Damage Tolerance Approach in the Concept of Quality Control Workflow of Passive CFRP Strengthening of Reinforced Concrete Structures

The use of Carbon Fiber Reinforced Polymer (CFRP) is one of the most popular methods of strengthening existing reinforced concrete structures nowadays. Many civil engineering structures (e.g. bridges) are strengthened in this way. Due to the reduction of costs and environmental impact through the decrease of construction waste and lower production of new construction materials (e.g. cement responsible for significant emission of CO₂), it is still economically and environmentally justified to strengthen existing structures rather than demolish old ones and build new ones. After many years of using CFRP strengthening techniques, more and more concerns are dictated by its lifetime durability, gradual deterioration as well as quality assessment of the bond between composite and concrete. Although available manufacturers' design guidelines for nondestructive evaluation strengthened reinforced concrete (RC) structures indicate possible inspection methods, in the practice, they are limited mainly to visual inspection and tap tests. So far, it is not clear if any acceptable level of defect size in FRP-bonded members exists; thus, if any air voids or delamination are detected, the laminate should be replaced with the new one. This process in the performed in situ technically simple tests relies mostly on the worker’s perception. The lack of qualitative and quantitative guidelines for acceptable defect levels may lead to conservative conclusions and unnecessary economic and environmental loss. On the other hand, recent studies (Bialozor in An Investigation into the Influence of Bond Defects on the Behaviour of RC Beams Strengthened with CFRP, PhD dissertation, 2022, [1]; Zhou et al. in Compos. B 113:80–90 [2]) show that FRP strengthening systems are capable of remain a specific value of member capacity even when some defects exist in the bond. The main aim of this study is to highlight the issue of extending the lifetime of the passive CFRP strengthening system of RC structures by adopting the damage tolerance approach and chosen concept of quality assessment workflow by pushing existing strengthening to its limits, simultaneously maintaining structure in acceptable conditions in terms of safety or serviceability. Facing the fact of the common and massive use of FRP strengthening over the last three decades around the world, such a new approach in the area of quantitative diagnostic may be considered one of the basic tools for the extension of the life of structures and reducing the environmental impact of the construction sector.