Modifying the engineering properties of naturally occurring bitumen for sustainable pavement construction

This study explores enhancing the engineering properties of naturally occurring bitumen by incorporating plastic waste, aiming for sustainable pavement construction. Key objectives include evaluating the rheological, stability, and microstructural characteristics to determine the optimal plastic content for modification. Experimental investigations involved ductility, flash point, penetration, Marshall stability, and flow tests, alongside scanning electron microscopy (SEM) and response surface analysis. Various polymer waste contents (0%–10%) were blended into bitumen using the wet method. Aggregate properties were analyzed per BS EN 933–1 standards, and the influence of polymer content was modeled to optimize the mix design. Incorporating 7.5% polymer waste significantly enhanced binder properties, improving stability, reducing air voids, and minimizing deformation. SEM revealed densified microstructures with reduced pore sizes. While 7.5% polymer addition optimized resistance to cracking and rutting, increasing content to 10% negatively affected penetration values and void content. Flash and fire points increased, enhancing safety against fire hazards. Response surface analysis confirmed the substantial impact of polymer content on stability, with a robust R-squared value of 0.9275. This study offers a cost-effective approach to utilizing waste plastic as a bitumen modifier, addressing environmental pollution while improving road construction materials, with promising implications for scalable and sustainable infrastructure development.

Introduction

Bitumen is a widely used engineering material with a global consumption of approximately 102 million tons annually (Yang et al., 2024a). It is a complex hydrocarbon mixture, either naturally occurring or pyrogenic, and is entirely soluble in carbon disulfide (ASTM International, 2021; Ajayi et al., 2023). Typically composed of 79%–88% carbon, 7%–13% hydrogen, and smaller amounts of sulfur, oxygen, and nitrogen (Kök et al., 2024), bitumen is extensively applied in road construction, roofing, and corrosion protection (Taneja and Kishore, 2024; Lo et al., 2019; Loto et al., 2021). Naturally occurring bitumen, which does not require distillation, is known for its durability and is found in various forms, including mineral deposits and asphalt lakes (Ajayi et al., 2023). Nigeria possesses significant bitumen reserves, with the first discovery recorded in Agbabu, Ondo State, in 1900 (Kök et al., 2024). The country holds the second-largest tar sand deposit globally, offering potential for road construction applications (Edoga et al., 2025). While research has explored synthetic modifiers like ethylene-vinyl acetate (EVA) and copolymers to improve bitumen properties (Guma et al., 2015; Pallavi et al., 2024a; Javadi et al., 2024; Francis, 2016; Ajayi et al., 2024; Milos, 2015; Ojeyemi et al., 2015; Yang et al., 2024b; Singh et al., 2024a; Kök et al., 2024), studies on the modification of naturally occurring bitumen remain limited.

A promising modification approach involves incorporating low-density polyethylene (LDPE) from Pure Water Sachets (PWS) to address plastic waste pollution. Nigeria generates approximately 5.96 million tons of plastic waste annually, with PWS contributing significantly due to their slow degradation (Asteray, 2024; Javadi et al., 2024; Singh et al., 2024b). These plastics often clog drainage systems and release toxic gases when incinerated (Singh et al., 2024c). Integrating PWS into bitumen presents a dual solution—enhancing pavement durability while mitigating environmental waste. Research indicates that plastic-modified bitumen improves load-bearing capacity, deformation resistance, and longevity while reducing road construction costs (Pallavi et al., 2024b; Taneja et al., 2024; Jexembayeva et al., 2024; Hamedi et al., 2020; Casey et al., 2008; Dalhat and Al-Abdul Wahhab, 2017; Piromanski et al., 2020; Alnadish et al., 2023; Rozeveld et al., 1997).

The incorporation of LDPE from PWS into naturally occurring Agbabu bitumen not only enhances pavement performance but also contributes significantly to environmental sustainability. By utilizing 5%–7.5% PWS, this study diverts approximately 50–75 kg of plastic waste per ton of bitumen modified, based on typical asphalt mix designs requiring 5%–6% binder content. This approach addresses Nigeria’s plastic waste challenge, where approximately 5.96 million tons are generated annually, with PWS constituting a significant portion. Recycling PWS into bitumen reduces landfill accumulation and mitigates pollution from incineration, which releases toxic emissions. Furthermore, the energy-intensive production of synthetic polymer modifiers, such as styrene-butadiene-styrene, emits approximately 2–3 kg CO2 per kg of polymer, whereas using waste LDPE reduces the need for virgin polymer production, potentially lowering CO2 emissions by up to 10%–15% per ton of modified bitumen, depending on processing efficiencies. These sustainability benefits, combined with the reduced demand for virgin materials, position PWS-modified bitumen as an environmentally viable solution for sustainable pavement construction, aligning with global circular economy goals.

