Development and Performance Assessment of a FRP Wrapped Balsa Wood Bridge Deck in Louisiana Steve C.S. Cai, Ph.D., P.E. Professor, Edwin B. Norma S. McNeil Distinguished Professor Dept. of Civil and Environmental Engineering Louisiana State University, Baton Rouge, LA 70803 Archana Nair, Miao Xia and Shuang Hou 1
Acknowledgments Federal Highway Administration IBRD Program and Louisiana Transportation Research Center (LTRC), LA DOTD for funding this project. Mr. Walid Alaywan for acting as project manager and Mr. Gill Gautreau, Mr. Nicholas Fagerburg, and other team members at LA DOTD for facilitating the project. The bridge deck was manufactured by Alcan Baltek Corporation and Dr. Kurt Feichtinger s effort for this project is greatly appreciated. The contents of this report reflect only the views of the writers who are responsible for the facts and the accuracy of the data presented herein. 2
Outline Introduction to Project OTDR-based slip monitoring FBG based moisture monitoring Field Instrumentation Plan Conclusions
Project Background CORIBM Bridge on route LA 70 in District 61, Assumption Parish Damaged bridge deck National Coast Guard requires a movable bridge. A light deck such as FRP deck is one of the options. 4
Final Selection: FRP-wrapped Balsa Wood Bridge Deck FRP Balsa wood bridge deck Damaged bridge deck 5
Bridge Deck Fabrication Balsa wood Fiber optic cable Hardwire Epoxy infusion
Concerns about the Bridge Integrity Debonding Proposing Fiber optic sensors for performance monitoring Moisture ingress 7
Issues of Traditional Sensors Model 4000 Vibrating Wire Strain Gage STS II Data Acquisition System and Intelliducer Foil strain gauges Fiber Optic Sensors 8
Fiber optic sensors have been used in structural health monitoring Advantages: Immunity to electromagnetic/radio frequency interference (EMI/RFI) Small size and light weight Long-term stability, no corrosion Distributed sensing capability Relatively safe in flammable environments 9
Fundamental structure of optic fiber Cladding: 125 m Core: Single mode: 9 m Multimode: 50/62.5 m Coating or buffer: 245 m The small size allows the optic fiber able to be easily coupled in FRP materials 10
Application of fiber optic sensors Transduction Mechanism Intensity OTDR ROTDR BOTDR Interferometer Fabry Perot(FP) Spectrometric Fiber Bragg Grating (FBG) Application Local FBG, FP Distributed OTDR, ROTDR, BOTDR Multiplexed FBG 11
Optic time-domain reflectometry (OTDR)- based fiber optic monitoring of slip 12
Principle of OTDR based monitoring AQ7250 mini-otdr Optic fiber Pulse Bending Connection Fiber end OTDR Back scatter Back scatter intensity Distance L=t v/2 t: time taken in the round trip V: light velocity in the waveguide 13
Optic fiber bending effect Optic power loss R: Curvature 1/ 2 AR exp( UR) 16 14 12 45 30 15 It has been used for concrete crack detection Power loss(db) 10 8 6 4 2 0 0 0.5 1 1.5 2 2.5 3 Crack width(mm) Concrete crack measurement 14
OTDR based slip monitoring method 15
Instrumentation plan of the project 16
Lab test setup Specimen Steel Two parts Interface 17
Sensor configuration 18
Lab tension test MTS machine Monotonic ramp load profile Resolution of displacement transducer: 0.5 m 19
OTDR traces Before the interface debonding the OTDR traces barely change 18.3 Power loss(db) 18.2 18.1 18 17.9 17.8 Due to original bent of optical fiber Induced by the debonding of the interface 17.7 17.6 17.5 120 125 130 135 140 145 150 Location(m) 20
OTDR traces At the final stage the fiber broke and there is a peak at the OTDR traces 40 35 30 Peak due to optical fiber breaking Power loss(db) 25 20 15 10 5 120 125 130 135 140 145 150 Location(m) 21
Interface slip vs. OTDR power loss 5 4 Power loss(db) 3 2 Interface failed 1 0 0 0.5 1 1.5 2 2.5 Slip(mm) 22
Fiber Bragg Grating(FBG)-based fiber optic monitoring of moisture 23
Principle of Fiber Bragg Grating sensor Strain & temperature S1 S2 S3 Sn 24
