3.1. High-Temperature Stability
Due to high summer temperatures, asphalt mixtures are prone to softening, which reduces their bonding ability. After frequent vehicle traffic, the asphalt mixture pavement can develop permanent deformations such as rutting. This study evaluates the high-temperature performance of fiber-reinforced asphalt mixtures using the rutting test. The results are shown in
Figure 3.
The experimental data show that basalt fiber enhances the high-temperature performance of asphalt mixtures. This is primarily due to the large specific surface area of the fibers, which allows them to adsorb a significant amount of asphalt, increasing the thickness of the asphalt film and enhancing the adhesion of asphalt to aggregates. Additionally, the basalt fiber and asphalt form a composite material that interacts with the aggregate to increase the internal friction angle, thus reducing the tendency for displacement and improving the high-temperature performance of the asphalt mixture. As illustrated in
Figure 3, the rutting deformation of the basalt fiber-modified asphalt mixtures after 45 min ranges from 1.734 to 2.742 mm, and after 60 min ranges from 1.926 to 2.998 mm. The dynamic stability ranges from 2079 to 3281 cycles/mm. In comparison, the control group S0 has values of 2.16 mm, 2.561 mm, and 1571 cycles/mm. Group S9 exhibits the smallest rutting deformation and the highest dynamic stability at 3281 cycles/mm, which is 2.09 times that of the control group and represents a 1.09-fold increase.
Hao, M. et al. [
39] reported that with an optimal oil-stone ratio of 5.3% and a fiber content of 0.3%, the average rutting depth after 60 min was 3.397 mm, which is greater than the maximum value of 2.998 mm found in this study. The average dynamic stability was 1428 cycles/mm, which is lower than the minimum value of 2079 cycles/mm observed in this study. In this experiment, the S9 group showed the smallest rutting deformation and the highest dynamic stability, indicating the most stable high-temperature performance. This fiber content ratio allows for efficient load transfer and dispersion to the aggregates and asphalt binder, increases the asphalt’s viscosity, and effectively reduces aggregate sliding and asphalt flow under high-temperature conditions.
Statistical analysis reveals the following: For the 45-min rutting deformation metric, groups S2, S3, S5, S7, S8, and S9 show significant differences compared to S0, indicating they significantly affect the 45-min rutting deformation. The remaining four groups show no significant differences. For the 60-min rutting deformation metric, groups S2, S4, S6, S7, S8, and S9 show significant differences compared to S0, indicating they significantly affect the 60-min rutting deformation, while the differences for the other four groups are not significant. For the dynamic stability metric, all groups show significant differences compared to S0, indicating that each group has a significant impact on dynamic stability. This is due to the bonding strength between basalt fibers and the asphalt binder, which restricts the relative movement of aggregate particles, significantly increasing the asphalt mixture’s toughness and effectively enhancing its deformation resistance. However, compared to S0, some groups (S3, S5, S7, S8) show a slight increase in rutting deformation. This may be attributed to poor dispersion of basalt fibers in these combinations, leading to negative effects. The agglomerated fibers may not provide the intended reinforcement effect and may occupy space meant for aggregates.
3.2. Moisture Susceptibility
The infiltration of moisture into the interface between asphalt and aggregates in mixtures is a primary cause of water damage in asphalt pavements. Repeated vehicle traffic, combined with moisture entering the mixture, can lead to the detachment of asphalt from aggregate surfaces, resulting in surface defects such as raveling and potholes. Therefore, this study evaluates the water stability of fiber-reinforced asphalt mixtures using the immersion Marshall test and freeze-thaw splitting test. The results are shown in
Figure 4 and
Figure 5.
As shown in
Figure 4, the Marshall test results indicate that as the basalt fiber gradation changes, the performance of asphalt mixtures with added basalt fiber shows some improvement. The stability of the basalt fiber-modified asphalt mixtures ranges from 12.04 to 13.76 kN, with water stability from 11.02 to 12.76 kN and water residual stability from 86.8% to 99.8%. The maximum values for these three indicators are S3 with a stability of 13.76 kN, S8 with a water stability of 12.76 kN, and S9 with a water residual stability of 99.8%. The control group S0 has values of 11.65 kN, 9.53 kN, and 81.8%, which are lower than the corresponding values of the basalt fiber-modified asphalt mixtures.
