Enhancing Asphalt Performance: The Combined Influence of Trinidad Lake Asphalt (TLA) and Styrene–Isoprene–Styrene (SIS) on PG 64-22 Binder

Abstract

This study investigates the combined impact of Trinidad Lake Asphalt (TLA) and styrene–isoprene–styrene (SIS) on the performance of PG 64-22 asphalt binder. The study specifically examines how the addition of TLA and SIS affects the stiffness, viscoelastic properties, and ability to resist deformation of the asphalt binder, using the Superpave test method. The findings demonstrate that the addition of SIS improves the ability to withstand deformation, with the best results seen in the mixture containing 20% TLA and 5% SIS. This blend achieves a harmonious combination of stiffness, pliability, and ease of use. Increasing the SIS content to 10% and combining it with a 20% TLA greatly enhances resistance to rutting, but it also decreases workability. In addition, a mixture consisting of 10% TLA and 10% SIS provides excellent resistance to fatigue and possesses viscoelastic properties that are well suited for reducing oxidative aging and cracking. Although SIS typically enhances the performance of binders, an excessive quantity can result in unwanted rigidity, requiring meticulous optimization. The results highlight the importance of customizing TLA–SIS combinations according to specific environmental conditions in order to optimize advantages. Subsequent studies should investigate the prolonged impacts and alternative combinations of additives to further improve the performance of binders.

1. Introduction

In response to the growing demand for roads that possess both durability and safety, researchers have placed great importance on making progress in the development of road construction materials. The emphasis on this topic arises from advancements in technology and the need to improve the physical and chemical characteristics of asphalt due to the difficulties presented by severe weather and climatic occurrences. In order to enhance performance under varying temperatures, modified asphalt mixtures are supplemented with different additives [1]. Trinidad Lake Asphalt (TLA) is notable among these additives due to its distinct properties. Derived from an indigenous lake in Trinidad and Tobago, TLA serves to address certain constraints associated with conventional petroleum-derived asphalt binders. Although TLA exhibits certain physical characteristics, such as elevated viscosity and density, it possesses a chemically distinct composition compared to traditional binders [2].

The thermal stability of TLA is outstanding, as it has low penetration rates and a high softening point. Additionally, it possesses remarkable chemical stability, displaying a high level of resistance to oxidation, as well as exceptional water resistance, thereby augmenting its adhesive properties. Due to its chemical resemblance to petroleum asphalt, TLA can be mixed with it. However, using TLA alone in asphalt concrete is frequently not feasible. As a result, TLA is commonly mixed with petroleum-based asphalt in specific ratios [3]. Integrating TLA improves the asphalt’s capacity to withstand deformation under high temperatures, but it also increases the likelihood of fatigue cracking under lower temperatures, thereby restricting its practicality [4].

In order to resolve this problem, polymers are incorporated into the asphalt mixture. Studies suggest that polymers such as plastomers and thermoplastic elastomers enhance the characteristics of asphalt by decreasing its sensitivity to temperature changes and enhancing its ability to withstand permanent deformation. Although concerns regarding the long-term storage of polymer-modified asphalt were acknowledged in 1978, subsequent field tests, including those carried out in Texas in 1976, demonstrated favorable outcomes [5]. Several polymers, such as styrene–butadiene–styrene (SBS), ethylene–vinyl–acetate (EVA), styrene–ethylene–butylene–styrene (SEBS), polyethylene (PE), polypropylene (PP), and polyvinyl chloride (PVC), have undergone testing in hot asphalt mixtures. These polymers increase the material’s ability to withstand changes in temperature and stress caused by traffic, thereby improving important characteristics such as stability, resistance to moisture, and strength against fatigue [6,7,8,9], and out of all polymers, SBS is commonly utilized in asphalt paving because of its dependable performance over a wide range of temperatures [10].

That is why the study investigates the potential use of styrene–isoprene–styrene (SIS) as a substitute for SBS, due to its comparable characteristics as a thermoplastic elastomer derived from petroleum. The primary distinction between SIS and SBS lies in their molecular structure. SIS replaces the butadiene component in the SBS molecular chain with an isoprene component, leading to a chain composed of alternating isoprene and styrene groups. SIS can be categorized as either linear or star-shaped, based on its molecular configuration. SIS possesses enhanced toughness and compatibility with other additives due to the inclusion of a branched methyl group in the isoprene [11].

Therefore, this study involved the combination of a petroleum-based binder, specifically PG 64-22, with Trinidad Lake Asphalt (TLA) in ratios of 10% and 20%, as well as the addition of SIS in ratios of 5% and 10%. Prior research [12] has established a maximum threshold of 20% for TLA content and a replacement limit of 10% for SIS, which aligns with similar findings [13]. We employed the wet process, specifically McDonald Technology, for the mixing procedure, which is widely acknowledged as one of the most efficient techniques for achieving a consistent mixture [14]. The mixing process lasted for one hour, with a shear rate of 700 rpm and a temperature of 177 °C. The comprehensive experimental plan is outlined in Figure 1.

2. Materials and Experimental Method

2.1. Materials

The primary binder used in this research was PG 64-22, sourced from the Texas Asphalt Pavement Association in Buda, Texas, USA. Its detailed properties are presented in

Table 1. Properties of primary binder (PG 64-22).

