Fatigue Tests of 27-Year-OldPrestressed Concrete BridgeBox Beams

Chetana RaoStructural EngineerGM2 AssociatesGlastonbury, Connecticut(Formerly, Graduate Student,Department of Civil andEnvironmental Engineering,University of Connecticut)

Two precast, prestressed concrete box beams, 36 in. wide x 27 in.deep x 56 ft long (914 x 686 mm x 17.1 m), were removed from adeteriorated multibeam bridge and subjected to fatigue testing.Visually, the beams appeared in generally good condition but showedsigns of water leaking through the longitudinal shear keys and somecorrosion of reinforcement. The beams were purposely precrackedprior to fatigue loading and had periodic overloads applied. Onebeam, cycled to a nominal bottom tension stress level of 6-\/i psi(0.50[j MPa) and a stress range of 0.06 retained excellentperformance after 1,500,000 cycles. Cycling to a nominal bottomtension stress level of 9[j psi (0.75Tj? MPa) and a stress range of0.11f, significantly reduced the strength of the other beam andcaused fatigue failure of strand wires after 145,000 cycles.

Gregory C. Frantz, Ph.D., P.E.ProfessorDepartment of Civil andEnvironmental EngineeringUniversity of ConnecticutStorrs, Connecticut

I n 1987, the deteriorated WalnutStreet Bridge (see Fig. 1) in EastHartford, Connecticut, which was

built in 1960, was replaced. The simpie span, multibeam bridge consistedof 13 precast, prestressed concrete boxbeams 36 in. wide x 27 in. deep x 56 ftlong (914 x 686 mm x 17.1 m) (seeFigs. 2 and 3). Four beams wereseverely deteriorated and had to be demolished in-place. Several of the remaining beams were saved for latertesting.

Previous papers2 discussed thestatic behavior of two of the beams

and the chloride ion content and distribution in five of the beams. This article discusses the fatigue behavior oftwo other beams from this samebridge. Ref. 3 provides a full discussion of this portion of the researchprogram.

Fatigue failure in prestressed concrete members can occur due to failure of the concrete from flexural compression, diagonal tension or shear,failure of strands, failure of bond, orfailure of end anchorages of posttensioned beams. When uncracked,the concrete, which typically remains

74 PCI JOURNAL

in compression, bears a large portionof the stress reversals and with thehigh limits of the stress range capacityof concrete, fatigue life can be assumed to be infinite.

Prior to flexural cracking, the stressrange in the strands is very small andfatigue need not be a concern as longas the concrete has not cracked. Because it is usually not cracked duringservice loading, prestressed concretegenerally resists fatigue effects betterthan reinforced concrete. However, forloading above the cracking capacity,the stress range in the strands is muchlarger because the strands must carryall of the tension.

The AASHTO Specifications4limitthe maximum concrete tensile stress to6,.fj psi (0.507J MPa), slightlybelow the concrete’s crackingstrength. However, if a beam is loadedto cracking by even one (unintentional) overweight truck, the flexuralcracks thereafter will reopen when thebottom fiber concrete compressionstress is decreased to zero. From thispoint on, the strands will have steepincreases in stress levels with increasing loads above the decompressionload. Thus, the stress range in thestrands can be much larger than originally anticipated even for the usualservice load ranges.

The AASHTO Specifications giveno particular requirements for fatiguein prestressed concrete bridge members. If the concrete tensile stressesexceed 37J psi (0.25 7[I MPa)under a realistic estimate of serviceloads, ACT Committee 2l5 recommends that the stress range in prestressed reinforcement shall not exceed 0.06f based on a crackedsection analysis, for minimum prestress levels of 60 percent of the strandtensile strength.

LITERATURE REVIEWShenoy and Frantz”6 performed

static tests on two beams that werecompanion beams to those tested inthis study. Measured prestress losseswere about 50 percent of the predictedlosses. The beams, designed by themore conservative 1957 AASHOSpecifications,7easily carried the factored static load levels required by

current specifications and reached thestrength predicted by a strain compatibility analysis.

Murray and Frantz28 did chloridetesting on five of the box beams fromthe Walnut Street Bridge. Constructedwithout a waterproofing membraneunderneath the bituminous wearingsurface, the beams had chloride levelsas high as two to three times the corrosion threshold level with active corrosion occurring.

