Porous asphalt mixture is a type of asphalt mixture with good drainage. However, it has poor tensile strength performance and durability. Chopped basalt fibers (CBF) have been proved to be an effective additive to improve the mechanical and fatigue performance of asphalt mixtures, but little attention has been paid on porous asphalt mixture. This paper examined the effect of chopped basalt fibers with different lengths (nonfiber, 3 mm, 6 mm, 9 mm, and 12 mm) and contents (3% and 4%) on the performance of the porous asphalt mixture. A series of tests were conducted to figure out the optimum fiber length and content, including draindown test, cantabro abrasion test, freeze-thaw split tensile test, wheel tracking test, low-temperature cracking resistance test, and four-point bending beam test. Thereafter, indirect tensile tests at different temperatures were conducted to investigate the tensile strength properties of porous asphalt mixtures with optimum fiber length and content. Besides, the macroscopic and microscopic morphology of fracture sections of the samples after indirect tensile tests were studied by using a single-lens reflex (SLR) camera and scanning electron microscopy (SEM) so as to further explore the reinforced mechanism of chopped basalt fibers. The results show that the addition of chopped basalt fibers can generally improve the performance of porous asphalt mixture since chopped basalt fibers form a three-dimensional network structure in the porous asphalt mixture.
Porous asphalt mixture is widely used for its remarkable water permeability, good skid resistance performance, and the function of reducing the pavement temperature [
Adding additives to the asphalt or asphalt mixture has become one of the technical means to solve this problem [
At present, most of the fibers used in asphalt mixtures include organic fibers (lignin fibers, polymer fibers, and polyester fibers) and inorganic fiber (chopped basalt fibers, etc.) [
Therefore, this paper examined the effect of chopped basalt fibers with varied lengths (nonfiber, 3 mm, 6 mm, 9 mm, and 12 mm) and contents (3% and 4%) on the performance of the porous asphalt mixture. And draindown test, cantabro abrasion test, freeze-thaw split test, wheel tracking test, low-temperature bending test, and four-point bending test were conducted, respectively. The tensile properties of porous asphalt mixture with chopped basalt fiber at different temperatures (−25°C, −15°C, −5°C, 5°C, 15°C, and 25°C) were explored by the indirect tensile test. Morphology analysis of fracture sections was also studied to further understand the reinforced mechanism of CBF by SEM.
A type of radial styrene-butadiene-styrene- (SBS-) modified asphalt was used in the research. The properties of asphalt are shown in Table
Properties of the asphalt.
Properties | Results | Specification requirements |
---|---|---|
Penetration (25°C, 0.1 mm) | 66.0 | ≥50 |
Softening point (°C) | 80 | ≥75 |
Ductility (cm) | 29 | ≥20 |
Viscosity (Pa·s) | 2.302 | 2.2∼3.0 |
Elastic recovery (25°C, %) | 98.7 | ≥90 |
Density (g·cm−3) | 1.023 | Measured |
Penetration ratio (25°C, %) | 85.7 | ≥65 |
Basalt coarse aggregates (minimum size of aggregate ≥4.75 mm) and limestone fine aggregates (maximum size of aggregate <4.75 mm) were used, respectively. The properties of the aggregates are listed in Tables
Properties of the coarse aggregates.
Properties | Results | Specification requirements | |
---|---|---|---|
5∼10 mm | 10∼15 mm | ||
Apparent density (g/cm3) | 3.002 | 2.955 | ≥2.70 |
Water absorption (%) | 0.43 | 0.57 | ≤2.0 |
<0.075 mm grain content (%) | 0.1 | 0.4 | ≤1 |
Soft stone content (%) | 0.1 | 0.1 | ≤1 |
LA abrasion value (%) | 9.9 | ≤20 | |
Crushing value | 11.0 | 13.0 | ≤18 |
Properties of fine aggregates.
Properties | Results | Specification requirements |
---|---|---|
Apparent density (g/cm3) | 2.936 | ≥2.60 |
Sturdiness (%) | 0.2 | ≤3 |
Angularity (s) | 33 | ≥30 |
The mineral filler was produced by limestone. Some properties are shown in Table
Properties of mineral filler.