Despite its benefits, challenges remain in optimizing naturally occurring bitumen with waste plastics. Research gaps include limited long-term performance evaluations, inadequate environmental and economic assessments, and the absence of standardized guidelines for incorporating plastic waste into bitumen. Addressing these concerns requires characterization studies, field trials, and standardized policies to ensure large-scale adoption. Strategically combining Nigeria’s natural bitumen with waste plastic modifiers presents a sustainable solution for road construction. Future research should focus on refining formulation techniques, ensuring compatibility, and validating long-term performance, ultimately contributing to a more resilient infrastructure system.

Materials and methods

Materials

The materials used in this study includes bitumen, coarse aggregates and PWS (LDPE). These materials were used in the production of the surface course in flexible pavements. The naturally occurring bitumen was extracted from Agbabu in Ondo state, Nigeria. The bitumen was collected from a well situated deep within the forest at the heart of the bitumen seepages were there had been 15 boreholes drilled earlier by past researchers and investigators. PWS of low-density polythene is predominantly characterized by hydrocarbon chains similar to that of bitumen with a melting point of approximately 110 °C and density range of 0.89–0.91 which followed a similar trend which was used as a modifier (Jimoh, 2010). It is a tough, insoluble material at room temperature but becomes soluble at high temperatures in the presence of aromatic hydrocarbons (Javadi et al., 2024). The aggregates used for this research were both coarse and fine aggregates obtained from Ota, Ogun state, Nigeria.

Sample preparation

The bitumen sample was of a lustrous black appearance, occurring in semi-solid form (Figure 1), it was heated to form a homogenous sample. The polymer modified bitumen (PMB) was prepared by blending the shredded pure water sachets Figure 2 with the bitumen through the wet blending method at 140 °C–160 °C to ensure fluidity and compatibility with the polymer’s melting point (∼110 °C). The wet method involves the polymer added directly to the heated bitumen before adding the aggregate to the binder. The method is also referred to as the ‘cooking method’ because it requires a simple cooking device, i.e., cooking stove. The set up comprised of a metal container, a heater, and a stirrer with shear blade operating at approximately 700 RPM to produce shear forces in the mixer blending process. It is important to note that not all types of polymers can be blended using this process as it is difficult to control the blending temperature and produce the required shear force. (Ayodele, 2017; Abul, 2012) However, dry mixing was not adopted because it does not facilitate a deep physico–chemical reaction between the bitumen and plastic polymer. The bitumen sample was heated and poured into 8 metal containers which had been weighed previously and then labelled accordingly: sample A 0%, 2.5%, 5%, 7.5% and 10%. Each bitumen was then weighed and the shredded PWS were added to their respective percentages according to the weight of the bitumen. In this research, the PWS were collected from the wastes of households in the community of Canaan Land, Ota Ogun State, Nigeria. After collection, the materials were washed, sun-dried and then shredded into fine pieces of 2–3 mm in size. During the mix, the shredded plastic sachets were added slowly and continuously stirred for 20–30 min until the material was homogenous throughout.

Figure 1

Figure 1. Bitumen sample.

Figure 2

Figure 2. Shredded PWS.

Test and methods

The aggregate gradations were determined in accordance with British Standards Institution (2012). Tests were carried out on the naturally occurring bitumen to determine the rheology. Softening point test was carried out according to ASTM International (2020) while the viscosity test was done according to ASTM International (2022). Penetration test (a measure of the consistency of the bitumen in a decimal meter) was carried out on the bitumen sample in accordance with BS EN 1426 (2015) (Airey, 1997; Brit ish Standards Institution. BS EN 1426, 2015). The test was carried at 0, 2.5, 5, 7.5% and 10% of the bitumen waste. The rheology was also done at the percentage additions.

Marshall stability was done on the asphalt mixture with the naturally occurring bitumen and the polymer modified bitumen according to ASTM International ASTM D6927 (2015). The samples (cores) for the Marshall test were prepared by blending the aggregates as shown in Table 1. Then, 1,200 g of the blended aggregates were weighed along with 4% bitumen according to the weight of the aggregates. The aggregates were heated to a temperature of 150 °C before being mixed with the bitumen. A crater was formed in the aggregate, and the 90/100 penetration grade bitumen heated to 150 °C was added. The aggregates and the bitumen were mixed until all the aggregates were properly coated (Ogundipe, 2019). The samples for the Marshall test were prepared by blending the aggregates as shown in Table 1. Tests were conducted in triplicate to ensure consistency and reliability.

