Research on the Preparation and Performance of Biomimetic Warm-Mix Regeneration for Asphalt Mixtures

Abstract

To determine the formula for biomimetic warm-mix regeneration and fulfill the requirements of a “high waste asphalt mixture content, high quality, and high level” for its usage in reclaimed asphalt pavement (RAP), this paper first determined the suitable preparation process and formula for biomimetic warm-mix regeneration based on orthogonal experiments and a gray correlation analysis. Then, the optimum dosage of the warm-mix regenerant was determined by a uniaxial penetration test, low-temperature splitting test, and freeze–thaw penetration test. The rutting test was conducted to characterize the high-temperature performance of the asphalt mixture. The Immersion Marshall Test and the freeze–thaw splitting test were used to characterize the water stability of the recycled asphalt mixture. The low-temperature small beam test was employed to study the low-temperature performance of the recycled asphalt mixture. The asphalt’s short-term and long-term aging processes were simulated using the rotary thin-film oven test (RTFOT) and the pressure aging test (PAV). The action mechanism of biomimetic warm-mix regeneration was revealed by Fourier-transform infrared spectroscopy (FTIR). Finally, a comprehensive thermal performance test was conducted on the aged asphalt after biomimetic warm-mix regeneration. The results showed that the self-made biomimetic warm-mix regeneration agent exhibited an excellent regenerative effect on RAP and significantly reduced the mixing temperature of the styrene–butadiene–styrene (SBS)-modified asphalt mixture. In addition, the self-made biomimetic warm-mix regeneration agent effectively improved the high- and low-temperature performance of the recycled asphalt mixture, but had no noticeable effect on the water stability. The suggested dosage of the biomimetic warm-mix regeneration agent was 6%, and the mixing temperature was 130 °C. The microscopic chemical analysis revealed that biomimetic warm-mix regeneration restored the performance of aged asphalt by supplementing the light component. The change rules of the chemical functional groups and the comprehensive thermal properties of the recycled mixture showed a good correlation with the change rules of its high- and low-temperature performance.

1. Introduction

With the continuous progress of China’s transport industry, more and more waste asphalt mixtures are generated from pavement overhaul and maintenance, and the industry is paying more and more attention to the recycling of RAP materials [1]. With advancements in the major and moderate repair processes of asphalt pavement, a large amount of RAP would be generated, which would take up land resources and cause environmental pollution. In addition, with continuous advancements in China’s urbanization process, the increasing shortage of natural sand and gravel resources and the accumulation of RAP stocks will gradually become China’s main contradiction in road engineering. To reduce the waste of mineral resources and achieve the sustainable development and utilization of resources, more and more attention is being paid to recycling and using RAP [2]. However, the construction temperature of traditional thermal regeneration technology is too high, causing this technology to produce a large number of toxic gases and consume a large amount of energy, which is not conducive to environmental protection. Therefore, it is critical for research on warm-mix regeneration technology to be conducted. Warm-mix regeneration technology can effectively utilize non-renewable resources such as RAP and significantly reduce the aging of asphalt. At the same time, during mixing, transportation, and other processes, the emission of toxic and harmful gases is reduced, energy is saved, and the environment is protected; additionally, the recycled pavement has a good road performance [3,4].

Aged asphalt in RAP loses part of its light components after long-term aging; it becomes hard and brittle. However, the use of traditional petroleum-based regenerants would cause environmental pollution. In recent years, bio-based regenerants have shown advantages in versatility and green environmental protection with the rapid development of biotechnology [5]. Tang Wen et al. [6] used a biological regenerant as a lubricant to improve the polymerization ability of each component in the asphalt and then achieve a regeneration effect. Ye Qunshan et al. [7] used tung oil and a regeneration agent to achieve fusion, and found that the regeneration agent can regenerate aged asphalt by supplementing the light components. They also found that the high- and low-temperature properties of the recycled asphalt mixture were higher than those of the original asphalt. With the proposal of green transportation, warm mix technology has gradually developed [8]. Currently, the three types of warm mix agents used in China are foam asphalt technology, organic additives, and surfactants [9]. Guo Peng et al. [10] concluded that a change in the adhesion between asphalt and stone is due to a change in the asphalt’s surface energy and cohesive energy by the warm mix agent. Currently, most of the research on warm-mix regeneration technology uses warm mix agents and regenerates. That is, the effect of warm-mix regeneration can be achieved by adding a warm mix agent to hot mix regeneration technology. Yang Jianggang et al. [11] used the Sasobit warm mix agent and the Evoflex regenerant and added a warm mix agent based on hot mix regeneration. The test results show that the water stability and low-temperature performance of warm mix recycled asphalt mixtures are similar to the results of hot mix recycled asphalt mixtures. The mixing of recycled asphalt has been significantly improved. Zhu Tanyong. [12] used two warm mix finished products based on hot mix recycled asphalt. They found that properties such as the stiffness and modulus did not change significantly with the addition of the warm mix additives.

The main material of the warm-mix regeneration agent used in this article was biomimetic mussel glue, which is artificially synthesized mussel glue. Biomimetic mussel glue is not completely derived from the biological materials of real mussel glue, but it imitates the structure of mussel glue. Materials designed and manufactured for specific properties or functions may use some non-biologically derived chemical components or synthetic methods to simulate the properties of natural materials, but they are essentially different in their source from real bio-based materials. The artificial material used in this study was synthesized based on the adhesiveness and other characteristics of bio-based mussel glue, and the biomimetic warm-mix regeneration agent made from it had the advantages of being green, environmentally friendly, convenient, and cheap. Because of this, this article determined the preparation process and formula for biomimetic warm-mix regeneration through the orthogonal design test method and used the uniaxial penetration test to determine the optimal mixing temperature of the biomimetic warm-mix regenerant in the external doping method. With the blending ratio, FTIR was used to reveal the mechanism of action of the biomimetic warm-mix regeneration agent. With the help of comprehensive thermal tests and mixture performance tests, the consistency of the microscopic thermal properties and the high- and low-temperature properties of the recycled asphalt mixture were characterized. This study’s success provides a theoretical basis for applying bionic-based warm mix recycled asphalt.