Statistical analysis reveals significant differences in water stability indicators compared to the S0 group, suggesting that these variations significantly impact water stability. However, for stability indicators, the impacts of groups S1, S2, S5, and S9 are not as pronounced, while other groups show significant effects. This is attributed to the basalt fibers’ low moisture absorption rate of less than 0.1%, which increases the optimal emulsion content in the mixture, resulting in a thicker asphalt film on the aggregates and enhanced bonding between them. Consequently, this reduces the erosion and damage caused by water to the asphalt mixtures.
Figure 5 illustrates that the freeze-thaw splitting strength and the residual strength ratio vary with the gradation of basalt fibers, and the asphalt mixtures incorporating basalt fibers demonstrated improved freeze-thaw splitting performance. This improvement is due to the dispersion of basalt fibers of various lengths within the asphalt mixture, which forms a spatial network structure capable of bearing and distributing part of the stress, thereby reinforcing the mixture and limiting crack propagation, which increases the splitting strength. The freeze-thaw splitting strength of the basalt fiber-modified asphalt mixtures ranges from 1.11 to 1.45 MPa, while the freeze-thaw splitting strength ranges from 0.99 to 1.30 MPa, and the freeze-thaw splitting strength ratio ranges from 84.03% to 89.66%. Group S7 (with basalt fiber lengths of 6 mm, 9 mm, and 12 mm at proportions of 40%, 50%, and 10%, respectively) has the maximum values, with a freeze-thaw splitting strength of 1.45 MPa, a freeze-thaw splitting strength of 1.30 MPa, and a freeze-thaw splitting strength ratio of 89.66%. Group S9 (with fiber length ratios of 6 mm, 9 mm, and 12 mm at 3:4:3) shows a freeze-thaw splitting strength of 1.40 MPa, a freeze-thaw splitting strength of 1.24 MPa, and a freeze-thaw splitting strength ratio of 88.57%. The control group S0 has a freeze-thaw splitting strength of 1.09 MPa, a freeze-thaw splitting strength of 0.86 MPa, and a freeze-thaw splitting strength ratio of 78.9%, all of which are lower than the corresponding values for the basalt fiber-modified asphalt mixtures.
Statistical analysis indicates that, compared to the S0 group, all groups except S5 show significant differences, suggesting that they all have a significant impact on both the freeze-thaw splitting strength and the freeze-thaw splitting strength ratio of the asphalt mixtures.
3.3. Low-Temperature Stability
Asphalt becomes stiff and brittle in cold environments, making asphalt pavements susceptible to cracking when subjected to the combined effects of thermal contraction and vehicle loading during the winter. Therefore, low-temperature splitting tests of asphalt mixtures with basalt fibers were conducted to study their crack resistance, with results shown in
Figure 6.
Figure 6 demonstrates that the tensile strength and failure modulus of asphalt mixtures vary with the gradation of basalt fibers, indicating that adding basalt fibers improves the low-temperature splitting performance of asphalt mixtures. The tensile strength of the fiber-reinforced asphalt mixtures ranges from 4.52 to 5.41 MPa, and the failure modulus ranges from 3044 to 3636 MPa. Group S7 (with basalt fiber lengths of 6 mm, 9 mm, and 12 mm in proportions of 4:5:1) achieves the maximum values, with a tensile strength of 5.41 MPa and a failure modulus of 3636 MPa. Group S9 (with fiber length proportions of 6 mm, 9 mm, and 12 mm at 3:4:3) shows a tensile strength of 5.36 MPa and a failure modulus of 3604 MPa. The control group S0 has a tensile strength of 4.35 MPa and a failure modulus of 2931 MPa, both lower than those of the basalt fiber-modified asphalt mixtures.
Statistical analysis reveals that, compared to the S0 group, all groups except S3 exhibit significant differences, indicating that they all significantly affect the tensile strength and failure modulus of the asphalt mixtures. This improvement is due to the network bonding effect formed by the basalt fibers and asphalt, which inhibits the formation of cracks in the asphalt mixtures under low temperatures and load conditions, thereby enhancing both the tensile strength and failure modulus.
3.4. Optimization of Basalt Fiber Gradation
Based on the results from
Section 3.1,
Section 3.2 and
Section 3.3, it is evident that different gradations of basalt fibers have varying impacts on the high-temperature performance, water stability, and low-temperature performance of asphalt mixtures.
Figure 2 indicates that group S9 exhibits excellent high-temperature stability.
