Keywords

1 Introduction

“Carbon peaking and carbon neutrality” is an important measure to address global climate change and promote sustainable development. We are committed to achieving this goal by accelerating the shift to renewable energy and promoting low-carbon transportation [1, 2]. The recycling and utilization technology of old road surfaces is one of the effective measures for energy conservation and emission reduction widely used in the process of highway construction both domestically and internationally. It improves the reuse rate of mineral resources and protects the development of new mineral resources. The high-value utilization of asphalt milling and recycled materials (RAP) for old road surfaces is still in the exploratory and experimental stage [3,4,5,6,7]. At present, high content hot recycled asphalt mixtures face challenges such as poor gradation stability and heating difficulties. The warm mix high content plant mix hot recycling technology is currently the mainstream research direction, with the majority being the addition of warm mix agents and mechanical foaming [8,9,10,11]. Research has shown that the high-temperature performance of hot recycled asphalt mixture increases with the increase of RAP content, but the upper limit of RAP content is unknown [12,13,14]. Due to the fact that hot recycled asphalt mixtures are mainly used in the middle and lower layers of pavement structures, this article uses AC-25C type mixtures to study the high-temperature performance of hot recycled asphalt mixtures with different RAP dosages. The high-temperature performance is evaluated through rutting tests and uniaxial penetration strength tests.

2 Raw Materials and Mix Design

2.1 Raw Materials

Due to the good high-temperature performance of modified asphalt, in order to reduce the impact of new asphalt on the performance of recycled mixtures, Qilu Petrochemical 70-A grade road petroleum asphalt was used in this study; Select typical limestone as raw materials: 20-30 mm, 10-20 mm, 5-10 mm, 3-5 mm, 0-3 mm; RAP uses a certain highway to recover old road surfaces. Considering the stability of the high content recycled mixture gradation, RAP is broken into three specifications: 10-20 mm, 5-10 mm, and 0-5 mm. The aggregates, asphalt, mineral powder, and other materials used in this study have been tested to meet the relevant technical requirements of the JTG F40–2004 specification. The RAP asphalt content from coarse to fine is 3.0%, 3.4%, and 7.1%, respectively.

2.2 Mix Ratio

The Marshall design method was used for the mix proportion design of the mixture used in this study. In order to achieve the high value utilization of RAP, in the mix design process, the amount of milling and planing materials in each stage should be as close as possible to the proportion of each stage in the secondary crushing and screening, to avoid excessive or insufficient situations. The content of milling material exceeds 30%, and the performance of old asphalt in the milling material is improved by adding a regenerant. The regenerant content accounts for 6% of the total asphalt content. During the mixing process, the regenerant and milling material are mixed first, and new aggregates are added for mixing to extend the dry mixing time. The wet mixing time for spraying asphalt should also be extended. This study compared the mix proportions of 25%, 40%, and 60% with 0%, as shown in Table 1. The grading curve is shown in Fig. 1, and the test specimens were formed according to the above ratio for experimentation. The experimental results are shown in Table 2.

(1) Due to the aging of RAP asphalt, the optimal asphalt content of plant mixed hot recycled asphalt mixture is usually higher than that of conventional asphalt mixture;

(2) With the increase of milling material content, the stability of asphalt mixture gradually decreases and the flow value gradually increases. The asphalt mastic content in hot recycled asphalt mixture is relatively high compared to conventional mixture, and the limited heating temperature of milling material results in the mixture not being able to achieve the uniformity of conventional mixture, resulting in strong rheological properties of recycled asphalt mixture, and the specimen still maintains a certain cohesion after instability.

Table 1. Mix proportions of hot recycled asphalt mixtures with different dosages
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Fig. 1.
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Grading Curve of Hot Regenerated Asphalt Mixture with Different Dosages

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Table 2. Volume Index of Hot Regenerated Asphalt Mixtures with Different Dosages
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3 High Temperature Performance Evaluation

3.1 High Temperature Anti Rutting Performance

This article uses rutting test to study the high-temperature resistance to rutting performance of asphalt mixtures. The rutting test is the most commonly used test method for evaluating the high-temperature performance of asphalt mixtures in China. The rutting test mainly studies the deformation of 45–60 min of rutting, analyzes the relationship between deformation and rolling times, and uses correlation equations to compare the rutting performance (Fig. 2).