Gage factor calibration (constraint free) Adjustable temperature chamber b b G G ε T T G : train gage factor, G T : temperature gage factor, FBG interrogator Wavelength shift(nm) FBG1 1548.000 1547.950 y = 0.0105x + 1547. 1547.900 1547.850 1547.800 1547.750 1547.700 20 25 30 35 40 45 Temperature( ) FBG1 FBG2 FBG3 FBG4 FBG5 Wavelength(nm) 1547.739 1544.906 1549.901 1524.088 1527.205 Cage factor(pm/ C) 10.49 9.61 9.89 10.12 9.79 Theoretical value 10.32 10.30 10.34 10.17 10.19 Averaged gage factor =9.92pm/ C 25
Balsa wood moisture monitoring FBG sensor Swelling occurs as wood gains moisture FBG sensors are used to monitor the hook expansion induced from moisture First loop: Measure hook expansion Second loop: External force free to measure temperature only
Balsa wood moisture monitoring FBG sensor lab test 0-5 FBG moisture sensor Refercece FBG sensor Optic fiber Specimen Wavelength change(pm) -10-15 -20-25 -30-35 -40-45 Temperature sensor Moisture content is close to saturated state -50 0 2 4 6 8 10 12 14 16 Time(hour) Moisture ingress direction Moisture reaches the cross section of the sensor
Temperature gage factor calibration temperature gage factor 38pm/ C
Hoop moisture expansion of the specimen Moisture content is 70 close to saturated state 60 50 Strain( ) 40 30 20 10 Moisture reaches the cross section of the sensor 0 0 2 4 6 8 10 12 14 16 Time(hour) The moisture expansion can be easily indentified by subtracting temperature induced strain
Manufacturing and Field Installation 30
Slip Monitoring Scheme By a single optic fiber, multiple places can be measured simultaneously
FBG Sensor Instrumentation Traffic response monitoring Integrity between the bridge deck and girder Moisture monitoring
Imbedded FBG Sensors
Protection of Connector
FRP Balsawood Slab in LaDOTD Yard
OTDR Slip Sensor on Top of Girder Flange Slip sensor Top flange with epoxy
Bonding of FRP Slab and Steel Girder
Complete FRP balsa wood steel girder deck panel Ready to go
Installation of Slab -Steel Girder Panel 39
Field Instrumentation and Wiring 40
Field testing trucks 41
LADOTD Engineers 42
Typical comparison between BDI strain gages and FBG sensors (c) Deck 3 (north end) for load case N_SR1 (d) Deck 3 (north end) for load case N_D1_30 43
Finite element modeling 1 ELEMENTS U F 1 ELEMENTS U F Y Z X 1 NODAL SOLUTION STEP=1 SUB =1 TIME=1 EPELX (AVG) RSYS=0 DMX =.220065 SMN =-.354E-03 SMX =.506E-03 MN 1 ELEMENTS MAT NUM U CP MX Y Z X -.354E-03 -.259E-03 -.163E-03 -.677E-04.279E-04.123E-03.219E-03.315E-03.410E-03.506E-03 44
Table 4 Strain comparisons Girder Deck SG9 SG 10 SG11 SG 12 SG3 SG4 SG15 SG16 S_SS1_a G1_Top G1_Bott G2_Top G2_Bott D_1(S) D_2(S) D_1(N) D_2(N) BDI -42.45 101.55-57.5 86.1 241.5 223.5 131.5 91.1 FEM (C)* -52.2 123.44-51.36 120.36 172.19 168.92 38.8 31.2 FEM(N_C)** -144.47 144.5-125.97 127.6 198.75 201.58 45 43.5 S_SS1_b BDI -55.5 164.5-76.7 134 50.5 34.65 30.4 47.3 FEM (C) -91.07 179.6-66.01 176.55 6.4 7.96 27.8 22.4 FEM N_C) -227.7 224.87-185.73 194.16 7.242 7.72 34.5 41.1 S_SS1_c BDI -41.3 83.6-53.2 69.65 18.25 2.9725 216.5 263.5 FEM (C) -48.7 96.45-39.64 93.21 1.78 2.34 163.58 168.5 FEM(N_C) -118.6 118.6-104.35 104.6 1.995 2.33 202.12 200.9 45
Acceleration measurement Acceleration from sensors A1-A6 for N_D1_55 load case (north bound) 46
Typical monitored strain data 47
Conclusions A distributed fiber-optic monitoring method based on optic time domain reflectormetry(otdr) has been proposed to monitor the interface slip between bridge deck and girder which are bonded together. From the preliminary work, it can be concluded that the occurrence of interface failure at the critical slip value can be identified by the proposed sensing system and the extent of slip can be estimated by the induced optic power loss. A moisture monitoring techniques using FBG sensors is put forward, by which volume change is measured to indicate the moisture. The expansion of the FRP wrapped Balsa wood due to moisture action is evident and can be used to indicate the moisture change. 48
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