2.1.3. Aggregates and Fillers

The coarse and fine aggregates that were used comprised crushed limestone. The particle size of the coarse aggregate is ≥2.36 mm, while that of the fine aggregate is 0.075–2.36 mm. The filler that was used was the mineral powder ground from the limestone; the results of its technical properties are shown in Tables 3–5.

**Table 3.** Technical properties of coarse aggregate.


**Table 4.** Technical properties of fine aggregate.




### *2.2. Mixture Gradation*

Gussasphalt concrete is a kind of asphalt mixture with high asphalt content, high mineral powder content, and a void ratio of less than 1%, which is mixed at high temperature (220~260 ◦C) and paved by the fluidity of the mixture itself without rolling. The CEBM has great flowability with a self-compaction property after paving, it has high bitumen and high mineral powder content, and the gradation of CEBM is similar to that of Gussasphalt

concrete. The gradation range of Gussasphalt concretes varies among different countries. The gradation of CEBM was selected from the range intersection of Gussasphalt concrete in China, Germany, and the EU [41]. The aggregate composite gradation is shown in Figure 1. Table 6 is the proportion of each mineral aggregate. The cement is used to replace the part of the mineral powder in equal volume, and the sum of cement and mineral powder accounts for 24% the weight of the mineral aggregates. The emulsified bitumen content is measured in the weight of aggregates, and its proportion is calculated with the residue bitumen after demulsification. ent countries. The gradation of CEBM was selected from the range intersection of Gussasphalt concrete in China, Germany, and the EU [41]. The aggregate composite gradation is shown in Figure 1. Table 6 is the proportion of each mineral aggregate. The cement is used to replace the part of the mineral powder in equal volume, and the sum of cement and mineral powder accounts for 24% the weight of the mineral aggregates. The emulsified bitumen content is measured in the weight of aggregates, and its proportion is calculated with the residue bitumen after demulsification.

**Parameters Unit Results Requirements Experimental Method** 

<0.15 mm 92.4 90–100 JTG E42-2005 T0351

Apparent specific gravity – 2.612 ≥2.5 JTG E42-2005 T0352

Plasticity coefficient – 3.5 <4 JTG E42-2005 T0354 Hydrophilic coefficient – 0.82 <1 JTG E42-2005 T0353

Gussasphalt concrete is a kind of asphalt mixture with high asphalt content, high mineral powder content, and a void ratio of less than 1%, which is mixed at high temperature (220~260 °C) and paved by the fluidity of the mixture itself without rolling. The CEBM has great flowability with a self-compaction property after paving, it has high bitumen and high mineral powder content, and the gradation of CEBM is similar to that of Gussasphalt concrete. The gradation range of Gussasphalt concretes varies among differ-

<0.6 mm 100 100

<0.075 mm 86.3 75–100

*Materials* **2022**, *15*, x FOR PEER REVIEW 5 of 22

**Table 5.** Technical properties of filler.

Particle size range (%)

*2.2. Mixture Gradation* 

**Figure 1.** Gradation curve. **Figure 1.** Gradation curve.

**Table 6.** Grading and proportion of mineral materials. **Table 6.** Grading and proportion of mineral materials.


#### *2.3. Mechanism Characterization Test 2.3. Mechanism Characterization Test*

2.3.1. Surface Micro-Morphology Testing (SEM) 2.3.1. Surface Micro-Morphology Testing (SEM)

A Scanning Electron Microscope (SEM, JSW-5510LV) was applied to investigate the surface micro-morphology of CEBM, and the test voltage was 15 mv. The sample was collected from the middle of the CEBM specimen, cured for 24 h, and dried in an oven at 45 ◦C. Then, a gold spraying treatment was conducted on the surface of the sample, to ensure the sample conduction. The sample was placed on the sample table of SEM for detection.

#### 2.3.2. Surface Free Energy Measurements

The surface free energy (SFE) of the residual bitumen from the emulsified bitumen, 70# base bitumen, styrene-butadiene-styrene (SBS) modified bitumen, limestone aggregate, and cement concrete were tested. For the preparation of the bitumen sample, the bitumen was dropped on a glass slide and placed horizontally in an oven at 150 ± 5 ◦C for 3 min to level the bitumen surface. Then, it was naturally cooled in a dust-free and dry environment for 6 h. The heated and liquified bitumen were spread onto a dried heat-resistant microscope slide to form a homogeneous film. The bitumen specimen is shown in Figure 2a. For the aggregate and cement specimen, limestone rock and 7 d cured cement block were cut into 50 mm × 50 mm × 5 mm and a polishing treatment of their surfaces was performed to obtain a smooth specimen. The specimen is shown in Figure 2b.

detection.

