Effects of Rejuvenator Dosage, Temperature, RAP Content and Rejuvenation Process on the Road Performance of Recycled Asphalt Mixture

Abstract

In this paper, the key technologies in the construction process of hot in-place recycling were investigated in order to improve the utilization rate of waste asphalt mixture; traditional lab tests including penetration, softening point and ductility tests, atomic force microscope test of recycled asphalt under different rejuvenator content, and the test of milling on grading at different temperatures were carried out. The influence of RAP content and rejuvenation processes on road performance were studied, and the low-temperature performance of mixture was analyzed by the energy analysis method, and the evaluation index was proposed. Test results indicated that the penetration and ductility increases, the softening point decrease with the rejuvenator content increasing, and the optimum rejuvenator content is 4%. The optimum mixing and compaction temperature will decrease by 2–6 °C on average for every 10% increase of RAP content by analyzing the mixture volume index. The results showed enhance rutting resistance of the mixture but lower moisture resistance and low-temperature crack resistance by adding the RAP content. The strain energy density of 10 KJ/m³ is proposed to evaluate the low-temperature performance of the mixture, and 30% RAP produces optimal mixture. The higher rutting resistance and moisture resistance can be obtained by using the construction process of RAP+ rejuvenator co-heating, and higher low-temperature crack resistance with RAP+ rejuvenator without heating.

1. Introduction

In recent years, with the rapid development of highways, particularly of asphalt pavement, older highways have gradually entered the maintenance period. A large number of pavement maintenance and repair work has resulted in such problems as the increase of reclaimed asphalt pavement (RAP). Especially, a large amount of waste asphalt pavement materials, if not timely recycled, not only take up a lot of land and is a waste of resources, but also pollute the environment [1,2,3]. Studies have shown that the reuse of RAP was significant in saving resources, protecting the environment and achieving sustainable development of the construction industry [4,5,6,7,8].

Generally, the asphalt pavement recycling technology is divided into four types, namely hot central plant recycling, hot in-place recycling, cold central plant recycling and cold in-place recycling. Among them, the hot in-place recycling technology has the advantages of rapid construction, maximum recycling of RAP, saving transportation costs, and benefiting interlayer bonding, which is of great significance to the development of environment-friendly highway maintenance technology [9,10,11,12,13].

Recently, extensive studies focused on the performance characterization and microscopic mechanism of recycled asphalt mixtures with different types or dosage of rejuvenator. The results showed that the content of the regenerating agent, the type of regenerating agent and the production technology all had influence on the properties of asphalt mixture. Ding, L.T. et al. [14] investigated the morphology and property changes of the aged asphalt with a rejuvenator. The results indicated that a rejuvenator was conducive to microstructure change and high temperature rheological properties. Yu, J. et al. [15] comparatively analyzed the rejuvenator effect of artificial aged asphalt with soft bitumen, liquid surfactant and bio-rejuvenator. The results indicated that the bio-rejuvenator performed better with additional content of artificial RAP asphalt. Ma, Y.T. et al. [16] presented the adding procedure of a rejuvenator would influence rheological and aging characteristics of aged asphalt. Yaseen, G. et al., Chen, A.Q. et al. and Bilema, M. et al. [17,18,19] surveyed the influence of rejuvenator content on penetration, softening point and ductility of recycled asphalt. Researchers investigated the microscopic mechanism of aged asphalt with rejuvenator, and it was found that the composition of aged asphalt changed by the rejuvenator, the light components decreased and the micelle number and proportion in the colloidal structure increased [20,21]. Based on the results of these analyses, there is a consensus that a rejuvenator can effectively control and reduce the performance loss of recycled asphalt after aging and change the colloidal structure of aged asphalt [22].

