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Non-Destructive Testing of Concrete with Ground Penetrating Radar

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Non-Destructive testing of concrete with ground penetrating radar Elias Hammarström Civil Engineering, master's level 2019 Luleå University of Technology Department of Civil, Environmental and Natural Resources Engineering Förord This thesis was done in collaboration with Department of Civil, Environmental and Natural Resources Engineering at Luleå University of Technology (LTU). I want to thank the department for the general support and my supervisor Cosmin Popescu for his support and input during the project. Furthermore, I would also like to thank Björn Täljsten and the other people on site for support during the field trips, the crew of Thysell’s Lab at LTU for their help during the work in the lab and Cristian Sabau for supplying the slab used in the laboratory tests. Last of all, I would like to thank the Explora- tion Geophysics division, Thorkild Maack Rasmussen and Saman Tavakoli on LTU for providing advice support on the GPR and its related software. Luleå 2018 Elias Hammarström I Abstract Concrete structures are susceptible to deterioration over time and it is vital to continually assess concrete structures to maintain the structural integrity and prolong the service life. In recent years there has been an increased interest in non-destructive testing of concrete, i.e. assessing the state of the concrete with- out causing any damage to the structure in the process. There are many different techniques that falls under the term non-destructive testing and one of these that have gained prominence during the last few years is Georadar or ground penetrating radar, often shortened as GPR. GPR is a technique where microwaves are sent into the surface of the concrete by a device, the waves will reflect back to the device when encountering interfaces of areas with different electric properties. The waves are then received by the same device indicating the internal structure of the concrete. This makes the technique an excellent way to find reinforcement bars as the electric properties of concrete and metal strongly differ. In theory though, the technique should also be able to detect other internal differences in concrete, such as voids and corrosion areas but further research is still needed in these areas. This aim of this report is to evaluate ground penetrating radar as a non-destructive technique for assess- ment of concrete structures. In order to do this different tests has been conducted to evaluate the gen- eral performance and usability with a literature review introducing the science behind and what conclu- sions other researches has reached and using a testing methodology to reach the results. The tests can in a simple way be divided into two parts, first lab tests on a slab in a controlled setting where the internal structure was known, and then two shorter field trips in order to evaluate the performance properly in- situ. The results were, to some extent, ambiguous. Although it was found that GPR is an excellent method for finding and locating near-surface reinforcement it was also concluded that the results could vary significantly depending on the location. In one of the field trips the performance of the GPR technique was compared to the performance of traditional cover meter and in this case the portability of the cover meter outperformed the somewhat clunky handling of the GPR. The concrete cover measurement using post-processing of the radar data gave a rough estimate, but once again evaluation still relied on the in- situ conditions and the estimate were sometimes questionable. Finding reinforcement below the first layer yielded differing results and it was concluded that further tests were needed to fully evaluate the capabilities of the technique in this regard. The conclusions of the thesis was that although the tests show some potential for the method the results expected from GPR would strongly depend on suitability of the project and experience of the user. One important limiting factor was the availability of devices. For the current project only one specific device was used, it was theorized that another GPR device could get better results depending on the purpose. Furthermore, the lack of experience was also considered to be a limiting factor that might have had an effect on the results. For future research more tests on lower reinforcement and tests on detection of deterioration were suggested. Comparative studies with other similar non-destructive techniques were also considered to be an area of possible interest. Keywords: Concrete, Non-destructive testing, Ground Penetrating Radar (GPR), Assessment, Inspec- tion II Notations and abbreviations Notations Symbol Description 휀 Dielectric constant (farad/meter) −12 휀0 Dielectric constant of vacuum or air (= 8.85 ∗ 10 farad/meter) 휀푟 Relative dielectric constant t Time (s) 훼 Signal attenuation (dB/m) −1 −1 Conductivity (Ω 푚 ) 푣 Velocity of the wave underground (m/s) 푥0 Horizontal position of object (m) 8 퐶0 Speed of light in vacuum/air (= 3 ∗ 10 m/s) Abbreviations NDT Non-destructive Testing GPR Ground Penetrating Radar, Georadar, Impulse radar III Table of content FÖRORD .......................................................................................................................................... I ABSTRACT ..................................................................................................................................... II NOTATIONS AND ABBREVIATIONS ...................................................................................... III TABLE OF CONTENT ................................................................................................................ IV 1 INTRODUCTION ................................................................................................................. 1 1.1 Background ............................................................................................................................... 1 1.2 Aim ............................................................................................................................................ 2 1.3 Research question ..................................................................................................................... 2 1.4 Limitations ................................................................................................................................ 2 1.5 Structure of thesis ..................................................................................................................... 2 2 LITERATURE REVIEW ....................................................................................................... 3 2.1 Defects in concrete structures ................................................................................................... 3 2.1.1 Concrete defects ............................................................................................................. 3 2.1.2 Steel reinforcement defects ............................................................................................ 3 2.1.3 Brittle failure of post-tensioned concrete caused by voids in tendon ducts ................. 3 2.1.4 Deterioration phases for concrete structures ................................................................ 4 2.2 Ground penetrating radar ........................................................................................................ 4 2.2.1 Dielectric properties ....................................................................................................... 6 2.2.2 Dielectric properties of concrete and steel .................................................................... 7 2.2.3 Instrumentation ............................................................................................................. 8 2.2.4 Data analysis of GPR-recording ..................................................................................... 9 2.2.5 Reinforcement in concrete .......................................................................................... 10 2.2.6 Hyperbola fitting .......................................................................................................... 11 2.2.7 Bridge Surveys with GPR ............................................................................................. 11 2.3 Examples of projects using radar ............................................................................................ 12 2.3.1 Bridge in Poland ........................................................................................................... 12 2.3.2 Evaluation of military owned bridge in United States................................................ 13 2.4 Other Non-destructive testing methods ................................................................................. 14 2.4.1 Covermeter ................................................................................................................... 14 2.4.2 Ultrasonic testing and ultrasound pulse echo ............................................................. 15 3 METHODOLOGY AND TESTS ....................................................................................... 16 3.1 Equipment ............................................................................................................................... 16 3.1.1 Quantum mini radar device ......................................................................................... 16 3.2 Software ..................................................................................................................................
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