Aging States	Test Properties	Test Result
Original binder	Viscosity @ 135 °C (cP) Viscosity @ 180 C (cP)	757 285
	G*/sin δ @ 64 °C (kPa)	2.06
RTFO-aged residual	G*/sin δ @ 64 °C (kPa)	5.27
RTFO + PAV- aged residual	G*sin δ @ 25 °C (kPa)	3396
	Stiffness @ −12 °C (MPa)	182
	m-value @ −12 °C	0.32

. This binder serves as the baseline material for the study, known for its versatility and performance in various climatic conditions. The second modifier employed in the research was Trinidad Lake Asphalt (TLA), a natural asphalt obtained directly from the renowned Lake of Trinidad and Tobago. The distinctive properties of TLA, outlined in

Table 2. Properties of Trinidad Lake Asphalt.

Properties	Test Method	Test Result
Penetration (25 °C)	ASTM D5 [15]	1–3
Density	ASTM D70 [16]	1.41–1.43 g/m3
Softening point	ASTM D36 [17]	93 °C–99 °C
Flash point	ASTM D92 [18]	256 °C–267 °C
Fire point	ASTM D92 (Cleaveland open cup) [18]	307 °C–311 °C

, make it an excellent choice for enhancing the performance characteristics of asphalt mixtures, particularly due to its unique composition and natural origin. The third modifier utilized was SIS, a thermoplastic elastomer obtained from TSRC Specialty Materials LLC, Houston, TX, USA. SIS is known for its ability to improve the elasticity and durability of asphalt binders, and its properties are detailed in

Table 3. Characteristics of SIS polymer.

Polymer Attribute	Linear Structure
Styrene Content, wt %	15
Diblock Content, wt %	18
Melt Flow, g/10 min (200 °C/5 kg)	11
Solution Viscosity, cps	1240
Ash Content, wt %	0.3
Volatile Matter, wt %	0.2
Specific Gravity	0.92
Tensile Strength, psi (MPa)	3600 (25)
Elongation, %	1250
Hardness, Shore A	33

. The inclusion of SIS aims to enhance the overall mechanical properties of the asphalt, making it more resilient under various stress conditions.

2.2. Sample Preparation

The modified asphalt was prepared using a sequential process involving a low-speed shearer and a mechanical stirrer at high temperature. Initially, 300 g of base asphalt binder PG64-22 and Trinidad Lake Asphalt (TLA) was combined in proportions of 10% and 20% individually, heated, and fully melted in a container. Subsequently, styrene–isoprene–styrene (SIS) was added at two different ratios of 5% and 10%, respectively. The mixture was then sheared using a low-speed shearer at 700 rpm for 1 h to ensure thorough blending of the SIS with the TLA-modified asphalt binder. Throughout the entire process, the mixture temperature was maintained at 177 °C.

2.3. Test on Physical Properties

The rotational viscometer test, performed in accordance with ASTM D4402 [19], was employed to assess the workability of the asphalt binder. This evaluation was conducted at two critical temperatures, 135 °C and 180 °C, using a sample size of 10.5 g and a spindle number 27. A total of 20 readings were taken at one-minute intervals to accurately replicate field conditions. Testing at these temperatures is essential for determining the binder’s viscosity, which is a key factor affecting its handling and application during the mixing and compaction processes in pavement construction.

The viscosity of the binder at elevated temperatures provides insight into its flow characteristics, which are crucial for ensuring proper mixing with aggregates and achieving optimal compaction in pavement layers. By analyzing the binder’s performance at 135 °C, we gain an understanding of its behavior during typical hot-mix asphalt operations, where this temperature represents the standard mixing and laying conditions. Testing at 180 °C, on the other hand, allows us to evaluate the binder’s performance under more extreme conditions, such as those encountered during long-distance transportation or when using specific additives that may alter its thermal properties. These assessments are vital for ensuring that the asphalt binder maintains its workability, stability, and durability in real-world paving applications.

2.4. Thermal Aging of Asphalt: Short-Term and Long-Term Processes

Thermal aging of asphalt is a critical factor in determining the longevity and performance of pavement materials. It is divided into two main phases: short-term and long-term thermal aging. Each phase simulates different aspects of the aging process that asphalt undergoes throughout its lifecycle.

Short-Term Thermal Aging: This phase is primarily concerned with the initial aging that occurs during the asphalt production process. The Rolling Thin Film Oven Test (RTFO), as outlined in ASTM D2872-19 [20], is used to replicate the conditions experienced by asphalt when it is mixed with aggregates at high temperatures. During the RTFO test, asphalt is exposed to elevated temperatures of 163 °C and continuous air flow for 85 min, which simulate the volatilization of lighter hydrocarbons and the oxidation processes that occur during hot-mix asphalt production. This test is essential for understanding how asphalt will perform immediately after being laid, as it provides insight into the changes in viscosity and other physical properties that result from the initial exposure to heat and oxygen.

Long-Term Thermal Aging: Following the RTFO process, the asphalt binder is subjected to long-term aging using a Pressurized Aging Vessel (PAV), as specified in ASTM D6521-19 [21]. The PAV test simulates the oxidative aging that occurs during the pavement’s service life, typically spanning 5 to 10 years. This method involves exposing the asphalt to elevated temperatures and pressures, which accelerates the aging process, allowing researchers to predict the material’s long-term performance. The PAV procedure mimics the natural environmental stresses, such as heat, air, and UV radiation, that asphalt encounters over time, leading to the gradual hardening and embrittlement of the material. Understanding long-term thermal aging is crucial for assessing the durability and resilience of asphalt pavements, ensuring they meet performance expectations throughout their intended lifespan.

