DETERMINATION OF DESIGN PARAMETERS OF ASPHALT PAVEMENT BASED ON PG TECHNOLOGY

The design parameters are one of the important factors to ensure the quality of asphalt pavement design. In “Highway Asphalt Pavement Design Specification” (JTGD50-2017), the stander of China, used the asphalt mixture anti-pressure resilience modulus at a single temperature of 20 ℃ as the design metrics. However, asphalt mixture, as a sticky-bullet plastic material, shows different mechanical properties at different temperatures. China is a vast territory, and there are great differences between the high and low temperature value (m and n) of each region. Therefore, it is unreasonable to design asphalt pavement only with the asphalt mixture anti-pressure resilience modulus value at 20 ℃ . Studies show that the design parameters using PG technology can improve the high temperature anti-rutting and low temperature cracking performance of asphalt pavement.


INTRODUCTION
Some modern asphalt pavement designs (for example, in China) use the asphalt mixture's anti-pressure resilience modulus (20°C) as a criterion. The anti-pressure resilience modulus is one of the most important parameters for the mechanical properties of pavements [1][2]. A reasonable pavement thickness can be calculated from the anti-pressure resilience modulus [3]. The index of anti-pressure resilience modulus can also be used to evaluate the long-term service performance of asphalt pavements in terms of fatigue life [4], permanent deformation [5] and thermal cracking [6]. The anti-pressure rebound modulus of asphalt mixtures is therefore widely used as a parameter in the design of pavements. For example, Elliott R P et al. in the 1986 AASHTO Guide to Structural Design of Pavements stated that modulus of elasticity was one of the basic and reasonable indicators to be included in pavement design [7] Maher A et al. also used parameters such as dynamic modulus, elastic modulus and Poisson's ratio for flexible pavement design [8]; The effect of different factors on pavement performance was investigated by Su N et al. The study showed that the modulus of elasticity, load and temperature have a large effect on the variation in the performance of asphalt pavements. [9] According to Hossain Z et al., the modulus of elasticity is an important design element in the pavement design process. [10] The design parameters at a single temperature, on the other hand, are not reflective of the mechanical characteristics of the asphalt mixture at varied temperatures. This is due to the fact that both asphalt and asphalt mixtures have viscoelastic material qualities. [11][12] The material's mechanical characteristics change with temperature [13][14], resulting in totally distinct states and moduli at high and low temperatures [15][16]. The pavement performance of asphalt mixtures is clearly influenced by temperature. Therefore, in order to avoid the influence of temperature changes on asphalt pavement design, many countries and regions have started to adopt PG technology for asphalt pavement design. Based on the PG rated climate zones for road asphalt pavements, the high and low temperature design parameter values for each region are selected for road design. For example: Salem H A et al. found that the PG method was more reasonable for the design of Libyan desert roads and classified Libyan desert roads as PG70-10, PG76-10 and PG82-10. [17] Tan Y et al. used digital image correlation (DIC) analysis to obtain design parameters capable of characterizing the low-temperature performance of asphalt mixtures in cold regions, based on the theory of PG techniques. And they found that the low temperature performance of asphalt mixes was better when designed using this method. [18] Copeland A et al. also used PG technology to design the Florida Highly Recycled Asphalt Pavement -Warm Mix Asphalt project. [19] It can therefore be seen that the use of PG technology for asphalt pavement design allows for a range of high and low temperature variations in asphalt pavement design compared to the use of indicators such as anti-pressure resilience modulus at a single temperature for asphalt pavement design. This avoids the impact of temperature changes on the design of the pavement.
This study is an exploratory study of the design method for asphalt pavements with PG technology in the USA, based on the standard design method for road asphalt pavement design with parameters such as anti-pressure rebound modulus. On the basis of the climatic zones of China and the PG classification of asphalt pavements, asphalt pavement design parameters corresponding to the high and low temperature values (m and n) for each climatic zone were established. Refinement of asphalt pavement design parameters determined by a single temperature of 20°C in China. Design parameters using PG technology can improve the high temperature rutting resistance and low temperature cracking resistance of asphalt pavements.

