## 1 Introduction  
Concrete-filled steel tubular (CFST) arch bridges have been widely used in recent decades at home and abroad, and have been studied in the United States, France, Spain, and Japan. Most of the CFST arch bridges are exposed to the natural environment's conditions, and many bridges are in an extremely bad environment, likely damaged by acid or alkaline materials. According to previous investigations [1; 2], the most common problem of the concrete-filled steel tube arch rib is the interface void between the concrete and the steel tube. This problem is highly concealed and presents a very serious risk of harm, which directly affects the safety of concrete-filled steel tube arch bridges during operation [3; 4; 5]. Many research results show that an interface void is inevitable, especially in the vault. The main causes of the interface void are construction technology, gravity drop, temperature change, and concrete shrinkage. The existence of an interface void makes the structure lose the hoop effect of steel tube on concrete, and reduces the bearing capacity of the arch rib. As the void height and scope increase, the bearing capacity decreases even more. Existing nondestructive tests of CFST arch bridges with disengaging at the steel-concrete interface include surface wave detection [6; 7], partial quantitative self-excitation method [8], ultrasonic testing [9], optical fibers detection [10], infrared thermography [11], and so on [12; 13]. Surface wave detection is based on a spectral variation mechanism of transmitted pulse reacting to the degree of defect [14]. The advantage of detection is a fast testing speed and a relatively low cost. However, this technique is not mature and still at the stage of laboratory experiments. The partial quantitative self-excitation method applies an incentive to the structure through a hammer. The interface separation can be identified from the spectrum calculated by the time domain signals [15; 16]. This method is simple and easy to be integrated, but it is hard to conduct boundary recognition. Ultrasonic testing is one of the most widely used nondestructive testing methods. It can speculate the defects within the concrete through ultrasonic detectors and transducers. Measurements such as first wave amplitude, propagation velocity, and the main frequency of receipt signal are analyzed [17; 18]. The testing results are relatively reliable, and the procedures are simple. However, this method can only detect defects quantitatively. Optical fibers detection embeds sensing fiber in concrete during construction. The defects in bonding interface may cause the micro-bending of optical fibers. Defects in concrete can be detected through the consumption of the optical transmission and the change of the mode coupling characteristics of the optical fiber [19; 20]. The limitation of this approach is that it can only be applied to newly built bridges. Infrared thermography is a main method used in CFST nondestructive testing, which is based on the theory of heat transfer. The temperature distribution on the structure surface measured by a thermal imager is used to detect the defects [21; 22; 23; 24; 25]. However, the results of infrared radiation are influenced by factors like surface material and surroundings. In the research of nondestructive testing and monitoring technology for the interface void of concrete-filled steel tube arch bridges, much research at home and abroad has been focused on qualitative detection technology. Most of these methods are related to point detection, with a small detection range, expensive equipment and a huge volume. These factors make it impossible to carry out real-time monitoring of the interface voids of existing CFST arch bridges. The core issue of the damage evolution law of bridge engineering is \"damage identification and location\".