Together, these testing methods provide a comprehensive understanding of how asphalt materials respond to thermal aging, guiding the selection and formulation of binders that can withstand the rigors of both initial processing and extended use in road construction.

2.5. Rheological Properties at High Temperatures

The viscoelastic properties of the binder were evaluated at a high temperature of 64 °C using the Dynamic Shear Rheometer (DSR) test, in accordance with AASHTO T 315 [22]. This test is critical for assessing the binder’s resistance to deformation and rutting at elevated temperatures, where such issues are most prevalent. Conducted at a frequency of 10 radians per second (equivalent to 1.59 Hz), the test provides insights into the binder’s complex shear modulus (G*) and phase angle (δ), which are key indicators of its performance.

Initially, the test was performed on the unaged, modified binder to establish baseline performance metrics. Following this, the binder was subjected to short-term and long-term aging processes to simulate the effects of field aging. The G*/sinδ parameter, which is inversely related to the binder’s rutting susceptibility, was calculated at 64 °C for both the freshly modified and short-term aged binders. This parameter serves as an indicator of the binder’s ability to resist rutting by quantifying the elastic portion of the complex modulus.

Additionally, a fatigue cracking test was conducted at 25 °C on the long-term aged binder. This test aims to assess the material’s ability to withstand repeated loading cycles without cracking, which is essential for predicting pavement durability. The fatigue resistance is reflected in the G*.sinδ value, calculated at the lower temperature, to ensure that the binder maintains adequate flexibility and resilience over its service life. These tests collectively provide a comprehensive evaluation of the binder’s performance, ensuring its suitability for high-temperature applications while maintaining resistance to fatigue damage.

2.6. Rheological Properties at Low Temperatures

The low-temperature rheological behavior of the sample was evaluated using the Bending Beam Rheometer (BBR) in accordance with AASHTO T 313 [23], conducted at a temperature of −12 °C. In this test, a modified asphalt sample with dimensions of 125 mm in length, 12.5 mm in width, and 6.25 mm in thickness was submerged in cold alcohol for a duration of 1 h to ensure thermal equilibrium. The specimen was then supported at both ends by metal fixtures, and a constant force of 980 mN was applied at the midpoint of the sample over a period of 240 s. The resulting continuous deflection (in millimeters) was meticulously recorded throughout the test. The creep stiffness (S) and creep rate (m) were subsequently calculated to assess the low-temperature cracking resistance of the asphalt. These parameters provide critical insights into the material’s ability to withstand thermal stresses and prevent cracking under low-temperature conditions, which is crucial for ensuring pavement durability in cold climates.

2.7. Multiple Stress Creep and Recovery (MSCR) Test

The Multiple Stress Creep and Recovery (MSCR) test is designed to simulate the high-stress conditions encountered by pavement structures, particularly during the early stages of their service life when rutting is most prevalent. This test uses Rolling Thin Film Oven Test (RTFOT)-aged asphalt to more accurately reflect the binder’s resistance to rutting, as this aging process replicates the initial aging that occurs in the field. By evaluating the binder under these conditions, the MSCR test provides a realistic assessment of its performance in resisting permanent deformation.

Two critical parameters are derived from the MSCR test data: non-recoverable creep compliance (J_nr) and percent recovery (rec%). The non-recoverable creep compliance (J_nr) is calculated as the ratio of unrecoverable shear strain to the applied stress, reflecting the binder’s susceptibility to permanent deformation under sustained loading and is shown below as Equation (1). And the percent recovery (rec%) is determined as the percentage difference between peak strain and unrecovered strain relative to the peak strain, which indicates the binder’s ability to recover after loading and is shown below as Equation (2) [24].

$$J_{nr}={\displaystyle \frac{Unrecoverable shear strain}{Applied Stress}}$$

(1)

$$\mathit{rec} (\%)=100 \times {\displaystyle \frac{Peak strain-Unrecovered strain}{Peak strain}}$$

(2)

The MSCR test involves applying 20 cycles of stress at 0.1 kPa, followed by 10 cycles at 3.2 kPa, with each cycle consisting of 1 s of shear creep loading and a subsequent 9 s recovery period. The 0.1 kPa stress level evaluates the binder’s behavior in the linear viscoelastic region, while the 3.2 kPa stress level assesses performance in the non-linear region. This approach ensures a comprehensive understanding of the binder’s performance across different stress levels, which is crucial for predicting long-term pavement durability.

