Assessment of Asphalt Mixtures Enhanced with Styrene–Butadiene–Styrene and Polyvinyl Chloride Through Rheological, Physical, Microscopic, and Workability Analyses

Abstract

This study investigates the performance improvement of asphalt binders through the incorporation of two polymers, polyvinyl chloride (PVC) and styrene–butadiene–styrene (SBS), with asphalt grade (60–70), to address the growing demand for durable and climate-resilient pavement materials, particularly in areas exposed to high temperatures like Iraq. The main objective is to improve the mechanical characteristics, thermal stability, and workability of typical asphalt mixtures to extend pavement lifespan and lessen maintenance costs. A thorough set of rheological, physical, morphological, and workability tests was performed on asphalt binders modified with varying content of PVC (3%, 5%, 7%, and 9%) and SBS (3%, 4%, and 5%). The significance of this research lies in optimizing binder formulations to enhance resistance to deformation and failure modes such as rutting and thermal cracking, which are common in extreme climates. The results indicate that PVC enhances performance grade (PG), softening point, and viscosity, although higher contents (7% and 9%) exceeded penetration grade specifications. SBS-modified binders demonstrated marked improvements in softening point, viscosity, and rutting resistance, with PG values increasing from PG64-x (unmodified) to PG82-x at 5% SBS. Fluorescence microscopy confirmed optimal polymer dispersion at 5% concentration for both SBS and PVC, ensuring compatibility with the base asphalt. Workability testing revealed that SBS-modified mixtures exhibited higher torque requirements, indicating reduced workability compared to both PVC-modified and unmodified binders. These findings offer valuable insights for the design of high-performance asphalt mixtures suitable for hot-climate applications and contribute to the development of more durable and cost-effective road infrastructure.

1. Introduction

Many asphalt roads in Iraq are affected by rutting due to increased traffic volume, which may be related to the asphalt binder itself. By employing polymer-modified asphalt to satisfy the required performance standards, this problem may be fixed. Using asphalt treated with polymer in pavement construction is an essential step in increasing the lifespan and durability of asphalt roadways [1]. Plastomers and elastomers are the two main categories of polymers used to modify bitumen for road use. The primary purpose of plastomers, such as polyvinyl chloride (PVC), is to modify bitumen by creating a strong, stable three-dimensional network that provides resistance to deformation. Marshall’s mix design was used by Rahman et al. [2] to improve the properties of asphalt mixes by using waste plastics such as polyethylene and PVC. Waste PVC is added to base asphalt to boost its viscosity and stiffness, which enhances the modified resistance of asphalt to rutting. However, elastomers with high elasticity, such as styrene–butadiene–styrene (SBS), may stretch and regain their original shape, making them resistant to persistent deformation. In addition, they have high tensile strengths when stretched and may regain their original state following stress exposure [3]. Shearing, stirring, and other mixing techniques that guarantee the uniform dispersion of SBS in the asphalt are used to create SBS-modified asphalt, made from asphalt blended with a certain amount of SBS modifier [4]. Słowik [5] evaluated the rheological characteristics of asphalt treated with SBS. The findings demonstrated that the SBS concentration significantly affected the asphalt binders’ decreased sensitivity to temperature. Since it is difficult to define polymer-modified asphalt by empirical testing, it is necessary to determine fundamental technical qualities that may be used to forecast asphalt mixtures’ performance with accuracy. Moreover, there is an increasing need to comprehend the reasons behind polymer-improved asphalt actions. This has led to studies that examine the relationship between polymer-improved asphalt morphology and performance to gain a better understanding of how all of the components of polymer-modified binders (PMBs) interact to create binders with predictable behaviors.

The bitumen phase and the polymer dispersion have often been distinguished using fluorescent microscopy [6]. This has allowed researchers to ascertain if the PMB mix is homogenous and stable throughout storage and/or aging [7]. One of the first studies on the effects of blending parameters, polymer concentration, and base binder chemical composition on the bituminous blend morphology changed by styrene, butadiene, and styrene (SBS) was carried out by [8]. It is widely acknowledged that to prevent separation during handling and storage, there must be a certain degree of compatibility between the bitumen phase and the polymer. The microstructural investigation of polymer-modified binders in bituminous mixtures has also been carried out using fluorescent microscopy. This might be seen as a significant option given the numerous drawbacks connected to the use of extraction solvents in bituminous laboratories. Because of this, microscopy is a perfect and secure way to provide information on the in situ condition of the binder inside asphalt. In this study, fluorescence microscopy is presented for assessing polymer dispersion in asphalt during production. The sample conditioning and preparation significantly impact the morphology of modified asphalt. As a result, it is critical to design an exact preparation procedure with well-defined and regulated temperature, shearing, and chilling [9].

