Green Technology: Performance of Sustainable Asphalt Mixes Modified with Linear Low-Density Polyethylene Waste

Abstract

This experimental study evaluated the performance of modified asphalt mixtures prepared by incorporating 2%, 4%, and 6% linear low-density polyethylene (LLDPE) by weight of asphalt binder through a series of tests. The microstructural analyses using scanning electron microscopy (SEM) were conducted on asphalt samples to assess the engineering properties of the asphalt mixes. Finally, ANOVA statistical analysis has been employed to determine the statistical significance of the differences in all tests’ means. Based on laboratory findings, the Marshall stability test result showed that the modified asphalt mixes up to 4% LLDPE had enhanced performance by 12.7% compared to the control mix. A significant decrease (up to 31.3%) in binder penetration was demonstrated due to the incorporation of LLDPE into the asphalt mix. The softening point of the LLDPE–asphalt mixes was increased by up to 17.6%. It was also demonstrated that the incorporation of such LLDPE dosages maintains the flow limits within the specified range; however, the flow of the asphalt mix with 4% LLDPE was 3.17 mm which is the nearest to the average value of the upper and lower acceptable limits. The air voids of mixes with LLDPE content more than 4% by was decreased to less than 4% which is not recommended in high-temperature climates to control mixture bleeding. Microscopic analysis revealed an improvement in the densification of asphalt microstructures, attributed to the LLDPE particles significantly changing the rheology and viscosity of the base mixture and making the hot asphalt mixture more homogeneous. Based on the physical and rheological properties investigated in this study, it could be concluded that 4% LLDPE produces the best performance in asphalt mixtures. Overall, the ANOVA analysis demonstrated that the incorporation of LLDPE into asphalt mixes has a significant impact on all of their properties.

1. Introduction

Recently, traffic volume on road networks has witnessed an increase, so the need for periodic maintenance work has become more significant than ever [1]. The inability of some countries to handle road maintenance costs has led to cracks in cold areas and rutting in hot areas [2,3]. So, the trend toward using alternative, sustainable, and environmentally friendly materials to preserve natural raw materials and reduce the financial costs to the responsible authorities has become indispensable [4]. Recently, investigating the potential of integrating waste and recycled materials as a partial replacement of original components in asphalt mixes becomes a growing interest to pavement researchers. For instance, Joumblat et al. [5] provided a review covering the findings of the research studies conducted on utilizing bottom ash and fly ash, resulting from the incineration of municipal waste, as fillers in asphalt mixtures. Researchers seek to enhance the performance of asphalt mixtures by utilizing sustainable additives such as mass-produced waste polymers from plastic landfills [6,7,8]. Utilizing plastic waste in bitumen mixes increases the life of the road, reduces the consumption of petroleum materials, and reduces harmful environmental effects [9]. Consequently, the modification of bitumen through polymerization is commonly utilized within the asphalt industry to enhance the characteristics of the binder, thereby influencing the pavement’s ability to resist thermal cracking, fatigue, and rutting [10].

The polymers utilized as modifiers for bitumen can be classified into three distinct categories based on their chemical composition and characteristics. These categories include plastomers, elastomers, and reactive polymers [11]. Polyethylene stands out among plastomers as being highly regarded for its effectiveness in modifying asphalt binders. The superior resistance to fatigue and rutting exhibited by polyethylene-modified asphalt binders compared to traditional asphalt binders is a well-documented fact [12]. Polyethylene, a type of plastomers, can be found in various forms such as high-density polyethylene (HDPE), medium-density polyethylene (MDPE), low-density polyethylene (LDPE), and linear low-density polyethylene (LLDPE) [13,14]. The incorporation of LLDPE resulted in notable enhancements in the base bitumen’s physical, chemical, rheological, and thermal characteristics [15]. As the concentration of recycled LLDPE was augmented, viscosity, softening point, and penetration index increased significantly, with a simultaneous decrease in penetration value [16]. Mohammed et al. [17] conducted a thorough review on the role of low-density polyethylene (LDPE) in asphalt mixes. They have concluded that the incorporation of LDPE into asphalt mixes is a promising approach for pavement design and performance as it could significantly enhance the properties of asphalt mixes in specific conditions. However, they have demonstrated that more studies are needed to optimize the design and application of LDPE in asphalt mixes and to investigate its long-term behavior.