The average values of J_nr and rec% are calculated using the following Formulas (3)–(6) [24].

$$J_{nr\left(\mathrm{0,1}\right)} ={\displaystyle \frac{\sum _{n=11}^{20}{[J}_{nr }\mathrm{0,1}n]}{10}}$$

(3)

$$J_{nr\left(\mathrm{3,2}\right)} ={\displaystyle \frac{\sum _{n=21}^{30}{[J}_{nr }\mathrm{3,2}n]}{10}}$$

(4)

$${rec}_{\mathrm{0,1}} ={\displaystyle \frac{\sum _{n=11}^{20}{[rec}_{\mathrm{0,1} }n]}{10}}$$

(5)

$${rec}_{\mathrm{3,2}} ={\displaystyle \frac{\sum _{n=21}^{30}{[rec}_{\mathrm{3,2} }n]}{10}}$$

(6)

These parameters are essential for determining the appropriate high-temperature performance grade (PG) of asphalt binders according to traffic loading conditions. The PG classification—Standard (S), Heavy (H), Very Heavy (V), and Extremely Heavy (E)—is based on J_nr3.2 values, with specific thresholds set at 4.5 kPa⁻¹, 2.0 kPa⁻¹, 1.0 kPa⁻¹, and 0.5 kPa⁻¹, respectively. Additionally, the difference in non-recoverable creep compliance (J_nr diff) between the 0.1 kPa and 3.2 kPa stress levels must not exceed 75%, ensuring the binder’s stress sensitivity remains within acceptable limits. This stringent evaluation ensures that only binders with sufficient resilience and stability under varying stress conditions are selected, thereby enhancing the durability and longevity of pavement structures [24].

2.8. Statistical Analysis

We utilized SPSS software (version 27, IBM, San Marcos, TX, USA) to conduct an Analysis of Variance (ANOVA) and subsequently employed Fisher’s Least Significant Difference (LSD) test, with a significance level of α = 0.05. An Analysis of Variance (ANOVA) was employed to identify any statistically significant disparities among the means of the samples. A significance level of 0.05 indicates a 5% chance of erroneously concluding that there is a difference when there actually is not (Type I error).

After identifying significant differences using ANOVA, the Fisher’s Least Significant Difference (LSD) test was used to identify the specific groups that showed dissimilarities. This test computes the minimum discrepancy between two sample means required to assert that their population means are statistically distinct. If the absolute difference between any two sample means surpasses this threshold, it signifies that the population means are significantly different.

A significance level of 0.05 signifies a 5% chance of observing the results, or more extreme data, if the null hypothesis is true, rather than a 95% “probability of being true.” The ANOVA was employed as an initial technique to detect significant variations, whereas Fisher’s LSD test was utilized for pairwise comparisons to precisely identify differences between groups [25].

3. Results and Discussion

3.1. Physical Properties

Figure 2 and

Table 4. Statistical analysis yielded results of the viscosity of modified binders at 135 °C and 180 °C as a function of different combinations in the original condition (=0.05).

Viscosity at 135 °C and 180 °C
		135 °C							180 °C
	Combination	PG64-22	PG64-22+10% TLA	PG64-22+20% TLA	PG64-22+10% TLA+5% SIS	PG64-22+20% TLA+5% SIS	PG64-22+10% TLA+10% SIS	PG64-22+20% TLA+10% SIS	PG64-22	PG64-22+10% TLA	PG64-22+20% TLA	PG64-22+10% TLA+5% SIS	PG64-22+20% TLA+5% SIS	PG64-22+10% TLA+10% SIS	PG64-22+20% TLA+10% SIS
135 °C	PG64-22	-	S	S	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	S	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	S	S	S	S	S	S	S	S	S	S
	PG64-22+20%TLA+5%SIS	-	-	-	-	-	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA+10%SIS	-	-	-	-	-	-	S	S	S	S	S	S	S	S
	PG64-22+20%TLA+10%SIS	-	-	-	-	-	-	-	S	S	S	S	S	S	S
180 °C	PG64-22	-	-	-	-	-	-	-	-	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	-	-	-	-	-	-	-	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	-	-	-	-	-	-	-	S	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	-	-	-	-	-	-	-	S	S	S
	PG64-22+20%TLA+5%SIS	-	-	-	-	-	-	-	-	-	-	-	-	S	S
	PG64-22+10%TLA+10%SIS	-	-	-	-	-	-	-	-	-	-	-	-	-	S
	PG64-22+20%TLA+10%SIS	-	-	-	-	-	-	-	-	-	-	-	-	-	-

S—Significant and N—Non-significant.

demonstrate how different amounts of TLA and SIS affect the viscosity of PG64-22 asphalt binder at two important temperatures, 135 °C and 180 °C. Viscosity plays a crucial role in evaluating the ease of use and effectiveness of asphalt binders, especially in the presence of elevated temperatures during pavement construction and usage. The red arrow on the graph represents a viscosity limit of 3000 cP. This limit is crucial for maintaining the workability of the binder while also providing enough stiffness to resist deformation.

The PG64-22 binder has a viscosity of 755 cP at 135 °C and 285 cP at 180 °C, which falls within the acceptable range for workability and handling. Upon the addition of TLA to the binder, a discernible increase in viscosity is noted. More precisely, the introduction of 10% TLA increases the viscosity to 855 cP at a temperature of 135 °C and 305 cP at a temperature of 180 °C. On the other hand, when 20% TLA is added, the viscosities become 866 cP and 311 cP at the same respective temperatures. The observed increases indicate that the addition of TLA causes a slight increase in the rigidity of the binder, enhancing its ability to resist deformation while not significantly affecting its ease of use.

However, the addition of SIS to the TLA-modified binders results in a more noticeable increase in viscosity, emphasizing its important role in improving binder stiffness and performance. As an example, when 10% TLA and 5% SIS are combined, the resulting viscosity is 1775 cP at 135 °C and 575 cP at 180 °C. The significant increase in value indicates that the addition of SIS improves the elasticity of the binder, thereby increasing its ability to withstand permanent deformation. This characteristic is particularly important for achieving optimal performance at high temperatures.

