Condition assessment of GFRP-retrofitted concrete cylinders using electromagnetic waves

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Condition Assessment of GFRP-Retrofitted

Concrete Cylinders Using Electromagnetic Waves

by

Tzu-Yang Yu

Submitted to the Department of Civil and Environmental Engineering

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy in the field of Structures and Materials

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

June 2008

c

Massachusetts Institute of Technology 2008. All rights reserved.

Author . . . .

Department of Civil and Environmental Engineering

May 8, 2008

Certified by . . . .

Oral Buyukozturk

Professor of Civil and Environmental Engineering

Thesis Supervisor

Accepted by . . . .

Daniele Veneziano

Chairman, Departmental Committee for Graduate Students

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Condition Assessment of GFRP-Retrofitted Concrete

Cylinders Using Electromagnetic Waves

by

Tzu-Yang Yu

Submitted to the Department of Civil and Environmental Engineering on May 8, 2008, in partial fulfillment of the

requirements for the degree of

Doctor of Philosophy in the field of Structures and Materials

Abstract

The objective of this study is to develop an integrated nondestructive testing (NDT) capability, termed FAR NDT (Far-field Airborne Radar NDT), for the detection of defects, damages, and rebars in the near-surface region of glass fiber reinforced poly-mer (GFRP)-retrofitted concrete cylinders through the use of far-field radar mea-surements (electromagnetic or EM waves). In this development, two far-field mono-static ISAR (inverse synthetic aperture radar) measurement schemes are identified for collecting radar measurements, and the backprojection algorithm is applied for processing radar measurements into spatial images for visualization and condition assessment. Reconstructed images are further analyzed by mathematical morphol-ogy to extract a numerical index representing the feature of the image as a basis for quantitative evaluation. The components of the development include dielectric modeling of materials, laboratory radar measurements, numerical simulation, and image reconstruction. It is found that using the developed technique the presence of near-surface defects can be detected by the oblique incidence measurements. Radar signals in the frequency range of 8 GHz to 18 GHz are found effective for damage detection in the near-surface region of the specimens. Numerical simulation using the finite-difference time-domain (FDTD) method is conducted to understand the propa-gation and scattering of EM waves from the defects and inclusions in two-dimensional and three-dimensional GFRP-concrete models. The FDTD simulation is capable of predicting the far-field response of GFRP-concrete cylinders and beneficial to bet-ter understanding the patbet-tern of field measurements in the application of the FAR NDT technique. Dielectric properties of materials are investigated for their use in numerical simulation and for improving the precision of reconstructed images. Re-constructed images of GFRP-concrete cylinders with and without artificial features (rebar and defect) clearly indicate the presence of these features. Normal incidence scheme is found to be effective for rebar detection, and the oblique incidence scheme can discover near-surface defects such as GFRP debonding and delamination. The proposed FAR NDT technique is found to be capable of detecting near-surface de-fects in GFRP-concrete cylinders and potentially applicable for the field condition

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as-sessment of GFRP-retrofitted reinforced concrete and other reinforced concrete civil infrastructure systems.

Thesis Supervisor: Oral Buyukozturk

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Acknowledgments

It is Professor Oral Buyukozturk (Course I) who led me into the research field of con-dition assessment of concrete structures using electromagnetic waves through working on a research project several years ago, on which my doctoral dissertation is essentially based. It is also through working on this project, my research interests on various topics were subsequently developed/discovered. His guidance, encouragements, and supports are indispensable to me for making this dissertation possible. I am deeply indebted to the time he spent with me at nights and on weekends, and to his tolerance in the process of forging my research attitude and enhancing my research capabilities. It would not have been possible for me to accomplish this work without his training on many aspects. For that, I truly appreciate this precious opportunity he gave me at MIT.

Professor Jerome J. Connor (Course I) was kind enough for supervising my Mas-ter’s thesis when I came to MIT in 2001 and for joining my thesis committee in 2005. It is very difficult not to be encouraged and inspired by him in every discussion we have made through his infectious passion on research and teaching.

I am also indebted to the late Professor Jin Au Kong (Course VI) for his leading me into the intriguing world of electromagnetism and for his valuable suggestions made in my committee meetings. His extraordinary sense of humor proportionally reflects the magnitude of his knowledge. His research philosophy inspires me and has made me a good friend of ”SAM” ever since. His sudden decease on March 13, 2008, is an unmeasured loss to me and everyone who knows him, while his lecturing and words still vividly survive in our memories.

It is my pleasure to have Dr. Tomasz M. Grezgorczyk (Course VI) serving on my thesis committee. His insightful suggestions to the electromagnetic problems I have encountered in conducting this research are most helpful and valuable. Dr. Grezgorczyk has also been very supportive to the completion of this research in many aspects. I am deeply grateful for his willing to guide me in exploring the world of electromagnetism.

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I should like to take this opportunity to express my gratitude to Professor Michael C. Forde (University of Edinburgh, Scotland) serving as a member on the thesis committee. His constructive suggestions and questions I have received during his stay at MIT in 2004 and during my thesis defense in 2008 are valuable to the further improvement of this thesis. I am grateful for his supporting this research on many aspects and for his sharing his perspectives and thoughts on many critical problems in civil engineering.

Special thanks go to Dr. Antonis Giannopoulos for the use of GPRMax2D/3D and his suggestions in the numerical simulation.

Many productive and interesting discussions with Professors E. Kausel and D. Veneziano (both in Course I) are greatly appreciated.

I would like to extend my thanks to a number of people for their help of various kinds; to Dennis Blejer and Alex Eapen (MIT Lincoln Laboratory) for their help on laboratory radar measurements and data interpretation; to Patricia Dixon and Cynthia Stewart when I was in need of help in 2004; to Donna Hudson for her help on proposal budgets; to Patricia (Patty) Glidden, Kris Kipp, Jeanette Marchocki, and Donna Beaudry for their everyday relentlessly greetings on the aisles in Building 1. This journey would have been much more colder without their warm smiles.

I have been lucky enough to make many friends in Course I; O. Gunes, C. Au, E. Karaca, R. Sudarshan, J. Pei, A.E. Sew, M.A. Nikolinakou, J.A. Ortega, K. Ishimaru, S. Cheekiralla, P. Dohnalek, C. Tuakta, S. Lin, I. Tsai, J. Park, Y. Moriyama, I.(Aki) Choo, D. Lau, as well as in other Departments including K. Lee (Course VIII), J. Chen (Course VI), and M. Nikku (Course VIII). The time with my classmates including Marc, Bora, Carmen, Jason, Tashan, Vimal, Luca, Chinghuei, and Sakda was also memorisable.

Last but not least, I will always have special gratitude and love for my grandma, Shun Jen, my father, Jr-Shen Yu, my mother, Kuei-Yin Shu, my brother, Shun-Hwa Yu. Their endless, unconditional supports warm my heart as always. Finally, I want to dedicate this thesis to my wife, Kaiwen Chen, who has been my Muse on many aspects ever since she walked into my life.