Table 1

Table 1. Aggregate tests.

Result and discussion

Test on the aggregate

The particle shape of the aggregate determines the performance and workability of asphalt mixtures. It can be deduced from the results of the dry mechanical analysis of aggregates for the shredded PWS samples according to AASHTO American Association of State Highway and Transportation Officials (2016), and American Association of State Highway and Transportation Officials (2022) that the gravel is uniformly graded. The majority of the particles fall between 4.75 and 9.5 mm sieves which is the nominal aggregate size. When particles are angular rather than thin and flat, they exhibit better performance against stress; therefore, engineers recommend the application of angular particles. Particles with angular shapes, which is a typical property of crushed stones, have stronger interlocking and better performance than round ones, and, consequently, better resistance against the rutting brought about by heavy and repetitive loading. The highest passing percentage was within 19.0 mm sieve which is the maximum aggregate size (Figures 3, 4). The aggregate gradations were determined in accordance with British Standards Institution (2012).

Figure 3

Figure 3. Sieve Analysis of the coarse aggregate.

Figure 4

Figure 4. Sieve Analysis of the fine aggregate.

Physical tests on the aggregates

The composition of the asphalt concrete mix composition was prepared to fall within the Federal Government of Nigeria’s Specifications for Road and Bridges (Federal Government of Nigeria, 1997). The asphalt mixture was prepared as shown in Table 1. The calculations and results are presented and calculated according to the specified standard by AASHTO T84 for fine aggregate samples and AASHTO T85 for coarse aggregate (American Association of State Highway and Transportation Officials, 2016; American Association of State Highway and Transportation Officials, 2022). Table 1 showed the result of the bulk specific gravity, aggregate bulk SSD specific gravity, and apparent specific gravity.

Penetration test result for agbabu bitumen

The result for the penetration value of naturally occurring bitumen is shown in Table 2. The natural Agbabu bitumen exhibited an average value of 78 mm at 25 °C, classifying it as 70/80 grade, which aligns with AASHTO standards for suitability in pavement construction. Upon modification with PWS (LDPE), penetration values increased up to 7.5% polymer content before decreasing at 10%. This initial increase may be attributed to the partial solubilization and dispersion of LDPE chains into the bitumen matrix at moderate concentrations, which temporarily enhances fluidity and reduces stiffness by disrupting asphaltene aggregations without fully cross-linking the structure. However, at higher contents (e.g., 10%), polymer agglomeration likely occurs, leading to phase separation and a stiffer, less penetrable binder.

Table 2

Table 2. Penetration value for natural bitumen sample.

This trend partially deviates from typical findings in polymer-modified bitumen studies, where penetration generally decreases linearly with increasing plastic waste due to the formation of a rigid polymer network that hardens the binder. For instance, in a study by Javadi et al. (2024) on waste plastic bag-modified bitumen, penetration decreased from 70 to 80 dm m to lower values with 2%–8% addition, attributed to enhanced intermolecular forces and viscosity. Similarly, Alnadish et al. (2023) reported reduced penetration in LDPE-modified binders, improving rutting resistance. In contrast, our results with natural Agbabu bitumen suggest a unique interaction, possibly due to its higher natural resin content (as noted in Ojeyemi et al. (2016), for Agbabu samples), which facilitates better initial compatibility with LDPE at 7.5%, leading to an optimal balance. This highlights the need for site-specific optimization when using natural bitumens, as opposed to refined ones.

Penetration value for polymer modified bitumen

The result from the penetration tests performed on the polymer modified bitumen are specified in Table 3. The result of the analysis showed that the bitumen sample is of 70/80 penetration grade this is in accordance with previous findings of Fadairo et al. (2015), and Ojeyemi et al. (2016). However, the addition of polymer to the sample showed an improved penetration at 7.5%. Nevertheless, there is a decrease in the penetration value on the addition of 10% polymer content. Therefore, it can be concluded that the addition of plastic waste to bitumen increase the penetration, but higher plastic waste content from 10% and above would have an adverse effect by reducing the values.

Table 3

Table 3. Penetration value for polymer modified bitumen.

