Laboratory Evaluation and Finite Element Modeling of SBS and Basalt Fiber Modified Mixtures

Abstract

The incorporation of basalt fiber into asphalt mixtures offers potential improvements in their viscoelastic properties. This study explores the addition of basalt fiber to Styrene Butadiene Styrene (SBS)-modified asphalt mixtures with varying SBS contents. Specifically, 0.3% basalt fiber was added to an asphalt mixture containing 3% SBS, and its performance, measured in terms of dynamic stability and flexural strength, was compared with a mixture with 7% SBS content. Additionally, finite element analysis using the Modified Burger’s Logit model was conducted to assess rutting and fatigue behavior. Given the high cost associated with increasing the SBS content, basalt fiber presents a cost-effective alternative without sacrificing performance. Laboratory tests, including the Marshall stability test, dynamic stability, flexural strength, and fatigue tests, were conducted to evaluate both mixtures. Results indicate that the mixture with 0.3% basalt fiber and 3% SBS outperforms the 7% SBS mixture, showing a 47% improvement in dynamic stability and rutting resistance and a 16% increase in flexural strength.

1. Introduction

Different modifications in asphalt mixtures can be performed to enhance the high and low temperature performance of asphalt mixtures. Polymer and crumb rubber-modified bitumen mixtures have been used to enhance the viscoelastic properties of asphalt mixtures. Although polymer and crumb rubber-modified mixtures exhibit promising performance in terms of fatigue and rutting resistance, due to rising material costs, some alternative options are available for further optimization of the asphalt mixtures’ performance.

The alternative is to reduce the percentage of polymer or crumb rubber used in the mixtures and, furthermore, the reduction in polymer or crumb rubber can be compensated for by adding fibers to the mixture without compromising the performance of the asphalt mixtures. Furthermore, the performance of completely pure SBS-modified asphalt mixtures and fiber-reinforced-modified asphalt mixtures can be compared in terms of rutting and fatigue damage. Different forms of fibers are commercially available involving basalt fiber, polyester fiber and xylogen fiber [1]. The usual percentage of these fibers varies from 0.15% to 0.45% depending on the characteristics of the asphalt mixtures.

In terms of the significance of this research, an alternative option is necessary for decreasing the procurement costs of SBS-7% mixtures, since these types of mixtures are mostly used in major highways and airport projects. Therefore, an alternative option is the use of fiber modifications to asphalt mixtures and subsequently decreasing the amount of SBS polymer, thereby improving the economic efficiency of the mixtures. Lab tests and finite element modeling further enhance the rutting depth performance comparisons of SBS-modified mixtures with different percentages and fiber modifications.

The performance of basalt fibers (BFs) is sensitive in terms of the diameter of fibers used in the asphalt mixtures. Commercially available BFs are available with diameters of 7 µm, 16 um and 25 um. The performance of basalt fibers also varies with the proportions of BFs used in the mixtures, with a recommended percentage of 0.3% [2]. When compared with polyester fiber and xylogen fiber, BF exhibits superior performance in terms of high and low temperature strength enhancement and decreased water sensitivity [3].

Commercially available fibers can also be obtained in organic and inorganic forms. Lignin fiber has been previously used in the modification of the properties of asphalt mixtures due to its readiness to decompose early [4]. However, inorganic fibers outperform organic fibers in terms of increased water damage resistance. Furthermore, the use of organic fibers results in increased tensile strength, increased stiffness modulus, increased elasticity, and constancy in physical and chemical properties of asphalt mixtures [5].

In terms of fibers with inorganic origins, BF is produced from natural basalt. Basalt fiber offers added strength and durability to asphalt mixtures and has good alkali and acidity resistance [6]. BFs provide better strength than conventional steel-formed fibers and have lower environmental impact when it comes to CO₂ emissions [7]. Furthermore, in terms of cost efficiency, basalt fibers are less expensive to procure than carbon fibers [8]. The properties of basalt fibers include being incombustible and resistance against extreme temperature variations.

