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30 June 2023

Experimental Analysis to Evaluate the Impact of Styrene-Butadiene-Styrene and Crumb Rubber on the Rutting and Moisture Resistance of Asphalt Mixtures

and
1
Civil Engineering Department, College of Engineering, University of Sulaimani, Sulaymaniyah 46001, Iraq
2
Civil Engineering Department, College of Engineering, University of Duhok, Duhok 42001, Iraq
*
Author to whom correspondence should be addressed.
Sustainability2023, 15(13), 10387;https://doi.org/10.3390/su151310387
This article belongs to the Section Sustainable Materials
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Abstract

The most severe distresses in asphalt pavement are rutting, fatigue, and low-temperature cracking; therefore, to solve these problems, it is essential to modify asphalt binders in asphalt concrete mixtures. In this study, a comparison between using styrene-butadiene-styrene (SBS) and crumb rubber (CR) as modifiers for asphalt binders to overcome distress issues was conducted. Base and SBS or CR-modified binders were subjected to all conventional and Superpave binder tests. Engineering tests such as the Hamburg wheel tracker and indirect tensile strength ratio were also run to evaluate the engineering properties. The used SBS percentages were 1, 2, 3, 4, and 5%, while CR percentages were 3, 6, 9, 12, and 15% by total weight. The results showed lower penetration, higher softening point, viscosity, and elastic recovery for both additives. In addition, dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests showed increasing values of both high and low temperatures of modified asphalt performance grade (PG) with increasing SBS or CR percent. The tensile strength ratio and Hamburg wheel tracker results showed the best engineering properties at 3% SBS or 9% CR, the optimum percent. A triple percentage of CR is needed to get the same effect of SBS for the asphalt mixture.

1. Introduction

Nowadays, traffic loads on roads are growing and becoming heavier. Increased loads shorten the service life of the pavement when combined with unfavorable weather [1,2]. Consequently, developing more durable asphalt mixtures is vital to prevent pavement degradation [3].
Several techniques have been developed to enhance the qualities of the asphalt mixture used in road building; one of these techniques involves combining the asphalt mixture with different additives. Polymers are the additives that are most frequently used in polymer-modified asphalt (PMA), a novel substance created when polymers are combined with asphalt that has different qualities from regular asphalt.
Styrene-butadiene-styrene (SBS) and crumb rubber (CR), styrene-butadiene rubber (SBR), ethylene-vinyl acetate (EVA), and polyethylene are among the polymers that are frequently utilized in asphalt modification [4,5]. With these modifications, it is feasible to lessen fatigue damage, lessen rutting sensitivity, increase the hot asphalt mixture’s aging resistance, and prevent the issue of the binder running off with the aggregate by enhancing adhesion qualities [6]. The produced SBS-modified mixes significantly improved fatigue cracking performance than the conventional mix [7].
The rutting resistance is improved when asphalt is modified with SBS and it has higher performance properties [8,9]. Generally, SBS can be dispersed in bitumen as its content is not more than 5% [10]. The wheel tracking test has been used effectively to identify the rutting susceptibility of bituminous mixes. Ranging specifications are available for wheel tracking rut depths specific to the typical mixes, climatic, and traffic loading conditions [11].
Vulcanized rubber has been applied in the bitumen industry for a long time. It is perceived to have the potential to improve bitumen performance by improving thermal sensitivity, flexibility, stability, and stripping. While it helps dissipate the developed stress at low temperatures, preventing asphalt concrete cracking, at high temperatures, it acts as a membrane and controls the asphalt flow, enhancing shear resistance [12]. The rubber of worn-out tires has been shown to decelerate asphalt aging [13].
A smart solution for sustainable development is by reusing waste materials, and it is believed that crumb rubber modifier could be an alternative polymer material in improving hot mix asphalt performance properties [14].
In this research, the effects of using both SBS and CR separately on asphalt properties were conducted to produce modified asphalt, which can be used to overcome the common pavement distresses by using the Superpave mixture design method and its specification. The modified asphalts were subjected to conventional and performance-grade laboratory tests. The tensile strength ratio (TSR) and Hamburg wheel tracker (HWT) test are conducted on modified asphalt mixtures to find each additive’s effect on the modified asphalt mixture’s engineering properties and their optimum content. Finally, a comparison between these two additives was also conducted to find a replacement percentage of CR which is cheap, available, and has less environmental pollution instead of somewhat costly SBS regarding that recycled CR cannot have completely the same effect as virgin SBS.

