Alkhrdaji, T and A. Nanni, "Flexural Strengthening of Bridge Piers Using FRP Composites," ASCE Structures Congress 2000, Philadelphia, PA, M.

Transcription

1 Alkhrdaji, T and A. Nanni, "Flexural Strengthening of Bridge Piers Using FRP Composites," ASCE Structures Congress 2000, Philadelphia, PA, M.Elgaaly, Ed., May 8-10, CD version, # , 8 pp.

2 FLEXURAL STRENGTHENING OF BRIDGE PIERS USING FRP COMPOSITES Tarek Alkhrdaji, Graduate Research Assistant/Ph.D. Candidate Antonio Nanni, Ph.D., P.E., V&M Jones Professor of Civil Engineering, FASCE Center for Infrastructure Engineering Studies (CIES) University of Missouri-Rolla Abstract: The effectiveness of FRP jackets for increasing the shear capacity and the flexural ductility of reinforced concrete (RC) columns was demonstrated in many studies. However, for smaller axial loads, the contribution of FRP jackets to flexural strength is minimal. Using FRP sheets in the direction of a column with end anchorage to improve its flexural capacity at the base is not easily achieved. This paper reports on a research project aimed at upgrading the flexural capacity of RC piers using near-surface mounted (NSM) FRP rods. Flexural strengthening and testing to failure of the piers were carried out on a bridge that was scheduled for demolition during the Spring of Three of the four piers of the bridge were strengthened with different configurations using FRP rods and jackets. The flexural strengthening was achieved using NSM carbon FRP rods that were anchored into the footings. The piers were tested under static push/pull load cycles. An analytical model was developed to determine the net forces acting on a bridge pier at a given load level based on the measured response. Strengthening techniques, test results, modes of failure, and sample analytical results of tested bridge piers are described and the effectiveness of this technology is demonstrated. Keywords: Bridge piers, Carbon fibers, Fiber Reinforced Polymer (FRP), Flexural strengthening, Near-surface mounted (NSM) reinforcement, Structural modeling. 1

3 INTRODUCTION Many reinforced concrete (RC) bridge piers, constructed in the first half of this century, were designed as gravity piers with minimal flexural capacity. The potential risk of failure of these piers under a moderate earthquake is becoming a growing concern to states DOT s RC piers can be seismically deficient in shear and flexural strength, and flexural ductility. Due to lack of seismic detailing requirement, it is common to find minimal amount of transverse reinforcement in gravity piers constructed prior to However, they can be adequate to resist the earthquake induced shear forces due to their large cross sections. Inadequate flexural strength, on the other hand, may arise from the low seismic lateral forces that were typically considered in earlier designs. Inadequate flexural strength may also arises from the premature termination of the main reinforcement or its inadequate splicing. One method for retrofitting piers with flexural strength deficiency consists of the addition of a RC jacket. This method is also effective in improving the shear strength and the ductility of a pier. However, it may not be very practical due to undesirable section enlargement or construction constraint. Previous work on strengthening of columns with FRP composites has demonstrated the effectiveness of jacketing with FRP in the hoop direction in improving the shear capacity and the flexural ductility of RC rectangular columns (Seible et al, 1995). Since some gravity piers are designed to carry axial loads that are only a small fraction of their axial load capacity, the influence of jacketing on enhancing the flexural capacity is minimal. This is because jacketing can only improve the flexural capacity through concrete confinement if failure was governed by concrete crushing (compression-controlled failure). Strengthening of columns for flexure using FRP sheets with the fibers aligned in the column direction is not practical due to anchorage requirement at the base of column. New techniques for the flexural retrofit of RC piers, especially gravity piers, are therefore required. In an attempt to improve the flexural capacity of columns jacketed with FRP sheets, researchers have used steel plates with bolt connections accompanied by section enlargement at the base of the column (Hakamada, 1997). This method resulted in a slight improvement of the flexural capacity. However, such mechanical anchors, although effective in the laboratory, are not very practical for field application due to drawbacks such as stress concentration, which can cause the premature rupture of FRP. In addition, where carbon FRP is used, the likelihood of galvanic corrosion due to steel-carbon fiber contact is an additional concern. Strengthening of RC members with near-surface mounted (referred to as NSM) FRP rods is another technique that consists of embedding FRP rods in grooves made on the surface of the concrete and bonded in place with epoxy. This technique was successfully used to upgrade Pier 12 at the Naval Station in San Diego, CA to meet demand of operational changes accompanied by higher vertical loads (Naval Facilities, 1998). The use of NSM rods is more practical than externally bonded FRP laminates when the end anchorage of the FRP reinforcement is an essential design requirement or when the installation of laminates involves extensive surface preparation work. 2