Properties | Results | Specification requirements |
---|---|---|
Apparent density (g/cm3) | 2.700 | ≥2.60 |
Hydrophilic coefficient (%) | 0.7 | ≤1 |
Water content (%) | 0.4 | ≤1 |
High-viscosity agent (HVA) and chopped basalt fiber were used as additives in this study. HVA is a commonly used additive for porous asphalt mixture to enhance the viscosity of asphalt and improve the bonding between asphalt and aggregates, as shown in Figure
High-viscosity agent (HVA).
Properties of the high-viscosity agent (HVA).
Properties | Results | Specification requirements |
---|---|---|
Density (g/cm3) | 0.985 | 0.90∼1.00 |
Single particle quality (g) | 0.029 | ≤0.03 |
Melt index (g/10 min) | 9.0 | ≥2.0 |
Chopped basalt fiber was obtained from Jiangsu Province (Figure
Chopped basalt fibers.
Properties of chopped basalt fiber.
Properties | Results | Specification requirements |
---|---|---|
Relative density (g·cm−3) | 2.71 | — |
Length (mm) | 3, 6, 9, 12 | — |
Diameter ( |
13 | — |
Water content rate (%) | 0.13 | ≤0.2 |
Oil absorption rate (%) | 52 | ≥50 |
Melting point (°C) | 1600 | — |
Tensile strength (MPa) | 2218 | ≥1200 |
Figure
SEM micrograph of chopped basalt fibers.
Besides, some crystalline particles can be observed on the fiber surface, which results in increased surface roughness, and the anchoring effect can be formed when the fibers interact with the asphalt binder, thereby effectively improving the interfacial adhesion between the fiber and the asphalt and improving the overall performance of the asphalt mixture subsequently.
The test methods are illustrated in Figure
Flowchart of test methods.
The gradation of porous asphalt mixture is presented in Table
Porous asphalt mixture gradation.
Sieve size (mm) | 16 | 13.2 | 9.5 | 4.75 | 2.36 | 1.18 | 0.6 | 0.3 | 0.15 | 0.075 |
|
||||||||||
Passing ratio (%) | 100.0 | 92.8 | 60.8 | 13.6 | 11.6 | 9.6 | 8.4 | 7.2 | 6.2 | 5.1 |
The porous asphalt mixtures with basalt fibers were fabricated for laboratory tests according to the Chinese technical specification [
Design parameters of each porous asphalt mix.
Mixture type | No. | Fiber length (mm) | Fiber content (%) | Binder content (%) | Air void (%) |
---|---|---|---|---|---|
Porous asphalt mixture | 01 | — | — | 4.6 | 20.3 |
02 | 3 | 0.3 | 4.8 | 20.1 | |
03 | 3 | 0.4 | 4.8 | 20.1 | |
04 | 6 | 0.3 | 4.8 | 19.9 | |
05 | 6 | 0.4 | 4.8 | 19.8 | |
06 | 9 | 0.3 | 4.8 | 19.6 | |
07 | 9 | 0.4 | 4.8 | 19.5 | |
08 | 12 | 0.3 | 4.9 | 19.3 | |
09 | 12 | 0.4 | 4.9 | 19.2 |
The draindown test and cantabro abrasion test based on JTG E20-2011 were used to evaluate the draindown resistance and antishedding ability of the porous asphalt mixture [
The freeze-thaw split test was performed according to the JTG E20-2011 procedure to evaluate the water stability of the porous asphalt mixtures [
The wheel tracking tests were performed according to the JTG E20-2011 procedure to evaluate the high-temperature stability of the porous asphalt mixture [
The low-temperature bending beam test based on JTG E20-2011 was used to evaluate the low-temperature cracking resistance of the porous asphalt mixture [
In this study, the four-point bending beam fatigue test was performed using an UTM-25 testing apparatus according to JTG E20-2011 and a rectangular beam with the dimension of 380 mm × 63 mm × 50 mm was used. A haversine loading with the frequency of 10 Hz was used at 15°C, and the experiment was conducted by constant strain mode at different strain levels of 650, 850, and 1050 microstrain. Three replicates are used for each test, and the final data are the average value of every experiment.