Different approaches have been proposed in the literature to use epi-fluorescence microscopy to analyze the morphology of asphalt treated with polymer [10,11,12]. The research suggested that the morphology that results from rapid cooling is probably not representative of the final morphology in the asphalt mix, as PMB morphology is dependent on preparation temperature and cooling history [13,14]. The in situ binder undergoes high-temperature mixing and transit during asphalt pavement construction, and then it naturally cools down during installation. During PMB performance testing, this issue should be taken into consideration, particularly if the test specimen is tiny, as in the case of dynamic shear rheometer (DSR) tests [15], where it has been discovered that rheological characteristics and morphology have a strong relationship [16]. To achieve the appropriate hot mix asphalt (HMA) density and smoothness in a compacted pavement, satisfactory workability is crucial. Smooth pavement construction may be more challenging for combinations that are extremely abrasive and, thus, have limited workability. Because of their large void content, compacted pavements can have serious performance issues [17]. The life of the pavement is shortened if the binder is not adequately compacted, which increases the risk of permeability issues and accelerates the pace of oxidative aging.

Recent research has concentrated on enhancing pavement performance and sustainability through the use of substitute or waste-derived materials. Acrylonitrile styrene acrylate (ASA) waste composites and stamp sand, for instance, have been tested as asphalt-free surface materials [18]. Others, on the other hand, have investigated rubber-modified asphalt that contains aggregates produced from tires to enhance subgrade performance [19]. Further, pyrolysis wax and waste plastics have been used to wet-modify asphalt, which has demonstrated potential in improving binder qualities [20]. They are in line with the goal of the current study, which is to enhance the workability and mechanical behavior of hot mix asphalt mixes in Iraqi high temperatures by using SBS and PVC waste polymers.

The usage of polymer-modified binders has expanded globally as a result of their performance characteristics. Nevertheless, because polymer-modified binders tend to make binders more viscous, the workability of HMA significantly drops at a given temperature when using them. When using modified binders, compacting HMA to the required density may be more challenging than when using unmodified binders. The workability of HMA is essentially a function of binder qualities at a particular temperature if the compositional parameters of a mix, such as aggregate physical properties and gradation, are maintained constant. The workability of the mixture improves with increasing temperature since the binder’s viscosity lowers with rising temperatures. It is not always ideal to raise the mix temperature to get the appropriate workability, though.

The following issues arise from high temperatures: (1) damage to additives; (2) damage to asphalt (heat hardening); (3) higher fuel consumption; and (4) increased generation of smoke and volatile organic compounds [21,22].

Binder viscosity has historically been used to calculate the mixing and compaction temperatures of HMA. The workability of the mix is directly impacted by the compaction temperatures attained using this approach since equiviscous binder conditions are employed. However, issues with appropriate mixing and compaction temperature have been noted with the growing usage of modifiers and novel HMA mix types (such as Superpave and stone matrix asphalt). As a result, research was required to analyze the methodologies for assessing the workability of HMA mixes, as well as the necessity of evaluating the application of workability to determine mixing and compaction temperature. The pavement industry has been emphasizing how important it is to measure workability value easily and reasonably with consistency. However, there is a need for further research into the viability of hot mix due to the recent surge in property development. The impact of mixing temperature on determining the workability value of HMA has not received much attention, according to the literature [17,21].

Several researchers have assessed the workability of asphalt concrete using torque or various indications derived from the gyratory compactor and porosity from earlier studies that focused on determining the workability of HMA, although several researchers gauge workability using torque [22].

This study evaluates the rheological properties and workability of modified asphalt binder, which includes PVC and SBS, and is mainly utilized in hot mix asphalt (HMA) for the surface layer of road pavements. Thus, different proportions of SBS (3%, 4%, and 5%) and PVC (3, 5, 7, and 9) were mixed with virgin bitumen. When creating the asphalt concrete mixes, a predetermined optimum asphalt proportion of 5.3% was followed by an evaluation of the workability of the mixture, the workability device was manufactured based on recommendations from earlier research [23], and the mixing bowl was made of material that keeps the mixture from clumping to produce precise data based on field experiences.

The objectives of this research are as follows:

• Evaluate the physical and rheological properties of PVC- and SBS-modified asphalt and compare them. PVC and SBS are the most common polymers available in the market.
• Develop a methodology for evaluating the workability of asphalt modified with polymer by a manufacturing device for measuring workability, depending on previous research.

Asphalt improved with SBS is the most often utilized in street paving in Iraq, and finding an alternative to this polymer is critical due to its high cost. To determine the optimal temperature for spreading and compacting polymer-modified asphalt mixtures, workability data (torque required for mixing) should be compared to physical and rheological tests for PVC and SBS polymers. The best polymer should be chosen based on its resistance to high temperatures, ease of use for asphalt field workers, and cost.

3.

The polymer-modified asphalt mixture may be classified depending on its workability. Workability may be used to determine the optimal mixing temperature as well as mixing and compaction torque.

4.

Asphalt pavements in Iraq suffer from rutting and cracking due to extreme heat and traffic loads, exposing the limitations of conventional binders. This study explores the use of polymer-modified asphalt (PMA) using styrene–butadiene–styrene (SBS) and polyvinyl chloride (PVC) to enhance binder performance. While SBS offers elasticity and thermal resistance, its high cost limits its use. PVC, a cheaper alternative, shows promise but requires further study, particularly at higher dosages. This research fills a gap by comparing the physical, rheological, morphological, and workability properties of binders modified with SBS and PVC, aiming to develop an effective, economical solution for hot-climate pavements.

