The rehabilitation, upgrading and retrofitting of existing reinforced concrete (RC) structures is one of the most difficult challenges facing structural engineering today. One retrofitting method that has gained widespread acceptance over the last decade involves externally bonding fiber reinforced plastic (FRP) patches on to the tension face of a concrete beam or slab, thus increasing the beam's (or slab's) flexural stiffness and loading capacity. The increased usage of these FRP patches in these applications has created the requirement for reliable nondestructive evaluation (NDE) techniques capable of characterizing these bonded patches. Any candidate NDE technique must be capable of providing quantitative and meaningful information (from an engineering design perspective) about the characteristics of an in situ FRP patch. The most critical element from a quality assurance (and thus NDE inspection) point of view is the adhesive bond layer - as opposed to the FRP patch itself. Concrete repair procedures typically use a high quality FRP patch that is manufactured in a controlled environment. This is in contrast to the adhesive bond layer, which is field assembled, often under adverse conditions. There are two important quality assurance/NDE inspection problems for these adhesively bonded FRP patches that can be addressed with guided wave techniques. The first is concerned with the measurement of the material properties of the adhesive layer. Guided waves can be used to determine in situ adhesive bond properties, such as the shear modulus of an epoxy bond. The second inspection problem is the measurement of the adhesion properties, the quality of the bonding at the two interfaces between the adhesive and the adherends. This is a critical issue for bonded FRP patches, especially the adhesion properties of the concrete-to-adhesive interface. The proposed research develops a NDE methodology that uses guided ultrasonic waves to characterize the in situ bond properties of a FRP patch in three tasks: Task 1. Understand the underlying mechanics of the propagation of guided ultrasonic waves in these bonded components. This task examines the behavior of ultrasonic waves in a FRP patch bonded to a concrete component, quantifying the effect of certain bond parameters on the propagation of guided waves in the bonded assembly. This forward problem involves both experimental studies and numerical simulation of FRP patches, concentrating on directly measurable acoustic parameters. Task 2. Interpret the experimental/numerical results from Task 1 in terms of relevant, engineering parameters in order to develop a quantitative relationship between these acoustic measurements and critical FRP patch performance metrics. This task relates these directly measurable, guided wave attributes (such as dispersion curves that provide wave speed/frequency relationships) to material properties that are essential to engineering design. The specific parameters quantified in this research are: the bulk properties of the in situ adhesive layer, including thickness and stiffness; and the adhesion properties of this adhesive layer (particularly the concrete-to-adhesive interface), including detection of any voids, gaps or regions of disbonds. Task 3. Conduct a preliminary study that establishes the effectiveness of an inversion technique for the evaluation of these guided waves. This inverse problem uses neural networks to determine the bulk elastic properties, thickness and adhesion characteristics of an in situ adhesive bond from experimentally measured guided waves. The results of this proof-of-principle study are critical for the development of an efficient, real-time field inspection methodology. This research project operates under the premise that education and research are equally important in developing innovative NDE methodologies for civil infrastructure. In addition to performing basic and applied research, academic institutions have a primary responsibility to train students so that they can meet the requirements of the workplace. With this in mind, an education program is proposed that will educate and train a new generation of engineers to address issues relating to NDE of repaired and rehabilitated RC components.
