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, #40492-046-008, 8 pp.

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FLEXURAL STRENGTHENING OF BRIDGE PIERS USING FRP COMPOSITES

Tarek Alkhrdaji, Graduate Research Assistant/Ph.D. CandidateAntonio 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 theflexural ductility of reinforced concrete (RC) columns was demonstrated in manystudies. However, for smaller axial loads, the contribution of FRP jackets to flexuralstrength is minimal. Using FRP sheets in the direction of a column with endanchorage to improve its flexural capacity at the base is not easily achieved. Thispaper reports on a research project aimed at upgrading the flexural capacity of RCpiers using near-surface mounted (NSM) FRP rods. Flexural strengthening andtesting to failure of the piers were carried out on a bridge that was scheduled fordemolition during the Spring of 1999. Three of the four piers of the bridge werestrengthened with different configurations using FRP rods and jackets. The flexuralstrengthening was achieved using NSM carbon FRP rods that were anchored into thefootings. The piers were tested under static push/pull load cycles. An analyticalmodel was developed to determine the net forces acting on a bridge pier at a givenload level based on the measured response. Strengthening techniques, test results,modes of failure, and sample analytical results of tested bridge piers are describedand 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.

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INTRODUCTION

Many reinforced concrete (RC) bridge piers, constructed in the first half ofthis century, were designed as gravity piers with minimal flexural capacity. Thepotential risk of failure of these piers under a moderate earthquake is becoming agrowing concern to states DOT’s

RC piers can be seismically deficient in shear and flexural strength, andflexural ductility. Due to lack of seismic detailing requirement, it is common to findminimal amount of transverse reinforcement in gravity piers constructed prior to1970. However, they can be adequate to resist the earthquake induced shear forcesdue to their large cross sections. Inadequate flexural strength, on the other hand, mayarise from the low seismic lateral forces that were typically considered in earlierdesigns. Inadequate flexural strength may also arises from the premature terminationof the main reinforcement or its inadequate splicing. One method for retrofittingpiers with flexural strength deficiency consists of the addition of a RC jacket. Thismethod 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 orconstruction constraint.

Previous work on strengthening of columns with FRP composites hasdemonstrated the effectiveness of jacketing with FRP in the hoop direction inimproving 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 thatare only a small fraction of their axial load capacity, the influence of jacketing onenhancing the flexural capacity is minimal. This is because jacketing can onlyimprove the flexural capacity through concrete confinement if failure was governedby concrete crushing (compression-controlled failure). Strengthening of columns forflexure using FRP sheets with the fibers aligned in the column direction is notpractical due to anchorage requirement at the base of column. New techniques forthe flexural retrofit of RC piers, especially gravity piers, are therefore required.

In an attempt to improve the flexural capacity of columns jacketed with FRPsheets, researchers have used steel plates with bolt connections accompanied bysection enlargement at the base of the column (Hakamada, 1997). This methodresulted in a slight improvement of the flexural capacity. However, such mechanicalanchors, although effective in the laboratory, are not very practical for fieldapplication due to drawbacks such as stress concentration, which can cause thepremature rupture of FRP. In addition, where carbon FRP is used, the likelihood ofgalvanic corrosion due to steel-carbon fiber contact is an additional concern.

Strengthening of RC members with near-surface mounted (referred to asNSM) FRP rods is another technique that consists of embedding FRP rods in groovesmade on the surface of the concrete and bonded in place with epoxy. This techniquewas successfully used to upgrade Pier 12 at the Naval Station in San Diego, CA tomeet demand of operational changes accompanied by higher vertical loads (NavalFacilities, 1998). The use of NSM rods is more practical than externally bonded FRPlaminates when the end anchorage of the FRP reinforcement is an essential designrequirement or when the installation of laminates involves extensive surfacepreparation work.

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A research program at the University of Missouri-Rolla was tailored toinvestigate the applicability and effectiveness of NSM rods in improving the flexuralcapacity of RC piers. Bridge J857, located in Phelps County-Missouri, wasscheduled for demolition during the Spring of 1999. The bridge was, therefore,considered for the strengthening and testing to failure of its RC piers. A structuralmodel was developed to reflect the observed behavior of the bridge piers. The modelwas analyzed using the matrix displacement method of structural analysis todetermine the internal forces (moments) and external force acting on the pier at anyapplied 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 typicalconditions of existing bridges in mid-America. It consisted of three simply supportedsolid RC decks with an original roadway width of 7.6 m (25 ft). Each simplysupported deck spanned 7.9 m (26 ft). The bridge bents (see Figure 1) consisted oftwo piers connected at the top by a RC cap beam. The piers had a 0.6 × 0.6 m (2 × 2ft) square cross-section and were reinforced with four 19 mm (#6) deformed steelrods. The transverse reinforcement consisted of 6 mm (#2) steel ties spaced at 457mm (18 in). Each pier was supported by 1.2 × 1.2 × 0.75 m (4 × 4 × 2.5 ft) squarefooting. The actual length of the piers varied from 1.8 to 3.4 m (6 to 11 ft). Nocorrosion of reinforcement or concrete spalling was observed on the bridge piers.