As the levels of TLA and SIS increase, the viscosity consistently increases. Specifically, when the mixture contains 20% TLA and 5% SIS, the viscosity is 2212.5 cP at 135 °C and 787.5 cP at 180 °C. As long as these values stay below the 3000 cP limit, it shows that the binder can still be worked with. However, the increased stiffness indicates better ability to withstand rutting and deformation, which is crucial for ensuring that the pavement remains durable over time, especially when subjected to heavy traffic loads.

However, when the content of SIS is increased to 10%, the viscosity that is obtained exceeds the threshold of 3000 cP, especially at a temperature of 135 °C. At this temperature, viscosities of 3812.5 cP (with 10% TLA) and 4975 cP (with 20% TLA) are observed. Although high viscosities suggest excellent resistance to deformation at high temperatures, they can also present difficulties in handling and applying the binder, as it may become excessively rigid for efficient mixing and compaction.

To summarize, the use of SIS is essential for enhancing the performance of TLA-modified PG64-22 binders, specifically by improving stiffness and elasticity. These properties are crucial for withstanding deformation under high temperatures. Nevertheless, it is crucial to carefully balance the trade-off between enhanced performance and reduced workability. To achieve the desired pavement performance while maintaining workability during construction, it is important to determine the optimal concentrations of SIS and TLA. Having a proper balance is crucial in order to fully optimize the advantages of SIS in improving the durability and ability to withstand stress of pavements.

3.2. Results of Rheological Properties at High Temperatures

The study examines the impact of TLA and SIS on the rheological behavior of PG 64-22 asphalt binder, with a specific emphasis on the G*/sinδ parameter. This parameter, known as the complex shear modulus (G*) combined with the phase angle (δ), is a crucial indicator of a pavement’s capacity to withstand deformation, particularly in high-temperature conditions. A higher G*/sinδ value signifies enhanced resistance to rutting, a prevalent and detrimental type of permanent deformation in asphalt pavements.

Figure 3 and

Table 5. Statistical analysis results of the G*/sin δ at 64 °C of modified asphalt binders as a function of different combinations in original and RTFO conditions (α = 0.05).

Viscoelasticity Original and RTFO Conditions at 64 °C
		Original							RTFO
	Combination	PG64-22	PG64-22+10% TLA	PG64-22+20% TLA	PG64-22+10% TLA+5% SIS	PG64-22+20% TLA+5% SIS	PG64-22+10% TLA+10% SIS	PG64-22+20% TLA+10% SIS	PG64-22	PG64-22+10% TLA	PG64-22+20% TLA	PG64-22+10% TLA+5% SIS	PG64-22+20% TLA+5% SIS	PG64-22+10% TLA+10% SIS	PG64-22+20% TLA+10% SIS
Original	PG64-22	-	S	S	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	S	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	S	S	S	S	S	S	S	S	S	S
	PG64-22+20%TLA+5%SIS	-	-	-	-	-	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA+10%SIS	-	-	-	-	-	-	S	S	S	S	S	S	S	S
	PG64-22+20%TLA+10%SIS	-	-	-	-	-	-	-	S	S	S	S	S	S	S
RTFO	PG64-22	-	-	-	-	-	-	-	-	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	-	-	-	-	-	-	-	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	-	-	-	-	-	-	-	S	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	-	-	-	-	-	-	-	S	S	S
	PG64-22+20%TLA+5%SIS	-	-	-	-	-	-	-	-	-	-	-	-	S	S
	PG64-22+10%TLA+10%SIS	-	-	-	-	-	-	-	-	-	-	-	-	-	S
	PG64-22+20%TLA+10%SIS	-	-	-	-	-	-	-	-	-	-	-	-	-	-

S—Significant and N—Non-significant.

demonstrate that the base binder, PG 64-22, shows the lowest G*/sinδ values in both its original and RTFO-aged conditions. Without any alterations, PG 64-22 exhibits limited resistance to deformation at elevated temperatures, rendering it unsuitable for environments where rutting is a significant issue.

TLA implementation in PG 64-22 results in a significant enhancement in the binder’s efficacy. For example, when 10% TLA is added, the G*/sinδ values of the original binder increase from 2.05 kPa (original) and 5.35 kPa (RTFO) to 2.55 kPa and 5.90 kPa, respectively. By increasing the TLA content to 20%, the binder’s stiffness and rutting resistance are further improved, as indicated by the values of 3.10 kPa (original) and 6.50 kPa (RTFO). The results demonstrate that TLA, a naturally occurring asphalt, greatly enhances the binder’s capacity to withstand deformation.

The most notable enhancements are observed when SIS is added, especially when combined with TLA. Adding 5% SIS to the PG 64-22 + 10% TLA binder significantly enhances the G*/sinδ, which reaches 8.14 kPa (original) and 15.30 kPa (RTFO). When the SIS content is increased to 10%, the G*/sinδ values significantly increase to 12.10 kPa (original) and 20.80 kPa (RTFO). This illustrates that the use of SIS is crucial in improving the binder’s ability to withstand deformation.

