Three main objectives have been set for this research. The first is to develop an accurate method for measuring the dielectric constant of PCC using a capacitor probe (C-Probe) that has been recently developed at Virginia Tech and validate it for field application to detect internal PCC flaws such as delamination. The C-Probe consists of two flexible conducting plates, connected to a Network Analyzer, with a specific separation between them. The second is to optimize the C-Probe design configuration for different PCC slab thicknesses. The third objective is to develop a predictive model that correlates the bulk dielectric constant of PCC with its critical parameters (cement, aggregate, and air content). Five calibration methods have been developed and evaluated for the C-Probe to measure the dielectric properties of PCC. This evaluation has demonstrated that open, short, Teflon material (OSM) calibration method is the most appropriate one for the C-Probe. The selected calibration method was used to validate the C-Probe fixture for field application by measuring 1.5 x 1.5 m PCC slabs prepared with different mix properties, thicknesses, and induced deterioration. The C-Probe has been proved to detect induced voids in the PCC slabs. In addition, the effect of steel reinforcement on measurements can be mastered by controlling the penetration of electromagnetic (EM) field in the PCC slabs. The effective penetration depth of the EM field for different C-Probe design configuration was optimized by computer simulation. The results have been used to develop a predictive model that correlates the effective penetration depth with the plates' size, separation between them, and the dielectric constant of the PCC under test. Thus, an optimum design for different desired penetration depth was achieved. Two experimental designs were developed to identify the critical parameters that affect the bulk dielectric constant of PCC. A computer simulation was used to identify the significance of each parameter. A predictive model has been developed to correlate the PCC bulk dielectric constant to the critical parameters. The estimated dielectric constant of PCC using the predictive model was correlated to that obtained by other theoretical mixture models; the predictive model has found to correlate well with Looyenga theoretical mixture model. / Ph. D.

Portland cement concrete (PCC) structures deteriorate with age and need to be maintained or replaced. Early detection of deterioration in PCC (e.g., alkali-silica reaction, freeze/thaw damage or chloride presence) can lead to significant reductions in maintenance costs. Portland cement concrete can be nondestructively evaluated by electrically characterizing its complex dielectric constant in a laboratory setting. A parallel-plate capacitor operating in the frequency range of 0.1 to 40.1 MHz was developed at Virginia Tech for this purpose. While useful in research, this approach is not practical for field implementation. In this study, a capacitor probe was designed and fabricated to determine the in-situ dielectric properties of PCC over a frequency range of 2.0 to 20.0 MHz. It is modeled after the parallel-plate capacitor in that it consists of two conducting plates with a known separation. The conducting plates are flexible, which allows them to conform to different geometric shapes. Prior to PCC testing, measurements were conducted to determine the validity of such a system by testing specimens possessing known dielectric properties (Teflon). Portland cement concrete specimens were cast (of sufficient size to prevent edge diffraction of the electromagnetic waves) having two different air contents, two void thicknesses, and two void depths (from the specimen's surface). Two specimens were cast for each parameter and their results were averaged. The dielectric properties over curing time were measured for all specimens, using the capacitor probe and the parallel-plate capacitor. The capacitor probe showed a decrease in dielectric constant with increasing curing time and/or air content. In addition to measuring dielectric properties accurately and monitoring the curing process, the capacitor probe was also found to detect the presence and relative depth of air voids, however, determining air void thickness was difficult. / Master of Science