The observed trends in penetration values can be attributed to the chemical interactions between LDPE and the bitumen matrix. At 2.5%–7.5% PWS, LDPE’s hydrocarbon chains partially solubilize into the bitumen’s lighter fractions (aromatics and resins), disrupting asphaltene aggregations and enhancing fluidity, as evidenced by the peak penetration at 7.5% (Table 3). The high resin content in Agbabu bitumen, as noted by Ojeyemi et al. (2016), acts as a compatibilizer, promoting uniform dispersion of LDPE and enhancing compatibility. However, at 10% PWS, phase separation occurs due to polymer agglomeration, where excess LDPE exceeds the bitumen’s capacity for solubilization, forming undispersed clumps that increase stiffness and reduce penetration. Optimizing blending conditions, such as increasing shear rate or mixing time, could mitigate agglomeration and enhance compatibility at higher polymer contents.

Softening point result for the polymer modified bitumen

Figure 5 presents the variation in the softening point of polymer-modified bitumen with increasing waste plastic content. The softening point of the unmodified bitumen was 35.5 °C, which slightly decreased to 34.75 °C at 5% PWS before rising to a peak of 41.5 °C at 7.5% and dropping to 38 °C at 10%. The initial minor decrease at low polymer content could result from incomplete melting and dispersion of LDPE, creating localized soft spots within the matrix. At 7.5%, the polymer likely achieves uniform integration, forming a cross-linked network that increases the binder’s molecular weight and thermal resistance, requiring higher temperatures to induce flow. The decline at 10% may stem from oversaturation, where excess polymer forms undispersed clumps, reducing overall homogeneity and thermal stability.

Figure 5

Figure 5. Variation of softening point for sample a with PWS (%) content.

These changes are consistent with the stiffening effect of polymers observed in prior research, though the non-linear trend in our study adds nuance. Yang et al. (2024a) found that polymer-modified asphalts exhibit increased softening points (up to 10 °C–15 °C higher) due to enhanced viscoelasticity from polymer entanglement, which mirrors our peak at 7.5%. Likewise, Kök et al. (2024) reported softening point elevations in waste photopolymer-modified bitumen, linking it to improved high-temperature performance and reduced deformation. However, studies on natural bitumens, such as Fadairo et al. (2015) on Nigerian deposits, show similar variability, with softening points increasing only after optimal modifier dosages (e.g., 4%–6% for copolymers), supporting our identification of 7.5% as the threshold for LDPE in Agbabu bitumen. This comparison underscores the environmental benefits of waste LDPE, as it achieves comparable enhancements to synthetic polymers while addressing plastic pollution.

The increase in softening point up to 7.5% PWS reflects the formation of a cross-linked polymer network, where LDPE chains interact with bitumen’s aromatic and resin fractions through van der Waals forces and weak polar interactions, increasing the binder’s molecular weight and viscoelasticity. The high resin content in Agbabu bitumen enhances compatibility, facilitating uniform polymer dispersion and maximizing thermal stability at 7.5% PWS, as supported by Yang et al. (2024a). However, at 10% PWS, oversaturation leads to phase separation, where undispersed LDPE clumps disrupt the matrix’s homogeneity, reducing thermal stability (Figure 5). This phase separation arises from the saturation of bitumen’s lighter fractions, limiting further polymer integration, as noted by Kök et al. (2024). To address this, additives like maleic anhydride could enhance LDPE-bitumen bonding, reducing the risk of agglomeration at higher polymer contents.

Ductility of polymer modified bitumen

Figure 6 illustrates the variation in bitumen ductility with increasing waste plastic content. The results show that at 0% plastic, the ductility is highest at 20 cm. A sharp decline to 8 cm is observed at 5% plastic, followed by a partial recovery to approximately 14 cm at 7%–10%. These trends indicate that the addition of plastic affects bitumen flexibility. The significant drop at 5% suggests a stiffening effect, while the increase at 7%–10% may be attributed to improved plastic dispersion and interaction within the bitumen matrix. Overall, the findings suggest that waste plastic reduces ductility, with a critical decrease at 5%, whereas 7%–10% appears to balance flexibility and strength more effectively. The optimal plastic content for maintaining ductility is likely within the range of 5%–7.5%, providing a compromise between mechanical performance and workability. The ductility or brittleness of bitumen is influenced by its rheological properties, grading, and hardness, which collectively determine its structural behavior under varying conditions.

Figure 6

Figure 6. Variation of Ductility with PWS content (Sample B).

Flash and fire point test values for the polymer modified bitumen

The flash and fire points of polymer-modified bitumen increase with rising PWS content, demonstrating enhanced thermal stability and safety. At 0% PWS, these values are approximately 175 °C and 200 °C, respectively, reaching around 250 °C at 10% PWS. This consistent rise suggests that PWS significantly improves bitumen’s resistance to ignition and combustion, thereby reducing fire hazards on pavements and contributing to safer road surfaces (Figure 7).