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List of Figures

1-1 Several FRP strengthening/repair scenarios of concrete structures . . 25

1-2 Intact multi-layer cementitious composite systems . . . 26

1-3 Damaged multi-layer cementitious composite systems . . . 26

1-4 Detected air voids at various locations in a bridge box-girder wall and their repair . . . 27

1-5 Several failure modes in GFRP-concrete systems . . . 28

1-6 Modeling of construction defects and structural damages using artificial anomaly . . . 31

1-7 Organization of the dissertation . . . 32

2-1 Procedure of NDT Techniques . . . 37

3-1 Configuration grid in a two-dimensional domain . . . 70

3-2 Derivation of global and local ABCs . . . 74

3-3 Quantization error Er (k∆x) vs. k∆x . . . 76

3-4 Geometry of a 2D model for validation . . . 79

3-5 Gaussian current source and the reflection from PEC . . . 80

3-6 Sinusoidal current source and the reflection from PEC . . . 81

3-7 Gaussian current source and the reflection from PEC . . . 82

3-8 Gaussian current source and the reflection from PEC – Close-Up . . . 83

3-9 Theoretical curve and the FDTD solution of the amplitude reflection coefficient of a 2D dielectric plate . . . 84

3-10 Relative difference/error between the theoretical curve and the FDTD solution . . . 85

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3-11 Sinusoidal current source and the reflection from a 2D dielectric plate 86 3-12 Numerical domain for simulating actual far-field radar measurements 87 3-13 Incident field, total field, and net reflection field of a 9 GHz signal . . 88 3-14 Scattered (net reflection) field of Ez – Intact, lossless concrete cylinder 91

3-15 Total field of Ez – Intact, lossless concrete cylinder . . . 92

3-16 Total field of Ez – Intact, lossy concrete cylinder . . . 93

3-17 Scattered field of Ez, φ = 0◦ – Damaged, lossless concrete cylinder . . 94

3-18 Scattered field of Ez, φ = 0◦ – Damaged, lossy concrete cylinder . . . 95

3-19 Scattered field of Ez, φ = 30◦ – Intact, lossless concrete cylinder . . . 96

3-20 Scattered field of Ez, φ = 30◦ – Damaged, lossless concrete cylinder . 97

3-21 Thirteen 2D models for studying the effect of defect width in normal incidence . . . 98 3-22 Variation of reflected electric field with respect to different defect widths

in normal incidence . . . 98 3-23 Sixteen 2D models for studying the effect of defect depth in normal

incidence . . . 99 3-24 Variation of reflected electric field with respect to different defect depths

in normal incidence . . . 99 3-25 Total field of Hx – Plain concrete cylinder with a center rebar . . . . 100

3-26 Total field of Hy – Plain concrete cylinder with a center rebar . . . . 101

3-27 Total field of Ez – Plain concrete cylinder with a center rebar . . . . 102

3-28 Scattered field of Hx – Plain concrete cylinder with a center rebar . . 103

3-29 Scattered field of Hy – Plain concrete cylinder with a center rebar . . 104

3-30 Scattered field of Ez – Plain concrete cylinder with a center rebar . . 105

3-31 Scattered field of Ez – Plain concrete cylinder with a center rebar . . 106

3-32 A 3D lossless dielectric cylinder model . . . 107 3-33 Comparison between the 2D and 3D responses of a PEC cylinder model108 4-1 Specimens CON and GFRP . . . 114 4-2 Specimens CRE and 4RE . . . 114

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4-3 Specimens AD1 and AD2 . . . 115 4-4 Specimen AD3 . . . 115 4-5 Compact RCS antenna range facility in the MIT LL [Courtesy of the

MIT LL] . . . 119 4-6 Schematic of the compact RCS/antenna range facility in the MIT LL 125 4-7 Far-field monostatic ISAR normal incidence scheme. Note that the

angular zero is referred to a selected point on the cylinder. . . 126 4-8 Far-field monostatic ISAR oblique incidence scheme. Note that the

angular zero is in alignment with the axis of the cylinder. . . 127 4-9 PEC specimen – Aluminum tube [Courtesy of the MIT Lincoln

Labo-ratory] . . . 128 4-10 Frequency-angle response of the Aluminum tube – Amplitude (dBsm),

X-band . . . 128 4-11 Frequency-angle response of the Aluminum tube – Amplitude (dBsm),

Ku-band . . . 129 4-12 Mean amplitudes and their standard deviations of the reflection

re-sponse at different frequencies – Aluminum tube, X-band . . . 129 4-13 Mean amplitudes and their standard deviations of the reflection

re-sponse at different incident angles – Aluminum tube, X-band . . . 130 4-14 Mean amplitudes and their standard deviations of the reflection

re-sponse at different frequencies – Aluminum tube, Ku-band . . . 130 4-15 Mean amplitudes and their standard deviations of the reflection

re-sponse at different incident angles – Aluminum tube, Ku-band . . . . 131 4-16 Correlation coefficients of the reflection response at different

frequen-cies – Aluminum tube, X-band . . . 131 4-17 Correlation coefficients of the reflection response at different

frequen-cies – Aluminum tube, Ku-band . . . 132 4-18 Lossy dielectric specimen – Plexiglass rod . . . 132 4-19 Frequency-angle response of the plexiglass specimen – Amplitude (dBsm),

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4-20 Simulated and measured responses — Plexiglass rod, HH (upper level) and VV (lower level) polarizations [Courtesy of the MIT LL] . . . 134 4-21 Frequency-angle response of specimen CON (plain concrete) –

Ampli-tude (dBsm), X-band . . . 136 4-22 Frequency-angle response of specimen CON (plain concrete) –

Ampli-tude (dBsm), Ku-band . . . 136 4-23 Frequency-angle response of the specimen GFRP (plain concrete with

GFRP) – Amplitude (dBsm), X-band . . . 137 4-24 Comparison between the power responses of specimens CON and GFRP,

X-band . . . 139 4-25 Photos of the specimen CON showing surface roughness . . . 140 4-26 Misalignment between the cylinder specimen and the Styrofoam tower

in the normal incidence scheme . . . 142 4-27 Frequency-angle response of specimen AD1B – Amplitude (dBsm),

X-band . . . 142 4-28 Frequency-angle response of specimen AD1F – Amplitude (dBsm),

X-band . . . 143 4-29 Comparison between the power responses of the intact and damaged

sides of the specimen AD1, X-band . . . 144 4-30 Photos showing the concave region in the specimen . . . 145 4-31 Frequency-angle response of specimen AD2 – Amplitude (dBsm), X-band147 4-32 Frequency-angle response of specimen AD3 – Amplitude (dBsm), X-band147 4-33 Frequency-angle response of specimen CRE – Amplitude (dBsm), X-band148 4-34 Comparison between the power responses of specimens CON and CRE,

X-band . . . 149 4-35 Frequency-angle response of specimen 4RE (rebar 1) – Amplitude (dBsm),

X-band . . . 150 4-36 Frequency-angle response of specimen 4RE (rebar 2) – Amplitude (dBsm),

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4-37 Frequency-angle response of specimen 4RE (rebar 3) – Amplitude (dBsm),

X-band . . . 151

4-38 Frequency-angle response of specimen 4RE (rebar 4) – Amplitude (dBsm), X-band . . . 151

4-39 Frequency-angle response of the intact side of the specimen AD1 – Amplitude (dBsm), X-band . . . 153

4-40 Frequency-angle response of the damaged side of the specimen AD1 – Amplitude (dBsm), X-band . . . 154

4-41 Frequency-angle response of the damaged side of the specimen AD2 – Amplitude (dBsm), X-band . . . 154

5-1 Configuration of SAR measurements . . . 170

5-2 Monostatic radar and its footprint . . . 171

5-3 r¯s, ¯r0, and ¯rj in the far-field region . . . 171

5-4 Configuration of ISAR measurements . . . 184

5-5 Two cases of the reflection of radar signals . . . 185

5-6 Time shifting error . . . 185

5-7 Variables in ˜I(kx, ky) and their relationship . . . 186

5-8 Far-field ISAR measurement of the specimen AD1 – HH polarization, φi = −15◦ . . . 188

5-9 Complex form of the far-field ISAR measurement . . . 189

5-10 Desired sidelobe pattern with different sidelobe levels (SLLs) ranging from 30dB to 90dB . . . 190

