## 3 Experimental Study  
### Specimen Size and Embedded Sensor  
For this study, experiments were conducted on special HSC specimens that were designed for use in nuclear power plants. When concrete with a compressive strength of 64.5 MPa was used to construct of the Civaux-2 NPP in France, decrease in the temperature were observed to cut off 25% [24]. With reference of this NPP concrete these specimens, processed at 5 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, and 40 \({}^{\circ}\)C, showed strengths of 49.56, 60.98, and 70.02 MPa, respectively. The compressive strength of this concrete was expected to be 55.16 MPa. After conducting experiments, the estimated strength was 49.56 MPa, 60.98, and 70.02 MPa for three different samples. These three samples were placed in different temperature zones of 5 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, and 40 \({}^{\circ}\)C in order to measure the strength under various weather conditions. PZT materials were used as sensors and inserted into experimental specimens of concrete. With the changes in mechanical strength and elastic stiffness, structures can dynamically response well to vibrations and ultrasonic waves [3].  
The size of each specimen was kept at 1200 \(\times\) 2000 \(\times\) 1000 mm\({}^{3}\), as shown in Figure 3a,b. Though the concrete specimens were small but for real time concrete strength estimation of NPP, it was  
Figure 2: A schematic diagram of the sensor in concrete and the data acquisition process.  
necessary to use the actual size of steel reinforcement bars. For this purpose, the bar of 35 mm was used as reinforcement in the experimented samples.  
The experimented concrete was specially designed for NPPs, as the toxic chemical reaction and explosions take place in NPP, so some qualities of this special concrete must be highly selective for the reliable operations of NPP [24]. The compressive strength, mixing ratio, durability, density, resistance to weather, wear resistance, and permeability are the key factors to select for the HSC of NPP. Strength can be used as per requirement; normal, moderate, and high strength; similarly, density can be low, medium, and high. To gain the desired properties, some admixtures were added, the ratio of this HSC mix is given below in Table 2. Here, FA denotes fly ash, GGBS denotes ground granulated blast-furnace slag, SF denotes silica fume, AD denotes admixture, and AE denotes air entraining.  
The experimental setup was prepared before casting concrete. After pouring concrete, data were acquired by a single PZT sensor in each specimen; these sensors were already embedded in the specimens before casting concrete. Using the sensors embedded in the concrete specimens,  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline
**Specimen** & \multirow{2}{*}{**W/C \%**} & \multicolumn{8}{c}{**Unit Weight kg/m\({}^{3}\)**} & \multirow{2}{*}{**AD \%**} & \multirow{2}{*}{**AE \%**} & \multirow{2}{*}{**Temperature for**} \\
**No** & & & & & & & & & & & & \\ \hline
1 & 40 & 162 & 324 & 81 & 0 & 0 & 722 & 965 & 0 & 0 & 5 \\
2 & 34 & 155 & 328 & 0 & 2  
the dynamic responses of the structures to the ultrasonic waves and vibrations created by actuators were measured every hour for 28 days. At the same time, the internal temperature of the concrete was measured. The specimens were also simultaneously examined via compressive strength testing using cylinder cores taken from the main concrete at 1 st, 3 rd, 7 th 14 th, and 28 th days. Figure 4 shows some details of the sensor placement, spacing, and arrangement, which were set up before casting concrete.