FATIGUE BEHAVIOR OF IMPACT DAMAGED PRESTRESSED CONCRETE BRIDGE GIRDER REPAIRED WITH CFRP SHEETS

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1 FATIGUE BEHAVIOR OF IMPACT DAMAGED PRESTRESSED CONCRETE BRIDGE GIRDER REPAIRED WITH CFRP SHEETS Anthony Miller, Owen Rosenboom, Sami Rizkalla North Carolina State University, United States of America Abstract Replacement of damaged prestressed concrete bridge girders due to accidental impact by overheight vehicles is a major expense for bridge maintenance departments. The use of Carbon Fiber Reinforced Polymer (CFRP) materials for repair has an excellent potential to provide a fast and efficient technique as opposed to total girder replacement. This paper presents the behavior of a 16.7 meter impact damaged AASHTO Type II prestressed girder extracted from a bridge and repaired by CFRP material. The impact damage included a substantial loss of the concrete section approximately three meters from midspan along with one ruptured prestressing strand. The concrete section was restored with a polymer modified mortar and repaired using CFRP wet lay-up sheets configured to regain the ultimate load capacity of the original undamaged girder. The repaired beam was tested under cyclic loading designed to simulate the loads under which the girder would encounter in service. After the successful completion of the fatigue testing regimen, the girder was tested monotonically to failure to determine the behavior under the different limit states and mode of failure. 1. Introduction Bridge maintenance units are very frequently facing the dilemma of replacing or repairing prestressed concrete (PC) bridge girders that have been struck by overheight vehicles. Replacing the damaged girder is often the most expensive, time-intensive solution, resulting in lengthy traffic disruptions. Repairing the damaged girder using innovative techniques is becoming a more practical solution than replacement. Limited research has been done on the assessment of impact damage and subsequent repair methods to PC bridge girders. Several impact-damaged PC girders have been repaired in the field, but a limited number of studies have been conducted in a laboratory MILLER, Impact Damaged Prestressed Bridge Girder, 1/10

2 setting. One common repair technique is to splice the steel prestressing strands; however, this was found to perform poorly during fatigue, and in many cases is unable to restore the ultimate strength of the girder 1,2. Other laboratory investigations have been conducted to not only evaluate strand splicing, but to also evaluate the application and performance of other traditional repair techniques 3. A more innovative technique was needed that could not only restore the ultimate strength capacity of the damaged girder, but withstand the repetitive service loadings that all bridge girders undergo. The use of fiber-reinforced polymer (FRP) has emerged as an alternative to traditional strengthening techniques (external post-tensioning and externally bonded steel plates) of both reinforced concrete (RC) and PC structures. The use of FRP can be very effective at increasing the structural capacity of PC members 4,5. However, the repair of impactdamaged PC girders using carbon FRP (CFRP) laminates has not been explored by many previous researchers. The present research is the result of a tractor-trailer carrying excavating equipment that impacted the PC bridge superstructure of Robeson County, NC Bridge 169, USA. The girder had a significant loss of concrete on the bottom flange and lower web, as well as one ruptured prestressing strand in the bottom flange. The damaged section was restored with a polymer modified mortar and repaired using CFRP wet lay-up sheets configured to regain the ultimate load capacity original girder. Both the damaged and repaired sections are shown in Figure 1. The test results demonstrate that CFRP repair is able to restore the damaged girder s stiffness to that of a prediction of the undamaged girder. The CFRP repair was also able to increase both the ultimate strength and ultimate ductility of the repaired girder compared to the undamaged girder prediction. 2. Experimental Program 2.1 Specimen geometry and repair The test girder was a standard AASHTO Type II prestressed concrete girder, with a span of 16.7 m. The girders were spaced at 2.1 m and were in composite action with a 152 mm deck slab. During extraction of the girder from the bridge superstructure, the deck was cut to approximately the same width of the top flange. The test girder was prestressed with 16 low-relaxation, 1860 MPa, seven-wire, 12.7 mm diameter straight steel strands. Figure 2 shows test girder cross section and the prestressing strand pattern. As a result of the vehicular impact, a volume of concrete of approximately 0.1 m 3 was lost from the concrete section approximately 2.5 m from midspan, as shown schematically in Figure 3. The extent of damage was more prevalent on the front side of the girder than the back side, as shown in the same figure. On the heavily damaged side of the girder, one prestressing strand on the lower level was ruptured as indicated in MILLER, Impact Damaged Prestressed Bridge Girder, 2/10