2. Materials and Methods

2.1. Materials

2.1.1. Asphalt

The current study used a local asphalt binder with a penetration grade of (80/100) or (PG 58–22) from the Phoenix refinery. This low-viscosity asphalt is rarely used in pavement construction for hot climate regions; however, after mixing SBS or CR with it, higher viscosity asphalt resulted due to the additive’s effect. Table 1 shows the results of the conventional tests of the base asphalt (0% CR or SBS). The penetration range was ±2 for up to 49 penetration values and ±4 for penetration values of 50 to 149. The test repeatability in softening point test was from 0.8 to 1 °C. The standard deviation for ductility test was around 10% and for specific gravity is 0.00082 at 25 °C. The test repeatability in viscosity was 3.5% of the mean and for flash point was 8 °C. All the mentioned limits were considered during base and modified asphalt testing.
Table 1. Characteristics of the base asphalt.

2.1.2. Styrene-Butadiene-Styrene (SBS)

The SBS used was Phoeprene 1211 porous crumb, high quality powdered form, 30/70 Styrene/Butadiene percentages, thermoplastic copolymer, and radial structure. The specific gravity of the SBS was 0.79. The grain size distribution of SBS is shown in Figure 1. The SBS percentages used with the base asphalt were 1, 2, 3, 4, and 5% by total weight of the asphalt.
Figure 1. SBS and CR grain size distribution.

2.1.3. Crumb Rubber (CR)

The CR used in this research is a product of vehicles scrap tires obtained from a local factory that uses specified crushing machines to cut and shred scrap tires into small particles at normal room temperature (ambient grinding method). Figure 1 shows the grain size distribution of the CR. The measured specific gravity of the CR was 0.434. The CR percentage used with asphalt was 3, 6, 9, 12, and 15% by total weight, which is the most common percentage in previous studies. The main reasons for its use are the availability, environmental issue, and low cost of this additive, and their effectiveness even at high percentages.

2.1.4. Aggregate

The used aggregate was crushed aggregate produced from boulder crushing. Gravel, sand, and filler were subjected to the necessary tests according to Superpave specifications [16] as presented in Table 2. The asphalt concrete course that was prepared in this study was the surface course type 3A according to Iraqi specifications [17] or ASTM D3515/D-5 [15] because it is the most common and strongest course in high-traffic volume highways.
Table 2. Aggregate test results.

2.2. Methodology

2.2.1. Mixing Asphalt with SBS or CR

The mixing process of base asphalt with the SBS or CR was conducted using an oil bath supplied with a digital thermometer and internal agitator to spread heat, as shown in Figure 2. A vertical stirring mixer was used after adding the specified percentages of total weight. The mixing temperature was 180 °C, a stirring speed of 600 rpm was used to ensure mixing and prevent segregation, and the mixing time was 2 h. These mixing conditions are similar to the study of researchers [18,19,20].
Figure 2. The asphalt and SBS or CR mixing process.
Softening point samples were taken to track the change every half hour during this process. After finishing the mixing process, the conventional penetration tests, softening point, elastic recovery, rotational viscosity (RV), flash point, and specific gravity were made. The penetration, softening point, and loss of heat tests were also conducted after the RTFOT and storage stability tests. Microscope images were taken for the samples to ensure SBS or CR dispersion in the asphalt binders. Finally, PG tests that included DSR, RTFOT, PAV, and BBR were conducted to find their PG values and G*, δ, stiffness, and m values.

