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Ground penetrating radar (GPR) technology has been used for the past 20 years for a variety of applications to assess transportation infrastructure. However, the main issue after all these years remains: "How well does GPR work and under what conditions?" Results show that GPR works well for some situations, but is not an appropriate tool for other situations. It is not used currently on a routine basis by the US Departments of Transportation (DOTs) due mainly to difficulties encountered with data interpretation. Data interpretation difficulties are mainly attributed to the fact that images obtained from the reflected signals are not photographs of the features that are beneath the surface being investigated. The images show the amplitude of the radar-reflected signals from the interfaces with different dielectric properties. Therefore, a considerable amount of experience and operator skill may be required to correctly interpret sub-surface radar results. To better understand reflected GPR signals, this research was conducted with the following objectives: to determine the dielectric properties of concrete over the used GPR frequency range; to synthesize the reflected air-coupled radar signals and compare them with measured waveforms; to model and study the effects of simulated defects in concrete on the reflected air-coupled and ground-coupled radar signals; and to validate the research results in the field by predicting layer thicknesses of flexible pavements and detecting moisture in flexible pavement systems. Several concrete slabs, 1.5x1.5 m, were constructed with known thicknesses, simulated defects, and different reinforcement configurations. The concrete mixes included four different bridge deck mixes and one concrete pavement mix used in the State of Virginia. Results have shown that the dielectric constant of concrete is frequency and mix dependent. However, modeling the reflected signals using an average complex dielectric constant over the entire radar frequency range led to modeled waveforms comparable to the measured waveforms. Although air- and water-filled voids did distort the reflected waveforms, a model was developed to predict the reflected waveforms from the simulated defects. Reinforcement was found to affect the reflected waveforms only when it was oriented in a direction perpendicular to the GPR antennas. A model was also developed to predict the GPR waveforms obtained from flexible pavements. This model could be used in a procedure to measure layer thicknesses more accurately by including losses that occur inside the pavement materials. Two different case studies, where a ground-coupled GPR system was used to locate moisture at different layers, have led to the conclusion that the ground-coupled GPR is a feasible tool to detect moisture inside pavements.

There are two basic methods used to analyze and design reinforced concrete members for shear, the American Concrete Institute Code method (used in the United States) and the truss model method (used in different European Codes and in the Canadian Codes). The ACI Code method is a semi-empirical solution since it builds on fitting actual test results to the analytical mechanism method. Although it will lead to safe results, the ACI method lacks a physical model to represent the actual forces in the beam. For this reason, attention is increasingly being given to develop a mechanical-mathematical model to show the actual behavior of the beam failing in shear. The truss model theory provides a more promising way to treat shear since it can model the structural action in the beam. It was first proposed by Ritter and Marsh at the turn of the twentieth century. The concept has been extended by recent work of Lampert and Thürlimann (1971), Collins and Mitchell (1980), MacGregor (1988), and others. The purpose of this study is to compare the ACI method and the truss model method for the design of reinforced concrete slender and deep beams. The results of this study suggest that the truss model is better suited for the design of deep reinforced concrete beams because it models the dominant mechanism that happens in the beam which is the force transfer from load to reaction by the direct compression struts. For slender reinforced concrete beams, either method is suitable for design.