## 1 Introduction  
Due to the rapid development of more demanding concrete infrastructures worldwide, these structures need to be upgraded and enlarged in terms of their size, design, shape, stability, and strength. To meet these new design requirements, the strength of concrete is important; these structures rely on concrete as a construction agent due to its good compressive strength, durability, and cost efficiency [1; 2]. Because concrete is a heterogeneous material, consisting of a mixture of water, cement, fine aggregates, coarse aggregates, fiber/steel, and different other admixtures, it is sensitive to the curing process that is used to gain the design strength [3]. Thus, concrete needs to be monitored precisely from right after it is poured until the 28 th day to determine whether it has achieved the appropriate design strength [4; 5]. Wide range of novel technologies have been adopted to construct concrete structures to decrease costs and increase safety in NPPs and other construction industry. Some admixtures have also been developed to add into the concrete mixtures to obtain higher compressive strength and durability or to improve the workability. When constructing NPPs, durable concrete with high compressive strength is needed to safely run toxic chemical processes and protect human life and the environment [6]. Thus, when casting HSC, it is necessary to monitor the strength development at an early age and determine whether concrete gains the design strength. To estimate the early-age compressive strength of concrete, different types of tests can be used. NDT (non-destructive testing) methods are usually considered to be cost effective and time-saving approaches to estimating the state of structures [7; 8; 9]. NDT methods for concrete include the rebound/Schmidt hammer test, ultrasonic wave propagation, and techniques that use PZT (piezoelectric) materials [5; 10; 11; 12; 13; 14]. Due to the accuracy of its result, the strength estimation of concrete using EMI has been widely studied [13; 14; 15; 16; 17; 18; 19]. In our work, experiments are conducted on the basis of NDT by analyzing the dynamic response of structures to vibrations using a PZT material sensor and analyzing EMI results [20] with fuzzy logic. In this study, NDT based a single-sensor technique is used to obtain data from concrete specimens. Here, the PZT material performs as an actuator and a sensor simultaneously. After concrete casting, the PZT sample actuates (actuator mode) to produce vibrations and then receives (sensor mode) the structure's dynamic response to ultrasonic waves and vibration [21]. Moreover, after acquisition of the data, the cross correlation (CC) was calculated and the EMI was analyzed as a function of the curing time of concrete using a fuzzy logic simulation. The fuzzy logic simulation tool in MATLAB was used to simulate the data in order to obtain clear results. The required rules of fuzzy logic were set according to strength estimation, and then the input of data was run to obtain results. After this process, based on the growth pattern of the frequencies of ultrasonic waves from the EMI and fuzzy logic-treated data, the curves for strength prediction were established; this gave us a flow line that could be used to predict the concrete compressive strength for 28 days. When these simulation data were compared with the actual strength results of concrete, which were observed during 28 days of curing by testing cylindrical cores taken from specimens, the results were almost similar.  
Our proposed NDT method to estimate the strength of concrete by analyzing the EMI using fuzzy logic saves time and provides a secure and satisfactory procedure to monitor the structural health of concrete. This method can be modified in the future by using other related parameters.