# Experiment on the Tensile Strength of Concrete Joint Surface at Early Ages
## 2 Materials and Methods
### Preparation Process  
1. Weighing Materials: According to the proportions listed in Table 1, accurately measure and prepare sufficient quantities of each component raw material. Subsequently, sequentially add these raw materials into the mixer as per the specified order. Throughout the process, strict measures must be taken to prevent any foreign impurities from contaminating the mixture, ensuring the purity of the concrete mix, and the accuracy of the test data. 2. Dry Mixing Stage: Start the mixer and ensure it operates smoothly. Begin with a period of dry mixing, where cement and aggregates are blended into a uniform mixture. 3. Wet Mixing Stage: Add the pre-measured water and admixtures, and the concrete enters the wet mixing stage. During this phase, the cement and water are thoroughly combined to form a uniform slurry mixture. (Each mixing cycle lasts 60 s, and three cycles are required). 4. Pouring the Test Blocks: After the mixing process is completed, pour the uniformly mixed concrete onto a flat surface. Then, evenly apply a layer of machine oil to the bottom and sides of the 150 mm \(\times\) 150 mm \(\times\) 150 mm cubic molds. However, ensure that the side where the grout stopping tape is placed is not oiled, as this facilitates the formation of a good bond at the joint surface. Use a shovel to fill the molds with concrete, ensuring the material is evenly distributed. Next, use a vibrating device to thoroughly vibrate the concrete in the molds. This helps the concrete flow \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Concrete Design Mix** & **Cement** & **Mineral** & **Fly Ash** & **Expansive** & **Natural** & **Manuf-** & **Pebble** & **Large** & **Admixture** & **Water** \\
**Ratio** & & & & **Agent** & **Sand** & **Sand** & **Gravel** & **Gravel** & **Admixture** & **Water** \\ \cline{2-10}
**(kg/m\({}^{3}\))** & **241** & **55** & **17** & **19** & **145** & **822** & **198** & **792** & **7** & **146** \\ \hline \multicolumn{10}{c}{Apparent density (kg/m\({}^{3}\))} & \multicolumn{4}{c}{Chloride ion content (\%)} & \multicolumn{4}{c}{Total alkali content} \\ \cline{4-10}  & & & & & & (kg/m\({}^{3}\)) & & Slump (mm) \\ \hline \multicolumn{10}{c}{2280} & \multicolumn{4}{c}{0.056} & \multicolumn{4}{c}{0.89} & \multicolumn{4}{c}{170} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Concrete Mix Proportion and Performance Indicators. Figure 2: Roughened interface. adequately, and removes internal air bubbles, thereby enhancing its density and overall strength. After completing the vibrating process, use a scraper to level the surface of the concrete specimens. This step helps ensure the flatness of the specimens, effectively eliminating any potential surface irregularities and achieving a uniform surface. Once all of the above operations are completed, place the prepared concrete specimens in a room temperature environment for initial setting (Figure 3), ensuring they attain the necessary strength and durability during the subsequent curing phase. Here, it is particularly important to ensure that the amount of new concrete poured is slightly overfilled. This allows the concrete to achieve a level surface directly during the vibrating process, avoiding the need for multiple filling and vibrating operations due to insufficient filling. Repeating the vibrating steps can cause the machine oil to seep into the concrete, severely hindering the effective bonding between the new and old concrete. 5. Curing of Test Blocks: After successful demolding of the test blocks (Figure 4), all samples are transferred to a curing room that adheres to strict temperature and humidity standards. The room consistently maintains a temperature within the range of 20 \({}^{\circ}\)C \(\pm\) 2 \({}^{\circ}\)C, and ensures that the air humidity remains above 95% at all times. The curing room is shown in Figure 5.
6. Pouring the Central Test Block: According to the experimental design, remove 10 concrete test blocks at the age of 3 days, 5 days, and 10 days.