4 A research program at the University of Missouri-Rolla was tailored to investigate the applicability and effectiveness of NSM rods in improving the flexural capacity of RC piers. Bridge J857, located in Phelps County-Missouri, was scheduled for demolition during the Spring of The bridge was, therefore, considered for the strengthening and testing to failure of its RC piers. A structural model was developed to reflect the observed behavior of the bridge piers. The model was analyzed using the matrix displacement method of structural analysis to determine the internal forces (moments) and external force acting on the pier at any applied load level by using the measured deformations as an input. DESCRIPTION OF THE BRIDGE PIERS Bridge J857, was build during the early 1930 s and represented typical conditions of existing bridges in mid-america. It consisted of three simply supported solid RC decks with an original roadway width of 7.6 m (25 ft). Each simply supported deck spanned 7.9 m (26 ft). The bridge bents (see Figure 1) consisted of two piers connected at the top by a RC cap beam. The piers had a m (2 2 ft) square cross-section and were reinforced with four 19 mm (#6) deformed steel rods. The transverse reinforcement consisted of 6 mm (#2) steel ties spaced at 457 mm (18 in). Each pier was supported by m ( ft) square footing. The actual length of the piers varied from 1.8 to 3.4 m (6 to 11 ft). No corrosion of reinforcement or concrete spalling was observed on the bridge piers. STRENGTHENING SCHEMES Seismic performance category (SPC) B was selected for the analysis of the bridge piers since it is relevant to Missouri (AASHTO, 1996). Under SPC B condition, the seismically induced lateral load at the top of the piers was determined to be 160 KN (36 kips). The computed shear capacity of the piers was 338 KN (76 kips). For flexure, the capacity for lateral load applied at the top of the piers in the longitudinal direction varied from 98 KN (22 kips) to 53 KN (12 kips) for the shortest and tallest piers, respectively. The piers were therefore adequate in shear and deficient in flexure. Three piers were strengthened and the fourth pier was used as a benchmark. Two piers were strengthened for flexure using near-surface mounted carbon FRP rods. One pier was strengthened with 14 NSM rods, mounted on two opposite faces, seven on each face. A second pier was strengthened with six NSM rods, three on each face. The NSM rods considered for this application were 11 mm (7/16 inch) diameter smooth carbon rods with surface roughened by sandblasting. The rods were fully anchored (minimum 380 mm (15 in.)) into the footing of each pier. Finally, the two piers were wrapped with 4-ply of carbon FRP jacket. The third pier was externally jacketed with six plies of glass FRP sheets. As will be discussed later, the test setup was designed such that the lateral movement of the piers was allowed at the top while restrained at the base. Therefore, it was expected that the maximum moment would occur at the base of the pier. Consequently, the NSM rods were only anchored to the footing. In addition, anchoring the NSM rods to the top flare can not 3

