| Author: | Lin, Zhen |
| Title: | Development of an embeddable fiber optic sensing system for monitoring chemical parameters inside concrete |
| Advisors: | Ni, Yi-qing (CEE) |
| Degree: | Ph.D. |
| Year: | 2026 |
| Department: | Department of Civil and Environmental Engineering |
| Pages: | 1 volume (various pagings) : color illustrations |
| Language: | English |
| Abstract: | Hong Kong, a coastal metropolis, critically depends on marine infrastructure such as bridges and artificial islands to facilitate transportation and underpin economic development. However, those marine infrastructures are inherently impermanent as concrete structures deteriorate due to phenomena such as corrosion, carbonation, and chemical degradation. Chemical parameters like relative humidity, chloride, and sulphate will accelerate deterioration. Proactive monitoring of these parameters can enable early intervention to prevent catastrophic structural failure. Among all the sensing technologies for monitoring chemical parameters inside concrete, fiber optic sensing systems have attracted great interest due to their advantages such as small size, waterproofness and high corrosion resistance, remote monitoring and multiplexing. Nevertheless, field application of fiber optic sensing system for monitoring chemical parameters inside concrete remains rare and the relationship between the fiber optic sensing system signals and chemical parameters is complex. To address these gaps, this thesis developed two types of embeddable fiber optic sensing systems for monitoring chemical parameters inside concrete including fiber Bragg grating (FBG)-based sensing system and fluorescence-based sensing system, validated their feasibility for the field applications in real marine concrete structure, and employed machine learning to analyze the data collected from the long-term field test of the fiber optic sensing system. First, an embeddable FBG-based sensing system for monitoring relative humidity inside concrete was developed based on the strain change induced by the swelling or shrinking of the polyimide (PI) coating on FBG sensors under varying humidity conditions. After calibration, the FBG-based sensing system was installed in a concrete beam sample placed in an exposure site to validate long-term field performance. The FBG-based sensing system worked well for more than two years, including under extreme weather conditions. Then a Gaussian Process (GP) model was trained using the data collected from the long-term field tests of FBG-based relative humidity sensing system and the data collected from an automatic weather buoy to predict the relative humidity inside the concrete beam sample. The GP model provided predictions with 90% confidence level, which consistently encapsulated most measured relative humidity values across multiple channels and seasonal conditions. The results indicated that the GP model provided a viable method for estimating the relationship between the signals from FBG-based sensing system and relative humidity inside the concrete. Also, an embeddable fluorescence-based sensor for monitoring chloride ions inside concrete was developed, utilizing quinine sulphate as the chloride ion fluorescent indicator. The performance of the developed fluorescence-based sensor for monitoring chloride was tested in chloride ion solutions, concrete simulation solutions with different pH values, solutions containing diverse ions for selectivity assessment, and mortar specimens. The results showed that the fluorescence intensity of the developed fluorescence-based sensor proportionally correlated with the chloride ion concentration in different chloride ion solutions in accordance with the Stern-Volmer law. The fluorescence-based sensor exhibited a remarkable stability across the pH range (8.6-12.6) and had a high selectivity for chloride ions. In the mortar specimens the fluorescence-based sensor's sensitive depths are better than 3 mm and the sensing accuracy for chloride concentration is better than 0.1%. The results indicated substantial potential for field application of the embeddable fluorescence-based sensing system for monitoring chloride inside concrete in the field. Then a fluorescence-based sensor for monitoring sulphate was developed using scandium-morin complex as the sulphate ion fluorescent indicator. The molarity ratio of scandium and morin was set at 2:1 due to the highest peak intensity in the excitation-emission maps. The scandium-morin complex solution was immobilized as a film on glass substrates using cellulose acetate. Performance tests in sulphate solutions showed that the fluorescence intensity of the developed fluorescence-based sulphate sensor proportionally correlated with the sulphate ion concentration in different sulphate ion solutions in accordance with the Stern-Volmer law. Furthermore, a fluorescence-based sensor for monitoring chloride and sulphate was developed with dual-ion sensing element which had an isolation layer to prevent two fluorescent indicators from contacting. The performance of the sensor was tested in the chloride and sulphate solutions with different concentrations. The results showed that the fluorescence-based sensor for monitoring chloride and sulphate had two peaks and the peaks were sensitive to chloride ions and sulphate ions, respectively. The performance of the embeddable fluorescence-based sensing system for monitoring chloride and sulphate was tested in the wave wall project at Lei Yue Mun. The results showed that the chloride sensitive peak and sulphate sensitive peak had a slow downward trend during the commission. The intensity fluctuation of chloride sensitive peak was larger than that of sulphate sensitive peak. This study underscores the considerable potential of the field applications of fiber optic sensing systems for monitoring chemical parameters inside concrete enabling a more profound understanding of deterioration mechanisms. And it demonstrates the critical role of machine learning in analyzing the substantial datasets generated by fiber optic sensing system to elucidate relationships between optical signals and target chemical parameters. |
| Rights: | All rights reserved |
| Access: | open access |
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