Ullah et al. [18] examined the effects of incorporating waste plastic aggregates of LDPE and HDPE in the asphalt mix. Results indicated enhancements in stability and flow values with 15% replacement. LDPE samples demonstrated reduced rut depth, and HDPE displayed a notable rise in resilient modulus. LDPE and HDPE waste plastic aggregates decreased asphalt mixture density, attributed to increased air voids, thereby improving dynamic modulus values. Also, Abduljabbar et al. [19] reported that integrating waste low-density polyethylene into a thin asphalt overlay enhanced its mechanical characteristics, including heightened rigidity and decreased vulnerability to moisture. The researchers confirmed that using reclaimed plastic waste could benefit the functionality of asphalt components, suggesting a promising sustainable resolution for the construction field. Mazouz and M’Hammed [20] found that the LDPE in the asphalt mix acted as a binder, enhancing adhesion between bitumen and aggregates, filling voids in the mix, and improving resistance to permanent deformation, ultimately increasing road longevity. Rincón-Estepa et al. [21] studied the mechanical properties of hot asphalt mixes incorporation wet-modified low-density polyethylene waste (LDPE). The LDPE was modified using a 150 °C temperature to reduce the emissions of asphalt cement (AC) oxidation. After investigating the rheological and physical properties of the asphalt binder containing different contents of modified LDPE, it has been found that the best performance could be achieved by incorporating 5% LDPE by mass of the AC. The authors concluded that such mixtures with wet-modified LDPE could be used in high-temperature regions to hinder the pavement rutting.

Likewise, Kishchynskyi et al. [22] concluded in an experimental study that the use of plastic waste helps substantially improve the abrasion and slip resistance of flexible pavement and also allows the obtaining of values of splitting tensile strength to satisfy the specified limits when plastic waste content is 30% by weight of mix. Shaikh et al. [23] indicated that using an EE-2 polymer modifier in Hot Mix Asphalt (HMA) enhanced rutting resistance, resilient modulus, and stability. Overall, using various waste materials enhances several critical characteristics of traditional bitumen. These enhancements may suggest favorable performance under moderate to high temperatures and an enhancement in resistance to fatigue and rutting [24].

On the other hand, studies have recently focused on microscopic analysis of asphalt mixtures using energy-dispersive X-ray spectroscopy (EDX) technology. Adding polymers to asphalt mixtures led to changes in structure and modifications in the samples’ properties, which are associated with an increase in the temperature for decomposition [25]. Adding plastic to the hot aggregate immediately before combining it with the bitumen increases durability against permanent deformation at high temperatures [26]. The stiffness of chemically modified bitumen mixtures (waste plastics) increased by 10%. This enhancement is related to the growth of the cohesive forces between the aggregate and the bitumen. Plastic coating decreases porosity and moisture absorption capacity [27]. Using waste plastics in asphalt reinforcement enhances engineering properties by improving resistance to rutting, fatigue, deformation, and moisture stability [28]. The compression values exhibited by asphalt mixtures containing LDPE were lower than those of conventional asphalt mixtures [29].

Although extensive research has been conducted on polymer-modified bitumen, the polymer needs to enhance its rheology, morphology, and creep evaluation [24]. Researchers have recently reported some critical issues concerning utilizing polymers as modifiers in the wet process. These issues include mixing problems related to high viscosity at high temperatures, the affinity of the polymers for bitumen, and the high cost of modifying it [30]. However, the bond between polymer particles and bitumen still needs further investigation regarding microstructure analysis.

So, this study seeks to explore the potential utilization of linear low-density polyethylene (LLDPE) waste plastics as an enhancer to asphalt mix performance and cut down on expenses. Despite a limited studies on the utilization of mixed-source and recycled LLDPE in bitumen, the optimum LLPDE content is not yet demonstrated and more studies are needed to optimize the LLDPE–asphalt mix design and explore its long-term performance [17]. Hence, the main objective of this study is to provide a comprehensive investigation of the stability, penetration, softening point, flow, AVs, VMAs, GMM, and flash point of asphalt mixes containing 2%, 4%, and 6% of LLDPE by weight of asphalt binder. Based on such tests’ results, the optimum LLDPE content is suggested for further investigation of the long-term behavior.