STRENGTHENING SCHEMES

Seismic performance category (SPC) B was selected for the analysis of thebridge piers since it is relevant to Missouri (AASHTO, 1996). Under SPC Bcondition, the seismically induced lateral load at the top of the piers was determinedto be 160 KN (36 kips). The computed shear capacity of the piers was 338 KN (76kips). For flexure, the capacity for lateral load applied at the top of the piers in thelongitudinal direction varied from 98 KN (22 kips) to 53 KN (12 kips) for the shortestand tallest piers, respectively. The piers were therefore adequate in shear anddeficient in flexure. Three piers were strengthened and the fourth pier was used as abenchmark.

Two piers were strengthened for flexure using near-surface mounted carbonFRP rods. One pier was strengthened with 14 NSM rods, mounted on two oppositefaces, seven on each face. A second pier was strengthened with six NSM rods, threeon 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 werefully anchored (minimum 380 mm (15 in.)) into the footing of each pier. Finally, thetwo piers were wrapped with 4-ply of carbon FRP jacket. The third pier wasexternally jacketed with six plies of glass FRP sheets. As will be discussed later, thetest setup was designed such that the lateral movement of the piers was allowed at thetop while restrained at the base. Therefore, it was expected that the maximummoment would occur at the base of the pier. Consequently, the NSM rods were onlyanchored to the footing. In addition, anchoring the NSM rods to the top flare can not

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be easily achieved due to its shape (see Figure 1). The mechanical properties of theFRP sheets and rods are given in Table 1. Figure 2 summarizes the strengtheningschemes of the bridge piers.

Table 1: Mechanical properties of FRP reinforcement

FRP TypeDimension✝

mm[in]

DesignStrengthMPa [ksi]

Design Strainmm/mmor in/in

TensileModulus GPa

[ksi]Glasssheets*

0.353[0.0139]

1520[220] 0.0210 72

[10,500]Carbonsheets*

0.165[0.0065]

3800[550] 0.0170 228

[33,000]Carbonrods**

11[7/16]

1240[180] 0.0105 119

[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 weremade using conventional hand-held tools. The grooves were cleaned using sandblasting to remove all loose particles and dust. Surface preparation is important sincethe tensile stresses are transmitted from the concrete to the FRP rod through thebinding 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 onthe pier sides. The grooves and the drilled holes were then filled halfway with aviscous epoxy grout and the carbon FRP rods were installed. Another layer of epoxygrout was then applied and the surface was leveled.

All FRP jackets were installed by the wet lay-up process. The carbon andglass FRP sheets covered the entire height of the piers with the fiber directionperpendicular to the pier axis. The corners of the rectangular piers were rounded to 13mm (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 loadmuch larger than theoretically predicted. This was done to account for the possibilityof higher actual material strengths than initially presumed as well as for thestrengthening effect. The desired level of loading could only be applied by means ofhydraulic jacks. The test setup was designed such that it could induce reversingloading cycles in which the piers were allowed to displace laterally at the top. Toachieve this, a 250-mm (10-inch) strip of the deck was saw cut and removed. Thecentral portion of the cap beams were also saw cut and removed and a hydraulic jackwas inserted in the gap. A schematic of the test setup is shown in Figure 3. Thefunction of the internal jack was to apply the outward push force to the piers cap

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beam. Saw cutting the bridge deck and the cap beam allowed for the relativemovement of the cap beams and the topping deck. To pull the piers together, areaction frame was constructed such that it confined the cap beams. A set of twohydraulic jacks was then attached to the reaction frame. The internal and externaljacks were used alternately to create a static lateral loading cycles.

INSTRUMENTATION

The bridge piers were instrumented with electric strain gages installed on themounted rods as well as on existing steel reinforcement. Strain gages were alsoinstalled on the FRP sheets. An 890 KN (200 kips) capacity load cell was used tomeasure the applied force. The lateral displacement of the each pier was measured atmid-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 footingusing conventional drop anchors. The rotation of the cap beam, the pier, and thefooting was measured by means of inclinometers at three locations (see Figure 4).