5-11 Weighted complex form of the far-field ISAR measurement . . . 191

5-12 Shifted 1D DFT of the weighted complex signal . . . 192

5-13 Poynting vector of the radar . . . 192

5-14 Projected image of the transformed far-field ISAR measurement . . . 193

5-15 Full-bandwidth far-field ISAR measurements with an azimuth vector of (−14.8◦, −15◦, −15.2◦) . . . 194

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5-16 Full-bandwidth far-field ISAR measurements with an azimuth vector of (−14.8◦, −15◦, −15.2◦) . . . 195 5-17 Full-bandwidth far-field ISAR measurements with an azimuth vector

of (−14.6◦, −14.8◦, −15◦, −15.2◦, −15.4◦) . . . 196 5-18 Full-bandwidth far-field ISAR measurements with an azimuth vector

of (−14.4◦, −14.6◦, −14.8◦, −15◦, −15.2◦, −15.4◦, −15.6◦) . . . 197 5-19 Full-bandwidth half-aperture backprojection image – ∆φi× [−150 : 1 :

1] (∆φi = 0.2◦) . . . 197

5-20 Full-bandwidth full-aperture backprojection image – ∆φi× [−150 : 1 :

150] (∆φi = 0.2◦) . . . 198

5-21 Sub-bandwidth full-aperture backprojection image – [8, 11] (GHz) . . 198 5-22 Sub-bandwidth full-aperture backprojection image – [8, 10] (GHz) . . 199 5-23 Sub-bandwidth full-aperture backprojection image – [8, 9] (GHz) . . . 199 6-1 Dielectric dispersion of several types of polarization (Modified after

Knight and Nur (1987) [135]) . . . 213 6-2 A two-dimensional free-space transmission model . . . 220 6-3 Overview of the proposed methodology . . . 221 6-4 Transmitting horn and the network analyzer for the free-space

mea-surement [Courtesy of the MIT Lincoln Laboratory (MIT LL)] . . . . 223 6-5 Transmitting and receiving horns for the free-space measurement

[Cour-tesy of the MIT LL] . . . 224 6-6 Error surfaces of the dielectric measurements collected from Teflon,

Lexan, Bakelite, and Portland cement concrete . . . 225 6-7 Error surfaces of the dielectric measurements collected from Teflon,

Lexan, Bakelite, and Portland cement concrete . . . 226 6-8 Convergence of estimates of 0_r at different frequency bandwidths using

TDOA . . . 232 6-9 Normalized phase velocity vs. loss factor 00_r . . . 233 6-10 Behavior of the Cole-Cole model (s = 2, ∞= 1) . . . 240

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6-11 Behavior of the Davidson-Cole model (s= 2, ∞= 1) . . . 241

6-12 The Cole-Cole diagram of Cole-Cole’s model . . . 243

6-13 The Cole-Cole diagram of Davidson-Cole’s model . . . 244

6-14 The Cole-Cole diagram and the equivalent circuit of Fr¨ohlich’s model 244 6-15 The Cole-Cole diagram and the equivalent circuit of Cole-Cole’s model 247 6-16 The Cole-Cole diagram and the equivalent circuit of Davidson-Cole’s model . . . 248

6-17 Performance of the Cole-Cole model for free water . . . 253

6-18 Considered sorption model in the pore structure of hcp . . . 254

6-19 Jellium distance in the sorption model . . . 257

6-20 Used chemical potential accounting for the formation of the bound water on the surface of hcp . . . 258

6-21 Measurements and model prediction of non-porous and porous speci-mens of hydration products . . . 259

6-22 Calculated bonding potential of free water molecules . . . 260

6-23 Calculated relaxation time distribution over the bound water later . . 261

6-24 Performance of the curve-fitted model for the specific surface area of hcp by nitrogen adsorption . . . 264

6-25 Performance of the Cole-Cole model for epoxy . . . 266

6-26 Performance of the Cole-Cole model for E-glass fabric . . . 268

6-27 Performance of the Cole-Cole model for E-glass fabric – Real part and imaginary part . . . 269

6-28 Unidirectional GFRP layer . . . 269

6-29 Performance of six mixing models for GFRP-epoxy . . . 271

6-30 Relationship between the w/c ratio and product of the w/c ratio and dielectric constant of oven-dried cement paste specimens . . . 279

6-31 Curve-fitting results of the oven-dried hcp in the frequency range of 3 GHz to 24 GHz . . . 284

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7-2 Normal and oblique incidence inspection schemes of the FAR NDT

technique . . . 288

7-3 Defect and rebar detection . . . 289

7-4 Specular dominant circumstance . . . 290

7-5 Specular recessive circumstance . . . 290

7-6 Computed far-field distances at various frequencies and two antenna apertures . . . 291

7-7 Inspection procedure of the FAR NDT technique . . . 292

7-8 Bridge inspection – Beam and column . . . 293

7-9 Two types of frequency bandwidth integration . . . 296

7-10 Improvement of image resolutions Progressive image focusing – Fre-quency integration, HH polarization . . . 297

7-11 Prediction error of cross-range resolution formulae . . . 298

7-12 Progressive image focusing – Frequency integration using shifting cen-ter frequency, HH polarization, θ = 15◦ . . . 309

7-13 Improvement of image resolutions progressive image focusing – Angular integration, HH polarization . . . 310

7-14 Comparison of images of the intact and damaged surfaces of the spec-imen AD1 at different incident angles (30◦ ∼ 10◦_{) . . . .} ₃₁₁

7-15 Comparison of images of the intact and damaged surfaces of the spec-imen AD1 at different incident angles (−10◦ ∼ −30◦_{) . . . .} ₃₁₂

7-16 Description of the used far-field ISAR measurements and specimens for damage detection . . . 313

7-17 Maximum amplitudes of the backprojection images of the specimen AD1 – Full bandwidth (8GHz∼12GHz), HH polarization . . . 313

7-18 Differential maximum amplitudes of the backprojection images of the specimen AD1 – Full bandwidth (8GHz∼12GHz), HH polarization . . 314

7-19 An eight-node element for morphological operations . . . 314 7-20 Backprojection images of the specimen AD1 – HH polarization, θ = −15◦315 7-21 Backprojection images of the specimen AD1 – VV polarization, θ = −15◦315

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7-22 Variation of nE with respect to nthv of the intact-side of the specimen

AD1 . . . 316 7-23 Variation of nE with respect to nthv of the damaged-side of the

speci-men AD1 . . . 316 7-24 Backprojection images and their feature-extracted version of the

spec-imen AD1 (HH polarization, full bandwidth, θ = −15◦) . . . 317 7-25 Original nE(θ) curves of the intact and damaged surfaces of the

speci-men AD1 (HH polarization, full bandwidth) . . . 317 7-26 Filtered nE(θ) curves of the intact and damaged surfaces of the

speci-men AD1 (HH polarization, full bandwidth, L = 3) . . . 318 A-1 A layer underlain a solid half space . . . 326 B-1 A two-dimensional two-layer model with infinite boundary (TE waves) 332 B-2 TE and TM waves and incident wave vector . . . 334 B-3 A two-dimensional three-layer model (TE waves) . . . 335 B-4 A three-dimensional infinite dielectric cylinder impinged by plane waves 337

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List of Tables

2.1 NDT techniques utilizing mechanical waves . . . 38

2.2 Electromagnetic spectrum . . . 39

2.3 NDT Techniques . . . 41

2.4 Types of mechanical wave . . . 45

2.5 Microwave and radar NDT techniques . . . 57

2.6 Microwave and radar NDT applications in civil engineering . . . 59

4.1 Designed specimens . . . 113

4.2 Used materials and their suppliers . . . 117

4.3 Manufacturing standards . . . 117

4.4 Statistical parameters of the frequency-angle amplitude measurement of the Aluminum tube (PEC) . . . 123

4.5 Statistical parameters of the frequency-angle response of the plexiglass rod specimen . . . 125

4.6 Peak RCS of simulated and measured responses of the plexiglass rod specimen . . . 126

4.7 Signal contents of the radar measurements of laboratory specimens . 135 4.8 Maximum and minimum powers (dBsm) of specimens CON and GFRP, X-band . . . 137