In this study, it was found that the binder with 20% TLA and 10% SIS had the highest G*/sinδ values, with 26.50 kPa (original) and 33.10 kPa (RTFO). These findings indicate that the simultaneous application of TLA and SIS produces a synergistic outcome, resulting in a notable enhancement in the stiffness and resistance to rutting of the binder. This makes it particularly well suited for pavements that are exposed to elevated temperatures.

The enhancements observed with the incorporation of SIS can be ascribed to its distinctive polymeric architecture. SIS, being a thermoplastic elastomer, improves the elasticity and stiffness of the binder. This enables the binder to retain its structural integrity when subjected to load, especially in high-temperature conditions. This leads to increased G*/sinδ values, which indicate a greater ability to withstand permanent deformation or rutting. This is a crucial factor for the long-term durability and lifespan of asphalt pavements.

To summarize, the results of this study clearly show that adding SIS to TLA-modified PG 64-22 binders greatly improves the flow characteristics of the asphalt, making it more resistant to deformation at high temperatures. The significant enhancements in G*/sinδ observed in different compositions highlight the crucial role of SIS in enhancing pavement performance, making it a valuable additive for achieving more durable and resilient asphalt pavements.

3.3. Results of Rheological Properties at Ambient Temperature

The rheological properties at room temperature are assessed through a fatigue resistance test, using the parameter G*.sin δ (in kPa). Higher values indicate a greater vulnerability to fatigue cracking. All tested binders underwent PAV aging, which replicates the oxidative aging process that takes place throughout the lifespan of the pavement. Beginning with the original binder, PG64-22, which displayed a G*.sin δ value of 3400 kPa, we can observe a fundamental level of fatigue resistance (as shown in Figure 4). The implementation of a 10% TLA led to a substantial increase in the G*.sin δ value, reaching 5090 kPa. This indicates that the binder became more rigid and susceptible to fatigue cracking. The trend persists with a 20% modification in TLA, resulting in a G*.sin δ value of 6548 kPa. Although the greater rigidity resulting from a higher TLA content can be beneficial in preventing rutting, it also raises concerns regarding the possibility of fatigue cracking, particularly in binders that have been aged over a long period of time.

SIS, a thermoplastic elastomer, is added to asphalt binders to enhance their elasticity and fatigue resistance. This addition is important in counteracting the stiffening effects of TLA. Upon adding 5% SIS to the 10% TLA-modified binder, the G*.sin δ value decreases to 3980 kPa. This exemplifies the capability of SIS to enhance flexibility and enhance the binder’s durability against fatigue cracking. Furthermore, the inclusion of 5% SIS in the 20% TLA-modified binder results in a decrease in the G*.sin δ value to 5480 kPa. Increasing the SIS content to 10% in both TLA-modified binders leads to lower G*.sin δ values. The 10% TLA binder has a value of 2530 kPa, while the 20% TLA binder has a value of 4970 kPa. This demonstrates the significant effect of SIS in improving fatigue resistance, and

Table 6. Statistical analysis results of the RTFO + PAV of modified asphalt binders at 25 °C as a function of different combinations (α = 0.05).

RTFO + PAV Condition at 25 °C
		RTFO + PAV
	Combination	PG 64-22	PG64-22+10%TLA	PG64-22+20%TLA	PG64-22+10%TLA+5% SIS	PG64-22+20%TLA+5% SIS	PG64-22+10%TLA+10% SIS	PG64-22+20%TLA+10% SIS
RTFO+PAV	PG64-22	-	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	S	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	S	S	S
	PG64-22+20%TLA+5% SIS	-	-	-	-	-	S	S
	PG64-22+10%TLA+10% SIS	-	-	-	-	-	-	S
	PG64-22+20%TLA+10% SIS	-	-	-	-	-	-	-

S—Significant and N—Non-significant.

shows that all values are significantly different from each other.

From a scientific standpoint, the findings emphasize the significance of SIS in contemporary pavement engineering. SIS reduces the likelihood of fatigue cracking by improving the viscoelastic properties of the binder, which counteracts the increased stiffness caused by TLA. The equilibrium between rigidity and adaptability is crucial for the durability of pavements, especially in older binders that have experienced substantial oxidative aging. SIS enhances the flexibility of the asphalt matrix, enabling it to efficiently absorb and disperse strain energy. This is essential for preventing the formation and spread of cracks throughout the lifespan of the pavement.

3.4. Results of MSCR Test on Unaged and Short-Term Aged Binders

The results of the MSCR test provide important information about the performance of different binders. This includes the original binderPG64-22, as well as its modifications with TLA and further enhancements with SIS. The MSCR test evaluates the performance of these binders in both their unaged state and after short-term aging using the RTFO. Figure 5 and

Table 7. Statistical analysis results of the % recovery at 64 °C of modified asphalt binders as a function of original and RTFO conditions (α = 0.05).

