The Use of Flax Fiber-Reinforced Polymer (FFRP) Composites in the Externally Reinforced Structures for Seismic Retrofitting Monitored by Transient Thermography and Optical Techniques

Natural fibers constitute an interesting alternative to synthetic fibers for the production of composites due to their environmental and economic advantages. Even though their strength is on average lower compared to their synthetic counterparts, natural fibers such as flax, among other bast fibers (jute, kenaf, ramie, and hemp), are serious candidates for seismic retrofitting applications, given that their mechanical properties are more suitable for dynamic loads. Strengthening of structures is performed by impregnating flax fiber-reinforced polymers (FFRP) fabrics with epoxy resin and applying them to the component of interest, increasing in this way the load and deformation capabilities of the building, while preserving its stiffness and dynamic properties. The reinforced areas are, however, prompt to debonding if the fabrics are not properly mounted. Nondestructive testing is therefore required to verify that the fabric is installed uniformly and that there are no air gaps or foreign materials that could instigate debonding. In this chapter, the use of active infrared thermography was investigated for the assessment of a laboratory specimen with artificial defects, an actual FFRP-retrofitted beam, and a part of an external masonry wall. Results coming from the academic specimen were compared to digital speckle photography and holographic interferometry images.