1. Introduction
Asphalt is a smooth material consisting of more than a thousand chemical types and has vital significance for the transportation infrastructure [
1]. Asphalt is used extensively as a binding in flexible pavement construction worldwide due to its excellent viscoelastic properties and superior service performance [
2,
3]. Due to asphalt’s temperature sensitivity, it changes in climatic conditions, reducing the quality of asphalt pavement, which causes different distresses, such as rutting at high temperatures and cracking at low temperatures [
4]. When asphalt pavement is exposed to heavy traffic loads, its quality is considerably reduced [
5]. The suitable modification of the asphalt binder must be perfect for promoting safe, reliable, and environment-friendly flexible pavement materials [
6]. The physical properties and medium- and high-temperature performances (mechanical performance) can be enhanced by using asphalt modifiers [
7,
8]. Among various bitumen modifiers, crumb rubber modifier (CRM), tafpack super (TPS), and polypropylene (PP) have attracted enormous attention in the field of asphalt pavements [
9,
10].
Al-Hadidy et al. investigated the influence of accelerated ageing on the engineering properties of modified asphalt mixtures with polypropylene (PP). They concluded that the addition of polypropylene to asphalt concrete mixes enhanced its resistance to permanent deformation, fatigue, cracking, and ageing [
11].
Khabaz and Khare conducted their experimental tests and simulations using styrene-butadiene rubber (SBR) and molecular dynamics (MD), respectively, to verify glass transmission and molecular mobility in modified asphalt. Their results pointed out that the volumetric features of the SBR modified asphalt were found to be insensitive to the presence of SBR additives. Further, the addition of SBR increased the aggregation of the asphaltene molecules. Likewise, the addition of SBR led to a decrease in the mobility of the asphalt particles constituent. The proposed outcomes show the potential for polymeric modifiers as enhancements to the dynamic mechanical features of asphalt, but not its volumetric properties [
1].
The laboratory tests and the volume-temperature behavior of the asphalt systems in the simulation exhibited the same glass transition phenomenon. The glass transition temperature, room temperature density, and coefficient of volume thermal expansion of the virgin asphalt systems agreed with the experimental data when calculating the effect of the high cooling rate used in the simulation [
1,
12].
Ting et al. conducted a study on shear resistance properties of TPS and styrene-butadiene-styrene (SBS) modified bitumen binders and mixtures by measuring the laboratory creep data over a broad domain of the constant shear stresses and examining the permanent deformation of the porous asphalt mixtures for fixed compressive stress. They conducted a comparison between the shear strain rate and shear creep modulus of the TPS and SBS modified asphalt binders that concluded in a similar trend [
9].
Bouraima et al. utilized different modifiers such as TPS, high-viscosity additive (HVA), road-science technology (RST), and SBS additives, summarizing that the conventional binders did not meet the requirements of a 60 °C dynamic viscosity with TPS and HVA modifiers. Besides, the SBS modified asphalt binder has better viscosity. Their results concluded that the SBS modifier is a suitable choice [
10].
Barco et al. compared the performance of the crumb rubber modifier and the SBS modifier binders. The results showed that the crumb rubber modifier (CRM) contents have to be higher than that of the SBS to obtain the same performance. Additionally, the crumb rubber modifier (CRM) binders are more stable than the SBS modified asphalt in terms of modifier concentration. Furthermore, the CRM binders exhibited less water susceptibility and similar thermal and ageing susceptibility to SBS [
13].
The crumb rubber types are different; their sources are extensive and their components are various, so their modified influence on the asphalt binders will not be the same [
13]. CRM was utilized in the modification of asphalt binders to improve its rutting resistance properties by increasing the stiffness and ductility of the asphalt at high service temperatures [
14,
15]. It was also used to extend the fatigue life of asphalt through the thickening of asphalt film and accordingly decrease the asphalt ageing rate [
16]. Moreover, the application of the crumb rubber modifier as an additive reduces the stiffness of the asphalt binder at low service temperatures, which are convenient for its low service temperature performance [
17,
18]. Because of these advantages, there is considerable attention regarding the utilization of tire rubber for modified asphalt binders, which could potentially enhance flexible pavement [
18].