In addition, it was observed that appropriate RAP content has a significant effect on the performance of hot recycled asphalt mixture [23,24,25,26,27]. Bilema, M. et al. [28] concluded that this enhanced stiffness and rutting resistance of the RAP but lowered moisture resistance, and the pavement sustainability and rutting performance of RAP could be restored by adding WFO and CR. Yin, P. et al. [29] found that with the addition of RAP, high-temperature performance of HRAM increased and the low-temperature performance and moisture susceptibility decreased. Zhu, J.Q. et al. [30] surveyed high modulus asphalt mixture containing RAP, and it was found that the dynamic modulus values and moisture damage susceptibility of asphalt mixture were optimal with 40% RAP. Based on the results of vehicle load simulator, Zaumanis, M. et al. [31] demonstrated that the crack resistance of recycled high modulus asphalt mixture was deficiency when RAP content was 100%. Ma, Y.T. et al. [32] revealed that mixtures heated with a higher temperature had better cracking and moisture resistance performances. Li, X.L. et al. [33] indicated that the heating temperature of RAP and mixing time could improve the homogeneity of RAP mixture. Furthermore, the rejuvenation process was taken into considered as key factor of performance of hot in-place recycling asphalt mixtures. Xie, Z.X. et al. [34] investigated the effect of rejuvenator types and mixing procedures on volumetric properties of mixtures. The results indicated that some volumetric properties of mixtures changed under different mixing procedures and rejuvenator types. Lei, Y. et al. and Lei, Y. et al. [35,36] investigated that the high temperature performance, compaction property and workability have obvious differences with different mixing sequences.

Overall, the rejuvenator, temperature, RAP content and rejuvenation process are all crucial factors to achieve the highest performance of the hot in-place recycling asphalt mixtures [37]. However, most studies mainly focused on the effect of the rejuvenator on aged asphalt and the effect of RAP content on aged asphalt mixture, lacking research on the performance of recycled asphalt mixture systematic research. In this paper, the optimum dosage of rejuvenator was studied first, and then the optimum mixing and compaction temperature was determined based on four kinds of hot recycled asphalt mixture with different RAP content. Furthermore, the influence of different RAP content and rejuvenation process on the performance of the mixture was discussed. The research results were of great significance to improve the construction quality of hot in-place recycling.

2. Materials and Methods

2.1. Materials

The materials in this study included asphalt extraction from old road milling materials, No. 90 virgin asphalt, one type of permeable rejuvenator, and reclaimed asphalt pavement (RAP). Aged asphalt and RAP were provided by a local old road in Henan, China. The permeable rejuvenator was LT-2, independently developed by a company in Wuhan, Hubei, China. The test methods were carried out in accordance with Chinese standard JTG E20-2011 [38] and the properties of permeable rejuvenator are shown in

Table 1. Properties of permeable rejuvenator.

Property Indices	Test Results	Test Methods
Viscosity at 60 °C (mm2/s)	87	T0619-2011
Flash point (°C)	238	T0633-2011
Saturates (%)	7.3	T0618-1993
Aromatics (%)	69.5	T0618-1993
Viscosity ratios before and after RTFOT	2.3	T0610-2011 T0619-2011
Mass ratios before and after RTFOT	1.5	T0610-2011
15 °C Density (g/cm3)	0.94	T0603-2011
Appearance	Black viscous liquid

.

2.2. Testing Method

2.2.1. Optimum Dosage of Rejuvenator Test

• Traditional laboratory tests of asphalt

In this paper, the performance of recycled asphalt mixed with different rejuvenator ratio (0, 3%, 4%, 5%) was evaluated by penetration, softening point and ductility tests. One group, two groups and three groups of recycled asphalt specimens were formed for penetration, softening point and ductility tests for each rejuvenator content, respectively, where three test points were selected on the penetration sample. The process of testing specimen was carried out in accordance with Chinese standard JTG E20-2011 [38].

2.

