## 4 Experimental Results  
Tests were performed to identify the influence of the unbonding methods (sleeve, paper tape, and Teflon tape) on the measured bond properties. The results show that their influence on bond strength can be considered negligible, being at most around 0.3% of the bond strength obtained from the Cored type. The failure mechanisms resulting from the pull-out tests performed on the Reference, the Teflon, and the Cored specimens are shown in Figure 7a, Figure 7b, and Figure 7c, respectively, for both tested bonded lengths. \begin{table}
\begin{tabular}{c c c c c} \hline Water content & 173.2 kg/m\({}^{3}\) & Aggregate type & Quartz \\ Cement content & 275.0 kg/m\({}^{3}\) & Aggregate shape & Round \\ Aggregates content & 1976.1 kg/m\({}^{3}\) & Max. aggregate size & 16 mm \\ \hline \end{tabular}
\end{table} 
Table 1: Mixed design of the analysed concrete. \begin{table}
\begin{tabular}{c c c c c c} \hline
**Prop.** & **Unit** & **28 Days** & **70 Days** & **400 Days** & **ECa** & **ECb** \\ \hline \(f_{cu}\) & MPa & \(34.3\pm 6.2\%\) & \(38.7\pm 7.6\%\) & - & 39.7 & 43.4 \\ \(f_{cy}\) & MPa & \(32.3\pm 7.7\%\) & \(36.5\pm 2.9\%\) & - & 37.4 & 40.9 \\ \(E\) & GPa & \(30.7\pm 8.1\%\) & \(31.2\pm 7.5\%\) & - & 32.1 & 33.0 \\ \(f_{t,B}\) & MPa & \(3.3\pm 9.8\%\) & \(3.4\pm 8.6\%\) & - & 3.64 & 3.86 \\ \(G_{f}\) & N/m & \(105.8\pm 14.7\%\) & \(117.8\pm 7.3\%\) & - & - & - \\ \(f_{cy}^{*}\) & MPa & \(26.1\pm 8.0\%\) & - & \(34.0\pm 6.5\%\) & 30.2 & 33.0 \\ \hline \end{tabular}
\end{table} 
Table 2: Summary of concrete properties for different specimens and ages. Predictions made according to Eurocode ageing functions using cement-related \(s\) parameter (ECa) and using \(s\) parameter calibrated (\(s=0.32\)) on cubes’ results (ECb). Figure 7a shows the failure mechanisms of the Reference anchors having 30 mm and the 40 mm of unbonded length. For the Reference configuration, since the embedment depth is small, the concrete cone developed during the pull-out tests cannot be neglected since it covers a relevant fraction of the bonded length. This is reflected in a strong reduction of the calculated bond strength. Thus, the method suggested in the guideline is conservative and may hide a reserve of capacity which is bigger when smaller embedment depths are considered. Figure 7b shows the Teflon configuration failure. Since the unbonding layer behaves as a crack located on the threads of the threaded bar, the failure starts and propagates along the inner interface until the anchor's end. For the Cored configuration specimens, the failure does not start at the inner interface but rather at the outer one. The failure immediately crosses the adhesive mortar layer since the aged concrete is too strong, and the outer physical interface is stronger than the inner one. This is confirmed by the Reference configuration, where the failure starts at the concrete surface as the concrete cone and progresses through the mortar layer to the inner interface without the outer interface's involvement. In this case, little difference can be expected between the Cored and Teflon cases since, for both of them, the failure mostly involves just the inner interface. Figure 8 shows the experimental results for the three configurations. For each one of the three configurations, the results of the tests performed on both bonded lengths are plotted in the same diagram. The results are also presented in Table 3 in terms of the bond strength obtained by assuming the uniform bond model. As can be seen in Figure 8a,b, both the Cored and Teflon configurations lead to the same peak load and, consequently, to the same bond strength. The Cored configuration, however, shows very consistent results, having a smaller scatter both in terms of peak load and stiffness. Additionally, the post-peak behaviour of the tests performed on the Cored configuration is very consistent compared with the remaining test results. This can be explained by the fact that the cored section excludes contact and, therefore, excludes possible friction within the system.