## 4 Final Setting Judgment Based on Strain Monitoring  
### Changes of Early Strain  
Strain sensors can reflect the change of concrete from a plastic flow state to the final setting state from the beginning of embedding to the process of synchronous deformation with concrete. Therefore, it is necessary to analyze the early monitoring data to better understand the concrete condensation process. Vibrating string strain sensors use the relationship between the vibration frequency and the strain to reflect the strain condition by measuring the vibration frequency change of steel string under stress. Taking two and eight measuring points as examples, the initial frequency modulus and temperature reading of the strain from pouring are shown in Figure 4.  
As shown in Figure 4, the initial reading of the strain sensor from the beginning to the end of the pouring has obviously regular changes. According to the fluctuation trend and the mutation position, the data can be divided into the following three stages. The three stages are represented by 123 in the figure. The first stage (0-13 h of point 2, 0-29 h of point 8):  
In this stage, the temperature data of the sensor fluctuates up and down according to the sine curve, which is consistent with the fluctuation trend of the environmental tem \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Materials**} & \multirow{2}{*}{**Cement**} & **Mineral** & **Coal** & \multirow{2}{*}{**Sand**} & \multirow{2}{*}{**Stone**} & **Water** & **Water Reducing** \\  & & **Fines** & & **Fly Ash** & & & & **Agent** \\ \hline Mixing proportion & \multirow{2}{*}{170} & \multirow{2}{*}{50} & \multirow{2}{*}{120} & \multirow{2}{*}{798} & \multirow{2}{*}{1070} & \multirow{2}{*}{160} & \multirow{2}{*}{7} \\ (kg/m\({}^{3}\)) & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mix proportion of concrete. Figure 4: Early data of measuring. perature. The strain frequency modulus and temperature show the same change trend, indicating that the sensor has not been covered by concrete during this period. The temperature reading of the strain sensor is the environmental temperature. Currently, the frequency reading of the strain sensor only contains the thermal expansion and cold contraction deformation of the steel string itself due to the change of the environmental temperature. The second stage (13-21 h of point 2, 29-38 h of point 8):  
At the beginning of this stage, the temperature and frequency modulus change abruptly, and the temperature after the abrupt change is about 30 \({}^{\circ}\)C, the concrete molding temperature. This indicates that the strain sensor begins to be buried in the concrete at this time. At this stage, the concrete is in a plastic flow state, and the material properties, such as the elastic modulus and the thermal expansion coefficient, change dramatically. At this time, the elastic modulus of the concrete is less than the elastic modulus of the strain sensor, the thermal expansion coefficient is greater than the thermal expansion coefficient of the strain sensor, the strain sensor is not completely constrained, and the steel string is affected by its own temperature deformation and partial structural deformation. The frequency modulus change fluctuates, and the overall trend is consistent with the temperature change trend. At this time, the strain sensor still cannot reflect the true strain of the structure. The third stage (21-60 h of point 2, 38-60 h of point 8):  
After the strain sensor is embedded in the concrete for a period, the concrete condenses and releases a large amount of hydration heat, and the temperature of the strain sensor begins to increase rapidly. At the same time, the frequency modulus corresponding to the strain enters a stable change and decreases with the increase of temperature.