Figure 7

Figure 7. Flash and fire point test vs. PWS content (sample b).

Asphalt test on the polymer modified bitumen

The bulk density analysis (Figure 8) shows a decrease in compacted asphalt mix density from 2.79 g/cm³ to 2.32 g/cm³ as plastic waste content in bitumen increases. This reduction suggests that the lower-density plastic decreases overall compactness, potentially enhancing air void formation or altering the mixture’s structural arrangement, which in turn affects its weight, stability, and mechanical performance. The observed trend indicates that incorporating plastic waste influences the internal structure of the asphalt mix, potentially modifying its durability and long-term performance characteristics (Ojeyemi et al., 2016).

Figure 8

Figure 8. Bulk density against PWS content sample B.

Marshall Stability and flow test for the polymer modified sample

The plot of Marshall Stability values versus % PWS is shown in Figure 9. The stability values increase up to 5% PWS, reaching approximately 8 kN, before slightly decreasing at 7.5% and 10%. The flow values remain relatively constant, while air void percentages show a slight reduction as PWS content increases. These observed trends are summarized in Table 4. The results indicate that incorporating 5% PWS achieves the highest stability, suggesting an optimal balance between load-bearing capacity and resistance to flow. In contrast, higher PWS levels may lead to reduced stability, potentially due to saturation effects or inadequate dispersion of the plastic within the mix.

Figure 9

Figure 9. Marshall stability and flow properties.

Table 4

Table 4. Summary of the bitumen test in relation with standards.

Figure 10 shows that voids in mineral aggregate (VMA) increase with PWS content up to 6%, peaking at approximately 18%, before decreasing at 10% due to increased plastic-bitumen cohesion, which restricts void formation. This suggests that moderate PWS content enhances air void accommodation and improves workability, but excessive polymer addition reduces permeability and compactibility. An optimal PWS content of 6% ensures a balance between air voids and structural integrity. Similarly, Figure 11 illustrates that voids filled with bitumen (VFB) increase from 58% at 0% PWS to 75% at 10%, indicating enhanced bitumen coating and better compaction. This progressive increase improves pavement durability by minimizing unfilled voids, thereby reducing susceptibility to moisture-induced damage and enhancing long-term performance. In conclusion, The void in the mineral aggregate increases from 2.5% PWS content to 7.5% (Figure 11). At further addition of the plastic waste, the void in the mineral aggregate reduced. However, the lowest percentage of the voids filled with mineral was at 2.5% addition of PWS but it the highest was at 10%.

Figure 10

Figure 10. Voids in mineral aggregate.

Figure 11

Figure 11. Voids filled with bitumen.

Proposed pavement application

Increasing PWS content enhances stability and reduces flow, significantly improving resistance to cracking, deformation, shear stress, and rutting. Consequently, PWS-modified bitumen binders exhibit greater stiffness, which strengthens the asphalt matrix and enhances long-term rutting resistance, making it more suitable for high-traffic pavements and extreme weather conditions.

Optimal plastic content variations and practical implications

The study identified different optimal plastic waste (PWS, LDPE) contents for various properties of the modified Agbabu bitumen: 5% for Marshall stability (Figure 9) and 7.5% for penetration (Table 3) and softening point (Figure 5). These variations arise due to the distinct sensitivities of each test to the polymer-bitumen interaction and the specific performance attributes they measure. The Marshall stability test evaluates the load-bearing capacity and resistance to deformation of the asphalt mixture, which is influenced by the aggregate-binder interaction and the overall mix cohesion. At 5% PWS, the polymer enhances the binder’s stiffness and improves aggregate coating, leading to peak stability (∼8 kN), as the polymer chains form a reinforcing network within the bitumen matrix without causing excessive stiffness. Beyond this point, at 7.5% and 10%, stability slightly decreases, likely due to polymer agglomeration or reduced workability, which disrupts uniform dispersion and weakens the mix’s structural integrity.