4.9 Maximum and minimum RCS (dBsm) of the specimens AD1 . . . 141

4.10 Configuration of the specimen 4RE . . . 148

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6.1 Types of magnetic materials . . . 206 6.2 Types of dielectric materials . . . 208 6.3 Types of materials defined by conductivity . . . 210 6.4 Civil engineering applications of dielectric properties . . . 216 6.5 Thicknesses of the specimens . . . 227 6.6 TDOA measurements and dielectric constants 0_r of the specimens . . 229 6.7 Loss factors 00_r of the specimens . . . 229 6.8 Estimated 0_r by TDOA using different frequency bandwidths (GHz) . 231 6.9 Comparison of internal field approach and external field approach . . 242 6.10 Measured complex permittivity of water at 20◦C . . . 252 6.11 Fitted parameters in Debye’s and Cole-Cole’s models of free water . . 253 6.12 Averaged statistical thickness, t (˚A), of adsorbed water layer on hcp . 259 6.13 Relaxation time, τw (ps), of free water molecules . . . 260

6.14 Specific surface area of hcp [83] . . . 263 6.15 Comparison of two GFRP-epoxy systems . . . 271 6.16 Performance of six mixing models for a GFRP-epoxy system . . . 272 6.17 Volumetric fractions of cementitious composites . . . 277 6.18 Density of cement paste specimens [83] . . . 277 6.19 Parameters in the oven-dried hcp model . . . 280 7.1 Image resolution formulae . . . 297

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Chapter 1 Introduction and Research

Motivation

”All difficult things have their origin in that which is easy, and great things in that which is small.”

—– DaoDeJing, Lao Tzu (∼ 600 B.C.)

Deterioration of manmade structures is an inevitable and non-stopping process, no matter how carefully structures are maintained or reserved. Among all manmade structures, civil infrastructures such as buildings, bridges, tunnels, dams, pipelines, roads, airports suffer from the severe attacks from the environment during their design lifespan which may range from 30 to 50 years (bridges designed for 120 years). In order to meet the expected lifespan and performance for civil infrastructures, strengthening and repair of concrete structures has become an important issue for public safety and for effective infrastructure management. Engineering technologies are developed and introduced for extending the service life of concrete structures by means of restoring their design capacity for continuous use and/or upgrading them for possible future challenges from the environment, and for meeting the demand for increased service load conditions.

Strengthening and repairing of concrete structures can be conducted either inter-nally or exterinter-nally. Internal strengthening techniques such as injection techniques use

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adhesive materials to fill in interconnecting cracks and voids in concrete. In this type of techniques, targeted infiltration/penetration depth of injected adhesive may not be easy to achieve, and the effectiveness of construction sometimes remains question-able. External strengthening techniques, on the other hand, are more effective than internal techniques and can achieve a significant level of strengthening, especially for column-type structures. At present time, the use of fiber reinforced polymer (FRP) composites as an externally bonded element to confine the concrete in order to secure the integrity of concrete structures has been proven, both theoretically and practically, to be an effective strengthening/repair approach. FRP composite jacketing systems have emerged as an alternative to traditional construction, strengthening, and re-pair of reinforced concrete columns and bridge piers. A large number of projects, both public and private, have used this technology and escalating deployment is ex-pected, especially in seismically active regions. Integration of the new FRP composite with the existing concrete substrate results in the formation of a new structural sys-tem. Differences in the material properties of the two structural components (FRP and concrete) pose challenging problems of predicting the behavior of the integrated structural system. Extensive research effort has been devoted to this active field as reported in the literature on structural engineering, and composite materials and construction.

Compared with the traditional materials such as steel, general advantages offered by FRP composites include high strength-to-mass ratio, high stiffness-to-mass ratio, ease in handling, and resistance to corrosion. Ample research activities and applica-tions of FRP strengthening in civil engineering have been reported in research papers [44] and reports [193, 49] in conjunction with developed design codes [53, 87, 198] and industrial manuals [253, 55, 152].

Typical FRP composites used in civil engineering applications are carbon FRP (CFRP), glass FRP (GFRP) and aramid FRP (AFRP). Among these strengthening materials, GFRP composites have been widely adopted since the 1990s [228, 202, 145, 225], particularly in retrofitting of reinforced concrete (R/C or RC) members, such as slabs [145], beams [45], walls [132] and columns [258] [160] to increase/restore

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their mechanical capabilities (flexural, shear and compressive). GFRP composites can also be applied to masonry [250], metal [56] and wood structures [62]. Figure 1-1 demonstrates several strengthening/repair scenarios on concrete structures. In this dissertation, GFRP strengthened and retrofitted concrete columns are considered and modeled by GFRP-retrofitted concrete cylinder specimens. Figure 1-2 and Figure 1-3 show the schematics of intact and damaged GFRP-concrete column systems.

(a) Beam strengthening for shear

for flexure FRP sheet/layer

(b) Column strengthening for axial &

flexure

(c) Slab strengthening for flexure

(d) Wall strengthening

for shear

Figure 1-1: Several FRP strengthening/repair scenarios of concrete structures

Prior to the strengthening/repair construction, it is important to know (1) the location(s) for strengthening and (2) the level of strengthening at a specific location in order to properly and effectively strengthen the structure. Therefore, ap-propriate and field applicable nondestructive testing (NDT) or evaluation techniques need to be introduced to assess the current condition (level of damage) of the deteri-orated/damaged structure.

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(a) FRP-wrapped concrete column FRP-epoxy (b) FRP-wrapped reinforced concrete column Concrete Steel reinforcement Epoxy

Figure 1-2: Intact multi-layer cementitious composite systems

(a) FRP-retrofitted concrete column FRP-epoxy (b) FRP-retrofitted reinforced concrete column Steel reinforcement Epoxy Concrete Concrete cracking Delamination

Figure 1-3: Damaged multi-layer cementitious composite systems

After the strengthening/repair construction, the integrated concrete struc-ture with the externally bonded GFRP composite forms a multi-layer composite sys-tem. Construction defects and structural/environmental damages may occur within the Gretrofitted concrete structures, and especially, in the vicinity of FRP-concrete interfaces. construction defects such as air voids/pockets being trapped between the GFRP wrap and the concrete substrate may be encountered. The pres-ence of these air voids creates a region at which the shear stresses are discontinuous. The stress discontinuity will further encourage the formation and development of FRP debonding at and in the vicinity of the interface of FRP and concrete under

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associ-ated loading conditions. Therefore, in practice, the defected areas must be identified and repaired. Figure 1-4 shows the detected air voids and their repair in a bridge rehabilitation project (the Jamestown Bridge, Rhode Island).

