BEHAVIOUR OF FRP-TO-CONCRETE INTERFACE: THEORETICAL MODELS AND EXPERIMENTAL RESULTS

Abstract

Fiber-reinforced polymers (FRP) are more and more commonly employed for structural strengthening existing structures of both reinforced concrete (RC) and masonry. Since FRP are externally bonded on a concrete or masonry substrate, the issue of adhesion on those materials generally controls the effectiveness of strengthening in members stressed either in bending or shear. Understanding the behaviour of FRP-to-concrete joints tested under pull-out actions is of paramount importance for describing the key mechanical properties of the adhesive interface between FRP and concrete, which plays a key role in the possible debonding failure of externally strengthened beams. With this aim the present dissertation deals with the theoretical models and the experimental results related to the behaviour of the FRP-to-concrete adhesive interface. It consists of eight chapters: The first chapter introduces the topic of the use of fiber-reinforced polymer (FRP) for the design and the construction of Externally Bonded FRP Systems for Strengthening Existing Structures. Particular attention is paid to the issue of the adhesion of external FRP reinforcement applied to concrete elements. A brief state of the art referring to national and international literature is also reported. The aims and the outline of this doctoral thesis are clarified. In the second chapter an analytical model will be firstly presented for determining both shear and normal stresses throughout the adhesive interface in the linear range. The main parameters that govern the phenomenon of adhesion will be analyzed. The nonlinear behaviour of the FRP-to-concrete interface will be also addressed by discussing the ultimate bearing capacity of FRP laminates bonded on concrete members. The third chapter presents the experimental program that was conducted to compare the interface behavior, under monotonic and cyclic actions, of two main types of commercial external FRP reinforcement, namely sheets and plates. In particular, the set-up designed to perform the 58 bond tests is accurately described and the main results obtained, with particular reference to the strains recorded on each reinforcement during the test, are reported and discussed. The fourth chapter presents two alternative methods of identification of interface laws defined respectively direct method (DirIM) and indirect method (IndIM). The comparison between the bilinear bond laws obtained by both methods showed the greater effectiveness of IndIM method than the DirIM one. Moreover, an extensive interpretation of the experimental readings made by the method IndIM allowed to obtain useful results to define a proposal for updating the instructions CNR-DT200/2004. The fifth chapter describes the intermediate debonding phenomenon, due to the detachment of the reinforcement between the cracks in the concrete member. The main contributions from the national and international literature, related to both the theoretical interpretative models and the results of tests performed on full-scale member, are collected. A comparison between theoretical and experimental results highlights that it is necessary to continue with further investigations on the geometric and mechanical parameters governing the phenomenon, and, at the same time, to validate the design formulas currently in use on the basis of the latest experimental results. In the sixth chapter a statistical procedure for the Calibration of resistance models from experimental data, in accordance with the guidelines provided in Eurocode 0 is presented. The procedure has been implemented on the basis of the results of the bond tests carried out and analyzed in the third and fourth chapters, and based on the results of tests on full-scale members found in literature and discussed in the fifth chapter. The application of the procedure allowed a proposal for updating the design formulae provided in the Instructions CNR-DT200 (2004) to be formulated. In the seventh chapter, the results obtained by applying the IndIM procedure to obtain simplified bond laws and the statistical procedure calibrating the bond strength relationships were summarized and integrated, in order to prepare an updating proposal for instructions CNR-DT200 (2004). In the eighth chapter the conclusions of the dissertation were summarized as well as the expected future developments downstream of the activity.