Study on road performance of asphalt mixture based on POE/PA6 modifier

A POE/PA6 modifier with increased reactivity was prepared using the twin-screw extrusion granulation process and added to the asphalt mixture. The effect of the POE/PA6 modifier on high-temperature stability, low-temperature anti-cracking, and water stability of asphalt mixture was studied. The results show that compared with the asphalt mixture mixed with recycled PE, the asphalt mixture mixed with POE/PA6 has nearly four times the rutting resistance of the former at 70°C and 1.0 MPa, the fracture energy and flexibility index are twice the former at -10°C, and the maximum bending strain and water stability are also significantly improved. This modifier shows a good engineering application prospect in highway construction and maintenance under high temperatures and heavy load traffic environments.


Introduction
In recent years, through the large-scale application of polymer materials such as SBS and recycled PE in highway maintenance, the service life of ordinary national and provincial arterial highway asphalt pavement in China has been greatly extended, and the quality of highway maintenance has been significantly improved.However, with the rapid economic and social development, heavy highway traffic is becoming more common.The early damage phenomenon dominated by asphalt pavement ruts and cracks is still prominent [1] , especially in the long longitudinal slope, heavy lanes, intersections, bus lanes, toll station squares, and other sections under heavy traffic conditions.Conventional treatment solutions are difficult to solve this industry problem fundamentally.Some areas try to use semi-flexible grouting road surfaces, epoxy asphalt, and other technical solutions but also face the problems of complex construction technology, long closed traffic time, and high project costs.
As an engineering plastic with excellent performance, PA6 has many applications, featuring high strength, wear, oil, and chemical resistance [2] .Still, it also has problems such as high brittleness at low temperatures and poor compatibility with non-polar materials [3] .Ethylene-octene copolymer (POE) is widely used in wire, cable, daily necessities, and other industries because of its high elasticity and good processing properties [4][5][6] .Toughening of PA6 with POE can improve the comprehensive properties of PA6 [7] , but no relevant studies on POE/PA6 in asphalt mixtures have been seen.Therefore, the author prepared a POE/PA6 modifier with increased reactivity and added it to the asphalt mixture to investigate its influence on comprehensive road performance and provide information for asphalt pavement maintenance under heavy traffic.

Preparation of POE/PA6 modifier
The PA6, capacity enhancing POE, antioxidant, and other additives used in this paper are commercially available products.Firstly, all kinds of raw materials are mixed and dispersed according to the set proportion, and the capacity enhancing POE occupies 25% of the mass of PA6.Then, the premix is extruded and granulated through the twin screw extruder, and the particle size of the material is kept below 5 mm, and it is used after drying.

Preparation of asphalt mixture
The bitumen used in this experiment is grade A 70# petroleum bitumen, and the mineral material used is diabase of 10-15 mm and 5-10 mm produced in Shanxi, limestone of 0-5 mm, and mineral powder.The test results of raw materials all meet the requirements of relevant regulations.The grading composition and curve of the mineral material are shown in Table 1 and Figure 1.
Table 1.The pass rate of each sieve for AC-13 gradation.
Sieve size (mm) 16 13.Mineral grade curve of asphalt mixture.Marshall design method confirmed the optimum asphalt-stone ratio of the AC-13 asphalt mixture.The forming conditions of the specimen are as follows.Mixing temperature is 175℃-185℃, compaction molding temperature is 170℃-180℃, and double-sided co MPaction is 75 times.The mixing process involves adding the coarse and fine aggregates along with a PA6/POE modifier, which accounts for 0.5% of the total ore mass, into the mixing pot and mixing for 90 seconds.The measured petroleum asphalt mixing 90 s, and finally, the mineral powder mixing 90 s.The experimental results of asphalt mixture are shown in Table 2, and 5.4% is finally adopted as the optimal asphalt-stone ratio of the AC-13 asphalt mixture.

Performance test
To fully appraise the road performance of asphalt mixture with POE/PA6, in this study, the performance of the mixture was compared with that of the recovered PE asphalt mixture with the same content and tested the stability, anti-cracking, and water stability of asphalt mixture according to the optimal asphalt-stone ratio determined above.

High-temperature stability
Marshall stability and dynamic stability tests were carried out on the above two mixtures to characterize the anti-rutting performance of the asphalt mixture at high temperatures, and a specific experimental method was referred to the relevant requirements in JTG E20-2011.The dynamic stability test adopts two different health temperatures and test pressures, 60℃, 0.7 MPa, and 70℃, 1.0 MPa, respectively.The former simulates general temperature and traffic load conditions, while the latter simulates high temperature and overloading environment.The test results are shown in Table 3.As seen from Table 3, compared with the matrix asphalt mixture, the stability and dynamic stability of the mixture after the addition of recycled PE have been greatly improved.The shear strength of the mixture has been significantly enhanced, while the addition of the POE/PA6 modifier has a more significant improvement in the stability and dynamic stability indicators.It is worth mentioning that the asphalt mixture with recycled PE shows excellent rutting resistance at 60℃ and 0.7 MPa.Still, the performance decreases significantly at 70℃ and 1.0 MPa.The matrix asphalt mixture cannot measure the test result because the result is too small, which is also the main reason it cannot meet the high temperature and heavy load environment.There are two possible reasons why the asphalt mixture with POE/PA6 modifier has excellent high-temperature rutting resistance.On the one hand, reactivity enhancement POE contains active groups such as epoxide, which can react chemically with the amine or imine group of the nylon molecular chain during the melt extrusion blending process.Thus, the interface binding force between the two phases is enhanced [8][9] .On the other hand, the epoxy group can also react with the hydroxyl and carboxyl groups in the asphalt and hydroxyl groups on the aggregate, resulting in the asphalt mixture forming a mixture with a certain spatial network structure.The possible chemical reaction is shown in Figure 2.

