# Durability Evaluation Study for Crumb Rubber-Asphalt Pavement
## Materials and Methods
### Experimental Methods Setup
#### 2.2.3 Moisture Stability Evaluation Method of Crumb Rubber-asphalt Mixture  
Owing to the mechanical and deformational properties of the crumb rubber-asphalt mixture, the harsh environmental conditions, and repeated traffic load to which it is exposed, the study of the moisture stability of the crumb rubber-asphalt pavement is significantly important [48]. Conventional evaluation methods for asphalt mixture moisture stability, such as the Marshall water immersion test and the indirect tensile strength test under freeze-thaw cycle, are not sufficient to evaluate the long-term moisture stability of crumb rubber-asphalt pavement under special environmental conditions and repeated traffic load [49; 50]. Based on the characteristics of rubber crumbs, the actual environmental conditions of the pavement and load effects to which it was exposed [38], this study proposed the indirect tensile strength test under a freeze-thaw-boil cycle as the main evaluation method for the moisture stability of the crumb rubber-asphalt mixture. Furthermore, considering the deformation performance of the crumb rubber-asphalt mixture, the Cantabro abrasion test [46] under water immersion was used as an auxiliary evaluation method. This test method can be used to comprehensively evaluate the long-term water stability of the crumb rubber-asphalt mixture. 1. Indirect Tensile Strength Test under Freeze-Thaw-Boil cycle The proposed indirect tensile strength test differs from the conventional indirect tensile strength test under freeze-thaw (as ASTM D 4867) [51] in that a set of test specimens are subjected to freeze-thaw and boiling treatment, and the water stability performance of these specimens is analyzed compared to untreated test specimens and test specimens under special treatment. The test method is based on the following principles: the water that enters the interior of the mixture in the ice and snow season is simulated by exposing a vacuum-saturated test specimen to a freeze-thaw cycle; and, the repeated scouring action of the dynamic water pressure generated by dynamic wheel load is simulated by continuous boiling in water at 100 \({}^{\circ}\)C. By incorporating these two aspects, the improved method test can better evaluate the water stability of crumb rubber-asphalt mixture than the conventional freeze-thaw split test. Further, as described above, in order to more objectively evaluate the long-term water stability performance of the crumb rubber-asphalt mixture, a test specimen that has been aged for a long term (refer to \"Practice for Shortened Long Term Aging of Hot Mix Asphalt \" T 0734) [46] is also subjected to the indirect tensile strength test under a freeze-thaw-boil cycle, and the durability of this mixture specimen against water damage is obtained by comparative analysis. The test implementation process is shown in Figure 4.  
The specific steps in the indirect tensile strength test under a freeze-thaw-boil operation are as follows:  
1. The test specimens, which correspond to the samples of the mixture that have been subjected to standard Marshall compaction molding, are randomly divided into three groups, each group comprising no less than four specimens. 2. The average value of the splitting strength of the first group of specimens at 25 \({}^{\circ}\)C, recorded as \(\overline{R_{T1}}\), is obtained directly. The second and third sets of specimens are vacuum-saturated for 15 min at 97.3-98.7 kPa and then returned to normal pressure for 0.5 h in water. The test specimens are taken out of the water and placed at a temperature of -18 \({}^{\circ}\)C for 16 h.
3. The second set of test specimens are thawed to 25 \({}^{\circ}\)C, and the average value of the splitting strength of the set is determined and recorded as \(\overline{R_{T2}}\). After the freeze-thaw cycle, the third set of test specimens are immediately placed in boiling water for 2 h and then cooled to 25 \({}^{\circ}\)C before measuring the splitting strength, which is recorded as \(\overline{R_{T3}}\). Figure 4: Schematic illustration of the test process.