In contrast, penetration and softening point tests assess the rheological properties of the bitumen binder alone, focusing on its consistency and thermal stability. The penetration test measures the binder’s hardness, with higher values indicating a softer material. At 7.5% PWS, penetration peaks, suggesting an optimal balance where LDPE chains partially solubilize into the bitumen, reducing asphaltene aggregation and enhancing fluidity without compromising cohesion. Similarly, the softening point, which indicates resistance to thermal deformation, reaches its maximum (41.5 °C) at 7.5% PWS due to the formation of a cross-linked polymer network that increases the binder’s molecular weight and viscoelasticity. The decline at 10% PWS in both tests likely results from oversaturation, where excess polymer forms undispersed clumps, reducing homogeneity and causing phase separation, as supported by Yang et al. (2024), who noted similar saturation effects in polymer-modified asphalts.

The difference in optimal PWS content (5% vs. 7.5%) reflects the distinct roles of the binder and the asphalt mixture in pavement performance. Marshall stability prioritizes the composite behavior of the asphalt mix, where aggregate interactions dominate, and excessive polymer content may hinder compactibility, as seen in the increased voids in mineral aggregate (VMA) at higher PWS levels (Figure 10). Conversely, penetration and softening point focus on the binder’s standalone properties, where higher polymer content (7.5%) optimizes molecular interactions before reaching saturation. This aligns with findings by Javadi et al. (2024), who reported optimal LDPE contents of 4%–6% for stability in asphalt mixes but higher percentages (up to 8%) for binder-specific rheological improvements. Similarly, Kök et al. (2024) found that waste polymer modifiers enhance thermal properties at slightly higher dosages than those optimal for mix stability, supporting our observations.

For practical use in pavement construction, the 5% PWS content is recommended as the more reliable option. This recommendation is based on the Marshall stability test’s direct relevance to asphalt mixture performance under traffic loading, which is critical for pavement durability and resistance to rutting and cracking. The 5% PWS content achieves the highest stability (8 kN) and balances flow properties, ensuring workability and structural integrity, as evidenced by the reduced air voids and optimal voids filled with bitumen (VFB) at this level (Figure 11). While 7.5% PWS enhances binder-specific properties like penetration and softening point, these improvements are less critical for overall pavement performance, as the asphalt mix’s behavior under load is primarily governed by stability and aggregate interactions. Furthermore, the slight decline in stability at 7.5% and the risk of phase separation at higher contents (e.g., 10%) suggest that exceeding 5% PWS may compromise long-term performance due to reduced flexibility and potential brittleness, as noted in Alnadish et al. (2023). Thus, 5% PWS offers a practical and robust compromise, aligning with field-applicable mix design requirements and ensuring cost-effective, sustainable pavement construction with enhanced environmental benefits through plastic waste utilization.

Analysis of variance

The input parameter which is most significant on the marshal stability is input parameter B which is the addition of polymer waste. It has the highest F-value of 23.5 and minimum Prob > F value (see Table 5). The R-Squared value of 0.9275 showed the robustness of the model. The mathematical relationship between the input parameters is as shown in Equation 1. The ANOVA table evaluates the significance of input parameters A (Penetration) and B (Polymer Waste) on Marshall Stability. Parameter B (Polymer Waste) is the most significant factor, with the highest F-value of 23.5 and the lowest Prob > F value (0.04), indicating its strong influence. Parameter A shows a lower F-value of 12.75 and a marginal Prob > F value (0.0702), making it less significant. The model’s R-squared value of 0.9275 and Adjusted R-squared of 0.8549 demonstrate that the model explains 93% of the data variability, indicating strong reliability. The derived equation shows Polymer Waste has a greater impact on the response, with a coefficient of 25.4145 compared to 4.1955 for Penetration. This highlights the substantial effect of increasing polymer content on improving Marshall Stability. Overall, the results confirm that polymer addition is the key factor for enhancing bitumen mix performance.

Table 5

Table 5. ANOVA for response surface linear model.

$\begin{array}{ll}\hfill \text{Response}& =+314.84687+{4.1955}^{*}\text{\hspace{0.17em}Penetration}\hfill \\ \hfill & +{25.4145}^{*}\text{\hspace{0.17em}Polymer}\hfill \end{array}(1)$

