REPORT FHWA/NY/SR-02/138

Strengthening of Church Street Bridge PierCapbeam Using Bonded FRP CompositePlates: Strengthening and Load Testing

OSMAN HAG-ELSAFIRYAN LUNDSREENIVAS ALAMPALLI

SPECIAL REPORT 138TRANSPORTATION RESEARCH AND DEVELOPMENT BUREAUNew York State Department of TransportationGeorge E. Pataki, Governor/Joseph H. Boardman, Commissioner

Strengthening of Church Street Bridge Pier Capbeam Using Bonded FRP CompositePlates: Strengthening and Load Testing

Osman Hag-Elsafi, Engineering Research Specialist IRyan Lund, Civil Engineer ISreenivas Alampalli, Engineering Research Specialist II

Research Report 138April 2002

TRANSPORTATION RESEARCH AND DEVELOPMENT BUREAUNew York State Department of Transportation, State Campus, Albany, New York 12232-0869

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ABSTRACT

In this report, application of FRP laminated plates to strengthen the capbeam of Pier 3 of East ChurchStreet Bridge in Chemung County, New York, is discussed. Addition of a concrete wearing surfaceand a median barrier to the bridge superstructure increased dead load, contributing to deficiency inmoment and shear capacities of the capbeam structure. As a result, the concrete capbeam sufferedflexural and shear cracking, and was considered for strengthening using bonded FRP composite plates.Service load tests were performed before and after the plates were installed, to investigateeffectiveness of the strengthening system. Installation of the plates and results from the load tests arediscussed in the report.

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CONTENTS

I. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

A. BACKGROUND AND OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1B. REPORT ORGANIZATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

II. PLATES INSTALLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

A. STRENGTHENING PLATES SYSTEM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5B. SURFACE PREPARATION AND INSTALLATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

III. INSTRUMENTATION AND LOAD TESTING . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

A. INSTRUMENTATION AND DATA ACQUSITION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9B. LOAD TEST PLANS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

IV. RESULTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

A. STEEL REINFORCEMENT AND CONCRETE STRAIN RESULTS . . . . . . . . . . . . . . . . . 13B. FRP STRAINS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15C. DISCUSSION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

V. CONCLUSIONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31

I. INTRODUCTION

Transportation agencies are continuously searching for efficient ways to rehabilitate deterioratedconcrete bridges. Left untreated, such bridges may be load-posted or replaced for having insufficientcapacity to carry current legal loads. An alternative to posting for lower loads or replacement maysometimes be achieved by improving the load-carrying capacity of these bridges by retrofittingsuspected deficient members using bonded Fiber Reinforced Polymer (FRP) materials (1-11). Thesematerials offer many advantages, such as high strength, light weight, and non-corrosive properties.Additionally, installation of FRP materials for strengthening purposes is generally faster thanconventional rehabilitation methods and requires minimal interruption to traffic (1-5).

The New York State Department of Transportation (NYSDOT) has recently used, on experimentalbasis, FRP composite plates to strengthen the capbeam of Pier 3 of East Church Street Bridge inElmira, New York. This application and the load tests conducted to evaluate effectiveness of thestrengthening system are discussed in this report.

A. BACKGROUND AND OBJECTIVES

The East Church Street bridge was built in 1954. It carries State Route 352 in the City of Elmira, NY,over State Route 17, the Southern Tier Expressway and future Interstate 86. It is a four-span, multi-stringer steel structure, with a total length of about 67 m. It has two 3.66-m wide lanes, and two 1.07-m wide and 0.23-m high curbs. The average daily traffic counts on Routes 352 and 17 are about 9,000and 20000 vehicles, respectively. A side view of the bridge is shown in Figure 1.

An inspection in1997, during the Concrete Vulnerability Assessment Program, revealed excessivecracks at the capbeams of Piers 2 and 3 of the bridge. The two piers and photos of the cracks at Pier3 capbeam are shown in Figures 2 and 3, respectively. Each of the capbeams has a 1.1 m x 1.1 msquare cross-section, and is supported by three 1.1- m diam circular piers. An increase in dead load,as a result of recent additions of deck overlays and a median concrete barrier, was a suspected causeof the cracking. The structure, which was originally built with a 292-mm thick concrete deck and nobarrier, now has a 318-mm thick deck and a median concrete barrier weighing about 12kN/m. Apreliminary analysis indicated possible capbeams’ deficiencies in both moment and shear capacitiesunder current service loads. A decision was made to use bonded FRP materials to strengthen thecracked beams, to demonstrate the materials use and investigate their feasibility as a cost-effectivetechnique for bridge rehabilitation. Two types of materials were considered: bonded-FRP laminatesfor Pier 2 capbeam and pre-cured FRP plates for Pier 3 capbeam. Rehabilitation of Pier 2 capbeam

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was documented in Reference 7. For Pier 3 capbeam, design objectives were set, based on thepreliminary analysis, to increase the beam’s shear and moment capacities by about 10 and 20 percent,respectively, using the FRP plates system shown in Figure 4. The structure was instrumented withstrain gages and load-tested twice to evaluate effectiveness of the FRP retrofit system.