Leon et al.9 subjected four 20-year-old prestressed bridge girders to fatigue to study techniques to repair impact damage. Girders repaired withadditional post-tensioning were unableto regain their predamaged stiffness.The tests suggested that the originalbeams were in excellent condition.

Knudsen and Eney’° tested a newpretensioned, precracked concrete boxbeam. Approximately 1,300,000 cycles of HS2O loading had only minoreffects, but 100,000 cycles with 54percent overload increased deflectionsby 30 percent.

Russell and Burns” studied threenew pretensioned concrete girderswith draped or debonded strands. Thebeams were precracked during the firststatic test and two of the beams werealso precracked with web shearcracks. The beams with shear crackswere tested to over 225,000 cycles andthe other beam to about 700,000 cy

des. The girders performed very welland did not fail during the fatiguetests.

Rabbat et al.’2 conducted fatiguetests of new Type II AASHTO PCIgirders. Three girders were tested with5 million cycles with a maximumstress of zero in the bottom fibers andthen statically loaded to their full ultimate strength and showed no fatiguedistress. Three precracked girderswere loaded to a maximum stress of67J psi (0.507fj MPa) in the bottom fiber and showed fatigue distressbetween 3.2 and 3.8 million cycles.

Overman, Breen, and Frank’3 studied the fatigue behavior of 11 new pretensioned concrete girders with Un-shored cast-in-place slabs andconcluded that even a small number ofcycles of modest overloads can causehigh stress ranges and sharply reducefatigue life. They concluded thatadopting 6J psi (0.507fj MPa) asthe maximum tensile stress in the bottom fiber was not conservative enoughwithout being supplemented by welldistributed confined reinforcement.They recommended that a maximumtensile stress of 3 psi (0.25 JF?MPa) would be sufficiently conservative to prevent fatigue damage.

Roller et al.’4 tested two prestressed,high strength concrete girders, onewith long-term loading and one withfatigue loading. They reported lower

Fig. 1. Panoramic view of Walnut Street Bridge.

September-October 1996 75

Fig. 2. Cross section of bridge.

measured prestress losses than predicted. One full depth crack, formedduring girder fabrication, was presentnear midspan during the fatigue testing. After 5 million cycles of fatigueloading to 6J7 psi (0.50[j MPa),the beam still satisfied the AASHTOstrength requirements.

DESCRIPTION OF BEAMSThe two beams tested in this study

were Beam 10, which was next to twovery badly deteriorated Beams 11 and12 that had to be demolished in placeprior to removal, and Beam 8, whichwas next to the center beam (see Fig.2). The BI-36 box beams were reinforced with 22 7/ in. (11.1 mm) diameter Grade 250 strands (see Fig. 3).

Beam 10 had large patches of whitestains from salt deposits on the sides.On one side, it had two 1 ft (300 mm)long horizontal cracks at strand levelindicating corrosion of the strands.Beam 8 had very little surface stainingfrom water and salt on its sides andbottom. There were a few rust stainedportions indicating corrosion of somestirrups or strands. The beam also hadtwo large vertical cracks extendingdown from the top about 6 ft (1.8 m)from one end, probably caused duringremoval of the beam from the bridgeor during later handling. The beamshad not experienced any flexuralcracking and otherwise seemed to bein good condition.

Fig. 3. Cross section of box beam.

Original design plans were notavailable. Current traffic counts indicated approximately 5500 vehicles perday on this bridge. Cores indicated aconcrete compressive strength of approximately 6700 psi (46 MPa), closeto the value from the previous work.’The properties of the strands were assumed to be the same as in the previous study.

EXPERIMENTAL PROGRAMDetails of the test setup, instrumen

tation, definition of service load andtest procedure are given below.

Test Setup

Figs. 4 and 5 show the test setup.The 56 ft (17.1 m) long test beamswere simply supported on a 54 ft (16.5

2” Asphaltwear i ngsurface

Iy

19’—6”

1 ft. 12 in. 305mm

(\J

76 PCI JOURNAL

m) span and subjected to two pointloads 11 ft (3.4 m) apart centeredabout midspan. One actuator appliedload to a steel beam that distributedthe load into four loads applied overthe beam webs.