2. Preparation and Formula Optimization of Biomimetic Warm-Mix Regeneration Agent

2.1. Main Raw Materials

This article used a self-made biomimetic warm-mix regeneration agent. Its formula mainly consists of base oil, extracted oil, rubber oil, self-made biomimetic mussel glue, activated penetrant, antioxidants, etc. Based on the above materials,

Table 1. Composition and molecular weight of each component of homemade biomimetic mussel gum.

Serial Number	Each Component	Molecular Weight	Manufacturer
1	Polytetramethylether glycol (PTMEG)	2000	Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China
2	2,6—Toluene diisocyanate (TDI)	250.25	Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China
3	1,4—Butanediol (BDO)	90.12	Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China

shows the main raw materials of each component of the synthesized self-made biomimetic mussel glue.

The components of bionic mussel glue include component A and component B. Component A is a compound with an isocyanate group at the end and a side chain containing a catechol group, and component B contains NCO. Monomer R1 (the molecular structure is shown in Figure 1) and TDI were mixed and reacted at a molar ratio of 1:2 for 2 h; then, BDO was added and stirred evenly to obtain component A.

The isocyanate index is controlled to 0.5 (the molar ratio of hydroxyl group and isocyanate is 1:2), and PTMEG and TDI are stirred and reacted for 3 h by which component B is obtained. The bionic mussel glue is obtained by mixing A and B at 1:1.

2.2. Preparation Process of Bionic-Based Warm-Mix Regenerant

2.2.1. Preparation Process

The bionic-based warm-mix regeneration agent is prepared using YH-S312 laboratory special shearing equipment (YH-S312 from Shenyang University of Architecture). The preparation process is shown in Figure 2, where T₁ (°C) represents the temperature of epoxidized soybean oil, and T₂ (°C) represents the stirring temperature during the mixing process. S₁ (r/min) and S₂ (r/min), respectively, represent the two shear rates r/min after adding epoxidized soybean oil and other components; t₁ (h) and t₂ (h) correspond to the stirring time under S₁ and S₂, respectively. First, slowly pour the preset amount of homemade biomimetic mussel glue into the epoxy soybean oil preheated to T₁ along the side wall of the ceramic cylinder and stir for t₁ at a stirring speed of S₁. After sufficient shearing, add the preset proportion of extracted oil, rubber oil, activated penetrant, and antioxidant to keep the entire system at T₂, adjust the rotation speed to S₂, and stir for t₂. After shearing is stopped, a bionic-based warm-mix regeneration agent is prepared, and the prepared sample is put into a sealed container for testing.

2.2.2. Orthogonal Design

Reaction temperature, stirring time, and stirring rotation speed were control factors. Three levels were set for each control factor and the design was carried out using L₉ (3⁴) orthogonal table (

Table 2. Orthogonal experiment of three factors at three levels.

Test Number	A. Temperature Reflex (°C)	B. Cut Time (h)	C. Shearing Revolutions (r/min)	Horizontal Combination
1	T1: 45, T2: 50	t1: 0.3, t2: 0.3	S1: 5000, S2: 3000	A1B1C1
2	T1: 45, T2: 50	t1: 0.5, t2: 0.3	S1: 3000, S2: 3000	A1B2C2
3	T1: 45, T2: 50	t1: 0.5, t2: 0.5	S1: 5000, S2: 5000	A1B3C3
4	T1: 55, T2: 60	t1: 0.3, t2: 0.3	S1: 3000, S2: 3000	A2B1C2
5	T1: 55, T2: 60	t1: 0.5, t2: 0.3	S1: 5000, S2: 5000	A2B2C3
6	T1: 55, T2: 60	t1: 0.5, t2: 0.5	S1: 5000, S2: 3000	A2B3C1
7	T1: 65, T2: 70	t1: 0.3, t2: 0.3	S1: 5000, S2: 5000	A3B1C3
8	T1: 65, T2: 70	t1: 0.5, t2: 0.3	S1: 5000, S2: 3000	A3B2C1
9	T1: 65, T2: 70	t1: 0.5, t2: 0.5	S1: 3000, S2: 3000	A3B3C2

). The process parameters for the preparation of biomimetic warm-mix regenerant were determined based on the 60 °C viscosity, aromatic content, saturation content, and viscosity ratio before and after the film oven test.

The 60 °C viscosity, aroma content, saturation content, and viscosity ratio of the biomimetic warm-mix regenerants before and after the film oven test were obtained through conventional performance tests to evaluate the performance of the biomimetic warm-mix regenerants under different preparation process conditions. To ensure good flow and processability of the bitumen, the initial reaction temperature was set at 45 °C. It can be seen in Figure 3 and Figure 4 that the saturation content of the biomimetic warm-mix regenerant decreases as the reaction temperature increases. According to this trend, the aromatic content increases, decreases, and finally stabilizes. The viscosity and viscosity ratio at 60 °C do not change significantly before and after the film oven test.

With the increase in stirring time, the viscosity and saturation of the biomimetic warm-mix regenerant samples first increased and then decreased, and the aromatic hydrocarbon content increased. This indicates that, when the mixing time is too short, the modification effect is not obvious; while, when the mixing time is too long, the biomimetic warm-mix regenerant and oxygen contact time is too long, which accelerates the light component of the deterioration of the speed. As can be seen from Figure 4, the viscosity ratio before and after the film oven test increases with the increase in reaction temperature, first increasing, and then decreasing, and finally tending to stabilize. Through experiments, it was found that, when S₁ is 3000 r/min and 5000 r/min, the performance of the biomimetic warm-mix regenerant is not significantly affected. When S₂ is 5000 r/min, after the biomimetic mussel glue is incorporated, the reaction temperature will increase significantly, and the viscosity and saturation content of the bionic-based warm-mix regenerant will decrease slightly. It shows that the temperature of biomimetic mussel glue rises rapidly when stirred at high speed. Stirring too fast will cause the temperature to rise sharply and accelerate the volatilization of light components.