Figure 1. Church Street Bridge.

Pier 3 Pier 2 Pier 1

Pier 3 Capbeam

Rte 17 West Rte 17 East

14.78 m 20.65 m 18.44 m 11.73 m

North

Rte 352 (East Church Street)

Pier 2 Capbeam

Figure 2. Bridge elevation.

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(a) South elevation of Piers 2 and 3 capbeams. (b) Cracks at center of Pier 3 capbeam.

Figure 3. Pier 3 capbeam.

7.13 m

Notes: 1. All plates applied 2. Moment plates are 254 mm x 13 mm. on both beam sides. 3. Shear plates are 6-mm thick.

1.1 m diam.1.1 m x 1.1 m

Beam: Column:

Positive momentregion plate (Typ.)

Negative moment region plate

Bridge deckShear platesShear plates

7.13 m

East

West

Steel girder (Typ.)

Figure 4. FRP strengthening system.

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B. REPORT ORGANIZATION

In the next chapter, the FRP strengthening system is described, and procedures followed for surfacepreparation and plates installation are explained. Instrumentation and data acquisition system usedduring the load tests are also described in the same chapter. The results for before and afterinstallation tests are presented in Chapter 3. The chapter also includes discussion on consistency ofthe test data and structural behavior of the beam during the testing, neutral axis location in the positivemoment region, bond between the FRP plates and the capbeam concrete, and effect of thestrengthening system on shear behavior. Conclusions and future efforts are described in the finalchapter of the report.

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II. PLATES INSTALLATION

A. STRENGTHENING PLATES SYSTEM

Flexural and shear plates, made of carbon/glass hybrid and glass materials, respectively, were usedfor strengthening of Pier 3 capbeam (Figures 5a and 5b). The flexural plates are 13-mm thick, 254-mm wide, and 5.18-m long. The shear plates are 7-mm thick, 1.1-m wide, and 1.7-m long, shaped

as shown in Figure 5b. The modulus of elasticity of the [0 carbon, 0 glass] hybrid flexure platesmaterial is 69000 MPa and its ultimate strain is 0.5%. The shear modulus for the [+45, -45] glassshear plates material is 6900 MPa and its maximum shear strain is 0.3%.

(a) Flexure plate. (b) Shear plate.

Figure 5. Strengthening system.

B. SURFACE PREPARATION AND INSTALLATION

The capbeam was pressure washed with water to remove any loose materials on the beam surface thatcould lead to plate debonding. The beam surface was then allowed to dry. On the day of theinstallation, plates were positioned and aligned on the beam sides, marked, and holes were pre-drilledthrough the plate and concrete at various locations. A proprietary epoxy mix, consisting of a resinand a hardener, was then applied to the plate or the concrete surface. Anchor bolts, installed at thepre-drilled hole locations, were used to mechanically maintain tight contact between the plates andconcrete surface (Figure 6). The epoxy mix was prepared as per the manufacturer's specifications. Onone side of the beam, the epoxy was first applied to the plates before they were mechanically fastenedto the concrete. On the other side, a different method was attempted. First, the epoxy was applied tothe concrete surface and then the plates were held tight against the beam using a wood frame clamping(Figure 7). During and after installation photos of the capbeam are shown in Figure 8. After the plates

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were installed, a tar coat was applied on top of them to prevent water from seeping between the platesand concrete, in case air voids were present. The plates were then painted for aesthetics reasonsand protection from ultraviolet light. The after load test was conducted 14 days after installation ofthe plates; the manufacturer recommended a 7-day minimum cure time.

(a) Flexure plate. (b) Shear plate.

Figure 6. Mechanical clamping.

Figure 7. Wood frame clamping of shear plates.

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(a) During installation. (b) After installation.

Figure 8. FRP plates system.

