Fourier analysis applied to Infrared Thermography of Fiber Composites used for the Strengthening of Structural Elements

Abstract

Fiber reinforced polymers (FRP) composites are an efficient method in the strengthening of civil structures. FRP systems are usually applied by means of a wet lay-up method. National guidelines recognize that Infrared Thermography (IRT) is a possible non-destructive evaluation (NDE) technique for the detection of bonding defects, voids and damages and the characterization of the surface layers without a direct contact between the device and the observed area. Conversely, in the guidelines no clear indications are given about the inspection procedure. Fiber Reinforced Polymer (FRP) composites are widely used for the strengthening and seismic retrofitting of concrete, masonry or steel structures. FRP systems are applied using a wet lay-up method in which the fibers are impregnated on site. In this case, the performance of the system is due not only to the quality of the materials but also to the accuracy of the application since the system may contain air voids or non-uniform distributions of the epoxy resin. The efficiency of this technology is strongly dependent on the correct positioning and bonding of the fibers on the surface. Therefore inspection and monitoring are essential. A series of regulatory documents and guidelines were issued for the use of FRP for strengthening existing civil constructions. In the European fib [1] it is explained that thermographic NDTs may be successfully applied for bonding evaluation and that defects are located as hot (or cool) spots due to different thermal properties of degraded bonding. The ACI Committee [2] defines that thermographic tests may indicate signs of progressive delamination in the inspection and assessment of reinforced structures. In the Italian Guideline CNR DT200/2004 [3], thermographic tests are considered effective for the evaluation of substrate deterioration prior to FRP application to ensure proper quality of the support and for the execution of non- destructive tests. All the guidelines recognize that the control of the application may be performed with Infrared Thermography (IRT) among different Non Destructive Evaluation (NDE) techniques. Nonetheless, at present, there are no standard procedures to assess the quality of the applications in civil engineering structures. Infrared thermography represents a valid tool for the detection and measurement of bonding defects or damage in the composite strips and can be used for the definition of possible damage progression. The paper presents a series of results obtained using infrared thermography for applications masonry walls and concrete beams. For the analysis of the thermographic sequence of recorded images an algorithm based on the Fourier frequency analysis was set.