3 Figure 2. Many other strands were exposed due to the spalled concrete as shown in Figure 1 however, they were considered intact in the design. The repair of the damaged PC girder began by restoring the cross section to its original dimensions. The repair mortar used was Tyfo Type P, two-component, quick setting, polymer modified cementitious mortar. Following the concrete restoration, the following steps were carried out the installation of the FRP material: 1) grinding to round the edges and remove irregularities caused during concrete repair, 2) sandblasting the concrete surface, 3) application of primer to fill in bug-holes, 4) saturation of CFRP sheets prior to installation, 5) application of first longitudinal layer, and any subsequent layers, 6) installation of the FRP U-wraps. (a) (b) Fig. 1- Damaged and repaired PC girder sections (a) and (b), respectively RUPTURED PRESTRESSING STRAND Φ12.7 STRANDS Fig. 2- Specimen cross section (dimensions in mm). 457 MILLER, Impact Damaged Prestressed Bridge Girder, 3/10

4 16.71m 4.04m 3.63m 9.04m ELEVATION FRONT 10.80m 1.93m 5.20m ELEVATION BACK Fig. 3- Location of impact damage to PC girder U-wraps spacing 0.4 m (typ) 1.8 U-wrap spacing 0.4 m (typ) THICKENED EPOXY BEDDING WITH SMOOTH TRANSITION (1) LAYER 305mm WIDE CFRP SHEET, ORIENTED LONGITUDINALLY (TYP) 8.84 ELEVATION FRONT ELEVATION BACK Fig. 4- FRP repair configuration (dimensions in mm). (3) LAYERS 406mm WIDE CFRP SHEETS, ORIENTED LONGITUDINALLY (TYP) (1) LAYER 305mm WIDE CFRP SHEET, ORIENTED VERTICALLY (TYP) (1) LAYER 152mm WIDE CFRP SHEET, ORIENTED LONGITUDINALLY (TYP) Unidirectional externally bonded CRFP wet lay-up sheets were bonded to the bottom flange and web, as shown in Fig 4. The sheets were chosen based on their proven record in strengthening PC girders as well as their excellent performance in a cost-effective analysis 5,6. Continuous CFRP U-wraps were required at the damaged concrete zone to stabilize crack growth in that region. U-wraps were also applied at approximately 0.4m intervals along the girder, extending to the top flange, to prevent possible delamination of the longitudinal sheets. The original ultimate capacity of the undamaged section was determined using a cracked section analysis approach. The capacity of the repaired section was also predicted in a similar manner with the assumption that the damage occurred at midspan. The predicted load versus deflection relationship is compared to the measured values as discussed in Section Material Properties The characteristics of the concrete, steel and FRP laminate were determined according to ASTM standards. Results of the measured values are summarized in Table 1. The concrete strengths shown are estimates from original drawings, as testing is currently ongoing. The effective prestressing stress was estimated to be 1.14 GPa, as specified by the original drawings, which corresponds to an effective prestressing force of P e = 1962 kn. The CFRP used in the repair was Fyfe SCH-41 with Tyfo Type S epoxy. MILLER, Impact Damaged Prestressed Bridge Girder, 4/10

5 Witness panels of the CFRP laminate used in the repair were tested, and the results are shown in Table 1. Table 1: Material properties Prestressing Steel Mild Steel Concrete Carbon fiber Strand Type Low-relaxation Units Strand tensile strength 1862 Mpa Nominal diameter 12.7 mm Strand area 127 mm 2 Modulus of Elasticity 200 Gpa Bar nominal diameter 12.7 mm Bar area 129 mm 2 Stirrup nominal diamter 9.5 mm Stirrup area 71 mm 2 Tensile Strength 413 MPa Modulus of Elasticity 200 GPa PC girder 41.4 MPa Concrete deck 24.1 MPa Cementitious mortar 43.4 MPa Nominal ply thickness 1 mm Ultimate tensile strength 986 MPa Modulus of Elasticity 95.8 GPa 2.3 Test setup and instrumentation The girder was tested in three-point bending using an MTS hydraulic actuator. The clear-span distance between supports was 16.4 m. An electronic data acquisition system was used to measure and record the real-time structural response of the system. The displacement profile of the PC girder was measured using string potentiometers placed at L/8 points. Strain gauges and PI gauges were used to determine the strain profile in six different locations, as shown in Figure Fatigue load determination The fatigue loading range for the repaired girder was designed to simulate loads which would be encountered during the service life of the original girder. Initially two different fatigue loading schemes were considered: one which would simulate the original AASHTO HS-15 truck design loading, and the other which would simulate an induced tensile stress at the extreme bottom of the concrete flange equal to ' 0.25 f c MPa, which is specified as the limit stress for service loading in corrosive environments 7. Analysis indicated that the original girder was overdesigned, therefore the HS-15 type loading did not govern the maximum loading condition. The final design of the fatigue loading was based on the live load level which would simulate a ' tensile stress at the bottom flange of the concrete equal to 0.25 f c MPa. MILLER, Impact Damaged Prestressed Bridge Girder, 5/10