Firstly, this study focuses on the methods of incorporating LLDPE in asphalt. Subsequently, it discusses the interaction of LLDPE with bitumen, evaluates the rheological behavior of the mixtures and suggests the optimum LLDPE content. The modified hot asphalt mixture’s microstructural characteristics were also assessed using Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM/EDX). By achieving these aims, this research can offer valuable information about how to determine the workability of asphalt and specially modified mixtures quantitatively. A viable strategy for mix design and pavement construction will also be developed, enhancing the workability of asphalt mixtures containing recycled plastics.

2. Experimental Program

In this study, raw materials were chosen from known sources in Jordan. Laboratory tests were conducted to estimate the quality of the utilized materials.

2.1. Materials

• Bitumen:

This research used bitumen grade 60/70 from the Jordanian Petroleum Refinery as an asphalt binder. The laboratory tests performed to evaluate the bitumen properties were specific gravity, softening point, flash point, and penetration. The properties of the asphalt binder are presented in

Table 1. Properties of asphalt binder.

Test	Test Result
Specific Gravity	1.019
Flash Point (°C)	327
Penetration (0.1 mm)	64
Ring and Ball Softening Point (°C)	51

.

• Aggregate:

The coarse and fine aggregates used in the present study were crushed granite imported from the Aqaba area. Limestone powder was selected as the mineral filler and obtained to supplement the size of the fine material in the hot mix asphalt (HMA) mixture design.

Table 2. Composition of The Aggregate.

Aggregate Samples	Description	Recommended Proportions (by Weight)
Coarse Aggregate (Folleyeh)	Crushed granite materials	5.0%
Coarse Aggregate (Homseyeh)	Crushed granite materials	14.0%
Medium aggregate (Adaseyeh)	Crushed granite materials	36.0%
Fine Aggregate	Limestone	45.0%
Total		100%

shows the best aggregate blending percentages based on previous investigations to meet the Jordan Ministry of Public Works and Housing (MOPWH)’s standards.

• Linear Low-Density Polyethylene (LLDPE):

The plastic waste material utilized in the modified asphalt mix is linear low-density polyethylene LLDPE-218WJ, sourced from Saudi Basic Industries Corporation (SABIC) as shown in Figure 1. The LLDPE-218WJ represents a butene linear low-density polyethylene resin commonly employed in various general-purpose applications. Films crafted from LLDPE-218WJ demonstrate good tensile strength, favorable hot tack properties, and contain antioxidants, slip agents, and anti-blocking agents. These thin-walled containers have a softening temperature at 98 °C and a 0.918 g/cm³ density. The mechanical and physical tests of the used LLDPE are presented in

Table 3. Mechanical and Physical Test of the LLDPE-118WM.

Test Method (ASTM)	Property	LLDPE
D 1505	Density kg/m3	918
D 1525	Vicat softening point °C	98
D 882	Stress at break, MD Mpa	35
D 882	Stress at break, TD Mpa	29
D 1709	Dart impact strength g/μm	85

.

2.2. Preparation of Specimens

Figure 2 illustrates the diagrammatic representation of the specimen preparation and testing in this study. Firstly, four experimental mixtures were prepared to achieve the optimal Asphalt Content (AC%), incorporating the recommended combined grading and increasing percentages of binder content from 3% to 4.5% relative to the overall mixture weight (samples before adding LLDPE plastic), as shown in Figure 3. Three specimens were made for each trial mixing percent, and the average of these three was used.

Secondly, the process involved the utilization of the Marshall method, a widely accepted standard, for both the preparation and evaluation of the specimens according to the ASTM-D6926 [31] standard, as shown in Figure 4. The Marshall hammer delivered 75 impacts on each side of the specimens (representative of heavy trucks). Subsequently, the determined values of unit weight, stability, flow, stiffness, air voids, and voids in mineral aggregate were shown with the binder content. By analyzing these plots, the optimal Asphalt Content (AC%) of 4% was ascertained, and this particular percentage was selected for blending with LLDPE. Results of the trial mix for stability, density, and air voids are presented in

Table 4. AC% with asphalt mixture properties.

