## 1 Introduction  
Characterizing the quality of a concrete surface is a major problem for the concrete industry. Surface quality varies depending on the application envisaged for concrete. The surface needs to be not only visually appealing but also structurally sound. Quantifying surface defects is essential for ensuring material quality and durability, and it is equally crucial to understand how these surfaces initially form and how they may be altered. Surfaces may indeed evolve freely during their solidification, but they may also incur damage, or they may take on a closed shape due to the wall effect when in contact with formwork [1, 2]. In assessing the severity of surface defects in concrete, it becomes evident that the gravity of these imperfections is not solely determined by the area that they occupy on the surface but also by their depth. Deeper defects can have a more profound impact on the structural integrity and performance of concrete [3, 4]. Additionally, the depth of defects can influence their potential to accumulate contaminants and facilitate the growth of microorganisms that might compromise the material's long-term durability [5, 6]. Therefore, for aesthetic and functional considerations, the combination of both surface area and depth provides an objective assessment of concrete surface quality. Surface quality evaluation tends to be subjective and, hence, more difficult, as current specifications allow for visual evaluation, which can lead to disagreements in the detection and evaluation results [7]. The primary quality inspection methods for concrete surfaces are based on manual inspection--size measurement, counting, and classification of bugholes on the surface of concrete [8; 9; 10]. The Quality Surface Index (QSI) serves as an illustrative example of a simplified method for evaluating the surface area occupied by pores. This approach, as depicted in Figure 1, which provides a visual representation of this surface evaluation method, involves analyzing pore groups based on their diameter [11]. The sand patch method, according to [PERSON], involves a pre-defined amount of sand to be applied on the surface to be measured, and it is distributed evenly in circular movements [12]. Despite the fact that manual evaluation is a powerful inspection tool and plays a significant role in detecting and evaluating the quality of concrete, several limitations have been found. First, manual inspection methods are performed by inspectors walking along the surface of a structure while using only their naked eye. It has also been pointed out that large quantities of concrete are widely used. Secondly, the results are not always reliable and are even prone to errors, the process of inspection is time-consuming, and the requirement of experienced inspectors exacerbates the pressing shortage of a highly skilled inspection workforce in the construction industry [10]. As such, the main disadvantage of visual inspection is that it is time-consuming, labor-intensive, and impractical [9; 13]. A person's ability to judge faults is limited in that it cannot quantify the value of a given defect. Subjective evaluation depends on individual experience and differing perceptions without a uniform standard; in fact, the same person might even make different judgments on different days. Moreover, it is not possible for a person to determine all possible defects [14]. This technique, however, is extremely laborious and is incapable of representing surface quality [7]. Therefore, this method was replaced with an improved method recommended by the Concrete International Board (CIB) [15], which proposed the comparison of actual concrete surfaces with photographs of reference samples representing seven scales with different degrees of bughole coverage.