5 be easily achieved due to its shape (see Figure 1). The mechanical properties of the FRP sheets and rods are given in Table 1. Figure 2 summarizes the strengthening schemes of the bridge piers. FRP Type Table 1: Mechanical properties of FRP reinforcement Dimension mm [in] [0.0139] [0.0065] 11 [ 7 / 16 ] Design Strength MPa [ksi] 1520 [220] 3800 [550] 1240 [180] Design Strain mm/mm or in/in Tensile Modulus GPa [ksi] 72 [10,500] 228 Glass sheets* Carbon sheets* [33,000] Carbon rods** [17,200] Sheet thickness or bar diameter * Fiber properties ** Rod properties STRENGTHENING PROCEDURE The NSM FRP rods were embedded in grooves that were 19 mm (¾ in.) deep, and 14 mm ( 9 / 16 in.) wide cut along the length of the piers. The grooves were made using conventional hand-held tools. The grooves were cleaned using sand blasting to remove all loose particles and dust. Surface preparation is important since the tensile stresses are transmitted from the concrete to the FRP rod through the binding paste by means of tangential stresses. To anchor the rods, 400 mm (16-in) deep holes were drilled into the footings. The holes were aligned with the grooves on the pier sides. The grooves and the drilled holes were then filled halfway with a viscous epoxy grout and the carbon FRP rods were installed. Another layer of epoxy grout was then applied and the surface was leveled. All FRP jackets were installed by the wet lay-up process. The carbon and glass FRP sheets covered the entire height of the piers with the fiber direction perpendicular to the pier axis. The corners of the rectangular piers were rounded to 13 mm (0.5 in.) radius to prevent stress concentrations in the FRP sheets. TEST SETUP The loading system was designed such than it could apply a maximum load much larger than theoretically predicted. This was done to account for the possibility of higher actual material strengths than initially presumed as well as for the strengthening effect. The desired level of loading could only be applied by means of hydraulic jacks. The test setup was designed such that it could induce reversing loading cycles in which the piers were allowed to displace laterally at the top. To achieve this, a 250-mm (10-inch) strip of the deck was saw cut and removed. The central portion of the cap beams were also saw cut and removed and a hydraulic jack was inserted in the gap. A schematic of the test setup is shown in Figure 3. The function of the internal jack was to apply the outward push force to the piers cap 4

6 beam. Saw cutting the bridge deck and the cap beam allowed for the relative movement of the cap beams and the topping deck. To pull the piers together, a reaction frame was constructed such that it confined the cap beams. A set of two hydraulic jacks was then attached to the reaction frame. The internal and external jacks were used alternately to create a static lateral loading cycles. INSTRUMENTATION The bridge piers were instrumented with electric strain gages installed on the mounted rods as well as on existing steel reinforcement. Strain gages were also installed on the FRP sheets. An 890 KN (200 kips) capacity load cell was used to measure the applied force. The lateral displacement of the each pier was measured at mid-height and at the top of the pier using linear variable displacement transducers (LVDTs). The LVDTs were mounted on steel towers that were fixed to the footing using conventional drop anchors. The rotation of the cap beam, the pier, and the footing was measured by means of inclinometers at three locations (see Figure 4). TESTING PROCEDURE Once the setup was erected, instrumentation was connected to the data acquisition system and zero reading were taken. For safety reasons, the first loading cycle was always a pull-in load condition. When the desired lateral force was achieved, the system was unloaded and the hydraulic hoses were disconnected from the external jacks and connected to the internal jack. A push-out force was then applied. These two loading cycles were repeated until the weaker pier failed. To test the second pier of the same bent, a diagonal bracing was installed against the failed pier, as shown in Figure 4. Prior to the testing of the piers of the second bent, the deck slabs resting on the bent were jacked up using hydraulic jacks lubricated steel plates were inserted between the cap beam and the deck slabs. This action was intended to eliminate some of the frictional forces at the top of the piers. TEST RESULTS AND OBSERVATIONS The failure loads of the bridge piers exceeded in magnitude the predicted loads. In addition, all the piers underwent a double curvature type of behavior. The rotation restraint of the superstructure on the cap beam was larger than expected even for the bent with reduced friction. For the piers with reduced friction, larger rotations were measured on the cap beam. In these piers, as the cap beam rotated it pushed the topping decks upward. As a result, the point of application of the vertical force due to the deck weight shifted to the edge of the cap beam. This behavior resulted in an additional moment that acted at the top of the pier. For the unstrengthened pier, the applied lateral load at failure was 351 KN (79 kips) and the measured maximum lateral displacement at the top of the pier was around 15.5 mm (0.61 in.). Figure 5(a) illustrates the measured rotations at the last loading cycle (push-out) of the unstrengthend pier. The continuous rotation under constant force was related to the yielding of the reinforcement as well as soil failure, which is represented by the continuous rotation of the footing at failure. One major 5