2.3.2. Surface Free Energy Measurements

**Figure 2.** Contact angle test of bitumen and cement mortar. ((**a**) The bitumen specimen; (**b**) limestone rock specimen). **Figure 2.** Contact angle test of bitumen and cement mortar. ((**a**) The bitumen specimen; (**b**) limestone rock specimen).

A Scanning Electron Microscope (SEM, JSW-5510LV) was applied to investigate the surface micro-morphology of CEBM, and the test voltage was 15 mv. The sample was collected from the middle of the CEBM specimen, cured for 24 h, and dried in an oven at 45 °C. Then, a gold spraying treatment was conducted on the surface of the sample, to ensure the sample conduction. The sample was placed on the sample table of SEM for

The surface free energy (SFE) of the residual bitumen from the emulsified bitumen, 70# base bitumen, styrene-butadiene-styrene (SBS) modified bitumen, limestone aggregate, and cement concrete were tested. For the preparation of the bitumen sample, the bitumen was dropped on a glass slide and placed horizontally in an oven at 150 ± 5 °C for 3 min to level the bitumen surface. Then, it was naturally cooled in a dust-free and dry environment for 6 h. The heated and liquified bitumen were spread onto a dried heatresistant microscope slide to form a homogeneous film. The bitumen specimen is shown in Figure 2a. For the aggregate and cement specimen, limestone rock and 7 d cured cement block were cut into 50mm × 50mm × 5 mm and a polishing treatment of their surfaces was

performed to obtain a smooth specimen. The specimen is shown in Figure 2b.

The bitumen (film), limestone, and cement concrete were tested using three liquids with known SFE parameters, namely distilled water, ethylene glycol, and glycerol, respectively. The distilled water, ethylene glycol, and glycerol were used to measure the contact angle; the surface free energy parameters of these three kinds of liquid are shown in Table 7. The pendant-drop method was applied and the contact angle was measured with a contact angle tester, as shown in Figure 3, the test temperature was 25 °C. The experiment was carried out using an SDC-100 contact angle device that was sourced from Dongguan Dingsheng Precision Instrument Co., Ltd. The bitumen (film), limestone, and cement concrete were tested using three liquids with known SFE parameters, namely distilled water, ethylene glycol, and glycerol, respectively. The distilled water, ethylene glycol, and glycerol were used to measure the contact angle; the surface free energy parameters of these three kinds of liquid are shown in Table 7. The pendant-drop method was applied and the contact angle was measured with a contact angle tester, as shown in Figure 3, the test temperature was 25 ◦C. The experiment was carried out using an SDC-100 contact angle device that was sourced from Dongguan Dingsheng Precision Instrument Co., Ltd.


**Table 7.** SFE parameters of test liquid (25 ◦C, mJ/m<sup>2</sup> ).

## *2.4. Manufacturing of CEBM*

CEBM adopts a Gussasphalt concrete gradation, based on a C/EB formula system with high asphalt content, high mineral powder (including cement) content, and a small amount of water, which grants it great flowability. Using its great flowability "pouring, leveling, and compacting", it can form a uniform pavement with high density and low voids without rolling.

Firstly, Gussasphalt concrete gradation was adopted to ensure that the air voids of the molding mixture sample were less than 1%. The emulsified bitumen, cement, water, aggregate, and mineral powder were measured according to the proportion ratio of CEBM. In addition, the water-cement ratio was determined to be 0.5:1 [18]. The Additional water quantity of CEBM was then calculated according to Equation (1).

$$\mathbf{C}\_{W} = \mathbf{R}\_{w/\mathfrak{c}} \mathbf{C}\_{\mathbf{C}} - (\mathbf{1} - \mathbf{C}\_{Br}) \mathbf{C}\_{B} \tag{1}$$

where *C<sup>W</sup>* is the additional water content in CEBM, *Rw/c* is the water-cement ratio, *C<sup>C</sup>* is the cement content in CEBM, *C<sup>B</sup>* is the emulsified bitumen content in CEBM, and *CBr* is the evaporation residue of the emulsified bitumen. *Materials* **2022**, *15*, x FOR PEER REVIEW 7 of 22