Supports under the loading beamand at the test beam ends were carefully designed to permit rotation andlongitudinal movement while preventing excessive longitudinal movement.A 22 ft (6.7 m) high steel frame carried the actuator reaction into thestructural strong floor.

Instrumentation

Near midspan, the bottom cover onfive strands was carefully removedwithout nicking the strands. Seven toten SR-4 strain gauges (3.18 mmgauge length) were applied with epoxyto strand wires. A linear variable differential transducer (LVDT) measuredmidspan deflection. Dial gauges monitored any slipping of strands at thebeam end. Crack widths were measured to ±0.00 1 in. (0.025 mm) with a50-power microscope. A computercontrolled data acquisition systemrecorded strains and deflections.

All tests were performed as load-control tests. The hydraulic loadingand data collection systems werecomputer monitored and controlled

Note: 1 kip = 4.448 kN.(S) = Static loading(F) = Fatigue loading

with very reliable safety limit checksand system shutdown procedures.Therefore, cyclic testing frequentlycontinued throughout the night whenno one was present to observe thetest.

Definition of Service Load

For these tests, “service load”(equivalent to dead load plus live loadplus impact) was defined to be theload that produced a nominal bottomfiber stress, fb’ of 6 /j psi (based onan uncracked section), which occurred

at a moment of 709 kip-ft (961 kNm). The beam’s actual nominal capacity, of 1117 kip-ft (1514 kN) defined the required strength at factoredloads, M.

These two conditions set, for testingpurposes, the dead load moment, MD,and live load moment, MLL+I , asfollows:

MD + MLL+1 = 709 kip-ft (961 kN-m)

forfb = 6.fj psi (0.50j MPa)

1.3 (MD + l.67MLL+J) = 1117 kip-ft

(1514 kN-m) forM =

Fig. 4. Test setup. Fig. 5. Loading system.

Table 1. Load history for Beam 10.

Test identification Load cycles Load range (kips)

A 1 0to55(S)

B 2 0to55(S)

C 104,003 20 to 41 (F)

D 104.004 010 50(S)

E to T 909 014Repeat fatigue and static sequence

with 100.000 cycle increments

-

- U 1 109,014 20 to 41 (F)

.____ V - 1,109.015 Oto5O(S) —_____

W -- - 1,309,015 20 to 41 (F)

X 1,309,016 0 to 50 (S)

Y 1509,016 20 to 41 (F)

Z 1,509,017 0 to 73.8 (S)

September-October 1996 77

Table 2. Load history for Beam 8. Test Procedure

Note: 1 kip = 4.448 kN.(S) = Static loading(F) = Fatigue loading

Fig. 6. Load vs. deflection for Beams 10 and 8.

Solving these two equations simultaneously indicated that actuatorloads of 20 and 41 kips (89 and 182kN), plus the weight of the steel loaddistribution beam plus the bridgebeam’s self weight, were required toproduce the dead load and the serviceload conditions, respectively. An actuator load of 50 kips (220 kN) produced a nominal bottom fiber stressof psi (0.75...fj MPa). Hereafter, “applied load” or “load” willmean the load applied by the actuatorwithout the 5 kip (22 kN) deadweight of the steel load distributionbeam.

These beams were originally designed using the 1957 AASHO Specifications, which had more conservativedesign and load criteria than presentspecifications. For comparison purposeswith the test loads applied in this research, when these beams were in service, the design loads required by the1992 AASHTO Specifications wouldcause a service load moment of 494kip-ft (670 kN-m). This corresponds toan actuator load of 21 hips (92 kN), andthe required strength would be 840 hip-ft (1140 kN-m). The fatigue loadingused in this research was a very severeloading for these beams.

Tables I and 2 define the load sequences for both beams. Each beamwas first tested in a static condition inapproximately 5 hip (22 kN) load increments. Loading continued to amaximum load of 50 or 55 kips (220or 240 kN), which was above thecracking load. During this initial test,the effective prestress level was determined by observing the reopening offlexural cracks as was done in the firststudy.’ Crack development, strains,and deflections were monitored. Asecond static test was then performed.A multiple sequence of cyclic testingfollowed by a static test (to simulateoverloads) then commenced. A finalstatic test completed the program.

