Cracks are ubiquitous phenomena in concrete structures, especially in road pavements and bridge decks. Locating and measuring the concrete cracks are among the most time-consuming practices in structural health monitoring and bridge inspection. Similar to other aspects of living in the modern era, many structural inspection practices are improving thorough automation. One common approach is to equip ground/aerial platforms with visual cameras and capture inspection images from the structure of interest. The number of acquired images can be burdensome and, due to their size and complexity, they require an excessive amount of manual post-processing to count, measure and classify the cracking on the structure. Developing and selecting a post-processing algorithm(s) to automatically discover cracks can solve the issues with large datasets and excessive inspection times. In this paper three popular edge detectors, Sobel and Roberts edge detectors in spatial domain, and Gaussian high pass filter method in frequency domain, are implemented in a Matlab program to detect cracks in 100 captured images by an Unmanned Aerial Vehicle from bridge decks (50 defected and 50 sound). The Sobel method provided the most accurate results with 92% successful detection while this number was 83% and 81% for Gaussian and Roberts, respectively. In terms of sensitivity, the minimum crack width detectable by Sobel method was 2.5 pixels which was 0.5 pixel less than Gaussian and 1 pixel less than Roberts. Another perks of using Sobel method was its speed. With 0.95 second per image, Sobel was slightly faster than Roberts, with 0.98 second per image, and significantly faster than Gaussian high pass filter with 1.33 seconds per images. Therefore, the Sobel edge detector was the most appropriate edge detector among the studied methods for crack detection in concrete structures.
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