Using elastic-wave-based NDT methods to detect defects in RC structures is difficult because of the interference of rebars. In this study, the interference effect on the quality of SAFT images was quantitatively explored using a series of numerical simulations. Furthermore, two concrete specimens were fabricated for experimental verification. The experimental results exhibited strong agreement with those of quantitative analyses used in numerical simulations. These results indicated that using SAFT images to expose the defects under a layer of rebars is feasible. Moreover, a useful guideline for selecting the impacting source to produce a high-quality SAFT image is proposed.