Beam 10 was tested first. The cyclicloading increment was for 100,000 or200,000 cycles between loads of 20 and41 hips (89 and 182 kN), which corresponded to the dead load and the service load, at a frequency of 1 Hz. Thisproduced a maximum nominal bottomfiber stress of 6fj psi (0.50fMPa). Because the beam had been purposely precracked, flexural cracks reopened prior to this upper load level.The theoretical strand stress range wasabout 15 ksi (100 MPa) or O.o6fPU’based on a cracked section analysis.Strains and deflections were monitored.

Beam 8 was cycled between loadsof 20 and 50 hips (89 and 220 kN) at afrequency of 0.66 Hz. The 50 kip (220kN) load was slightly higher than theservice load to simulate the effect offrequent overloads. This level of loadproduced a total midspan moment13.6 percent higher than the serviceload moment in Beam 8. The maximum cyclic load produced a maximum nominal bottom fiber stress of9f psi (0.75.J MPa). The theoretical strand stress range was 32 ksi(220 MPa) or 0.12f, based on acracked section analysis. BecauseBeam 8 had a more severe stress rangethan Beam 10, it had fewer cycles inthe fatigue sequences than Beam 10.

TEST RESULTSDynamic effects of the cyclic loads

were small for these beams. Althoughnot measured, the predicted effect wasless than 4 percent for Beam 10 and

Test identification

AA

BB

Load cycles

cc

Load range (kips)

DD

EE

2

19,252

19,253

73,903

FF

________Oto5O(S)

Oto SO(S)

_____

20 to 50 (F)

________

Oto5O(S)

20 to 50 (F)

73,904

GG

HH

II

JJ

KK

145,098

145,099

0 to 50 (S)

250,636

250,637

264,188

20 to 50 (F)

0 to 50 (S)

20 to 50 (F)

0 to SO(S)

20 to 50 (F)

LL 264,189 0to55.7(S)

80

70

60

500)

f 40

30

20

10

0

0 1 2 3 4 5 6 7 8

Midspan Deflections (inches)

78 PCI JOURNAL

less than 2 percent for Beam 8. Theloads, stresses, and stress ranges reported in this article were calculatedas for static loading without dynamiceffects.

Predicted strand stresses were calculated using a cracked section analysis for load stages above bottom fiberdecompression (for all load cyclesother than the first cycle). Predictedload-deflection response was based onmoment-curvature and moment-areaanalyses. Beam behavior was evaluated based on response to the staticloadings.

Figs. 6 and 7 show the load-deflection curves for some of the static testsfor the beams. During the first statictests, both beams followed the predicted response very closely. The second static tests were very similar tothe first tests except the flexuralcracks reopened at a lower load ofabout 25 kips (110 kN), which wasless than the service load. Duringthese static tests, Beams 10 and 8 hadmaximum loads of 55 and 50 kips(240 and 220 kN), respectively. Theseloads are higher than the service loadlevel of 41 kips (180 kN) but stillwithin the elastic response range ofthe strands.

Beam 10 showed little effect of fatigue during its more than 1,500,000cycles of testing. Due to lack of observed fatigue effects, the test of Beam10 was terminated after 1,509,016 cy

des of this very severe loading.Beam 8, which had a more severe

load range, definitely showed signs offatigue distress after about 145,000cycles. At that time, the sound of astrand wire breaking in Beam 8 washeard. After 261,000 cycles, six morewires were heard failing. A total of 16wires were heard breaking in Beam 8over the entire cycling. Testing continued even when no one was present toobserve the beam, and it is likely thatmore wires ruptured during that time.

Deflections

During the first sequence of fatiguecycles (approximately 1 00,000 forBeam 10 and 19,000 for Beam 8), bothbeams had a significant increase in deflections, with a greater amount in Beam8. With further cycling, the response ofBeam 10 did not change much between100,000 and 1,500,000 cycles. Beam 8,with its more severe loading, showedmuch more of an increase in deflectionwith additional cycles.

Fig. 8. Deflection vs. number of cycles for Beams 10 and 8. Fig. 9. Deflection vs. number of cycles for Beam 8.

