiber Reinforced Plastic Rod Producedby Pultrusion Method, Proceedings of the international Conference on Engineering Materials, Ottawa, Canada, June 8-11,1997 (Editors: Al-Manaseer et al.), pp. 327-340.

March-April 2003 107