*2.5. Property Characterizations of L-DMA-Modified Asphalt* 2.5.1. DSR Test

DSR (MCR-101, Anton Paar, Graz, Austria) examined the rheological properties of L-DMA-A. Asphalt's viscoelasticity is significantly impacted by exposure temperature changes as a typical viscoelastic material [26,27]. Specific test parameter settings for the temperature sweep are shown in the Table 2.


**Table 2.** Test parameters of temperature sweep.

Asphalt pavement will be affected by vehicle load during the service process. Loading with different frequencies will cause different deformations of asphalt pavement, and the deformation size determines the high-temperature performance of asphalt pavement. Viscosity is an essential parameter of asphalt and the addition of L-DMA will affect the viscosity of asphalt binder. Therefore, it is necessary to study the variation trend of the viscosity of 70# asphalt and L-DMA-modified asphalt under different load frequencies. The specimens were subjected to a frequency sweep in the range of 0.01–10 Hz. In order to meet the practical application of pavement, the temperature was set to 58 ◦C.

Additionally, L-DMA-modified asphalt with different L-DMA contents were tested by loading–unloading mode in the Multiple Stress Creep and Recovery (MSCR) test, which can be used to characterize the high-temperature performance of asphalt [28]. In this paper, the MSCR tests under stresses of 0.1 kPa and 3.2 kPa were carried out at 58 ◦C. The whole test contained 20 cycles of creep and recovery. Specifically, the first 10 cycles were conducted under the stress of 0.1 kPa, and the following 10-cycle test under the constant stress of 3.2 kPa was further conducted to finish the whole test. Figure 5 shows the creep and recovery (loading–unloading) curves of asphalt in one cycle.

**Figure 5.** Change of strain with time during one creep—recovery cycle.

According to AASHTO M 332 Standard [29], the two evaluation indexes obtained by the MSCR test are the creep recovery rate *R* (%) and nonrecoverable creep compliance *Jnr*, which are used to evaluate the viscoelastic characteristics of asphalt under high-temperature conditions. The nonrecoverable creep compliance *Jnr* is the ratio of ultimate unrecoverable strain and applied stress in each loading cycle, which describes the contribution of asphalt to high-temperature deformation of asphalt mixtures. The creep recovery rate R (%) is the percentage of recoverable strain and total creep strain in the loading cycle, which characterizes the high-temperature elastic recovery performance of asphalt and reflects the influence of delayed elastic deformation of asphalt to a certain extent. According to the Equations (1)–(3) below, *R* (%) and *Jnr* under different stresses can be calculated.

$$J\_{nr100}\left(\frac{1}{kPa}\right) = \frac{1}{10}\left\{\sum\_{1}^{10} \frac{non\text{-recoverable\ strain}}{0.1}\right\} \tag{1}$$

$$J\_{nr3200}\left(\frac{1}{kPa}\right) = \frac{1}{10}\left\{\sum\_{1}^{10} \frac{non\text{-}recoverable\ strain}{3.2}\right\} \tag{2}$$

$$R(\%) = \frac{1}{10} \left\{ \sum\_{1}^{10} \frac{recoverable \, strain}{peak \, strain} \times 100\% \right\} \tag{3}$$

#### 2.5.2. BBR Test

Low-temperature cracks are inevitable in the use of asphalt pavement. Improving the low-temperature performance of asphalt pavement is the top priority of road workers. In general, BBR is most commonly used for the low-temperature performance evaluation of asphalt. According to the specification AASHTO T313-12 [30], 70# asphalt and L-DMAmodified asphalt beams were prepared, respectively, then held 1 Dh at the test temperature and were subjected to a 4 min loading test to measure the creep stiffness modulus S and creep rate m. S and m were selected at 60 s, if S ≤ 300 MPa and m ≥ 0.3 at this condition, then the asphalt is applicable at this temperature condition.