Trial #	AC%	Stability (kg)	Density (g/cm3)	Air Voids %
1	3	1528	2.23	6.9
2	3	1546	2.24	7.1
3	3	1576	2.25	7.3
4	3.5	1584	2.29	5.8
5	3.5	1604	2.30	5.7
6	3.5	1639	2.31	5.9
7	4	1625	2.36	3.9
8	4	1638	2.37	4.1
9	4	1660	2.38	4
10	4.5	1563	2.25	2.6
11	4.5	1573	2.26	2.7
12	4.5	1589	2.27	2.8

and Figure 4.

For blending the LLDPE with bitumen, the bitumen was initially subjected to preheating at a temperature of 170 °C within an oven, after which the LLDPE particles were incrementally introduced into the preheated bitumen. The bitumen/LLDPE composites were formulated by combining bitumen and LLDPE at concentrations of 2%, 4%, and 6% LLDPE loading at a mixing temperature of 170 °C. The bitumen–recycled plastic composite was agitated using a Silverson L5M-A high-speed shear mixer (Silverson Machines Ltd., Chesham, UK), maintaining a consistent mixing temperature of 170 °C and a rotational speed of 3500 rpm for a duration of 1.5 h.

It is worth noting that the ratios of LLDPE were determined based on the outcomes of the stability, flow, density, and air void ratio assessments. For instance, three samples were prepared at a concentration of 2%, and the aforementioned evaluations were conducted (stability, flow, density, and air void ratio). Subsequently, the mixing process was repeated at 4% and 6%, respectively. Ultimately, when it became evident that the value for stability, density, and air voidsbegan to decline with the increasing percentage of the modifying agent (Figure 4), the incorporation of the modifying material ceased.

2.3. Microstructural Analysis

Microstructural analysis was performed on the modified and unmodified asphalt mixtures samples to understand the impact of LLDPE on the properties of asphalt mixes. This was carried out using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX). To achieve high-resolution images, gold–palladium was used to coat the sample surfaces in order to eliminate any surplus charges after being dried in a vacuum chamber. The FEI-Nova Nano SEM microscope (LabX, Midland, ON, Canada) was used for scanning and imaging. Additionally, EDX analysis was utilized to investigate the elemental composition of all samples.

2.4. Statistical Analysis (ANOVA)

In order to determine if the effect of incorporating LLDPE into asphalt mixes has a statically significant impact on their properties, statistical analysis has been incorporated. As this study investigated three different LLDPE percentages in addition to the control mix (more than two groups or treatments), the analysis of variance (ANOVA) F-test is employed. In this method, the null hypothesis assumes that there is no significant difference between the groups’ means. If the resultant F statistical value is more than the critical F value (extracted from the F-distribution table), then the null hypothesis is rejected implying that the difference between the group means is statistically significant.

3. Results and Discussion

Following the most suitable asphalt content (AC%) at 4% from among the trial mixtures, four mixtures were prepared with different LLDPE contents (0, 2, 4, and 6%) of the asphalt content. The results showed that the best results for the modified mixture were achieved at 4% LLDPE.

Table 5. Properties results of asphalt mixes containing LLDPE particles.

LLDPE Content	Properties Results of Asphalt Mixes
	Stability (kg)	Penetration (0.1 mm)	Softening Point (°C)	Flow (mm)	AV (%)	VMA (%)	GMM (g/cm3)	Flash Point (°C)
0%	1641	64	51	2.58	4.8	15	2.54	327
2%	1740	49	55	2.82	4.6	14.7	2.43	330
4%	1849	47	57	3.17	4	14.1	2.39	334
6%	1571	44	60	3.37	0.4	18.2	2.28	339

shows the average results of the tests conducted on these mixtures and their comparison with the reference mixture.