2.2.2. Aggregate Structure Selection

Three aggregate gradations were designed (A, B, and C) according to the Superpave control point’s limits and specifications [16], as shown in Figure 3, after passing all the required gravel, sand, and filler tests. For each aggregate, five samples were prepared to be mixed with asphalt using five percentages (4, 4.5, 5, 5.5, and 6%) of asphalt. The Gmm for each aggregate mixture was measured in the laboratory. Then all the mixes were compacted using gyratory compacter, The results were used to find voids in mineral aggregate VMA, voids filled with asphalt VFA, density at an initial number of gyration, design number of gyration and a maximum number of gyration, dust to binder ratio, and specific gravity of the compacted mixture at 4% air voids. Finally, depending on the aggregate result and Superpave specifications [16], the passed aggregate B was selected for all the later modified asphalt mixtures with SBS or CR.
Figure 3. Aggregates gradations for 12.5 mm nominal max size [16].

2.2.3. Determining the Modified Asphalt Mixture Optimum Contents

Using aggregate B, six asphalt mixtures were prepared for all modified asphalts with SBS or CR, including the base sample, using a mixer supplied with the heating element. The asphalt percentage for each sample was 4% to 6.5%, with an increment of 0.5% depending on SBS or CR percent. All the samples were compacted using gyratory compacter, and the optimum asphalt content for each SBS or CR percent was found after volumetric analysis according to Superpave specifications and procedure.

2.2.4. Tensile Strength Ratio Test (TSR) (ASTM D4867 [15], AASHTO T283 [21])

This test was conducted on base, SBS, or CR-modified asphalt mixture samples. Six samples for each percent were prepared for the TSR test. The sample diameters were 10 cm, sample height was around 60 mm, and three samples were tested in dry conditions and the other three were tested conditioning through one cycle of freezing and thawing. The samples were prepared using 7% air void as specified, then tested to find the indirect tensile strength of the samples. The TSR was also calculated. Figure 4 shows the sample saturation process.
Figure 4. TSR samples saturation process.

2.2.5. Hamburg Wheel Tracker Tests (AASHTO T324) [21]

This test was conducted on a base and modified asphalt mixture with SBS or CR. The samples had a diameter of 15 cm and a height of 6 cm; they were tested using a wheel tracker machine (manufactured by Control Company). This was conducted to find the rutting value versus the number of wheel passing repetitions (1 cycle = 2 passes). The passes continued until 20,000 passes or the rut depth of 20 mm was reached, as shown in Figure 5. A curve was drawn between rut depth in mm and number of passes; the stripping inflection point (SIP) and number of passes to failure (Nf) were found from this curve.
Figure 5. Hamburg wheel tracker machine.

3. Results and Discussions

3.1. SBS and CR- Modified Asphalt Binders

Table 3 and Table 4 show the results of tests conducted on the SBS and CR-modified asphalt. Penetration values decreased while the softening points increased with increasing SBS% or CR%, as expected which means that the asphalt became harder. These results are agreed with those obtained by [19].
Table 3. Test results for SBS-modified asphalt samples.
Table 4. Test results for asphalt mixed with CR.
The ductility test became inapplicable, excluding the base sample as the breaking point was not reached and replaced by the elastic recovery test. The increasing elastic recovery values indicated that the mix was gaining more elasticity. In the elastic recovery test, the sample was pulled out to an elongation of 20 cm, then after maintaining for 5 min, the sample was cut into two halves in the midpoint using scissors. Then, after 60 min, the recovery length was measured. The viscosity increased clearly [22]. CR 12% and CR15% had a viscosity greater than 3 Pa·s which is the limit of maximum viscosity according to Superpave specifications. Additionally, the flashpoint and specific gravity values increased slightly. The penetration index increased from a negative to a positive value due to the change in penetration and softening point, which means that the SBS or CR is an effective additive.
After the storage stability test for the up and down part of the mold, the softening point increased with the increase of SBS or CR due to absorbing maltenes component from the asphalt and swelling of SBS or CR [23]. This can be overcome by continuous mixing before using the modified asphalt. Penetration and softening point values after the rolling thin film oven test (RTFOT) changed as expected due to the hardening effect of short-term aging. The loss% value decreased but was still within Superpave limits; this is reasonable as the modified asphalt became harder [19].
The penetration values after the RTFO test decreased compared with the unaged samples as expected, since aging leads to the evaporation of volatile components in asphalt. The same thing happened with softening points which increased after the RTFO test for the same reason.
The rotational viscosity tests were conducted on SBS or CR-modified asphalt percentages at 135 °C and 170 °C. These results were used to find mixing and compacting temperatures for the modified asphalt mixtures. This can be achieved using two temperatures -viscosity values and a viscosity bar chart.