Atomic force microscopy (AFM) test

The surface morphology of the recycled asphalt mixed with different rejuvenator ratio (0, 3%, 4%, 5%) was observed by NanoMan vs. AFM. The specimen was tested at a temperature of 25 °C and a relative humidity of 25%. The micro images in the range of 20 um × 20 um were presented by tapping mode at no less than 3 positions on each sample surface. Further, the data such as surface roughness were calculated by NanoScope Analysis software. The calculation formulas are:

$$R_{max}=h_{max}-h_{min}$$

(1)

$$R_q=\sqrt{\frac{{{\displaystyle \sum }}_1^N{Z}_i^2}{N}}$$

(2)

$$R_a=\frac{1}{N}{{\displaystyle \sum }}_{j=1}^N\left|Z_j\right|$$

(3)

where $R_{max}$ is the maximum vertical distance between a highest and a lower image data point in an image processed. $R_q$ and $R_a$ represent the roughness of the image, $R_q$ is the root mean square average of the adhesion deviation of the image data plane, $R_a$ represents the arithmetic average of the absolute value of the surface adhesion deviation measured from the average plane, and n is defined as the number of ‘bee structures’ in the image range. $h_{max}$ and $h_{min}$ are the maximum and minimum height in the atomic force microscope image, respectively, and $Z_i$ and $Z_j$ represent adhesion deviation.

2.2.2. Temperature Control Test

• Milling temperature control test

The environmental temperature, wind speed and solar radiation of the test site were tested by the automatic environmental monitoring system, and the surface temperature were tested by infrared thermometer. The relationship between various factors and heating temperature was analyzed, and the influence of temperature on the hot in-place rejuvenation was obtained. The gradation design of milling hot recycled asphalt mixture was carried out at 120 °C, 130 °C and 140 °C, and the influence of milling on the gradation of recycled asphalt mixture at different temperatures was analyzed.

2.

Optimum mixing and compaction temperature test

In this paper, the mixture gradations with each RAP content are the same mineral aggregate as the Standard median gradation. The optimum asphalt–aggregate ratio of each mixture was determined by the Marshall design method according to the Chinese standard JTG E20-2011 [38], which was 3.95%, 3.67%, 3.45% and 3.32% for the mixture with RAP content (30%, 40%, 50%, and 60%), respectively. For each RAP content, five groups of Marshall specimens were formed with 165 °C as the median change of mixing temperature and 145 °C as the median change of compaction temperature, respectively, with four parallel specimens in each group. The Marshall test was conducted on hot recycled asphalt mixture with RAP content of 30%, 40%, 50% and 60% by using the volume index control method. The Marshall test was conducted on hot recycled asphalt mixture with RAP content of 30%, 40%, 50% and 60% by using the volume index control method. The optimum mixing and compaction temperature was when the air void content was 4%.

2.2.3. Road Performance Test

In this paper, the effects of RAP content (30%, 40%, 50%, 60%) on high-temperature stability, moisture-induced damage and low-temperature crack resistance of hot recycled asphalt mixture were evaluated by rutting test, immersion Marshall test, freeze-thaw split test and low-temperature bending test. However, only the flexural tensile strength or flexural tensile strain determined in the low-temperature bending test was used as a comprehensive evaluation of the performance of recycled asphalt mixture under low-temperature conditions, which can only replace its unilateral flexural tensile strength. Therefore, the strain energy density was introduced from the perspective of energy as a comprehensive consideration to evaluate the performance index of recycled asphalt mixture under low-temperature conditions. The calculation formula is shown as Equation (4).

$$W_f=\frac{dW}{dV}={{\displaystyle \int }}_0^{{\epsilon }_0}{\sigma }_{ij}d{\epsilon }_{ij}$$

(4)

where $W_f$ is the strain energy density, ${\sigma }_{ij}$ and ${\epsilon }_{ij}$ are the stress and strain components, ${\epsilon }_0$ is the strain critical value.

2.2.4. Effect of Rejuvenation Process on Road Performance of Recycled Asphalt Mixture

In this paper, the recycled asphalt mixture with 30% RAP content were molded according to three different rejuvenation construction processes under the Optimum mixing and compaction temperature, and the road performance of the recycled asphalt mixture with different mixing processes were evaluated by the rutting test, freeze-thaw split test and low-temperature bending test. The different rejuvenation construction processes are shown in

Table 2. Different rejuvenation construction processes.