3.1. Stability

Figure 5 shows the average value of all stability results for the asphalt mixes in this study. It was noted that increasing the LLDPE content up to 4% in the mixtures improved the stability values of the samples, but when the percentages of LLDPE content increased beyond this limit, the stability of the asphalt mixtures decreased compared with the control mixture. Findings revealed that the stability value of the modified mixture with 2% and 4% LLDPE achieved an increase of 6% and 12.7% compared to the control mixture with 0% LLDPE content. Such results agree with the findings of Abduljabbar et al. [19] who reported 10.62% and 33.6% increases in the stability of Thin Asphalt Overlay (TAO) containing 2% and 4% of waste low-density polyethylene (w-LDPE), respectively. Meanwhile, adding LLDPE content in percentages of 6% decreased the stability results by 4.3% compared to the unmodified mix, as shown in Figure 5.

Findings show that adding LLDPE to asphalt mixtures can improve their stability. This is attributed to the remaining LLDPE not melting in the bitumen, which is essential to seal the mixture gaps [32]. Also, LLDPE that is not dissolved acts as an aggregate, so both types of polyethylene (dissolved and undissolved) help increase the Marshall stability of the asphalt. This stability is good for increasing the overall performance of the asphalt pavement. The amount of additive materials, the mixing process, and the mixing conditions are essential for improving the mixture’s stability and enhancing the modified asphalt’s stiffness [33].

3.2. Penetration

Figure 6 illustrates the profound impact of LLDPE particle modification on the penetration property of both modified and unmodified mixtures, which indicates mixture consistency. The results reveal a significant decrease in binder penetration due to LLDPE, particularly at 6% LLDPE content, where penetration decreases by almost 31.3% compared to the unmodified mix. This result agrees very well with the findings of Nizamuddin et al. [15] who reported a 32% decrease in the penetration value of asphalt mixes containing 6% of recycled LLDPE. This significant reduction confirms the remarkable effect of LLDPE on the controlling binder, by increasing the consistency of the mixture and thus making it more rigid and resistant to deformations [34]. This was due to the strong network formation of the polymer dispersed in the asphalt matrices. The decrease in penetration is also related to the diffusion of the oil fraction within the bitumen into the polymer phase, causing more significant interactions and swelling between the LLDPE polymer modifier and the polar molecules of the asphalt, as reported by Taibo et al. [35].

3.3. Softening Point

The softening point test evaluated the asphalt binder’s stability under high-temperature conditions. Figure 7 shows the softening point values for the modified mixtures with LLDPE and compares them with the reference mixture. Incorporating LLDPE significantly enhances all mixtures’ resistance to permanent deformation under high-temperature conditions. Asphalt modified with LLDPE at 2, 4, and 6 wt.% increased softening point by 7.8, 11.8, and 17.6%, respectively, compared to the reference mixture. Such enhancement percentages are lower than the findings of Nizamuddin et al. [15] who reported increases in the range of 12.24% and 89.8% in the softening point values of asphalt mixes containing 3% to 6% of recycled LLDPE. This difference might be attributed to the recycling of LLDPE which has not been performed in this study. This modification is attributed to the bitumen’s rubbery and elastic behavior, which makes it more resistant to temperature cracking and permanent deformation and enhances the material’s robustness, as reported in a similarly previous study conducted by Casey et al. [36].

3.4. Flow

As shown in Figure 8, the flow values increased for all asphalt mixes modified by polyethylene compared with the conventional unmodified asphalt mixture but still within the acceptable limits of the AASHTO and ASTM specifications (normally 2–4 mm as recommended by the General Specification for Roads and Bridges, GSRB.). Incorporating an LLDPE content of 2% led to increased flow values compared with the control mix by 9.3%. Also, increasing these particles by 4 and 6% of asphalt content increased the flow value compared with an unmodified mix by 22.9% and 30.6%, respectively. Such observations are in agreement with the finding of Abduljabbar et al. [19] who had demonstrated that utilization of waste LDPE polymer maintains the flow limits within the specified ranged recommended by GSRB, i.e., (2–4) mm. The increase in flow is attributed to the melting of some parts of the polyethylene in hot asphalt mixtures which enhances the flow properties of LLDPE asphalt mixes without altering the bitumen content [37]. As per the ASTM-D6927 standard [38], there is no ideal value for the mix flow; however, it should be within the acceptable limits (2–4 mm). Flow values below the upper limit indicates a plastic or unstable mix while mixes with flow values below the lower limits are considered brittle. In this study, the flow of the asphalt mix with 4% LLDPE is 3.17 mm which is the nearest to the average value of the upper and lower acceptable limits, i.e., 3 mm. This may indicate that such a mix with the average standard flow value is neither brittle nor plastic.

