# Dielectric Characterization of Chinese Standard Concrete for Compressive Strength Evaluation
## 1 Introduction  
Concrete is a heterogeneous cementitious material and the second most consumed material in the world. Concrete has long been used for all types of civil structures worldwide. In China, abundant megaprojects use cement concrete as basic building blocks, or as precast beam components in civil structures. According to the Chinese National Bureau of Statistics, China's commodity concrete production was 1.64 billion cubic meters (BCMs) in 2015, and when compared to the value of 0.74 BCMs in 2011, it has increased by 222% [1]. This value indicates that China is the largest producer and consumer of concrete in the world, and its production roughly occupies about 15% of global production. The concrete which is mixed and cast to conform to the Chinese standard is termed the Guo Biao (GB) concrete [2; 3]. There are many similarities between the Chinese standard concrete (JGJ55) and the European one (EN206-1). For example, the compressive strengths are both classified as C30, C35, C40, and so on, indicating that the minimum values of the corresponding compressive strengths are 30 MPa, 35 MPa, 40 MPa, and so on. However, the use of superplasticizer is recommended by the EN206-1, whereas the Chinese concrete does not strictly require this in JGJ55. The main differences among the worldwide standards are the physical properties (e.g., density) of the raw materials used, such as cement, sand, and coarse aggregate. Ordinary cement concrete behaves like a brittle material, where cracks occur in the course of its long-term use. Such cracks are inevitable due to a variety of natural or man-made factors,and cause the compressive strength to decline, leading to structure deformation, and thus affecting the performance of structural integrity. Numerous nondestructive testing or evaluation (NDT/E) methods have been developed for the prediction of compressive strength, development monitoring, and the evaluation of standard concretes [4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Conventional NDT/E techniques, such as visual inspection, rebound hammers, the pull-off resistance correlation method [4; 5], and the maturity test [6], etc., often suffer from inaccuracy, being cost-intensive and time-consuming. Recently, state-of-the-art NDT techniques were developed for the monitoring, prediction, and evaluation of the mechanical deformation of concrete structures [7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. In particular, the piezoceramic-based smart aggregates play a significant role in concrete structural health monitoring (SHM) [7; 8; 9; 10], wherein the determination of concrete properties was performed by using the travel time of longitudinal waves over a known distance, between smart aggregates. Electrical resistance and resistivity measurements were employed as a relation to the microstructure development of concrete mix, such as pore structure, porosity, and pore-size distribution, etc., and hence, for compressive strength prediction and evaluation [11; 12; 13; 14; 15]. Electrical conductance at microwave frequency has also been used in [16], as a function of the mobility and concentration of the ions in the pore solution of cementitious composites. More recently, novel methods were proposed, based on the communication of electromagnetic waves at low (stress wave) [17; 18], highly nonlinear solitary wave [19], and high (microwave) [20] frequencies, used for concrete characterization, concrete surface moisture detection, and strength estimation, respectively. Their ultimate purpose was to detect damages, evaluate health conditions, alleviate hazards, and extend the working life of civil structures. All of the described DNT techniques have their relative pros and cons, and no unique technique has been identified as perfect in all aspects, including the reduction of the life-cycle cost (LCC) [21]. The LCC comprises the initial cost, maintenance or repair cost, and cost of failure, power efficiency, and effectiveness.