| The United States has the largest roadway infrastructure system in the world. However, this roadway infrastructure is deteriorating every year, causing increasingly serious hazards. Hence, periodic condition assessment of the roadway infrastructure provides better preventive and/or corrective planning. Also, continuous monitoring of the underground condition is necessary for obtaining environmental factor effects on deterioration. All these will help to preserve the roadway infrastructure integrity and reduce its life-cycle costs. In this dissertation, Electromagnetic (EM) sensors for subsurface characterization are introduced.; Ground penetrating radar (GPR) is used to measure the thickness and dielectric constant of the highway pavement based on the time lag between the direct wave and sub-layer reflection signal. The frequency-modulated continuous wave (FMCW) GPR is operated in the frequency modulation mode for thin over-layer pavement or asphalt pavement measurements, while the pulse GPR is operated in the time domain to provide larger penetration depth for concrete pavements.; In this dissertation, for each GPR technique, both path models and signal processing methods are introduced. The path models demonstrate the relationship between the travel time and the sub-layer dielectric constant and thickness, and signal processing methods are applied to received signals to get the travel times. Though the common middle point (CMP) method in seismic applications is widely used in the GPR data interpretation, the difference between these two conditions is not addressed properly. Here a new model is set up and applied for the FMCW radar. Both Fourier transform and power spectrum density analysis are introduced to get the propagation time and increase the resolution. For the pulse GPR, the existence of the underground direct wave in certain conditions is addressed and verified by simulation, which can provide reliable information about the pavement material. Also, time-frequency signal processing methods are introduced to get the reflection and cancel the rebar effects.; The radio-frequency (RF) phase shift moisture sensor is introduced for continuous moisture monitoring in pavement sub-grades, which is an important factor affecting the pavement quality. Simulation is applied for the sensor head structure optimization. Implementation and test results are also introduced. |
