## 5 Discussion and Practical Implications  
Asphalt concrete is used for impervious cores in dams, and it is commonly called hydraulic asphalt concrete (HAC) in dam engineering. A core should be flexible and ductile to adjust to deformations in the dam and its foundations. This requires the mix design for HAC to consist of more fine aggregates, more filler content, and more bitumen content. The aggregates are \"supersaturated\" with bitumen. Furthermore, HAC is compacted into a  
Figure 13: Typical comparison of the long-term axial creep strain curves (solid line) using the material stress–strain–time–temperature creep model (SSTTC) and the test results under different radial confining stresses and different temperatures. (**a**) Confining stress of 1.0 MPa at 5 \({}^{\circ}\)C. (**b**) Confining stress of 1.0 MPa at 15 \({}^{\circ}\)C. (**c**) Confining stress of 0.5 MPa at 20 \({}^{\circ}\)C (refer to Figure 3a). (**d**) Confining stress of 0.5 MPa and at 25 \({}^{\circ}\)C (refer to Figure 4a). (**e**) Confining stress of 0.8 MPa at 30 \({}^{\circ}\)C (refer to Figure 5). dense state with a vibratory roller in the field, and there are significant kneading effects. The skeleton structure of aggregates in an asphalt core is weak, and the performance of the core is dominantly governed by the asphalt mortar in the HAC. The test results showed that the magnitude of confining stress levels had insignificant effects on the stress-strain relationship, which was linear in the creep-stable state. Therefore, the viscoelasticity of asphalt concrete can be modeled with the generalized Kelvin model in a linear form of creep compliance. When HAC is designed to have less bitumen content (for instance, 6.1% of the total weight) and is compacted in the laboratory in a steel mold, the HAC is constrained in the steel mold when subjected to compaction. The skeleton structure in the aggregates in compacted asphalt specimens is strong. The behavior of asphalt concrete is affected by the aggregate's skeleton structure and the interlocking thereof. The creep-stable modulus \(\mathrm{E_{cs}}\) was high [10]. From Equation (7) and the test results shown in Table 2, the relationship between the creep-stable modulus \(\mathrm{E_{cs}}\) and temperature is shown in Figure 14.  
Based on the axial and volumetric strains in the creep-stable state during the tests, the equivalent Poisson ratio at different temperatures was found to be approximately 0.37, as shown in Figure 15.  
Under conditions of high deviator stress, asphalt concrete is unable to reach a creep-stable state of equilibrium and presents plastic flow. The creep strain continues to increase, and the asphalt concrete eventually fails. There must be a stress-state boundary where asphalt concrete changes from a creep-stable state to a creep-unstable state. This boundary was defined as the plastic yield boundary (PBY) by [PERSON] and [PERSON] [10]. The PBY can be estimated by using creep tests with stepwise loading. The deviator stress at the PBY  
Figure 14: Creep-stable modulus \(\mathrm{E_{cs}}\) versus temperature for dam core specimens. Figure 15: Radial strain versus axial strain in the “creep-stable state” in triaxial compression tests. is larger than the last step of creep-stable deviator stress but is smaller than the step of creep-unstable deviator stress. Applying the Mohr-Coulomb criterion, the relationships of the cohesion intercept and friction angle with temperature for the PBY are shown in Figure 16.  
It should be noted that the relationship of the PYB with the cohesion intercept and friction angle shown in Figure 16 does not mean that the asphalt concrete fails. It is a stress state at the PYB in which the creep strain continues to increase. When used in impervious cores in dams, asphalt concrete is recognized to fail when it is cracked and loses its watertightness. Asphalt cores are in the center of embankment dams and are constructed in a hot state. Load is increasingly applied on an asphalt core when raising an embankment.