Fatigue Assessment For The Failed Bridge Deck Closure Pour .
Fatigue Assessmentfor the Failed Bridge DeckClosure Pour at MileMarker 43 on I-81http://www.virginiadot.org/vtrc/main/online reports/pdf/14-r12.pdfELIAS RIVERAGraduate Research EngineerEBRAHIM K. ABBASGraduate Research EngineerWILLIAM J. WRIGHT, Ph.D., P.E.Associate ProfessorRICHARD E. WEYERS, Ph.D., P.E.ProfessorC.L. ROBERTS-WOLLMANN, Ph.D., P.E.ProfessorVia Department of Civil and Environmental EngineeringVirginia Polytechnic Institute and State UniversityFinal Report VCTIR 14-R12VIRGINIA CENTER FOR TRANSPORTATION INNOVATION AND RESEARCH530 Edgemont Road, Charlottesville, VA 22903-2454www. VTRC .net
1. Report No.:FHWA/VCTIR 14-R12Standard Title Page - Report on Federally Funded Project2. Government Accession No.:3. Recipient’s Catalog No.:4. Title and Subtitle:Fatigue Assessment for the Failed Bridge Deck Closure Pour at Mile Marker 43on I-815. Report Date:April 20146. Performing Organization Code:7. Author(s):Elias Rivera, Ebrahim K. Abbas, William J. Wright, Ph.D., P.E., Richard E. Weyers,Ph.D., P.E., and C.L. Roberts-Wollmann, Ph.D., P.E.8. Performing Organization Report No.:VCTIR 14-R129. Performing Organization and Address:Virginia Tech Transportation Institute3500 Transportation Research PlazaBlacksburg, VA 24061-010510. Work Unit No. (TRAIS):11. Contract or Grant No.:9782512. Sponsoring Agencies’ Name and Address:Virginia Department of TransportationFederal Highway Administration1401 E. Broad Street400 North 8th Street, Room 750Richmond, VA 23219Richmond, VA 23219-482513. Type of Report and Period Covered:Final Contract14. Sponsoring Agency Code:15. Supplementary Notes:16. Abstract:Fatigue of reinforcing steel in concrete bridge decks has not been identified as a common failure mode. Generally, thestress range occurring in reinforcing steel is below the fatigue threshold and infinite fatigue life can be expected. Closure pourjoints, however, may be vulnerable to fatigue if some specific design details are present. This research shows that fatigue was alikely contributor to the I-81 closure pour failure. It is much less likely that corrosion directly caused a strength failure but it isvery likely that corrosion accelerated the onset of fatigue.The joints in the I-81 deck had vertical joint faces that did not provide any means for shear transfer across the joint. Thejoints were located under a wheel load path and were located away from beams or other means of deck support. This createdatypical conditions where shear forces across the joint due to wheel loads were carried only by the reinforcing steel. The stressrange in the reinforcing steel is greatly magnified under this scenario thereby making fatigue a possibility.New closure pour joints can easily be designed to prevent fatigue by providing structural support for both sides of thejoint. Existing joints, however, need to be evaluated to determine if fatigue vulnerability exists. Lacking knowledge of the jointinternal details, a simple differential deflection test can be performed to detect fatigue vulnerability. If the two sides of the jointare deflecting vertically relative to each other under wheel loads, than fatigue can be considered a possibility. No deflectionindicates that fatigue is unlikely.17 Key Words:Corrosion, reinforcement, decks, closure pours, fatigue, failure19. Security Classif. (of this report):UnclassifiedForm DOT F 1700.7 (8-72)18. Distribution Statement:No restrictions. This document is available to the publicthrough NTIS, Springfield, VA 22161.20. Security Classif. (of this page):21. No. of Pages:22. Price:Unclassified40Reproduction of completed page authorized
FINAL REPORTFATIGUE ASSESSMENT FOR THE FAILED BRIDGE DECK CLOSURE POURAT MILE MARKER 43 ON I-81Elias RiveraGraduate Research EngineerEbrahim K. AbbasGraduate Research EngineerWilliam J. Wright, Ph.D., P.E.Associate ProfessorRichard E. Weyers, Ph.D., P.E.ProfessorC.L. Roberts-Wollmann, Ph.D., P.E.ProfessorVia Department of Civil and Environmental EngineeringVirginia Polytechnic Institute and State UniversityVCTIR Project ManagerMichael M. Sprinkel, P.E.Virginia Center for Transportation Innovation and ResearchIn Cooperation with the U.S. Department of TransportationFederal Highway AdministrationVirginia Center for Transportation Innovation and Research(A partnership of the Virginia Department of Transportationand the University of Virginia since 1948)Charlottesville, VirginiaApril 2014VTRC 14-R12
DISCLAIMERThe project that is the subject of this report was done under contract for the VirginiaDepartment of Transportation, Virginia Center for Transportation Innovation and Research. Thecontents of this report reflect the views of the authors, who are responsible for the facts and theaccuracy of the data presented herein. The contents do not necessarily reflect the official viewsor policies of the Virginia Department of Transportation, the Commonwealth TransportationBoard, or the Federal Highway Administration. This report does not constitute a standard,specification, or regulation. Any inclusion of manufacturer names, trade names, or trademarks isfor identification purposes only and is not to be considered an endorsement.Each contract report is peer reviewed and accepted for publication by staff of the VirginiaCenter for Transportation Innovation and Research with expertise in related technical areas.Final editing and proofreading of the report are performed by the contractor.Copyright 2014 by the Commonwealth of Virginia.All rights Reserved.ii