2. Materials

2.1. Asphalt Binder

The (60–70) penetration-graded bitumen utilized in this investigation was supplied by the Al-Dura refinery located in Baghdad, Iraq. The performance properties of the original asphalt binder were determined following the ASTM standards [24] and the Iraqi Standard Specification for Roads and Bridges (SCRB, 2003) [25]. The test results and specification limits are presented in

Table 1. The characteristics of asphalt binder by Iraqi and ASTM standards.

Test	ASTM Designation [24]	Test Results	SCRB
			Specification (2003) [25]
Penetration (25 °C, 100 g, 5 sec)	D5	66	60–70
Ductility (25 °C, 5 cm/min)	D113	130	>100
Softening Point,	D36	46	49–56
Flash Point, °C	D92	251	>232
Fire Point, °C	D92
Specific Gravity of Asphalt	D72	1.03
Rotational Viscometer, (Pa. sec)	D4402	0.425@ 135 °C
		0.101@ 165°C
Loss on Heating (5 h at 163°C)	D1754	0.47
After Thin Film Oven Test (ASTM D-1754, 2020)
Retained Penetration, % of Original	(ASTM D5, 2020)	65	>52
Ductility @ 25 °C, 5 cm/min, (cm)	(ASTM D113, 2018)	101	>40

.

2.2. Aggregate

The mineral aggregates (coarse and fine) were brought from the Al-Nibaie quarry, which is a popular asphalt mix source in Baghdad. It includes one type of coarse aggregate, crushed quartz, and two types of fine aggregate, which are crushed gravel and natural river sand, to prepare the sample.

Table 2. Physical properties of fine and coarse aggregate.

Property	ASTM Specification [24]	Result	SCRB Specification [25]
Coarse Aggregate
Bulk Specific Gravity	ASTM C127	2.605
Apparent Specific Gravity	ASTM C127	2.659
percent wear (loss of Angel’s abrasion)	ASTM C131	15	≤30
Water Absorption, %	C127	0.53
Soundness Loss by Sodium Sulfate Solution, %	C88	3.12	≤12
Degree of Crashing, %		96	≥95
Flat and Elongated Particles, %	D4791	1.4	≤10
Fine Aggregate
Bulk Specific Gravity	ASTM C128	2.593
Apparent Specific Gravity	ASTM C128	2.679
Water Absorption, %	C128	0.72
Sand Equivalent	D 2419	54	≥45

displays the findings of the utilized aggregate’s physical characteristics. Two aggregate blends, a coarse blend and a fine blend, with a nominal maximum size of 12.5 mm, were chosen for this study’s aggregate structure design to examine the impact of gradation on the mixes’ performance characteristics. The aggregate gradation satisfies the range of gradation restrictions for densely graded paving mixes of wearing course as stipulated by the State Commission of Roads and Bridges (SCRB) in Iraq [24].

Table 3. Gradation of the aggregate for the surface course.

Sieve Size		% Passing as by (SCRB/R9, 2003) [25] Wearing Course Type IIIA
					Work Choice
Standard Sieves	English Sieves	Min.	Max.	% Passing	% Retaining
19.00 mm	3/4″	---	100	100	0
12.50 mm	1/2″	90	100	95	5
9.500 mm	3/8″	76	90	83	12
4.750 mm	#4	44	74	59	24
2.360 mm	#8	28	58	43	16
0.300 mm	#50	5	21	13	30
0.075 mm	#200	4	10	7	6
Pan	---				7

and Figure 1 show the gradations that were employed along with their specified limitations.

2.3. Mineral Filler

Hydrated lime is supplied by the local market to prepare the asphalt mixture. Hydrated lime enhances Marshall properties and reduces the sensitivity of asphalt mixes to moisture [26,27]. According to a previous study, using hydrated lime as a filler improved the fatigue life [28]. The filler is a non-plastic material that passes sieve No. 200 (0.075 mm) free of dry lumps or fine particulates, with a specific gravity of hydrated lime of 2.50.

Table 4. Chemical composition of mineral fillers.

Filler Type	Chemical Composition, %
	Lime (Cao)	Silica (SiO2)	Alumina (Al2O3)	Magnesia (MgO)	Ferric Oxide (Fe2O3)	Sulfuric Anhydride (SO3)	Loss on Ignition (L.O.I)
Hydrated Lime	69	1	----	2	----	1	27

displays the chemical composition of the filler.

2.4. Utilizing Polymeric Additives

In this investigation, two different kinds of polymers were employed. The first is plastomer polymer, or polyvinyl chloride (PVC), which was purchased from the local market and produced by the Saudi Arabia Basic Industries Corporation (SABIC), Riyadh, Saudi Arabia. It is a white powder with a density of 1.42 g/cm³ and a melting point range of 160–210 °C. The PVC we used had a tensile strength of 60 MPa and a maximum of 4% passing through a 200-mesh sieve. It was selected due to its availability, cost-effectiveness, and potential to enhance stiffness and deformation resistance. Polyvinyl chloride is depicted in Figure 2. The usual features of PVC are shown in

Table 5. Typical PVC attributes.

Density g/cm3	Melting Point °C	Tensile Strength (MPa)	Chemical Unit	Color	Passing Through Mesh 200
1.42	160–210 [29]	60	(C2H3CL) n	White powder	Max 4%

. Styrene–butadiene–styrene (SBS), Figure 3, an elastomer polymer, is the second component.