3.5. Air Voids (AVs)

The results in Figure 9 show the influence of increasing the polyethylene proportion in reducing air voids in asphalt mixtures. Incorporating the LLDPE particles by 2%, 4%, and 6% of asphalt content resulted in 4.6%, 4%, and 0.4% air voids values. This is attributed to the important role that the polyethylene plays in closing the mixture’s voids.

Abduljabbar et al. [19] reported air voids values of 3.1%, 3.6%, and 4.1% for Thin Asphalt Overlay (TAO) mixes containing 2%, 4%, and 6% of waste low-density polyethylene (w-LDPE), respectively.

Generally, an air void content below 4% is considered unsatisfactory due to the significance of air voids within the composition of asphalt mixtures to prevent bleeding under high temperatures and skid resistance [39]. Conversely, many air voids lead to harmful consequences such as layer weakening, asphalt oxidation, water seepage into underlying strata, and fissures and road collapses [40]. Therefore, the air void content should be around 4%. Based on the above, the best amount of LLDPE is 4%.

3.6. Voids in Mineral Aggregate (VMAs)

The voids in the mineral aggregate are critical factors in ensuring the durability of the asphalt mixture and improved rutting and cracking resistance. Figure 10 describes the relationship between the content of the LLDPE in mixes and the VMA values. The lowest average value of VMA was 14.1% for the 4% LLDPE content. The highest average of VMA was 18.2% for the 6% LLDPE content. The findings indicated that the value of VMA experienced a marginal decline as the concentration of LLDPE augmented, ultimately achieving a level of 14.1% at the optimal content of LLDPE at 4%. Subsequently, it increased to 18.2%, in contrast to 15% for the control mixture. So, it can be concluded that adding plastic to the bituminous mixture can overfill the voids between the mineral aggregates, thus improving the adhesion of the modified asphalt mixture and increasing durability [40].

3.7. GMM

Figure 11 presents the results of the theoretical maximum specific gravity, GMM, with LLDPE. The Maximum specific gravity of the non-modified mixture was documented as 2.487 g/cm³. At a 2% LLDPE content, there was a marginal increase in the GMM to 2.504. Introducing a 4% LLDPE content led to a slight decrease in the GMM to 2.495 g/cm³ (higher than the non-modified sample). Subsequently, increasing the LLDPE content to 6% resulted in a significant decrease in the GMM to 2.29 g/cm³ (representing an 8% decrease compared to the non-modified sample). That change will result in the modified asphalt having less weight, but the specific gravity of bitumen is still suitable [41].

3.8. Flash Point

All LLDPEs’ weight percentages fall within the standard range of flash point, but as decided based on the penetration test, 4% and 6% are optimum for LLDPEs. As shown in Figure 12, mixes modified with LLDPE at 2, 4, and 6 wt.% increased values by 0.9, 2.1, and 3.7%, respectively, compared to the reference mixture. The graphic trend confirms that the relationship between flash points and the addition of plastic particles aligns with what was found in previous studies [42].

