# Hybrid Metaheuristic-Neural Assessment of the Adhesion in Existing Cement Composites
## 2 Experimental Setup  
As mentioned previously in [29], the NDT tests were carried out on a surface of a two-layer cement composite with dimensions of 2500 \(\times\) 2500 mm\({}^{2}\) and with a constant thickness of the overlay equal to 25 mm (Figure 1a,b). The overlay was made of C20/25 class cement mortar with the maximum quartz aggregate grain size equal to 2 mm. The overlay was laid on the existing substrate with a constant thickness of 125 mm made of C30/37 class concrete. In this concrete, the maximum broken basalt aggregate grain size was equal to 8 mm. The surface of the existing concrete substrate was sandblasted. In order to achieve a wider range of adhesion between the overlay and the substrate, half of the surface of the substrate was covered with a bonding agent in the form of concentrate to be diluted with water. Table 1 shows the weight composition of the mixes that were used to make the substrate and the overlay.  
The grid of the testing areas was applied on the surface of the overlay 500 mm from its edge (Figure 1a) and the number of testing areas was equal to 256. 90 days after laying the overlay, tests using two acoustic NDT methods were carried out on its surface in 256 designated testing areas (Figure 2a,b).  
The values of the following parameters were determined in all these areas using the IR method: average mobility (\(N_{\mathrm{av}}\)), dynamic stiffness (\(K_{\mathrm{d}}\)), mobility slope (\(M_{\mathrm{p}}/N\)), and void index (\(v\)) according to [54; 55; 56]. The value of the frequency of the sound wave reflection from the bottom of a sample (\(f_{\mathrm{T}}\)), obtained using the I-E method, was also measured according to [57; 58; 59].  
After finishing the tests using acoustic NDT methods, tests using the pull-off method were conducted in the same testing areas in order to obtain the real values of the \(f_{\mathrm{b}}\) (Figure 2c). The  
Figure 1: The tested two-layer cement composite: (**a**) arrangement of testing areas on the surface of the overlay; (**b**) cross-section; and (**c**) view of the surface of the overlay after pull-off testing.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & & \multicolumn{3}{c}{**Components of Mixes (kg/m\({}^{3}\))**} \\ \cline{2-9}
**Layer** & **Poriland** & **Poriland** & & & & & \\
**Designation** & **Cement CEM** & **Cement CEM** & **Cement CEM** & **Fly Ash** & **Water** & **Aggregate** & **Aggregate** & **Plasticizer** \\  & **I 42.5 R** & **II A-LL 42.5 R** & & & **0-2 mm** & **2-8 mm** & **Flow 6920** \\ \hline Overlay & 276.0 & – & – & 138 & 1599.0 & – & – \\ \hline Substrate & – & 352.0 & 40 & 165 & 724.4 & 1086.6 & 2.0 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Weight composition of mixes.  
Figure 2: Exemplary view of tests carried out on the surface of the overlay using: (**a**) the IR method; (**b**) the I-E method; and (**c**) the pull-off method.  
values were then used as patterns for learning and testing the ANN. The data used for development of the models was obtained from past experiments [29]. In this article, the output value of the pull-off adhesion predicted by the ANN is denoted as\(f_{\text{c,b}}\). Exemplary data is presented in Table 2.