(3) Aging

Aging has relatively little effect on the low temperature performance of porous asphalt mixture. The factors of porosity and aging are together the least and the second-least influential among the four factors. From Figure 9, it can be seen that the flexural tensile strength, maximum bending strain, and bending stiffness modulus of porous asphalt mixture with short-term aging are respectively 0.9%, 1.1%, and 0.5% lower compared to those of porous asphalt mixture with no aging. The flexural tensile strength and bending stiffness modulus of porous asphalt mixture with long-term aging are respectively 4.4% and 6.2% lower whereas the maximum bending strain is 0.4% higher, compared to those of porous asphalt mixture with no aging. The modest decrease in low temperature performance of porous asphalt mixture through aging was in line with the findings by Ma et al. [45]. The slight effect of aging should be due to the effectiveness of the high viscosity SINOTPS modifier in improving the antiaging performance of porous asphalt mixture.

### (4) Test temperature

The influence of test temperature on low temperature bending test results is less significant than that of modifier content. At this juncture, it is noteworthy that the brittle point is a threshold temperature of asphalt mixture at which its behaviour is transitioned from plastic to brittle. In low temperature bending test, in order to prevent possible sudden brittle fracture from affecting the accuracy of the results, the test temperature is generally set at higher than the brittle point temperature. For porous asphalt, the brittle point is lower than that of ordinary dense graded asphalt [57], and the test temperature can generally be set at −10 ◦C to −20 ◦C. Consequently, in order to investigate the effect of temperature changes on the low temperature performance of porous asphalt mixture, three levels of test temperature, namely 0 ◦C, −10 ◦C, and −20 ◦C, were adopted in the test.

From Figure 9, it can be seen that when the test temperature was decreased from 0 ◦C to −20 ◦C, the flexural tensile strength increased by 5.0%, the maximum bending strain decreased by 7.7%, and the bending stiffness modulus increased by 13.9%. The results indicated that in this temperature range, the porous asphalt mixture still exhibited plastic deformation, and the brittle point temperature was not reached [58]. The maximum bending strain decreased with the test temperature, indicating that the lower the test temperature, the less capable the porous asphalt mixture in resisting deformation. At the same time, the bending stiffness modulus of porous asphalt mixture increased with decreasing test temperature, indicating a reduction in the low temperature crack resistance of porous asphalt mixture.

### *3.3. Influence of Porosity on Maximum Bending Strain*

The porosity is amongs<sup>t</sup> the most concerned indicator in the design of porous asphalt mixture. To further analyze the influence of porosity on the low temperature performance of porous asphalt mixture, the single factor influence test of porosity was conducted. The test selected the maximum bending strain as the evaluation indicator.

The experiment adopted the same mineral aggregate gradation and raw materials as those in the orthogonal test, the modifier content was 12%, and the bitumen/aggregate ratio was 4.8%. The mixing and moulding process of porous asphalt mixture was consistent with that in the orthogonal test, and the porous asphalt mixture was not subjected to aging. According to the results analysis of orthogonal test, as the porosity increased from 16.2% to 23.8%, the maximum bending strain decreased by 3.3% only. To attain higher statistical confidence, in the single factor influence test of porosity, on the basis of the orthogonal test results, another two groups of specimens with porosity of 18.4% and 26.3% were added. The five groups of specimens were subjected to low temperature bending test at −10 ◦C. With reference to the test results, the relationship between the maximum bending strain *εB* (in με) and the porosity *n*0 (in %) was obtained, as depicted in Figure 10.

**Figure 10.** Relationship between maximum bending strain and porosity.

It is evident from Figure 10 that there exists an approximately linear correlation between the maximum bending strain and the porosity of porous asphalt mixture, and the maximum bending strain decreases as the porosity increases. The relationship between the maximum bending strain and the porosity is expressed in Equation (2) with coefficient of determination *R*<sup>2</sup> = 0.986.

$$
\varepsilon\_B = 7527.7 - 100.9n\_0 \quad (16.2 \le n\_0 \le 26.3) \tag{2}
$$

when designing the mix proportion of porous asphalt mixture, in order to ensure good working performance of porous asphalt mixture in low temperature environment, the porosity should also conform to the required limits of the maximum bending strain in relevant technical standards. In the cold regions of northern China, when applying porous asphalt pavement, the maximum bending strain of porous asphalt mixture should not be less than 5000 με in accordance with JTG F40-2004 [50]. From Equation (2), it is noted that when the porosity is controlled within 25%, thus the limit requirement on maximum bending strain is satisfied.

Finally, the possible use of recycled asphalt pavement to enhance the sustainability of highway infrastructure has attracted vast research interest in recent years [59–61]. Recycled asphalt mixtures, which are prepared by blending reclaimed asphalt pavement, virgin bitumen, and mineral additives, offer various advantages including resources recycling, cost saving, and reduced environmental impact [62]. Further research on the utilization of reclaimed asphalt in porous asphalt pavements and their low temperature performance is recommended.
