## 1 Introduction  
Asphalt concrete is a viscoelastic material. Asphalt core embankment dams (ACEDs) have been adopted and used for more than 60 years. With the advantages of asphalt concrete becoming apparent, this dam type is being increasingly adopted, and the number of ACEDs built was about 120 in 2010 and about 250 in 2020 [1; 2]. The highest ACED was about 125 m before 2005, and it was increased to about 170 m in 2017. Many high ACEDs located in deep compressive overburdens are being designed.  
The width of the asphalt core from the top to the bottom of a dam is commonly in the range of 0.5-1.2 m. An asphalt mixture is either manually placed in pre-erected steel formworks or is placed by a machine power. Once placed, the asphalt mixture and adjacent transition zone materials are compacted by vibratory rollers. The core is constructed at a common rate of two layers per day. The layer thickness of the core and adjacent transition zones is in the range of 0.2-0.25 m. It is inevitable that construction must be stopped for a few days per month to conduct quality checks or for a few months during rainy seasons or winter. Typically, 2-3 years are spent constructing a 100 m high ACED. The dam construction procedures and processes suggest that asphalt cores are subjected to stepwise loading creep during construction.  
The temperature of the asphalt mixture is usually between 140 and 160 \({}^{\circ}\)C during its placement and compaction when using bitumen type B70 or B90. The temperature may be 10-20 \({}^{\circ}\)C lower when the softer bitumen type B180 is used. The asphalt core is in the center of the embankment dam and is protected by the adjacent transition zones. It is overlaid layer by layer with hot asphalt concrete when raising the embankment. The temperature in the core is reduced gradually with time. The reduction rate depends on the dam construction rate and weather conditions. Field observations at Quxue Dam indicated that the temperature in the core was around 40 \({}^{\circ}\)C after one month and around 20 \({}^{\circ}\)C after half a year [3]. During reservoir impounding, the temperature changes relatively quickly and gradually approaches a constant temperature. In some situations, a dam is constructed, and the reservoir is simultaneously impounded. In such cases, the temperature in the core cools quickly.  
The performance of asphalt cores in dams during their construction is subjected to stepwise loading creep and is simultaneously affected by changes in temperature. In previous studies, long-term triaxial creep tests were conducted at a constant temperature, and the test results showed that the stress-strain reached a creep-stable state over time [4; 5; 6; 7; 8; 9]. When the test results from [PERSON] [4] and [PERSON] [5] were reanalyzed, the deviator stress and lateral or axial strain presented a good linear relationship. In the reanalysis of the results from [PERSON], [PERSON], and [PERSON] [6; 7; 8], this relationship was also found to be roughly linear with a larger scatter. This relationship in the reanalyzed results from [PERSON] et al. [9] also presented roughly linear characteristics, covering about 70% of the deviator stress strength. Based on a reanalysis of test results in the literature and on their own test results, [PERSON] and [PERSON] [10] proposed a creep-stable model (CS model) for analyzing the behavior of the cores of dams. To extend previous studies [10] and establish a material stress-strain-time-temperature creep model (SSTTC model), systematic investigations were conducted.