ABSTRACTFatigue of reinforcing steel in concrete bridge decks has not been identified as a commonfailure mode. Generally, the stress range occurring in reinforcing steel is below the fatiguethreshold and infinite fatigue life can be expected. Closure pour joints, however, may bevulnerable to fatigue if some specific design details are present. This research shows that fatiguewas a likely contributor to the I-81 closure pour failure. It is much less likely that corrosiondirectly caused a strength failure but it is very likely that corrosion accelerated the onset offatigue.The joints in the I-81 deck had vertical joint faces that did not provide any means forshear transfer across the joint. The joints were located under a wheel load path and were locatedaway from beams or other means of deck support. This created atypical conditions where shearforces across the joint due to wheel loads were carried only by the reinforcing steel. The stressrange in the reinforcing steel is greatly magnified under this scenario thereby making fatigue apossibility.New closure pour joints can easily be designed to prevent fatigue by providing structuralsupport for both sides of the joint. Existing joints, however, need to be evaluated to determine iffatigue vulnerability exists. Lacking knowledge of the joint internal details, a simple differentialdeflection test can be performed to detect fatigue vulnerability. If the two sides of the joint aredeflecting vertically relative to each other under wheel loads, than fatigue can be considered apossibility. No deflection indicates that fatigue is unlikely.iii
FINAL REPORTFATIGUE ASSESSMENT FOR THE FAILED BRIDGE DECK CLOSURE POURAT MILE MARKER 43 ON I-81Elias RiveraGraduate Research EngineerEbrahim K. AbbasGraduate Research EngineerWilliam J. Wright, Ph.D., P.E.Associate ProfessorRichard E. Weyers, Ph.D., P.E.ProfessorC.L. Roberts-Wollmann, Ph.D., P.E.ProfessorVia Department of Civil and Environmental EngineeringVirginia Polytechnic Institute and State UniversityINTRODUCTIONIn 1992, several bridge decks on I-81 near Marion, Virginia, were replaced, using stagedconstruction, as part of a bridge rehabilitation project (Sprinkel et al., 2010). Figure 1 presents atransverse section showing the width and location of the closure pour. Epoxy-coatedreinforcement was used as the reinforcing steel. There was no formed keyway at the jointbetween the previously cast deck and the closure pour. After 17 years in service, a 3 ft by 3 ftclosure pour section punched through, as shown in Figure 2. All of the bars along theclosure/deck interface on both sides of the closure were severed.The closure pour was positioned under the left wheel path of the southbound right lane ofthe bridge deck, so the joint was subject to a very large number of wheel loads. Observations atthe bridge site indicated that the joint had opened slightly. In this case it is possible that thereinforcing bars alone were carrying shear and moment across the joint. The open joint alsoprovides a more direct path for deicing salts to penetrate to the reinforcing bars and inducecorrosion. Figure 3 shows a portion of the closure pour joint where the concrete was removedaround the bars spanning across the joint. The brown coloration indicates degradation of theepoxy coating, there is evidence of corrosion in the bars, and the bars are fractured verticallyclose to the plane of the joint. There is little evidence of ductile deformation of the bars prior tofracture. This indicates that the bars failed in a brittle mode possibly due to fatigue and fracture.It is therefore likely that both fatigue and corrosion played a role in the failure of the closure.1
Figure 1. Transverse Section of Bridge Deck With Closure PourFigure 2. Failed Section of Closure Pour2
Figure 3. Condition of Reinforcing Bars Spanning Across Joint Showing Corrosion and Vertical BreakThrough BarsPURPOSE AND SCOPEThe purpose of this study was to investigate the influence of the fatigue and strengthoverload on the overall failure mechanism that occurred in the I-81 bridge deck slab. A separatereport characterized the influence of corrosion on the failure process (Abbas et al., 2014). Thisstudy focused on strength and fatigue testing of specimens to determine the mechanical effectson the failure process. Four 4.5 ft by 10 ft slab sections, containing the closure, were removedfrom the failed bridge deck to perform a series of tests. Specimens tested from these slabs havesome pre-existing level of corrosion and fatigue damage from their service in the bridge. Threenew slabs were fabricated in the lab, with the same design as the slabs removed from the actualbridge deck. These specimens were used to assess shrinkage and the cause of the joint openingobserved in the field. In addition, these specimens served as undamaged controls for the fatigueand strength tests since there was no pre-existing damage.Two types of testing were performed in this study. Strength tests were performed toevaluate the slab strength across the closure pour joint. The failure mode was studied to helpdetermine if overloads contributed to the failure mechanism. A second series of tests subjectedthe slab specimens to cyclic loading to determine if fatigue contributed to the failure.3
