# Assessment of Existing Bond Models for Externally Bonded SRP Composites
## 2 Bond Performance of External Strengthening  
Debonding can be defined as a brittle loss of adhesion between composite overlay and substrate. This is the most common failure mode of externally bonded fiber-reinforced strengthening systems. Existing design guidelines distinguish between four models of debonding. Originally, they were defined by [PERSON] et al. [10]. Those are plate end interfacial debonding, concrete cover separation, intermediate flexural crack-induced debonding, and critical diagonal shear crack-induced debonding. Schematically they are presented in Figure 1. In the case of SRP composites, what can also occur is interlaminar failure at the matrix-fiber interface [11]. Bond performance may be determined in double-shear, single-shear, and notched beam test. In the first two cases, depending on how the force is applied, there can be an indicated push-shear test and pull-shear test. In the push-shear test, clamping the sample is easier; therefore, it is more popular. However, the pull-shear test better reflects the real conditions under which debonding occurs along the tensed surface. Bond performance is influenced by many factors related to geometry, mechanical properties of materials, environmental conditions, and application conditions. Design methods take into account only some of them. Undoubtedly, the most important factor is the bond area expressed by bond width and bond length. Some bond-strength models introduce the upper limit of the real bond length (called effective bond length) beyond which no further increase in bond force occurs. The second important geometric parameter is the thickness of the composite. The real thickness includes the fibers and the  
Figure 1: Debonding failure modes: (**a**) plate end interfacial debonding; (**b**) concrete cover separation; (**c**) intermediate flexural crack-induced debonding; (**d**) critical diagonal shear crack-induced debonding. epoxy matrix. Its direct adoption into the calculations could limit their comparability for SRP tapes of different densities; therefore, the thickness should be taken as the dimension of the cross-sectional area of the fibers distributed over the width of the composite. The debonding of an SRP composite usually occurs at a thin layer beneath the concrete surface, sometimes with local removal of cover pieces. Concrete is usually the weakest material in a joint; therefore, its strength is another critical factor affecting the bond strength. Typically, compressive strength needs to be determined, less often tensile strength or surface tensile strength. Composite mechanical properties also affect the bond strength. In general, the higher stiffness, the better bond. The bond strength models are most commonly classified into three categories [12; 13]:  
* Empirical models, based on the regression of test data, such as [PERSON] [14], [PERSON] and [PERSON] [15], and [PERSON] [16]. * Fracture mechanics based models, including [PERSON] [17], [PERSON] [18], [PERSON] and [PERSON] [19], [PERSON] and [PERSON] [20], and [PERSON] et al. [21]. * Design models, usually based on simple assumption, such as [PERSON] et al. [22] [PERSON] and [PERSON] [23], [PERSON] et al. [24], [PERSON] et al. [25], [PERSON] and [PERSON] [26], [PERSON] et al. [27], [PERSON] and [PERSON] [28], [PERSON] et al. [29], [PERSON] et al. [30], and [PERSON] and [PERSON] [31]. The length of the anchorage seems to be of key importance for the bearing capacity of the bonded joint. As was earlier mentioned, experimental studies and some fracture mechanics analyses showed that above a certain length (called effective or active bond length), the further increase in load capacity was negligible. To indicate this phenomenon, [PERSON] et al. [32] proposed an alternative classification for models independent of the bond length, models not considering the effective bond length, and models considering the effective bond length.