## 2 Materials and Methods
### Experimental Methods and Testing Set-up
#### 2.2.1 Countered Double Cantilever Beam (CDCB) test  
In general, tensile bond tests can be subdivided into direct and indirect methods. The main flaw of direct tensile tests is their high sensitivity to the specimen anchoring system and to slight eccentricities, which might lead to bending effects at the interface or premature material failure rather than bond failure [20]. Indirect tensile tests, such as flexural and splitting methods, on the other hand, can help overcome some of these issues, although the indirect nature of the stresses applied may introduce some computational challenges. There is a long tradition of splitting fracture tests to determine the fundamental mechanical behavior of materials and of bi-material systems [13; 19]. The technique using a Double Cantilever Beam specimen was first developed for structural adhesives and eventually adapted to studying fracture properties of monolithic materials [21] as well as interfaces in cement-based bi-material systems [15]. The DCB test consists of applying a splitting load to propagate a crack in a pre-notched specimen while recording the applied load and the opening displacement of the crack faces, with the major benefit that several fracture parameters can be investigated. In addition, more reliable compliance measurements are possible (as described in the following sections) as the test geometry promotes the application of smaller splitting loads and larger displacements compared to other test setups [21]. Further studies suggested tapering the samples in order to make the rate of the strain release theoretically independent of the crack length, hence leading to a more stable crack propagation. This specific condition can be achieved by shaping the sample so that the rate of change of the compliance is theoretically constant during the crack growth (as detailed in the following sections). The resulting geometry was re-named the Contoured DCB (CDCB) test (Figure 1), also known as the linear compliance test [21]. Geometrical proportions and size of the specimen tested are shown in Figure 1a,b and follow the set-up introduced by [15]. A side groove along the desired crack plane promotes a proper crack growth and hinders any unfavorable crack deviation in monolithic specimens (Figure 1a). In this study, however, the test was also adapted to the fracture analysis of interfaces, with the specimen subdivided into two symmetrical parts, that is, the concrete substrate on one side, the repair material on the other side, and the bond plane in between, running exactly along the CDCB middle section (Figure 1b). For those bi-material tests, the interface represents a plane of weakness by itself, and to avoid excessive damage during handling, the side groove was removed from the middle line (Figure 1b). This aspect was properly computed in the back-calculations discussed below. The test was performed using a servo-hydraulic Instron universal testing machine (Figure 1e). Specimens were supported by a hinge fixed to the lower plate of the testing machine, while the upper part were supported by a steel profile with two identical wedges, which were used to apply the splitting load. Each wedge was positioned between two low-friction bearings mounted on both sides of the CDCB sample by means of steel rods passing through the section (and placed above the crack so that this would not be affected, Figure 1d). The steel profile was connected to a load cell, which was connected to the cross head of the machine. Although the specimen was subjected to a vertical load in addition to the horizontal splitting load (SL), this vertical component can be ignored by keeping the angle of the wedge, \(\alpha=15^{\circ}\) (Figure 1d), small and by employing low friction needle bearings so that the coefficient of friction, \(\mu\), between the wedge and bearings can be reasonably neglected [15]. The displacement rate was kept on average at 0.02 mm/min, to promote more stable crack propagation. During the test, load and Crack Mouth Opening Displacement (CMOD) were recorded continuously.