# Repair Composite Adhesion Strength: A Comparison of Testing Methods
## 2 Materials and Methods
AF-S failure directly indicates problems with the interface bond between the repair composite and the substrate, while CF-S failure highlights weaknesses in the repair material's internal cohesion. These failure modes are thus considered the most relevant indicators of overall system performance, providing more reliable data for evaluating the effectiveness of repair strategies than other less critical failure modes. For each of the two adhesion testing methods of repair composites to the surface of concrete slabs, 20 samples were made for each of 10 reference slabs. To obtain statistically robust results for comparison, a total of 150 samples per adhesion testing method were selected for analysis. These samples were chosen from an initial set of 200 samples (20 samples per slab, across 10 slabs) after careful inspection to eliminate any showing defects or signs of improper application that could affect adhesion strength. The indicator of the value of adhesion between the repair composites and the surface of the reference plates \(f_{\text{A}}\) (MPa) was determined as follows:  
\[f_{\text{A}}=P_{\text{n}}/F \tag{1}\]  
where \(P_{\text{n}}\)--pull-off force (failure load, N); \(F\)--the contact area of the test specimens with the surface of the dolly (mm\({}^{2}\)). \begin{table}
\begin{tabular}{c c} \hline
**Properties** & **Specifications** \\ \hline Consolidation & 90 s on a vibrating table at 50 Hz \\ Endurance & 24 h under normal conditions \\ Water absorption & on the surface 2.0–8.0 cm\({}^{3}\) after 4 h \\ \hline \end{tabular}
\end{table} 
Table 2: Properties of reference concrete slabs. Figure 5: Reference concrete slabs prepared for pull-off testing with applied repair composite and cut-out specimens. of various factors such as material quality, application technique, and environmental conditions on the bonding process of repair composites. Moreover, it commonly happens that the close spacing of pull-off adhesion samples can weaken the nearest areas, causing a wide range of adhesion values due to localized damage from testing forces. Significant variations in adhesion were also observed by the authors [31]. [PERSON] and colleagues emphasize that the acceptable coefficient of variation (COV) for adhesion is not universally defined; however, a COV of 40% is suggested to be acceptable, provided there are at least five valid pull-off tests. It is important to establish an upper limit for each application, ideally determined by the manufacturers. After calculating the average adhesion strength for each plate, the deviation in individual measurements from these average values was determined:  
\[\sigma_{\overline{\text{R}}_{\text{A}}}=\sqrt{\frac{\Sigma(\text{X}_{\text{i}} -\overline{\text{X}^{2}})}{\text{n}-1}} \tag{2}\]  
where \(\text{X}_{\text{i}}\) represents each individual mean adhesion strength, \(\overline{\text{X}}\) is the sample mean adhesion strength, \(\text{n}=10\) is the sample number. According to the data, the dispersion of the average values of adhesion on all reference plates is 0.00456, which indicates the high repeatability of the results of the measurement method. The number of samples that showed deviations from the average adhesion index (2.411 MPa) by 30% or more was 71 units from 200; the values are marked in gray in the table. Taking into account the requirements for the adhesive strength index according to the Ukrainian standard [24], the distribution of the received classes for mixtures used in the repair of concrete and mortar surfaces, as well as for mortar mixtures and the mortars based on them is as follows: PM1--132 samples (88%), PM2--132 samples (88%), and PM3--148 samples (98.6%). As shown in Figure 6, the standard deviation values across the specimens range from approximately 0.25 to 0.55. Notably, specimens on plates 4 and 8 exhibit higher deviations, both around 0.55, indicating significant variability in bond strength for these samples. In contrast, plates 6 and 9 show the lowest deviations, around 0.25, suggesting a more consistent bond quality for these 30 samples.