#### 2.5.3. FTIR Spectroscopy Analysis

To study the chemical structure changes of L-DMA after modified asphalt and the modification mechanism, the FTIR Spectroscopy test was carried out. The sample was dissolved in carbon disulfide and then dropped onto a clear KBr lens. After drying for a moment, the sample was prepared and put into the instrument for testing.

#### **3. Results and Discussion**

#### *3.1. Structural Analysis and Thermal Stability Analysis of L-DMA*

The infrared spectra of L-DMA are shown in Figure 6. The absorption peak at 3370 cm−<sup>1</sup> was the vibration peak of catechol phenolic hydroxyl, and the formation of hydrogen bonds widened the peak area. The N-H stretching vibration peak appeared at 3202 cm−<sup>1</sup> . The C=O stretching vibration peak at 1730 cm−<sup>1</sup> originated from the amide group, and the hydrogen bond made the electron shift outward and the bond force weaken. The bending vibration and deformation vibration of sec-amide appeared at 1655 cm−<sup>1</sup> and 1520 cm−<sup>1</sup> , respectively. However, the stretching vibration of C=C overlapped with the bending vibration of N-H, so the absorption peak appeared at 1655 cm−<sup>1</sup> . At the same time, the absorption peaks at 1605 cm−<sup>1</sup> and 1450 cm−<sup>1</sup> were also observed, which were the stretching vibration of the L-DOPA aromatic ring. The single peak at 1260 cm−<sup>1</sup> is the C-N stretching vibration.

Compared with the spectra of the reaction monomers MAA and DOPA, in the FTIR spectra of L-DMA, the absorption peaks of methacrylic anhydride in MAA at 1787 and 1725 cm−<sup>1</sup> had obviously disappeared and changed into a single peak of C=O at 1730 cm−<sup>1</sup> , and two amino peaks at 3210 and 3075 cm−<sup>1</sup> at L-DOPA became one, indicating that the primary amine in L-DOPA reacted to form the secondary amide. From the above analysis, MAA and L-DOPA reacted successfully to form L-DMA.

Figure 7 shows the thermal weight loss curve of the L-DMA monomer powder, the initial thermal decomposition temperature of L-DMA is 221.5 ◦C with a mass loss of 5%, which is mainly due to the volatilization of bound water. There was a sharp mass loss between 224 ◦C and 478.8 ◦C, which corresponded to the decomposition of the C-N bond and methacrylate. Subsequently, the slow mass loss of the sample from 480 ◦C to 800 ◦C was the decomposition of the L-DMA main structure. The maximum temperature during asphalt mixing and paving is about 180 ◦C. Therefore, L-DMA can be used for asphalt modification.

**Figure 6.** FT-IR spectra results of MAA, L-DOPA, and L-DMA.

**Figure 7.** The TG curve of L-DMA.

#### *3.2. Modification Mechanism of L-DMA*

The modification mechanism of L-DMA-modified asphalt was analyzed by FTIR. Figure 8 compared the infrared spectra of L-DMA, 70# asphalt, and L-DMA-modified asphalt. It can be found that the characteristic absorption peaks of 70# base asphalt show asymmetric and symmetric stretching vibration of methylene at 2925 and 2854 cm−<sup>1</sup> . C=C and C=O stretching vibrations appear at 1600 cm−<sup>1</sup> ; the absorption peaks of the benzene ring stretching vibration are 866 and 809 cm−<sup>1</sup> . Combined with the analysis of the infrared spectrum of L-DMA in Figure 6, it can be inferred that the modification process of L-DMA on asphalt is mainly a physical modification process. With the addition of L-DMA, the conjugated effect between catechol and the aromatic benzene ring in asphalt resulted in the redshift of the vibration absorption peak of the benzene ring. The peak area widened at about 3370 cm−<sup>1</sup> , suggesting that L-DMA can generate many hydrogen bonds