Number	Rejuvenation Construction Process
Process I	RAP Heating + Rejuvenator
Process II	RAP + Rejuvenator Co-heating
Process III	RAP +Rejuvenator not heating

.

3. Results and Discussion

3.1. Effect of Optimum Dosage of Rejuvenator

3.1.1. Rejuvenator Content

Table 3. Asphalt performance of different rejuvenator dosage.

Rejuvenator Dosage (%)	Penetration (25 °C, 0.01 mm)	Softening Point (°C)	Ductility (10 °C, cm)	Remark
0	24.6	70	8.7	Permeable Rejuvenator
3	30.8	66.3	12.2
4	35.5	64.2	15.6
5	41.2	60.9	18.7

show the penetration, softening point and ductility of different samples obtained from three index tests as a function of rejuvenator dosage. From Table 3, it can be concluded that the penetration and ductility of the recycled asphalt increases and the softening point decrease with increasing rejuvenator dosage. During the production process of long-term repair and application of asphalt binder, slight component loss, relative decrease of colloidal element content and significant increase of asphaltene content will occur, which will cause the increase of asphalt softening point and decrease of penetration and ductility, and the permeable rejuvenator has a protective effect.

3.1.2. Nano-Morphology of Asphalt Sample

The two-dimensional height scanning results of the samples are shown in Figure 1a–e. From Figure 1, it can be clearly seen that the asphalt samples with different rejuvenator dosage have a ‘bee structure’, and the microstructure of aged asphalt and the nano-morphology in the AFM image have also changed to varying degrees with the increase of rejuvenator dosage. Compared with the virgin asphalt, the aging asphalt ‘bee structure’ agglomeration phenomenon was more serious, the size was larger, the number was less, and as a whole showed a wide and rare phenomenon. With the increase of the rejuvenator dosage, the size of ‘bee structure’ decreased and the number increased, which tended to the microstructure of virgin asphalt, while there was still a small amount of large ‘bee structures’. During asphalt aging, the light components in asphalt would gradually decrease, and the proportion of asphaltene in asphalt would gradually increase. The existence of asphaltene was the main reason for the formation of ‘bee structure’, so the ‘bee structure’ of aging asphalt was more obvious than that of virgin asphalt. The composition of the aged asphalt was changed by the rejuvenator, which also reduced the relative content of the aged asphaltene, so that the original dense ‘bee structure’ development was more loose and smaller in size, closer to the virgin asphalt.

Figure 2a–e shows the fluctuation trend of ‘bee structure’ of asphalt samples with different rejuvenator dosage from a three−dimensional perspective. From Figure 2, it can be concluded that with the increase of the rejuvenator dosage, the coarse columnar structure of the aged asphalt gradually became finer and more abundant, and the surface roughness was obviously improved, which was closer to the virgin asphalt, indicating that the aged asphalt had a better rejuvenation effect.

The number, $R_{max}$, roughness and other parameters of ‘bee structure’ of asphalt samples with different rejuvenator dosage were analyzed, and the average value of each test result was taken. The statistical results are shown in

Table 4. The results of parameter analysis.

Sample Shape	Aged Asphalt	Asphalt Mixed with
		3% Rejuvenator	4% Rejuvenator	5% Rejuvenator	Virgin Asphalt
n	34	38	76	59	79
$R_{max}/\mathrm{nm}$	118	108	98.6	115	82.1
$R_q/\mathrm{nm}$	9.42	9.5	5.04	8.15	5.48
$R_a/\mathrm{nm}$	5.68	7.58	2.68	4.82	2.93

. It is concluded from Table 4 that with the increase of the rejuvenator dosage, roughness indexes $R_q$ and $R_a$ of recycled asphalt gradually decreased, and $R_{max}$ decreased first and then increased. Based on the changes of $R_q$, $R_a$ and $R_{max}$, it can be seen that the $R_{max}$ of recycled asphalt mixed with 4% rejuvenator was closer to that of virgin asphalt, indicating that the addition of recycled agent had caused a certain rejuvenation of aged asphalt. Combined with the three index test results, it can be obtained that the optimum content of rejuvenator was 4%.