Response surface analysis

Figures 12, 13 demonstrate how penetration grade and polymer waste content affect Marshall Stability. The contour plot (Figure 12) reveals a positive correlation between penetration grade and stability, with higher penetration values enhancing load-bearing capacity. The 3D surface plot (Figure 13) confirms this trend, showing stability peaks at about 7.5% polymer content, after which it plateaus or slightly declines. This suggests that higher penetration improves binder cohesion and flexibility, strengthening the mixture’s structural integrity. The peak at 7.5% polymer content points to a synergistic effect between the polymer and bitumen, likely improving polymer dispersion and bonding. Beyond this point, stability may decline due to saturation effects, incomplete polymer dispersion, or polymer agglomeration, which could disrupt mixture uniformity and reduce cohesion. The response surface analysis emphasizes the need to optimize polymer content for maximum mechanical performance. Exceeding the optimal polymer dosage may reduce stability due to material incompatibilities or excessive stiffness. Therefore, balancing penetration grade and polymer content is crucial for enhancing the mixture’s durability and load-bearing capacity. These findings suggest that a higher penetration grade combined with 7.5% polymer waste content maximizes Marshall Stability. However, exceeding the optimal polymer level can degrade performance by causing poor dispersion, agglomeration, increased stiffness, and higher air voids, all of which compromise flexibility and structural integrity. Chemically, excessive polymer content saturates bonding capacity, leading to phase separation and reduced compatibility, making the binder more brittle and prone to cracking (Fadairo et al., 2015).

Figure 12

Figure 12. Contour plot for the marshal stability.

Figure 13

Figure 13. Contour plot for the marshal stability.

Marshall Flow

The graph in Figure 14 illustrates the effect of polymer content and penetration grade on the Marshall Flow of the mixture. It is evident that Marshall Flow increases with the addition of polymer and higher penetration grade. At lower polymer levels (0%–2.5%) and lower penetration grades (231–240), the flow values remain minimal, ranging between 71.43 and 76.39. As both parameters increase, the contour lines indicate a steady rise in Marshall Flow, reaching its highest values in the upper-right region (95.4–95.9), where polymer content is 10% and penetration grade is around 276. The steep gradient of contour lines in the upper-right portion highlights the significant sensitivity of Marshall Flow to changes in polymer and penetration grade. This suggests a synergistic effect when both parameters are increased simultaneously. Conversely, regions with low polymer content and penetration grade show gradual changes, implying a weaker influence on Marshall Flow in those areas.

Figure 14

Figure 14. Contour plot for the marshal stability.

In practical applications, increasing polymer content enhances Marshall Flow, which improves stability and rutting resistance. However, a balance must be maintained, as excessively high values could affect other properties like stiffness. Therefore, an optimal range of polymer content (5%–7.5%) and moderate penetration grade (245–260) is recommended for a stable asphalt mixture. The result in Table 6 shows the penetration grade has the highest influence on the marshal flow of the modified sample from the F-value. The R-squared value of 0.95 showed the robustness of the model developed. Equation 2 shows the mathematical relationship between the assessed parameters.

Table 6

Table 6. Analysis of variance table [Partial sum of squares].

$\text{Marshall}\text{Flow}=-81.52554+0.64067*\text{PG}+15.7934*\text{PW}-0.056886*\text{PG}*\text{PW}(2)$

Scanning electron microscopy result of the modified sample

The SEM images of the unmodified and polymer-modified bitumen provide critical insights into the microstructural changes induced by the addition of 7.5% low-density polyethylene (LDPE) from Pure Water Sachets (PWS). Figure 15 illustrates the microstructure of unmodified Agbabu bitumen, revealing a heterogeneous surface characterized by an irregular, wrinkled texture with prominent folds and voids. Quantitative analysis of the SEM image indicates an average void size of approximately 5–10 µm in diameter, with an estimated void area fraction of 15%–20% across the scanned surface. These voids suggest weak points that could compromise mechanical stability under load, as they act as stress concentration sites prone to crack initiation. Minor cracks, ranging from 2 to 5 µm in width, are also visible, alongside localized areas of densification with reduced pore sizes (1–3 µm). These structural heterogeneities likely contribute to variable mechanical performance, with voids reducing load-bearing capacity and densified regions enhancing localized strength, as noted in prior studies on natural bitumens (Rozeveld et al., 1997).

Figure 15

Figure 15. Microstructure of the unmodified bitumen.

In contrast, Figure 16 depicts the microstructure of bitumen modified with 7.5% PWS, showing a significantly smoother and more uniform surface with enhanced compactness. The void area fraction is reduced to approximately 5%–8%, with average void sizes decreasing to 2–4 μm, indicating a substantial densification effect. This reduction in void content is attributed to the LDPE acting as a filler, filling interstitial spaces within the bitumen matrix and promoting a more cohesive structure. The modified bitumen exhibits a finer texture, with a random string-like fibrillar network, likely resulting from polymer-bitumen interactions at the molecular level. These string-like structures, approximately 1–2 µm in width, suggest improved polymer dispersion and bonding, enhancing the material’s resistance to deformation and moisture infiltration. Notably, cracks are nearly absent in the modified sample, with only occasional micro-fissures (<1 µm) observed, indicating improved structural integrity.

