Twenty six percent of reinforced and prestressed concrete highway bridges in the United States need repair or replacement. Proper use of available and promising sustainable technologies plays a critical role in the nation’s economy and the safety of the traveling public. Employing fiber-reinforced polymer (FRP) advanced materials has recently been accepted as a rational and sustainable rehabilitation option for structurally deteriorated infrastructure. Despite the advancement in FRP techniques, inspection of its installation presents a significant challenge to its widespread use. To ensure durability and capacity of externally bonded FRP structures, it is critical to evaluate the potential for debonding failure and surface defects. In this study, experimental and theoretical investigations on employing ground-penetrating radar (GPR) and infrared tomography (IRT) methods were carried out to evaluate reinforced concrete bridge deck slabs externally bonded with glass FRP (GFRP), carbon FRP (CFRP), and a combination thereof. Eight externally bonded FRP concrete deck slab specimens were prepared: three with CFRP, four with GFRP, and one with hybrid CFRP/GFRP. Cracks, voids, and debonding were artificially made on the surface of the concrete deck slabs. Test variables include location and size of surface voids, and rebar diameters and debonding. Improved 2-D and 3-D image reconstruction was established. The results showed that the developed software, using the enhanced image reconstruction technique, provide clear images of the FRP-strengthened deck slabs. Test data revealed that the GPR technique could accurately determine rebar diameters, as well as size and location of voids. The GRP, however, could not well predict debonding and cracks. Results obtained from the IRT indicated that it can detect and locate near-surface defects with a good accuracy. The study suggests that the combination of the GPR and IRT techniques can be effectively employed to image internal defects of FRP-strengthened concrete bridges.