3.9. Microstructural Analysis

This study utilized the SEM technique to investigate the micromorphology of asphalt mixtures. The microstructures, rheological properties, and modification mechanism of structural properties were also analyzed, as well as modified physicochemical and viscoelastic properties. Figure 13 and Figure 14 illustrate SEM images of modified and unmodified asphalt mixtures with various LLDPE content (0, 2, 4, and 6%) by weight of asphalt. SEM analysis is characterized by the ability to detect cohesion and adhesion between the binder and additive materials within the asphalt matrices [43]. It also reveals the optimum content to stabilize the bitumen and polymer network. The micrographs of the binder incorporating less than 4% polymer particles present a good dispersion in the asphalt matrix. This reassuring finding suggests that a lower LLDPE content can maintain appropriate dispersion. That is, a low LLDPE content of up to 4% is suitable for pavement engineering, as it significantly changes the rheology and viscosity of the base mixture and makes the hot asphalt mixture with added polymers more homogeneous. As demonstrated in literature, LLDPE particles have irregular shapes with highly porous structures, a cross-linked network of flakes, and rough surfaces in different interlayer systems [44]. These fine flakes contain columnar vertical crystalline SiO₂ whiskers on their surface and edges [45]. However, the activated amorphous SiO₂ truly impresses with its crucial role in the strong interaction with bitumen. This interaction ensures the polymer particles are evenly dispersed in the modified asphalt compound, a fundamental behavior of the material that showcases its impressive properties and could potentially inspire new approaches in the field. Also, with the help of EDX, the chemical composition can help pavement engineers understand the physiochemical behavior of the asphalt [46].

On the other hand, adding a higher concentration of LLDPE particles leads to accumulation, narrows the polymer’s dispersion, and produces various asphalt morphologies. The presence of air voids and irregular agglomerations is easily observed in images, which can reduce the performance and properties of the mixtures. The agglomeration phenomenon in bitumen may be attributed to the van der Waals attraction force, where the mixtures modified with large amounts of LLDPE particles appeared with rough surfaces that were unable to resist the crack propagation but were able to resist interface sliding [44]. So, the limited diffusion of polymer molecules on the asphalt surface indicates the low engineering properties of these binders.

These results agree with previous studies. In a study conducted by Olukanni et al. [47], it was observed that 5% LLDPE concentration was optimal for achieving greater dispersion. Microscopic analysis showed that the high LLDPE content was irregularly distributed in the asphalt binder and had rough surfaces. Other materials in the asphalt matrix were less compact and cohesive to each other. Because of this, the LLDPE-modified asphalt has an uneven particle shape and fragmented porous surface morphology. Thus, it forms an interconnected network, which reduces the crack length and increases the tensile strength of the asphalt binder.

Generally, using recyclable plastic materials in road construction is an environmentally friendly solution that reduces the amount of plastic waste in landfills and non-decomposable material discharge into the environment. It also extends the life of roads and reduces the usage of petroleum products, especially road paving materials. Adding polymeric plastics to bitumen mixtures improves their efficiency in terms of strength, crack resistance, durability, etc. This will reduce the increasing demand for bitumen, thus reducing costs and providing job opportunities for plastic waste collectors.

3.10. ANOVA Statistical Analysis Results

The stability, penetration, softening point, flow, AVs, VMAs, GMM, and flash point test results of all samples from different mixes are summarized in

Table 6. Stability results of all tested samples (kg).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
1625	1701	1825	1565
1638	1735	1857	1596
1660	1784	1865	1552

,

Table 7. Penetration values of all tested samples (0.1 mm).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
64.2	48.9	47.4	44.6
63.8	48.6	46.5	43.9
64	49.5	47.1	43.5

,

Table 8. Softening point results of all tested samples (°C).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
51	55	56.1	60
50	54	57.8	60.4
52	56	57.1	59.6

,

Table 9. Flow values of all tested samples (mm).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
2.55	2.80	3.19	3.38
2.61	2.84	3.14	3.40
2.58	2.82	3.17	3.34

,

Table 10. Air voids (AVs) values of all tested samples (%).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
4.8	4.6	3.9	0.5
4.7	4.7	4.1	0.3
4.9	4.5	4	0.4

,

Table 11. Voids in Mineral Aggregates (VMAs) Values of all Tested Samples (%).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
14.7	14.8	13.9	17.9
15.2	14.5	14.2	18.3
15.1	14.7	14.1	18.3

,

Table 12. Theoretical maximum specific gravity (GMM) values of all tested samples (g/cm³).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
2.542	2.438	2.389	2.275
2.536	2.428	2.398	2.282
2.542	2.430	2.395	2.286

and

Table 13. Flash point results of all tested samples (°C).

0% LLDPE (Control)	2% LLDPE	4% LLDPE	6% LLDPE
328	331	334	340
327	329	335	338
326	330	333	339

, respectively.