Basalt fiber has been previously used for performance enhancement of pavements in terms of avoiding raveling in pavements, specifically for porous asphalt pavements and open-graded friction course [9]. In case of stone matrix asphalt (SMA) mixtures, the introduction of BF can also improve the ductility of asphalt mixtures and enhance the dynamic modulus, compressive strength and temperature sensitivity by providing an adequate operational temperature range [10].

Due to increasing traffic volume and axle loads, the use of polymer- or crumb rubber-modified mixtures is essential to reduce the amount of maintenance interventions required. Since the use of only polymer and crumb rubber modifiers is costly for the same amount of performance gains for asphalt mixtures, it is essential to introduce cost-effective alternatives by reducing the amount of SBS or CR modifiers needed for asphalt mixtures. Therefore, the percentage of SBS or CR used in asphalt mixtures can be compensated for by adding basalt fiber without compromising the strength of the mixtures. The addition of basalt fiber to the mixtures can further reinforce the polymer matrices in the mixtures, thereby providing adequate strength and stability to the polymer matrices at reduced costs, leading to resistance in raveling, fatigue damage and rutting of the asphalt mixtures [11].

In terms of rutting performance gains for basalt fiber-modified asphalt mixtures, Celauro et al. [12] evaluated the rutting progression of asphalt mixtures modified with 0.35 basalt fiber. A wheel tracking test was carried out with loading cycles of 20,000. Results showed that basalt fiber-modified asphalt mixtures exhibited 28% less rutting when compared with conventional HMA mixtures. Zhang et al. [13] performed the dynamic stability evaluation of asphalt mixtures modified with 0.4% basalt fiber using wheel rutting rests. Results showed a 35% gain in dynamic stability of 0.4% basalt fiber-modified mixtures. Hnag et al. performed a rutting resistance evaluation of 0.4% basalt fiber-modified asphalt mixtures. Results suggested the use of a stone matrix asphalt mixture along with a combination of 0.4% basalt fiber content for increased rutting resistance. Li et al. [14] evaluated the effect of 16 um and 26 um diameter basalt fiber on low-temperature cracking resistance and high-temperature stability of asphalt mixtures. Wheel tracking tests, semi-circular bending tests and the bending beam test were employed to study the behavior of two different diameter scenarios. Results showed that 16 um diameter basalt fibers outperformed the 26 um diameter type in terms of increased high-temperature stability and increased cracking resistance in low-temperature conditions.

Xing et al. [15] evaluated the effect of bundled basalt fibers on the performance of asphalt mixtures in terms of rutting resistance using bending beam rheometer and dynamic shear rheology tests. Results showed increased toughness of BF-modified mixtures in high-temperature conditions when compared with lignin fibers. Chen et al. [16] evaluated the effect of basalt fiber on viscosity and the high-temperature performance of asphalt mixtures. Results showed that adding basalt fiber to the asphalt mixture increased the viscosity and high-temperature performance of asphalt mixtures by creating a strong three-dimensional network. Zheng et al. [17] performed a dynamic stability and low-temperature performance evaluation of basalt fiber-modified asphalt mixtures at percentages of 0.15%, 0.3% and 0.45%. Results showed improvement in performance of asphalt mixtures by adding an optimum 0.3% basalt fiber in terms of ultimate tensile strength and maximum bending strain of tested specimens. Zhao et al. [2] carried out a performance evaluation of basalt fiber-modified SBS mixtures and conventional SBS-modified mixtures. Wheel tracking tests and the three-point bending test were used to evaluate the effect of basalt fiber at different percentages. Result showed an optimum percentage of 0.3% basalt fiber, where the dynamic stability and tensile strength ratio increased by 25.5% and 6% when compared with conventional SBS-modified asphalt mixtures. Furthermore, it was found that basalt fiber outperformed both lignin- and polyester fiber-modified asphalt mixtures. Guo et al. [18] evaluated the high and low temperature properties of asphalt mixtures with fiber proportions of 1%, 2%, 3%, 4% and 5% with fiber lengths of 6 mm, 9 mm and 15 mm. A dynamic shear rheometer and bending beam rheometer were employed to study the rheological properties of various mixture types. Results showed that basalt fiber 2% with 9 mm fiber size outperformed other mixture types, with temperature performance and low-temperature stress dissipating ability. Chauhua et al. [5] evaluated the effects of basalt fibers on bitumen and asphalt mixtures using various percentages of basalt fiber. Different tests including viscosity, water stability and splitting tensile strength tests were performed on prepared specimens. Results showed an increased stiffness modulus of asphalt mixtures and increased deformation resistance at the recommended basalt fiber percentage of 0.3%.

