Radar has the potential of becoming a powerful and effective tool for the nondestructive testing of concrete structures. An advancement of the method can be achieved through the understanding of the interaction between electromagnetic waves and concrete, and the identification of optimum radar measurement parameters for probing concrete. For the determination of optimum parameters, systematic radar measurements are needed in tandem with the implementation of proper signal processing techniques for imaging. This paper presents the results of such radar measurements, which are made using a wideband imaging radar with different frequency ranges. Inverse synthetic aperture radar is used for the measurements of laboratory-size concrete specimens having three different types of internal configurations. The signal processing algorithms implemented to obtain two-dimensional and three-dimensional imagery of concrete targets from radar measurements are also discussed. For the concrete specimens and measurement setup used in this study, 2-3.4 GHz waveforms are found to be adequate for the concrete thickness measurement, 3.4-5.8 GHz waveforms are adequate for the detection of delamination, and 8-12 GHz waveforms are adequate for the detection of inclusions embedded inside concrete.

In this work, circular concrete-filled steel tubular (CCFT) columns, rather than the more popular H-shaped columns, were suggested as pre-founded columns for top-down construction. In addition, a novel shear connection system with headed stud shear connectors between the CCFT columns and the flat slabs was developed. It was noted that a CCFT column with a design compressive strength similar to that of an H-shaped column without consideration of length effects can be easily installed, even into a smaller borehole. Furthermore, compared to the H-shaped column, less steel is required for the CCFT column. It was shown that the amount of steel needed can be reduced by decreasing the wall thicknesses or diameters of the CCFT column depending on the exposed length of the column during excavation. The fillet-welded joint of the developed shear connector system was also tested before its in-situ application. The test results revealed that the joint possessed sufficient shear and deformation capacities. The CCFT column with the developed shear connection system was ultimately applied to an actual top-down construction process. The good constructability of the CCFT column system and reductions in construction costs and time were confirmed.

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