Ground Penetrating Radar (GPR) has recently gained momentum to nondestructively detect embedded objects and defects in concrete structures. Quality of the scans and reconstructed images are directly correlated to dimensions and depths of the concrete defects and embeded objects, as well as the frequencies of the GPR antennas. Laboratory reinforced concrete slab specimens, with embeded defects and objects, were constructed and nondestructively tested. Four GPR antennas of 400, 900, 1,600, and 2,600 MHz were used in the nondestructive testing. Objects in the concrete slabs include #3 to #8 rebars located at diferent depths and various artificial voids and delaminated areas. Synthetic aperture forcusing technique was applied to effectively intepret the GPR data, as well as to improve both the two-dimensional and three-dimensional reconstructed images. The study concluded that all the four antennas could detect the embedded objects near the concrete slab surface. It was observed that the higher the frequency of the antenna the better the quality and sharpness of the reconstructed images. GPR signal attenuation assisted concluding a varying detecability scale for embedded objects in concrete slabs.