Shi et al. [19] evaluated the low-temperature performance of basalt fiber-reinforced asphalt mixtures in terms of freezing and thawing cycles. Basalt fiber rubber composite was used for modifying the asphalt mixtures. Results showed further improvement in performance of the basalt fiber-reinforced asphalt mixtures in terms of freezing and thawing action by 0.3%. Shi et al. [20] evaluated the high-temperature properties of bundled basalt fiber, lignin fiber and polyester fiber mixed with asphalt mixtures. Results showed increased fracture energy of the asphalt mixtures mixed with basalt fiber and increased high-temperature performance. Hui et al. [21] evaluated the effect of adding basalt fiber to asphalt mixtures in terms of fatigue cracking, thermal cracking and rutting. Results showed increased cracking and rutting resistance of the asphalt mixtures when reinforced with basalt fiber. Phung et al. [22] used basalt fiber asphalt concrete for the performance enhancement of asphalt mixtures measured using machine learning models including Gradient Boosting, Particle Swarm Eagle and Bald Eagle Search techniques. Results showed increased performance of the basalt fiber asphalt concrete in terms of Marshall stability, with the highest level of prediction exhibited by the Bald Eagle Search model. Cai et al. [23] evaluated the impacts of adding basalt fiber on the viscosity, stability and splitting tensile strength of asphalt mixtures. Result showed an increased tensile strength and stiffness modulus of asphalt mixtures with a percentage of 0.3% basalt fiber. Zhu et al. [24] evaluated the effects of adding basalt fiber, lignin fiber and polyester fiber on the toughness, wear resistance and mechanical strength of asphalt mixtures. Results showed superior performance of asphalt mixtures with basalt fibers in terms of increased rutting and fatigue cracking resistance predicted by using the Random Forest model. Yan et al. [25] evaluated the effects of adding basalt fiber to SBS-modified asphalt mixtures on aging, freezing and thawing, rutting and low-temperature bending of asphalt mixtures. Results showed increased high-temperature stability and low-temperature cracking resistance of asphalt mixtures with added basalt fiber. Wang et al. [26] evaluated the effects of adding copped basalt fiber (CBF) on tensile strength, durability, fatigue damage and mechanical strength of asphalt mixtures. Mixtures were prepared with chopped basalt fiber percentages of 3% and 4%. Low-temperature cracking tests, freeze thaw tests and wheel tracking tests were used to evaluate the performance of different percentages and length variations of basalt fiber. Results showed an increased performance of basalt fiber with an optimum fiber length of 6 mm and fiber percentage of 3%. Liu et al. [27] evaluated the effects of adding basalt fiber to asphalt mixtures in terms of the flexural tensile strength of asphalt mixtures. Results showed increased tensile strength and stiffness modulus of fiber-reinforced asphalt mixtures.

Li et al. [28] evaluated the effects of basalt fiber on hot-in-place asphalt recycling mixtures. Recycled asphalt mixtures consisted of 70% Recycled Asphalt Pavement (RAP) and 30% new mixture. Dynamic stability and bending beam tests were carried out to evaluate the mixtures’ resistance to rutting and low-temperature cracking. Results showed that the addition of basalt fiber to the recycled asphalt mixtures improved the stripping resistance, rutting resistance and cracking resistance by 46.9%, 105.2% and 102.3%, respectively.

