| Author: | Cham, Yui Kei |
| Title: | Cover-depth modelling and error correction of object mapping in reinforced concrete structures in the near-field of ground penetrating radar |
| Advisors: | Lai, Wallace (LSGI) Poon, C. S. (CEE) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Subject: | Ground penetrating radar Ground penetrating radar -- Mathematical models Reinforced concrete construction -- Inspection Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Land Surveying and Geo-Informatics |
| Pages: | xv, 119 pages : color illustrations |
| Language: | English |
| Abstract: | The quality of construction works and integrity of building structures are interrelated with the reliability, durability, safety, lifespan and cost efficiency of maintenance for all built structures and elements throughout their entire life. To ensure the quality of work and identify the potential defects as early as possible, inspection and certification of quality of works and material for new works, addition and alteration (A&A) works and modification works, and mandatory regular routine inspection of existing buildings are performed on-site. To save project time and cost, the modern diagnostic technique provides quick, reliable and non-subjective traceable evidence to the engineer prior to making a proper engineering judgment. The modern non-destructive testing and evaluation technique neither not damages the appearance and function of the building element nor interferes with the enjoyment of the user. With the advanced technology development in sensing and intelligent data processing, ground penetrating radar (GPR) offers a powerful nor invasive approach to accurately locate and measure unseen objects inside non-metallic objects and below ground. GPR measures the travelling time of the reflected wave and estimates the object's depth. Thus, GPR is an indirect measurement method, and its vertical measurement accuracy and quality level (QL) are ±40% under QL-B2/2P as set out in Specification for Underground Utility Detection, Verification and Location, PAS:2014 published by the Institution of Civil Engineers (ICE). For cover depth measurement by GPR, relevant code, standards, and specifications regarding accuracy are still in the nurturing stage. The objective of this study is to develop a standard model to recognize the potential sources of error, minimize and correct the error, and evaluate the uncertainty in the GPR measurement of concrete reinforcement cover. In the beginning, the study focused on recognizing the potential errors from various sources and classified them into three different areas including the host material, the ray-path geometry, and equipment & signal processing (Chapter 3). Then, the study evaluates potential errors through (1) different ray-path models with associated mathematical models, (2) and experimental validation in both air and controlled reinforced concrete samples. Having reviewed the deficiency of the current single trilateration model under ASTM, models taken into account of the antenna separation and oblique angle, and target object size and depth in semi-trilaterated and full trilaterated ray-path models with associated mathematical expression respectively were validated by experiment (Chapter 4). The potential sources of error and uncertainty mentioned (Chapter 3) were evaluated based on the experimental results (Chapters 4 and 5). A refined algorithm based on a semi-trilateration model and root mean square error algorithm was developed and tested concerning the confidence level and interval (Chapter 6). According to the experimental results, the following conclusions were drawn, (1) Effect of accurate concrete cover depth measurement in the reactive near field and Fresnel region using high-frequency 2GHz antenna – an error ranging from 34.5-136.1% in the near field in comparison to the 1.2-28.2% in the Fresnel region. Accurate measurement in near-field regions is highly limited and is not recommended. (2) Effect of ages of concrete on accuracies of cover depth measurement – the variation of average estimated depth is 0.13mm and 0.02mm at early Day 56 and at Day 90 and afterwards respectively. The result also showed that the dielectric constants and Two-Way Travel Time were decreasing and became steady, and the peak frequencies were increasing steadily for different cover depths at Day 90 and afterwards over the age of hardened concrete. The result is closely related to the cement hydration process which converts free water to bound water. (3) Uncertainty, confidence level and interval of refined algorithm based on semi-trilateration method will be defined after experimental validation on real concrete elements. The abovementioned work provides a standard model and measurement method to accurately estimate the concrete cover depth with comprehensive consideration of different potential sources of errors from the formation of host materials, i.e. fresh concrete to hardened concrete, the use of equipment and signal process, and ray-path and mathematical models. The findings also provide clear guidance to stakeholders, including clients, engineers, surveyors, and operators, on achieving accurate and reliable cover depth measurement with confidence in GPR measurement. |
| Rights: | All rights reserved |
| Access: | open access |
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