with free hydroxyl and amino groups in asphalt through catechol groups, thus forming a strong hydrogen bond and promoting its combination with asphalt. The number of hydrogen bonds increased significantly with an increase in L-DMA content. The hydroxyl characteristic peak area of modified asphalt with 10% L-DMA content was larger than that of other contents. Moreover, due to the existence of the benzene ring, L-DMA can interact with the aromatic ring in asphalt through the Π-Π accumulation [31–33]. These can improve the adhesion performance of modified asphalt. At the same time, dense hydrogen bonds can also enhance the self-healing performance of asphalt [34–37]. Infrared spectra also show that L-DMA does not decompose and exists stably in asphalt.

**Figure 8.** FT-IR spectra results of L-DMA, 70# base asphalt, and L-DMA-A.

#### *3.3. Rheological Properties of L-DMA-Modified Asphalt*

When L-DMA is added to asphalt as a modifier, physical and chemical modifications will inevitably occur, thus affecting the original structure and the viscoelastic properties of asphalt. In general, the composite shear modulus G\* is the measurement of the total resistance of the material during repeated shear deformation [38,39]; the phase angle δ is the relative index of the number of recoverable and irreversible deformations.

The composite modulus and phase angle curves of L-DMA-modified asphalts with different contents of L-DMA in the temperature range of 30–80 ◦C are shown in Figure 9. It is clear that with an increase in L-DMA content, the composite modulus G\* of modified asphalt increases and the phase angle δ decreases, indicating that the addition of L-DMA improves the shear deformation resistance of asphalt, and the elasticity of asphalt is obviously enhanced. The modified asphalt with 10% content has the most excellent hightemperature performance, which is mainly caused by the good adhesion role of L-DMA in asphalt. The catechol group in L-DMA generated a large number of hydrogen bonds with the free hydroxyl and amino groups in asphalt, thus forming a strong hydrogen bond and promoting the integration with asphalt [40,41]. Moreover, catechol and the asphalt aromatic ring accumulated to produce cohesion [42,43], increasing the proportion of elastic components in the modified asphalt and improving the shear deformation resistance of asphalt.

The evaluation index of anti-rutting performance of asphalt is quantified by the rutting factor, which is expressed as G\*/sinδ. In general, the asphalt binder with a larger rutting factor shows stronger anti-rutting ability [44]. According to Superpave specification, AASHTO: MP1, at the highest pavement design temperature, the G\*/sinδ value of matrix

asphalt should be at least 1.0 kPa [45]. Figure 10 shows the rutting factors of base and different modified asphalts at the temperature range of 30–80 ◦C. The results show that the rutting factors of modified asphalts rise with an increase in L-DMA content. The modified asphalt with 10% content has the largest rutting factor because the incorporation of L-DMA will introduce phenolic hydroxyl groups, which will form hydrogen bonds with the aromatic components in the light component of the asphalt, thereby increasing the adhesion of the material. At high temperatures, the catechol group in L-DMA will be oxidized into semiquinone and quinone structures, which improves the overall cohesive energy [46,47]. In addition, the benzene ring can also produce Π-Π conjugate with the aromatic ring in asphalt, which increases the cohesive energy. These effects increase the cohesion and adhesion of modified asphalt. These enhanced bonding structures will hinder the free flow of modified asphalt, enhance the elastic recovery ability, and improve the rutting resistance.

