**2. Materials and Methods**

#### *2.1. Materials*

Styrene–butadiene–styrene modified asphalt (named as SBS asphalt for short) was used in this research, which was acted as virgin asphalt. This was mainly because the recycled asphalt mixtures (RAM) in this paper was mainly used in the upper layer of the asphalt pavement. The upper layer usually had strict requirements on the performance of asphalt mixtures, thus SBS asphalt was commonly adopted [33]. SBS asphalt has a penetration of 68 dmm at 25 ◦C and a ductility of 486 mm at 5 ◦C and a softening point of <sup>56</sup> ◦C. Rejuvenator with a density of 0.943 g/cm<sup>3</sup> and a viscosity of 1.780 Pa·s at 60 ◦C were included. SBS asphalt and rejuvenator were obtained from Hohhot and Nanjing, China. Steel slag and basalt as virgin aggregates were supplied from Wuhan and Jingshan. Steel slag coupled with SBS modified asphalt were conducted to prepare RAM with excellent performance, so as to create implementation of the maximum additional value of steel slag. The rejuvenator in this study was mainly used to restore the performance index of aged asphalt in RAP [34]. Their properties were displayed in Table 1.


**Table 1.** The properties indexes of steel slag and basalt.

#### *2.2. Methods*

#### 2.2.1. Preparation of RAM

RAP were acquired from surface pavement on Wu-Huang highway of China. RAM with 10%, 20%, 30%, 40% and 50% RAP and 13.2 mm nominal maximum size were prepared based on Marshall design method. The dosage of the rejuvenator in RAM was 6 wt% (mass ratio of rejuvenator and aged asphalt) [35]. Steel slag and basalt were conducted as virgin coarse aggregates and limestone was conducted as virgin fine aggregate to fabricate two types of RAM. Limestone powder as filler was involved. Figure 2 depicted the grading curve of RAM with steel slag and basalt.

**Figure 2.** Grading curve of asphalt concrete (AC)-13: (**a**) steel slag; (**b**) basalt.

#### 2.2.2. Performance Evaluation of Recycled Asphalt Mixtures

The bulk density, air voids, VMA and VFA were employed to investigate the volume performance of RAM as the evaluation indexes. Tests of Marshall stability, indirect tensile strength and Cantabro spatter loss were applied to assess the moisture susceptibility of RAM. The standard wheel tracking test with a rolling speed of 42 cycles/min, a testing temperature of 60 ◦C and a load strength of 0.7 MPa were conducted to discuss the high temperature stability performance. Low temperature performance was characterized through a three-points bending test with a temperature of −10 ◦C and a loading rate of 50 mm/min. Beam specimens with a size of 250 × 30 × 35 mm were involved. Texture depth (TD) and British Pendulum Number (BPN) were used to evaluate the skid resistance.

A fatigue resistance test combined with a Hamburg wheel tracking (HWT) test were carried out to study the durability of RAM. Indirect tensile fatigue test with a testing temperature of 15 ◦C, the stress level of 0.35, 0.40, 0.45 and 0.50 MPa and a Poisson's ratio of 0.35 were used. The testing device and schematic diagram of HWT test are described in Figure 3. The creep slope and stripping slope were slope of the tangent line of the creep curve and stripping curve. By calculating the intersection point of the two curves, the stripping inflection point (SIP) could be acquired. The testing temperature and rolling rate were 50 ◦C and 45 cycles/min [36].

**Figure 3.** Testing device (**a**) and schematic diagram (**b**) of HWT test.

2.2.3. Radar Chart Evaluation Method

The radar chart method is a common graphical method to display multiple variables, which can map a multidimensional space point to two-dimensional space, indicating the feature of qualitative evaluation of each evaluation object. In the traditional radar map evaluation, the area and perimeter of the graph were extracted as feature vectors, while the feature vector area and perimeter revealed the disadvantage of varying with the ranking of indicators. Therefore, an improved radar chart evaluation method with uniqueness feature was conducted to quantitatively assess the comprehensive performance of RAM.