Figure 16

Figure 16. Microstructure of the modified bitumen.

Quantitative density changes were further assessed by comparing the relative surface compactness between the two samples. Using image analysis software, the modified bitumen showed a 25%–30% increase in dense phase coverage compared to the unmodified sample, corroborating the observed reduction in voids and improved load distribution. This densification aligns with findings by Alnadish et al. (2023), who reported that LDPE modification reduces porosity in bitumen, enhancing rutting resistance due to a more interconnected matrix. Similarly, Jexembayeva et al. (2024) noted that recycled PET plastics in bitumen create a denser microstructure, reducing crack propagation under cyclic loading. The spherulitic texture of the LDPE, as observed in our study, further supports the coarsening of the fibrillar network, which enhances mechanical strength and thermal stability, consistent with Rozeveld et al. (1997).

However, challenges remain in achieving uniform polymer dispersion, as minor localized agglomerations were observed in some regions of the 7.5% PWS sample, potentially due to variations in mixing conditions. These findings underscore the importance of optimizing blending parameters to ensure homogeneity. Compared to unmodified bitumen, the 7.5% PWS-modified sample demonstrates superior microstructural integrity, with reduced voids, minimal cracking, and enhanced densification, making it more suitable for pavement applications under heavy loads and extreme weather conditions. Future studies should quantify crack propagation rates and void distribution under dynamic loading to further validate these improvements.

The overall results from the experimental test shows that plastic-modified bitumen, mixed with aggregates for asphalt, improved stability, load-bearing capacity, and resistance to deformation and cracking, with 7.5% plastic content proving optimal. This modification enhanced Marshall stability, thermal resistance, and microstructural integrity while reducing air voids. Challenges included achieving uniform dispersion of plastic, with higher contents (e.g., 10%) causing poor mixing, phase separation, and increased stiffness, reducing flexibility. These improvements make pavements more durable and resistant to rutting and cracking under heavy loads, supporting sustainability. However, careful optimization of polymer content and blending methods is essential to ensure uniformity and maximize the benefits of modified asphalt mixtures. Based on the results of the study, the polymer-modified bitumen demonstrated significantly higher Marshall stability compared to unmodified bitumen, enhancing resistance to deformation and cracking.

This improvement stems from better densification and the reinforcing effect of the polymer. The flow properties of the modified bitumen were also optimized, achieving an ideal balance between deformation and stiffness. The modified bitumen showed a higher softening point (41.5 °C), indicating superior thermal stability and resistance to high-temperature deformation, reducing risks such as rutting. While unmodified bitumen had greater ductility, the modified version balanced flexibility and strength, ensuring improved load-bearing capacity and durability. Microstructural analysis revealed the modified bitumen’s compact structure with fewer voids, enhancing load distribution and resistance to deformation. It is evident that the addition of 7.5% polymer significantly enhances bitumen’s thermal stability, mechanical strength, and structural integrity, addressing the limitations of unmodified bitumen. These improvements highlight the potential of polymer-modified bitumen for more durable and sustainable pavement solutions.

Conclusion

This study evaluated the rheological and mechanical properties of naturally occurring Agbabu bitumen modified with low-density polyethylene (LDPE) from pure water sachets (PWS) for sustainable pavement construction. The addition of 7.5% PWS optimized bitumen properties, achieving a 70/80 penetration grade, a softening point of 41.5 °C, and enhanced Marshall stability, indicating improved resistance to deformation. Microstructural analysis via SEM revealed a denser, smoother matrix with reduced voids at 7.5% PWS, suggesting better load-bearing capacity. However, 10% PWS led to reduced penetration and stability, likely due to polymer agglomeration. These findings demonstrate that 5%–7.5% PWS enhances bitumen’s thermal stability and mechanical performance while repurposing plastic waste, supporting sustainability.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

RL: Formal Analysis, Investigation, Validation, Visualization, Writing – original draft, Writing – review and editing. AB: Conceptualization, Formal Analysis, Investigation, Methodology, Project administration, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing. HB: Formal Analysis, Investigation, Methodology, Supervision, Validation, Visualization, Writing – original draft. OD: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Resources, Validation, Visualization, Writing – original draft.

Funding

The authors declare that no financial support was received for the research and/or publication of this article.

Acknowledgments

Acknowledgements

The authors sincerely appreciate Covenant University for their support towards the success of the research.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Generative AI statement

The authors declare that no Generative AI was used in the creation of this manuscript.

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