# Repair Composite Adhesion Strength: A Comparison of Testing Methods
## 2 Materials and Methods  
Repair composite samples were prepared using European cement PC-I 500 as a binder, while quartz sand with a 0.63 fraction served as the fine aggregate. The rheological properties of the solutions were controlled by incorporating water-redispersible vinyl acetate/ethylene copolymer powder DA 1400 VA-E RDP (Dairen, China) at 2% by mass of the mixture. Admixtures of SAP copolymer of anionic polyacrylamide and potassium acrylate (Sika, Aquagel, Denver, CO, USA) 0.2% by weight of the mixture and 0.5% of pure sodium aluminate NaAlO\({}_{2}\) (Density 2.602 g/cm\({}^{3}\) Melting point 1650 \({}^{\circ}\)C) were used to correct the consistency and hardening time, Figure 2.  
Cement, quartz sand, and additives were placed in the mechanical mixer bowl and slowly mixed to create a homogeneous composition. Water was then added to the bowl and mixing continued until all ingredients were fully combined, Figure 3a. The quantity of water required to achieve the target working consistency was determined following the procedure outlined in [26]. This consistency was maintained across all solutions using a cone penetration test. The average immersion depth, which is the difference between the initial and one-minute readings (with an error of up to 1 mm), was averaged over two tests and rounded to the nearest centimeter to determine the mixture's mobility grade, as shown in Figure 3. Water was added to achieve a 7 cm cone immersion depth, as specified in DSTU B B.2.7-23 [27]. Figure 2: Raw materials of the experiment. The adhesive strength indicators for all studied compositions of cement samples were determined using the DYNA Z16 apparatus (Proceq SA, Schwerzenbach, Switzerland) (Figure 4) in accordance with [24; 28; 29]. The load was applied to the metal puller with an increase in the rate of load application of 250 \(\pm\) 50 N/s. Reference concrete slabs of size 450 \(\times\) 450 \(\times\) 45 mm, manufactured following the requirements of the standard [30], were used as a concrete substrate; composition specification and properties are given in Tables 1 and 2. \begin{table}
\begin{tabular}{c c} \hline
**Component** & **Specifications** \\ \hline Binding & Portland cement type PC II/A–S–400 \\ Aggregate & gravel sand, particle size 0–8 mm, continuous \\  & curve of granulometric composition A-B \\ Binder/aggregate mass ratio & 1:5 \\ W/C ratio & 0.5 \\ \hline \end{tabular}
\end{table} 
Table 1: Composition of concrete slabs. Figure 3: Use of mechanical mixer for (**a**) mixing process and (**b**) determining the consistency by cone device: 1—bowl with mortar; 2—reference cone; 3—lock screw; 4—scale; 5—holders. In accordance with [25], a primer based on a dispersion of synthetic resins and Ceresit CT 19 fillers was used as a contact layer. To evaluate adhesion parameters according to [24], repair composite strips (maximum thickness 10 mm) were applied to pre-prepared concrete slabs using a 6 \(\times\) 6 mm square-toothed spatula. Ceramic tiles were then glued to the composite according to [30], and then the dollies of the pull-off tester were placed on the tiles. For adhesion testing, 050 mm diameter \(\times\) 10 mm thick repair composite specimens were prepared as per EN 1542:1999 [25]. These specimens were cored from a continuous layer applied to concrete slabs [30] (Figure 5). Pull-off tester dollies were then attached to the hardened specimens using a two-component epoxy adhesive [25]. The most reliable data on the quality of adhesion of repair composites to concrete slabs were obtained by considering only the results of AF-S and CF-S types of adhesive bond loss in both cases (Table 3). The selection of AF-S (adhesive failure along the contact layer) and CF-S (cohesion failure along the substrate) failure types for analysis prioritizes the most critical aspects influencing the long-term durability and performance of the repair system.