4.2.1. Effect of MBF Content on the Performance of Emulsified Asphalt Evaporation Residue
- I.
Conventional Performance Test
(1) Injectivity and softening point
The effect of MBF on the penetration and softening point of evaporation residue of matrix-emulsified asphalt is illustrated in
Figure 10.
Figure 10 demonstrates that the content of MBF increased from 0% to 2.5%. Needle penetration of the evaporated residue of the matrix-emulsified asphalt decreased from 67.3 (0.1 mm) to 49.7 (0.1 mm). The softening point increased from 50.8 °C to 65.9 °C, with a percentage increase of 29.7%. Due to the addition of MBF, the consistency of the evaporated residue increased, which restricted the free flow of asphalt and thus led to a decrease in the needle penetration. At the same time, the surface area of MBF is relatively enormous, and the adsorption effect on asphalt is produced after it is mixed into the evaporated residue of emulsified asphalt, making it form a stable structure asphalt film, which improves the softening point and high temperature stability.
(2) Ductility
The effect of MBF on the 15 °C ductility of evaporation residue of matrix-emulsified asphalt and SBR emulsified asphalt is shown in
Figure 11.
Figure 11 demonstrates that the percentage of MBF increased from 0% to 2.5%. The ductility of the emulsified asphalt evaporated residue with MBF decreased significantly from 63.5 cm to 12.6 cm. When the MBF dose was greater than 1.5%, the ductility decreased sharply. 3% SBR latex could dramatically improve the ductility of the emulsified asphalt evaporated residue. With the increase of the proportion of MBF, the SBR improvement effect gradually decreases. The analysis found that when the proportion of MBF was greater than a specific value, the dispersion effect of MBF in the evaporated residue became worse. The phenomenon of agglomeration, and thus stress concentration, easily occurs, resulting in the reduction of elasticity. This suggests that the proportion of MBF should not be too large.
- II.
Dynamic Shear Rheology Test
In order to further investigate the optimal dosing of MBF, the high and low-temperature rheological properties of MBF emulsified asphalt evaporated residues were evaluated. The dynamic shear rheological test parameters were: strain control mode; strain control of 12%; frequency of 10 rad/s; temperature range of 46 °C−76 °C; temperature interval of 6 °C; G*/sinδ ≤ 1.0 kpa after stopping.
(1) Phase angle and complex shear modulus
The phase angle δ and complex modulus G* of MBF emulsified asphalt evaporation residue at different temperatures are shown in
Figure 12.
Figure 12 shows that with the increase of temperature, the phase angle δ of MBF emulsified asphalt evaporation residue increases. The complex modulus G* moderately decreases, and the decrease tends to be flat. In the high-temperature part, the index of evaporation residue of MBF emulsified asphalt is similar to that of No. 70 matrix asphalt. The analysis demonstrates that the viscosity component in emulsified asphalt increases with the increase of temperature. The elastic composition of evaporation residue of MBF emulsified asphalt is relatively reduced, as it has similar temperature sensitivity to matrix asphalt. At the same temperature, MBF greatly reduces the phase angle δ of emulsified asphalt and increases the complex modulus G*. The viscosity composition of emulsified asphalt matrix increases, which improves the high temperature performance of emulsified asphalt and enhances the matrix.
(2) Rutting factor
Figure 13 shows that the rutting factor of evaporation residue of MBF emulsified asphalt gradually decreases with the increase of temperature. After the temperature exceeds 58 °C, the decline gradually becomes gentle. With the increase of temperature, the high-temperature anti-rutting effect of MBF slightly decreases. This shows that the evaporation residue of MBF emulsified asphalt has obvious temperature sensitivity. At the same temperature, the rutting factor increases with the increase of the content of the coupling-modified fiber. When the temperature reaches 58 °C, the ratio of MBF increases from 0 to 1.5%, and G*/sinδ increases by 1.3 kPa. It shows that MBF can enhance the high-temperature rutting resistance of emulsified asphalt. Analysis suggests that the specific surface area of MBF can absorb the light components in the emulsified asphalt evaporation residue. At the same time, the surface of MBF is rough, which increases the mechanical engagement ability with asphalt when forming a fiber-interleaved structure. MBF reduces the fluidity of the emulsified asphalt evaporation residue and improves the high-temperature rutting resistance.
- III.
Low-Temperature Performance
The low-temperature performance of emulsified asphalt evaporation residue is closely linked to the low-temperature crack resistance of cold recycled mixture. The effect of modified fiber on the low-temperature rheological properties of emulsified asphalt was studied by the force ductility test (FDT) and bending beam rheological test (BBR).