2.2.3. Gray Correlation Analysis

(1)

Determine the reference sequence and comparison sequence

In this section, the corresponding experimental data are extracted from the level changes of three factors: shear time, shear revolutions, and reaction temperature. Metrics analyzed include 60 °C viscosity, aromatic content, saturation content, and viscosity ratio before and after the film oven test as a comparison sequence X_i.

$$X_i=(x_i(1),x_i(2),\dots \dots ,x_i(k))$$

(1)

In the formula, i = 1, 2, …, m; k = 1, 2, …, n. Among them, i represents the type of influencing factors, and k represents the level of influencing factors. The mean method is used; see Equation (2). The variables are dimensionless, as shown in

Table 3. Dimensionless processing of data.

Test Number	60 °C Viscosity (mm2/s)	Aroma Content (%)	Saturated Content (%)	Viscosity Ratio Before and after Film Oven Test (%)
1	76	58	28	75.4
2	79	61	34	78.5
3	70	62	20	78.7
4	73	48	19	74.6
5	76	58	31	75.9
6	72	59	16	76
7	75	59	30	76.7
8	76	58	31	76.1
9	70	60	10	77.5

for details.

$$x_i(k)=\frac{x_i(k)}{\frac{1}{m}{\displaystyle {\sum }_{k=1}^m{x}_i(k)}}$$

(2)

$$X_0=(x_0(1),x_0(2),\dots \dots ,x_0(k))$$

(3)

In the formula, i = 1, 2, …, m; k = 1, 2, …, n. Take {x₀} = {59 62 54 79} as a reference.

(2)

Formula (4) is used to calculate the absolute difference corresponding to the data series of the evaluated object and the reference data series one by one. The calculation results are shown in

Table 4. Absolute deviation.

Test Number	60 °C Viscosity (mm2/s)	Aroma Content (%)	Saturated Content (%)	Viscosity Ratio Before and after Film Oven Test (%)
1	3	4	6	3.6
2	0	1	0	0.5
3	9	0	14	0.3
4	6	14	15	4.4
5	3	4	3	3.6
6	7	3	18	3
7	4	3	4	2.3
8	3	4	3	2.9
9	9	2	14	1.5

.

$$\mathsf{\Delta }i(k)=\left|x_0(k)-x_i(k)\right|$$

(4)

(3)

Calculate the maximum and minimum differences between the two poles. See Formulae 5 and 6 for calculations.

$$M=\underset{i}{\max }\underset{k}{\max }\mathsf{\Delta }i(k)$$

(5)

$$m=\underset{i}{\max }\underset{k}{\max }\mathsf{\Delta }i(k)$$

(6)

In the formula, i = 1, 2, …, m; k = 1, 2, …, n.

(4)

Calculate the correlation coefficient and correlation degree. See Formulae (7) and (8) for calculations. The calculation results are shown in

Table 5. Correlation index and correlation.

Test Number	60 °C Viscosity (mm2/s)	Aroma Content (%)	Saturated Content (%)	Viscosity Ratio Before and after Film Oven Test (%)	Correlation
1	0.422	0.345	0.253	0.373	0.348
2	1.000	0.600	1.000	0.759	0.838
3	0.542	1.030	0.343	1.000	0.729
4	0.501	0.750	0.143	0.578	0.493
5	0.143	0.111	0.143	0.139	0.134
6	0.517	0.714	0.294	0.714	0.559
7	0.175	0.221	0.175	0.269	0.210
8	0.155	0.121	0.155	0.159	0.148
9	0.443	0.518	0.333	0.871	0.616

.

$${\gamma }_{0i}(k)=\frac{m+\rho M}{{\mathsf{\Delta }}_i(k)+\rho M}$$

(7)

$${\gamma }_{0i}=\frac{1}{n}{\displaystyle \sum _{k=1}^n{\gamma }_{0i}}(k)$$

(8)

In the formula, ρ = 0.5, i = 1, 2, …, m; k = 1, 2, …, n.

(5)

Correlation sorting: γ₂ > γ₃ > γ₉ > γ₆ > γ₄ > γ₁ > γ₇ > γ₈ > γ₅.

In summary, it can be seen that No. 2 has the most significant correlation. Combined with Table 1, the determined preparation process parameters are as follows: T₁ and T₂ are 45 °C and 50 °C, respectively; t₁ and t₂ are 0.5 h and 0.3 h, respectively; and S₁ and S₂ are 3000 r/min and 3000 r/min.

2.3. Formula Optimization of Bionic-Based Warm-Mix Regeneration Agent

The content of base oil (A), extracted oil (B), and self-made biomimetic mussel glue (C) were control factors. Three levels were taken for each factor, and the L₉ (3⁴) orthogonal table was used. The specific orthogonal experimental design plan is shown in

Table 6. Orthogonal experiment, with three factors and three levels.

Test Number	A	B	C	Horizontal Combination
1	35	20	10	A1B1C1
2	35	17	8	A1B2C2
3	35	15	5	A1B3C3
4	40	20	8	A2B1C2
5	40	17	5	A2B2C3
6	40	15	10	A2B3C1
7	45	20	5	A3B1C3
8	45	17	10	A3B2C1
9	45	15	8	A3B3C2

. The formulae of each biomimetic warm-mix regeneration agent component were determined based on the 60 °C viscosity, aromatic content, and saturation content.

The performances of the warm-mixed regeneration agent were measured through the 60 °C viscosity, saturation content, and aromatic content. Then, the best warm-mix regenerant formula was determined. The test results are shown in

Table 7. Test results of biomimetic warm-mix regenerate.

Test Number	60 °C Viscosity (%)	Aroma Content (%)	Saturated Content (%)
1	66	61.3	13.1
2	67	61.2	14.3
3	70	61	12.7
4	66	62.5	15.1
5	69	64.7	14.8
6	64	61.2	16.2
7	64	66.5	17.6
8	65	65.1	19.6
9	67	65.3	18.3

and Figure 5.