6 16.71m 4.67m 1.14m 1.09m 1.35m 2.44m 1.14m 4.67m DL D DR CL DO DLO L/8 L/8 L/8 L/8 L/8 L/8 L/8 L/8 ELEVATION FRONT Fig. 5- Location of strain and PI gauges along length of PC girder Once the nominal tensile stress was selected, the live load moment which induces this tensile stress for the actual bridge girder with a full composite deck was determined. The resulting stress profile was created for the midspan section, and the stress at the location of the bottom prestressing strands was determined. The final step was to determine the level of applied load that induced a stress in the lower prestressing strands of the test girder equal to the value determined above. The lower load level was determined based on the equivalent dead loads of the bridge and the self-weight of the test girder. The equivalent concentrated load range used in this study was 83.2 kn and kn as the maximum and minimum load, respectively. 3. Experimental Results 3.1 Overall behavior Before fatigue testing began, an initial static test was conducted to determine the initial stiffness of the member, as well as the initial load deflection relationship. The initial static test loaded the specimen to 227 kn. The first visible crack was detected just outside the location of the transverse U-wraps encapsulating the damaged region, towards midspan, at a load of kn. Two other cracks formed nearby as the girder was loaded up to 227 kn. Following the initial static test, the specimen was subjected to 2 million cycles of fatigue loading, as specified in section 2.4. At various intervals, the fatigue cycling was stopped and a static test was performed to monitor the behavior of the girder. The load-deflection relationship for each of these static tests are shown in Figure 6. As shown, the girder completed the 2 million cycles with very little residual deflection. After completion of the fatigue cycling, a final static test was performed up to failure. The final load-deflection relationship is shown in Figure 7. Final failure of the repaired AASHTO girder was caused by propagation of flexure-shear cracks, which originated outside the termination point of longitudinal CFRP in the undamaged region, towards the compression zone and leading to failure of the girder near midspan (Figure 8). Crushing of the concrete occurred before propagation of the flexure-shear cracks MILLER, Impact Damaged Prestressed Bridge Girder, 6/10

7 completely into the top flange of the girder. The measured ultimate load was kn and the midspan displacement was 147 mm. 250 Load (kn) Initial Cycles 2.8 mm 2000k cycles Displacement (mm) 0.25sqrt(f'c) HS15 Initial Cycles Initial Cycles (2) at 5k cycles at 10k cycles at 50k cycles at 100k cycles at 250k cycles at 500k cycles at 750k cycles at 1000k cycles at 1250k cycles at 1500 cycles at 1750 cycles at 2000k cycles Fig. 6 Load-deflection curve after various intervals of fatigue cycling. Load (kn) Damaged and Repaired Prediction Initial Cycles Final Cycles (1) 0.25sqrt(f'c) Initial Cycles Initial Cycles (2) at 2000k cycles Final Cycles (1) Final Cycles (2) Undamaged Girder Prediction Damaged and Repaired Prediction Displacement (mm) Final Cycles (2) Undamaged Girder Prediction Fig. 7 Actual and predicted load vs deflection for repaired AASHTO girder. MILLER, Impact Damaged Prestressed Bridge Girder, 7/10