80

70

60

,_ 50C,,

‘ 40

30

20

10

0

0 1 2 3 4 5 6

Midspan Deflections (inches)

Fig. 7. Load vs. deflection for Beam 8.

C

t=

4.0

3.5

3.0

2.5

2.0

1.5

1.0

0,5

0.0

rl)

C

0 3 6 9 12 15 18

Number of Cycles (x 100,000)

0.0 0.5 1.0 1.5 2.0 2.5 3.0

Number of Cycles (x 100,000)

September-October 1996 79

Figs. 8 and 9 show the effect of number of cycles on the static detlections ofboth beams. Deflections tended to increase with increasing cycles. At comparable load levels and number of cycles, Beam 8 had significantly higherdeflections than Beam 10.

Strand Stresses

For both beams, the effective prestress at the start of testing was takenas 153 ksi (1050 MPa), which was approximately the value determined inthese tests and in the previous tests.’

C

cj3

“Measured” strand stresses were determined using the strain due to the initialeffective prestress, the strain incrementmeasured during loading, and the measured strand stress-strain curve.

Figs. 10 and 11 show how the strandstress varied during the various statictests. Strand stresses remained in theelastic range except during the finalstatic test of Beam 10 and during thefinal fatigue series and final static testof Beam 8. During the initial pair ofstatic tests, the stress increased linearlyuntil the cracks formed or reopened.

As Fig. 11 shows, the strand

stresses in Beam 10 changed very little due to the fatigue cycles (whichalso indicates very little “drift” in thestrain gauge readings). Beam 8 hadmuch larger changes in stress levelsas a result of cycling. At each comparable load level, the stresses in Beam8 were higher than those in Beam 10.

The test data for Beam 8 showed aslight decrease in stress during the19,252 to 145,009 cycles at the higherload levels. It is not clear why this stressdecrease in Beam 8 occurred. At zeroapplied load, the strand stress increasedfor Beam 8 but not for Beam 10. Towards the end of the life of Beam 8, aswires began to rupture, the stresses increased very rapidly and were in the inelastic range at the higher loads.

Fig. 12 shows how the strand stressrange varied with the number of fatiguecycles. It was expected that the measured stress range would increase withcycles. Beam 10, loaded between 20 and41 kips (89 and 182 kN), had a measured stress range of 16.4 ksi (113 MPa)or 0.062f at the beginning of testing.By the end of 1,500,000 cycles, therange had increased only slightly to 18.5ksi (128 MPa), a 13 percent increase.

Beam 8, loaded between 20 and 50kips (89 and 222 kN), had a measuredstress range of 28.8 ksi (199 MPa) or0.1 lf at the beginning of the test. Atthe end of 264,000 cycles, the rangehad increased to 38 ksi (262 MPa), a32 percent increase. An increase in the

80

70

60

— 50‘6’

— 30

20

10

0

Predicted Curv

BeamlO mrn@ I cycle

—•--— @ I cycle —O-

@2 cycles —--- 2cycles —0-—

@909,013 —•--- @145,099 —‘-—

@1,509,017 —.-— @250,637 —0—-

@264,189 —0—-

150 175 200 225

Stress (ksi)

Fig. 10. Variation of strand stress for Beams 10 and 8.

250

250

225

200

175

150

125

100

Beam 10 Beam 8@ Okips —,--— —c’-—@ 20 kips —•—- —0-—@3Okips —•— —0-—

@40 kips —A--- —a-—50kiP

1 kip=4.448kN1 ksi = 6.895 MPa

I-c...—.--.... ..—..••••• . . .yyyyVVVVV ,. V ‘V

50

I kip 4.448 kN

I ksi = 6.895 MPa

20

‘6’C6’

10 - BeamlO Beam8 Predicted

20-41 kips —‘——

20-50 kips ° —

0— I I

0 3 6 9 12 15 18

Number of Cycles (x 100,000)

0 3 6 9 12

Number of Cycles (x 100,000)

15 18

Fig. 11. Strand stress vs. No. of cycles (Beams 10 and 8). Fig. 12. Strand stress range vs. No. of cycles (Beams 10 and 8).