2.3.1. Range Analysis

The range analysis method (R method) has the advantages of simple calculation, accurate results, and intuitive image. It is usually used to analyze the results after orthogonal experiments [13]. The larger the R-value, the more significant the impact of this value on the test results. The range calculation formulae are shown in Equation (9) and Equation (10):

$$R_{\mathrm{j}}=\max ({\overline{K}}_{j1},{\overline{K}}_{j2,}{\overline{K}}_{j3})-\min ({\overline{K}}_{j1},{\overline{K}}_{j2,}{\overline{K}}_{j3})$$

(9)

$$K_{mn}=\frac{1}{N}{\displaystyle \sum _{i=1}^N{P}_i}$$

(10)

In the formula, K_mn is the average value of the indicator value corresponding to the nth level of the mth factor; P_i is the indicator value; and R_m is the range value of the mth factor. The calculation results are shown in

Table 8. Range analysis (60 °C viscosity).

Test Number	A	B	C
K1	203.00	201.00	203.00
K2	199.00	201.00	200.00
K3	196.00	196.00	195.00
${\overline{K}}_1$	67.67	67.00	67.67
${\overline{K}}_2$	66.33	67.00	66.67
$\overline{K}3$	65.33	65.33	65.00
R	2.33	1.67	2.67

and

Table 9. Range analysis (saturation content).

Test Number	A	B	C
K1	50.40	50.80	48.70
K2	46.60	48.20	44.30
K3	44.80	42.80	48.80
${\overline{K}}_1$	16.80	16.93	16.23
${\overline{K}}_2$	15.53	16.07	14.77
$\overline{K}3$	14.93	14.27	16.27
R	1.87	2.67	1.50

.

It can be seen from Table 8 and Table 9 that, for 60 °C viscosity, homemade bio-mimetic mussel glue > base oil > extracted oil, and the optimized formula is A₁B₂C₁ or A₁B₁C₁; for saturation content, the degree of influence of each component content is as follows: base oil > extracted oil > homemade bio-imitation mussel glue, and the best formula is A₃B₂C₁; and, for the aroma content, the influence of each ingredient content is as follows: extracted oil > base oil > homemade bio-imitation mussel glue, and the best formula is A₁B₁C₃. In summary, the optimized formula cannot be determined at present, as the optimized formulae considering the three factors are different. Therefore, the variance analysis is needed to further optimize the formula.

2.3.2. Variance Analysis

Firstly, calculate the deviation and degree of freedom based on the previous orthogonal and range tests, and then calculate the test basis according to the range formula in

Table 10. Calculation table of variance basis.

Test Number		A. Base Oil (%)	B. Extract Oil (%)	C. Homemade Biomimetic Mussel Glue (%)	D. Blank Group	60 °C Viscosity	Saturated Content	Aroma Content
x1		1	3	3	3	65	62.3	13.1
x2		1	2	2	2	69	61	14.3
x3		1	1	1	1	73	61.2	12.7
x4		2	3	2	2	70	68	15.1
x5		2	2	1	3	71	62	14.8
x6		2	1	3	1	66	64.8	16.2
x7		3	3	1	1	71	62.3	17.6
x8		3	2	3	2	67	61.1	19.6
x9		3	1	2	3	68	59.3	18.3
60 °C viscosity	K1	203.00	201.00	203.00	198.00	$Q={\displaystyle \sum _{\mathrm{i}=1}^9{\mathrm{x}}_i^2}=157604$ $\mathrm{T}=598$ $P=\frac{{\mathrm{T}}^2}{n}=64.444$
	K2	199.00	201.00	200.00	198.00
	K3	196.00	196.00	195.00	202.00
	${\overline{K}}_1$	67.67	67.00	67.67	66.00
	${\overline{K}}_2$	66.33	67.00	66.67	66.00
	$\overline{K}3$	65.33	65.33	65.00	67.33
Saturated content	K1	193.70	186.50	189.10	187.80	$Q={\displaystyle \sum _{\mathrm{i}=1}^9{\mathrm{x}}_i^2}=35948.16$ $\mathrm{T}=568.8$ $P=\frac{{\mathrm{T}}^2}{n}=63.2$
	K2	183.70	188.90	185.40	186.40
	K3	182.30	184.30	185.20	185.50
	${\overline{K}}_1$	64.57	62.17	63.03	62.60
	${\overline{K}}_2$	61.23	62.97	61.80	62.13
	$\overline{K}3$	60.77	61.43	61.73	61.83
Aroma content	K1	50.40	50.80	48.70	49.80	$Q={\displaystyle \sum _{\mathrm{i}=1}^9{\mathrm{x}}_i^2}=2275.69$ $\mathrm{T}=141.8$ $P=\frac{{\mathrm{T}}^2}{n}=15.667$
	K2	46.60	48.20	44.30	45.10
	K3	44.80	42.80	48.80	46.90
	${\overline{K}}_1$	16.80	16.93	16.23	16.60
	${\overline{K}}_2$	15.53	16.07	14.77	15.03
	$\overline{K}3$	14.93	14.27	16.27	15.63

. The variance analysis is shown in

Table 11. Analysis of variance.

Source of Variance		Bias Error	Degrees of Freedom	Sum of Mean Squares	F Value
60 °C viscosity	Factor A	8.22	2	4.11	2.31
	Factor B	5.56	2	2.78	1.56
	Factor C	10.89	2	5.44	3.06
	Error	3.56	2	1.78
Saturated content	Factor A	30.65	2	15.32	21.18
	Factor B	2.29	2	1.14	1.58
	Factor C	3.71	2	1.85	2.56
	Error	1.45	2	0.72
Aroma content	Factor A	5.45	2	2.72	1.45
	Factor B	11.10	2	5.55	2.96
	Factor C	4.40	2	2.20	1.17
	Error	3.75	2	1.87

.

Taking the significance level α = 0.05, the test value of F is found in the table as F_0.05 (2, 2) = 19. The F values of 60 °C viscosity and aromatic content are less than 19, and the influence is insignificant. The influence value of the saturation content factor A is greater than 19, indicating that the influencing factors are significant. The results were compared with the range analysis, and it was concluded that the optimal formula was A₃B₂C₁. Each formula contains 45% base oil, 17% extraction oil, and 10% bionic mussel glue.

3. Performance of Simulated Bio-Based Warm-Mix Recycled Asphalt Mixture

3.1. Materials and Mix Ratio Design Test Materials and Test Methods

3.1.1. Experiment Material

(1)

New asphalt

The new asphalt used during the test is SBS-modified asphalt. Its technical indicators are shown in

Table 12. Main technical indicators of SBS-modified asphalt.