METHODSSpecimensThe study was conducted on a total of eleven specimens, eight from the actual I-81 failedbridge and three new specimens fabricated and cast in the lab.I-81 Deck SlabsSections measuring approximately 4.5 ft by 10 ft were saw cut, removed from the bridgedeck, and delivered to the Thomas Murray Structures Laboratory at Virginia Tech. The sectionscontained the entire closure pour section along with approximately 9 in of the adjacent deck slab,as shown in Figure 4. After examining the sections, eight test specimens, 22 in wide, were cutfrom the four slabs to be used for fatigue and strength testing. The 22-in specimens eachcontained two truss bars, one top straight bar, and one bottom straight reinforcing bar. Adetailed characterization of the condition of the slabs and the specimen cut locations is providedin a separate report (Abbas et al., 2014).9"10'-0"9"Stage I Deck4'-6" 3'-0"SawCutClosure PourJoint1'-10"Sectionscut fortesting atvariouslocationsSaw CutsStage II Deck9"Plan ViewSideViewFigure 4. Approximate Dimensions of Slab Sections Showing Cuts Made to Extract Typical Test SpecimenLab Cast SpecimensThree slab specimens were fabricated and cast in the lab. The concrete used in castingthe slabs was A-4 ready mixed concrete, which is standard for Virginia bridge decks (4,000 psi at28 days, Virginia Department of Transportation [VDOT] standard identification). The slabswere constructed to mimic the actual configuration of the slab, including the support beams. Theactual deck and the lab specimen dimensions and reinforcing are presented in Figure 5.The specimens were cast in two stages with the center section (closure pour) cast 30 daysafter the two end sections. The end sections were bolted to support beams to simulate theconstraint of the two sides of the bridge. This resulted in some opening of the joints due toshrinkage of the closure pour concrete. Details of the casting sequence are provided in a separatereport (Abbas et al., 2014).4
2'-6"8'-0"3'-0"2'-6"8 1/2"Actual Configuration SectionLab Configuration Section#5 Truss Bars1'-10"#5 Straight Bars Top and BottomPlan View of Lab Cast SpecimenFigure 5. Lab Cast Slab Specimens Showing Reinforcing Steel LayoutMaterial PropertiesThe material properties of the I-81 and the lab cast slab specimens were measured priorto testing (Abbas et al., 2014). Core samples were taken and tested from the I-81 slabs that werein service for 17 years. Test cylinders were prepared during casting of the lab cast specimens.Table 1 shows the average values from multiple tests.5
Specimen TypeI-81 SlabsLab Cast Slabs(90-day strength)Table 1. Average Concrete Strength Properties in Test , ksiModulus, ksiStrength, ksiCenter7.654,4200.655End Blocks5.663,7600.563Center6.406,1100.650End Blocks6.654,5800.760The reinforcing steel for the lab cast slabs was No. 5 Grade 60 bars with a measured yieldstrength of 67.5 ksi. All bars were epoxy coated to match the actual bridge conditions. Theproperties of the reinforcing steel for the I-81 specimens were not measured but it was assumedto be typical strength for No. 5 Grade 60 bars.Strength and Fatigue TestingBasic Test Set-upA three-point loading configuration was used for all tests to approximate the moment andshear occurring across the closure pour in the actual bridge. In the bridge, the slab is attached tomultiple girders and some degree of two-way action can be expected. Since the failure occurredalong the closure pour joint line it is reasonable to assume that one-way action dominated thefailure. The test specimens, shown in Figure 6, are strips cut from the slab and are tested underone-way action. The point load shown in the figure was actually applied over a 10 in by 20 inelastomeric pad to simulate a truck wheel load in the actual bridge.22in54inP8 1/2inFigure 6. Testing Configuration of 22-in Slab SpecimensFigure 7 shows the approximate moment and shear diagrams that would be expectedfrom a point load at the center of the closure pour in the I-81 bridge. The roller supports for thetest specimens are located at inflection points in the moment diagram; therefore the momentdiagram for the single span specimens approximates the moment diagram for the continuousslab. This results in moment and shear forces across the joint in the test specimens that aresimilar to what is expected in the actual bridge. However, this set-up does not consider any axial6