80 PCI JOURNAL

stress range of about 5 ksi (35 MPa)had occurred within the first 20,000cycles and shows that a relatively lownumber of overloads can have a significant effect on a structure.

Fig. 12 also shows the stress rangefor Beam 8 between the same deadload and live load levels, 20 and 41kips (89 and 182 kN), as for Beam 10.Even at this level, Beam 8 had a significantly higher stress range thanBeam 10. Not too many cycles ofoverload were needed to cause a significant increase in the stress range atnormal load levels.

Cracking

Fig. 13 shows the crack pattern inthe middle 24 ft (7.3 m) of Beam 10.The initial static tests created a welldeveloped and similar series of flexural cracks in both beams. The averagecrack spacing was about 12 to 14 in.(300 to 350 mm), which approximately coincided with the stirrup spacing of 14 in. (350 mm). During the initial static tests, each load incrementcaused an increase in crack width orcrack length. The heavy, short horizontal lines on some cracks in Fig. 13indicate the final crack length in thefirst static test at 55 kips (240 kN).

Cycling caused some cracks to extend and some new cracks to form.The final crack length at completionof testing is also shown in Fig. 13.

Fig. 13. Crack pattern for Beam 10.

The crack widths of several major

cracks were read throughout the tests.Figs. 14 and 15 show how the width oftwo cracks, one on each beam, variedwith applied load and number of cycles. There was some variation inthese crack widths due to the continuing development of additional nearbycracks during the testing.

Crack widths increased due to applied fatigue loads, especially in Beam8. At 50 kips (222 MPa), the crackwidth in Beam 10 increased from0.009 in. (0.20 mm) in the first statictest to 0.0 15 in. (0.38 mm) after1,500,000 cycles. At the same loadlevel, the crack width in Beam 8 in-

creased from 0.011 in. (0.28 mm) inthe first static test to 0.032 in. (0.81mm), nearly a 200 percent increase, atthe end of 264,000 cycles.

During the final static test of Beam10, the cracks extended into the topflange of the beam. Shear in thesebeams was not critical and fatigue didnot cause the cracks to become inclined nor cause shear problems.

Final Static Tests

After more than 1,500,000 cycles,the response of Beam 10 during thefinal static test was very similar to itsearlier static tests (see Fig. 6). It had

C”

0

70 -

60

50 -

40

30

20

10

0

I I

0 10 20 30 40

Crack Widths ( x 0.00 1 inches)

50

Fig. 15. Crack widths vs. number of cycles (Beams 10 and 8).

1 ft. — 12 in. — 3O5u

C’.’

+ I t) t1,15’ —0”

I kip = 4.448 kNIn. = 25.4mm

35

30

25

20

15

10

5

0

C”

C

C”‘C

C

c-)

Beam 10@lcycle —•—-

@2 cycles —•—

@ 900,000 —A-—

@ 1,500,000 —•-—

Beam 8@lcycle —0——

@2 cycles —D—

@ 141,500 —a-—@250,640 —c-—@264,820 —0-—

Fig. 14. Crack widths vs. applied load (Beams 10 and 8).

0 3 6 9 12 15 18

Number of Cycles (x 100,000)

September-October 1996 81

essentially the same response up to 50kips (220 kN). The final load stagewas at 74 kips (330 kN) with a deflection of 7.4 in. (190 mm). The beamdid not show any signs of failure evenat this stage and additional loads werenot applied due to the actuator’s strokelimitations. The predicted ultimate capacity was 79 kips (350 kN) at a deflection of 23.2 in. (589 mm). Beam10 still showed the good ductility desired in bridge structures.

The final static test of Beam 8 wasdone after 264,000 cycles and after atleast 16 wires had broken. At a maximum load of 55.7 kips (248 kN), ithad deflected about 4 in. (100 mm),about 35 percent more than Beam 10at that same load. Beam 10 hadreached this same deflection at about62 kips (276 kN). Further loading wascausing additional wire failures andthe test was stopped for safety reasons.