Using TPS to modify asphalt has broad applications. In recent years, it has been shown to improve the rutting resistance to permanent deformations [
19]. TPS commonly holds the thermoplastics elastomer as an essential component, which is hard to melt in a bitumen binder. However, when combining the polymer implementation with asphalt, it is possible for a plasticizer to become soluble in the virgin bitumen, wherein it modifies it to the high viscous bitumen for a higher quality [
9,
19].
The polypropylene (PP) is interacted with virgin asphalt and enhance the asphalt performance characteristics, which are transported by changing the rheological properties of the modified asphalt. When the PP is introduced to the bitumen binder, the flow decreases with an increase in the modified binder concentration, which reflects the increase in the viscosity of the modified binder [
3].
The objectives of this study were for the laboratory experiments to evaluate the physical properties and mechanical performance of the modified asphalt binders using CRM, TPS, and PP modifiers. Secondly, the comparison was conducted between the results to determine the most appropriate additive for enhancing the medium- and high-temperature performance and physical properties of the bitumen binders. To achieve these goals various tests such as ductility, rotational viscosity, toughness, and tenacity tests as well as a rheological test such as the dynamic shear rheometer (DSR) test were applied on the modified asphalt binders using CRM, PP, and TPS modifiers.
4. Conclusions
From the outcomes of this study, the following conclusions can be drawn:
With the addition of 4% CRM, the increase in viscosity is more than two times the viscosity of the neat asphalt, and about 31% and 34% more than the addition of 4% of PP and TPS, respectively. Thus, to enhance the viscosity in an asphalt binder, use CRM, as it is the preferred additive for improving rutting resistance and fatigue/reflection cracking resistance in flexible pavement, followed by PP and TPS modifiers.
The results showed that significant reduction in ductility occurred by the addition of CRM, PP, and TPS modifiers. Moreover, TPS-modified asphalt binder was found to have a high ductility and thus TPS is more flexible. Further, it concluded that TPS behaves differently from both CRM and PP for all concentrations.
The results of the toughness and tenacity test concluded that the maximum force increased linearly with the increase of CRM, PP, and TPS concentrations. The consequences indicated that the maximum strength obtained from the addition of CRM is better than those derived from PP and TPS modifiers. The significant dissipated energy during the fracturing process was achieved by adding CRM. Outcomes also showed that CRM has a small value of tenacity. Thus, CRM is the most appropriate modifier for enhancing the mechanical properties of the asphalt binder behavior.
The DSR test results showed that the G*/sin(δ) and G* values increased with the modifier content and decreased when the temperature increased. The values obtained from the CRM-modified bitumen binders were higher than those of the PP- and TPS-modified binders. Thus, CRM-modified asphalt binders were the most preferable for enhancing the medium and high-temperature performance and permanent deformations.
The DSR stress-strain relationship results indicate that the medium- and high-temperature features (stress) increased when CRM content increased. However, in the case of PP and TPS, the properties increased at 2% and 3.5% of PP and the stress decreased considerably at 4% of PP. Further, the properties increased at concentrations of 2% and 3% TPS and the stress reduced substantially at 3.5% and 4% TPS. Because the CRM-modified binder has a wide linear viscoelasticity area, CRM is the most appropriate modifier to enhance medium- and high-temperature performances.
Note that this study investigated only medium- and high-temperature effects. In future studies, the authors will investigate the low-temperature properties of asphalt binders modified by CRM, PP, and TPS, as well as the effect of these modifiers on the physical properties and the low-temperature performance (mechanical performance) of the modified asphalt binders, in order to prove which additive is preferable when enhancing bitumen binders.