**Figure 3.** Diagram of contact angle test. **Figure 3.** Diagram of contact angle test.

**Table 7.** SFE parameters of test liquid (25 °C, mJ/m2). **Reagent SFE () Dispersion Component ( ) Polarity Component ( ) SFE Acidity Parameter (** ା**) SFE Alkalinity Parameter (** ି**)**  Distilled water 72.8 21.8 51.0 25.5 25.5 Then, the aggregates were mixed in dry conditions for an even mixture; the designed amount of water was sprayed to the uniformly mixed aggregate, it was stirred quickly and evenly, thus granting the mixture the desired flowability, and flowability experiment's outflow time was less than 20 s (JTG/T3364-02—2019). The mixture was conditioned at room temperature (25 ◦C) for curing for more than 24 h. The molding process and specimens of CEBM are shown in Figure 4. The Volume parameters of CEBM are shown in Table 8. *Materials* **2022**, *15*, x FOR PEER REVIEW 8 of 22

Ethylene glycol 48.3 29.3 19.0 3.0 30.1

cement content in CEBM, *CB* is the emulsified bitumen content in CEBM, and *CBr* is the evaporation residue of the emulsified bitumen. **Figure 4.** Manufacturing of CEBM ((**a**) Marshall specimen appearance; (**b**) apparent section of Marshall specimen). **Figure 4.** Manufacturing of CEBM ((**a**) Marshall specimen appearance; (**b**) apparent section of Marshall specimen).

**Volume Parameter Emulsified Bitumen Contents (%)** 

Bulk specific gravity 2.594 2.589 2.581 Theoretical maximum density 2.610 2.605 2.599

Air voids (%) 0.61 0.61 0.69

The Marshall stability of the Marshall specimen with different cement and emulsified bitumen contents and the curing time were detected to investigate the influential parameters of the mechanical property test of CEBM. Each group had 8 specimens, 4 of which were conditional and 4 were non-conditional. After curing them at 25 °C for 6 h, 12 h, and 24 h, respectively, the Marshall stability of CEBM was tested under the drying and bath

The size of the rutting specimen of CEBM was 300 mm × 300 mm × 50 mm, and the specimens were cured at 25 °C for 24 h and 72 h (Complete curing). Before the test, the samples were placed in the rutting instrument for more than 5 h to maintain a constant temperature, and the test temperature, wheel loading, and rate were 60 °C, 0.7 MPa, and

The Universal testing machine (UTM-100) was used for testing the low-temperature crack resistance of CEBM. The specimen size was 250 mm × 30 mm × 35 mm. Each group had 3 specimens. The test temperature was −10 °C, the span of the beam was 200 mm, and

the loading rate was 50 mm/min. The schematic diagram is shown in Figure 5.

Then, the aggregates were mixed in dry conditions for an even mixture; the designed amount of water was sprayed to the uniformly mixed aggregate, it was stirred quickly and evenly, thus granting the mixture the desired flowability, and flowability experiment's outflow time was less than 20 s (JTG/T3364-02—2019). The mixture was conditioned at room temperature (25 °C) for curing for more than 24 h. The molding process and specimens of CEBM are shown in Figure 4. The Volume parameters of CEBM are

**8 10 12** 

shown in Table 8.

*2.5. Experimental Methods* 

**Table 8.** Volume parameter of CEBM.

2.5.1. Mechanical Property Test of CEBM

2.5.2. Wheel Track Test of CEBM

42 times/min, respectively.

water conditions of 25 °C and 60 °C, respectively.

2.5.3. Low-Temperature Bending Test of CEBM


**Table 8.** Volume parameter of CEBM.

#### *2.5. Experimental Methods*

#### 2.5.1. Mechanical Property Test of CEBM

The Marshall stability of the Marshall specimen with different cement and emulsified bitumen contents and the curing time were detected to investigate the influential parameters of the mechanical property test of CEBM. Each group had 8 specimens, 4 of which were conditional and 4 were non-conditional. After curing them at 25 ◦C for 6 h, 12 h, and 24 h, respectively, the Marshall stability of CEBM was tested under the drying and bath water conditions of 25 ◦C and 60 ◦C, respectively.