The influence of fibers on asphalt mixtures and their comparison against the different percentages of basalt fibers have not been addressed before with the aid of finite element modeling in previous research; therefore, in this current research, a 3% SBS mixture with basalt fiber was tested against the 7% SBS conventional mixture. Furthermore, the performance evaluation of lignin, basalt and polyester fibers has been compared against the SBS mixture previously; however, in this research, the 3% SBS mixture was used in order to evaluate the performance directly with the SBS-7% mixture in terms of dynamic stability.

2. Experimental

In this research, two variations of samples, one consisting of a 0.3% BF with 3% SBS mixture and the second one consisting of only a 7% SBS mixture are tested using the flexural strength test and wheel rutting test in the lab, since a conventional 7% SBS mixture would outperform the conventional 3% SBS mixture. Furthermore, the properties of these two variations are further employed in finite element modeling to perform predictions based on lab performance of these mixtures. Moreover, predictions based on finite element modeling are made for two distress mechanisms including rutting and fatigue damage.

2.1. Material Properties

In terms of the asphalt mixture used, a 3% SBS-modified asphalt mixture with Penetration Grade PG 76-22 is used with a 0.3% BF modification and another set of specimens without BF modification with a 7% SBS mixture. Further properties of the asphalt mixtures are shown in

Table 1. Properties of asphalt mixtures.

PG Grade	Penetration at 25 °C [0.1 mm]	Softening Point [°C]	Separation [°C]	Elastic Recovery [%]	Ductility [°C]	Viscosity [135 °C Pa·s]
76-22 with 3% SBS 0.3% basalt fiber	72	79	1.5	74	42.1	0.82
76-22 with 7% SBS	76	84	1.8	68	37.4	0.74

.

The technical properties of the basalt fiber are shown in

Table 2. Properties of basalt fiber.

Property	Unit	Value
Diameter	μm	16.1
Length	mm	4–6
Elastic modulus	GPa	83.2
Heat resistance	%	92.1
Elongation	%	4
Ferrous content	%	7.43
Acidity content	%	3.6
Fracture strength	MPa	2359
Relative density	g/cm3	2.72

.

A dense gradation of aggregates was selected, with limestone used as a mineral filler. Aggregate gradation with control points and restricted zones is shown in

Table 3. Aggregate properties.

Sieve Size [mm]	Percentage Passing [%]	Control Point [%]	Restricted Zone [%]
16	100	-	-
13.2	94.9	90–100	-
9.5	78.4	Less than 90	-
4.75	46	-	-
2.36	36.4	28–58	39.1
1.18	25.8	-	25.6–31.6
0.6	19.4	-	19.1–25.1
0.3	15.2	-	15.5
0.15	11.8	-	-
0.075	8.4	2–10	-

.

2.2. Dynamic Stability, and Marshall and Flexural Testing

The mixing methodology consists of using a dry mix of a stone matrix asphalt SMA-13 asphalt mixture, where 0.3% basalt fiber and aggregates are premixed before bitumen is added to the mixture and heated to 165 °C for 2 h. This process removes excessive moisture from raw materials and improves workability during sample preparation. Mineral filler is later added to asphalt and mixed at a rate of 500 rotations/min at 165 °C. The 7% SBS asphalt mixture was procured directly from the manufacturer.

For rutting performance evaluation of the mixtures, rutting tests are carried out using wheel tracking apparatus developed by the company Infratest (Infrastest, Berlin, Germany) as per the American Society for Testing and Materials ASTM D8292. Tests are carried out at 60 °C, with the size of the specimen being 300 mm × 300 mm × 50 mm. The equipment used is shown in Figure 1.