3.2. Relationship of SBS and CR with Penetration, Softening Point, and Viscosity

Figure 6 and Figure 7 show the SBS and CR percent versus penetration, softening point, and viscosity with their regression equations and correlation coefficients. These model equations can be used to find conventional test values for any required SBS or CR percent or to predict any value out of the range.
Figure 6. Relationship between SBS% and (a) penetration, (b) softening point, and (c) viscosity.
Figure 7. Relationship between CR and (a) penetration, (b) softening point, and (c) viscosity.

3.3. Microscopic Images of the SBS and CR-Modified Asphalt Binders

To check the dispersion of SBS or CR in the asphalt during the mixing process, fluorescent microscope images with magnification power (40×) were taken for SBS while a light microscope was conducted for CR. Figure 8 and Figure 9 show the asphalt particles represented by yellow (SBS) or black (CR) spheroid particles. For SBS 4% and 5%, the SBS particles was agglomerated.
Figure 8. Microscope images (40×) of SBS-modified asphalt samples.
Figure 9. Microscope images (40×) of CR- modified asphalt samples.

3.4. Performance Grade (PG) Tests

This test consisted of two rheometer tests, dynamic shear rheometer (DSR) and bending beam rheometer (BBR). These two tests are essential to determine performance grades’ high and low temperatures (PG).

3.4.1. Dynamic Shear Rheometer (DSR) Test

This test gives four important parameters, complex shear modulus (G*), which is the slope of shear stress to shear strain representing the stiffness of the asphalt, (G*/sinδ) which represents the rutting parameter, (G*.sinδ) represents the fatigue parameter, and the phase angle (δ) represents the slope angle of the viscous to the elastic phase of the asphalt.
The DSR tests were not conducted completely for SBS 5%, CR12%, and CR15% because the original samples passed 94 °C, beyond the PG specifications. This was expected due to their high viscosity values.
Table 5 and Table 6 show the DSR test results for SBS and CR samples. There was an increase in high temperature of PG with an increase in SBS or CR percentage. This will satisfy the requirements of the hot regional climate of Middle East countries. The values after RTFOT were greater due to the aging of the asphalt binder.
Table 5. DSR results for SBS asphalt binder samples.
Table 6. DSR results for CR asphalt binder samples.

3.4.2. Bending Beam Rheometer (BBR) Test

This test had two outcomes; the stiffness (S) and the m-value. Table 7 and Table 8 present these two values for SBS% and CR%. The stiffness values increased and the m-value decreased with increasing SBS or CR% at the same temperature. These results indicate that the increase in additive percentages will raise the low-temperature grade of asphalt PG.
Table 7. BBR results for asphalt samples with different SBS percentages.
Table 8. BBR results for asphalt samples with different CR percentages.
Table 9 shows that increasing the SBS or CR percentages increase the high and low-grade temperatures. These results are confirmed by [22] and by [19]. SBS 3%, CR 6%, and CR 9% gave the PG value of PG 76-16, which is suitable for most hot country climates.
Table 9. PG results for SBS and CR asphalt mix [15].

