Application of digital image correlation in investigating the bond between FRP and masonry Bahman Ghiassi a,⇑ , Jose Xavier b , Daniel V. Oliveira a,1 , Paulo B. Lourenço a,2 a ISISE, University of Minho, Department of Civil Engineering, Azurém, 4800-058 Guimarães, Portugal b CITAB, University of Trás-os-Montes e Alto Douro, Vila Real, Portugal article info Article history: Available online 25 June 2013 Keywords: Digital image correlation Bond behavior FRP composite Masonry abstract Full characterization of the bond behavior between Fiber Reinforced Polymers (FRPs) and masonry in externally bonded reinforcement (EBR) technique is crucial at the design stage or structural performance prediction. In this regard, a full-field assessment technique seems to be valuable for an adequate charac- terization of the bond behavior. The digital image correlation (DIC) and feature tracking techniques have been used in this study for investigating the evolution of strains and deformation during uniaxial tensile tests and shear debonding tests in FRP-masonry systems. The results show that the DIC is a valuable technique for characterization of the bond behavior and investigating its three-dimensional aspects. The DIC was also found applicable for following the matrix crack development in Steel Reinforced Grout (SRG) specimens. Feature tracking method was used for monitoring the strains development on the steel fibers in SRG specimens during tensile tests. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Composite materials such as Fiber Reinforced Polymers (FRPs) or Steel Reinforced Grouts have been extensively used for external strengthening of masonry structures. The effectiveness of this strengthening technique is intrinsically dependent on the bond performance between the composite material and the masonry substrate. As the bond is a key mechanism in transferring the stres- ses from the structural element to the composite material, any bond loss leads to deterioration of the strengthening system and possible premature failure. Therefore, complete understanding of the governing bond mechanisms is necessary for design and qual- ity control purposes. Significant progress has been achieved in the last years regard- ing experimental and computational investigation of the debond- ing mechanism and damage in FRP-strengthened masonry elements [1–6]. However, aspects such as failure initiation, effective bond length, distribution of interfacial strains, the role of mortar joints and three-dimensional nature of the bond behav- ior require further investigation. A comprehensive bond-slip model, for numerical modeling approaches, is also missing for FRP-masonry systems. The bond-slip law can be experimentally obtained during the debonding tests from the distribution of strains, obtained from the strain gauges attached to the FRP’s sur- face, along the bonded length [2]. However, the measurements are limited to the location of the strain gauges [2] and the bond behav- ior cannot be investigated precisely. Use of a full-field measurement technique seems to be valuable in better understanding the above mentioned aspects of the bond behavior. This paper addresses the applicability of the optical mea- surement techniques for characterization of the tensile and bond behavior in FRP-masonry systems. For this reason, uniaxial tensile tests and shear debonding tests have been performed on previ- ously prepared specimens and the evolution of strains on the spec- imens’ surfaces has been measured with digital image correlation (DIC) and feature tracking methods. These methods have been widely used for measurement of displacements or strains in differ- ent fields of solid mechanics [7–9]. However, only a few studies can be found using these techniques for investigating the interfacial bond behavior, e.g. [10,11]. To the knowledge of authors, existing studies have been devoted to FRP-strengthened concrete elements and information on full-field assessment of bond in FRP-strength- ened masonry has not been attempted. Uniaxial tensile tests were performed here on Glass Fiber Rein- forced Polymer (GFRP) and Steel Reinforced Grout (SRG) speci- mens. GFRP composites, compared with other conventional FRP materials, have lower axial stiffness which makes them more suit- able for masonry structures. SRG has been chosen as a composite 0263-8223/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.compstruct.2013.06.024 ⇑ Corresponding author. Tel.: +351 253 510 499; fax: +351 253 510 217. E-mail addresses: bahmanghiassi@civil.uminho.pt (B. Ghiassi), jmcx@utad.pt (J. Xavier), danvco@civil.uminho.pt (D.V. Oliveira), pbl@civil.uminho.pt (P.B. Loure- nço). 1 Tel.: +351 253 510 247; fax: +351 253 510 217. 2 Tel.: +351 253 510 209; fax: +351 253 510 217. Composite Structures 106 (2013) 340–349 Contents lists available at SciVerse ScienceDirect Composite Structures journal homepage: www.elsevier.com/locate/compstruct