Wheel speed is set at 45 passes/min and a load of 0.7 MPa is applied. For permanent deformation evaluation, dynamic stability function is used, as shown in Equation (1).

$$\mathrm{D}\mathrm{S}\equiv {\displaystyle \frac{\left(t_2-t_1\right)\times N}{d_2-d_1}}\times C_1\times C_2$$

(1)

where DS is the dynamic stability in times/mm, $t_1$ and $t_2$ are test timings (min), $d_1$ and $d_2$ are rutting depths at timings $t_1$ and $t_2$ (mm), $N$ is the wheel speed at 45 passes/min, and $C_1$ and $C_2$ are experimental coefficients of value 1.0. Furthermore, percentage rut depth can be calculated with regards to the dynamic stability as shown in Equation (2).

$$PRD\equiv {\displaystyle \frac{∆L}{D}}\times 100$$

(2)

where $∆L$ is the rutting depth at specimen timing in mm and $D$ is the thickness of the specimen in mm. For low-temperature performance evaluation, the bending beam test is used. The test is carried out at −10 °C using a sample size of 200 mm × 25 mm × 30 mm, as per ASTM D8237-21. The equipment used in shown in Figure 2.

The loading rate is set at 50 mm/min in order to determine the low-temperature cracking resistance; the failure strain is calculated using Equation (3).

$${\epsilon }_B\equiv {\displaystyle \frac{6h_{2}l}{L^2}}$$

(3)

where $h_2$ is the height of specimen in mm, $l$ is the deflection in mm, and $L$ is the span of the specimen in mm. Furthermore, the fatigue damage evaluation can be performed using a half sine wave load at 10 Hz with the temperature kept at 25 °C using Equation (4).

$$N_f\equiv k{\left({\displaystyle \frac{1}{{\sigma }_0}}\right)}^n$$

(4)

where $N_f$ is the number of times of loading until specimen damage, ${\sigma }_0$ is the initial bending tensile stress in MPa, and $k$ and $n$ are regression coefficients obtained during controlled stress mode. The Marshall test results for both variations of samples are shown in

Table 4. Marshall tests results.

Type	Optimum Binder Content (%)	Voids (%)	Voids Filled with Asphalt (%)	Voids in Mineral Aggregate (%)	Marshall Stability (kN)	Flow (0.1 mm)
76-22 with 3% SBS and 0.3% BF	4.6	4.83	66.2	14.1	13.9	32.6
76-22 with 7% SBS	4.3	4.95	65.1	14.3	11.2	30.2

.

Marshal tests were carried out on two sample variations. As observed in Table 4, higher gains in Marshall stability and a slightly low optimum binder content for the BF-modified asphalt mixture can be observed. Furthermore, a higher number of voids filled with asphalt exist for the BF-modified mixture due to the absorption of fiber. The addition of basalt fiber reduces the permanent deformation of asphalt mixtures with adequate flow parameters.

Dynamic stability tests were conducted, with the results shown in

Table 5. Dynamic stability test results.

Type	Dynamic Stability (Times/mm)
76-22 with 3% SBS and 0.3% BF	2483
76-22 with 7% SBS	1524

. As observed from the table, dynamic stability further improves with the addition of 0.3% basalt fiber content, showing a 48% performance improvement, despite using 3% SBS in the mixture rather than 7% SBS. This is due to the fact that fibers reinforce the asphalt mixture by improving its stability and creates interlocking among the fibers, and the fibers aggregate to resist permanent deformation.

The percentage rut depth for each scenario was calculated and is shown in Figure 3. As observed, the highest percentage is exhibited by the 7% SBS mixture, showing up to 4% of rut depth compared with only 2.5% for 3% SBS with 0.3% BF. The addition of basalt fiber further optimizes high-temperature stability, thereby increasing the rutting and permanent deformation resistance. Rutting performance is also further improved using a stone matrix asphalt mixture.

Low-temperature cracking resistance tests were conducted for a total of 5 specimens per each scenario and the results with average values are shown in

Table 6. Flexural properties of different scenarios.