SBS is a block copolymer produced through the polymerization of polystyrene and polybutadiene. There are three components to the chemical chain of SBS: butadiene helps to give asphalt its flexibility at low temperatures, while styrene gives asphalt its performance and durability at high temperatures. Figure 4 shows the chemical structure of SBS.

Table 6. Physical, mechanical, and chemical characteristics of linear SBS polymer (KRATON).

Bulk Density Kg/m3	Molecular Structure	Vinyl Content (%)	Styrene/Butadiene Ratio	Di-Block Content	Elongation%	Specific Gravity	Tensile Strength MPa	Melting Point °C	Color
0.4	Linear triblock	$\le 15$	$\approx$ 30/70	$<15$	88	0.94	32 MIN	180	White pellet

lists the characteristics of SBS, whereas Figure 4 shows the SBS that was employed in this investigation. There have been three distinct SBS percentages used: 3, 4, and 5% by binder weight. These three SBS percentages are utilized because SBS is a costly form of polymer and is thus most likely to be the most economical.

3. Preparation of Polymer-Modified Asphalt

Wet blending was used in this study to concentrate on the properties of polymer-modified asphalt.

3.1. Blending of PVC with Bitumen

We used four PVC plastic percentages: 3%, 5%, 7%, and 9% by bitumen weight. The hot asphalt was combined with gradually added particles, and the mixture was mixed for two hours at 2000 rpm with a temperature maintained at 165 °C using a mechanical mixer [31,32].

3.2. Blending of SBS with Bitumen

To ensure compatibility and uniformity, pure asphalt was mixed with three different percentages of styrene–butadiene–styrene (SBS) (3%, 4%, and 5%) by weight of bitumen before adding the SBS polymer. The asphalt binder was heated to 135 °C to achieve a free-flow state. The mixture was then kept in the oven at 180 °C and continuously stirred at high speed until expansion was observed [33]. After expansion, it was sheared for 40 min at a speed of 2500 rpm in the shearing machine [34]. Finally, it was returned to the oven at 160 °C for two hours of continuous development.

3.3. FM Image

Fluorescence microscopy (FM) is the most widely used method to examine the morphology and compatibility of asphalt–polymer blends [35,36]. In this study, FM images were obtained using a Leica DM2500 fluorescence microscope located at the Materials Research Department, Ministry of Science and Technology. The microscope operated under the following conditions: an excitation filter of 450–490 nm, an emission filter of 515 nm, and a magnification of 100 times. These settings were chosen to optimize contrast between the polymer and asphalt phases. FM was employed to confirm the dispersion of the material within the asphalt binder during mixing. The primary forms present in the compounds include the asphalt phase and the additive phase. The distribution characteristics of additives in asphalt can significantly affect the compatibility of modified asphalts, leading to variations in road performance. To prepare the FM samples, the modified asphalt was first heated to a flow state [37]; then, a small amount of asphalt was placed on a glass slide and covered with a cover glass. After the asphalt samples had cooled down, they were examined using the fluorescence microscope. The most representative FM images were captured.

3.4. Asphalt Tests

Penetration, softening point, ductility, rotating viscosity, dynamic shear rheometer (DSR), and standard rolling thin film oven (RTFO) tests were performed on the original and modified asphalt by ASTM D5, ASTM D36, ASTM D4402, ASTMD717508, ASTM D7405, and ASTM D2872 [25], respectively. Using a Brookfield rotational viscometer, the viscosities of bitumen mixtures were measured at a rotational speed of 20 rpm according to ASTM D4402-15 [38], and viscosities were evaluated at 135 °C and 165 °C. Figure 5A, B show the DSR device and rotational viscosity device.

Dynamic Shear Rheometer Test

Bitumen is a viscoelastic material that exhibits characteristics of both a viscous liquid and an elastic solid [39]. The dynamic shear rheometer (DSR) test is used to evaluate the viscoelastic characteristics of the binder between 4 and 88 °C [40]. To judge the performance of each sample, the DSR tests in this study were conducted at high temperatures, ranging from 58 to 82 °C. To define the asphalt binder’s performance grade, DSR examined the primary rheological parameters (complex shear modulus, G*), phase angle, δ, and the rutting parameter (G*/sin(δ)) for both the pure and RTFO-aged samples throughout a predetermined temperature range. The performance level of the unaged and RTFO-aged samples was estimated using the complex shear modulus (G*) and phase angle (δ) measurements at high temperatures. According to AASHTO M320, the Superpave design limit dictates that the minimum value of G*/sin(δ) for unaged samples is 1 kPa, and for RTFO-aged materials, it is 2.2 kPa. The viscosity and elastic properties of asphalt binders at different temperatures are investigated using the DSR test; see Figure 5B. The test temperatures are determined by taking into account the average weather in the area where the asphalt binder will be built.

4. Workability Tests

4.1. Bituminous Mix Design

The Marshall mix design method was used to determine the optimum asphalt binder content (OAC) for hot mix asphalt (HMA), following standard procedures tailored to local requirements. Asphalt binder contents were evaluated at five levels: 4%, 4.5%, 5%, 5.5%, and 6% by weight of the total mix. For each binder content, three replicate specimens were prepared using AASHTO T245 and ASTM D6926-10 standards [41,42,43,44]. The OAC was determined to be 5.3% by the weight of aggregates, calculated as the average binder content corresponding to the following:

• Maximum Marshall stability;
• Maximum bulk unit weight;
• Four percent air voids.

