**1. Introduction**

In reinforced concrete structures, the workability and durability of concrete are critical for the structures to meet desired structural performance [1–3]. Furthermore, the evaluation of properties, such as the setting time of fresh concrete, is critical for assuring quality and reducing construction time. Conventionally, the penetration test that measures the shear resistance of the cementitious material was widely used to determine setting times [4]. In addition, research also showed the possibility of using ultrasonic body waves (longitudinal and shear waves) to characterize the early age properties of cementitious material [5–14].

Lots of research has been conducted to correlate the body wave velocities and material properties. For instance, many efforts were made, aiming to find the relationship between primary wave velocity and setting times [9,11,15–19]. Dumoulin et al. [19] used embedded piezoelectric patches as smart aggregates, to monitor the primary wave velocity evolution during the setting and hardening phases of concrete, and it was found that it was hard to extract the primary wave velocity with a good accuracy at very early ages. Besides, water in the cementitious materials leads to a high primary wave velocity at the early age, which will shield the velocity originating from the solid portion of the material during setting [20]. Moreover, recent research showed that the primary wave velocity in cementitious material is affected by the presence of air void [11,21]. Research by Zhu et al. showed that the presence of air void has limited influence on the shear wave propagation, and the shear wave velocities are almost consistent at the initial setting [5,21,22], and this consistency was verified on cement pastes and mortar samples. Liu et al. [5] used an embedded

**Citation:** Wang, D.; Yu, G.; Liu, S.; Sheng, P. Monitoring the Setting Process of Cementitious Materials Using Guided Waves in Thin Rods. *Materials* **2021**, *14*, 566. https:// doi.org/10.3390/ma14030566

Academic Editors: Mathieu Bauchy, Francesca Lionetto and Sanjay Mathur Received: 1 December 2020 Accepted: 21 January 2021 Published: 25 January 2021

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bender element to monitor the shear wave velocity change during the setting of mortar and concrete samples, and a strong correlation between the shear wave velocity and penetration resistance was found. Carette determined the setting time of mortars containing two types of fly ashes based on the combined monitoring of P and shear waves, and it was found that shear wave velocity and dynamic elastic properties are the most accurate indicators of the setting process [23]. However, when it comes to the complexity of the implementation of the measurement system, the techniques above seem not that attractive.

Ultrasonic guided waves are elastic waves propagating in a plate or rod. Due to its long propagation range, the guided wave is frequently used in material characterization and damage detection in plates and pipes [24–28]. For instance, Zima and Kedra [27] carried out a series of numerical studies to find out the effect of a concrete mesostructure on lamb wave propagation in concrete plates, and it was found that the displacements associated with wave motion are affected by the mesostructure of the concrete plates. Lee et al. [28] formed an embedded guided wave sensor system by placing two pairs of collocated identical piezoelectric patches on a steel plate with uniform thickness. The system was then used for the hardening process monitoring of the ultra-high performance concrete. It was found that two features of the guided wave, namely the amplitude attenuation and the time-of-flight, are more suitable to monitor the behavior of the ultrahigh performance concrete. When a rod is embedded in another medium, part of the guided wave energy will propagate through the interface, and leaky waves are excited in the surrounding medium. The attenuation caused by the leakage depends on the properties of the surrounding materials [29]. Thus, it is possible to evaluate the properties of the surrounding materials by monitoring the attenuation change. The through-transmission method was adopted by Sharma and Mukherjee [30,31] to measure guided waves in a rebar, and the signal amplitude was used to correlate with P wave velocity and compressive strength in concrete. Ervin et al. [32] used this method to monitor corrosion of rebar embedded in mortar. However, this method is not applicable when only one side could be accessed. Vogt et al. [29] investigated the scattering of ultrasonic guided waves at a point where a free cylindrical waveguide enters an embedding material, and the lowestorder longitudinal mode was recommended for embedded guided wave monitoring. As an example, Vogt et al. [33] used a steel wire for guide wave propagation, in a reflected manner, to monitor the curing process of epoxy resins, and the reflected guided wave signals were analyzed using both the reflection coefficient and attenuation method. It was found that both methods are sensitive to the shear properties at low frequencies. Sun and Zhu [34] improved the dispersion calculation of the embedded rebar and used the embedded rebar for guide wave propagation to monitor the early age properties of cement and mortar samples, and in this study the guided wave was generated using an electric coil and received by a commercial ultrasonic transducer. However, to the authors' knowledge, only a steel rebar or wire with a fixed diameter was investigated in the studies above; it would be interesting to make comparisons between rods of different materials and diameters.

In this paper, a series of experimental tests on mortar and concrete samples with various mix designs were performed. Four different kinds of metallic rod were tested simultaneously to find out the optimal guided wave setup to monitor the setting process. Meanwhile, shear wave velocities of the mortar and concrete were monitored, and the time of setting was also measured from these samples. The relationship between these measurements was also discussed.