TESTING PROCEDURE

Once the setup was erected, instrumentation was connected to the dataacquisition system and zero reading were taken. For safety reasons, the first loadingcycle was always a pull-in load condition. When the desired lateral force wasachieved, the system was unloaded and the hydraulic hoses were disconnected fromthe external jacks and connected to the internal jack. A push-out force was thenapplied. These two loading cycles were repeated until the weaker pier failed. To testthe second pier of the same bent, a diagonal bracing was installed against the failedpier, as shown in Figure 4. Prior to the testing of the piers of the second bent, thedeck slabs resting on the bent were jacked up using hydraulic jacks lubricated steelplates were inserted between the cap beam and the deck slabs. This action wasintended 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 predictedloads. In addition, all the piers underwent a double curvature type of behavior. Therotation restraint of the superstructure on the cap beam was larger than expected evenfor the bent with reduced friction. For the piers with reduced friction, larger rotationswere measured on the cap beam. In these piers, as the cap beam rotated it pushed thetopping decks upward. As a result, the point of application of the vertical force dueto the deck weight shifted to the edge of the cap beam. This behavior resulted in anadditional moment that acted at the top of the pier.

For the unstrengthened pier, the applied lateral load at failure was 351 KN (79kips) and the measured maximum lateral displacement at the top of the pier wasaround 15.5 mm (0.61 in.). Figure 5(a) illustrates the measured rotations at the lastloading cycle (push-out) of the unstrengthend pier. The continuous rotation underconstant 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

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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). Forthe pier strengthened with 7 CFRP bars on opposite sides and CFRP jacketing, thefailure was initiated by a crack occurred at the pier-flare intersection where themounted rods were terminated. After the crack occurred, the pier went through acontinuous rotation with no increase in load carrying capacity. The applied lateralload at failure was 360 KN (81 kips) and the measured lateral displacement at the topof 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 loadingcycle (push-out) of this pier. This figure indicates that at maximum load, the wholepier experience a rigid body rotation for a while, then a crack occurred at the top ofthe pier causing the cap beam rotation to reduce significantly due to the yielding ofthe reinforcement and the formation of a plastic hinge. After the formation of theplastic hinge and the redistribution of moments, the pier and the footing continued torotate at a faster rate indicating soil failure.

For the pier strengthened with three CFRP bars on two opposite sides andCFRP jacketing, cracks occurred at the top and the base of the pier, as shown inFigure 7. The footing of this pier was originally cast in a bedrock therefore norotation was measured at this footing, as shown in Figure 8. The figure illustrates themeasured rotations at the last loading cycle (push-out) of this pier. As a result of thelarge rotational stiffness of the footing, larger moment were developed at the base ofthe pier. Failure was initiated by the rupture of the FRP rods at the base of the pier ata load level of 382 KN (86 kips) with a maximum lateral displacement measured atthe top of 21.8 mm (0.86 in.). This indicates that the full capacity of the NSM rodscan be achieved, giving that the rods are adequately anchored. As for the pier withGFRP jacket only, the pier started to rotate as a rigid body at 222 KN (50 kips). Thetest was terminated when the lateral displacement exceeded 38.1 mm (1.5 in.). Thefailure mode of this pier was, therefore, a soil failure. It should be mentioned that theabove given displacements at maximum loads are the absolute displacement withoutaccounting to the rotation of the footing. The variation in failure modes and lateralload capacity may be related to the influence of superstructure/substructureinteraction, variation in the boundary conditions of each pier (e.g., footing rotationstiffness and friction forces), and the skew effect of the bridge bents.

ANALYTICAL MODELING

The basic objective of modeling is to provide the simplest mathematicalformulation of the true behavior of the pier, which satisfies a particular set of knownvalues (in this case, the measured response) for quantitative determination of theinternal forces. The developed structural model simulating the observed behavior ofa bridge pier is shown in Figure 9(a). The pier is simulated by a column, which isfree to displace laterally at the top and is restrained laterally at the bottom. Theflexibility of the footing is represented by a rotational spring with unknown constantk1 at the base of the column, which models the effect of footing rotation due to soildeformation. Another spring with unknown stiffness k3 is used at the top of thecolumn to model the effect of cap beam rotation due to the applied loading. The

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frictional force between the deck and the cap beam is represented by a linear springconnected to the top joint. The spring constants, k5, may vary at each load level dueto softening of the boundary conditions under repeated loading cycles. The model isanalyzed using the matrix displacement method. The overall structural stiffnessmatrix, internal forces, and external loading due to a given structural response can,therefore, be determined by simple matrix operations. For simplicity, the unknowninternal forces are determined at the locations of the nodes were displacements androtations were measured experimentally. Accordingly, each pier is represented by atwo-element, three-node column, as shown in Figure 9(b). The unknown rotations ofthe joints are denoted as X1, X2, and X3 and the unknown displacements of the jointsare denoted as X4 and X5. The nodal displacements are used as the degrees offreedom (DOFs) of the column. Thus, the column has five degrees of freedom. Thismodel is only applicable prior to pier cracking, after which a non-linear analysis isrequired. For the current investigation, the capacities of the piers were slightly largerthan the cracking capacity and for some cases failure was governed by the rigid bodyrotation of the pier, therefore the elastic analysis approach was valid at higher loadlevels.