(1) Force ductility test (FDT)
Figure 14b shows that when MBF is added to the emulsified asphalt evaporation residue, the maximum tensile force corresponding to the ductility at 5 °C continuously increases. The maximum tensile force is 138.7 N. Comparing with the base emulsified asphalt, the increase rate reaches 206%. This shows that during the stretching stage, the distribution of MBF in the emulsified asphalt is relatively uniform. The site effect of MBF on the emulsified asphalt leads to an increase in the maximum tensile force. MBF can greatly improve the shear resistance of the emulsified asphalt matrix. In order to fully evaluate the effect of MBF on the low-temperature rheological properties of emulsified asphalt matrix, tensile flexibility
f: Fmax/Lmax, yield strain energy
E: Fmax ×
Lmax, and toughness ratio
RT/V: ST/SV indexes are introduced. The calculation formula and schematic diagram of the corresponding parameters is shown in
Figure 14a. Slope Sa of the AB section straight line is the stiffness modulus of the test piece.
Figure 14b shows that with the increase of MBF content, the stiffness modulus is continuously increases. The ductility value drops rapidly, and the yield strain energy increases. MBF deteriorates the low-temperature performance of emulsified asphalt.
(2) Toughness ratio RT/V: ST/SV
Figure 15 shows that with the increase of MBF content, the
RT/V index first increased and then decreased.
RT/V and MBF doping have a highly nonlinear relationship. When the MBF content is 1.5%, the toughness ratio index is the greatest. SBR has good viscoelasticity. Adding SBR latex increases the toughness ratio of the evaporation residue of MBF emulsified asphalt. However, with the increase of MBF content, the improvement effect is gradually reduced. Analysis suggests that when the amount of MBF is small, MBF can absorb the emulsified asphalt evaporation residue. This process will form a stable structure asphalt membrane and enhance the mechanical properties of emulsified asphalt. MBF has little effect on the elastic properties of emulsified asphalt evaporation residue. When the MBF content exceeds 1.5%, the dispersibility of MBF in the emulsified asphalt evaporation residue is poor. Although the tensile force increases, stress is concentrated and weak interface points appear, which leads to a substantial decrease in its ductility. Adding SBR can improve the low-temperature ductility of the evaporation residue of MBF emulsified asphalt, but the content of MBF should not exceed 1.5%.
(3) Bending beam rheological test (BBR)
The low-temperature creep properties of emulsified asphalt evaporation residue with different MBF content were analyzed by SYD-0627 bending beam rheometer (Deshe Precision Instrument Co., Ltd. Changzhou, China). The test temperatures were −12 °C and −18 °C. The specimen size was 127 ± 2 mm long, 12.7 ± 0.05 mm high, 6.35 ± 0.05 mm wide, and the load was 980 ± 50 mN. The test results are presented in
Table 6.
Table 6 shows that at the same temperature, with the increase of MBF content, the creep stiffness S continuously increases. When the MBF content is low, the initial creep stiffness increases slowly. When the MBF content exceeds 1.5%, the creep stiffness increases faster. Under the condition of −12 °C, the proportion of MBF increased from 0.5% to 2.5%, and the creep stiffness of the evaporation residue of MBF emulsified asphalt increased by 7%, 18%, 24.8%, 57.1%, and 78.1%. At 18 °C, the creep stiffness of the evaporation residue of MBF emulsified asphalt is greater than 300 Mpa.
Adding SBR latex will restore part of the low-temperature creep properties. When the MBF content exceeds 1.5%, the creep rate m of the emulsified asphalt evaporation residue is markedly reduced. Under the condition of −12 °C, the creep rate m of 1.5% MBF compounded SBR latex increased from 0.331 to 0.342. Analysis suggests that MBF can be adsorbed with the emulsified asphalt matrix, which will produce a more durable structured asphalt membrane and increase the interfacial bonding force. Because MBF has the characteristics of high modulus and high tensile performance, it has a weak influence on the low-temperature performance of the emulsified asphalt evaporation residue. Nevertheless, when the amount of MBF is too much, MBF will cluster inside the matrix, which will adversely affect the low-temperature crack resistance of the emulsified asphalt matrix. Adding SBR latex will improve the low-temperature creep performance of the emulsified asphalt evaporation residue. The recommended amount of MBF is 1.5%.