Technical Indicators	Test Results	Indicator Requirements
Penetration (0.1 mm)	66	60~80
Softening point (°C)	56	≥55
Ductility 5 °C (cm)	37	≥30
Elastic recovery (%)	72.7	≥65
Adhesion grade	5	≥4

.

(2)

RAP

The aged asphalt in this test was recycled and reused using the upper layer RAP of a city’s main road. The upper layer used SBS asphalt mixture AC-13. The upper layer material can be divided into three grades after screening. The grades of 0~5 mm, 5~10 mm, and 10~15 mm are divided into three categories: coarse RAP, medium RAP, and fine RAP. The RAP screening results are shown in

Table 13. RAP screening results of milling materials.

Sieve hole size (mm)		16.0	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
Passing rate (%)	Coarse RAP	100	96.1	52.4	16.2	10.6	7.6	4.8	1.4	0.5	0.2
	Medium RAP	100	100	99.8	61.8	40.7	28.7	16.9	7.7	3.1	1.2
	Fine RAP	100	100	100	89.8	57.5	37.6	19.5	7.7	2.9	1.1

.

According to Table 12, the coarse aggregate content in RAP is lower. The reason may be that the coarse aggregate is crushed and refined under repeated traffic load actions. Since the coarse aggregate is in the asphalt, the mixture acts as a skeleton, which decreases the pavement’s performance and weakens its ability to resist deformation [14].

(3)

Asphalt content in RAP

The asphalt content in RAP is shown in

Table 14. Asphalt content in RAP test results.

Asphalt Content in RAP Test Results
Sieve hole size	0~5 mm	5~10 mm	10~20 mm
Asphalt content (%)	7.71	6.68	3.90

.

(4)

Aggregate

The mineral materials are made of limestone and divided into three grades: 0~5 mm, 5~10 mm, and 10~20 mm. The filler was limestone mineral powder.

(5)

Biomimetic warm-mix regenerant

The optimum warm-mix regenerant formulation is then determined. The biomimetic warm-mix reclaimer was formulated according to the formulations and mixing methods identified in Chapter 1.

Table 15. Performance index of biomimetic warm-mix regenerate.

Serial Number	Test Items	Test Results	Indicator Requirements	Reference Specification
1	60 °C viscosity (mm2/s)	74	50–175	JTG E20: T0619
2	Flash point (°C)	237	≥220	JTG E20: T0611
3	Saturated content (%)	16.6	≤30	JTG E20: T0618
4	Aroma content (%)	60.8	Actual measurement	JTG E20: T0618
5	Viscosity ratio before and after film oven test	1.018	≤3	JTG E20: T0619
6	Quality changes after film oven test (%)	−1.24	≤4 ≥−4	JTG E20: T0609 or JTG E20: T0610
7	Density 15 °C (g/m3)	1.001	Actual measurement	JTG E20: T0603

lists the performance specifications and test criteria for the biomimetic warm-mix regenerant.

3.1.2. Mix Design

To carry out the mix performance test, the ratio of RAP and new aggregate need to meet the requirements of asphalt mixture grade design specification, using 30% of RAP instead of new material, raw material grade and grade range is shown in

Table 16. Raw material grading and grading range.

Raw Materials	Passage Percentage (%) of Each Sieve Hole (mm)
	16	13.2	9.5	4.75	2.36	1.18	0.6	0.3	0.15	0.075
10~15	99	93.8	18.5	0.3	0.3	0.3	0.3	0.3	0.1	0.1
5~10	100	99.9	95.7	11.2	2.1	1.3	1.1	1.0	0.8	0.6
0~5	100	100	100	98.3	75.5	52.4	29.6	15.5	6.8	2.9
Mineral powder	100	100	100	100	100	100	100	100	100	100
RAP 0~5	100	96.1	52.4	16.2	10.6	7.6	4.8	1.4	0.5	0.2
RAP 5~10	100	100	99.8	61.8	40.7	28.7	16.9	7.7	3.1	1.2
RAP 10~20	100	100	100	89.8	57.5	37.6	19.5	7.7	2.9	1.1
Grading online	100	90	70	40	26	17	12	9	7	5
Graded offline	100	100	83	58	40	30	23	18	13	8
Grading median	100	95	76.5	49	33	23.5	17.5	13.5	10	6.5

, grade curve is shown in Figure 6, using the median of the AC-13 type grade as the synthetic grade.

3.1.3. Best Oil–Stone Ratio

The optimal asphalt dosage for mineral material gradation is used as the optimal oil dosage for the warm-mix recycled asphalt mixture. Mixing at 130 °C and conducting a standard Marshall test shows that, when the optimal asphalt ratio is 4.8%, the optimal asphalt ratio is 4.8%. The test results of each indicator are shown in

Table 17. Marshall experimental results.

Grading Type	Whetstone Ratio (%)	Stability (kN)	Flow Value (0.1 mm)	Void Ratio (%)
AC-13	4.8	8.16	2.41	2.528

.

3.2. Research on Road Performance of Warm-Mix Recycled Asphalt Mixture

3.2.1. Optimum Dosage of Bionic-Based Warm-Mix Regenerant

The high-temperature performance, low-temperature performance, and water stability performance of the warm-mix reclaimed asphalt mixtures were verified by a uniaxial penetration test, low-temperature splitting test, and freeze–thaw penetration test, respectively, to determine the optimum dosage of the best biomimetic warm-mix reclaimer. A warm-mix reclaimer with 4%, 6%, 8%, and 10% of old asphalt content was added to the RAP to determine the performance of the mixes. The results of the mix performance tests are shown in

Table 18. The performance of the SBS-modified asphalt mixture after externally adding the biomimetic warm-mix regenerant agent.

Dosage of Bionic-Based Warm-Mix Regenerant (%)	0	4	6	8	10	Brand New Material
Uniaxial penetration test (MPa)	1.99	1.60	1.61	1.68	1.38	1.60
Freeze–thaw splitting test (MPa)	4.316	4.844	4.846	4.504	4.531	4.847
Freeze–thaw penetration test (%)	0.703	0.809	0.839	0.922	1.264	0.838

and Figure 7. The new material represents the asphalt mix in normal mixing conditions.

