Use of infrared thermography for quantitative non-destructive evaluation in FRP strengthened bridge systems

The increasing use of fiber reinforced polymer (FRP) composites as externally bonded reinforcement for strengthening of concrete structures has created a need for the development and implementation of rapid methods of field level non-destructive evaluation (NDE) both for quality assurance during installation of the material and also for long-term monitoring of in-field performance. These methods must provide inspectors with the ability not only to inspect FRP strengthened structures for defects, but also should provide the means to quantify the shape, size and severity of the defect for continued monitoring and comparison. Infrared (IR) thermography, which uses data from temperature differentials to detect and characterize defects and anomalies, presents a potentially efficient technique for non-contact, real-time inspection and quantitative data interpretation. In this research a three-girder two bay reinforced concrete bridge deck segment was loaded under field representative loading conditions and the girders and slabs were sequentially strengthened with FRP composites to study the effect of FRP strengthening at systems level. The two slabs were strengthened with two different composite systems, prefabricated strips and field impregnated fabric laminates. The appearance and progression of damage in the FRP systems and at the FRP-concrete interface with an increase in the level of loading was quantitatively monitored using IR thermography. Based on the thermal intensity, the damage could be classified into types such as interlaminar debonding inside the composite or composite-concrete interface debonding. Also based on the magnitude of the thermal intensity, the severity of the damage could be quantitatively monitored. The results were also correlated to the visual observations of crack patterns and failure mechanisms as well as to strain and displacement data measured during the test.