An example of the analytical results is given in Figure 10 for the pierstrengthened with 7 NSM-rods on two opposite sides. Figure 10(a) shows themeasured deformations at the three nodes of the model due to a lateral load of 178KN (40 kips). The effect of footing rotation was included in determining the lateraldisplacements (X4 and X5) of the joints. Figure 10(b) shows the calculated externalloads and reaction of the column while Figure 10(c) shows a plot of the momentdiagram along the length of the column. The results indicated that the frictional forceis in the order of 57.6 KN (13 kips). However, this value was found to be reduced athigher load levels. The maximum moment occurred at the top of the pier due to thelarger rotational stiffness exerted by the superstructure on the cap beam than therotational stiffness of the soil. The analytical behavior correlates well with theexperimental results where the first crack on this pier occurred at the pier-flareintersection, the location of the maximum moment. Due to limited space, acomprehensive documentation of the structural modeling, structural analysis andanalytical results will be reported in a future publication.

CONCLUSION

The objective of this research program was to demonstrate the use near-surface mounted FRP rods to improve the flexural capacity of rectangular RC piers.Prior, to demolition, full-scale bridge piers were strengthened with FRP rods andsheets and tested to failure. Test results indicate that this strengthening technique iseffective in increasing the flexural capacity of the piers. Test results also indicate thatthe capacity and failure modes of the bridge piers are closely related to thesuperstructure/substructure interaction and the pier boundary conditions. Flexuralstrengthening of piers may cause the structural deficiency problem to shift anotherlocation within the structure. Therefore, flexural strengthening may require theretrofitting of beam-pier joints and foundations to account for the upgraded flexuralcapacity of the pier. In general, the determination of the elastic structural response

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under any load value is quite achievable with a reliable model in terms of well-defined boundary conditions and reasonably accurate material properties andstiffness.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge the funding provided by the MissouriDepartment 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 StructuralPreservation Systems, Baltimore, MD, provided and installed the FRP systems,respectively.

REFERENCES

“Standard specifications for Highway Bridges.” (1996). American Association ofState Highway and Transportation Officials (AASHTO), Washington, D.C.

Hakamada, F. (1997), “Experimental Study on Retrofit of RC Columns Using CFRPSheets.” Proc., Third Int. Sym. on Non-Metallic (FRP) Reinforcement forConcrete Structures (FRPRCS-3), Japan Concrete Institute, Tokyo, Japan, 1,419-426.

Macrae, G. A., Nosho, K., Stanton, J., and Myojo, T. (1997), “Carbon Fiber Retrofitof 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, 371-386.

Naval Facilities Engineering Service Center. (1998), “Navy Advanced Compositetechnology in Waterfront Infrastructure.” Special Publication Sp-2046-SHR.

Seible, F., Hegemier, G., Priestly, M. J. N., and Innamorato, D. (1995), “RectangularCarbon Fiber Jacket Retrofit Test of a Shear Column with 2.5% Reinforcement.”Report No. ACTT-95/05, University of California, San Diego.

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Figure 1. The Two Bents of the Bridge

Figure 2. Strengthening Schemes of the Piers

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)

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26’-3”

2W14 × 90

HydraulicJack

HydraulicJack

Cut throughbridge deck

DywidagRod

25’ Saw Cut 3’-6”

Pier

Bent 1 Bent 2

Above bridge deck Below bridge deck

Figure 3. Schematic of the Test Setup (1 in. = 25 mm)

Figure 4. Diagonal Bracing of the Failed Pier.

6” steel pipe

5’ x 5’

54o

P

LVDT

Inclinometer

F

steel pipe

RC footing

Failed

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Figure 5. Behavior of the Unstrengthened Pier (1 KN = 0.225 kip)

Figure 6. Behavior of the Pier with 14 NSM rods (1 KN = 0.225 kip)

(a) Measured Rotations (b) Final crack

(a) Measured Rotations (b) Final crack at flare

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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 = 0.225 kip)

(a) Top crack at pier-flare intersection (b) Pier base crack showing FRP rupture

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Figure 9. Analytical model of a bridge pier

Figure 10. Measured deformation and analytical results for pier with 14 NSM rods

P

k1

k3k5

L

(a) structural model

X1

X2X4

X3X5

2

1

①

②

③

L/2

L/2

(b) two-elements analytical model

156.9 KN-M

194.7 KN-M120.4 KN

(b) calculated external forces(a) measured response

P = 178 KN

2.97 m

X3 = 0.022 deg.

X5=0.515 mm

X2 = 0.046 deg.

X4=0.304 mm

X1 = 0.0306 deg.

194.7 KN-M

156.9 KN-M

(c) moment diagram