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 2 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 pattern 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 as- 3 sessment of GFRP-retrofitted reinforced concrete and other reinforced concrete civil infrastructure systems. Thesis Supervisor: Oral Buyukozturk Title: Professor of Civil and Environmental Engineering 4 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. 5 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. 6 Contents 1 Introduction and Research Motivation 23 1.1 Research Objective . 29 1.2 Research Approach . 29 1.3 Organization of the Dissertation . 31 2 Literature Review 35 2.1 Nondestructive Testing (NDT) Techniques . 36 2.1.1 Optical Methods . 42 2.1.2 Acoustic Methods . 44 2.1.3 Thermal Methods . 48 2.1.4 Radiographic Methods . 51 2.1.5 Magnetic and Electrical Methods . 53 2.1.6 Microwave and Radar Methods . 56 2.2 Summary . 61 3 Numerical Simulation 63 3.1 Maxwell’s Curl Equations and Linearly Polarized EM Waves . 65 3.2 Finite Difference Time Domain Solution and Yee’s Algorithm . 68 3.3 Absorbing Boundary Condition – Perfectly Matched Layer . 73 3.4 Stability Criteria in Discretization . 74 3.4.1 Discretization in Space . 75 3.4.2 Discretization in Time . 76 3.5 Two-Dimensional and Three-Dimensional Simulations . 77 7 3.5.1 Validation of the Code . 77 3.5.2 Actual Far-Field Simulation . 82 3.6 Simulation Results . 85 3.6.1 Damage Detection in Normal Incidence . 86 3.6.2 Damage Detection in Oblique Incidence . 86 3.6.3 Effect of Defect Width in Normal Incidence . 88 3.6.4 Effect of Defect Depth in Normal Incidence . 89 3.6.5 Rebar Detection in Normal Incidence . 89 3.6.6 2D and 3D Responses . 107 3.7 Summary . 107 4 Laboratory Radar Measurements 111 4.1 Experimental Program . 112 4.2 Manufacturing of the Specimens . 116 4.3 Experimental Configuration and Parameters . 118 4.3.1 Monostatic ISAR Normal Incidence Scheme . 120 4.3.2 Monostatic ISAR Oblique Incidence Scheme . 121 4.4 Calibration of Laboratory Radar Measurements . 121 4.4.1 PEC Specimen . 122 4.4.2 Lossy Dielectric Specimen and Its Optical Model . 124 4.5 Frequency-Angle Measurements . 127 4.5.1 Monostatic ISAR Normal Incidence Scheme . 133 4.5.2 Monostatic ISAR Oblique Incidence Scheme . 152 4.6 Summary . 155 5 Image Reconstruction 159 5.1 Single Scattering and Synthetic Aperture Radar . 160 5.2 Inverse Synthetic Aperture Radar . 172 5.3 Backprojection Algorithms .