3.5. Tensile Strength Ratio (TSR)

This test result gave three important outcomes, dry indirect tensile strength (S dry), conditioned indirect tensile strength (S cond.), and the tensile strength ratio between conditioned to dry samples. The conditioned strength and tensile strength ratio are more important for severe environments than dry ones. Table 10 and Figure 10 present the TSR test values for SBS and CR-modified asphalt mixtures. The degree of saturation was kept between 70 and 75% to obtain homogeneous samples, while the Superpave specification value was 55–80%. The TSR values increased with the increase of SBS% or CR% until the optimum percentage was reached; the TSR values decreased. This trend is similar to the results obtained by [19,24]. The TSR values for both control samples are very close to the specification limit of 80%, this ensures the necessity to use modified asphalt.
Table 10. TSR results for SBS and CR-modified asphalt.
Figure 10. TSR results for SBS and CR-modified asphalt.

3.6. Hamburg Wheel Tracker

The test outcomes were passes versus rut depth in millimeters. Then, a curve was drawn between passes and rut depth to find stripping inflection points (SIP) and failure points (Nf). SIP represents a point when the stripping occurs in the sample. The Nf point represents the failure point when the sample reaches 20 mm rut depth. Table 11 and Figure 11 present the Hamburg wheel tracker test results for SBS and CR%. It is clear that rutting resistance increased until the optimum percent of additives was reached and then decreased; this result is similar to the conclusion of [25].
Table 11. Wheel tracker results for SBS and CR-modified asphalt mixtures [21].
Figure 11. Hamburg wheel tracker results for SBS and CR asphalt mixtures.

4. Conclusions

This study used styrene-butadiene-styrene and crumb rubber to modify asphalt binder and overcome the distress problem of flexible pavement. After conducting the required tests on the modified mixtures and the control samples, the following points were concluded:
  • Adding SBS or CR to asphalt raises the viscosity, which leads to higher mixing and compacting temperature of the asphalt mixture, this must be considered in the highway construction process.
  • The optimum CR% to get maximum moisture and rutting resistance is CR 9%, while for SBS% is SBS 3%.
  • Approximately adding 1% of SBS or 3% CR each to asphalt will raise the high-temperature PG one grade.
  • Adding 9% CR or 3% Table SSBS changes the PG from PG 58-22 to PG 76-16, this is very beneficial for hot climate regions.
  • The percentage increase in TSR for 9% CR compared to the control sample is 20%, and for 3%, SBS is 22%. These are valuable amounts in moisture resistance.
  • The percentage increase in the rutting parameter for 9% CR is 246%, for 3% SBS is 350%. These are remarkable improvements in the rutting resistance of asphalt concrete.
  • The extra cost of using SBS or CR in asphalt mixture is approximately 10% and 5%, respectively; this is a reasonable amount regarding their engineering benefits, decreasing maintenance cost, and providing a sustainable asphalt concrete pavement with an extended service life.
  • To replace costly SBS additives with cheap and environment-friendly CR, triple the amount of CR is needed to get improvement in engineering properties of asphalt concrete mixture without forgetting that 3% SBS-modified asphalt had higher conventional asphalt tests values (Table 3 and Table 4) and higher asphalt concrete tensile strength and rutting parameter values (Table 10 and Table 11) than 9% CR-modified asphalt.
  • TSR values for the control sample are very close to Superpave specifications. This ensures that using a modified asphalt mixture is crucial.

Author Contributions

A.O.M.: Conceptualization, methodology, software, data analysis, writing—original draft preparation. R.A.K.: Supervision, reviewing and editing. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

Compiled data and calculations are stored in Excel files in Highway lab./Civil department/college of engineering/university of Sulaimani and will be made available upon request to the corresponding author.

Acknowledgments

The authors would like to thank the College of Engineering—University of Sulaimani for supporting this study.

Conflicts of Interest

The authors declare that they have no competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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