## 1 Introduction  
Concrete is one of the most commonly and widely used construction materials over the world thanks to its high compressive strength and economic superiority [1]. However, a concrete structure is inherently susceptible to cracks due to its low tensile resistance. The crack propagation often causes the abrupt brittle failure of the concrete structure without any forewarning. Thus, the brittle failure leading to heavy losses of lives and properties is difficult to be prevented. In order to prevent such brittle failure of the concrete structures, a number of structural health monitoring (SHM) techniques have been developed. For example, strain gauge, acoustic emission, impact test and lead zirconate titanate (PZT)-based ultrasonic techniques have been widely used as contact-type sensor-based crack detection techniques. [PERSON] et al. utilized vibrating wire strain gauges for measuring the small strain in a concrete structure [2]. Then, [PERSON] et al. used fiber optic sensors to monitor concrete cracks using the acoustic emission technique [3; 4]. Also, [PERSON] et al. tried to characterize the cracking behavior in strain-hardening cement-based composite using acoustic emission technique [5]. [PERSON] et al. performed the impact test using an impact hammer to detect a crack in a concrete slab [6]. [PERSON] et al. performed ultrasonic-based inspection for detecting the defect in concrete [7]. [PERSON] et al. experimentally assessed crack in a reinforced concrete structure using PZTs [8; 9; 10; 11], and [PERSON] and [PERSON] also utilized PZTs for detecting splitting failure of a reinforced concrete structure under incrementally increased loading conditions [12]. [PERSON] et al. tried to numerically and experimentally observe PZT-generated surface wave propagation in a concrete structure to detect cracks [13]. [PERSON] and [PERSON] proposed a combination of embedded PZTs with surface bonded PZTs to assess the cracks in a reinforced concrete beam [14]. The aforementioned contact-type sensor-based SHM techniques have the advantages of low cost and high accuracy, but there are technical limitations such as sensor durability, maintenance related to sensor replacement, massive cabling issues and so on. In particular, the performance of the sensor might be degraded much earlier than the target structure itself [15; 16]. Furthermore, the spatial sensing range is typically limited [17]. To overcome the aforementioned technical limitations, noncontact crack detection techniques have been proposed. As representative tools, digital camera [18; 19; 20], laser displacement sensor [21] and infrared (IR) thermography [22; 23] have been reported. Recently, unmanned aerial vehicle (UAV) has been employed for incorporating with the noncontact crack detection techniques, because UAV can effectively inspect broad target areas including even inaccessible areas. For instance, UAV-mounted vision cameras were used for evaluating concrete cracks [24; 25]. [PERSON] and [PERSON] also used UAV with an embedded image processing system for automatic recognition of cracks on a concrete surface [26]. IR camera incorporated with laser excitation was also utilized for detecting multiple concrete cracks [27]. More recently, [PERSON] et al. proposed the fully noncontact hybrid sensing system combining a continuous-wave line laser, IR and digital cameras for enhancing deep learning-based crack detectability of a concrete structure [28]. Although the noncontact crack detection techniques can address the technical limitations of the contact-type sensing mechanism, they are more suitable for periodic inspection rather than real-time monitoring of a target structure. In a SHM point of view, real-time monitoring is one of the most important issues for preventing abrupt structural failure. As an alternative, embedded sensing techniques such as embedded PZT-based smart aggregates and self-sensing concrete have been proposed for concrete crack detection. For example, [PERSON] et al.