## 1 Introduction  
External tendons are widely used in the girders of post-tensioning bridges. Tendon failure can have a serious impact on the safety of a bridge system [1], so assessing the tendon condition is critical to ensuring bridge safety [2]. The most common tendon failure is caused by corrosion of the steel strands of the tendon. In particular, external tendons are more susceptible to corrosion than internal tendons embedded in hardened concrete because external tendons are more exposed to the surrounding environment, characterized by parameters such as humidity, salinity, and temperature [3]. Importantly, voids in the grout reduce the strength of the tendon system due to the lack of stress redistribution [4]. A more severe problem resulting from the voids is that they expose the steel strands to corrosive ions and moisture, accelerating the corrosion of the steel strands [5]. A grout void is a common defect caused by unskilled workmanship, low-quality grout material, sudden changes in the duct shape, and long-term grout sinking. Therefore, it is important to regularly inspect external tendons for voids and monitor changes in their shape and size, which requires a suitable inspection method. In general, an external tendon in a post-tensioning system consists of several steel strands, cement grout, and a protective duct pipe surrounding them. Smooth high-density polyethylene (HDPE) is widely used as a duct in the tendon [6]. In recent decades, several methods have been proposed to inspect external tendons for voids in the grout. Some of these methods are based on mechanical waves, such as sounding inspection [7] and ultrasonic testing [8]. The sounding method is carried out by tapping an impactor on the outside of the duct, and a professional expert listens to the tapping sound to determine the presence of damage. This is a simple and fast method but is not suitable for detecting relatively small voids. For ultrasonic testing, ultrasonic probes are used to transmit and receive ultrasonic waves. Depending on the ultrasonic mode used and the arrangement of the probes, there are several techniques such as pulse-echo, through-transmission, linear array, guided waves, etc. It is essential to maintain coupling between the probe and the duct using a liquid couplant for ultrasonic transmission, which can make scanning for imaging and sizing of voids difficult. Other methods are based on electromagnetic waves, such as infrared thermography and radiography, which allow high-resolution imaging of voids [9]. In infrared thermography, voids act as thermal barriers and are detected by thermal imaging. Passive thermography using natural ambient heat is highly dependent on ambient temperature conditions and has limited detection capability, whereas active thermography requires artificial heat sources to transfer heat to tendons. Radiography using X-ray or gamma-ray radiation is a tool to provide high-detectability and high-resolution images of voids. However, radiography is often subject to heavy regulations due to concerns about radiation exposure during testing. Terahertz (THz) electromagnetic waves in the frequency region of 0.1-10 THz can be used for nondestructive inspection of nonconductive materials owing to their transparency to THz waves [10; 11]. This study presents a method based on THz waves to inspect external tendons for voids in the grout below the duct. An A-scanner using electromagnetic waves may be a promising tool for the real-time detection of voids between the duct and the grout in tendons. Electromagnetic waves characterized by low attenuation in the HDPE duct and a significant reflectivity difference between the duct/grout and duct/void interfaces are required to identify the presence of voids below the duct through A-scan measurements. THz waves exhibit the above characteristics so that a THz A-scanner can distinguish well between voids and cement grout in contact with the HDPE duct.