8 (a) (b) Fig. 8 Crushing of the concrete (a) before ultimate failure of the specimen (b) 3.2 Discussion of test results As a result of the impact damage imparted on the AASHTO girder and the concrete restoration process, an accurate estimation of the effective prestress force in the prestressing strands was difficult to determine. Using an estimate of the prestress losses of 24.0 percent, calculated using the lump-sum method 7, the stress ratio in the lower prestressing strands at midspan was determined using the following equation: f ps2 f ps1 SR ps = 100 (3.1) f pu where f ps2 and f ps1 are the upper and lower stress values in the prestressing strand due to the applied fatigue loading, and f pu is the ultimate strength of the strand. The upper and lower stress values in the prestressing strands were determined by combining the resulting strain profile of the member at the desired loads along with the Ramberg Osgood coefficients of the prestressing strands determined during material testing. The corresponding stress ratio was found to be 1.7%, which is considerably lower than the 5% limit recommended for straight prestressing strands from an earlier study 5. Throughout the entire fatigue cycling regime, no cracks were observed within the repaired region. It is believed that the presence of the tension-strut CFRP reinforcement and transverse U-wraps controlled propagation of the cracks within the damaged zone. The experimental load versus strain curves at various cross sections during the final static test are shown in Figure 9. Flexural cracks occurred initially within the undamaged section, symmetrical to the damaged section, at an applied load of 420 kn. The maximum ultimate strain measured in the CFRP at the damaged section was 0.7%, which is less than the specified 1.0% rupture strain provided by the manufacturer. The measured strain at failure was greater than the ultimate strain capacity using the bond coefficient, κ m, recommended by ACI 440 design guidelines 440.1R.04. MILLER, Impact Damaged Prestressed Bridge Girder, 8/10

9 700 PI_DO_B, Tensile strain in concrete at undamaged region 600 Load (kn) PI_CL_BF, Tensile strain in CFRP at midspan 200 PI_D_BF, Tensile strain in CFRP at damaged region PI_CL_BF 100 PI_D_BF PI_DO_B Tensile Strain (%) Fig. 9- Load versus tensile strain during final static test Figure 9 shows a residual strain of 0.07% in the repaired region after completion of the fatigue loading regime. This was caused by crack opening and aggregate interlock in the restored section. Figure 9 also shows that the stiffness of the damaged and undamaged regions are almost equal after flexural cracking occurred in the undamaged region. In a similar test of a damaged AASHTO Type II girder 8, U-wraps were extended only around the bottom flange and a mode of failure within the CFRP system was found. At ultimate load, neither the U-wraps nor longitudinal CFRP delaminated prior to crushing of the concrete. It is believed that premature debonding did not occur in the longitudinal CRFP as a result of the presence of the closely spaced U-wraps, which extended completely to the top of the girder. 4. Conclusions The following conclusions may be drawn from this experimental and analysis investigation: An impact damaged AASHTO Type II girder with one ruptured prestressing strand and significant loss of concrete section can be repaired using externally bonded CFRP sheets to restore the original flexural capacity. Detailing of the CFRP repair system should be carefully considered to restrain crack opening in the damaged region and to prevent debonding failures. Failure occurred in the undamaged section, meaning that the section repaired with CFRP material outperformed the original section. MILLER, Impact Damaged Prestressed Bridge Girder, 9/10

10 5. Acknowledgments The authors would like to acknowledge the support of the North Carolina Department of Transportation through project Special thanks to Fyfe Corporation for donation and installation of the FRP materials. The authors would also like to thank Jerry Atkinson, the technician at the Constructed Facilities Laboratory, whose help was invaluable. 6. References 1. Olson, S.A., French, C.W., and Leon, R.T., Reusability and Impact Damage Repair of Twenty-Year-Old AASHTO Type III Girders, Research Report No , University of Minnesota, Minneapolis, Minn Zobel, R.S. and Jirsa, J.O., Performance of Strand Splice Repair in Prestressed Concrete Bridges, PCI Journal v.43 (6) (1998) (72-84). 3. Zobel, R.S., Carrasquillo, R.L., and Fowler, D.W., Repair of Impact Damaged Prestressed Bridge Girder Using a Variety of Materials and Placement Method, Construction and Buildings Materials v.11 (5-6) (1997) Takács, P. F., and Kanstad T., Strengthening Prestressed Concrete Beams with Carbon Fiber Reinforced Polymer Plates, NTNU Report R-9-00, Trondheim, Norway Rizkalla, S.H., Rosenboom, O.A., and Miller, A.D., Value Engineering and Cost Effectiveness of Various FRP Repair Systems, a Report for the North Carolina Department of Transportation, Rosenboom, O.A. and Rizkalla, S.H., Fatigue Behavior of Prestressed Concrete Girders Strengthened with Various CFRP Systems, ACI SP-230 FRP RCS7, November American Association of State Highway and Transportation Officials, AASHTO LRFD Bridge Design Specifications, AASHTO, Ludovico, M.D., Nanni, A., Prota, A., and Cosenza, E., Repair of Bridge Girders with Composites: Experimental and Analytical Validation, ACI Structural Journal v. 102 (5) (2005) ( ). MILLER, Impact Damaged Prestressed Bridge Girder, 10/10