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III. INSTRUMENTATION AND LOAD TESTING A. INSTRUMENTATION AND DATA ACQUISITION

The main objective of the testing program was to evaluate effectiveness of the FRP plates system inreducing current live load stresses in the capbeam structure. Instrumentation and load test plans weretherefore devised to provide data that would assist in this evaluation. For the before installation loadtest, flexural strains were measured in the steel reinforcement bars and concrete in both positive andnegative moment regions. Concrete strains, in a rosette setting, were also measured near the centerpier at the beam mid-depth for shear investigation. For the after installation load test, in addition tothe strains measured in the before installation test, flexural strains in the FRP plates in both positiveand negative moment regions were measured. Flexural concrete strains along a vertical line passingthrough the center of the center pier were also measured during the after installation load test. Strain-gage locations for the before installation test are shown in Figure 9, and the locations of those addedin the after installation test are shown in Figure 10. In these figures the prefix letters C, S, and F ingage numbers stand for the surface the gage is mounted on, i.e. concrete, steel, and FRP, respectively.All gages were mounted on the North face of the Northeast side of the capbeam. Gage locations arefurther described in Tables 1 and 2, where “+ve” and “-ve” stand for positive and negative,respectively.

Using the data recorded by the above gages, flexural strains for all materials in both positive andnegative moment regions, neutral axis location, relative slip between the FRP plates and the concrete,and shear strains in the concrete and the plates near the center pier can be obtained.

East

C-2C-3

C-9C-4

C-8C-7

S-3 S-4

C-10, C-11

C-5C-0

C-1 C-6

S-1, S-2

S-3, S-4

West

C-11

S-1 S-2

C-2C-3

C-9C-4

C-8 C-7

C-0, C-5

C-1, C-6

A-AB-BC-C

C-10

1.83 m

3.43 m

89 mm

70 mm 70 mm

89 mm

533 mm

C.L. C.L.

Center bars (Typ.)

C.L.

Figure 9. Before installation instrumentation plan.

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GageNumber

Description

C-0 +ve mom. region, top-left gage, on concrete

C-1 +ve mom region, mid-depth, left gage, on concrete

C-2 Shear region, top rosette, vertical gage, on concrete

C-3 Shear region, top rosette, 45E gage, on concrete

C-4 Shear region, top rosette, horizontal gage, on concrete

C-5 +ve mom. region, top-right gage, on concrete

C-6 +ve mom. region, mid-depth, right gage, on concrete

C-7 Shear region, bottom rosette, vertical gage, on concrete

C-8 Shear region, bottom rosette, 45Egage, on concrete

C-9 Shear region, bottom rosette, horizontal gage on concrete

C-10 -ve mom. region, bottom-left gage, on concrete

C-11 -ve mom. region, bottom-right gage, on concrete

S-1 +ve mom. region, North side gage, on bottom steel bar

S-2 +ve mom. region, South side gage, on bottom steel bar

S-3 -ve mom. region, North side gage, on top steel bar

S-4 -ve mom. region, South side gage, on top steel bar

Table 1. Before installation gage locations.

F-8

F-1

F-9East

West

F-2

A-AB-BC-C

F-2

F-3F-4

F-8F-5

F-7F-6

F-1

F-3F-4

F-8F-5

F-7 F-6 C-12

C-12

3.43 m

1.83 m

146 mm

89 mm

76 mm

Center bars (Typ.)

1.1 m

FRP moment plate (Typ.)

FRP shear plate (Typ.)

FRP shear plate (Typ.)

C.L.C.L. C.L.

533 mm

Figure 10. After installation instrumentation plan.

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GageNumber

Description

F-1 +ve mom. region, gage in-plane with gages S-1 and S-2, on plate

F-2 +ve mom. region, gage at mid-depth of plate

F-3 Shear region, top rosette, vertical gage, on shear plate

F-4 Shear region, top rosette, 45E gage, on shear plate

F-5 Shear region, top rosette, horizontal gage, on shear plate

F-6 Shear region, bottom rosette, vertical gage, on shear plate

F-7 Shear region, bottom rosette, 45E gage, on shear plate

F-8 Shear region, bottom rosette, horizontal gage, on shear plate

F-9 -ve mom. region, gage in plane with gages S-3 and S-4, on plate

C-12 -ve mom. region, bottom gage, on concrete

Table 2. After installation gage locations.