This OAC was used to prepare all mixtures, both unmodified and polymer-modified, for subsequent workability testing.

4.2. Workability Device

A custom-designed workability testing device was developed for this study to evaluate the mixing behavior of asphalt mixtures modified with SBS and PVC. This device improves upon previous torque-measuring systems by addressing limitations in mixing capacity and rotational speed.

4.2.1. Key Innovations Compared to Prior Studies

• Previous devices were limited to 20 kg capacity and 25 rpm [45,46,47].
• The new device allows testing of larger batch sizes up to 100 kg and mixing speeds up to 50 rpm.
• This enables a more realistic simulation of industrial-scale mixing processes.

4.2.2. Device Components

The device’s basic components are shown in Figure 6, and a shovel or paddle is displayed in Figure 7 [27].

• The motor box houses a variable-speed electric motor (1.5 HP, 220 V) capable of speeds from 1 to 50 rpm, controlled via an inverter panel.
• The mixing column is a central steel shaft (52 mm diameter) with three mixing arms (18 cm long, 9 cm wide), welded with E7018 arc-welding wire. The bottom two arms are set at 40° angles, and the top arm is angled in the opposite direction to ensure efficient upward and downward material flow.
• The mixing bowl is a cylindrical steel container (40 cm diameter × 40 cm height), thermally insulated, with two external 2000 W electric heaters for temperature control up to 220 °C.
• A dynamic torque sensor (range 0–2000 Nm, accuracy ± 0.1%) is installed at the shaft base to continuously measure resistance during mixing. It captures the real-time torque required for mixing and sends data to the acquisition system. Range: 0–2000 Nm.
• The data acquisition system is connected to a digital logger and software interface, which records torque values in real time at 1 s intervals.
• The control panel allows for the regulation of speed and temperature, as well as visual monitoring of operating parameters.

4.3. The Experimental Procedure

Physical and rheological properties were performed to determine the optimum content for both additives; each test was performed in triplicate. Before adding the additive to the heated aggregate, they were premixed with base asphalt. Hydrated lime was used as a filler. The tested mixture in the workability device weighed 15 kg, and its speed was 15 rpm [20]. The test was run in temperature increments of 10 °C between 120 °C and 200 °C. Average torque was determined over a certain period, and the mixing time was 5 min per test temperature, to provide a general overview of performance. It is useful for evaluating overall workability and uniformity in material behavior at different temperatures. In asphalt concrete studies, average torque can indicate how well the material performs under different compaction and mixing conditions, as demonstrated by Arshad et al.’s workability model, which explains 95% of torque variation through temperature and compaction factors [27,48].

5. Results and Discussion

5.1. Impact of SBS on the Characteristics of Asphalt Binder

Figure 8 illustrates how the microstructure of the asphalt binder changes as SBS content increases. Base asphalt (0% SBS) shows no polymer structure. At 3% SBS, initial polymer dispersion is observed alongside some agglomeration. Next, 4% SBS demonstrates better dispersion but is not fully homogeneous. At 5% SBS, a uniform, dense polymer distribution is evident with a clear separation between SBS-rich and bitumen-rich phases, indicating optimal mixing and compatibility.

5.2. Physical Properties of SBS-Modified Asphalt

Figure 9 shows the results of the penetration tests, ductility, softening point, and penetration index. The values of penetration fall as the proportion of SBS rises. Previous investigations have revealed similar outcomes elsewhere [49]. The ductility was increased as the percentage of SBS increased, as seen in the figure. In comparison to the control asphalt binder, the SBS-modified asphalt binders have a greater softening point temperature. Moreover, it has been noted that when the SBS content rises, so does the softening point temperature. The stiffening impact of the SBS-modified asphalt binder is shown by an increase in softening point, which is advantageous since higher-softening-point asphalt binders may be less prone to rutting or irreversible deformation [50,51]. The addition of modification raises the asphalt’s penetration index and lowers its susceptibility to temperature. It is recommended to use a lower SBS modifier dose (3–5%) to reduce production costs while still meeting performance specifications for SBS-modified binders, including viscosity requirements. According to AASHTO M320 and ASTM D4402, the rotational viscosity of the modified binder at 135 °C should not exceed 3 Pa·s (3000 cP) to ensure proper workability during mixing and compaction. Similarly, local guidelines, such as the Iraqi General Specifications for Roads and Bridges Section R/9 (2003), adopt comparable limits for hot mix asphalt applications.

Note: While this study evaluated key physical and rheological properties of SBS-modified binders, elastic recovery, an essential parameter reflecting the binder’s ability to resist permanent deformation, was not measured due to equipment limitations. Future research will include this parameter to provide a more complete performance profile of SBS-modified asphalt.