3.2. Effect of Temperature Control

3.2.1. Milling Temperature Control

The relationship between ambient temperature, wind speed, solar radiation and surface temperature of structure layer is shown in Figure 3a–c. It can be concluded from Figure 3 that the ambient temperature, wind speed and solar radiation had a significant effect on the heating temperature. The surface temperature of the structural layer increased significantly with the increase of ambient temperature and solar radiation, and decreased with the increase of wind speed.

It is necessary to control a reasonable heating temperature during construction, so that a certain temperature gradient is formed on the road surface and the temperature can be effectively transferred for a long enough time, as shown in Figure 4.

The influence of milling on gradation at different temperatures is shown in Figure 5. It can be concluded from Figure 5 that when the pavement was milled under different temperature conditions, the gradation of the pavement after milling changed greatly. The lower the milling temperature, the more the gradation after milling deviates from the upper limit of gradation. The higher the milling temperature, the closer the gradation after milling was to the original gradation.

Figure 6 shows the effect of pavement milling at 120 °C. The temperature interferes in the breakdown of the old asphalt layer and in the dimensions of the “black aggregates” obtained. At low temperature, the aggregate milling crushing of asphalt pavement increases, and the aging of old asphalt decreases. At high temperature, it can effectively reduce the damage of milling to aggregate and increase the rate of old asphalt recovered, effectively ensure the uniformity of the grading of the original pavement.

3.2.2. Optimum Mixing and Compaction Temperature

The change of air void content of Marshall specimens under different RAP content, mixing and compaction temperature is shown in Figure 7a,b. The optimum mixing and compaction temperature test results of hot recycled asphalt mixture is shown in Figure 8. From Figure 7 and Figure 8, it can be concluded that optimum mixing and compaction temperature of hot recycled asphalt mixtures decreased with RAP content, when the content of RAP was 30%, 40%, 50% and 60%, the optimum mixing temperature was 175 °C, 170 °C, 165 °C and 155 °C, and the optimum compaction temperature was 152 °C, 150 °C, 144 °C and 139 °C; for every 10% increase in RAP content, the mixing and compaction temperature decreased by 2–6 °C.

The possible reason was that asphalt film has a remarkable bonding effect when high RAP is added to the hot in-place recycling technology, the aged asphalt in RAP was well bonded with the old aggregate, and the recycled asphalt formed after adding the rejuvenator and virgin asphalt was easy to be wrapped with the old aggregate and pressed into a dense structure; with the increase of RAP content, recycled asphalt needs lower heat to soften itself to improve fluidity under the bonding effect of asphalt film, so as to bond with new aggregate. The higher the RAP content, the lower the mixing and compaction temperature of hot recycled asphalt mixture.

3.3. Effect of RAP Content on Road Performance

3.3.1. High-Temperature Stability

The calculated results of the rutting test are shown in

Table 5. Dynamic stability (DS) with different RAP content.

RAP Content (%)	Mixing Temperature (°C)	Compaction Temperature (°C)	DS (Time mm−1)
			Average Value	Specification Requirement
30	175	152	2872	≥800
40	170	150	3216
50	165	144	3753
60	155	139	3989

. It shows that the dynamic stability (DS) of the hot recycled asphalt mixture met the technical requirements of 800 times/mm according to the Chinese standard JTG F40-2004 [39]. With the increase of RAP content, the dynamic stability of the hot recycled asphalt mixture in the rutting test continues to increase. When the RAP content increases from 30% to 60%, the dynamic stability increased by 38.9%. This was probably because the aged asphalt was softened under the coordination of the rejuvenator and the virgin asphalt, but its viscosity value was still higher than the virgin asphalt, thus improving the rutting resistance of the recycled pavement.

