Aging concrete structures are increasing worldwide and the preservation of their structural integrity is a crucial issue. The first step in such a procedure is the estimation of damage parameters. Stress wave techniques contribute to this issue since they are non-destructive and can be used before and after the repair work to provide information for the internal condition. In the present paper, seismic tomography was applied for the characterization of a full-scale concrete structure. After repair, which was conducted by cement injection, the velocity distributions of several structure cross sections exhibited decrease, a trend that was not expected. Therefore, an investigation with scattering theory was undertaken to provide an explanation through the property mis-match of the stiff concrete matrix and the soft newly placed cementitious material. Additionally, laboratory experiments on grouted porous concrete specimens revealed that an immediate increase in velocity should not always be expected, especially at low temperatures, and care should be taken in interpreting ultrasonic data. Finally, suggestions about the improvement of characterization are made through the use of different frequencies and more promising features like the wave amplitude.