5.3. Impact of SBS on Asphalt Binder Viscosity

Figure 10 displays that at 135 °C, the viscosity of bitumen rose from 0.425 Pa·s to (1.414, 1.689, 1.9614) Pa·s when the percentage of SBS polymers was increased from 0% to 3%, 4%, and 5%. Additionally, at 165 °C, the basic binder’s viscosity increased from 0.101 Pa·s to (0.337, 0.433, 0.529) Pa·s, respectively. This increase results from the bitumen’s oily component being absorbed by the polymer when an appropriate elastomer (such as SBS) is added, which causes the bitumen to expand (usually up to eight times its initial volume) SBS absorbs the light components of bitumen, causing it to swell and form a developed network structure. Time, production method, and additives are a few examples of variables that might have an impact on this physical process [52,53]. The viscosity of the ordinary asphalt binder significantly increased upon the addition of SBS. The interface between SBS and asphaltene in the asphalt is responsible for this behavior. In asphalt manufacturers, raising the viscosity to an extremely high level is not advised since the modified asphalt must be heated using electricity, and the high viscosity cannot be pushed through pipes. The rotating viscosity findings proved that blended asphalt with 3, 4, and 5% was within the limit and did not exceed 3 Pa·s at 135 °C according to AASHTO M320. The maximum viscosity, 1.964 Pa. s at 135 °C, was obtained with 5% SBS. In Iraqi asphalt factories, the viscosity of polymer-modified asphalt mixed with 5% SBS was more appropriate for mixing.

5.4. Temperatures of Compaction and Mixing for the Modified Asphalt Binder

Viscosity was limited by the Superpave system to (0.17 ± 0.2) Pa. sec for mixing and (0.28 ± 0.3) Pa.sec for compaction. Studying the link between the viscosity and temperature of asphalt binders with SBS modification and control showed different mixing and compaction temperatures, as shown in

Table 7. Mixing and compaction temperature for the base and SBS-modified asphalt.

Binder Type	Mixing Range, °C	Compaction Range, °C
Base Binder	152–158	142–146
3%SBS	178–185	165–173
4%SBS	185–190	173–178
5%SBS	190–195	178–185

. In comparison to the control binder, it was observed that the SBS-modified asphalt binder had higher mixing and compaction temperatures. This is because SBS increased the viscosity of the asphalt, increasing mixing and compaction temperatures, boosting the ability of asphalt to withstand high temperatures in the summer, and improving rutting resistance.

5.5. Dynamic Shear Rheometer Test for SBS-Modified Asphalt

Figure 11 displays that the complex modulus (G*) increases as the percentage of the SBS increases and decreases when the temperature rises. In contrast, Figure 12 illustrates that the phase angle reduces when the SBS content increases and rises as the temperature rises. The cause of the phase angle values’ decline suggests a change in behavior from viscoelastic to elastic. As a result of the polymer’s efforts to improve its elastic qualities, the phase angle is reduced in comparison to pure asphalt. Figure 13 shows that when the SBS content increases, the G*/sinδ increases due to the ability of SBS to harden the asphalt (increasing the viscosity of the asphalt binder, resulting in increased resistance to high temperatures) and deformation. All SBS-modified asphalt binders have rutting parameters (G*/sin δ) higher than the basic asphalt binder at all test temperatures. As the proportion of the SBS component rises, the binder’s resistance to rutting increases. Additionally, the rutting resistance is raised by more than 3.4, 6.8, and 9.5 times at 64 °C by adding 3, 4, and 5% SBS to the basic asphalt, respectively. This suggests that greater SBS content will result in more elastic behavior. Several results from earlier research [54,55,56] suggest that this is a noteworthy indicator to reduce the permanent deformation caused by the applied stress in the asphalt binder.

5.6. Impact of SBS Modifier on Performance Grade

Table 8. Performance grade for original asphalt and SBS-modified asphalt.

Binder Types	Aging Status	Temp.(°C)	G* (kPa)	δ°	G*/sin δ (kPa)	Superpave Spec. Limit (kPa)	PG
Original Binder	Unaged	64	1.0601	88.3	1.0605	≥1	PG64-X
		70	0.5285	89	0.5285
	RTFO Aged	64	2.211	87.4	2.2122	≥2.2
		70	1.4001	87.7	1.4011
3%SBS	Unaged	70	1.7801	80.3	1.8058	≥1	PG70-X
		76	0.8901	82.6	0.8975
	RTFO Aged	70	2.355	79	2.3991	≥2.2
		76	1.1775	81.3	1.1912
4%SBS	Unaged	76	1.7150	75.6	1.7707	≥1	PG76-X
		82	0.8575	77.6	0.878
	RTFO Aged	76	2.6275	74.3	2.7294	≥2.2
		82	1.3138	76.3	1.3522
5%SBS	Unaged	82	1.8101	70	1.9262	≥1	PG82-X
		88	0.9051	72	0.9516
	RTFO Aged	82	2.4638	68.7	2.6444	≥2.2	PG82-X
		88	1.2319	70.7	1.3052

shows that for the SBS-modified asphalt, there is a one-degree rise in performance grade for every 1% increase in the SBS percentage, as indicated by the DSR coefficients G*, δ, G*/sin δ, and PG. The percentage gain (PG) rose from 64 to 70 at 3% SBS, from 70 to 76 at 4% SBS, and to 82 at 5% SBS. The performance grade (PG) for the base asphalt and 3%, 4%, and 5% SBS asphalts are determined to be PG 64-x, PG 70-x, PG 76-x, and PG 82-x, respectively. The SBS change effectively increased the asphalt binders’ resistance to rutting by lowering δ and increasing G*. The outcomes of the DSR test under the RTFO aging condition support the conclusions drawn from the results of the unaged DSR test, as shown in Figure 14, Figure 15 and Figure 16. The PG and rutting parameter “(G*/sin δ)” values were successfully improved by the SBS, indicating a higher level of rutting resistance. It is also important to note that the rutting parameter increases with increased SBS content.