**Figure 9.** The result of the temperature sweep test (30–80 ◦C).

**Figure 10.** Rutting factor and temperature curves of different asphalts.

1

The low-temperature rheological property of asphalt binder is one of the important factors for its cracking resistance. The SHRP standard defines G\*sinδ as the evaluation index of fatigue cracking. The smaller the value of G\*sinδ, the better the ability to resist fatigue damage [48–50]. In this paper, the low-temperature rheological properties of 70# base asphalt and modified asphalt with different contents of L-DMA were tested in the temperature range of −10–30 ◦C and the test results are shown in Figure 11. The composite modulus of modified asphalt decreases with an increase in temperature, while the phase angle increases. When the content of L-DMA is 1%, the modified asphalt has the highest composite modulus. Then, with an increase in L-DMA content, the composite modulus of modified asphalt shows a downward trend. When the content of L-DMA is 10%, the composite modulus of modified asphalt is the lowest and is lower than that of 70# asphalt. The reason is the change of L-DMA's effect, and when the content is low, it mainly plays a role of filling in asphalt, which makes the low-temperature performance of modified asphalt worse. However, with an increase in L-DMA content, unsaturated rings such as benzene, quinone, and pyridine in aromatic components of asphalt will conjugate with the benzene ring in L-DMA, and the rings overlap with each other to form a layered structure, which reduces the composite modulus G\* of modified asphalt.

**Figure 11.** The result of the temperature sweep test (−10–30 ◦C).

Figure 12 shows the changing trend of the fatigue factors of 70# asphalt and modified asphalts with different contents of L-DMA at different temperatures. The results show that the fatigue factor of modified asphalt increases first and then decreases with an increase in L-DMA content. At a high content of 10%, the addition of L-DMA decreases the fatigue factor and enhances the fatigue resistance of asphalt due to the conjugation between the unsaturated rings of L-DMA and asphalt.

**Figure 12.** Fatigue factor and temperature curves of different asphalts.

The viscosities of 70# asphalt and L-DMA-modified asphalts mixed with different contents of L-DMA are shown in the Figure 13. It can be seen from the figure that with an increase in loading frequency, the viscosity of asphalt at 58 ◦C shows a downward trend. The viscosity of 70# asphalt is the lowest, while 10% L-DMA-modified asphalt has the highest viscosity. The viscosity can reflect the deformation resistance of the material to a certain extent, and the viscosity of 10% L-DMA-modified asphalt can still maintain a high level at 58 ◦C, which indicates that the addition of L-DMA contributes greatly to the improvement of the high-temperature performance of asphalt.

**Figure 13.** Frequency—Viscosity Curve at 58 ◦C.

The strain changes of 70# base asphalt and L-DMA-modified asphalt under different stress levels at 58 ◦C are shown in Figure 14. The result shows that the 70# base asphalt has the largest cumulative strain. With an increase in L-DMA content, the cumulative strain of the modified asphalt gradually decreases. This is because the cohesion between L-DMA and asphalt improves the elasticity of asphalt materials, significantly improving the recoverable deformation ability of asphalt and thus improving the high-temperature performance of asphalt.

**Figure 14.** Strain during the MSCR test at 58 ◦C.

Table 3 shows the experimental results of creep recovery rate *R* (%) and unrecoverable creep compliance (*Jnr*) of base asphalt and modified asphalts at different stress levels. The results show that under the action of 100 Pa and 3200 Pa, the creep recovery rate R (%) of L-DMA-modified asphalt increases with an increase in L-DMA content, indicating that the addition of L-DMA enhances the recovery deformation ability of asphalt. The unrecoverable creep compliance (*Jnr*) decreases with an increase in L-DMA content, indicating that the incorporation of L-DMA improves the permanent deformation resistance of asphalt, which is consistent with the results of the rutting factor test. Comparing the unrecoverable creep compliance (*Jnr*) of asphalt and L-DMA-modified asphalts under different stress levels, it can be found that the unrecoverable creep compliance (*Jnr*) of 70# base asphalt changes greatly under 100 Pa and 3200 Pa, while the unrecoverable creep compliance (*Jnr*) of L-DMA-modified asphalts does not change significantly under different stress levels. It indicates that the addition of L-DMA reduces the dependence of asphalt on shear stress and enhances the shear deformation resistance of asphalt. The hydrogen bonds generated by the addition of L-DMA have certain reversibility, so the creep recovery rate *R* (%) of L-DMA-modified asphalt has greatly improved.