The indirect tensile test is used to determine the tensile properties of the asphalt mixture at a specified temperature and loading rate. This test was carried out in accordance with JTG E20-2011 [
Indirect tensile test.
The fractured section of the specimens after the indirect tensile test was chosen to observe the fracture mode by using a Nikon D5300 SLR camera and an XL-30ESEM environmental scanning electron microscope. The size of each observation section was approximately 20 mm × 20 mm × 20 mm. The sample should be gold coated before the SEM test, as shown in Figure
Sample after gold plating.
The pavement performance results of porous asphalt mixtures are shown in Figures
Draindown test results.
Cantabro abrasion test results.
Freeze-thaw split test results.
Wheel tracking test results.
Low-temperature bending test results.
Four-point bending beam fatigue test results.
It can be seen from Figures
The results of freeze-thaw split strength test are shown in Figure
From Figure
The results of the wheel tracking test are shown in Figure
Based on Figure
The results of low-temperature bending beam test are shown in Figure
It can be seen from Figure
The results of the four-point bending fatigue test are shown in Figure
From Figure
From Figures
The tensile properties of the porous asphalt mixture can indicate the thermal cracking resistance performance. Therefore, it is important to further study the effects of chopped basalt fiber on the tensile properties of porous asphalt mixtures. Indirect tensile tests were explored to evaluate the tensile properties of porous asphalt mixtures at different temperatures. According to the results of Figures
Results of indirect tensile strength.
As can be seen from Figure
The morphology of fracture sections of the samples is highly related to the temperatures. When the temperature is over 10°C, the samples cannot produce a fracture section by indirect tensile tests. Therefore, typical fracture sections of samples from indirect tensile tests (ITTs) at the temperatures of −15°C, −5°C, and 5°C were selected for morphology analysis, and the results are shown in Figures
Typical sections of no-fiber samples. (a) 5°C. (b) −5°C. (c) −15°C.
Typical sections of samples with fibers of 9 mm + 0.3%. (a) 5°C. (b) −5°C. (c) −15°C.
Typical sections of samples with fibers of 12 mm + 0.3%. (a) 5°C. (b) −5°C. (c) −15°C.
It can be seen from Figures
Samples with 9 mm length and 0.3% of fibers after −5°C ITT were selected for microstructure analysis by XL-30ESEM environmental scanning electron microscopy. Typical fiber distribution in porous asphalt mixture is shown in Figure
SEM micrographs of the crack of the porous asphalt mixture.
SEM micrographs of bonding interface between the fiber and asphalt. (a) Asphalt wrapped with chopped basalt fiber; (b) distribution of fibers in asphalt.
It can be observed from Figure
In Figure
In Figure
According to the results of this study, the following conclusions can be drawn: The addition of chopped basalt fibers can improve the pavement performance of porous asphalt mixture, and the recommended length and content of fiber are 9 mm and 0.3%, respectively. The addition of chopped basalt fiber can improve the indirect tensile strength of porous asphalt mixture. No obvious difference was observed between the 9 mm and 12 mm length of fibers. The indirect tensile strength reaches the maximum value at −5°C. At −5°C, the strength of aggregates contributes to the indirect tensile strength of the test sample, resulting in the maximum indirect tensile strength. The chopped basalt fiber is well combined with the asphalt and distributed in a three-dimensional network structure in the porous asphalt mixture, which can reinforce the performance of asphalt mixture significantly.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest regarding the publication of this paper.
The authors would like to appreciate the financial support of the National Natural Science Foundation of China (nos. 51578481 and 51578480), the Practice and Innovation Plans for Graduates of Yangzhou University (XSJCX17_023), and Natural Science Foundation of the Higher Education Institutions of Jiangsu Province (16KJB580010).