Asphalt mixture design
The optimal asphalt content of different asphalt mixtures is obtained through the Mashall mix ratio design ( Table 3). The static pressure method and the compaction method were used to form the test specimens (100 mm in diameter and 160 mm in height), and two ends of the two test specimens were cut by 30 mm to form a standard test specimen. Then the compressive resilience modulus of the two test pieces was tested in the temperature range of -30 ℃ -60 ℃.

RELATIONSHIP BETWEEN DYNAMIC MODULUS AND STATIC MODULUS OF ASPHALT MIXTURE
The "Testing Rules for Asphalt and Asphalt Mixtures of Highway Engineering" (JTG E20-2011) stipulates that the specimens used in the single-axis compression anti-pressure resilience molding of asphalt mixtures shall be formed by static pressure method or by using a core drill from the molding plate. The "Specifications for Design of Highway Asphalt Pavements" (JTG D50-2017) stipulates that test specimens formed by impact compaction, static compaction, vibration compaction and other methods can be used to perform dynamic compression resilience modulus tests of asphalt mixtures.

Relationship between static compressive resilience modulus of asphalt mixture and temperature
The test results are shown in Figure 1. The regression relationship and suggested values at different temperatures were obtained by fitting the curve of the relationship between the static modulus and the temperature (Table 4).

Establishment of the relationship between dynamic modulus and static modulus of asphalt mixture
Studies have shown that the dynamic modulus of asphalt mixtures is approximately two to three times the static modulus. The resilience modulus value of asphalt mixture is greatly affected by temperature and loading frequency. Under different temperatures and loading frequencies it will be several times different. There is usually a large multiple relationship between the dynamic and static modulus at low temperature and high frequency, but the difference is not large at room temperature and low frequency; and a slightly larger at the general temperature and frequency compared with room temperature and low frequency. Different loading frequencies represent vehicles of different speeds on the road. The speed range specified by the "Technical Standards for Highway Engineering" of China is 30-120 Km/ h. China's current stander "Code for Design of Highway Asphalt Pavements" (JTG D50-2017) stipulates that the loading frequency of the asphalt surface layer modulus is 10Hz (translates to a driving speed of approximately 67Km/ h), which can truly reflect the most actual speed of road vehicles in China.
Through experiments, this study found that the main factors affecting the resilience modulus of asphalt mixtures are temperature and frequency, while the gradation, the amount of asphalt, and the type of asphalt have little effect. And there are different multiples gap between the dynamic modulus and the static modulus of the asphalt mixture at different temperatures and loading frequencies. Therefore, this study mainly considers the two influencing factors of temperature and frequency. Based on the "Code for Design of Highway Asphalt Pavements" (JTG D50-2017), the relationship between the dynamic and static modulus of asphalt mixtures at 10 Hz and different temperatures is established. It can be found in Figure 1 that the decrease rate of the resilience mode value is larger at -30-0 ℃. In this temperature interval, the resilience mode value of 70# asphalt mixes with three grades decreased by an average of 7266.67Mpa. The resilience mode value of SBS modified asphalt decreased by 9666.81Mpa on average. The resilience mode value of 90# matrix asphalt mix decreased by 6783.33Mpa on average, The resilience mode value of SBS modified asphalt mix (90#) decreased by 9133.46Mpa on average. But the rate is reduced at 0-30 ℃, and the change of the modulus value slows down at 30-60 ℃.Compared with 0°C, the resilience mode value of the four asphalt mixes with different grades at 30°C decreased by 2373.41 MPa, 1675.21 MPa, 2276.69 MPa, and 1800.04 MPa on average, respectively. The average variation of the resilience mode value is lower in the 30-60 °C range. The slope change curve is shown in Figure  2. Therefore, this study divided the range of -30-60 ℃ into three temperature ranges of -30-0 ℃, 0-30 ℃, and 30-60 ℃. And the three temperatures of -20 ℃, 20 ℃ and 40 ℃ were taken as the representative to obtain the multiple relationships between the dynamic and static modulus. Because the impact of gradation and asphalt type on the resilience modulus value of asphalt mixtures was not obvious, the average values of the static modulus of different grades and asphalt types at -20 ℃, 20 ℃, and 40 ℃ were used as modulus value at this temperature, respectively (Table 5). Relevant research shows that the dynamic modulus of the asphalt mixture obtained at 10 Hz loading frequency and 20 ℃ is 16507 Mpa [20], which is 10.  It can be seen from Tables 5 and 7 that the dynamic compression resilience modulus is 5 to 10 times the static modulus, and the suggested value for the dynamic compression resilience modulus is 5-8 times. In this study, the top surface method was be used to test the spring deformation of the specimen at all heights, so the obtained resilience modulus value will be small. In order to obtain a more accurate relationship between dynamic and static modulus, combined with domestic and foreign research results, standard recommended values, measured data of this study, etc., it is concluded that that there is 4 times, 5 times, 7 times of the relationship between asphalt mixture dynamics and static modulus in the three temperature ranges (-30-0 ℃, 0-30 ℃ and 40-60 ℃), respectively.