	Flexural Tensile Strength [MPa]	Failure Strain ${\mathit{\epsilon }}_{\mathit{u}}/{10}^{-3}$	Curving Stiffness Modulus [MPa]
76-22 with 7% SBS	5.31	3.15	1851
76-22 with 3% SBS and 0.3% BF	6.24	4.32	1685

. As observed, a higher component of flexural strain and flexural stress exists for the basalt fiber-modified mixture. The addition of basalt fiber allows the test sample to be repeatedly loaded with a higher accumulation of strains when compared with the SBS only-modified specimen. The addition of basalt fiber allows for higher fatigue durability and decreased stiffness modulus in low-temperature conditions.

A visual comparison of some of the properties including dynamic stability, flexural tensile strength and failure strain is shown in Figure 4. As observed, the addition of 0.3% BF significantly increases the performance of the asphalt mixture in terms of improved permanent deformation resistance with a 47% higher magnitude of dynamic stability. Furthermore, in terms of low = temperature performance and higher use of load repetitions, the addition of 0.3% BF further optimizes the asphalt mixture, leading to a 16% increase in flexural tensile strength. The magnitude of failure strain also improves by 31% using the 0.3% BF with 3% SBS mixture.

3. Finite Element Modeling

The Modified Burger’s Logit model has been used to perform finite element analysis of the 3% SBS with 0.3% BF and 7% SBS mixtures, as shown in Equation (5). The ABAQUS version 2018 is used for modeling [29].

$$\epsilon \left(t\right)=\left[{\displaystyle \frac{1}{E_0(1-\omega )}}+{\displaystyle \frac{1}{E_2}}\left(1-e^{\frac{E_2}{n_2}}\right)+{\displaystyle \frac{1}{A^{e^{B_I}}}}\right]$$

(5)

where $\epsilon \left(t\right)$ is the creep strain of the Modified Burger’s model, $E_0$ is the dynamic modulus, $E_2$ is the internal damper parameter, n₂, $A$ and $B$ are the model viscoelastic fitted parameters and $\omega$ is the damage value. Values are obtained at a temperature of 30 °C in order to reduce the bias against rutting progression in higher-temperature conditions, as shown in

Table 7. Burger’s Logit model parameters.

	Mixture Type	n2 [MPa]	A [MPa]	B
76-22 with 7% SBS	6437.46	155.12	0.001023	0.9907
76-22 with 3% SBS and 0.3% BF	7102.58	161.38	0.001014	0.9989

. This model uses the external damper and external spring by simulating the material decay, thereby increasing the material modeling accuracy in predictions.

A dual wheel loading setup is used to simulate the traffic loading with a magnitude of the cumulative axle load of 100 kN, with a traffic speed of 90 Km/h and traffic volume of 50,000 axle passes, as shown in

Table 8. Loading details.

Parameters	Values
Tire type	Dual tire
Axle load	100 kN
Speed	90 Km/h
Number of axle passes	50,000
Ground pressure	0.6 MPa
Single loading time	0.02149 s
Cumulative loading time	1,152,433 s

. Poisson’s ratio for the asphalt mixture is 0.41 and the elastic modulus is 950,000 kPa.

3.1. Model Details

A 2D model was used to simulate the axle load of 100 kN with 50,000 passes, with a 15 cm asphalt layer resting on a rigid foundation. Microstrain analysis was performed for each mixture type and corresponding simulation results were used to evaluate rutting and fatigue damage separately from the lab-tested mixtures. Furthermore, a convergence study was performed and an appropriate mesh size was selected accordingly, as shown in Figure 5.

The model type is CPE4R with reduced integration and hourglass control. The total number of elements is 6100, with 5 mm of element size selected after performing the convergence study. As observed from Figure 6, movements along the vertical direction are allowed and movements along the normal horizontal direction are restricted in order to restrict the bias for concentrated strains and principal stress values under wheel loading.

3.2. Simulation Results

Simulations were performed for a total of 50,000 passes for each mixture type in ABAQUS. As shown in Figure 7, both the deformed zones and upheaval zones as a result of extensive rutting damage can be observed for the 7% SBS mixture. Von Mises stresses were determined under different depths of concentrated loading for both asphalt mixtures. The 7% SBS mixture shows higher stress concentrations when compared with the 3% SBS with 0.3% BF mixture.

