Experimental Mechanics (2011) 51:303–313 DOI 10.1007/s11340-010-9358-y Fracture Properties of Concrete–Concrete Interfaces Using Digital Image Correlation S.G. Shah · J.M. Chandra Kishen Received: 30 July 2009 / Accepted: 4 April 2010 / Published online: 8 May 2010 © Society for Experimental Mechanics 2010 Abstract The mode I and mode II fracture toughness and the critical strain energy release rate for different concrete–concrete jointed interfaces are experimen- tally determined using the Digital Image Correlation technique. Concrete beams having different compres- sive strength materials on either side of a centrally placed vertical interface are prepared and tested under three-point bending in a closed loop servo-controlled testing machine under crack mouth opening displace- ment control. Digital images are captured before load- ing (undeformed state) and at different instances of loading. These images are analyzed using correlation techniques to compute the surface displacements, strain components, crack opening and sliding displacements, load-point displacement, crack length and crack tip lo- cation. It is seen that the CMOD and vertical load-point displacement computed using DIC analysis matches well with those measured experimentally. Keywords Concrete interface · Digital image correlation · Bimaterial fracture toughness · Energy release rate Introduction An understanding of fracture behavior of a bimaterial interface has been a major area in current research. Understanding the behavior of an interface formed be- tween old and new concrete is very important in order S.G. Shah · J.M. Chandra Kishen (B ) Department of Civil Engineering, Indian Institute of Science, Bangalore 560 012, India e-mail: chandrak@civil.iisc.ernet.in to predict the performance of a repaired structure. In composite structures and materials, the weakest part is often the interface between different materials [1]. An interface appears when a repair material is applied to an infrastructure system after rehabilitation. Usually the interface is relatively weaker than the material on either side of it, in a repaired system. The performance of the repaired system under loading is strongly de- pendent on the performance of the interface. Com- patibility between repair material and substrate con- crete is recognized to be important for prevention of cracking but reliable quantification of the required parameters is lacking [2]. Further, in order to protect concrete constructions, quite often layers of concrete or cement bonded materials are used, thereby forming an interface [3]. The bonding at interfaces in concrete structures is important for safety and durability [4]. The chances of failure by cracking along the interface are higher because of stress concentration and rapid change of stress levels along the interface. Concrete is a heterogeneous material, wherein its fracture behavior is complicated and the quantification of fracture parameters becomes difficult. To obtain microscopic information on the failure processes in concrete, a robust full-field measurement method is required [5]. Direct observation of the fracture process is difficult because of the small scale at which the mi- crostructural features interact with the failure process. When cracks first initiate, their openings may be less than a micron. In addition, the microcracks develop in a widely distributed manner which requires a large field of examination. This necessitates the use of tech- niques with a much greater resolution than that which is possible with our human eye. Moire interferom- etry [6], various forms of electronic speckle pattern