5.7. Impact of PVC on the Characteristics of Asphalt Binder

Figure 17 represents the microstructural evolution of PVC-modified asphalt binder under a UV microscope as the percentage of PVC increases. At 3% PVC, the image displays scattered fluorescent spots, indicating that PVC is poorly dispersed and appears as scattered spots. At 5% PVC, the morphology shifts to a filamentous structure, suggesting that PVC begins to interact and elongate, indicating partial network formation. With 7% PVC, a partially connected net-like structure is observed. This implies improved dispersion and interaction between PVC particles in the bitumen matrix. At 9% PVC, the image reveals a fully developed network structure, indicating strong compatibility and interaction between PVC and bitumen, enhancing the overall homogeneity and stability of the modified binder. This suggests that a higher PVC content enhances dispersion and network formation in the asphalt binder.

5.8. The Effect of PVC on Penetration, Softening Point, and Ductility

As the PVC content in asphalt increases, the penetration and ductility values drop, as illustrated in Figure 18, while the softening point and penetration index increase. Reduced penetration and increased softening point values suggest that bitumen treated with PVC binder is suitable for usage in hot climates as it is rutting-resistant, especially in Iraqi summers [57,58,59,60].

5.9. Effect of PVC on the Viscosity of Asphalt Binder

The results of blending the asphalt binder pen (60–70) with 3%, 5%, 7%, and 9% PVC were measured using a Brookfield rotational viscometer [61,62]. Figure 19 shows the relationship between the viscosity of the asphalt binder and temperature based on the rotational viscosity test results at temperatures of 135 °C and 165 °C for the base and PVC-modified asphalt. The viscosity of asphalt increases as the PVC content increases. Asphalt with 9% PVC gives the highest viscosity value of 1.452 Pa·s at 135 °C, and all PVC-modified asphalts are within the limit. As a result of the rising viscosity of PVC, mixing and compaction temperature will increase, and that makes the mixture more resistant to rutting.

Table 9. Mixing and compaction temperature for the base and PVC-modified asphalt binder.

PVC%	Mixing Temp. °C	Compaction Temp. °C
Base binder	152–158	142–146
3%	182–192	164–170
5%	186–195	170–177
7%	193–200	175–185
9%	195–205	180–186

summarizes the mixing and compaction temperatures and viscosity of the PVC-modified asphalt.

5.10. Dynamic Shear Rheometer Test

The complex modulus, phase angle, and shear modulus (G*/sin(δ)) for materials changed by PVC are displayed versus temperature in Figure 20, Figure 21 and Figure 22 for unaged samples and Figure 23, Figure 24 and Figure 25 for aged samples, respectively. PVC modifiers were introduced to pure 60–70 bitumen in different percentages, and the complex shear modulus values increased gradually for both the unaged and RTFO-aged samples. The increase in complex shear modulus values indicates that PVC addition reinforced the bitumen’s shear strength. The graphs show that the addition of PVC enhanced the pure bitumen’s PG values and fundamental rheological parameters (G*, δ, and G*/sin(δ)). One grade was raised. When 3% and 5% PVC are added to pure bitumen, it moves from PG64-X to PG70-X, and two grades improve. The PG was enhanced from PG 64-X to PG76-X with the addition of 7% and 9% PVC to the asphalt pen (60–70). The bending beam rheometer (BBR) test must be used to ascertain the low-temperature value, represented by the letter “X.” However, BBR equipment is not readily available. The results are also presented in

Table 10. Superpave performance grade (PG) for original and PVC-modified samples.

Binder Types	Aging Status	Temp. (°C)	G* (kPa)	δ°	G*/sin δ (kPa)	Superpave Spec. Limit (kPa)	PG
Original Binder	Unaged	64	1.0601	88.3	1.0605
		70	0.5285	89	0.5285
		76	0.2565	89.5	0.2565	≥1	PG64-X
		82	0.12825	89.9	0.1282
	RTFO-Aged	64	2.21	88.3	2.2109
		70	1.4	88.5	1.4004
		76	0.654	89.2	0.6541	≥2.2	PG76-X
		82	0.2565	89.3	0.2565
3% PVC	Unaged	64	1.6301	84.3	1.6382
		70	1.1285	85	1.1328
		76	0.5565	85.5	0.5582	≥1	PG70-X
		82	0.2783	85.9	0.2789
	RTFO-Aged	64	3.41	84.3	3.4915
		70	2.8	84.5	2.8704
		76	1.4	85.2	1.4267	≥2.2	PG70-X
		82	0.7	85.3	0.7097
5% PVC	Unaged	64	2.9301	80.3	2.9726
		70	1.5285	81	1.5475
		76	0.8565	82.5	0.8639	≥1	PG70-X
		82	0.4283	83.8	0.4307
	RTFO-Aged	64	4.91	80.3	4.9812
		70	3.20	80.5	3.2445
		76	2.00	82.2	2.0187	≥2.2	PG70-X
		82	1.00	83.2	1.0071
7% PVC	Unaged	64	4.1301	77.6	4.2288
		70	1.9285	77.8	1.9731
		76	1.2165	79.2	1.2384	≥1	PG76-X
		82	0.6083	81.1	0.6156
	RTFO-Aged	64	6.71	77.6	6.8703
		70	3.6	77.3	3.6903
		76	2.36	78.9	2.4051	≥2.2	PG76-X
		82	1.18	80.5	1.1964
9% PVC	Unaged	64	8.7789	82.1	8.8631
		70	4.4435	82.09	4.4862
		76	1.8772	82.6	1.8929	≥1	PG76-X
		82	0.9211	82.8	0.9284
	RTFO-Aged	64	5.1301	76	5.2872
		70	2.3585	76.4	2.4265
		76	1.6165	77.1	1.6584	≥2.2	PG76-X
		82	0.8083	79.9	0.8209