%Recovery (Original and RTFO Conditions) at 64 °C
		Original							RTFO
	Combination	PG64-22	PG64-22+10% TLA	PG64-22+20% TLA	PG64-22+10% TLA+5% SIS	PG64-22+20% TLA+5% SIS	PG64-22+10% TLA+10% SIS	PG64-22+20% TLA+10% SIS	PG64-22	PG64-22+10% TLA	PG64-22+20% TLA	PG64-22+10% TLA+5% SIS	PG64-22+20% TLA+5% SIS	PG64-22+10% TLA+10% SIS	PG64-22+20% TLA+10% SIS
Original	PG64-22	-	N	S	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	N	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	S	S	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	S	S	S	S	S	S	S	S	S	S
	PG64-22+20%TLA+5%SIS	-	-	-	-	-	S	S	S	S	S	S	S	S	S
	PG64-22+10%TLA+10%SIS	-	-	-	-	-	-	S	S	S	S	S	S	S	S
	PG64-22+20%TLA+10%SIS	-	-	-	-	-	-	-	S	S	S	S	S	S	S
RTFO	PG64-22	-	-	-	-	-	-	-	-	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	-	-	-	-	-	-	-	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	-	-	-	-	-	-	-	S	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	-	-	-	-	-	-	-	S	S	S
	PG64-22+20%TLA+5%SIS	-	-	-	-	-	-	-	-	-	-	-	-	S	S
	PG64-22+10%TLA+10%SIS	-	-	-	-	-	-	-	-	-	-	-	-	-	N
	PG64-22+20%TLA+10%SIS	-	-	-	-	-	-	-	-	-	-	-	-	-	-

S—Significant and N—Non-significant.

demonstrate that the original binder, PG64-22, had the lowest percentage of recovery. Its original state showed a value of 0.04%, which increased to 1.85% after RTFO aging. The low recovery values, along with the highest J_nr values (5.11 kPa⁻¹ unaged and 1.92 kPa⁻¹ after aging), suggest a lack of resistance to deformation. The binder’s substantial non-recoverable stress when subjected to a load indicates that it is likely to experience rutting and other types of permanent deformation when exposed to traffic loading. When the TLA was added to the PG64-22 binder, there was a slight increase in the % recovery. For example, after undergoing RTFO aging, the PG64-22 + 20% TLA mixture exhibited a small increase in the % recovery to 2.87%, while its J_nr value remained relatively high at 1.47 kPa⁻¹. This suggests that although TLA improves the performance of the binder to some degree, the enhancement is not significant enough to significantly change the binder’s overall ability to resist deformation.

The introduction of SIS into the binder formulations resulted in the most notable improvements in performance. The inclusion of SIS resulted in significant enhancements in % recovery and reductions in J_nr values, indicating improved elasticity and increased resistance to deformation. As an illustration, the combination of the PG64-22 binder with 10% TLA and 5% SIS showed a significant increase in the percentage of recovery, reaching 16.17% when unaged and 30.92% after undergoing RTFO aging. Simultaneously, the J_nr values experienced a substantial decrease, indicating a notable enhancement in the binder’s capacity to regain its original form after being exposed to stress, consequently diminishing permanent deformation.

However, a noteworthy pattern arises as the amount of SIS content is further augmented. First, when the SIS content increases, there is a significant improvement in % recovery after aging. This indicates that the binder becomes more resilient and less likely to experience permanent deformation. The reason for this is most likely the formation of an elastomeric network by SIS within the binder matrix. This network enables the binder to deform when subjected to stress and then regain its original shape more efficiently. However, once the SIS content reaches 10%, even though the percentage of recovery remains high, the rate of improvement starts to decrease after aging. This trend indicates that there is an ideal level of SIS content, beyond which further SIS does not continue to improve performance in a linear manner. Excessive SIS can hinder the flow and self-healing properties of the binder, resulting in a slight decline in its performance enhancements.

In summary, the findings from the MSCR tests emphasize the crucial importance of SIS in improving the performance of binders, specifically by increasing the percentage of recovery and decreasing J_nr values. The enhancements render SIS-modified binders highly suitable for application in pavement engineering, particularly in high-traffic environments where resistance to rutting is of utmost importance. Nevertheless, the noticeable shift in pattern at increased SIS contents emphasizes the significance of fine-tuning the composition of binder modifiers to attain the desired equilibrium between rigidity and pliability, guaranteeing optimal pavement functionality.

3.5. Results of Rheological Properties at Low Temperatures

The rheological characteristics of the asphalt binders were assessed at low temperatures using stiffness (measured in MPa) and m-value as the main parameters. The performance of pavement in cold climates is heavily influenced by these properties, as they play a crucial role in its ability to withstand cracking caused by thermal stresses. The stiffness results from Figure 6 and

Table 8. Statistical analysis results of the stiffness (RTFO + PAV) of modified asphalt binders at −12 °C as a function of different combinations (α = 0.05).

Stiffness at −12 °C
		RTFO + PAV
	Combination	PG64-22	PG64-22+10%TLA	PG64-22+20%TLA	PG64-22+10%TLA+5% SIS	PG64-22+20%TLA+5% SIS	PG64-22+10%TLA+10% SIS	PG64-22+20%TLA+10% SIS
RTFO+PAV	PG64-22	-	S	S	S	S	S	S
	PG64-22+10%TLA	-	-	S	S	S	S	S
	PG64-22+20%TLA	-	-	-	N	S	S	S
	PG64-22+10%TLA+5% SIS	-	-	-	-	S	S	S
	PG64-22+20%TLA+5% SIS	-	-	-	-	-	S	S
	PG64-22+10%TLA+10% SIS	-	-	-	-	-	-	S
	PG64-22+20%TLA+10% SIS	-	-	-	-	-	-	-

S—Significant and N—Non-significant.