7 crack was observed on the pier, which occurred at the upper third of the pier height, close to the termination point of the flare reinforcement, as shown in Figure 5(b). For the pier strengthened with 7 CFRP bars on opposite sides and CFRP jacketing, the failure was initiated by a crack occurred at the pier-flare intersection where the mounted rods were terminated. After the crack occurred, the pier went through a continuous rotation with no increase in load carrying capacity. The applied lateral load at failure was 360 KN (81 kips) and the measured lateral displacement at the top of the pier varied from 1.5 mm (0.058 in.) just before cracking to 2.9 mm (0.116 in) at loading termination. Figure 6 illustrate the measured rotation at the last loading cycle (push-out) of this pier. This figure indicates that at maximum load, the whole pier experience a rigid body rotation for a while, then a crack occurred at the top of the pier causing the cap beam rotation to reduce significantly due to the yielding of the reinforcement and the formation of a plastic hinge. After the formation of the plastic hinge and the redistribution of moments, the pier and the footing continued to rotate at a faster rate indicating soil failure. For the pier strengthened with three CFRP bars on two opposite sides and CFRP jacketing, cracks occurred at the top and the base of the pier, as shown in Figure 7. The footing of this pier was originally cast in a bedrock therefore no rotation was measured at this footing, as shown in Figure 8. The figure illustrates the measured rotations at the last loading cycle (push-out) of this pier. As a result of the large rotational stiffness of the footing, larger moment were developed at the base of the pier. Failure was initiated by the rupture of the FRP rods at the base of the pier at a load level of 382 KN (86 kips) with a maximum lateral displacement measured at the top of 21.8 mm (0.86 in.). This indicates that the full capacity of the NSM rods can be achieved, giving that the rods are adequately anchored. As for the pier with GFRP jacket only, the pier started to rotate as a rigid body at 222 KN (50 kips). The test was terminated when the lateral displacement exceeded 38.1 mm (1.5 in.). The failure mode of this pier was, therefore, a soil failure. It should be mentioned that the above given displacements at maximum loads are the absolute displacement without accounting to the rotation of the footing. The variation in failure modes and lateral load capacity may be related to the influence of superstructure/substructure interaction, variation in the boundary conditions of each pier (e.g., footing rotation stiffness and friction forces), and the skew effect of the bridge bents. ANALYTICAL MODELING The basic objective of modeling is to provide the simplest mathematical formulation of the true behavior of the pier, which satisfies a particular set of known values (in this case, the measured response) for quantitative determination of the internal forces. The developed structural model simulating the observed behavior of a bridge pier is shown in Figure 9(a). The pier is simulated by a column, which is free to displace laterally at the top and is restrained laterally at the bottom. The flexibility of the footing is represented by a rotational spring with unknown constant k1 at the base of the column, which models the effect of footing rotation due to soil deformation. Another spring with unknown stiffness k3 is used at the top of the column to model the effect of cap beam rotation due to the applied loading. The 6

8 frictional force between the deck and the cap beam is represented by a linear spring connected to the top joint. The spring constants, k5, may vary at each load level due to softening of the boundary conditions under repeated loading cycles. The model is analyzed using the matrix displacement method. The overall structural stiffness matrix, internal forces, and external loading due to a given structural response can, therefore, be determined by simple matrix operations. For simplicity, the unknown internal forces are determined at the locations of the nodes were displacements and rotations were measured experimentally. Accordingly, each pier is represented by a two-element, three-node column, as shown in Figure 9(b). The unknown rotations of the joints are denoted as X1, X2, and X3 and the unknown displacements of the joints are denoted as X4 and X5. The nodal displacements are used as the degrees of freedom (DOFs) of the column. Thus, the column has five degrees of freedom. This model is only applicable prior to pier cracking, after which a non-linear analysis is required. For the current investigation, the capacities of the piers were slightly larger than the cracking capacity and for some cases failure was governed by the rigid body rotation of the pier, therefore the elastic analysis approach was valid at higher load levels. An example of the analytical results is given in Figure 10 for the pier strengthened with 7 NSM-rods on two opposite sides. Figure 10(a) shows the measured deformations at the three nodes of the model due to a lateral load of 178 KN (40 kips). The effect of footing rotation was included in determining the lateral displacements (X4 and X5) of the joints. Figure 10(b) shows the calculated external loads and reaction of the column while Figure 10(c) shows a plot of the moment diagram along the length of the column. The results indicated that the frictional force is in the order of 57.6 KN (13 kips). However, this value was found to be reduced at higher load levels. The maximum moment occurred at the top of the pier due to the larger rotational stiffness exerted by the superstructure on the cap beam than the rotational stiffness of the soil. The analytical behavior correlates well with the experimental results where the first crack on this pier occurred at the pier-flare intersection, the location of the maximum moment. Due to limited space, a comprehensive documentation of the structural modeling, structural analysis and analytical results will be reported in a future publication. CONCLUSION The objective of this research program was to demonstrate the use nearsurface mounted FRP rods to improve the flexural capacity of rectangular RC piers. Prior, to demolition, full-scale bridge piers were strengthened with FRP rods and sheets and tested to failure. Test results indicate that this strengthening technique is effective in increasing the flexural capacity of the piers. Test results also indicate that the capacity and failure modes of the bridge piers are closely related to the superstructure/substructure interaction and the pier boundary conditions. Flexural strengthening of piers may cause the structural deficiency problem to shift another location within the structure. Therefore, flexural strengthening may require the retrofitting of beam-pier joints and foundations to account for the upgraded flexural capacity of the pier. In general, the determination of the elastic structural response 7