(1)

Uniaxial Penetration Test

A 63.5 mm Marshall specimen of the specified height was made, and load was applied uniformly at a rate of 2 mm/minute until the specimen was damaged. The record of the test data is detailed in Table 18 and the calculation of compressive strength follows Equation (11).

$$R_c=\frac{4P}{\pi d^2}$$

(11)

Among them, P is the maximum load when the specimen fails (kN), R_c is the compressive strength of the specimen (MPa), and d is the diameter of the specimen (mm)

(2)

Splitting Strength Test

The test conditions were a test temperature of −10 °C and a loading speed of 1 mm/min.

(3)

Freeze–Thaw Penetration Test

The ratio of the uniaxial penetration strength after freezing and thawing to the uniaxial penetration strength without freezing and thawing was used as the freeze–thaw penetration strength ratio to evaluate the water damage resistance of the warm-mix recycled asphalt mixtures.

Table 18 and Figure 7, Figure 8 and Figure 9 show that, at the same temperature and RAP dosage, the 6% dosage of biomimetic warm-mix recycled asphalt mixtures has the best high-temperature stability, low-temperature cracking resistance, and water stability, so 6% was chosen. The biomimetic warm-mix recycled material was used to test the pavement performance of the mix at this dosage level.

In testing the performance of mixes with different warm-mix reclaimer dosages, two sets of specimens were made for each dosage and tested in parallel. The test results were averaged. The test procedure is shown in Figure 10, Figure 11 and Figure 12.

3.2.2. High-Temperature Performance

During use, asphalt mixtures are prone to irreversible deformation at high temperatures due to vehicle loading and environmental influences. Usually, the high-temperature performance of asphalt mixtures refers to the ability of asphalt pavements to resist rutting deformation in high-temperature environments [15]. The dynamic stability of recycled mixtures is calculated from the rut depth of a rutting test machine. A test axle with a wheel load of 70 MPa was repeatedly rolled over the specimen at a rate of 42 times/minute ± 1 time/minute at 60 °C. The test axle was then rolled over the specimen at a rate of 1 time/minute ± 1 time/minute. The dynamic stability of the asphalt mixture is assessed as the resistance to rutting at high temperatures. The calculation formula is shown in (12). In this test, three types of rutted slabs were pre-fabricated, using 0% and 6% of warm mix reclaimer, as well as the new material. Two pieces of each material were fabricated and rutted by the rutting test machine. The test procedure is shown in Figure 13. The results are shown in

Table 19. Rutting test results.

Warm-Mix Recycled Mix	DS (Second-Rate/mm)	Average Value	Requirements	Experiment Method
0%	3210	2860	≥2400	T0719-2011
	2510
6%	3231	4631
	6462
Brand new material	5836	6233
	6631

.

$$DS=\frac{(t_2-t_1)\times N}{d_2-d_1}\times C_1\times C_2$$

(12)

DS is the dynamic stability of the asphalt mixture (times/mm), t₁ = 45 min, t₂ = 60 min, the deformation of the d₁ specimen in 45 min, the deformation of the d₂ specimen in 60 min, N represents the number of rut rolling, C₁ is the correction coefficient, and C₂ is the specimen coefficient.

3.2.3. Low-Temperature Performance

When asphalt pavements are repeatedly subjected to travelling loads at low temperatures, the pavement is prone to fracture. When the temperature of the asphalt pavement is low, the phenomenon of cold shrinkage occurs. When the stress generated by the shrinkage of the mixture is greater than the maximum flexural and tensile strengths that the asphalt mixture can withstand, the internal materials of the asphalt pavement will be damaged, resulting in cracks [16,17,18,19,20]. When cracks appear, natural water and groundwater will infiltrate and cause more serious damage to the asphalt pavement. The low-temperature performance of warm-mix recycled asphalt mixtures was affected by the trabecular bending test. The prepared trabecular specimens were kept at a low temperature of −10 °C for 4 h and loaded at a 50 min/min and 40 kN loading rate. Rutted slabs were made with different doses of warm-mix regenerant and new materials. The rutted slabs were cut into beams of the same specifications with a cutting machine and subjected to bending tests. The test procedure is shown in Figure 14 and the calculation formula is shown in Equation (13). The two sets of test results of the bending test were averaged and shown in

Table 20. Low-temperature bending test results.

Warm-Mix Recycled Mixture	Maximum Bending and Tensile Strain (με)	Average Value	Requirements	Experiment Method
0%	2284	2370	≥3000	T0715-2011
	2456
6%	3382	3432
	3482
Brand new material	3182	3163
	3144

.

$${\epsilon }_B=\frac{6\times h\times d}{L^2}$$

(13)

In the formula, ${\epsilon }_B$ is the maximum tensile strain ($\mu \epsilon$), h is the mid-span height of the section (mm), d is the mid-span deflection (mm), and L is the span length (mm)

3.2.4. Water Stability

Asphalt mixtures consist mainly of aggregates and binders. Under the erosion of rainwater, they enter the asphalt mixture through the pores and cracks, erode the binder around the aggregates, and lead to adhesion between the aggregates. The performance is reduced, resulting in a loose mixture that is prone to damage such as water on the road surface. With the increase in the service life of asphalt pavement, under the action of traffic load, the asphalt pavement will be subject to the scouring of rainwater and the action of vehicle load. The erosion intensity will become larger and larger, and the degree of damage will increase dramatically. The water flooding Marshall test characterizes the water stability performance of asphalt mixtures.

According to the requirements, Marshall specimens were made, and the test specimens were randomly divided into two groups—the first group was kept in a constant temperature water tank for 30 min, and the second group was immersed in water for 48 h in a constant temperature water tank—and the Marshall stability test was carried out at a loading rate of 50 mm/min. The calculation of the residual stability of immersion MS₀ is shown in Equation 18, the test process is shown in Figure 15, and the test results are shown in

Table 21. Immersion Marshall test.

