Impact-echo (IE) and ultrasonic-echo (UE) are widely used elastic wave methods to determine voids in concrete and have successful applications in the field. However, because in most cases it is difficult to interpret the results, researchers have been studying on to develop imaging algorithms. In this study, SIBIE (Stack Imaging of Spectral Amplitudes Based on IE) and SAFT (Synthetic Aperture Focusing Technique) algorithms were used in order to evaluate voids within concrete blocks. For this purpose, concrete blocks containing post-tensioning ducts that include voids were produced and IE and UE tests were carried out to locate the voids. Moreover, apart from the other studies in literature, effect of thickness of the metal ducts on phase change of the ultrasonic signal was determined. As a result of the study, along with successful void visualization, material characterization by means of acoustic impedance characteristics of air and steel was made.