4.2.2. Mechanism Analysis of MBF Emulsified Asphalt Evaporation Residue
- I.
Principle of SBR-Enhanced Ductility of Emulsified Asphalt
The distribution of SBR latex in emulsified asphalt was observed by XDY-1 fluorescence microscope. The experimental results are presented in
Figure 16.
Figure 16 shows that the matrix-emulsified asphalt does not show fluorescence characteristics under the fluorescence irradiation of a mercury lamp. Emulsified asphalt is continuous and dark green. Some fluorescent points are emulsifiers. When 3% SBR latex is mixed with emulsified asphalt, a three-dimensional network structure is formed. The network structure can improve the ductility and flexibility of the matrix. The deformation resistance of the emulsified asphalt matrix is greatly improved. SBR emulsion is uniformly distributed in the emulsified asphalt matrix without agglomeration. SBR emulsion has excellent compatibility with emulsified asphalt, which enhances the low-temperature crack resistance of emulsified asphalt. The microstructure is in agreement with the macroscopic performance.
- II.
The Effect of MBF on Functional Groups of Emulsified Asphalt
The adsorption of functional groups on emulsified asphalt by MBF was investigated by Fourier transform infrared spectroscopy. The effect of MBF on the position and number of functional groups of emulsified asphalt was analyzed to study the mechanism of fiber increasing the adhesion of emulsified asphalt. The matrix-emulsified asphalt (EA), emulsified asphalt adsorbed by 1.5% basalt fiber (BF + EA), and emulsified asphalt adsorbed by 1.5% MBF (MBF + EA) were used for the infrared spectroscopy test. The test results are presented in
Figure 17.
Figure 17 shows that matrix-emulsified asphalt EA, BF + EA, and MBF + EA have relatively prominent peaks at 2926 cm
−1, 2847 cm
−1, 1462 cm
−1, and 1034 cm
−1. 2926 cm
−1 is the symmetric and asymmetric vibration of methylene (-CH2). 1462 cm
−1 is the shear vibration of (-CH2-) and the symmetric vibration of (-CH3). The absorption peak of 1034 cm
−1 is caused by (S=O) stretching vibration. The three emulsified asphalt peak positions are consistent, but the size of the absorption peak is different. BF emulsified asphalt in 2926 cm
−1, 2847 cm
−1, and 1462 cm
−1 peaks has different degrees of weakening. Nonetheless the magnitude changed little. The peak of MBF emulsified asphalt reduced significantly at the same position. It indicates that MBF can better adsorb the manageable components and unsaturated hydrocarbon chains in emulsified asphalt and has a better stability effect on emulsified asphalt.
- III.
Microstructure Analysis of MBF-Emulsified Asphalt
To further study the distribution and strengthening mechanism of the MBF system in emulsified asphalt evaporation residue, the evaporation residue of emulsified asphalt with 1.5% MBF content was observed by scanning electron microscopy (SEM). The distribution of MBF in emulsified asphalt is shown in
Figure 18.
Figure 18 shows that the distribution of MBF in emulsified asphalt is mostly monofilament. MBF does not display a large-scale clustering phenomenon and has good dispersion uniformity.
Figure 18a,b show that MBF plays a “bridging” role in the emulsified asphalt matrix. When the emulsified asphalt matrix is subjected to external stress and cracks, the fiber can block the development of cracks due to the high modulus and high tensile strength, thereby improving the crack resistance of the emulsified asphalt matrix.
Figure 18c,d show that the fibers are intricately allocated in the emulsified asphalt matrix and interspersed with each other. The fibers can constitute a three-dimensional network structure by overlapping, which plays a reinforcing role. Because MBF is a flexible material, it can improve the internal friction angle of the emulsified asphalt mixture. At the same time, the flow of loose asphalt is restricted and the stabilizing effect of the emulsified asphalt matrix is enhanced.
Figure 18e,f show that the surface of MBF is relatively rough, with scaly and massive bumps, which can be better adsorb asphalt. The theory of interface mechanics suggests that the combination of reinforced fiber and asphalt matrix can prevent MBF from breaking, falling off, and peeling off, which can enhance the mechanical properties of emulsified asphalt. A thick asphalt structure film is established on the surface of the MBF, which enhances the adhesion effect with the emulsified asphalt matrix. This makes the asphalt matrix maintain better temperature stability at high temperatures, improving the modulus of the emulsified asphalt matrix and enhancing the ability of the emulsified asphalt matrix to resist permanent deformation.