# Strength Correlation and Prediction of Engineered Cementitious Composites with Microwave Properties
## 1 Introduction  
Determining the compressive strengths at different curing ages of ordinary concrete in situ is of primary importance for ensuring safety in construction. The compressive strength estimated on the 28 th day has been widely used as a vital indication of the strength development of cementitious materials in industry [1]. Over the past few decades, nondestructive detecting and monitoring techniques have become popular in structure health monitoring (SHM) and evaluations as they offer many advantages over the conventional destructive techniques. As the microstructures of cement-based materials (CBMs), like pores and hydration compounds, govern their macroscale mechanical behavior, great efforts and various techniques have been employed to study and evaluate the process and the mechanism of microstructure during hydration in order to establish the strength prediction. Conventional thermal-based monitoring of heat generation leading to temperature increases during the hydration process was used in bridge and dam construction [2], whereas the compressive strength of CBMs in relation to the reflection loss of ultrasonic waves was investigated in [3]. An ultrasound technique was used to evaluate the compressive strength of CBM with added mineral admixtures, where compressive strength and ultrasonic pulse velocity were measured and correlated for days 3, 7, 28 and 120 of the curing period [4]. The relationship between pulse velocity and compressive strength was, hence, found to fall within the trend of exponential functions. A novel technique that made use of a fuzzy logic system to correlate the early-age CBM strength with harmonic amplitudes using smart aggregate was presented in [5]. Crack detection, hydration monitoring and energy harvesting using similar embedded piezoelectric transducers were reported in [6; 7; 8]. Recently, good correlation between acoustic emission energy at low frequency (25-100 kHz) and plastic strain energy of reinforcedconcrete specimens was established in [9; 10]. An active acoustic method at a lower frequency of around 6 kHz was also employed [11] to monitor setting and hardening in concrete at early ages, wherein the P-wave velocity and attenuation coefficient were used to characterize the development of calcium silicate hydrate (C-S-H) gel inside the concrete and, hence, predict strength growth during early ages. However, the aforementioned detecting techniques require in situ embedded sensors to be set up and or non-real time signal post-processing. For example, they need wired sensor pair matches, sensor locations and alignments, and so on. All of these require prior precise arrangement and calibration, and therefore will be high-cost and time-consuming, as they were applied to large-scale CBM's monitoring. Conversely, simple, direct detection of intrinsic electrical properties, e.g., direct current (dc) resistivity, of CBM during hydration was proposed [12; 13]. The four-probe (Wenner method) resistivity measurement used a simple setup, where no embedded sensors and alignment were required. However, the simple method suffers from accuracy problems due to the contact resistance between electrodes and CBM specimens being tested, as well as due to the possible shrinkage gap. To combat such problems, [PERSON] et al. introduced a non-contact measurement approach to monitor the hydration process of early-age concrete through electrical resistivity using alternative current (ac) transformer method [14]. It was found that non-contact resistivity measurement was appropriate for precise monitoring of CBM at very early-age period since the technique was sensitive to the ionic concentrations and mobility in the liquid or pore solution [15]. However, the transformer-based method has its limitations in terms of operating frequency because the accuracy of measured resistivity is a function of frequency [16]. The actual measured value is the complex impedance rather than the pure resistance, and thus could not truly reflect the hydration process, especially at higher frequency beyond 30 kHz.