In the improved radar chart evaluation method, the evaluation vector and evaluation function were constructed by extracting feature vectors to comprehensively reflect the level of RAM and the balanced development degree of each index [37]. Firstly, a matrix A = (*aij*)n × k for the evaluation indicators was established. Vector X = {*x*1, *x*2, *x*<sup>3</sup> . . . *x*n} and Y = {*y*1, *y*2, *y*<sup>3</sup> . . . *y*k} represent a group of objects and a set of indicators for the objects.

Secondly, the data in matrix A were standardized and non-linear transformed through Equations (1) and (2).

$$b\_{ij} = \frac{a\_{ij} - E\left(y\_j\right)}{\sigma(y\_j)}\tag{1}$$

$$r\_{i\bar{j}} = \frac{2}{\pi} \arctan(b\_{i\bar{j}}) + 1 \tag{2}$$

where *bij* and *rij* represent each indicator after standardization and non-linear transformation, respectively, and *E(y<sup>j</sup> )* and *σ(y<sup>j</sup> )* are the average value and standard deviation indicator *j*.

Thirdly, the characteristic vectors were calculated according to Equations (3) and (4).

$$u\_{\bar{i}} = [A\_{\bar{i}}, L\_{\bar{i}}] \tag{3}$$

$$\begin{cases} A\_i = \sum\_{j=1}^k \frac{1}{k} \pi r\_{ij}^2 \\\ L\_i = \sum\_{j=1}^k \frac{2}{k} \pi r\_{ij} \end{cases} \tag{4}$$

where *A<sup>i</sup>* and *L<sup>i</sup>* represent the area inside the arcs and sum length of arcs, respectively, and k represents the number of indicators.

Fourthly, the evaluation vector is defined based on the extracted characteristic vector, as shown in Equation (5).

$$\boldsymbol{\nu}\_{i} = \begin{bmatrix} \boldsymbol{\nu}\_{i1} \,\boldsymbol{\nu}\_{i2} \end{bmatrix} \tag{5}$$

where *νi*<sup>1</sup> and *νi*<sup>2</sup> are the relative area and perimeter of evaluation object. Calculation method of *νi*<sup>1</sup> and *νi*<sup>2</sup> are displayed in Equation (6).

$$\begin{cases} \nu\_{i1} = \frac{A\_i}{\text{Max}A\_i} \\ \nu\_{i2} = \frac{L\_i}{2\pi\sqrt{\frac{A\_i}{\pi}}} \end{cases} \tag{6}$$

Finally, the comprehensive evaluation function (f) was deduced through the geometric mean of *νi*<sup>1</sup> and *νi*2, as displayed in Equation (7).

$$\mathbf{f}(\boldsymbol{\upsilon}\_{\text{i1}} \; \boldsymbol{\upsilon}\_{\text{i2}}) = \sqrt{\boldsymbol{\upsilon}\_{\text{i1}} \times \boldsymbol{\upsilon}\_{\text{i2}}} \tag{7}$$

The road performance indexes of the RAM were obtained through the performance test. Through the above formula, the evaluation indicators of the two RAM with different RAP content were standardized and normalized, and the comprehensive performance of the two materials was evaluated by the obtained evaluation function.

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

#### *3.1. Volume Performance*

The volume performance parameters of RAM are presented in Table 2. When RAP content is lower than 30%, asphalt aggregate ratio of RAM with steel slag no change with the rising RAP content. As the RAP dosage continues to increase, asphalt aggregate ratio rises. The addition of RAP significantly reduces the bulk density of RAM with steel slag, while fluctuates little on the air voids, VMA and VFA. The asphalt-aggregate ratio of RAM with basalt demonstrates the similar change trend as the RAP content increases. There is a certain degree of increase in bulk density of RAM incorporated with basalt as the ascending RAP dosage. This is due to the diminution of virgin fine aggregate content of limestone with lower density. In general, the volume performance of steel slag and basalt RAM with different RAP content meet the specification requirements [38].



