IRT survey for the quality control of FRP reinforced r.c. structures

Abstract

Externally bonded Fibre Reinforced Polymers (FRPs) are widely used for strengthening of existing structures in recent years. However, the success of FRP reinforcement techniques is mainly subjected to the properly application of the FRPs, so a key role is played by the quality control of the application, to be performed by non – destructive techniques. At this aim, CETMA developed an InfraRed (IR) Thermography procedure and a post processing methodology able to detect and measure the area of bonding defects at the FRP – substrate interface. In this paper, the in – situ validation of the developed procedure on two r.c. structures retrofitted with FRP materials is presented. Since its first applications in Europe and Japan in the 1980s, t he use of bonded repair and retrofit of concrete structures with fibre reinforced polymer (FRP) systems has progressively increased. Thanks to their light weight, ease of installation, durability, minimal labour costs and site constraints, high strength/weight and stiffness/weight ratios, FRP repair systems can provide an economically viable alternative to traditional repair systems and materials, such as steel plate bonding, steel or concrete jackets, external post – tensioning and section enlargement. FRP systems can be used either to rehabilitate and restor e the strength of a weakened, damaged, or deteriorated structural member or to retrofit and strengthen a sound structural member to resist higher loads in case of a design or construction error, in case of a change in use or loading, or for a seismic upgrade. Additionally, these materials are readily available in several forms, ranging from factory made laminates to dry fibre sheets, that can be wrapped to conform to the geometry of a structure. The relatively thin profile is often desirable in applications where aesthetics or access is a concern. However, it is widely accepted that quality o f construction is one of the most important factors that affect long – term performance of FRP repair systems. In order to assure an effective FRP reinforcement, perfect adhesion between FRP and substrate (concrete or masonry) must be obtained; for this reason, it is essential to assess the quality of bonding with a Quality Control (QC) process, starting before the installation of the system (specific procedures for track and inspection all FRP components prior to installation, inspection of all prepared surfaces prior to FRP application) and continuing through and after the application (inspection of the work in progress to assure conformity with specifications, obtaining quality assurance samples, inspection of all completed work, performing tests for approval, repair of any defective work,…). For these purposes, visual inspection, acoustic tap testing , laboratory testing of witness panels or resin samples, direct pull – off testing are the most applicable methods of QC. Tap test seems to be the easier method to be used for the identification of defects at the FRP – substrate interface or inside the FRP; this because the infrastructures inspectors are quite familiar with tap tests and simply need to be trained to hear the difference between bonded and unbonded laminates, which is somewhat similar to the difference between sounding and damaged concrete. However scanning very large areas of reinforced structures with tap test could be very expensive in terms of time and safety. Infrared (IR) thermography can be a fast, reliable and confident n on destructive testing technique for the evaluation of bonding defects in FRP – reinforced concrete structures. However, the set – up of IR technique for the detection of a pa rticular defect (delamination, lack of bonding, …) in an FRP – reinforced r.c. structure needs a specific calibration, leading to the proper definition of the operational parameters (active/passive approach, distances, time – windows, …). Based on the results obtained in a large experimental program, an IR Thermography procedure, easily applicab le on – site, was developed. After the definition of acceptance criteria and reparation techniques for the defects, also a post processing software tool able to clearly define the defect contour was developed. This allows measuring the area of the bonding defect and then to compare it with the acceptance criteria, in order to establish whether the defect is acceptable or not, and, in case, what is the best way to repair it.