(a) Found air void defect with the removal of GFRP sheet

(b) Air void found near the corner

(c) Two found air voids (d) Repaired air void defect

Figure 1-4: Detected air voids at various locations in a bridge box-girder wall and their repair

FRP-concrete interface and concrete conditions cannot be fully revealed until physical removal of the FRP composite layer unless the member has already been subjected to apparent substantive damage. Partial or complete removal of the FRP composite layer for observation of the damage may pose a danger of structural col-lapse. A FRP-retrofitted concrete beam or column could appear safe without show-ing any sign of substantial damage underneath the FRP composite while containshow-ing a severely deteriorated region. Such scenario could happen when the structure has

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undergone a modest seismic event that has significantly damaged the FRP-concrete system while the system has not reached the failure stage. Failures of damaged FRP-concrete systems are often brittle, involving delamination of the FRP, debonding of concrete layers, and shear collapse, which can occur at load levels lower than the predicted theoretical strength of the retrofit system. Research results from large-scale retrofitted RC (reinforced concrete) beam tests [260, 34, 196] have also shown that failures of these systems may take place through various possible mechanisms, depending on the concrete grade, rebar provision, and properties of FRP. Identified failure modes include: (1) concrete crushing before steel yielding; (2) steel yield-ing followed by concrete crushyield-ing; (3) steel yieldyield-ing followed by FRP rupture; (4) shear failure; (5) concrete cover delamination; and (6) debonding in the vicinity of the FRP/epoxy/concrete bond interface. Brittle debonding has been particularly ob-served [227, 34, 9]. Similar failure modes can occur to FRP-strengthened RC columns. Figure 1-5 illustrates several failure modes in two GFRP-concrete specimens and one GFRP-RC column. Gradual debonding of the FRP composite (structural damage)

(a) Global shear cracking in a CFRP-wrapped concrete specimen (Au and Buyukozturk, 2005)

(b) Local concrete crumbling in a GFRP-wrapped concrete specimen (Au and Buyukozturk, 2005)

(c) GFRP rupture and concrete crumbling in a GFRP-wrapped concrete column (Sheikh and Yau, 2002)

Figure 1-5: Several failure modes in GFRP-concrete systems

under service load conditions may result in premature failures of the retrofitted sys-tem, leading to the total collapse of the structure. Thus, there is a need for inspection

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of debonding in such multi-layer systems using NDT techniques in field conditions. This dissertation deals with the development of a nondestructive testing/evaluation (NDT/E) technique for concrete columns/brideg piers retrofitted by externally wrapped glass fiber reinforced polymer (GFRP) composites using far-field radar measurements. In this development, research components including analytical approach, experimen-tal measurements, numerical simulation, and image reconstruction are described in detail in the following chapters. Research objectives, approach and the organization of the dissertation are given in the following.

1.1 Research Objective

The objective of this research is to develop a microwave-based NDT/E capability for the distant assessment of the physical condition of GFRP-retrofitted concrete structures with emphasis on the detection of anomalies and delaminations in the GFRP-concrete interface region and in the concrete cover areas. The research aims at the development of new knowledge on the interaction of GFRP-concrete systems with electromagnetic waves and an image reconstruction capability for physical imagery. A knowledge-based interpretation algorithm for an effective NDT/E technique as a basis for the condition assessment of GFRP-retrofitted concrete structures is developed.

1.2 Research Approach

The research approach to achieve the objective consists of four components:

1. Numerical simulation – Transmission and reflection of radar signals can be nu-merically simulated by the propagation and scattering of EM waves in a digital environment. Special models of the GFRP-concrete cylinders are constructed and impinged by plane EM waves using finite difference time domain (FDTD) methods. The purpose of the numerical simulation is to identify and study ma-jor system design parameters (measurement scheme, measurement frequency, and incident angle) in the development of a distant microwave-based NDT/E

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technique. Parametric study on the effects of artificial defect and dielectric properties in the reflected radar signals is conducted.

2. Laboratory experimentation – Physical radar measurements were made from laboratory GFRP-wrapped concrete cylinders manufactured with and without artificial defects and rebars. Artificial defects including a cube, a thin-plate, and a strip (made of Styrofoam whose electromagnetic properties are same as air) were embedded at the interface between the GFRP layer and the con-crete core. The selection of regular shapes for artificial defects represents the simplification to the complex shapes of real defects. Figure 1-6 shows this con-cept. The purpose of conducting radar measurements on laboratory specimens is twofold: First, a forward study can be performed using laboratory speci-mens whose defects are known. Radar measurements made on intact (without defect) and damaged (with defect) specimens can be evaluated based on their raw measurements and finally processed by image reconstruction as a basis for comparison with the real specimens. Secondly, the laboratory configuration provides a noise-free environment for the reflection radar measurements of the specimens. The removal of background noise is advantageous for better distin-guishing the signal due to the presence of defect at a higher signal-to-noise ratio (SNR). Such measurements are insightful for damage detection and convenient to deal with in terms of the need of denoising.

3. Image reconstruction – In this research, collected distant radar signals (measure-ments) were processed by tomographic reconstruction (TR) methods in order to reconstruct the spatial profile of GFRP-wrapped concrete cylinders for con-dition assessment. Fast backprojection algorithm was applied for implementing TR methods and for developing numerical codes. The purpose of signal pro-cessing is to establish an effective and efficient transformation to visualize radar measurements.

4. Modeling of Dielectric Properties – For the use of radar signals (electromagnetic (EM) waves) on probing materials, knowledge is needed regarding the dielectric

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The multi-layer cylinder structure Artificial defects GFRP-epoxy layer Concrete GFRP-epoxy layer Concrete Delamination between GFRP-epoxy and concrete

Structural cracks

width

depth A detected air void

Styrofoam anomaly

Figure 1-6: Modeling of construction defects and structural damages using artificial anomaly

properties of the medium in which EM waves travel. Wave-medium interac-tions including transmission, reflection, and scattering of EM waves can not be understood and simulated without the knowledge of the dielectric properties of materials. For the materials considered in this research, dielectric properties are modeled in the frequency range of 8 GHz to 18 GHz for water, epoxy resin, E-glass fabric, GFRP, and oven-dried hydrated cement paste. This modeling effort is important to an accurate numerical simulation, as well as in the im-age reconstruction for better locating defects (to be further explained in the chapters).

1.3 Organization of the Dissertation

This dissertation is organized in the following manner. Figure 1-7 illustrates the relations among the chapters in this dissertation.

Chapter 2 reviews the current development of NDT techniques for the condi-tion assessment of FRP-retrofitted concrete structures. Different NDT methods are compared for their current or potential use on GFRP-retrofitted concrete structures.

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Far-field Airborne Radar NDT Far-field monostatic ISAR measurements of GFRP-retrofitted concrete columns Backprojection processing and image reconstruction using

far-field ISAR measurements Condition assessment Image reconstruction Laboratory radar measurements Numerical simulation using FDTD Dielectric modeling of materials Literature review

Figure 1-7: Organization of the dissertation

Chapter 3 illustrates the numerical simulation results of electromagnetic waves propagation and scattering using finite difference time domain (FDTD) methods. Principles and implementation guidelines of FDTD methods are also introduced. Effects of system design parameters including measurement scheme, measurement frequency, and incident angle are studied.

Chapter 4 reports the radar measurements of GFRP-retrofitted concrete cylin-ders made in the MIT Lincoln Laboratory. Far-field ISAR (inverse synthetic aperture radar) measurements of GFRP-concrete cylinders were collected by a horn antenna operating in monostatic mode. Linearly polarized continuous wave (CW) radar sig-nals in the frequency range of 8 GHz to 18 GHz were used in probing the GFRP-concrete cylinders. Azimuthal (angular) range of 60 degrees were explored. Collected ISAR measurements were represented in the frequency-angle format.

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Chapter 5 introduces the data processing approach for image reconstruction. Backprojection algorithms are used for processing far-field ISAR measurements into spatial images. Reconstructed range–cross-range images are also provided in this chapter.

Chapter 6 addresses the dielectric modeling of the materials encountered in GFRP-concrete systems, including water, epoxy, GFRP, and oven-dried hydrated cement paste. Dielectric models applicable in the frequency range of 8 GHz to 18 GHz are developed based on the dielectric measurements of the materials reported in the literature.

Chapter 7 addresses the condition assessment methodologies for inspecting the near-surface defects/damages in GFRP-retrofitted concrete cylinders using recon-structed images.