## *3.2. Moisture Susceptibility*

## 3.2.1. Residual Marshall Stability

Figure 4 illustrates the results of residual Marshall stability (RMS) of RAM incorporated with steel slag and basalt. Compared with the virgin steel slag asphalt mixtures, the Marshall stability (MS) and immersion Marshall stability (MS1) of RAM increase with ascending RAP dosage in contrast to RMS. This is attributed to the enhancement of the overall elasticity of RAM with aged asphalt of high modulus. The result is match to the conclusion of Oldham et al. [39]. Meanwhile, aged asphalt reveals inferior adhesion property with aggregates, resulting in the reduction in RMS of RAM as RAP content increases. Steel slag RAM with 50% RAP exhibits an RMS of 90.5% and still remains at a high level. For basalt RAM, its variation rule of MS, MS<sup>1</sup> and RMS are consistent with that of steel slag RAM. While it demonstrates lower performance indexes under the same RAP content. RAM with steel slag and basalt all satisfy the requirements that the RMS of the modified asphalt mixture in the wet areas is not less than 85%.

**Figure 4.** RMS results of RAM: (**a**) steel slag; (**b**) basalt.

#### 3.2.2. Tensile Strength Ratio

Freeze-thaw splitting test can more truly reflect the water damage resistance of asphalt mixtures. Tensile strength ratio (TSR) reveals more stringent requirement than RMS, and TSR of asphalt mixture will not meet the requirements when RMS arrivals design constrain. Figure 5 presents the TSR results of RAM. It is stated that splitting tensile strength and TSR of RAM show a linear decrease with the ascending RAP dosage. This is because that RAP increases the modulus of asphalt in RAM, which diminishes the bonding force between asphalt and aggregate, resulting in the exfoliation of asphalt from the surface layer of the aggregate under water immersion, thus weakening the mechanical properties of the RAM [40]. Steel slag RAM with 50% RAP content exhibits a 6.2% reduction compared to virgin asphalt mixtures and reaches to 89.3%. While for RAM prepared with basalt, the corresponding values are 13.6% and 81.0%. This indicates that incorporating steel slag in RAM can reduce the potential moisture damage risks and elevate the water stability of RAM. This is consistent with the fact that steel slag embodied superior adhesive effect than basalt according to the analysis of molecular simulation [41].

#### 3.2.3. Cantabro Spatter Loss

The Cantabro spatter loss test is commonly applied to assess the adhesion between asphalt and aggregate in open-graded asphalt mixtures. Although a dense gradation was included in this study, the relative high RAP content may reveal a greater impact on the overall bonding of the asphalt mixtures due to inferior adhesion property between aged asphalt and aggregates. Given that AC-13 RAM was adopted as top layer of asphalt pavement, it suffers from surface stripping due to the dual action of rutting and rainwater.

Therefore, Cantabro spatter loss can be conducted to characterize resistance to water damage of RAM.

**Figure 5.** TSR results of RAM: (**a**) steel slag; (**b**) basalt.

Figure 6 depicts the Cantabro spatter loss results of RAM with steel slag and basalt. The spatter loss of steel slag and basalt RAM all boost as the ascending RAP dosage, while steel slag RAM exhibits a lower increment. Steel slag RAM involving 50% RAP content reaches a spatter loss of 5.5% and only increases by 1.9% in comparison with virgin asphalt mixture. This indicates that on the one hand, aged asphalt leads to a reduction in the adhesion of the aggregate to the asphalt in RAM, causing the surface binder to fall off under the action of water. On the other hand, the viscosity of the aged asphalt is restored by the action of the rejuvenator, which leads to an improvement effect of bond performance for overall asphalt in RAM. Comparative analysis of spatter loss results verified that steel slag RAM demonstrates superior moisture susceptibility than basalt RAM. Furthermore, the rising RAP content will not cause structural damage to RAM with steel slag and basalt due to their low spatter loss.

**Figure 6.** Spatter loss results of RAM: (**a**) steel slag; (**b**) basalt.

#### *3.3. High Temperature Stability Performance*

Figure 7 indicates the dynamic stability (DS) results of RAM. The participation of RAP boosts the DS of RAM and elevate its high temperature stability performance. It can be elaborated by the consequence that aged asphalt with high softening point and stiffness can prominently reduce the rutting depth of RAM and enhance its anti-rutting performance. Steel slag RAM reveals larger DS than RAM prepared with basalt, indicating its superior rutting resistance. This is attributed to the outstanding mechanical properties and abundant texture index of steel slag. The DS of steel slag RAM with 50% RAP is 5040 times/mm, which is 1.27 times that of the virgin asphalt mixtures. Steel slag and basalt RAM all exhibit

DS much higher than 2400 times/mm (minimum index in standard) [38], representing their excellent high temperature performance.

