# Durability Evaluation Study for Crumb Rubber-Asphalt Pavement
## Materials and Methods
### Experimental Methods Setup
#### 2.2.2 Adhesion Evaluation Method for Bitumen and Crumb Rubber Fine Aggregate  
At present, the test methods for evaluation of adhesion pertain to coarse aggregates. In contrast, fewer adhesion test methods have been developed for fine aggregates [34]. However, damage to the mixture and peeling off are more likely to arise from finer aggregates than from coarser aggregates because their adhesion with bitumen is poorer. Therefore, the influence of the adhesion of fine rubber-particle aggregate with bitumen on the moisture stability of the mixture should not be ignored, and it is necessary to propose an evaluation method for the adhesion of bitumen to fine aggregate. This study proposed that the adhesion of asphalt and fine aggregate can be evaluated by a combined boiling-sieving method, as follows:  
(1). Three 500-g samples of crumb rubber-asphalt mixture were prepared according to the design grade and the corresponding asphalt-aggregate ratio and were each placed on a flat bottom plate. The three samples were subjected to the following tests, and the average value was considered as the test result. (2). The hot asphalt mixture on the flat bottom plate was cooled to room temperature. While cooling, the crumb rubber asphalt mixture was gently turned with a shovel so as not to damage the agglomerate; it should not be separated by hand or broken with a hammer. (3). The mixture was collected, weighed to determine the mass \(m_{1}\), and then passed through a sieve with 4.75-mm-wide holes (as shown in Figure 3). A small shovel was used to guide the mixture gently so that it passed through the sieve holes freely without damaging its bonding state. The use of a shaker was strictly forbidden. The mixture on the 4.75 mm-square mesh was collected and weighed to determine the mass \(m_{2}\). (4). The collected mixture was immersed in a large beaker containing boiling water, and the degree of heating was adjusted to keep the water in a boiling state. The mixture was gently stirred with a glass rod during the boiling process, and a piece of paper was used to remove asphalt floating on the water surface. (5). After boiling for 20 min, the mixture was taken out, placed in a flat pan, and cooled to room temperature. While cooling, the mixture was gently tumbled with a shovel, as in step 2. (6). After the cooling of the mixture and the occurrence of complete evaporation of water, the mixture was again passed through the 4.75-mm-square mesh, using a shovel to help any free mixture to pass through the sieve, with the same requirements as in step 3. The part of the mixture that did not pass through the 4.75-mm sieve holes was weighed and its mass was recorded as \(m_{3}\). Figure 3: Schematic illustration of the boiling–sieving method. The design concepts and calculation methods of the boiling-sieving method are as follows:  
1. The mixture was in a freely bonded state after mixing and cooling. The crumb rubber asphalt mixture generally adopted intermittent gradation (2.36-4.75 mm). For convenience, this study used 4.75 mm as the boundary point between coarse and fine aggregates. 2. It was assumed that the mass of asphalt mastic (bitumen and mineral powder) coating the surface of the aggregate was directly proportional to the mass of the coarse and fine aggregate. In the mixture design, the mass ratio of the coarse aggregate was recorded as \(a_{1}\), and the mass ratio of fine aggregate was recorded as \(a_{2}\). \[m_{C}=m_{1}\times\frac{a_{1}}{a_{1}+a_{2}}\] (1) \[m_{F}=m_{1}\times\frac{a_{2}}{a_{1}+a_{2}}\] (2) In Equations (1) and (2): \(m_{C}\)--The mass of coarse aggregate covered with asphalt and mineral powder (g); \(m_{F}\)--The mass of fine aggregates coated with asphalt and mineral powder (g); \(m_{1}\)--Total mass of the mixture (g); \(a_{1}\)--The mass ratio of coarse aggregate in the mix design (%); \(a_{2}\)--The mass ratio of fine aggregate in the mix design (%);
3.