9 under any load value is quite achievable with a reliable model in terms of welldefined boundary conditions and reasonably accurate material properties and stiffness. ACKNOWLEDGEMENTS The authors gratefully acknowledge the funding provided by the Missouri Department of Transportation (MoDOT), Mid-America Transportation Center (MATC), and the University of Missouri-Rolla/University Transportation Center (UMR-UTC). Master Builders Technologies, Cleveland, OH, and Structural Preservation Systems, Baltimore, MD, provided and installed the FRP systems, respectively. REFERENCES Standard specifications for Highway Bridges. (1996). American Association of State Highway and Transportation Officials (AASHTO), Washington, D.C. Hakamada, F. (1997), Experimental Study on Retrofit of RC Columns Using CFRP Sheets. Proc., Third Int. Sym. on Non-Metallic (FRP) Reinforcement for Concrete Structures (FRPRCS-3), Japan Concrete Institute, Tokyo, Japan, 1, Macrae, G. A., Nosho, K., Stanton, J., and Myojo, T. (1997), Carbon Fiber Retrofit of Rectangular RC Gravity Columns in Seismic Regions. Proc., Third Int. Sym. on Non-Metallic (FRP) Reinforcement for Concrete Structures (FRPRCS-3), Japan Concrete Institute, Tokyo, Japan, 1, Naval Facilities Engineering Service Center. (1998), Navy Advanced Composite technology in Waterfront Infrastructure. Special Publication Sp-2046-SHR. Seible, F., Hegemier, G., Priestly, M. J. N., and Innamorato, D. (1995), Rectangular Carbon Fiber Jacket Retrofit Test of a Shear Column with 2.5% Reinforcement. Report No. ACTT-95/05, University of California, San Diego. 8

10 Figure 1. The Two Bents of the Bridge Bent 2, pier 1 (6 CFRP rods, 4 CFRP hoop plies) Bent 2, pier 2 (5 GFRP hoop plies) Bent 1, pier 1 ( 14 CFRP rods, 4 CFRP hoop plies) Bent 1, pier 2 (No Strengthening) Figure 2. Strengthening Schemes of the Piers 9

11 Above bridge deck Below bridge deck Hydraulic Jack Cut through bridge deck 25 Saw Cut 3-6 Pier Hydraulic Jack Dywidag Rod W14 90 Bent 1 Bent 2 Figure 3. Schematic of the Test Setup (1 in. = 25 mm) F P Failed 54 o 6 steel steel pipe pipe 5 x5 RC footing Inclinometer LVDT Figure 4. Diagonal Bracing of the Failed Pier. 10

12 Figure 5. Behavior of the Unstrengthened Pier (1 KN = kip) (a) Measured Rotations (b) Final crack Figure 6. Behavior of the Pier with 14 NSM rods (1 KN = kip) (a) Measured Rotations (b) Final crack at flare 11

13 (a) Top crack at pier-flare intersection (b) Pier base crack showing FRP rupture Figure 7. Cracks at Failure of the Pier with 6 NSM Rods Figure 8. Measured Rotation of the Pier with 6 NSM rods (1 KN = kip) 12

14 k 5 k 3 P X3 X5 3 L/2 2 L X2 X4 2 L/2 1 X1 k 1 1 (a) structural model (b) two-elements analytical model Figure 9. Analytical model of a bridge pier P = 178 KN X5=0.515 mm KN X3 = deg KN-M KN-M 2.97 m X4=0.304 mm X2 = deg. X1 = deg KN-M KN-M (a) measured response (b) calculated external forces (c) moment diagram Figure 10. Measured deformation and analytical results for pier with 14 NSM rods 13