Two types of strain gages were used. Type EA-06-250AE-350, general purpose 350 Ù, selftemperature-compensating, constantan foil-strain gages, were mounted directly on the steelreinforcement bars and the FRP plates. Type EA-06-20CBW-120, 120 Ù constantan strain gages withlarge measuring grids were bonded to the concrete using an epoxy resin. A total of 4 strain gageswere mounted on the steel bars and 12 on concrete in the before installation load test (Figure 9). Anadditional 10 gages were bonded to the FRP plates and 1 gage was mounted on the concrete for theafter installation load test (Figure 10). All the gages were made watertight and protected from theenvironment for long-term monitoring use. A general purpose strain gage measurement system wasemployed for data acquisition.

B. LOAD TEST PLANS

Four trucks, each closely resembling AASHTO M-18 Truck configuration (12), were used in both thebefore and after load tests (Figure 11a). Average weight of each of these trucks is approximately 196kN. By assigning a unique letter, A through D, to each of the four trucks (Figure 11b), the testing wassequenced as follows. Truck A, Trucks A & B, Truck B, Truck C, Trucks C & D, Truck D, TrucksA, B, C &D, Trucks B & D, and Truck D. Locations of the trucks on the bridge are shown in Figures11b and12. The stresses and existing cracking in the capbeam were closely monitored during thetesting, to decide on whether to proceed with the next loading sequence or not. The maximum momentsand shear forces developed in the capbeam under maximum loading conditions (Trucks A, B, C & D)were close to those produced by MS-18 AASHTO loading (12).

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4.57 m1.52 m 1.09 m

Truck ATruck B

Truck C Truck D

C.L. pier/capbeam

C.L. Bridge

Traffic

Traffic

South

North

0.305 m

0.305 m

(a) Configuration. (b) Truck positions on the bridge.

Figure 11. Test loading.

Figure 12. All four trucks on the bridge during the testing.

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IV. RESULTS

Strain gage data recorded during the before and after installation load tests were processed, andchecked for validity. The results obtained using these data are discussed in this chapter. Gage C-5recorded unrealistically low readings, and Gage C-11 malfunctioned during both tests. Gage C-10which malfunctioned during the before test was replaced and was operational during the afterinstallation test.

A. STEEL REINFORCEMENT AND CONCRETE STRAIN RESULTS

The strain results for the steel and concrete gages used in the before and after installation tests aresummarized in Figure 13 for the various truck sequences. These results clearly demonstrate a smalldifference in strains between the before and after installation tests. The truck loading sequencesinvolving only Trucks A and B (Figures 13a to 13c), which were positioned on the same side of thestructure where the gages were mounted, resulted in compressive strains in the top midspan concretegage (Gage C-0) and tensile strains in all other remaining non-rosette gages. As expected, Trucks A& B loading sequence (Figure 13b) produced the highest flexural strains in the midspan steel andconcrete gages in the positive moment region. For this loading sequence, the highest strains recordedfor the steel and concrete gages in this region were about 60 and 50 microstrain, respectively (Figure13b). Based on these strains, the maximum positive service load moment produced in the capbeamdue to this loading sequence can be estimated as 221 kN-m.

For the loading sequences involving only Trucks C and D (Figures 13d to13f, and 13i), which werepositioned on the opposite side of the structure where the gages were mounted, resulted in tensilestrains in the top midspan concrete gage (Gage C-0) and top steel reinforcement bars over the centerpier gages (Gages S-3 and -4); and compressive strains in all other remaining non-rosette gages. Thisis a state of negative bending in the positive moment region, resulting on an opposite effect to thatproduced by Trucks A and B combinations in this region. The highest compressive strain in the steelreinforcement bars (Gages S-1 and -2, measuring about 12 microstrain) and the highest tensile strainin the midspan top concrete (Gage C-0, measuring about 10 microstrain) were obtained for Trucks C& D loading sequence (Figure 13e), as expected. From these strains the maximum negative serviceload moment in the positive moment region can be estimated as 46 kN-m.

In the negative moment region, the highest strains recorded in the steel reinforcement (Gages S-3 and-4) for Trucks A, B, C & D loading sequence, measuring about 35 microstrain (Figure 13g).Generally, this loading sequences (Trucks A, B, C & D) is expected to produce maximum stresses inthe negative moment region steel reinforcement bar gages. However, from Figure 13g, it is clear thathighest strains were recorded by the positive moment steel reinforcement gages (Gages S-1 and -2)instead. This may be explained by the fact that negative moment steel gages (Gages S-3 and -4) were

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mounted slightly off-center of the center pier centerline. The slight difference between the results ofthe two Truck D sequences (Figures 13f and 13i) may be attributed to minor changes in truck positionsduring the before and after installation tests.

