# Dielectric Characterization of Chinese Standard Concrete for Compressive Strength Evaluation
## 2 Materials and Methods
### Method of Dielectric Constant Measurement  
Dielectric properties describe the physical-chemical properties related to the energy storage and energy dissipation of the concrete materials subjected to an external electric field excitation. A complex relative permittivity, which consists of real and imaginary parts (\(\epsilon_{r}{}^{\prime}-j_{\mathrm{\varepsilon}r}{}^{\prime\prime}\)), is commonly used to quantitatively and qualitatively define the dielectric characterization of concrete materials [27; 28; 29; 30; 31]. The real part of the complex permittivity is also known as the dielectric constant, which indicates the amount of energy stored inside the concrete element.  
In this study, we used the dielectric probe measurement method, wherein an open-ended coaxial (OEC) probe was used, to determine the dielectric constant of GB concrete after 28 days of curing. The OEC probe method is one of the most popular techniques for measuring the dielectric properties of cement-based materials [32; 33; 34; 35]. The method has the advantages of being simple, yet maintaining direct-contact, being nondestructive, and having a wide operating bandwidth in the microwave frequency regime. Its well-developed theory makes it possible to obtain sufficiently accurate results for both medium-loss and high-loss media [35; 36]. However, the 28-day concrete becomes low-loss dielectric material, and its loss factor (imaginary part of complex permittivity) is known to be small, approaching zero. Unfortunately, these loss values could not be accurately determined by using Keysight's measurement solutions, particularly at frequencies below 200 MHz [37]. In view of this obstacle, the research scope of this paper is therefore limited to the investigation of the real part of the relative complex permittivity. Hereafter, we simply use the term \"dielectric constant\" to indicate the real part of the relative complex permittivity (\(\epsilon_{r}{}^{\prime}\)).  
During the measurement, a Keysight vector network analyzer (E5080A) was used to measure the reflection coefficient. In order to minimize the measurement errors, air, a metallic short-circuit block, and distilled water were used for the calibration, as shown in Figure 3. A total of 20 randomly selected, non-overlapping positions on four side surfaces, were measured by using the OEC probe. Namely, five measurements took place on each surface while discarding the top and bottom faces, in order to avoid surface roughness. The complex permittivity was automatically computed from the measured reflection coefficient, via the Keysight materials measurement suite (N1500A). The dielectric constant was computed and recorded in the computer-based network analyzer E5080A, which connected with the OEC probe and an electronic calibration-kit through a 1-meter long flexible cable, as illustrated in Figure 3. Throughout the dielectric measurement, the average value, regarded as the dielectric constant, along with the standard deviation of each GB concrete specimen, was taken. The measuring frequency was set in the range of 10 MHz to 6 GHz, which covered the most popular cellular phone and WiFi communication frequencies.  
Figure 2: Photographs for GB concrete mixing and casting: (**a**) anchor type of concrete mixer; (**b**) elimination of entrapped air from specimens using vibration table.