*3.4. Low-Temperature Crack Resistance of L-DMA Modified Asphalt*

It can be seen from Figure 15 that with a decrease in temperature, the S modulus increases but the m value decreases, indicating that the lower the temperature, the worse

the low-temperature performance of asphalt. When the temperature is lower than −12 ◦C, the m of 70# asphalt and L-DMA-modified asphalt will be far less than 0.3, and the S is much higher than 300 MPa, indicating that when the temperature is lower than −12 ◦C, the low-temperature performance of 70# asphalt and L-DMA-modified asphalt cannot meet the standard. At −12 ◦C, the S and m of 70# asphalt basically meet the requirements of the specification. It can be seen that with an increase in L-DMA content, the S modulus increases first and then decreases gradually, indicating that less L-DMA content will affect the deformation capacity of asphalt. However, with an increase in L-DMA content, the deformation capacity of modified asphalt at low temperatures is becoming better and better, and the deformation capacity of modified asphalt is the best when the content of L-DMA is 10%, meeting the standard. The m decreases first and then increases, suggesting that with the incorporation of L-DMA, the stress relaxation ability of asphalt to external load decreases first and then increases. When the content of L-DMA is 10%, the m is the largest, and the modified asphalt has the best stress relaxation ability to deal with external loads.

Combined with the low-temperature rheological test results of DSR, it is found that the low content of L-DMA has an adverse effect on the low-temperature performance of asphalt, but with an increase in L-DMA content, it has a significant improvement on the low-temperature performance of asphalt. This is mainly because the low content of L-DMA plays a filling role in asphalt, increasing the composite modulus G\* and creep stiffness modulus S of L-DMA-modified asphalt, reducing the creep rate m, and worsening the low-temperature performance of modified asphalt. However, with an increase in L-DMA content, the unsaturated rings such as benzene, quinone, and pyridine in the aromatic components of asphalt will conjugate with the benzene ring in L-DMA, and the rings overlap with each other to form a layered structure. The interlayer friction is small, which reduces the G\* value of modified asphalt, decreases the S value, and increases the m value, thereby improving the low-temperature performance of asphalt. This is consistent with the results of the low-temperature sweep test in DSR.

**Figure 15.** The result of BBR.

#### **4. Conclusions**

In this study, the mussel adhesion protein material L-DMA was synthesized and used to modify asphalt. The deformation and cracking resistances of L-DMA-modified asphalt were studied by DSR temperature sweep, frequency sweep, multi-stress creep tests, and BBR tests. In addition, the modification mechanism of L-DMA-modified asphalt was analyzed by FTIR test. According to the test results, the following conclusions can be drawn.


Overall, the addition of L-DMA has a positive impact on the high-temperature rutting resistance and a high content of L-DMA will have some improvement on the lowtemperature cracking resistance of asphalt. The usage of L-DMA as a kind of mussel adhesion protein material improves the performance index of asphalt, and, theoretically, there is a possibility to improve water resistance and self-healing properties of asphalt, which will be explored in future research. Furthermore, due to the high cost and complex preparation process of L-DMA, further research is needed to decrease the cost and increase the preparation efficiency.

**Author Contributions:** Conceptualization, Q.L.; Investigation, J.W.; Methodology, J.W. and S.W.; Writing—original draft, J.W.; Writing—review and editing, Q.L. and S.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the National Natural Science Foundation of China (No.51978547), Technological Innovation Major Project of Hubei Province (2019AEE023) and Key R&D Program of Hubei Province (2020BCB064).

**Institutional Review Board Statement:** Not applicable. Studies do not involve humans or animals.

**Informed Consent Statement:** Not applicable.

**Acknowledgments:** The work presented in this paper is financially supported by the National Natural Science Foundation of China (No.51978547), Technological Innovation Major Project of Hubei Province (2019AEE023) and Key R&D Program of Hubei Province (2020BCB064).

**Conflicts of Interest:** The authors declare no conflict of interest.