Low-temperature anti-cracking
The low-temperature anti-cracking of the mixture was evaluated by using a three-point beam test at -10℃ and a semi-circular bending test at -10℃.The test methods were referred to the relevant provisions in JTG E20-2011 and AASHTO TP124-2016.The results of the three-point beam test are shown in Table 4.
Table 4.The results of the three-point beam test at -10℃.It can be seen from the above table that the fracture deflection and the maximum flexural strain of the asphalt mixture show the same variation law.The greater the fracture deflection is, the greater the bending strain is.The PA6/POE modifier mixture showed the best fracture deflection and bending strain.In contrast, the mixture with recovered PE showed a decrease in fracture deflection and maximum flexural strain compared with the matrix asphalt mixture.

Additive type
The semi-circular bending experimental data and curve of three different mixtures at -10℃ are shown in Figure 3 and Table 5.As can be seen from Figure 3 and Table 5, compared with the base asphalt mixture and the mixture mixed with recycled PE, the asphalt mixture mixed with the PA6/POE modifier shows excellent performance in terms of peak load, peak displacement, fracture energy, and flexibility index.Compared with the base asphalt mixture and the mixture mixed with recycled PE, the peak load of the asphalt mixture mixed with PA6/POE increased by 53.6% and 27%, respectively, because the addition of PA6/POE raised the flexural strength of the mixture.As several important indicators of fracture toughness, the increase of peak displacement, fracture energy, and flexibility index is more obvious, showing excellent low-temperature cracking resistance.It can be seen that POE with increased reactivity has a better toughening effect on PA6.

Water stability
This paper used two indexes of soaking stability residual strength ratio and freeze-thaw splitting residual strength ratio of asphalt mixture to appraise the water stability of different mixture types.The former was obtained by placing asphalt mixture samples in a 60℃ water tank for 48 h and testing the stability ratio before and after placement.The latter is obtained by adding a certain amount of water to the specimen, maintaining it at -18℃ for 16 h, then immediately placing it in water at 60℃ for 24 h, and testing the ratio of splitting strength.The specific test method is referred to the relevant provisions of JTG E20-2011, and the test results are shown in Figure 4.The results in Figure 4 show that the residual stability of immersed water and the ratio of splitting strength of the three mixtures meet the technical requirement of the industry standard (no less than 80% and 75%, respectively) when compared with the first two mixtures, the water stability of the asphalt mixture mixed with PA6/POE modifier is significantly better, especially the ratio of splitting strength.The reason may be that a modifier of the reactive group and aggregate hydroxyl groups on the surface of the chemical reaction happens, greatly improving the interface adhesion of asphalt-aggregate and resistance to moisture damage to the asphalt mixture of the interface.Therefore, the mechanical strength of the asphalt mixture is not significantly reduced by water immersion, resulting in a greatly increased residual strength ratio.

Conclusion
(1) Due to the chemical reaction between the active group of POE and the amine or imide group of the PA6 molecular chain and the hydroxyl group on the surface of asphalt and aggregate, the shear strength of the mixture is greatly improved.
(2) The fracture energy and flexibility index of asphalt mixture mixed with POE/PA6 is significantly greater than that of base asphalt mixture and asphalt mixture mixed with recovered PE, and POE with increased reactivity has a better toughening effect on PA6.
(3) The active groups in the POE/PA6 modifier react chemically with the hydroxyl group on the aggregate surface, which greatly improves the interface bonding force of the asphalt aggregate, thus improving the water stability of the mixture.
(4) In summary, the addition of the POE/PA6 modifier to the asphalt mixture can effectively slow down the occurrence of ruts, cracks, pits, and other diseases on the asphalt pavement.It has a good engineering application prospect in the field of highway construction and maintenance under high temperatures and heavy load traffic environments.

Figure 2 .
Figure 2. Schematic diagram of possible chemical reactions.

Figure 3 .
Figure 3. Semi-circle bending curve of mixture mixed with different additives.Table5.The results of the semi-circular bending test at -10℃.

Figure 4 .
Figure 4. Water stability of asphalt mixtures with different additives.The results in Figure4show that the residual stability of immersed water and the ratio of splitting strength of the three mixtures meet the technical requirement of the industry standard (no less than 80% and 75%, respectively) when compared with the first two mixtures, the water stability of the asphalt mixture mixed with PA6/POE modifier is significantly better, especially the ratio of splitting strength.The reason may be that a modifier of the reactive group and aggregate hydroxyl groups on the surface of the chemical reaction happens, greatly improving the interface adhesion of asphalt-aggregate and resistance to moisture damage to the asphalt mixture of the interface.Therefore, the mechanical strength of the asphalt mixture is not significantly reduced by water immersion, resulting in a greatly increased residual strength ratio.

Table 2 .
Test results of AC-13 asphalt mixture.

Table 3 .
Test results of stability and dynamic stability of different asphalt mixtures.

Table 5 .
The results of the semi-circular bending test at -10℃.