Unusual behavior in reinforced concrete structures, including those instrumented with embedded sensor networks, can often be detected by structural health monitoring (SHM) methods. This behavior can be caused by corrosion, cracking, delamination, or construction that differs from the design specifications. It is necessary to investigate unusual behavior to determine its cause. Nondestructive investigations are often applied to validate conclusions drawn from SHM methods, i.e. localization and description of features. Ground penetrating radar (GPR) has been used to investigate defects in structures, but has yet to be used to investigate the influence of material or mechanical properties of materials for the purposes of SHM. In this work, a post-tensioned concrete pedestrian bridge instrumented with fiber optic sensors was investigated (1) to quantify the differences between the concrete used in each of the two construction phases and (2) to locate and map continuous internal features of the bridge deck. A 2.6 GHz GPR antenna was used to survey the main span and two of four approaches of the bridge (one of which was constructed in the second construction phase). In addition to analyzing the data with standard processing and filtering techniques, additional attributes of the data were analyzed to quantify the difference between the concrete strength of each construction phase. The difference in mechanical properties of the concrete in each phase has been the subject of numerous SHM studies on the bridge and has been confirmed during this investigation. This discrepancy agrees with standard lab testing of concrete cores at 7 and 28 days, which is advantageous in associated SHM and analysis of the bridge. The results explore the relationship between electromagnetic and strength characteristics of the concrete, as well as the value of using GPR to complement SHM sensing approaches in understanding anomalies and as-built conditions of a structure.