Chapter 8 summarizes the research findings and discusses possible research topics for future work.

Appendix A provides the derivation of the phase velocity of Love waves in a layer underlain by a half space medium, as the basis for condition assessment of multi-layered systems using the phase velocity of Love waves.

Appendix B addresses the analytical investigation of several two-dimensional EM scattering problems in which a multi-layered dielectric medium is impinged by plane waves.

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Chapter 2 Literature Review

”Read not to contradict and confute, not to believe and take for granted, but to weight and consider.”

—– Essay on Of Studies, Francis Bacon (1561 ∼ 1626)

Condition assessment of materials and structures in a noninvasive manner is called nondestructive testing (NDT) or nondestructive evaluation (NDE). While the infor-mation regarding the condition of structural systems (e.g., bridge, buildings, dam, tunnel) designed for public use is not only crucial to the operation of the structure but also vital to public safety. Therefore, various NDT techniques have been de-veloped for obtaining such information. Reported NDT techniques demonstrate the research results and findings based on different approaches for assessing the condi-tion of a material/structure system. The purpose of this chapter is to review these techniques.

In this chapter, general description of NDT is first given. Current NDT techniques including optical NDT, acoustic NDT, thermal NDT, magnetic/electrical NDT, radio-graphic NDT, and microwave/radar NDT are reviewed with emphases on (1) physical principles regarding each NDT technique for understanding the characteristics (ad-vantages and constraints) of each technique, and (2) reported or potential application of these techniques on GFRP-wrapped concrete systems.

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2.1 Nondestructive Testing (NDT) Techniques

American Society for Nondestructive Testing (ASNT) defines NDT as

The testing of a specimen that determines its serviceability without damage that could prevent its intended use.

As indicated in the definition, NDT aims at providing information regarding the serviceability of a specimen (or material or structure) without damaging the specimen. Such a non-invasive scheme is favored for pertaining not only to the serviceability but also to the sustainability of the specimen under investigation. The specimen inspected by NDT techniques is sometimes termed the material under testing (MUT) which includes laboratory specimens and actual structures. General description of NDT is described in the following.

1. Purpose and Effectiveness of NDT Inspection — The purpose of NDT inspection is to understand the MUT via obtaining the information regarding its material properties through the knowledge of wave-medium or field-medium interaction. Serviceability of the MUT is based on the interpretation of mate-rial properties. Therefore, the effectiveness of NDT inspection in a particular application theoretically depends on the significance of such interaction and practically depends on the design of instrumentation. Should there be minor or no interaction between the chosen wave or field and the target MUT, the se-lected NDT method is theoretically ineffective. Should the signal-to-noise ratio (SNR) obtained through certain instrumentation be too small to be detectable, the selected NDT method is practically ineffective.

2. Procedure of NDT techniques — Figure 2-1 illustrates the general inspec-tion procedure of NDT techniques.

3. Classification of NDT Techniques — All NDT techniques are based on cer-tain types of physical law for probing and manifesting the characteristics (mate-rial properties or geometrical properties) of the MUT. Current NDT techniques are distinguished by the following features.

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Experimental configuration and installation

Generation of manifesting agent for probing the MUT

Selection of NDT methods

Measuring the response of the MUT

Interpretation of the measurement Natural or man-made source for manifestation

Natural source Man-made source (Instrumentation) Condition assessment (Manifestation-medium interaction) (Signal processing)

Figure 2-1: Procedure of NDT Techniques

(a) Underlying physical principle — The physical principle behind each NDT technique is the relationship between the transmitted/produced waves (dynamically) or fields (statically) and the material properties of the MUT. The essential concept is to interpret the change in the MUT through the change in the received waves/fields.

(b) Type of wave or field — Selected waves or fields for probing the MUT can be mechanical, thermal, electric, magnetic, or electromagnetic. These waves/fields can be produced by man-made devices (e.g., electronic trans-ducer and radar antenna) or by natural sources (e.g., thermal radiation and radiative decay) (Figure 2-1). It is obviously that one NDT technique utilizing mechanical waves is different from another technique utilizing electromagnetic waves. Table 2.1 shows several NDT methods utilizing mechanical waves. Table 2.2 lists the content of the electromagnetic

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spec-trum.

Table 2.1: NDT techniques utilizing mechanical waves

Technique Frequency (Hz)

Impact-echo and pulse-echo 5 × 104 ∼ 5 × 105

Acoustic emission (AE) 105 ∼ 106

Ultrasound —

Pulse velocity 2 × 104 _{∼ 2.5 × 10}5

Spectral analysis of surface waves ∼ 108

(c) Measurement device — Information resulting from wave-medium inter-actions is the response of a MUT and can be collected by electronic devices. Typically, an electronic device capable of manifesting a MUT (producing the manifesting agent) are equally capable of collecting the response of a MUT (reciprocal theorem). However, recent advances on novel mea-surement technology encourages the development of hybrid NDT whose manifesting agent is produced by one device and response is measured by another. Therefore, the use of a measurement device other than the one producing manifesting agent is considered a different NDT technique. (d) Instrumentation type — Information must be collected and transformed

by devices through instrumentation configuration (equipment). Several instrumentation types such as single-output and single-input-multiple-output are possible for different interpretation schemes leading to the materialization of different NDT techniques.

For generality the classification of NDT techniques in this thesis is made based on the underlying physical principles behind each NDT technique, rather than elaborating the details of each NDT technique.

4. Content of NDT techniques — While the NDT techniques are rapidly de-veloping and expanding, their physical principles can be generally classified into

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Table 2.2: Electromagnetic spectrum

Class Frequency

(Hz)

Wavelength, λ (m)

Energy (eV) Type of

inhomo-geneity Gamma rays 3 × 1019 ∼ 3 × 1022 10−11 ∼ 10−14 _{> 1.24 × 10}5 X rays 3 × 1016 ∼ 3 × 1019 10−8 ∼ 10−11 _1.24×102 _{∼ 1.24×} 105 Dislocations, pre-cipitates Ultraviolet (UV) 7.5×1014 _∼ 3 × 1016 7 × 10−7 ∼ 10−8 _{3.1 ∼ 1.24 × 10}2 Visible light 4.3×1014 _∼ 7.5 × 1014 (7 ∼ 4) × 10−7 1.77 ∼ 3.1 Infrared (IR) 3 × 1011 _∼ 4.3 × 1014 10−3 ∼ 4 × 10−7 _{1.24 × 10}−3 _{∼ 1.77} _Texture, _residual stresses,

crack-ing, grain size,

inclusions, fiber fracture, delamina-tions, porosity Microwaves 3 × 109 _∼ 3 × 1011 10−3 ∼ 10−1 _{1.24 × 10}−3 _∼ 1.24 × 10−5 Texture, residual stresses, cracking, delaminations Radio freq. (RF) 3 × 10 ∼ 3 × 109 107 ∼ 10−1 _{1.24 × 10}−13 _∼ 1.24 × 10−5 Cracking, delami-nations

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the following categories. Reported examples are provided under each category. • Optical methods — Including visual inspection [13], surface coating (e.g., photoelastic coating [41] and brittle coating), , fluorescent penetrants (e.g., dye penetrant [213], volatile liquid, and filtered particle), Moire inter-ferometry [284], optical holography [100], shearography [97], laser speckle metrology [218], optical heterodyne interferometry [19], borescope [204], fiber optical sensors [109], and machine vision [110].

• Acoustic methods — Including impact-echo [231], sonics [144], ultra-sonics [59, 247], acoustic emission [182], acousto-ultraultra-sonics [281], electro-magnetic acoustic transducer (EMAT) [161, 187], laser-ultrasonics [275], ultrasonic holography [136], acoustic microscopy [162], acoustography and vibro-acoustography [230, 189], and sonic signature analysis [112].