Von Mises stresses were calculated for both mixture types and calculated at depths of 50 mm, 450 mm and 800 mm, as shown in

Table 9. Von Mises stress values for each scenario.

Depth	76-22 with 7% SBS	76-22 with 3% SBS and 0.3% BF
50 mm	2.59	2.31
450 mm	2.24	2.20
800 mm	0.023	0.019

. The highest stress concentrations are shown with the 7% SBS mixture, with a magnitude of 2.5% at 50 mm and 0.023 at 800 mm. However, the SBS 3% with 0.3% BF mixture shows the least amount of stress concentrations, with magnitudes of 2.31 at 50 mm and 0.019 at 800 mm.

3.3. Fatigue and Rutting Damage Analysis

Fatigue damage occurs as a result of repeated numbers of axle passes. Fatigue damage starts with hairline longitudinal cracks under the wheel paths and gradually leads to alligator cracking. Fatigue damage occurs when 45% of the pavement surface is covered with alligator cracking. The Asphalt Institute’s model can be used to evaluate the number of passes leading to fatigue damage from the horizontal tensile strain under the asphalt layer using Equation (6).

$$N_f=0.0796\times {\epsilon }_t^{-3.291}\times E^{-0.854}$$

(6)

where $N_f$ is the number of load repetitions to fatigue damage, ${\epsilon }_t$ is the horizontal tensile strain at the bottom of the asphalt layer and $E$ is the elastic modulus of the asphalt mixture. Rutting damage in the pavement occurs as a result of further densification and shear deformation of the asphalt mixture, usually in high-temperature conditions. The Asphalt Institute provides a relationship between the number of passes to rutting damage and the vertical compressive strain, as shown in Equation (7).

$$N_f=1.365\times {10}^{-9}\times {\epsilon }_c^{-4.477}$$

(7)

where $N_f$ is the number of load repetitions to rutting damage and ${\epsilon }_c$ is the vertical compressive strain on top of subgrade. As observed from

Table 10. Number of passes to fatigue and rutting damage.

Mixture Type	Strain Types	Value [Microns]	Number of Passes to Damage
76-22 with 7% SBS	${\epsilon }_t$	18.2	7.48 × 1010
	${\epsilon }_c$	442.5	8.26 × 106
76-22 with 3% SBS and 0.3% BF	${\epsilon }_t$	15.3	9.34 × 1010
	${\epsilon }_c$	374.8	1.01 × 107

, the 3% SBS with 0.3% BF mixture significantly outperforms the 7% SBS mixture in terms of number of passes to damage for rutting and fatigue cracking. For the BF mixture, ${\epsilon }_t$ and ${\epsilon }_c$ stay at 15.3 and 374.8, respectively, showing the least strains compared with the SBS-7% mixture. The number of passes to fatigue damage for BF remains at 9.34 × 10¹⁰; however, for the same amount of damage, SBS-7% requires only 7.48 × 10¹⁰ passes. Furthermore, in terms so rutting damage, the 7% SBS mixture only requires 8.26 × 10⁶ passes.

The number of passes to fatigue damage for both mixture types are shown in Figure 8. As observed, the 3% SBS with 0.3% BF mixture exhibits a higher number of passes with an ${\epsilon }_t$ value of 15.3 compared with 18.2 for the 7% SBS mixture. Furthermore, a 25% increase in number of passes can be observed for the 3% SBS with 0.3% BF mixture when compared with the 7% SBS mixture; the addition of basalt fiber further improves the cohesion as well as interlocking of aggregates, thereby providing further resistance to fatigue damage.

Fatigue damage of the asphalt mixtures is shown in

Table 11. Fatigue damage parameters.