#### *3.4. Low Temperature Performance*

The flexural tensile strain of asphalt mixture can effectively reflect the possibility of brittle fracture at low temperature and characterize its crack resistance. Asphalt mixture with greater flexural tensile strain means the superior low-temperature crack resistance. The crack resistance indexes from bending test results of RAM are provided in Table 3. As the RAP content rises, the maximum load, tensile strength and tensile strain of RAM ascend in contrast to stiffness modulus, resulting in reducing effect on low temperature performance. It can be explained by the fact that RAM with high RAP content displays lower plasticity and is incline to become hard and brittle, which weakens the resistance to low temperature deformation [42]. Comparison results of steel slag and basalt RAM state that involving steel slag can elevate the flexural tensile strength and tensile strain of RAM. Steel slag RAM with 50% RAP reveals a tensile strain of 2548.4 µε. While the corresponding index is 2323.1 µε for basalt RAM, which is less than the minimum value of 2500 µε in the winter cold area in the specification. This indicates that when RAP content increases to 50%, steel slag RAM can be applied in severe cold areas instead of basalt RAM.


**Table 3.** The crack resistance indexes of RAM.

**Figure 7.** Dynamic stability results of RAM.


**Table 3.** *Cont.*

#### *3.5. Skid Resistance*

The texture depth and BPN results of RAM are illustrated in Figure 8. Virgin steel slag asphalt mixture demonstrates the highest texture depth and BPN, which reach to 0.95 mm and 77. Incorporating RAP decreases the texture depth and BPN of RAM. This is attributed to the deterioration of polished and weared stone value of RAP aggregate. When the same RAP dosage is involved, steel slag RAM displays larger texture depth and BPN than basalt RAM, indicating its superior skid resistance. This is attributed to the rich texture and impaction structure of steel slag [32]. Steel slag RAM with 50% RAP content exhibits the texture depth of 0.81 mm and BPN of 63, which far exceed the minimum value of 0.55 mm and 45 in requirements of the specification [38]. Texture depth of basalt RAM reduces from 0.86 mm, 0.83 mm, 0.81 mm, 0.78 mm and 0.75 mm to 0.73 mm as RAP content rises from 0 to 50% with an interval of 10%. BPN of basalt RAM with 50% RAP decreases by 20% and only reaches to 52, which is lower than 11 that of steel slag RAM and also arrival design constrain.

**Figure 8.** Texture depth and British Pendulum Number (BPN) results of RAM.

#### *3.6. Durability*

#### 3.6.1. Fatigue Resistance

Fatigue life, as the critical assessment indicator, can reflect the number of stress cycles of sample experienced failure. The fatigue life curves of RAM are depicted in Figure 9. Both of steel slag RAM and basalt RAM reveal a downward trend of fatigue life as RAP dosage increases. The conclusion is inconsistent with Yin et al.'s results [12]. This is due to the increase in the stiffness of blended asphalt and reduction in the response rate to stress for RAM [43]. Under the same RAP content, steel slag RAM embodies the higher fatigue life than basalt RAM, representing its superior fatigue performance.

**Figure 9.** Fatigue life curve of RAM: (**a**) steel slag; (**b**) basalt.

Fitting coefficient of fatigue equation for RAM are presented in Table 4. The satisfactory correlation is found in all RAM samples for their high correlation coefficient. RAM with larger K and smaller n means preferable fatigue resistance. Steel slag virgin asphalt mixture possesses the highest K value, indicating its excellent fatigue performance. The addition of RAP decreases the K value and boosts n value of steel slag RAM. Compared with basalt RAM, incorporating steel slag in RAM can elevate its K value and fatigue resistance. This conclusion is match to the analysis results of fatigue life. This is because steel slag emerges better interfacial adhesion for its high angularity and texture.


**Table 4.** Fitting coefficient of fatigue equation for RAM.