3.3.2. Moisture Susceptibility

The freeze-thaw splitting and immersion Marshall results of recycled asphalt mixtures with different RAP content are shown in

Table 6. Freeze-thaw splitting tensile strength ratio (TSR) and residual Marshall stability (MS₀) of recycled asphalt mixtures with different RAP content.

RAP Content (%)	Mixing Temperature (°C)	Compaction Temperature (°C)	TSR (%)		MS0 (%)
			Test Result	Specification Requirement	Test Result	Specification Requirement
30	175	152	89.6	≥75	87.6	≥80
40	170	150	85.9		86.6
50	165	144	79.3		83.1
60	155	139	76.7		81.3

. It shows that the freeze-thaw splitting tensile strength ratio (TSR) and residual Marshall stability (MS₀) of recycled asphalt mixtures all met the requirement of 75% and 80%. The moisture-induced damage of the hot recycled asphalt mixture decreased with the RAP content increases. It may be that the adhesion of the aged asphalt was reduced due to the addition of large amounts of aged asphalt raw materials, which led to the decrease of moisture-induced damage; when the RAP content reached 60%, the freeze-thaw splitting strength ratio and residual Marshall stability of recycled asphalt mixture were close to the specification value, which indicated that the excessive RAP content would reduce the asphalt cohesion and cause moisture damage, so the proportion of RAP should be controlled.

3.3.3. Low-Temperature Crack Resistance

The low-temperature bending results of recycled asphalt mixtures with different RAP content are shown in Figure 9. Figure 9 presents that with the increase of RAP content, the breaking strain decreased and the bending stiffness modulus increased, which indicated that the addition of RAP would adversely affect the low-temperature crack resistance. Based on the opposite change trend of breaking strain and bending stiffness modulus, the evaluation index of low temperature crack resistance of reclaimed asphalt mixture was the strain energy density of 10 KJ/m³.

3.4. Effect of Rejuvenation Process

The influence of 30% RAP content recycled asphalt mixture with different rejuvenation construction processes on the road performance of asphalt mixture are shown in Figure 10. It can be seen that under the condition of rejuvenation construction process II, the high-temperature stability and moisture-induced damage of recycled asphalt mixture were better than those of rejuvenation construction process I and III. However, the low-temperature crack resistance of recycled asphalt mixture under the condition of rejuvenation construction process II was far lower than that of rejuvenation construction process III.

As can be observed in Figure 10, different rejuvenation processes have different effects on the road performance of the mixture. Therefore, when determining the rejuvenation construction process, it should be selected in combination with the local natural conditions, so that the performance of the recycled asphalt mixture can be adapted to the natural conditions to achieve the purpose of prolonging the service life of the pavement.

4. Conclusions

The following conclusions can be drawn from this paper:

• As the content of the rejuvenator increases, the softening point of the recycled asphalt increases, the penetration and ductility decrease and the indexes of $R_a$, $R_q$ and $R_{max}$ gradually decrease. Based on the penetration, ductility, softening and Nanomorphology analysis of recycled asphalt, the optimum rejuvenator content is 4%.
• The damage of aggregate can be reduced by milling the pavement under high temperature and ensure the uniformity of the original pavement gradation. For every 10% increase in RAP content, the average mixing and compaction temperature of hot recycled asphalt mixture decreases by 2–6 °C.
• As the RAP content increases, the high-temperature stability of the hot recycled asphalt mixture increases, the moisture-induced damage and the low-temperature crack resistance decrease. The strain performance density of 10 KJ/m³ is proposed as the index to evaluate the low-temperature performance of recycled asphalt mixture. From comprehensive road performance results, the optimum percentage of RAP is 30%.
• High moisture-induced damage and high-temperature stability can be obtained by using RAP + rejuvenator co-heating construction process for recycled asphalt mixture. High low-temperature crack resistance can be obtained by using a RAP + rejuvenator not-heating construction process.