	$\mathit{k}$	$\mathit{n}$	${\mathit{R}}^2$
76-22 with 3% SBS and 0.3% BF	341	2.412	0.977
76-22 with 7% SBS	256	1.859	0.971

. As observed, the addition of 0.3% basalt fiber enhances the performance of the asphalt mixtures, where the k value is increased by 28% and the n value increases by 25% when compared with the SBS only-modified mixture. The use of basalt fiber decreases the stress concentration in asphalt mixtures by creating a stable film for increased cohesion between aggregates and bitumen, thereby forming a highly elastic mixture where a higher number of loading cycles is required for the specimen to become damaged. Furthermore, the addition of basalt fiber can be employed as a part of enhancing anti-fatigue properties of asphalt mixtures.

The number of passes to rutting damage is shown in Figure 9. As observed, the least number of passes is exhibited by the 7% SBS mixture, showing only 8.26 × 10⁶ passes for the same amount of damage compared with 1.01 × 10⁷ passes for the 3% SBS with 0.3% BF mixture. The 7% SBS mixture exhibits 47% higher rutting damage when compared with the 3% SBS with 0.3% BF mixture.

4. Conclusions and Findings

In this research, basalt fiber has been used with SBS-modified mixtures to evaluate the performance gains by decreasing the amount of SBS modifier required. Two variations of samples have been prepared, one consisting of 7% SBS and the second one consisting of 3% SBS with the addition of 0.3% basalt fiber. Since SBS modification increases in costs as its percentage is increased in mixtures, the addition of basalt fiber to polymer-modified mixtures provide a cost-effective option for enhancing asphalt mixture’s performance.

Marshall tests have been performed in order to analyze the volumetric properties of the two aforementioned variations. Dynamic stability tests have been conducted using wheel tracking equipment and rutting progression has been measured. Furthermore, low-temperature performance has been evaluated using bending beam equipment. Finite element analysis has been performed using the Modified Burger’s Logit model, and corresponding rutting and fatigue damage has been determined. The addition of 0.3% basalt fiber to 3% SBS-modified asphalt results in higher flexural strength and increased dynamic stability compared with the 7% SBS mixture. Based on the lab results and data from finite element simulations, improvement during high- and low-temperature conditions can be observed for fiber-reinforced asphalt mixtures. The use of fibers in asphalt mixtures further increases the cohesion and interlocking of aggregates and binders, leading to an increase in permanent deformation resistance. Magnitudes of microstrains further decrease rapidly for fiber-reinforced mixtures as a result of proper load distribution throughout the asphalt layer. Although SBS polymer enhances the interlocking and cross linking of carbon chains in the bitumen, which inherently leads to an improved elastic response, the use of basalt fiber also enhances the elastic response; however it also shows enhancements in terms of the interlocking and bonding of aggregates with the asphalt binder. Therefore, the same performance enhancement can be achieved by decreasing the percentage of SBS and adding 0.3% of basalt fiber, as is the case in this research. The use of basalt fiber increases the fatigue durability of asphalt mixtures in low-temperature conditions and increased stability and rutting resistance in high-temperature conditions. The findings are as follows:

• Addition of 0.3% basalt fiber enhances the rutting resistance and stability of asphalt mixtures, with the 0.35% SBS mixture showing a 47% improvement in dynamic stability of asphalt mixtures.
• Based on the Marshall tests results, usage of 0.3% basalt fiber with an SBS-modified mixture can further enhance the stability and permanent deformation of asphalt mixtures.
• Based on finite element simulations, fatigue damage is reduced by 25% when the 3% SBS with 0.3% BF mixture is used.
• Usage of 0.3% basalt fiber with 3% SBS-modified mixtures offers a cost-effective solution in terms of modification of asphalt mixtures.
• A 16% flexural strength gain can be observed with the addition of 0.3% BF when compared with SBS only-modified mixtures.
• Rutting magnitude increases by 47% in the case of the 7% SBS mixture when compared with the 3% SBS with 0.3% BF mixture.
• Use of the 0.3% BF with 3% SBS asphalt mixture show a 25% overall improvement in fatigue properties when compared with the 7% SBS mixture.
• Flexural strength increases by 16% when using the 0.3% BF with 3% SBS asphalt mixture.