Table 14. Summary of ANOVA analysis results of all tests.

Test	Sum of Squares (SS)			Degrees of Freedom (df)			Mean Square (MS)		F Value	F Critical
	Between Groups	Within Groups	Total	Between Groups	Within Groups	Total	Between Groups	Within Groups
Stability	131,768	6026	137,794	3	8	11	43,922.75	753.25	58.31	4.07
Penetration	714	1.54	716	3	8	11	238.00	0.19	1236.36
Softening Point	128	5.78	134	3	8	11	42.75	0.72	59.17
Flow	1.13	0.01	1.13	3	8	11	0.38	0.0007	523.33
AVs	38.25	0.08	38.33	3	8	11	12.75	0.01	1275
VMAs	30.32	0.34	30.66	3	8	11	10.11	0.04	237.82
GMM	0.10	0.0002	0.10	3	8	11	0.03	0.00002	1489.95
Flash Point	243	8	251	3	8	11	81	1	81.00

summarizes the ANOVA analysis of all the aforementioned tests including the calculated values of all parameters involved in such statistical analysis (sum of squares (SS), degrees of freedom (df), and the mean square (MS) between and within groups). The calculated F statistical values of all tests are also shown and compared to the F critical value provided in the F distribution tables for specific degrees of freedom.

It could be noted from Table 14 that the calculated F values of all tests exceed the F critical value which is 4.07. This indicates that the null hypothesis should be rejected and hence there is significant differences between the mean test values of all groups. This demonstrated that the incorporation of LLDPE into asphalt mixes has a significant impact on all of their properties.

4. Conclusions

In line with sustainability goals of utilizing polymers to reduce the demand for conventional bitumen, this study investigated the performance of asphalt mixes incorporating LLDPE waste as a promising pathway for advancement within the pavement industry. The following conclusions can be summarized based on experimental study findings for LLDPE-modified asphalt mixtures compared to pure asphalt mixtures:

• LLDPE particles can be suitably utilized as an enhancer for bitumen mixes, enabling the sustainable management of polyethylene waste and improved performance of asphalt mixes;
• Based on the physical and rheological properties investigated in this study, it could be concluded that 4% LLDPE produces the best performance in asphalt mixtures;
• Marshall stability findings for asphalt containing LLDPE particles indicated a significant increase in the strength and performance of roads, paving the way for more durable and resilient road surfaces;
• An asphalt mix modified with 4% LLDPE particles by weight has a better stability value than a conventional mix. However, the modified mix’s flow is increased as the LLDPE percentage increases;
• The findings reveal a significant decrease in binder penetration due to adding LLDPE, particularly at 6% content;
• The results showed that adding LLDPE to the bituminous mixture can overfill the voids between the mineral aggregates and reduce air voids in asphalt mixtures compared with the unmodified mixtures;
• Microstructural analysis demonstrated that incorporating LLDPE into asphalt mixtures significantly changes the rheology and viscosity of the base mixture and makes the hot asphalt mixture with added polymers more homogeneous;
• The ANOVA analysis demonstrated that the incorporation of LLDPE into asphalt mixes has a significant impact on all of their properties.

Overall, the study demonstrated that the incorporation of LLDPE plastic waste into asphalt mixes provides a significant enhancement to their properties. Such an approach does not only provide cost saving to asphalt mixes but also helps in reducing the LLDPE industrial waste and utilizes it as useful addition to the construction materials. This process is in line with the green construction, sustainability, and environmental protection goals.

5. Limitations of the Study and Future Work

This study investigated the properties of asphalt mixes modified with different percentages of the plastic waste, linear low-density polyethylene (LLDPE), by conducting different laboratory tests namely, stability, penetration, softening point, flow, air voids, VMAs, GMM, and flash point tests. However, it did not evaluate the deformation (rutting) of such mixes which is a key parameter in designing the asphalt concrete pavement. Hence, future work will be directed towards investigating the deformation and rutting resistance of LLDPE-modified asphalt mixes considering different loading configurations (time, rate, and temperature) by performing the dynamic creep test and applying the repeated load permanent deformation (RLPD) testing method.