## 3 Principles of the Surface Defect Index (SDI) Method  
The procedure implemented in the SDI method includes determining the area to be examined using a frame that specifies this region. This frame serves as an indicator for identifying and isolating the precise portion of the surface under scrutiny, allowing for a focused and systematic assessment of the targeted areas. By defining the boundaries of the analysis zone, the frame facilitates accurate measurements of the volume of defects, ensuring consistency and reliability in the results obtained. Furthermore, it enables the precise evaluation of the level of surface defects, further enhancing the accuracy of the assessment.  
The frame used to determine the surface defect area in this method is not standardized, meaning that there are no fixed or standardized dimensions for its measurements. Indeed, we can adjust the dimensions of this frame according to the specific needs of each sample or analysis. This flexibility allows the accurate targeting and quantification of surface defects by adapting the frame to the unique characteristics of each situation. Thus, we are able to obtain more precise and representative results by considering the variability in volume parameters, which are reflected by the three-dimensional measurement of surface defects, including their length, width, depth, and distribution within the material. Additionally, there is the flexibility to use more than one frame, allowing for a comprehensive assessment of surface defects across different areas or sections of the surface being studied. This approach enables us to capture a more complete picture of the surface characteristics and obtain accurate measurements of defect areas.  
Figure 3 describes the overall process of the new method proposed here. The SDI method first accurately delineates the surface defect area and then characterizes the present defects by quantifying their volume (length, depth, and width).  
The SDI method was developed using a four-step methodology, which is described in the following:  
* The first step in the SDI method, frame design, involves creating a frame adapted to the dimensions of the surfaces under analysis. This frame can be constructed from metal, plastic, or any other rigid material. Its dimensions are adjusted based on the size of the samples and the specific requirements of the study. The role of the frame as a crucial tool is to precisely delineate the measurement area for surface defects in the SDI method.
* In the second step, the placement of the frame for optimal evaluation, it is recommended to position the frame horizontally on the surface being analyzed, ensuring direct contact with the sample surface. This arrangement allows for the precise and effective delineation of the entire affected surface.
* In the third step, the application of RMP, a ready-mixed plaster (a pre-blended mixture of gypsum) was used for the tests to reveal the volume of defects present on the surface. This step involved applying RMP inside the frame at the interface between the frame and the surface under analysis, as well as applying the RMP on an ideal reference surface unaffected by defects (free from defects), which served as a baseline for comparison. The ideal surface was an estimation of the surface under analysis before the damage occurred.
* In the fourth and last step, surface defect quantification, after enclosing the entire damaged area within the frame (a rectangle bounding the length, width, and depth), all present defects were assessed by quantifying their volume. The surface defect quantification relied on the difference between the measurement obtained from the RMP applied on the surface under analysis and that applied on the reference surface. The quantification of the volume of surface defects is expressed by the Surface Defect Index (SDI), which represents the percentage of defects by volume.