## 1 Introduction  
Over the last few decades, the industry and researchers have been quite interested in investigating post-installed adhesive anchors for their business and for their safety relevance. Such anchors are used, among others, for rehabilitation and retrofitting purposes, and allow for a more flexible and sustainable design and construction. Steel and concrete are the materials typically involved in mechanical anchor systems [1]. In addition, for adhesive anchors, adhesive mortar is also included in the system (see Figure 1). The adhesive mortar layer transfers the load from the steel element to the concrete along the entire bonded length [2] via adhesion and interlocking [1]. In order to determine the tensile capacity of adhesive anchors, tests were carried out following the recommendation provided by the European Assessment Document (EAD [3]). The code distinguishes two methods: (i) unconfined tests, where the supports allow for an unrestricted formation of the concrete cone failure, and (ii) confined tests, which aim at eliminating the concrete cone failure by moving the test supports close to the anchor and assess the adhesion between the concrete and steel via the adhesive mortar layer. Figure 0(a) and Figure 0(b) show a visual representation of the unconfined and the confined test setup, respectively. A steel plate with a center hole of diameter \(1.5d_{0}<D<2d_{0}\) is used as a support in the confined configuration, where \(d_{0}\) is the concrete borehole diameter. The supporting area should be designed such that the compressive stress under the support does not exceed 70% of the concrete's compressive strength. In order to avoid boundary conditions influencing the failure process, the support distance for the unconfined configuration has to be larger than four times the anchor's embedment depth \(h_{ef}\). In order to avoid concrete cone failure, for high-bond-resistance adhesive anchors, confined configuration is recommended rather than unconfined. Steel failure needs to be avoided by reducing the embedment depth if necessary. If the resistance is obtained with a confined test, the result has to be converted to unconfined with the \(\alpha_{set-up}\) factor [3]. Confined testing is also preferred to determine the resistance of the bonding layer for numerical simulations [4]. In order to design single-adhesive anchors under a tensile load in uncracked concrete, different models have been proposed in the literature [5; 6]. The widely used uniform bond model defines the bond resistance as bond strength and assumes a constant stress along the bonded length [5; 6]. Failure occurs when the bond stress reaches the bond strength. Equation (1) provides the adhesive anchor tensile capacity \(N_{b}\), based on its embedment depth \(h_{ef}\), on the borehole diameter \(d_{0}\) and on the bond strength \(\tau_{0}\). The anchor diameter \(d\), together with the related bond strength \(\tau\), can be used alternatively in the equation. According to [PERSON] et al. [6], the uniform bond model is the one that provides the best fit to the large database considered. \[N_{b}=\tau_{o}\ d_{0}\ \pi\ h_{ef} \tag{1}\]  
The model was shown to be a valid behavioural model, both experimentally and numerically, by [PERSON] et al. [7], if \(d_{0}<1.5d\) and if \(4<h_{ef}/d<20\). [PERSON] et al. [6] showed that there is a slight improvement in the assessment when using the anchor diameter as opposed to the borehole diameter, but the results are not conclusive. Within this paper, the anchor diameter \(d\) is used for bond strength determination. The adhesive mortar, during installation, positions itself as a thin layer in the annular gap between concrete and steel. This leads to the development of two interfaces, the \"inner\" and the \"outer\" one (see Figure 2a). The so-called outer interface is the physical interface between the concrete and adhesive mortar.