demonstrate that the original PG64-22 binder possesses a stiffness of 186 MPa, which accurately reflects its fundamental performance. An incorporation of 10% Trinidad Lake Asphalt (TLA) into the binder resulted in a slight decrease in stiffness, with a value of 178 MPa. This decrease indicates an initial enhancement in flexibility, most likely because of the more pliable characteristics of the organic asphalt in TLA. Nevertheless, when the TLA content was elevated to 20%, the stiffness experienced a substantial increase, reaching 253 MPa. The increase in stiffness can be attributed to the elevated levels of asphaltene in TLA, which leads to a more rigid binder structure. This enhances the material’s ability to resist deformation but also increases its vulnerability to cracking. The function of SIS as a polymer modifier was also investigated. Upon adding 5% SIS to the binder that already contained 10% TLA, the stiffness was slightly decreased to 251 MPa. This slight decrease indicates that SIS, although it generally improves flexibility, could potentially create a network within the binder that either maintains or slightly decreases rigidity. However, when the SIS content was increased to 10% along with 10% TLA, the stiffness of the material further increased to 283 MPa. This suggests that the polymer network becomes more prominent, resulting in a stiffer material. The trend persisted in the case of 20% TLA blends, with stiffness values reaching 306 MPa and 314 MPa for the addition of 5% and 10% SIS, respectively. The results suggest that SIS plays a role in increasing stiffness, but its impact is intricate and influenced by the composition of the binder, which could result in a pavement structure that is more rigid.

Figure 7 depicts the m-value results, which provide valuable information about the binder’s capacity to alleviate stresses at low temperatures. The initial PG64-22 binder demonstrated a commendable m-value of 0.34, signifying favorable stress relaxation characteristics and consequently a reduced likelihood of thermal cracking. Nevertheless, when 10% TLA was introduced, the m-value dropped to 0.279, indicating a decrease in the binder’s ability to relax. Surprisingly, increasing the TLA content to 20% resulted in a small enhancement in the m-value, reaching 0.294. This improvement could be attributed to the higher amount of resin in TLA, which might counterbalance the adverse impact on stress relaxation to some extent.

The inclusion of SIS had an additional effect on the m-value. The addition of 5% SIS to the 10% TLA blend resulted in a decrease in the m-value to 0.257. Furthermore, increasing the SIS content to 10% led to a more significant reduction in the m-value, resulting in a value of 0.213. The binder with a composition of 20% TLA and 10% SIS exhibited the lowest m-value, which was measured at 0.184. These findings indicate that SIS increases the rigidity of the binder, but at the same time, decreases its capacity to release stress, which is essential for preventing cracking at low temperatures. The decline in m-value as the SIS content increases suggests that the polymer network created by SIS limits the relaxation mechanisms of the binder, thereby heightening the likelihood of thermal cracking in cold conditions.

In summary, the findings indicate that there is a reciprocal correlation between stiffness and m-value, implying a compromise between the ability to withstand deformation and the vulnerability to cracking. The inclusion of TLA and SIS improves the rigidity of the binder, making it better suited for high-temperature conditions. However, this also decreases the m-value, which increases the risk of cracking in low-temperature environments. Hence, it is crucial to strike a delicate equilibrium when employing SIS as a modifier, so as to guarantee that the advantages of enhanced rigidity are not achieved at the cost of diminished performance in low temperatures. The results are essential for enhancing the configuration of asphalt binders in frigid regions, where it is imperative to have both strong resistance to high temperatures and the ability to remain flexible at low temperatures in order to ensure the long-lasting durability of road surfaces.

4. Conclusions

The study investigated the effects of incorporating different proportions of Trinidad Lake Asphalt (TLA) and styrene–isoprene–styrene (SIS) into a PG 64-22 asphalt binder. The objective was to evaluate the impact of these additives on the performance of the binder, specifically in relation to physical and rheological parameters as per the Superpave test method. The study offers valuable insights into the impact of SIS on the overall performance of the binder. These insights can guide the selection of the most suitable formulations for various pavement engineering applications.

• The blend of 20% TLA and 5% SIS provides an optimal balance between workability and enhanced performance. This formulation significantly improves elasticity, while maintaining viscosity within critical limits, making it an ideal choice for durable pavements with excellent resistance to high-temperature deformation, without compromising ease of construction.
• The combination of 20% TLA and 10% SIS yields the highest G*/sinδ values, indicating superior resistance to rutting under high-temperature conditions making it particularly suitable for pavements in challenging environments requiring long-term resistance to deformation.
• The 10% TLA and 10% SIS formulation strikes a balance between stiffness and fatigue resistance, enhancing the binder’s viscoelastic properties. This combination improves the pavement’s ability to resist oxidative aging and mitigates fatigue cracking, ensuring a more durable pavement structure over time.
• Finally, 10% TLA and 5% SIS display promising results in the MSCR test, showing significant improvements in elasticity and resistance to permanent deformation. This combination is particularly well suited for high-traffic pavements, offering an optimal balance between rigidity and flexibility, ultimately improving pavement durability and overall performance.
• Although SIS typically improves performance, excessive concentrations can result in excessive stiffness, necessitating careful optimization to strike a balance between enhanced performance and manageable workability.

The results indicate that the performance of the binder is improved by SIS. However, the selection of the best combination should consider the specific environmental conditions and performance needs. This will ensure that the advantages of SIS are fully utilized without compromising other important properties. Additional research should examine the enduring consequences of these alterations and explore alternative combinations of additives with SIS to further enhance low-temperature performance.