Conversion of static-dynamic anti-pressure resilience modulus of asphalt mixture
The static modulus of asphalt mixture is converted to dynamic modulus according to the resulting multiple relationship in different temperature intervals. The conversion results are shown in Table 7. Through the conversion, it can be found that the modulus value of asphalt mixture at 20 ℃ is in the range of 6670-9607 MPa, which is in the dynamic modulus value of the new specification recommends at the loading frequency of 10Hz and 20 ℃. This shows that the conversion relationship between the static module of asphalt mixture and the dynamic modulus is reasonable.

Relationship between dynamic resilience modulus and temperature of asphalt mixture
The relationship between the dynamic anti-pressure resilience modulus and the temperature regression of asphalt mixture is shown in Table 8.

Relationship between splitting strength of asphalt mixture and temperature
The relationship between the asphalt mixture splitting strength and temperature is shown in Figure 2. According to the relationship curve fitting of Figure 3, the regression relationship formula and the suggested values at different temperatures (Table 9) are obtained.

DETERMINATION OF ASPHALT PAVEMENT DESIGN PARAMETERS BASED ON PG TECHNOLOGY (HEILONGJIANG PROVINCE, CHINA)
Based on the asphalt pavement design specification, the dynamic rebound modulus and the relationship between crack strength and temperature of asphalt mixtures, as well as the classification of PGm-n grades of asphalt pavements, were investigated by combining the PGm-n technology while retaining the design parameters of the specification. Based on the results of the previous study, this paper takes asphalt pavement in Heilongjiang Province, China, as an example to verify the validity of the results of the previous study.

Asphalt pavement PGm-n grade climate partition
Through the PGm-n grade analysis of asphalt pavement climate zoning in Heilongjiang Province, the PGm-n grade and the corresponding representative city (Table 10) are obtained. Based on the above research data, the design parameters of asphalt pavement in Heilongjiang province based on PG technology (Table 11 and (1) The resilience modulus and splitting strength of the asphalt mixture decrease with increasing temperature. When the temperature is between -30-0 ℃, these indicators are maintained at a high level; when the temperature is between 0-60 ℃, the mechanical index values are small and when the temperature is between -30-30 ℃, the drop of asphalt mixture indicators is large. In the range of 30-60 ℃, the downward trend of the mechanical index of the same class and different combination material types slows down, and the change of the mechanical index of asphalt mixture is not obvious.

CONCLUSIONS
(2) The asphalt mixture dynamics and static modulus have differences in terms of moulding method and loading method, and the test specimen preparation method has unevenness between static pressure and hard-hitting. Asphalt blend springing modulus is greatly influenced by temperature and loading frequency, and there are different multiplier relationships between dynamic and static module at different temperatures and loading frequencies. By dividing the entire temperature range (-30-60 ℃), combined with test data, domestic and foreign research results and specification recommendations, this study obtained 4 times, 5 times and 7 times relationship between dynamic and static modules in different temperature ranges (-30-0 ℃, 0-30 ℃ and 40-60 ℃), respectively. (3) There is an exponential function between the mechanical specifications of the asphalt mix and the temperature. According to the relationship between the indicators and temperature and the PGm-n grade of the asphalt pavement climate zone in China, the proposed value of asphalt pavement design parameters is established. The results of the study are of some theoretical and practical value to the design of asphalt pavement.