Abstract: A finite element model is developed for analyzing the interfacial behaviour for fibre-reinforced polymer (FRP) laminates externally bonded to concrete prisms and subjected to direct shear. The element sizes of the FRP, adhesive, and concrete at the interface were chosen to be very small (0.25-0.5 mm) so that the debonding behaviour could be properly captured. The behaviour at the interface between the FRP composite and the concrete is modelled using truss elements connecting the FRP laminate to the concrete block. The truss elements incorporate a nonlinear bond stress-slip relationship controlled by several parameters related to the characteristics of the FRP composite, adhesive, and concrete. Results are given in terms of the load capacity of the joint and the stress and strain distributions in the FRP, at the interface, and in the concrete. In addition, the transfer lengths, as well as the force transfer between the FRP laminate and the concrete block, are investigated. Comparisons between the finite element results and available experimental data are presented.
Key words: nonlinear finite element analysis, FRP-to-concrete bonded joints, interface elements, debonding, interfacial behaviour, transfer lengths.
Resume : Un modele par elements finis est developpe afin d'analyser le comportement de l'interface des lamines de polymeres renforces de fibres (PRF) colles sur l'exterieur de prismes de beton et soumis a un cisaillement direct. Les dimensions des elements de PRF, de l'adhesif et du beton a l'interface ont ete choisis tres petites (0,25 a 0,5 mm) afin bien saisir le comportement lors de la rupture de l'adherence. Le comportement de l'interface entre le composite PRF et le beton est modelise en utilisant les membrures des poutres en treillis reliant le lamine PRF au bloc de beton. Les membrures des poutres en treillis incorporent une relation non lineaire contrainte-glissement du lien controlee par plusieurs parametres associes aux caracteristiques du composite PRF, de l'adhesif et du beton. Les resultats sont donnes en termes de capacite portante du joint ainsi que les distributions de forces et de contraintes dans le PRF, l'interface et le beton. De plus, les longueurs de transfert et la force de transfert entre le lamine PRF et le bloc de beton sont etudiees. Les comparaisons entre les resultats d'elements finis et les donnees experimentales disponibles sont presentees.
Mots-cles : analyse non lineaire par elements finis, joints colles PRF-beton, elements d'interface, rupture de l'adherence, comportement de l'interface, longueurs de transfert.
Introduction
The strengthening of concrete structures by means of externally bonded, fibre-reinforced polymers (FRPs) is now routinely considered an effective method for enhancing the load capacity of existing structures. However, with this technique there is often a concern that possible failures may occur as a result of debonding of the FRP from the concrete. Laboratory investigations of FRP-strengthened concrete beams in flexure and shear have shown that despite the capability of achieving considerable increases in strength capacities, premature failures by debonding sometimes limit the effectiveness of the strengthening scheme. At present, our basic understanding of the mechanics of the bond and failure between the FRP and concrete for such applications is rather limited.
From various flexural tests and modified-beam tests on steel-concrete joints, as well as on FRP-concrete joints, it has been concluded that when laminate separation did occur it was due to high local interfacial bond stresses and peeling forces at the ends of the laminate (Van Gemert 1980; Quantrill et al. 1996a). These stresses were found to depend on the tensile strength of the concrete (Triantafillou and Plevris 1992; Quantrill et al. 1996b), the flexural rigidity of the cracked laminated section (Triantafillou and Plevris 1992), the concrete surface preparation (Van Gemert 1980; Ziraba et al. 1995), the strength and thickness of the adhesive (Quantrill et al. 1996b), and to a lesser extent, on the laminate aspect ratio b/t, where b is the width of the bonded laminate and t its thickness (Quantrill et al. 1996b). Although the debonding phenomenon has been investigated both experimentally and analytically by a number of authors, comprehensive finite element studies on FRP-concrete interfacial behaviour using suitable interface elements are scarce. An appropriate constitutive model for the interface elements is required, and it should reflect the characteristics of the FRP, concrete, and adhesive. It should conveniently be expressed in terms of a fairly simple bond-slip relationship.
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