Contents lists available at ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct A study of concrete cover separation failure in FRP-plated RC beams via an inter-element fracture approach Umberto De Maio a , Francesco Fabbrocino b , Fabrizio Greco a, ⁎ , Lorenzo Leonetti c , Paolo Lonetti a a Department of Civil Engineering, University of Calabria, Rende, Italy b Department of Civil Engineering, Pegaso University, Naples, Italy c Department of Engineering and Architecture, University of Parma, Parma, Italy ARTICLEINFO Keywords: Concrete cover separation Multiple crack propagation Strengthened RC structures FRP composite materials Cohesive interface model Inter-element fracture ABSTRACT The phenomenon of concrete cover separation failure in FRP strengthened RC elements, unlike plate-end and mid-span debonding failure modes, has been not analyzed in depth in the literature and simplified strength models are mainly available, which possess a limited predictive capability. To this end in the present work, a novelnumericalapproachtoinvestigatecoverseparationinFRP-platedRCbeamsisproposed,basedonaninter- element cohesive fracture approach able to simulate multiple crack onset, propagation and coalescence in concrete structures, used in combination with an embedded truss model for taking into account the interaction between concrete cracks and tensile steel rebars. This approach has been validated by performing complete failure simulations for benchmark crack propagation examples, and subsequently has been exploited for pre- dicting the load-carrying capacity and the related failure mode of real-scale retrofitted RC elements. Suitable comparisons with available experimental results have clearly shown the reliability and the effectiveness (in terms of numerical accuracy) of the proposed fracture approach. 1. Introduction External bonding of composite materials, such as fiber-reinforced polymer (FRP) systems, to the tension face of reinforced concrete (RC) members has been widely used in the flexural strengthening of existing concretestructures.Thepopularityofthistechniqueamongtheexisting retrofitting methods is attributable to the superior mechanical proper- ties of FRP systems over traditional ones (e.g. based on steel plates), such as minimum increase in structural size, excellent resistance to corrosionandfire,veryhighstrength-to-weightratios,aswellaseaseof handling and transportation. As a matter of fact, several experimental and theoretical studies have clearly shown that externally bonded FRP composites can be successfully used to improve the structural perfor- mances of concrete members, such as load-carrying capacity and stiff- ness, ductility, ultimate strength under cyclic and fatigue loading, and environmental durability, thus showing a notable extension of their service life and, ultimately, leading to beneficial effects in terms of overall sustainability [1–8]. However, from a structural point of view, an important issue con- cerningthereliabilityandthesafetyofthisstrengtheningmethodisthe occurrence of potential brittle failures of FRP systems, which may sig- nificantly reduce their effectiveness, unless explicitly considered in the design and assessment procedures. Indeed, the structural system re- sulting from the application of externally bonded FRP composites leads to modify some well-known failure modes of conventional RC struc- turesandtodisplayanumberofuniquefailuremodes,asaresultofthe great amount of research undertaken in the past decades (see, for in- stance [9], and references herein). These premature failure modes can be reduced to the three modes sketched in Fig. 1, i.e. the plate-end and intermediate crack-induced interfacial debonding failures and the concrete cover separation failure. Such catastrophic failures are strongly associated with the effec- tiveness of the stress transfer at the FRP-to-concrete interface. As a matter of fact, the bond of this interface is not perfect and its strength depends on both concrete strength and adhesive thickness. It has been shown that, in high-strength concrete members, failure typically occurs at the adhesive/concrete (AC) interface. In this case, the adhesive plays a notable role, meaning that greater thickness values lead to better stress redistributions and ultimately to greater ultimate load levels [10]. On the other hand, in low-strength concrete elements, failure occurs in the concrete phase and does not depend on the adhesive thickness. In this case, concrete cover separation is often observed, as reported in many experimental investigations (see [3,11–14]). Such a failure, which is promoted by the nucleation of an inclined crack at the https://doi.org/10.1016/j.compstruct.2019.01.025 Received 5 December 2018; Accepted 2 January 2019 ⁎ Corresponding author. E-mail address: fabrizio.greco@unical.it (F. Greco). Composite Structures 212 (2019) 625–636 Available online 04 January 2019 0263-8223/ © 2019 Elsevier Ltd. All rights reserved. T