## 3 Evaluation Methods of the Performance of GSP
### Computationand Simulation Method  
#### 3.3.1 Finite Element Method under Various Contact Models  
The Finite Element Method (FEM) is widely utilized in study of material engineering. FEM software such as ANSYS, ABAQUS, and BISAR are practiced in computation and simulation for GSP. Physical properties of GSP are calculated under various contact models according to different computational hypothesis and parameters from experience and tests, as shown in Tables 13 and 14.  
FEM can calculate load status of GSP structures using the Elastic Layered System model in three dimensions (3D) and two dimensions (2D). This model easily describes the relationship between the traffic load and the whole structure of GSP. It is found that the bonding of interlayers can reduce the shear stress, bottom tensile stress, and rebound deflection in GSP [133; 134]. Thus, the structure shows a better anti-shear capability than AC, which results in a high rutting resistance [120; 121; 122; 123; 124; 125; 126; 127; 128; 129; 130; 131]. This method also fits well and  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Author** & **Thickness H [cm]** & **Elastic Modulus E [Mpa]** & **Poisson’s Ratio [\(\mu\)]** \\ \hline [PERSON] et al. [129] & 15 & 1200 & 0.25 \\ [PERSON] et al. [130] & 10 & 2000 & 0.25 \\ [PERSON] et al. [131] & 15 & 2000 & 0.25 \\ [PERSON] et al. [132] & 20 & 2500 & 0.25 \\ [PERSON] et al. [133] & 5 & 2000 & 0.25 \\ [PERSON] et al. [122] & 30 & 3600 & 0.15 \\ [PERSON] et al. [40] & 6 & 7600 & 0.24 \\ \hline \hline \end{tabular}
\end{table}
Table 13: Computational parameters of GSP in FEM.  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Loading Mode** & **Fatigue Life** & **Correction Factor** \\ \hline Continuous loading & 17,739 & 1.0 \\
1000 cycles Loading and Equivalent & 40,980 & 2.3 \\ Intermittent Time & 50,823 & 2.9 \\
2 cycles loading + 1 cycle intermission & 140,050 & 7.9 \\
1 cycle loading + 2 cycles intermission & 734,395 & 41.4 \\ \hline \hline \end{tabular}
\end{table}
Table 12: Fatigue Correction Coefficient of GSP [117; 118].  
gains consistent conclusion in many circumstances, such as airports, which have different structures and requirements, [122; 132].  
The toughness of GSP is calculated using the Visco-Elastoplastic model as the contact model by the ABAQUS software. It is found that the toughness is affected by the OMA void: when it was 25%, GSP has the highest toughness of 11.358 kJ/m\({}^{2}\), showing the best anti-cracking capacity [135].  
A 2D sectional image model is established by a CCD digital camera and CAD software to analyze the position of cracking for GSP. OMA is assumed to be visco-elastoplastic, and cement mortar is defined as elastic in the computational process. Then, expansion force and contraction force in GSP are calculated by ABAQUS. The results show that expansion of cement mortar has little or no effect on GSP; but contraction can dramatically increase stress on asphalt-cement interfaces to lead to cracking [71]. In short, low-temperature shrinkage of cement mortar is the main reason causing cracking and asphalt-cement interfaces are the weak interfaces.