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4Gage Number

Before After

Truck A

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4

Gage Number

Before After

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(a) (b)

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4

Gage Number

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Trucks B

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4Gage Number

Before After

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4

Gage Number

Before After

Trucks C & D

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4

Gage Number

Before After

Truck D

(e) (f)

Figure 13. Concrete and steel strains for both tests.

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4

Gage Number

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Trucks A,B,C & D

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4

Gage Number

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Trucks B & D

(g) (h)

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C-0 C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8 C-9 S-1 S-2 S-3 S-4

Gage NumberBefore After

Truck D

(i)

Figure 13. Continued.

B. FRP STRAINS

Results for the FRP and the additional concrete gages mounted during the after installation test aresummarized in Figure 14. This figure clearly shows that the FRP plates participated in carrying theapplied loads, as evident by the recorded nonzero strains on the plates. Similar to concrete and steelgages results, the sequences involving Trucks A and B (Figures 14a to 14c), induced tensile strainsin both positive and negative moment regions FRP gages (Gages F-1 and -2, and Gages F-8 and -9).The highest tensile strains recorded for the positive and negative moment region plates, due to theseloading sequences were about 39 and 12 microstrain, respectively, obtained for Trucks A & Bsequence (Figure 14b).

For the sequences involving Trucks C and D, again similar to concrete and steel gages results, tensilestrains were obtained in the negative moment region FRP gages (Gages F-8 and -9), and compressivestrains in the positive moment region FRP gages (Gages F-1 and -2). The highest compressive strain(about 20 microstrain) in the positive moment region plate (Gage F-1) was obtained for Trucks C &D loading sequence (Figure 14e). This clearly illustrates reversal in the stress state in the positive

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moment region FRP plate, and its possible implications on plate design issues, such as fatigue.

The highest tensile strains recorded for the positive and negative moment region plates, respectively,were about 42 and 24 microstrain (Figures 14b and 14g), obtained for Trucks A, B, C & D loadingsequence. As expected, this loading sequence (Trucks A, B, C & D) resulted in maximum strains inthe negative moment region FRP plate gages. However, those strains were also expected to be higherthan those recorded by the positive moment region FRP plate gages. This discrepancy may beexplained by the fact that negative moment plate gages (Gages F-8 and -9) were mounted slightly off-center of the center pier centerline to match locations of existing steel reinforcement bars gages(Figures 9 and 10). Additionally, among all loading sequences, Trucks A & B sequence was expectedto result in the highest strains in the positive moment region plate (Figure 14b). However, the strainsobtained for Trucks A, B, C & D sequence were slightly higher than those recorded for Trucks A &B sequence (Gage F-1 in Figures 14g and 14b). The difference is small and may be attributed to slightchanges in truck positions.

Comparing Figures 14f and 14i, it is noted that the strain for Gage F-1 reversed direction during theunloading sequence in Figure 14i. To further investigate this issue, the cumulative plots in Figures 15were generated to observe for any inconsistency caused by this gage readings. These plots are labeledTrucks A+B, Trucks A+B+C, etc .., denoting superposition of the individual sequences strains for thetrucks joined by the “+” sign. For example, in Figure 15c, the results shown for the four truckssequence were obtained in two additional ways: as the sum of the sequences Truck A, Truck B, TruckC, and Truck D results, or as the sum of the sequences Trucks A & B and C & D results. From thesecumulative plots, it is clear that the highest variation in strain results (generally on the high side) areassociated with Gage F-1, signifying a bias in the strains recorded by this gage. Based on this andobserving the consistent results in Figures 13f and 13i obtained for Gages S-1 and -2, which weremounted in close proximity to Gage F-1, it can only be concluded that Gage F-1 completely driftedduring Truck D unloading sequence in Figure 14i.

The good agreement between compared strains for respective gages other than Gage F-1, reflected byFigure 15 clearly demonstrate linear structural behavior during the testing and applicability ofsuperposition principles.

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C-10 F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 C-12Gage Number

Truck C

(c) (d)

Figure 14. FRP and concrete strains during the after installation test.

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Trucks A,B,C & D

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C-10 F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 C-12Gage Number

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C-10 F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 C-12Gage Number

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(i)

Figure 14. Continued.