Additional Results

Prior to testing, Beam 10 had horizontal cracking at the strand level onthe east side that had been adjacent tothe two badly deteriorated beams. Alarge piece of concrete near midspanfell off after about 650,000 cycles, exposing two strands at the edge in thelower row. The corner strand wasbadly corroded and had a few brokenwires with rusted fracture surfaces thatthe authors concluded were not causedby the tests. At about 1,000,000 cyclesof loading, two more wires in thisstrand fractured due to fatigue.

After completion of the test, moreconcrete was removed from Beam 10in this same region. Two more wiresin the corner strand had fractured dueto corrosion prior to our testing. Thestrand immediately above the strandthat had fractured appeared to be infairly good condition with little rusting. The strand in the bottom row adjacent to the fractured strand had somerusting but little or no pitting. Theclear cover to the strands was 2.75 in.(70 mm) on the side and about 1.25 in.(32 mm) on the bottom. The stirrupnear this location had a cover of 2 in.(51 mm) to the side and 1.5 in. (38mm) on the bottom. The stirrup in thisregion was extremely rusted.

Towards the north end of the beam,

on the east side, there was also a longitudinal crack along the strand. Afterremoving the concrete, the strand appeared quite rusty. The stirrup here hada side cover of only about 0.625 in. (17mm), and it seemed like the rustinghere may have initiated at the stirrup.

Beam 8 had some surface stains dueto rusting of several stirrups, whichwere found to have little concretecover. The strands of Beam 8 were visually judged to be in good conditionand had the typical amount of surfacediscoloration due to minor rusting butthere was no pitting. The decreased fatigue strength of Beam 8 could not beattributed to corrosion of the strandsbecause they seemed to be in bettercondition than those of Beam 10.

There did not appear to be any significant change in the strand slip dialgauges in Beam 10. In Beam 8, onedial gauge, which was monitoring astrand that had ruptured during thetest, changed by 0.005 in. (0.1 mm),perhaps indicating some possible slipping for that strand.

CONCLUSIONS

It should be noted that these beamshad been in service for 27 years. Also,these tests subjected the beams tohigher loads than the beams had beenoriginally designed for or had beensubjected to in actual service. Thesetests support the following conclusions:

1. Fatigue need not be a concern aslong as the beam is uncracked. This canbe inferred due to the observed lowstrand stress range prior to cracking.

2. Beam 10, precracked during initial testing and subjected to more than1,500,000 cycles of loading thatcaused a nominal bottom fiber stressof 6Jj psi (0.50f MPa), did notshow any severe fatigue problems. Itsload-deflection and load-stress responses were satisfactory and close tothe predicted results.

3. Beam 8, subjected to loading thatcaused nominal bottom fiber stressesof 9\/j psi (0.75/j? MPa), startedfailing due to rupturing of strand wiresafter about 145,000 cycles.

4. The strand stress range limit of0.06f, recommended by ACT Committee 215, was consistent with the results. Beam 10, with a stress range of

O.O62fpIL, performed very well. Beam8, with a stress range of 0.1 lfpt(, performed very poorly.

5. After 264,000 cycles, the deflections of Beam 8 were 35 percent largerthan those of Beam 10 after 1,500,000cycles. After not many cycles of overload, Beam 8 had a significant increase in the stress range and increased crack widths even for normalservice loads.

6. Fatigue problems can occur withsmall increases in load range levels.Beam 8 had only 13 percent greatertotal applied moment than Beam 10did during the cycling. But after1,500,000 cycles, Beam 10 performedbetter than Beam 8 did after only145,000 cycles.

7. Beam 10, though it performedvery well in general, did experiencepropagation in strand failure and damages induced by corrosion.

8. These particular beams could beexpected to serve satisfactorily forlonger periods of time in a bridge provided the bridge does not experiencefrequent overloads. Bridge girders ina similar condition can be evaluatedto have satisfactory fatigue strength ifno corrosion is occurring.

RECOMMENDATIONS

1. Because an unintentional overload can cause flexural cracking,strand stresses and stress rangesshould be calculated assuming acracked cross section for loads abovethe decompression load.

2. Frequent loading which createsconcrete tensile stresses of more than6,fj psi (0.50J MPa) or withstrand stress ranges above 0.06fshould be avoided.

3. Overloads can have a significanteffect on fatigue strength and their effect on prestressed concrete beamsshould be studied further.

