# Repair Composite Adhesion Strength: A Comparison of Testing Methods
## 2 Materials and Methods
The results of measuring the bond strength values of repair composites with reference plates according to the European regulatory method [25] and the average values of the adhesion index are shown in Table 5. Contrary to the data reported for the method in [24],  
Figure 6: Relations between the calculated deviation and the experimental concrete plates made according to the Ukrainian standard. the results obtained by [25] revealed that 152 out of the initial 200 repair composite samples (76%) exhibited a loss of adhesive bond consistent with AF-S and CF-S type reference plates. The adhesion strength data demonstrate a considerable spread, with values ranging from a low of 0.124 MPa to a high of 2.558 MPa--a difference of over 20 times. These findings differ from previous studies made using the standard methods [24], which showed a smaller range spread of adhesion strength values. The variance in the average adhesion values for all reference plates of 0.0233 is 5.1 times higher than this indicator in the method described in the Ukrainian standard [24]. Nevertheless, low dispersion means the adhesion values are consistently close to the mean, which suggests uniform data quality, and reflects reliability and control in measurements. Significant variations in minimum and maximum adhesion values may be attributed to the coating preparation technique, as bond strength is sensitive to how the substrate surface is prepared. Some results indicate that the mean values of bond strength can varying from 6.1% to 15.8% [21; 23]. The average standard deviation values for the specimens range from approximately 0.25 to 0.55, as can be seen from Figure 7. Specimens of plates 4 and 6 exhibit the highest deviations, both close to 0.55, which is similar to the peaks observed in the Ukrainian standard. In contrast, the lowest deviations are found in specimens on plates 1, 2, and 9, with values between 0.25 and 0.3 indicating a relatively uniform bond strength. When the same binder is used with the same environmental conditions, the average bond strength deviations (Figures 6 and 7) for specimens of plates 1 and 2 according to the Ukrainian standard are approximately 0.42 and 0.32, respectively, indicating moderate variability from the average. In contrast, specimens of plates 1 and 2 tested according to the EU standard exhibit lower deviations of around 0.25, suggesting greater consistency. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline  & & & \multicolumn{8}{c}{**Number of Experimental Plate**} \\  & & **1** & **2** & **3** & **4** & **5** & **6** & **7** & **8** & **9** & **10** \\ \hline \multirow{8}{*}{**CIF-S**} & 1 & 1.657 & 1.727 & 0.561 & 0.270 & 1.565 & 0.124 & 0.370 & 0.742 & 1.547 & 1.582 \\  & 2 & 1.675 & 1.765 & 1.050 & 1.552 & 1.636 & 0.862 & 1.590 &  
The number of samples that showed deviations from the average adhesion index (1.986 MPa) by 30% or more was 69 units (the values are marked in gray in Table 5), which is almost similar to measurement results reported in the Ukrainian method [24]. Based on the adhesive strength index requirements outlined in the European regulatory document [25], the distribution of repair composite classes is as follows: R4--80 samples (53.3%), R3--141 samples (94%), R2--145 samples (96.6%), and R1--145 samples (96.6%). The above results showed that the adhesion measurement method according to [25] has comparatively lower repeatability. The possible reason for the reduction in adhesive strength is a certain degree of damage to the bonds between the applied repair composite and the concrete substrate in the process of forming washer samples using a crown, namely the internal mechanical fixation of the composite in the rough surface of the substrate at the microscale level [32; 33]. In addition, the thickness of the epoxy adhesive significantly influences the bond strength [34].