• Thermal methods — Including thermoelectric probe [71], impulse ther-mography [167], infrared therther-mography (IRT) [14], ultraviolet fluorescence (UVF) [165], and emission spectroscopy [92].

• Magnetic and electrical methods — Including magnetic field [170], magnetic particle [249], eddy current [222], nuclear magnetic resonance (NMR) [146], Kirlian photography [172], Barkhausen effect [264], scanning electron microscopy (SEM) [98], and impedance spectroscopy [128].

• Radiographic methods — Including X-ray radiography [200, 94], gamma-ray radiography [101], neutron radiography [116], proton radiography [127], synchrotron radiation [86], and Compton backscatter [46].

• Microwave and radar methods — Including microwave radiometry [273], waveguides, coaxial probes, radar horn antenna [35, 195].

• Miscellaneous — Including mechanical impedance (hardness testing [61]), embedded fiber-optic strain gauge [232], chemical, liquid penetrant testing, replication microscopy, SQUID (Superconducting Quantum Interference Device) [276], and spark testing [131].

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Table 2.3 summaries these methods classified by the elements mentioned pre-viously in this section. These methods are briefly described in the following sections. While some NDT methods are applied in fluid dynamics and aerody-namics, this thesis only concerns solid medium as the MUT. Review of NDT methods in civil engineering can also be found at [133] and [175].

Table 2.3: NDT Techniques Technique Type of wave/field Measurement device Material properties Optical EM waves in optical spectrum

Human eye, lasers, and optical cameras

Acoustic Mechanical waves Electronic transducers Mechanical properties Thermal EM waves in thermal radia-tion spectrum

Ultraviolet and in-frared cameras Thermal properties Magnetic/ electrical Magnetic field and electric field

Magnetic and electrical properties Radiographic X-rays,

Gamma-rays, neutrons

Photographic films Radiographic properties Microwave

and radar

EM waves in

microwave spec-trum and radio frequency range

Waveguides, coax-ial probes, radar antennas

Dielectric properties

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2.1.1 Optical Methods

Principles

Optical methods includes the earliest NDT method (visual inspection) and some mod-ern methods (e.g, laser speckle metrology and machine/computer vision), all relying on the electromagnetic radiation in the optical spectrum range, which can be detected by the human eye. The optical spectrum contains visible lights of wavelengths in air from approximately 400 nm to 700 nm (corresponding to frequencies from 750 THz to 428 THz) as the manifestation agent. Geometrical optics and physical optics can be applied for analyzing the propagation of visible light, while the former approach as-sumes zero-wavelength of light and considers no diffraction. However, the ray tracing technique in geometrical optics still serves as a convenient tool for providing insights in some problems. In physical optics, the scalar wave equation governs the motion of light waves in an isotropic medium [31].

∇2_{φ(z, t) =} 1

c2

∂2_{φ(z, t)}

∂t2 (2.1)

where z is the traveling distance of light waves, t is time, φ(z, t) is the space-time function of light waves (or wave function), and c is the speed of light waves in free space (c= 3 × 108 m/s). To account for the interference and diffraction phenomena of light waves during propagation, the Fresnel transform is used for determining the light intensity distribution on an observation plane (ξ, η) from a source plane (x, y).

g(ξ, η) = −ie−iωt λr Z ∞ Z −∞ 1 + cosθ 2 f (x, y)eiˆk · ˆrdxdy (2.2)

where i is the imaginary number, λ is the wavelength, ω is the radian frequency, ˆr is the observation vector between the source point (x, y) and the observation point (ξ, η) with length r, θ is the angle between normal vector, and ˆk is the wave vector with magnitude k (wave number). The Fresnel transform of light intensity distribution in the source plane f (x, y) is the observed g(ξ, η). Fresnel transform is also the basis for

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synthetic aperture radar (SAR) processing. Further investigation of optical analysis can be found at [31] and [265].

The first appearance of qualitative visual inspection technique may be dated long back in history without a record, while the practice develops and matures with time. Visual inspection is still by far the predominant technique for the assessment of transportation infrastructures such as bridges, although its reliability for routine and in-depth inspections can be doubtful [163]. Results of evaluation using human eye are qualitative in nature and could be very subjective. The quality of such evaluation strongly relies on inspectors’ experience and judgement. As such, ma-chine/computer/digital vision-based evaluation is introduced to eliminate the factor of subjectiveness and provides quantitative analysis. Signal analysis of recorded opti-cal images using artificial intelligence approach (e.g., fuzzy set theory, expert system [110], neural network, and genetic algorithm) and computer vision approach provides alternatives to physical approach.

Optical NDT for GFRP-concrete Systems

At the present time and to the author’s knowledge, there is no literature of optical NDT for GFRP-concrete systems, although field engineers do use visual inspection for the detection of significant GFRP debonding and delamination in practice. How-ever, visual inspection is inherently not applicable to the condition assessment of GFRP-concrete systems for the ineffectiveness of optical lights in penetrating GFRP sheets/layers. In some circumstances, significant GFRP debonding, GFRP delami-nation, and concrete cracking may incur discoloring on the surface of GFRP, which is detectable to optical NDT. However, at this stage of failure, severe damage levels are reached without precaution. Additionally, similar discoloring of FRP could also due to the on-site variation in the curing of epoxy, causing potential false-alarm detection for optical NDT. Some optical NDT utilizes computer/mechine vision analysis incor-porating artificial intelligence approach offers in-depth information through extensive simulation. But the approach is not physically sound, and the solution may not be unique.

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In summary, constraints of optical NDT methods for GFRP-concrete systems include: (1) physical limitation of visual lights in penetrating the opaque GFRP layer, and (2) insufficient credibility of optical signal (e.g., discoloring) for damage indication. These constraints hinder the use of optical NDT on GFRP-wrapped concrete systems for condition assessment.

2.1.2 Acoustic Methods

Principles

Acoustic methods are classified as the NDT techniques using mechanical waves as the manifesting agent to investigate MUTs. The use of mechanical waves, including body wave and surface wave, leads to the dynamic/vibrational response of MUTs. The governing equation of all acoustic methods from a microscopic point of view is the Navier equation of motion. For homogeneous and isotropic materials [171],

(λ + µ) ∇ (∇ · u) + µ∇2u + ρb = ρ∂ 2_u ∂t2 (2.3) where λ = νE (1 + ν)(1 − 2ν) and µ = E

2(1 + ν) are the Lame constants, u = Σuixˆi is the displacement field, and b is the body force field per unit mass. The traction boundary condition is needed for solving the observable displacement field at the boundary. λ (∇ · u) ˆn + µ ∂ui ∂xj + ∂uj ∂xi · ˆn = f (2.4)

where ˆn is the normal vector at the boundary and f is the prescribed function acting on the boundary. Displacement fields at the boundary in different conditions (differ-ent mechanical properties; λ, µ) are measured in order to retrieve the variation in the mechanical properties of materials. Velocity fields ∂u

∂t and acceleration fields ∂2_u

∂t2 can

be determined either analytically when displacement fields u are obtained by evaluat-ing the Navier’s equation, or numerically when displacement fields are measured over a period of time. Table 2.4 provides the relationship between wave velocity and me-chanical properties of materials. It is clear that changes in the meme-chanical properties

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(E, ν, ρ) of a MUT are reflected in the variation of these wave velocities. In Table 2.4, the direction of particle movement is determined with respect to the direction of wave propagation. Also, ρ denotes the density of the MUT, E the Young’s modulus, and ν the Poisson’s ratio. Another dispersive wave mode existing between body and surface waves is the Lamb wave whose wave velocities (phase and group) can only be numerically evaluated from the Rayleigh-Lamb equations [32].

