# Dielectric Characterization of Chinese Standard Concrete for Compressive Strength Evaluation
## 2 Materials and Methods  
### Design and Fabrication of GB Concrete  
In order to study the relationship between the compressive strength and respective dielectric constant of GB concrete, common ordinary GB concrete with a number of different water to cement (\(w\)/\(c\)) ratios, was investigated. The aim of this research was to study the dielectric behaviors of the ordinary concrete that only contained basic raw materials, namely water, cement, sand, coarse aggregate, and air (in the form of porosity). It is known that the dielectric constant of cementitious materials is influenced by the addition of chemical admixtures, supplementary cementitious materials, or fibers [23,24]. Therefore, as a primary study, no other admixtures, such as superplasticizer or chemical agents, were included in the concrete mix. Second to water, cement is the crucial, yet non-negligible, constituent for making concrete. The three major standards of internationally-used Portland cement are the American ASTM C159, the Chinese GB175, and the European EN197. In this study, the cement used was the ordinary type of Portland cement P.O. 42.5 that conformed to GB175-2007, and was supplied by Shanshui Cement Group Ltd. The minimum 28-day compressive strength of P.O. 42.5 is 42.5 MPa, which is equivalent to CEM 42.5N when considering the EN197. Medium silica sand was used, with mean and maximum sizes of 1.1 mm and 5.0 mm, respectively, whereas the maximum particle size of the coarse aggregate was 31.5 mm. In both the compressive strength test and dielectric test, relatively small sizes of concrete specimens were used, rather than large concrete structures. Throughout the experimental study, the authors consciously chose the required raw materials, and computed the required quantities in accordance with the design code of the Chinese standard concrete, JGJ55-2011 [2]. In order to maintain the flow-ability of the fresh concrete, the selected concrete unit water content was 205 kg/m\({}^{3}\) (\(m_{w0}\)), as superplasticizer was not used. Accordingly, the GB concrete specimens (C30, C35, C40, C45, and C50), which had a dimension of 150 mm \(\times\) 150 mm \(\times\)150 mm [3], with respective \(w\)/\(c\) ratios, were calculated as summarized in Table 1. The accurate amounts of mix proportions were determined by using the volume method, as given by [2]:  
\[\frac{m_{c0}}{\rho_{c}}+\frac{m_{g0}}{\rho_{g}}+\frac{m_{s0}}{\rho_{s}}+\frac{m _{w0}}{\rho_{w}}+0.01\alpha=1, \tag{1}\]  
\[\beta_{s}=\frac{m_{s0}}{m_{g0}+m_{s0}}\times 100\%. \tag{2}\]  
where  
\(m_{c0}\)--cement content per cubic meter of concrete;  
Figure 1: A big picture illustrating the cross-disciplinary work for compressive strength evaluation, using the dielectric constant of 28-day cement concrete. \(m_{g}\)0--coarse aggregate content per cubic meter of concrete;  
\(m_{s}\)0--sand content per cubic meter of concrete;  
\(m_{w0}\)--water content per cubic meter of concrete;  
\(\rho_{c}\)--density of cement, 3000 kg/m\({}^{3}\), the mid-range suggested by [2];  
\(\rho_{g}\)--density of coarse aggregate, 2700 kg/m\({}^{3}\);  
\(\rho_{s}\)--density of sand, 2650 kg/m\({}^{3}\);  
\(\rho_{w}\)--density of water, 1000 kg/m\({}^{3}\);  
\(\alpha\)--percentage of entrapped air in concrete mix, 1.0;  
\(\beta_{s}\)--sand rate, determined in accordance with the Chinese standard (GB) [2]. In addition to the proportions of raw materials (Table 1), both the physical and chemical properties of the cement affect the dielectric properties of GB concrete. The chemical composition as a percentage of weight, and the physical properties of the ordinary type of Portland cement P.O. 42.5, were considered, as listed in Tables 2 and 3, respectively. The slump values recorded for all types of fresh concrete fell in the range of 75-90 mm, which met the requirements of the standard JGJ55-2011 [2]. It is known that a high percentage of entrapped air will likely reduce the concrete's physical density, thus reducing the compressive strength, but increasing the water permeability.