.

5.11. Impact of PVC-Modified Asphalt and SBS on Workability Tests

According to the conducted experiments, the optimal amount for both additives was 5% of the asphalt’s weight. Figure 26 illustrates the relationship between torque and temperature for the base asphalt, 5% SBS, and 5% PVC-modified asphalt. The torque value decreases as the temperature increases. When mixing torque decreases, workability increases.

As demonstrated in Figure 26, the torque value of 5% SBS exceeds that of the original mixture by approximately (2.29, 2.56, 2.85, 3.22, and 3.73) Nm at temperatures of (120, 130, 140, 150, and 160) °C, respectively. Asphalt with 5% SBS also surpasses 5% PVC-modified asphalt by about (1.75, 1.79, 1.70, 1.75, 1.82, 1.78, 1.75, and 1.30) Nm at temperatures of (120, 130, 140, 150, 160, 170, and 190) °C, respectively.

A higher torque value signifies lower workability, indicating that the asphalt mixture with SBS is less workable than both the original asphalt mixture and the mixture enhanced with 5% PVC. The torque value for asphalt with 5% PVC is greater than that of the original mixture by about (0.19, 0.28, 0.43, 0.53, and 0.68) Nm at temperatures of (120, 130, 140, 150, and 160) °C, respectively. This means that asphalt with 5% PVC has lower workability compared to the original mix.

To explain the relationship between torque results, field compaction energy, and asphalt properties, the torque values obtained from the workability tests reflect the resistance of asphalt mixtures to mixing, which is strongly related to their compaction behavior in the field. Higher torque values, especially in SBS-modified mixtures, indicate increased binder viscosity and stiffness, as supported by rheological test results. While these properties enhance the mix’s resistance to rutting and improve performance at high temperatures, they also reduce workability and require more compaction effort to achieve the desired density. Higher torque mixtures often cool and stiffen more rapidly, which reduces the amount of time available for efficient compaction and necessitates faster or more intense rolling. The 5% SBS-modified mixture, which exhibited the highest torque, is expected to need more compaction energy due to its higher viscosity and elevated compaction temperature range (178–185 °C), compared to the base or PVC-modified mixes. Therefore, although SBS modification significantly enhances performance, it also requires adjustments to compaction practices to ensure long-term pavement quality.

6. Conclusions

This study assessed the impact of adding different concentrations of polyvinyl chloride (PVC) and styrene–butadiene–styrene (SBS) polymers to basic asphalt binder (60–70 penetration grade). The following important findings were obtained by physical testing, dynamic shear rheometer (DSR) analysis, RTFO aging, and workability assessment:

• Microstructural analysis: PVC and SBS were uniformly distributed in the asphalt binder at 5% concentration, according to fluorescence microscopy. Higher concentrations, however, caused non-uniform dispersion and agglomeration.
• PVC performance: Asphalt modified with 3% and 5% PVC met penetration grading criteria; however, 7% and 9% did not. PVC addition decreased penetration and ductility, while increasing the softening point, indicating enhanced temperature resistance at moderate dosages (3–5%). Rheological tests confirmed that PVC improved the complex modulus (G*) and reduced the phase angle (δ), improving the elasticity of binders and rutting resistance.
• SBS performance: SBS significantly improved physical properties by reducing penetration and increasing ductility and the softening point. The addition of 5% SBS raised the softening point from 46 °C to 64 °C. The DSR test exhibited that rutting resistance was enhanced with increasing SBS content, with the G*/sin(δ) values rising in both unaged and aged conditions. SBS-modified binders showed higher performance grades (PGs), improving from PG 64-x (base) to PG 82-x at 5% SBS. Viscosity values rose with SBS content, reaching acceptable Superpave thresholds, which enhanced high-temperature stability.
• Workability findings: The asphalt mixture with 5% SBS exhibited lower workability than the 5% PVC and base mixtures due to increased viscosity. Despite its lower workability and higher cost, SBS is more suitable for the hot Iraqi climate due to its superior thermal and rutting resistance.
• Recommendations: Further study is suggested to discover the relationship between workability and compactability, utilizing compactability windows to optimize asphalt pavement performance under field conditions.