Fiber-reinforced polymer (FRP) materials have been widely accepted to retrofit 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 groundpenetrating radar (GPR) and infrared tomography (IRT) methods were carried out to evaluate 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 depths. Improved 2-D and 3-D image reconstruction using synthetic aperture focusing technique (SAFT) was established to effectively interpret GPR data collected. The results showed that the developed software, using the enhanced image reconstruction technique, provides clear and high-resolution images of the FRP-strengthened deck slabs in comparison to those obtained from the device’s software. Test data revealed that the GPR technique could accurately determine rebar depths as well as depth, size, and location of artificial voids. The GRP, however, could not well predict debonding and cracking. Results obtained from IRT images indicated that it could 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 assess internal defects of FRP-strengthened concrete bridges.