tension force on the specimens due to constraint and shrinkage in the bridge. The presence of thejoint is a local stiffness discontinuity in the deck. In the actual bridge, localized rotation at thejoint is restricted due to the multi-span continuity of the slab. In the test specimens, higher localrotation occurs at the joint allowing the gap to close at the top of the joint. This allowed contactbetween the concrete on both sides of the joint and increased the local shear stiffness of the joint.The significance of this difference is discussed later in the report. However, given the limitedlength (9 in) available outside the closure pour, there was no feasible way to add constraint andaxial force to the I-81 slab specimens.Wheel Load(Proportioned for Specimen Width)Field Saw CutPField Saw CutClosurePourJoints P/2Shear-P/2-PL/8-PL/8Moment PL/8Figure 7. Comparison of Moment and Shear Diagrams for Continuous Slab in Bridge and Test SpecimensFigure 8 shows a schematic of the test set-up for both the fatigue and strength tests.Bearing plates were bolted on the top and the bottom of the end blocks to distribute the rollerpoint load into the concrete. Because of the short length of the end blocks in the I-81 slabspecimens, there was some concern that cracking and loss of development of the reinforcingsteel could occur and jeopardize the test. The bearing plates helped provide confinement to theconcrete. In addition, the bar ends were drilled and tapped to allow installation of end caps toimprove the development strength of the bars. These measures were successful; no bar slip wasobserved during testing.7
FrameCross HeadFrameCross Head50 kip MTSJack50 kip MTSJack10" x 20"ElastomericPad10" x 20" x 2"Bearing PlateSteel Plates with1/2" all thread at endsGrout plates to testspecimenRollers at 48" c-cRebar Caps1/4-20 Bolts & FenderWashers (typ.)epoxy to concrete &tightenPedestalsPedestalsFloorFigure 8. Schematic Showing Test Set-up for Strength and Fatigue TestsFigure 9 shows the test set-up for the fatigue tests. Two independent specimens weretested in parallel to accumulate cycles on two specimens simultaneously. The same test set-upwas initially used for the strength tests. However, somewhat unexpectedly, the 50-kip MTSjacks provided insufficient force to fail the specimens. The modified set-up, shown in Figure 10,replaced the MTS jack with a 100-kip static jack. The load patch and roller boundary conditionsremained identical for the two set-ups.As shown in Figure 5, the lab cast specimens had longer end blocks to simulate the actual8 ft girder spacing present in the bridge. One of the three lab cast specimens was strength testedwith the identical set-up shown in Figure 10. The support rollers remained at the 4 ft spacingand the longer end blocks cantilevered out on both ends. With some minor adjustment for selfweight, both the I-81 and one of the lab cast slabs were all tested using the same set-up.8
Figure 9. Testing Set-up Using 50-kip Servo-hydraulic Jacks for Fatigue TestsFigure 10. Modification of Test Set-up to Enable Higher Loads to Achieve Failure in Strength Tests9
Modified Axial Force Set-upA modification was made to the basic testing set-up to introduce axial force to thespecimen during testing. This configuration was used for one strength and one fatigue test. Thismodification allowed the joint to be jacked open during testing thereby minimizing the localizedjoint rotation effects previously discussed. Because of the limitations on end block length, it waspossible to test the lab cast specimens using only this configuration.The axial force was achieved by installing jacking struts on the two sides of thespecimens. This is essentially a reverse post-tensioning system that adds tension instead of theusual compression to the specimen. Jack placement on the sides prevented interference with theload actuators, instrumentation, and roller supports. The jacking struts consisted of square HSStubes with a manual screw jack welded to one end. Prior to application of the wheel load, thescrew jacks were adjusted to provide the desired level of axial force across the joint.Figure 11. Jacking Struts Installed on Sides of Specimens to Add Axial Force Across JointsThe jacking struts bear on end plate assemblies that were bolted to the concrete endblocks with concrete anchor bolts. The end plate assemblies consist of MC6x18 channels thatextend past the width of the specimen as shown in Figure 12. A 1-in steel bearing plate transfersthe jacking force to the channels. The screw jack has a swivel head in contact with the bearingplate and a rocker was placed on the end of the HSS tube. This allowed the anchor blocks torotate relative
Fatigue of reinforcing steel in concrete bridge decks has not been identified as a common failure mode. Generally, the stress range occurring in reinforcing steel is below the fatigue threshold and infinite fatigue life can be expected. Closure pour joints, however, may be vulnerable to fatigue if some specific design details are present.
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Application of ASME Fatigue Code: 3-F.1 ASME Smooth Bar Fatigue Curves Alternative Effective Stress vs Fatigue Cycles (S-N Curve) With your weld quality level assessed and an accurate FEA stress number, one can use the ASME fatigue curve to calculate the number of Fatigue Cycles. In our prior example, the fatigue stress could be 25.5 ksi or as .