4. Corrosion can be a serious problem when combined with fatigue. It is,therefore, absolutely essential that sufficient protection against corrosion beprovided in bridge structures.

5. Additional work should be doneto establish a correlation betweenmaintenance or rehabilitation requirements, observed deterioration, andstructural performance.

82 PCI JOURNAL

ACKNOWLEDGMENTSThis work was performed as part of

the research project “Tests on Pre

stressed Concrete Bridge Beams —

Fatigue Tests of the Bridge Beams,”sponsored by the Connecticut Department of Transportation. The authorsalso gratefully acknowledge the assis

tance provided by Blakeslee Prestress,Inc., Branford, Connecticut, for storing the beams prior to testing at theUniversity of Connecticut and for delivering and disposing of the beams.

REFERENCES1. Shenoy, C. V., and Frantz, G. C.,

“Structural Tests of 27-Year-Old Prestressed Concrete Bridge Beams,”PCI JOURNAL, V. 36, No. 5,September-October 1991, PP. 80-90.

2. Murray, V. E., and Frantz, G. C.,“Chloride Testing of 27-Year-OldPrestressed Concrete Bridge Beams,”PCI JOURNAL, V. 37, No. 5,September-October 1992, pp. 68-79.

3. Rao, C., and Frantz, G. C., “Tests onPrestressed Concrete Bridge Beams

Fatigue Tests of the BridgeBeams,” JHRAC Project Report 95-245, Department of Civil and Environmental Engineering, University ofConnecticut, Storrs, CT, December1995, 146 pp.

4. AASHTO, Standard Specificationsfor Highway Bridges, Fifteenth Edition, American Association of StateHighway and Transportation Officials, Washington, D.C., 1992.

5. ACT Committee 215, “Considerationsfor the Design of Concrete StructuresSubjected to Fatigue (ACT 215R1-R74),” American Concrete Institute,Farmington Hills, MI, 1992.

6. Shenoy, C. V., and Frantz, G. C.,“Tests on Prestressed ConcreteBridge Beams, Part 1 — StructuralTests of the Bridge Beams,” JHRACProject Report 91-198a, Departmentof Civil Engineering, University ofConnecticut, Storrs, CT, January1991, 120 pp.

7. AASHO, Standard Specifications forHighway Bridges, American Association of State Highway Officials,Washington, D.C., 1957.

8. Murray, V. E., and Frantz, G. C.,“Tests on Prestressed ConcreteBridge Beams, Part 2 ChloridePenetration in the Bridge Beams,”JHRAC Project Report 91-198b, Department of Civil Engineering, University of Connecticut, Storrs, CT,January 1991, 143 pp.

9. Leon, R. T., French, C. W., Olson,S. A., and Coggins, F. B., “Reusability of 20-Year-Old Prestressed BridgeGirders,” Proceedings of the Conference on Bridge Research in Progress,

Iowa State University, Ames, IA,September 1988.

10. Knudsen, K. E., and Eney, W. J.,“Endurance of a Full-Scale Pretensioned Concrete Beam,” Proceedingsof the 36th Annual Meeting, V. 36,Highway Research Board, Washington, D.C., January 7-11, 1957.

11. Russell, B. W., and Burns, N. H.,“Static and Fatigue Behavior of Prestressed Composite Bridge GirdersMade With High Strength Concrete,”PCI JOURNAL, V. 38, No. 3, May-June 1993, pp. 116-128.

12. Rabbat, B. G., Kaar, P. H., Russell,H. G., and Bruce, R. N., Jr., “FatigueTests of Prestressed Girders WithBlanketed and Draped Strands,” PCIJOURNAL, V. 24, No. 4, July-August1979, pp. 88-114.

13. Overman, T. R., Breen, J. E., andFrank, K. H., “Fatigue Strength ofPrestressed Concrete Girders,” FinalResearch Report, Center for Transportation Research, Bureau of Engineering Research, University ofTexas at Austin, Austin, TX, November 1984.

14. Roller, J. J., Russell, H. G., Bruce,R. N., and Martin, B. T., “Long-TermPerformance of Prestressed, Pretensioned High Strength ConcreteBridge Girders,” PCI JOURNAL,V. 40, No. 6, November-December1995, pp. 48-59.

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