I. Introduction
Crack is a common defect of pavement that directly affects pavement service ability and driving safety [1]. Traditionally, the cracks are counted and measured by engineers, but it is time-consuming and labor-intensive [2]. With the development of image-based technologies, pavement crack detection approaches have achieved critical improvements in accuracy and reliability [3]. Crack classification and segmentation are two main focuses of the pavement crack detection. The purpose of classification is to differentiate different crack types while segmentation is a pixel-level extraction of crack from the background [4]. Segmenting cracks from pavement background is a more challenging task than classification and is more attractive to engineers and researchers. During the past decades, a series of crack segmentation methods have been proposed and deep learning-based techniques have become the most interesting and advanced approach to segmenting pavement cracks. This is because deep learning models learn from large-scale data and require little human involvement during training, which can dramatically increase their accuracy and robustness in segmentation tasks [5].