# Establishment and Application of Crowd-Sensing-Based System for Bridge Structural Crack Detection
## 1 Introduction  
As an important part of transportation infrastructure, the health condition, safety and durability of bridge structures are of great concern to industry, academia, and the general public, as they are related to safe travelling for residents and the formulation of infrastructure maintenance decisions [1, 2]. Traditional bridge structure inspections are carried out by maintenance companies, which draw up inspection plans based on the distribution and quantity of bridges waiting for inspection [3, 4, 5]. On the one hand, with the development of China's economic development, the traffic flow on the roads is growing. The number of vehicles and their weights are constantly increasing, causing greater pressure on the structural safety of the existing bridges. On the other hand, as China's transportation infrastructure continues to develop, the number of bridge constructions rapidly increasing, and the distribution area of bridges is also gradually expanding, requiring more maintenance forces to carry out bridge maintenance [6, 7, 8, 9]. However, the number of specialized equipment for bridge inspections, such as bridge inspection vehicles and elevated work vehicles as well as professional inspectors, is insufficient, and the cost of labor is also increasing [10, 11]. Therefore, the insufficient detection force is unable to meet the constantly increasing demand for bridge inspection. In the case of limited force, theadjustments that the inspection units may need to make include prolonging the inspection cycle for bridges or using the limited force to inspect large bridges or critical routes, reducing the inspection of bridges on minor traffic routes. Failure to conduct timely inspections will have adverse effects on the safety and durability of bridges [12; 13; 14; 15]. Early crack detection is extremely important because cracks at this stage are fine in width and short in length, but as cracks expand, they can lead to structural failure [16; 17]. In the past, many crack detection and characterization techniques have been developed through research and development by experts in the fields of SHM [18; 19] (Structural Health Monitoring), NDE [20] (Non-Destructive Evaluation), etc. [PERSON] et al. [21] proposed a CNN-based damage identification method for hanger cables in a tied arch bridge and the result showed that the CNN trained by Fourier amplitude spectra was better than the CNN trained by time history data. [PERSON] et al. [22] used a CNN to establish a prediction model for evaluating ground motion and structural damage state. The training data were obtained from numerical simulating. [PERSON] et al. [23] used the CNN algorithm and designed a multi-scale module which could extract damage features from noisy and incomplete mode shape data. Measuring these temporal and spatial changes through traditional methods such as field observation or surveys is quite a challenging and time-consuming task [24]. Therefore, it is encouraged to explore new technologies for more efficient inspection methods, such as the utilization of smartphones with Global Position System (GPS), in order to reduce costs and increase participation of the public pedestrians [25; 26; 27]. Crowd sensing is a concept that emerged from the development of the Internet of Things and is a new way of acquiring monitoring data [28]. Crowd sensing can efficiently and cost-effectively collect a massive amount of heterogeneous data through portable mobile devices carried by a large number of end users on the mobile end [29; 30; 31]. Currently, smartphones equipped with various sensors such as image sensors, GPS sensors, and gyroscopes have become one of the basic items carried by the general public. The development of mobile networks such as 4G and 5G ensures that remote mountain areas can also achieve fast transmission of files, including images and even videos [32; 33; 34; 35; 36; 37]. Due to the demand for work, life, or entertainment, thousands of citizens naturally need to travel on various bridges [38; 39].