Fig. 2.
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Rutting deformation of mixtures with different dosages (45–60 min)

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Table 3. Rutting Test Results of Mixtures with Different Admixtures
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From Table 3, it can be seen that:

(1) The rutting deformation of plant mixed hot recycled asphalt mixture and conventional asphalt mixture tends to increase linearly within the range of 45–60 min, and the correlation R2 reaches above 0.99;

(2) According to the dynamic stability rutting deformation Eq. (45–60 min) of different types of mixtures, it can be seen that with the increase of RAP content, the k value first increases and then decreases, and the dynamic stability of 40% mixture reaches its highest. After the fusion of old asphalt and new asphalt in the milling material, it is equivalent to reducing the asphalt grade. When the content is 25%, the effect is not significant. When the content is 40%, the dynamic stability increases exponentially. When the content exceeds 60%, the dispersion uniformity of the milling material in the mixture decreases, resulting in a deviation between the actual grading and the theoretical grading, and a decrease in dynamic stability.

3.2 High Temperature Shear Failure Resistance

This article uses uniaxial penetration test to study the high-temperature shear failure performance, which was first proposed by renowned domestic road expert Professor Sun Lijun. During the test, the stress distribution of the test piece is consistent with that of the road surface. The data obtained from the uniaxial penetration test to determine the shear strength of the test piece is more practical. In the analysis of this article, the extreme point of axial pressure is used as the calculation point for the failure strength of the specimen.

Fig. 3.
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Axial deformation pressure curve of uniaxial penetration test

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The uniaxial penetration test specimens of asphalt mixtures have gone through five stages, including compaction stage (OA), elastic stage (AB), damage stage (BC), stress retention stage (CD), and complete failure (after point D) (Fig. 3). Due to the relatively high content of asphalt mastic in recycled asphalt mixture and the high cohesion C, there was no significant decrease in stress after specimen damage, and the mixture exhibited rheological characteristics.

Table 4. Rutting deformation of different admixtures (45–60 min)
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Fig. 4.
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Axial deformation pressure curve of uniaxial penetration test for mixtures with different dosages

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With the increase of RAP content, the penetration strength of asphalt mixture first increases and then decreases. The shear strength of mixture with 40% content increases most significantly. When the RAP content reaches 60%, the penetration strength decreases rapidly. The penetration test results are consistent with the rutting test results. Although the penetration strength increases with the addition of RAP, the extreme penetration depth continues to increase (Table 4).

From Fig. 4, it can be seen that: The compression deformation stage of asphalt mixture without RAP addition is very short, and the damage stage is closely connected with the cracking and failure stage, without obvious stress retention stage. After adding milling materials, the mixture exhibits obvious compression deformation, and the variation of compression deformation with the dosage is consistent with the penetration strength; as the dosage increases, the cohesion C of the mixture increases, and the stress holding stage and axial deformation of the asphalt mixture are prolonged. The axial deformation is consistent with the results of the Marshall flow test.

4 Conclusion

By conducting rutting tests and uniaxial permeability tests, 25%, 40%, and 60% RAP are added to the AC-25C asphalt mixture to comprehensively evaluate the high-temperature performance of the recycled mixture. Based on the results and discussions presented above, the conclusions are obtained as below:

(1) Rutting tests and uniaxial penetration tests show that the addition of RAP asphalt mixture can significantly improve its high-temperature stability and shear strength, but when the RAP content exceeds 40%, there is a significant downward trend, but it is still better than the asphalt mixture without RAP. When the milling material content exceeds 60%, the dynamic stability of the mixture does not meet the technical requirements of JTG F40–2004.

(2) Due to the presence of milling materials, the cohesive force C of hot recycled asphalt mixture increases, and the specimen still maintains a certain strength after instability. As the penetration depth continues to increase, the higher the RAP content, the longer the maintenance time of the mixture's penetration strength. Therefore, the uniaxial penetration test of hot recycled asphalt mixture is different from conventional mixture, which is divided into compaction stage, elastic stage, and damage stage. There are five stages: stress maintenance stage and complete failure stage.

(3) The problem faced by high-content hot recycled asphalt mixture is gradation stability. The milling process, secondary crushing and grading, storage, and other refined pre-treatment of the milling material are the foundation of RAP's high-value application, and ensuring the stability of the mixture gradation is the key.

From the perspective of high-temperature stability and shear strength, the optimal dosage of RAP is 40%. To ensure the grading stability of high-content RAP mixtures, it is crucial to study the RAP crushing process, which is also a prerequisite for the high-value utilization of RAP. Factory mixed hot regeneration technology can be vigorously promoted in future road maintenance.

Shortcomings and Prospects: This study only evaluates the high-temperature performance of hot recycled asphalt mixtures with different dosages. In the next step of research, the durability of the mixture can be comprehensively evaluated through its water resistance, low-temperature performance, dynamic modulus, and other properties, in order to further promote the high-value utilization of plant mixed hot recycled asphalt mixtures.