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C-10F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 C-12Gage NumberA & B A+B

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40

60

Mic

rost

rain

C-10 F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 C-12Gage Number

C & D C+D

(a) (b)

-60

-40

-20

0

20

40

60

Mic

rost

rain

C-10 F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9C-12Gage Number

A & B & C & D A+B+C+D A&B+C&D

-60

-40

-20

0

20

40

60 M

icro

stra

in

C-10F-1 F-2 F-3 F-4 F-5 F-6 F-7 F-8 F-9 C-12Gage Number

B & D B+D

(c) (d)

Figure 15. Cumulative truck loading strains.

C. DISCUSSION

a. Data Consistency and Linear Behavior

From the truck positions on the bridge and axle loads, the moments induced in the capbeam structureduring the various test sequences can be obtained and plotted against measured strains as shown inFigures 16 through 19. These figures demonstrate linear behavior during the testing and proveconsistency of the test data, reflected by the relatively small scatter of recorded strains about thebest-fit line. Comparing the before and after installation figures for the negative moment region(Figure 16 versus Figure 17), it can be concluded that the FRP plates system reduced service loadstresses in the steel reinforcing bars by about 10 percent. (For a 325 kN-m moment, for example,the steel bars strain is reduced from 28 to 25 microstrain). Pursuing a similar comparison for thepositive moment region (Figure 18 versus Figure 19), it can be concluded that the FRP plates systemslightly reduced service load stresses in the steel reinforcement bars at this region, by about 6

20

percent. (For a 175 kN-m moment, for example, the steel bars strain is reduced from 45 to 42microstrain). This is a very small difference which is also supported by the fact that the capbeamstiffness, as measured by the slope of the best fit line in Figures 18 and 19, did not exhibit asignificant change after installation of the plates.

5

10

15

20

25

30

35

Stee

l Mic

rost

rain

50 100 150 200 250 300 350 Moment (kN*m)

Gage S-3

Linear Fit

Negative MomentBefore

Figure 16. Before installation steel strain (εxx) in negative moment region.

5

10

15

20

25

30

35

Stee

l Mic

rost

rain

50 100 150 200 250 300 350 400 Moment (kN*m)

Gage S-3

Linear Fit

Negative MomentAfter

Figure 17. After installation steel strain (εxx) in negative moment region.

21

-20 -10

0 10 20 30 40 50 60

Stee

l Mic

rost

rain

-100 -50 0 50 100 150 200 Moment (kN*m)

Gage S-2

Linear Fit

Positive MomentBefore

Figure 18. Before installation steel strain (εxx) in positive moment region.

-20

0

20

40

60

Stee

l Mic

rost

rain

-100 -50 0 50 100 150 200 250 Moment (kN*m)

Gage S-2

Linear Fit

Positive MomentAfter

Figure 19. After installation steel strain (εxx) in positive moment region.

b. Neutral Axis Location

The intent here is to investigate changes in neutral axis location in the positive moment region afterthe capbeam was strengthened. The negative moment region was not considered in thisinvestigation because both Gages C-10 and -11, which were mounted to measure strain in bottomconcrete in that region, malfunctioned during the before installation load test. Calculated neutralaxis locations (measured from the top of the beam as shown in Figure 20) are given in Tables 3 and4, respectively, for the before and after installation loading sequences. Concrete stresses in bothtables were estimated based on Gage C-0 readings, because Gage C-5 gave unrealistically lowreadings during both tests. For steel reinforcement bar strains, the average of Gages S-1 and -2 wasused in the tables. From these tables, the average neutral axis locations (Figure 20) during thebefore and after installation load tests are estimated at 478 and 487 mm, respectively. The standarddeviations associated with these averages are 28 and 35 mm, respectively. Based on thisinvestigation, it may be concluded that the neutral axis moved slightly downwards after the FRPplates system was installed.

22

Figure 20. Beam section and strain diagram in positive moment region.

LoadingSequenc

e(Truck)

Strain(µε) Depth to

N/A(mm)Top

ConcreteBottomSteel

Estimatedat FRPLevel

A -32 37 31 483

A&B -45 60 51 432

B -33 33 28 508

C 5 -7 -6 -

C&D 8 -10 -8 457

D 5 -7 -6 -

A,B,C,&D -46 54 46 457

B&D -30 29 25 508

D 2 -5 -5 -

Average N/A Location* 478

Standard Deviation* 28* Excluding Trucks C and D individual sequences.

Table 3. Before installation neutral axis location.