Table 2.4: Types of mechanical wave

Class Particle

movement Wave velocity Name

Body Parallel vp = E(1 − ν) ρ(1 + ν)(1 − 2ν) 1/2 [255] P-wave, com-pressional wave, dilational wave wave Orthogonal vs= E 2ρ(1 + ν) 1/2

[255] S-wave, shear wave,

distortional wave, equivolumnial wave, rotational wave Surface Elliptical orbit (sym-metrical mode) vR ∼= 0.87 + 1.12ν 1 + ν E 2ρ(1 + ν) 1/2 [267] Rayleigh wave wave _Horizontally polarized shear mode

vL (See Appendix A) Love wave

Mechanical responses of the MUT need to be collected by electronic devices (trans-ducers) which translate mechanical responses into electrical signals for further analy-sis. Practical issues such as coupling between transducers and the surface of materials, multiple propagation paths, frequency-dependent characteristics of materials compli-cate the interpretation result.

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Acoustic NDT for GFRP-concrete Systems

In principle, acoustic NDT such as acoustic emission is applicable to brittle materials like concrete [26]. Since the integration of concrete with GFRP layers also forms a brittle system, acoustic NDT is theoretically applicable to GFRP-concrete systems.

Mirmiran et al. (1999) [1] applied acoustic emission (AE) technique to GFRP-confined concrete cylinder specimens to study the correlation between AE signals and the stress state in concrete. AE transducers were mounted on the surface of the speci-mens by a highly viscous coupling agent in order to ensure the close contact condition. The used dominant peak frequency of AE transducers was 150 kHz. GFRP-confined concrete column specimens with different lengths, cross sections, jacket types, and jacket thickness were manufactured and subjected to cyclic compressive loadings. The Felicity and Kaiser effects of AE signals were also discussed. They found that the AE activity can be correlated to the extent of damage within the specimen. Higher AE activities were observed on specimens with longer dimensions and thicker jack-ets. Although the frequency content of AE signals is a function of AE transducers’ frequency response, they considered spectral analysis ineffective for evaluating the condition of GFRP-concrete columns.

Kundu et al. (1999) [247] studied the scanned ultrasonic images of concrete plate specimens attached by GFRP and CFRP composites. The GFRP-concrete system was formed by gluing a GFRP sheet with epoxy and mounting it onto a concrete plate. A circular delamination region of about 50 mm diameter was introduced between the GFRP layer and the concrete substrate for inspection. The ultrasonic signals were generated with frequencies from 200 kHz to 800 kHz. Both monostatic and bistatic configurations of ultrasonic transducers were used in the applied two scanning modes; Lamb wave scanning (L-scan) and longitudinal wave scanning (pulse-echo and C-scan). It was found that the longitudinal scanning mode is not effective for distinguishing intact regions from damaged ones, while the Lamb wave scanning mode showed the damaged region (with delamination) as a bright spot in the produced image. They further concluded that the insensitivity of Lamb wave scanning mode to

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the small variations in the epoxy and concrete properties has made the Lamb wave scanning mode superior for GFRP-concrete systems.

Mirmiran and Wei (2001) [183] used ultrasonic pulse velocity (UPV) to investigate the extent and progression of damage in concrete cylinders with and without E-glass FRP tube, also used as a tool for damage indication. The GFRP-concrete system was formed by filling concrete into GFRP tubes. Cyclic compressive loadings were applied on the specimens whose mid-section was attached by two ultrasonic sensors on two opposite sides of the mid-section. They found that the UPV in GFRP-concrete systems were sensitive not only at lower stress levels but also after GFRP-concrete had significantly cracked, when compared with plain concrete specimens. Unlike the generally increasing trend of the AE signal in a GFRP-concrete system, the UPV signal exhibited a fluctuating pattern at different loading stages. They considered such difference as an opportunity to complement UPV with AE for the condition assessment of GFRP-concrete systems.

Bastianini et al. (2001) [80] used ultrasonic pulse amplitude (UPA) to locate the debonding defects in a polyurethane slab reinforced on both sides with GFRP, a concrete cylinder specimen wrapped with CFRP composites, and masonry columns wrapped with CFRP. An ultrasonic transducer was required in close contact condition with the surface of MUTs for effective measurements of UPA. They found that the use of UPA is rather independent from the MUT and from the defect nature.

In summary, the features of acoustic NDT methods for GFRP-concrete systems include: (1) mechanical waves can penetrate through the GFRP layer in GFRP-concrete systems. Surface waves such as Lamb waves are found effective on detecting the presence of unseen delamination, due to the change of GFRP thickness [247]; (2) properties of ultrasound waves traveling within GFRP-concrete systems can be used for locating defects within the systems [183, 80]; (3) the use of coupling agent is required to assure the contact condition between transducers and the surface of MUTs, suggesting that acoustic NDT is essentially a contact technique; (4) limited to the size of acoustic transducers, inspection must be conducted on a point-by-point basis; and (5) interpretation of results from reflection measurement is difficult, especially

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for heterogeneous materials like concrete. These constraints pose difficulties for the field application of acoustic NDT on GFRP-wrapped concrete systems for condition assessment.

2.1.3 Thermal Methods

Principles

Thermal methods measure the thermal radiation (radiometry) or temperature (ther-mometry) emitted/reflected from the surface of the MUT, as well as from the am-bient surroundings. Thermal radiation is the inherent electromagnetic radiation of materials above absolute zero temperature, which is a small fraction of the entire electromagnetic spectrum, containing electromagnetic waves ranging from ultraviolet light (3nm ∼ 400 nm) to far infrared radiation (300µm ∼ 1000µm). The radiant energy flux at a given temperature and at a given wavelength is determined by [122]

e(λ, T ) = Z λ

0

eλ(λ, T )dλ (2.5)

where eλ(λ, T ) is the distribution function of radiative flux at wavelength λ

(monochro-matic emissive power), and T is the Kelvin (absolute) temperature. Since a MUT emits a unique distribution of energy in wavelength at a given temperature, as the result of the thermal properties of MUT, such distribution (at a given temperature) can be used for determining the thermal properties of the MUT and, furthermore, for distinguishing one material from another. The relationship among eλ, λ, and T

can be demonstrated by the Plank law of emission for a black body which yields the maximum value of eλ that a material can attain.

eλb =

2π~c2

λhekB T λ~c _{− 1}

i (2.6)

where eλbis the eλ of a black body, ~ is Planck’s constant (~ = 6.62606876×10

−34_{J ·s),}

Condition assessment of GFRP-retrofitted concrete cylinders using electromagnetic waves

Condition Assessment of GFRP-Retrofitted

Concrete Cylinders Using Electromagnetic Waves

by

Tzu-Yang Yu

Submitted to the Department of Civil and Environmental Engineering

in partial fulfillment of the requirements for the degree of

Doctor of Philosophy in the field of Structures and Materials

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

June 2008

c

Massachusetts Institute of Technology 2008. All rights reserved.

Author . . . .

Department of Civil and Environmental Engineering

May 8, 2008

Certified by . . . .

Oral Buyukozturk

Professor of Civil and Environmental Engineering

Thesis Supervisor

Accepted by . . . .

Daniele Veneziano

Chairman, Departmental Committee for Graduate Students

Condition Assessment of GFRP-Retrofitted Concrete

Cylinders Using Electromagnetic Waves

by

Tzu-Yang Yu

Abstract

Acknowledgments

Contents

List of Figures

List of Tables

Chapter 1

Introduction and Research

Motivation

1.1

Research Objective

1.2

Research Approach

1.3

Organization of the Dissertation

Chapter 2

Literature Review

2.1

Nondestructive Testing (NDT) Techniques

2.1.1

Optical Methods

2.1.2

Acoustic Methods

2.1.3

Thermal Methods