Advanced microwave embedded sensors for infrastructure health monitoring

Microwave sensor systems have been widely investigated for many applications due to their ability to provide non-destructive, noncontact, one-sided and wireless testing. Among these applications infrastructure health monitoring of bridges, building, and dams using microwave sensors, which are mounted on or embedded in composite structures of infrastructure has been attracting an increasing interest. One of the current needs of infrastructure health monitoring includes the detection and monitoring of disbonding and gaps in concrete-based structures, which are also required for simultaneous characterization of concrete. A recently proposed microwave sensor technique exploiting a relatively simple waveguide sensor embedded in a concrete-metal structure such as a concrete-filled steel tube exhibited great potential. However, it suffers from a few drawbacks that need to be solved. This thesis aims to develop and investigate advanced microwave embedded sensors to solve main problems in the current microwave sensory technique including characterization of concrete in concrete-based structures at different stage of its life, size of the interface under inspection, detection and monitoring of a small gap between concrete and dielectric material surfaces and sensitivity to gaps. To achieve this aim the following five research contributions have been made: The first contribution is the methodology for the determination of the complex dielectric permittivity of concrete using both measurement data and simulation results at different stages (fresh, early-aged and dry) of its life. Firstly, it is developed and tested for a single flanged open-ended waveguide sensor with a hardened concrete specimen, and then the methodology is modified for the developed sensors embedded in concrete-based composite structures with fresh, early-age and dry concrete. Modern computational tool CST Microwave Studio and a performance network analyser are used for simulation and measurement, respectively, throughout this research work. The second contribution is a dual waveguide sensor, which is proposed, designed and applied for the detection and monitoring of a small gap in concrete-metal composite structures. It consists of two waveguide sections and a metal plate and uses the transmission of electromagnetic waves along gap when it occurs between the metal plate and concrete surfaces. It provides more measurement data than the single waveguide sensor for characterising concrete-metal structures such as transmission properties of guided waves along the gap and reflection properties of the metal–concrete interface at two different places at the same stage of concrete. As a result, the proposed sensor increases the size of the interface under inspection and sensitivity to the gap using the magnitude of reflection coefficient and magnitude of transmission coefficient together and/or independently. The third contribution is the design and application of a dual waveguide sensor with rectangular dielectric insertions that is proposed and tested for the characterisation of concrete–metal structures at different stages of the concrete life including its fresh stage. The dielectric insertions are designed and implanted in the waveguide sections in such a way that they create the resonant response of the sensor and prevent water and concrete entering the sections. The resonant properties of the sensor allow long-term monitoring of the concrete hydration, including the detection of the transition from fresh to hardened concrete on its first day. The proposed sensor along with the modified algorithm provides the determination of the complex dielectric permittivity of fresh concrete. The fourth contribution is a dual waveguide sensor with tapered dielectric insertions. Each tapered dielectric insertion is designed with a tapered part and rectangular part to reduce wave reflection from the insertions over an entire frequency band. The proposed sensor has improved performance at the resonant responses of a quarter-wavelength resonator formed by an open end at the tapered part and shorted end at the rectangular part of each insertion. The last contribution is the development of dual waveguide sensors with attached dielectric layer and their application for the detection and monitoring of gap between dielectric materials and concrete in metal-dielectric layer-concrete composites as well as the determination of complex dielectric permittivity of concrete at different stages of its life. One of the most attractive designs is the sensor with empty waveguide sections due to its simplicity and robustness, and capability of the layer for preventing penetration of the obstacles and water, and for optimization of the sensor. On the other hand, the sensors with dielectric insertions and the layer demonstrate a significantly higher magnitude of transmission coefficient. The proposed DWSs can be applied to characterise fresh concrete in a dielectric mould or on-line, and to investigate the shrinkage of different categories of concrete

Monitoring of microwave properties of early-age concrete and mortar specimens

Microwave noninvasive and evaluation techniques have shown to be promising for characterization of cement-based materials (CBMs). In the construction industry, the determination of initial water-to-cement (w/c) ratio and cure-state monitoring of early-age CBMs are still challenging tasks. In this paper, measurement results of monitoring of reflection coefficients, electrical conductance and susceptance of early-age concrete, and mortar specimens with different w/c ratios and constituent ratios are presented. The comparative measurements are conducted using a microwave near-field noninvasive testing technique at two frequency bands, namely, R-band ((1.7-2.6) GHz) and X-band ((8.2-12.4) GHz). Statistical analysis and exponential curve fitting have been performed on the temporal measurements of early-age (a few first days) CBMs. The results demonstrated that the dynamic ranges, the initial values, and the initial slopes of measurement curves can be used for cure-state monitoring, hydration rates prediction, and initial w/c ratio determination