Figure 3,
Figure 4 and
Figure 5 show that groups S7, S8, S9, and S10 demonstrate relatively superior performance, with statistical analysis revealing that the differences among these groups are not highly significant. Given that conventional data analysis may not effectively identify the gradation scheme with overall superior performance, a multi-objective grey target decision-making method is considered to address this issue. The calculation steps are as follows:
(1) According to the gray target decision method, the 11 experimental mix schemes form the set
S = {
S0,
S1,
S2,
S3,
S4,
S5,
S6,
S7,
S8,
S9,
S10}. The 12 experimental evaluation criteria form the set
A = {45 min deformation, 60 min deformation, dynamic stability, stability, water stability, water residual stability, unfrozen splitting strength, frozen splitting strength, residual strength ratio, maximum load, tensile strength, failure stiffness modulus}. The decision model’s sample matrix
X is constructed according to Equation (8) as follows:
(2) According to the theory of the entropy method, among the indicators in set
A, the 45-min deformation and 60-min deformation are cost-type indicators, which are normalized using Equation (10). The other indicators are benefit-type indicators, which are normalized using Equation (11). The consistent effect measurement matrix
Y is obtained as follows:
(3) By substituting the consistent effect measurement matrix
Y into Equations (12)–(15), the attribute matrix
R can be calculated as follows:
(4) Substituting the attribute matrix
R into Formula (16), the target center
P = [0.099, 0.111, 0.070, 0.132, 0.062, 0.065, 0.105, 0.076, 0.063, 0.059, 0.079, 0.079]
T is obtained. Using Equation (17), the target center distances
l for each evaluation indicator are calculated, as shown in
Table 6.
Therefore, the ranking of the 11 scenarios is LS6 = LS9 < LS10 < LS2 = LS4 < LS8 < LS7 <LS1 < LS3 < LS5 <LS0. The optimal experimental configurations are S6, S9, and S10, with proportions of 6 mm, 9 mm, and 12 mm basalt fibers as follows: 3:4:3, 0:5:5, and 3:3:4, respectively. Following these are S2 and S4, with fiber ratios of 0:10:0 and 5:5:0. The third set includes S8 and S7, with ratios of 5:3:2 and 4:5:1. The fourth set comprises S1 and S3, with ratios of 10:0:0 and 0:0:10. S5 represents the fifth set with a ratio of 5:0:5, and the least favorable is S0 with no fiber. Combining the results of high-temperature stability tests, water stability tests, and low-temperature splitting tests reveals that the performance of specimens containing 9 mm long fibers is superior to that without them, and within equivalent scenarios, higher quantities of 9 mm fibers correspond to better performance. Thus, 9 mm fibers play a crucial role in enhancing the properties of asphalt mixtures. Asphalt mixtures exhibit better performance when the three fiber lengths are continuously proportioned, whereas performance deteriorates when the proportions of the three fiber lengths are discontinuous, and without fibers, the performance of asphalt mixtures is the poorest.
According to
Table 6, the ranking of the 11 schemes is as follows: L
S6 = L
S9 < LS
10 < L
S2 = L
S4 < L
S8 < L
S7 <L
S1 < L
S3 < L
S5 <L
S0. The optimal experimental schemes are S6 and S9, with basalt fiber gradations of 0:5:5 and 3:4:3 for 6 mm, 9 mm, and 12 mm fibers, respectively. The next best schemes are S10, S2, and S4, with fiber gradations of 3:3:4, 0:10:0, and 5:5:0, respectively. The third tier includes S8 and S7, with fiber gradations of 5:3:2 and 4:5:1, respectively. The fourth tier includes S1 and S3, with fiber gradations of 10:0:0 and 0:0:10, respectively. The fifth scheme is S5, with a fiber gradation of 5:0:5. The least favorable scheme is S0, which has no fiber. Since the target distances for S6 and S9 are equal, it is necessary to further evaluate these two schemes based on the experimental data from
Figure 2,
Figure 3 and
Figure 4. For the performance indicators of the asphalt mixtures, it is evident that the performance under scheme S9 is superior.
Comparing the results from high-temperature stability tests, water stability tests, and low-temperature splitting tests, it is observed that the performance of asphalt mixtures with 9 mm fibers is better than those without 9 mm fibers. Additionally, within the same conditions, increasing the amount of 9 mm fibers improves performance. This indicates that 9 mm long fibers play a significant role in enhancing the performance of asphalt mixtures. When using a continuous gradation of fibers of three different lengths, the performance of asphalt mixtures is better. In contrast, when fibers of three different lengths are used in discontinuous gradation, the performance of the asphalt mixture is poorer, and mixtures without fibers exhibit the worst performance.