Meanwhile, the IE method is utilized to estimate the depth of such cracks using the time-of-flight (TOF) technique. The crack depth is then estimated by measuring the distance between the tip and the surface of the crack. The crack tip is identified when the strain of the concrete exceeds the cracking strain. During the loading procedure, the non-contact video-based method combined with visual inspection is adopted to monitor the occurrence of crack. In this study, several concrete beams are loaded in the laboratory to generate surface-opening cracks. However, in the current literature, the use of non-contact video-based methods mainly focuses on the structure displacement, thereby ignoring the strain of the structure. In view of this, several researchers have applied non-contact video-based measurement to field tests and verified the accuracy of this method by comparison with other conventional measurements. Moreover, this method does not need to access the critical areas of structures when these areas are untouchable. Among these technologies, video-based technology offers a way for researchers and engineers to access structure information considering multiple sections in a remote distance but with high resolution. Non-contact measurement systems mainly utilize laser, radar, and GPS technologies and images obtained via video technology and digital photography. With technical innovation, non-contact measurement systems have been recently invented and adopted by some researchers for monitoring the service condition of structures. Only few scholars have conducted experiments on concrete beams and slabs to apply this method for determining the depth of surface-opening crack. However, research regarding the depth of surface-opening crack is limited. The IE method was first used to detect concrete flaws and later widely applied to interpret the severity of delamination in bridge deck and detect large voids and corrosion damage.
A stress wave method called impact-echo (IE) method has been proposed and frequently adopted by researchers to identify the defects in concrete structures.
However, such methods cannot measure the depth of surface-opening cracks because they are used to identify the apparent and superficial problems while the depth of the surface-opening crack may be larger than the superficial profile of the crack.
For surface-opening cracks, audio-visual methods are the most extensively adopted because they are rapid and inexpensive in inspecting structures. These methods include audio-visual methods, stress wave methods, electromagnetic methods, and miscellaneous tests. Nondestructive test methods offer bridge managers and engineers fast and efficient approaches for bridge inspection and assessment. Nondestructive and destructive methods are commonly adopted in assessing the service condition of bridges between the two, destructive methods are often of low efficiency and poor economic effect because they may core, drill, or include load test on the bridge structure. Therefore, measuring the depth of surface-opening crack to realize appropriate maintenance schedule is of considerable importance. Surface-opening crack is one of the most common defects for concrete bridges it reduces stiffness and allows aggressive media, such as water and chloride, to corrode the reinforced bars, thereby causing the loss in load-carrying capacity. The USA spend approximately $10 billion annually on the repair and maintenance of their existing bridges. However, most of the bridges constructed decades ago have experienced different levels of degradation after being subjected to long-term environment attacks and load effects. Bridge engineering has considerably developed in the past few decades.