Warm-Mix Recycled Mix	Residual Stability (%)	Experiment Method	Requirements
0%	83.02	≥85	T0709-2011
6%	90.50
Brand new material	94.31

.

$$M{S}_0=\frac{M{S}_1}{MS}\times 100\%$$

(14)

MS₀ represents the residual stability of water immersion (%), MS₁ (kN) represents the stability of the specimen immersed in water for 48 h, and MS (kN) represents the stability of the specimen prepared by the standard test method.

According to the results of the water immersion Marshall test and freeze–thaw splitting test, the water stability of asphalt mixtures with a 6% warm-mix reclaimer meets the specification requirements. The water stability performance of the warm-mix reclaimer decreased, but the water stability performance of the bionic warm-mix reclaimer used in this paper decreased slightly. It is closer to the performance of the new material while meeting the specification requirements.

4. Microscopic Properties of Biomimetic Warm-Mix Regeneration Agent and Aged Asphalt

RTFOT and PAV were used to assess the short- and long-term aging effects of biomimetic warm-mix rejuvenators on SBS-modified asphalt, respectively [21]. The RTFOT was conducted at 163 °C with a test time of 75 min, the PAV was conducted at 100 °C with a storage time of 20 h, and the pressure in the container was 2.1 ± 0.1 MPa. The short- and long-term aged asphalt outputs of the different warm-mix rejuvenators were added to aluminum box.

4.1. FTIR Test

An IRT-100 infrared spectrometer from Japan’s Shimadzu Company was used to test the aged asphalt samples. The resolution of the FTIR test is 4 cm⁻¹, and the wavelength range is 4000~400 cm⁻¹, scanning 32 times.

4.1.1. RTFOT Test Results

The absorption peaks of different functional groups are shown in

Table 22. Different functional group absorption peaks.

Band (cm−1)	Functional Group
2850~3000, 1456	C-H [22]
1700	-OH [23]
1620	C=C and C=O [24]
1300–1400	-CH2 [25]
1000–1300	R—S(=O)—R′
725	C-H

.

The FTIR spectra of short-term aged SBS-modified asphalt with the bionic warm-mix reclaimer are shown in Figure 16. As shown in Figure 8, the FTIR curves of the bionic warm-mix reclaimer are similar to those of reclaimed asphalt, aged asphalt, and asphalt. The absorption peaks at 3000~2850 cm⁻¹ are caused by the asymmetric vibration of C-H in methylene (CH₂), and the absorption peaks at 1700 cm⁻¹ are caused by the vibration stretching of hydroxyl groups, and new hydroxyl groups appeared after the addition of the biomimetic warm-mix rejuvenator. This is due to the overlapping of hydroxyl and keto hydroxyl groups in the biomimetic warm-mix regenerant [26]. The C-H deformation vibration of the methylene group leads to the absorption peak at 1456 cm⁻¹. The one at 966 cm⁻¹ is caused by butadiene in the modifier. The one at 725 cm⁻¹ is caused by the C-H bending vibration in the benzene ring substituent. It can be seen from Figure 15 that the absorption peak at 2925 cm⁻¹ increases with the increase in the bionic warm-mix rejuvenator, which indicates that the bionic warm-mix rejuvenator acts on the aged asphalt, resulting in an increase in the saturated content of the aged asphalt to achieve a rejuvenation effect.

4.1.2. PAV Test Results

Studies found that the changes in the infrared absorption peaks of the carbonyl and sulfoxide functional groups can characterize the aging degree of asphalt [27]. The carbonyl index (CI) and sulfoxide index (SI) are shown in Formula (15), and the infrared spectrum is shown in Figure 17.

$$\begin{array}{l}CI=A_{1700}/{\displaystyle \sum A_{600}}~A_{2000}\\ SI=A_{1030}/{\displaystyle \sum A_{600}}~A_{2000}\end{array}$$

(15)

In the formula, A₁₇₀₀ represents the carbonyl area of 1700 cm⁻¹, and A₁₀₃₀ represents the sulfoxide group area of 1030 cm⁻¹.

The calculation results are shown in

Table 23. Long-term CI and SI.

Sample	CI	SI
SBS-modified asphalt	0	0.002
130 °C-0%	0.05	0.04
110 °C-6%	0.0047	0.0059
120 °C-6%	0.0045	0.0055
130 °C-6%	0.0045	0.0051
140 °C-6%	0.0048	0.0057
150 °C-6%	0.0036	0.0053
130 °C-4%	0.0046	0.0058
130 °C-8%	0.0043	0.0052

.

As can be seen from Table 23, the CI and SI of SBS-modified asphalt increased significantly after PAV aging, but, after the addition of the biomimetic-based warm-mix regenerant, the CI and SI showed a decreasing tendency with the increase in the added amount. This is because the C-H content increases in the wave number range of 2000–600 cm⁻¹, which reduces the carbonyl and sulfoxide indices. When the mixing temperature was 130 ℃ and the dosage was 6% and 8%, the CI and SI decreased most obviously, and the asphalt recovered the best from aging; with the decrease in temperature, the aging performance of asphalt could be recovered as well. It is further proven that the warm-mix effect of the biomimetic-based warm-mix rejuvenator is best at 130 ℃, and the optimum dosage is 6%.

4.2. Thermal Performance Analysis

4.2.1. TG Analyses

The high-temperature behavior of materials is reflected by the thermal decomposition of the material, i.e., the decomposition, volatilization, and mass loss with increasing temperature [28]. The tests were carried out using a Q50 thermogravimetric analyzer from TA (USA), heated from room temperature 25 °C to 800 °C at a rate of 20 °C/min using nitrogen as a protective gas. The test results were analyzed and processed by two methods: differential thermogravimetry (DTA) and thermogravimetric analysis (TGA).