23

c. Bond Between the FRP Plates and Concrete

This is an investigation to check that strain compatibility between the concrete and the FRP platesduring the after installation test was maintained. From Table 4, the strains at mid-depth of thepositive moment region FRP plate were estimated using the top concrete and bottom steelreinforcement strain readings (Figure 20). Estimated strain readings are also compared with thosemeasured at that location (Gage F-2) during the load testing. Comparing respective estimated andmeasured strain values in the table, it is noted that estimated strains are consistently higher (about100 percent) than those measured. This discrepancy may be attributed to sensitivity of computedFRP strains to the noise in measured steel and concrete strains and/or improper bonding of the FRPplate to the capbeam concrete. Actual recorded strains used in Table 5 indicate a better comparisonbetween recorded Gage F-1 and Gages S-1 and -2 strains (an average difference of about 30percent). Similar comparison in the negative moment region between Gage F-9 and Gage S-4 in thetable shows even better bond development between the FRP plate and capbeam concrete (an averagedifference of about16 percent). Thus, it can be concluded that the significant difference betweenestimated and actual Gage F-2 readings in Table 4 is largely due to sensitivity of calculatedestimates to the noise levels in the steel and concrete gage readings used in the analysis.

LoadingSequence

(Truck)

Strain(µε)

Difference(%)

Depth to N/A(mm)Top

ConcreteBottomSteel

Estimatedat FRPLevel

Actualat FRPLevel

A -34 37 31 15 51 483

A&B -50 58 50 26 48 483

B -34 36 30 15 50 508

C 4 -8 -7 -4 42 -

C&D 8 -11 -10 -5 50 432

D 4 -9 -8 -5 38 -

A,B,C, &D 44 48 41 22 45 483

B&D -33 28 23 11 52 559

D -1 -5 -5 -3 40 -

Average N/A Location* 49 487

Standard Deviation* 2 35* Excluding Trucks C and D individual sequences.

Table 4. After installation neutral axis location and FRP plate strains.

24

LoadingSequence

(Truck)

Positive Moment Region Negative Moment Region

Strain(µε) Difference

(%)

Strain(µε) Difference

(%)Gage F-1 BottomSteel

Gage F-9 TopSteel

A 23 37 38 7 9 22

A&B 37 58 36 11 13 15

B 26 36 28 7 6 16

C -5 -8 - 9 10 -

C&D -17 -11 55 13 14 7

D -20 -9 - 8 8 -

A,B,C, &D 44 48 8 27 28 4

B&D 32 28 14 16 23 30

D 14 -5 - 9 13 -

Average % Difference* 30 Average %Difference* 16

Standard Deviation* 15 Standard Deviation* 9* Excluding Trucks C and D individual sequences.

Table 5. After installation strain compatibility assessment. d. Shear Investigation

The effect of the FRP system on shear is summarized in Figures 21 to 23. In each of these figures,the average shear strain γxy is plotted against the shear force near the center pier due to some of thetruck loading sequences used in the tests. These figures also clearly demonstrate linear behaviorduring the testing. However, because of the low strain levels recorded by the rosette gages, thereis more scatter about the best-fit line in the shear data than that observed for the flexural steel resultsin Figures 16 through 19.

Comparing the before and after installation concrete shear strain results (Figure 21 versus Figure22), it can be concluded that installation of the FRP plates reduced shear strains by about 10 percent.The after installation shear strains on the FRP shear plate (Figure 23) were higher than thosemeasured in neighboring concrete because of the increase in shear force at the plate location due tothe presence of one of the bridge girders (see Figure 6b) between the two locations.

25

-0.5

0

0.5

1

1.5

2

Shea

r Mic

rost

rain

0 20 40 60 80 100 Shear Force (kN)

Data fit

Average

Figure 21. Before strengthening shear strain (γxy) on concrete.

0.2 0.4 0.6 0.8

1 1.2 1.4 1.6

Shea

r Mic

rost

rain

0 20 40 60 80 100 120 140 Shear Force (kN)

Data fit

Average

Figure 22. After strengthening shear strain (γxy) on concrete.

26

-1

0

1

2

3

4

Shea

r Mic

rost

rain

0 20 40 60 80 100 120 140 Shear Force (kN)

Data fit

Average

Figure 23. After strengthening shear strain (γxy) on composite.

27

V. CONCLUSIONS

Application of FRP plates for improving flexural and shear capacities of a cracked reinforcedconcrete, bridge capbeam was discussed in this report. The structure was load tested before andafter the plates were installed, to evaluate effectiveness of the strengthening plates system inreducing live load stresses in the capbeam. The test results indicated that the system moderatelyreduced live load stresses in the steel reinforcement bars in the negative and positive momentregions by about 10 and 6 percent, respectively. Based on further analysis using these results, thelocation of the neutral axis in the positive moment region was observed to have shifted downwards,as expected, after the beam was strengthened. Strain compatibility analysis proved that the flexuralplates were well bonded to the beam concrete, with a difference in strain between respectivelocations ranging from 16 to 30 percent on average. Analysis of shear strain data showed thatconcrete shear strain at the critical section near the center pier was reduced by 10 percent.

