## 3 Viscoelastic Adhesive Strength and Stability Testing Method
### Evaluation Method of Viscoelastic Adhesive Performance
#### 3.3.2 Adhesion Stability  
The attachment stability is reviewed using the peel load values for each section of the 4 types of NHV-SPRG composite waterproofing sheets derived in Section 3.3.1. Load and adhesion strength calculation methods were conducted in compliance to the standard format outlined in KS F 4934 \"Self-adhesive waterproof sheet.\"  
In general, when NHV-SPRG is peeled off from the interface of the substrate, stress is simultaneously generated in the interfacial adhesion between dissimilar materials and the cohesion interface, that the rate at which the two types of separation (adhesive or cohesive) can differ significantly based on the material composition, which in turn can have different effects on overall adhesion stability. For example, Figure 1 shows that when the sample waterproof sheet is peeled off from the surface of the mortar specimen, it is confirmed that  
Figure 8: 180 peel-off test method and data graph example; (**a**) testing process illustrated, (**b**) data procurement example illustrated. the adhesion load is read more closely at the interface of different materials. On the other hand, as shown in Figure 1, some materials display a different effect by showcasing when tension is applied to the cross-sectional area due to the viscoelastic properties, before stress is applied to the adhesion between the coating layer and the substrate. For this purpose, in this study, to analyze the relationship between viscoelastic adhesion strength and stability difference based on the peel load's coefficient of variation, grade of the linear regression line of the peel load, and peel load percent relative range. 1. Coefficient of Variation  
For this parameter, adhesion strength stability is determined by calculating the coefficient of variation based on the standard deviation of the 10 tensile load intervals (P1 to P10). Standard deviation refers to the value of how scattered one data value is from the mean, and the value obtained by subtracting the mean of the data from each variate of data is called the deviation from the mean of each variate. In this case, the larger the absolute value of the deviation means that the variance is far from the mean and the smaller the absolute value of the deviation is, the closer the variance is to the mean. In addition, the coefficient of variation is a technique used to compare data with different units of measurement, and generally refers to a value obtained by dividing the standard deviation by the mean. In the case of the non-hardened asphalt composite waterproofing sheet reviewed in this study, it is difficult to secure consistent adhesive strength over the entire area due to its viscoelastic properties. As confirmed in the above viscoelastic adhesive strength test result, the peeling load value decreased and increased, and increased and decreased for each section from P1 to P10; that is, when the non-hardening type viscoelastic waterproofing material adheres to the base concrete, it is judged that the adhesive load is lowered due to the variables (high and low compression load, voids, latency, protrusions, etc.) acting on the base concrete. The coefficient of variation calculated by reflecting these aspects will be able to judge the adhesion stabilization according to the size of the value. Based on the example graphs provided in Figure 9, it can be interpreted that there is adhesion stabilization when the value of the coefficient of variation is calculated by Equation (2):  
\[P_{cv}=\frac{\sqrt{\frac{\sum|x-\mu|^{2}}{N}}}{\mu} \tag{2}\]  
where;  
\(P_{cv}\): Coefficient of variation of peel load standard deviation  
\(x\): peel load sample \((\mathrm{N})\)  
\(\mu\): Average peel load \((\mathrm{N})\)  
\(N\): Number of data (number of P\({}_{t}\), 10)  
(2) Grade of the Linear regression Line  
Figure 9: A stability analysis comparison example of peel load data standard deviation; (**a**) a case with high standard deviation, (**b**) a case with low standard deviation.