The results of the tests are shown in Figure 17. All materials went through two stages of pyrolysis: no decomposition and violent decomposition. From 25 °C to 250 °C, there is zero decomposition. At this temperature, there is no change in the mass of the bitumen and the weight loss is zero. Between 250 °C and 500 °C, the bitumen undergoes violent decomposition with a mass loss of more than 75%. The lower the initial decomposition temperature of the modified bitumen, the wider the distribution of its components. On the contrary, the higher the decomposition temperature, the narrower the distribution of the components [29]. Bionic warm mixes achieve the recycling effect by increasing the light component of asphalt. From Figure 18, it can be seen that the initial decomposition temperature is in the order of aged asphalt > SBS asphalt > recycled asphalt. The initial decomposition temperature of warm mix recycled asphalt is in the order of 6% dosage > 8% dosage. Therefore, in the heating process, the bionic warm-mix recycled asphalt is first pyrolyzed, and then the recycled asphalt decomposes; and the light component is pyrolyzed, and then decomposes.

The DTA curves and thermal degradation parameters were plotted to better analyze the high-temperature performance of warm-mix recycled asphalt. The test results are shown in Figure 19 and

Table 24. DTA test results.

Sample	T5% (°C)		T10% (°C)		TD (°C)	Tmax (°C)	Residual Carbon Content (%)
(a) Degradation temperature and char residue of short-term aging
SBS	332.2		365.3		392.9	461.9	14.18
RTFOT	338.0		368.9		392.3	461.1	18.08
6%	371.3		352.2		385.9	460.4	17.70
8%	312.7		348.66		384.7	458.5	16.71
(b) Degradation temperature and char residue of long-term aging
SBS	332.2	365.3		392.9		461.9	14.18
RTFOT	338.9	370.2		387.5		459.8	19.41
6%	317.7	345.3		384.1		459.8	17.82
8%	308.2	344.2		385.0		458.9	17.21

Note: T_5% is the 5% weight loss rate, T_10% is the 10% weight loss rate, T_D is the extrapolated starting temperature, and T_max is the maximum weight loss temperature.

.

As can be seen from Figure 19 and the table in Table 24, the onset temperature of severe pyrolysis was found to be SBS asphalt > aged asphalt > 6% dosage > 8% dosage after regeneration with the addition of biomimetic warm-mix reclaimant under RTFOT and PAV conditions; the maximum pyrolysis temperature is as follows: SBS asphalt > aged asphalt > 6% dosage > 8% dosage. During pyrolysis, some of the biomimetic warm-mix recycled asphalt will be pyrolyzed first. Then, the asphalt will be pyrolyzed, so that the initial pyrolysis temperature of the recycled asphalt is lower than the initial pyrolysis temperature of the aged asphalt. In the thermal degradation process, the mass loss of warm-mix recycled asphalt is slower than that of aged asphalt and SBS asphalt; the residual material after pyrolysis is in the form of coke, and both are greater than 10%. The residual amount of warm-mix recycled asphalt is smaller than the aging residual amount, and the 8% dosage is smaller than 6% dosage. In summary, after adding the bionic warm-mix rejuvenator, the light component of aged asphalt was recovered, and the high-temperature resistance was improved, which verified that the bionic warm-mix rejuvenator has the advantage of improving the high-temperature resistance of asphalt. This result is consistent with the change trend of the mix test result.

The DTA curve and thermal degradation parameters were drawn to better analyze the high-temperature performance of warm-mix recycled asphalt. The test results are shown in Figure 20 and Table 24.

4.2.2. DSC Analyses

For the DSC test, an American Q20 differential scanning calorimeter was used. The sample was heated with the protective gas of nitrogen. It was heated from 30 °C to 120 °C at 10 °C/min, kept constant for two minutes, and then cooled to −7 °C at the same speed. The temperature was kept constant for 1 min, and then raised to 120 °C to obtain the heat flow curve.

It can be seen from

Table 25. Glass transition temperature.

Asphalt Type		Tg (°C)
SBS-modified asphalt		−28.5
RTFOT	0%	−24.4
	6%	−25.1
	8%	−25.3
PAV	0%	−27.8
	6%	−30.9
	8%	−29.3

that SBS asphalt has the smallest T_g and the best low-temperature performance. The T_g change trends of RTFOT and PAV are similar. However, the Tg of asphalt after RTFOT was significantly lower than that after PAV. In summary, the low-temperature performance of SBS asphalt decreases with the increase in aging time. After RTFOT, the low-temperature performance of the aged SBS-modified asphalt mixed with the biomimetic warm-mix regenerator is lower than that of the original asphalt. This is because the biomimetic warm-mix regenerant cannot fully act, so the T_g is smaller; in PAV, aged asphalt T_g > regenerated asphalt T_g. The results show that, with the addition of the biomimetic warm-mix regenerant, the low-temperature performance of aged asphalt is improved and higher than that of the original asphalt, which is consistent with the low-temperature test rules of recycled asphalt mixtures—8% mixed. The T_g of the dosage is greater than that of the 6% one, proving that adding a bionic-based warm mix causes poor low-temperature performance in short-term aging regeneration.

5. Conclusions

(1)

The optimized preparation process for biomimetic warm-mixed regeneration is as follows: T₁ and T₂ are 45 °C and 50 °C, respectively; t₁ and t₂ are 0.5 h and 0.3 h, respectively; and S₁ and S₂ are 3000 r/min and 3000 r/min, respectively. The optimal formula has 45% base oil, 17% extracted oil, 10% biomimetic mussel gum, 20% activator, and 8% antioxidant.

(2)

The biomimetic warm-mixed regenerant has an optimal content of 6%. At this content, the recycled asphalt mixture’s high-temperature stability, low-temperature crack resistance, and water stability meet the specification requirements, and compared with the new material, performance has increased to a certain extent. However, compared to the SBS asphalt mixture, the water stability performance has significantly decreased. Despite this reduction, it remains insignificant and is still higher than most current bio-based warm-mix regenerates.

(3)

It has been verified that this bionic-based warm-mixed regeneration agent can restore asphalt performance at a lower temperature than hot-mix regeneration. The FTIR test results reveal that the ideal content for the bio-based warm-mixed regenerates is 6%, aligning with the optimal content of the mixture. The high-temperature stability and low-temperature crack resistance of the bio-based warm-mixed regenerate on aged asphalt are significantly improved, which is consistent with the conclusion of the mixture.

(4)

All the experiments are now in the laboratory phase and require integration with practical engineering. Further research is needed to investigate the method and sequence of adding and mixing warm-mixed regeneration using bionic-based techniques in relation to real-world engineering applications.