This project provided an opportunity for the NYSDOT to demonstrate the feasibility of using FRPmaterials as a cost-effective bridge retrofit technique. Compared to conventional repair methods,the retrofit system used in this project proved to be a cost-effective solution ($18k vs $150k). Thesystem was relatively easy to install and caused minimal interruption to traffic. These are importantfeatures which make it attractive to bridge owners, specially for similar applications in highly-populated metropolitan areas.

29

ACKNOWLEDGMENTS

Instrumentation and data collection was conducted by George Schongar and Harry Greenberg of theTransportation Research and Development Bureau. Frank Owens, formerly with the Bureau,assisted in this project. Arthur Yannotti of Structures Division reviewed this report. JerryO’Connor, Regional Bridge Management Engineer of Region 6, was instrumental in therehabilitation of the bridge. He and his staff assisted with the traffic control and load testing, andalso reviewed the report. The contribution of Mark Norfolk and Hector Hoyos of Region 6 isparticularly acknowledged.

31

REFERENCES

1. Hag-Elsafi, O., Alampalli, S., Kunin, J., and Lund, R. “Application of FRP Materials in BridgeRetrofit,” Seventh Annual International Conference on Composites Engineering, Denver, CO,pp. 305-306, July 2000.

2. Alampalli, S., O=Connor, J.S., and Yannotti, A.P. “Advanced Composites for Cost-EffectiveRehabilitation of Bridges,” Advances in Composite Materials and Mechanics, ASCE SpecialPublication, pp. 76-84, 1999.

3. Alampalli, S., J. O’Connor, and Yannotti, A. “Design, Fabrication, Construction, and Testingof a FRP Superstructure,” Special Report 134, Transportation Research and DevelopmentBureau, New York State Department of Transportation, Albany, NY, December 2000.

4. Hag-Elsafi, O., Kunin, J., and Alampalli, S. “Evaluating Effectiveness of FRP Composites forBridge Rehabilitation Through Load Testing,” Structural Materials Technology: An NDTConference, Atlantic city, NJ, pp. 434-439, February-March 2000.

5. Hag-Elsafi, O., Kunin, J., and Alampalli, S. 2000. “Strengthening of Route 378 OverWynantskill Creek in New York Using FRP Laminates,” Research Report 135, TransportationR&D Bureau, NYSDOT, Albany, NY, March 2001.

6. Hag-Elsafi, O. and Alampalli, S., “Strengthening Prestressed-Concrete Beams Using FRPLaminates,” Structural Materials Technology. An NDT Conference, Atlantic city, NJ, pp.287-292, February-March 2000.

7. O’Connor, J., Hoyos, H., Yannotti, A., Alampalli, S., and Luu, K. “Reinforced Concrete Cap-Beam Strengthening Using FRP Composites,” Proceedings of the Fourth InternationalSymposium, Fiber Reinforced Polymer Reinforcement for Reinforced Concrete Structures,ACI, Baltimore, 1999.

8. Aref, A., and Alampalli, S. “Performance of a Skewed Fiber Reinforced Plastic Bridge,”Seventh Annual International Conference on Composites Engineering, Denver, CO, pp. 33-34,July 2000.

9. Halstead, J.P., O=Connor, J.S., Alampalli, S., and Minser, A. “Evaluating FRP Wrap with NDTMethods,@ Structural Materials Technology: An NDT Conference, Atlantic city, NJ, pp.275-280, February-March 2000.

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10. Hag-Elsafi,O., Lund, R., and Alampalli, S. “Strengthening of Church Street Bridge CapbeamUsing Bonded FRP Composite Plates: Finite Element Investigation and Comparison withLoad,” Research Report, Transportation R&D Bureau, NYSDOT, Albany, NY (in preparation).

11. Alampalli, S. “Modal Analysis of a Fiber-Reinforced Polymer Composite Highway Bridge,”XVIII International Modal Analysis Conference, San Antonio, TX, pp. 21-25, February 2000.

12. “Standard Specifications for Highway Bridges,” 1996 (16th ed.ition), American Association ofState Highway and Transportation Officials, Washington, D.C.