Evaluating and optimizing NBR-modified bituminous mixes: a rheological and RSM-based study

Abstract

Bitumen shows visco-elastic behavior, exhibiting both elastic and viscous properties as predicted by dynamic response and phase angle. Modern asphalt bituminous pavements face issues such as early-stage fatigue cracks, rutting, and permanent deformations due to low-temperature cracking, high-temperature deformation, moisture susceptibility, and overloading. These pavement distresses result in the formation of potholes, alligator cracks, and various deformations, which accelerate the need for rehabilitation and maintenance. To address these concerns, this study focused on utilizing Nitrile Butadiene Rubber derived from surgical gloves as an additive in conventional asphalt pavements to assess its effect on stiffness. Nitrile Butadiene Rubber was added in intervals of 2%, 4%, 6%, and 8% to conventional bituminous pavement. The rheological properties, marshall properties, dynamic modulus, and phase angle were evaluated for varying percentages of Nitrile Butadiene Rubber at different temperature, and frequency. The dynamic response was determined using a simple performance tester at four different temperatures (4.4 °C, 21.1 °C, 37.8 °C, and 54.4 °C) and six different frequencies (0.1, 0.5, 1, 5, 10, and 25 Hz). Response surface methodology was employed to establish a relationship between input and output variables and to optimize the amount of Nitrile Butadiene Rubber in the mix based on dynamic modulus and phase angle. The study concluded that adding up to 6% of Nitrile Butadiene Rubber improved Marshall stability, while higher percentages led to reduced stability. A similar trend was observed in the dynamic modulus, which peaked with the addition of 6% Nitrile Butadiene Rubber, regardless of frequency and temperature. The response surface methodology model indicated that coupling the percentage of Nitrile Butadiene Rubber with frequency increased the dynamic modulus at a constant temperature, with the highest value occurring at 4.4 °C. However, the dynamic modulus decreased as the temperature rose for the same combinations of Nitrile Butadiene Rubber percentages and frequencies. Numerical optimization suggested that a maximum of 5.9% Nitrile Butadiene Rubber should be added to achieve the highest dynamic modulus and lowest phase angle.

Introduction

Flexible pavements are vital to transportation infrastructure, designed to withstand varying loads and environmental conditions. Their performance and durability depend on the materials used, which must demonstrate adequate flexibility and resilience under traffic loads. Over the past several years, road infrastructure is often subjected to significant wear and tear due to heavy traffic loads, extreme weather conditions, and other environmental factors¹, which results in compromising the main aim of safe vehicle movement and comfort due to fatigue crack, rutting, potholes and several other type of pavement distresses^2–5. The combination of this heavier traffic with the harsh climatic conditions causes coatings to prematurely deteriorate, crack in cold weather and undergo rutting at high temperatures^6,7. Reports state that over 17 million tons of rubber trash is produced globally each year^8,9. The data from the global statistics for American Chemistry Council on the amount of plastic that is produced, recycled, burned with energy recovery, and dumped says that about 35.7 million tons of plastics were produced in the United States in 2018, accounting for 12.2% of the country’s municipal solid wastes (MSW) production. Only 8.7% of the plastics produced are recycled, while the remaining 75.6% and 15.7% are dumped into landfills and burned with energy recovery respectively¹⁰. As per UN evaluations, over 3.3 million tons of plastic are discarded annually in Pakistan. Without intervention, it is estimated that plastic waste will surge to 12 million tons per year by 2040. This escalation will result in severe environmental and public health repercussions¹¹. Hence, waste disposal poses a significant threat to the ecosystem, highlighting the necessity for a concerted effort to address the effective management of plastic waste¹². To advance transportation research, engineers are exploring optimal solutions for repurposing waste plastic¹³. Consequently, the use of waste materials in pavement construction has received a lot of attention recently.

Researchers have been making significant efforts to investigate the potential of utilizing various types of rubbers and thermosets to improve the performance of pavements as an alternative material for road construction¹⁴. To enhance the performance of bitumen, pavements have utilized various modifiers and additives, including polymers, oxidants, antioxidants, chemical modifiers, expanders, anti-stripping additives, and hydrocarbons¹⁵. Furthermore, several studies were carried out for pavement modification with the inclusion of recycled rejuvenators^16,17 where variety of virgin polymer types, including polyethylene, styrene-butadiene-styrene (SBS), crumb rubber, and silica fumes^18–21 were used which resulted in on hand increase in the characteristic of bitumen/pavements. Researchers have employed different types of rubber waste in their studies on thermal behavior²². In the present study, nitrile butadiene rubber (NBR) is used as an additive for modifying pavement. Over many years, NBR has been widely used in industry due to its cost benefits, high resistance to oils, good processibility, and low abrasions^23–26. A study shows that NBR inclusion significantly improves strength, where 7.5% of NBR increased fracture toughness up to 0.30 MPa²⁷. The utilization of waste nitrile butadiene rubber (NBR) combined with ethylene vinyl acetate (EVA) in a 5% NBR/EVA blend resulted in reduced penetration, suggesting an augmentation in the hardness of the modified bitumen²⁸.Besides this, incorporating NBR in flexible pavements there is a high likelihood of reduced maintenance when compared to bituminous roads²⁹. The improved road surface can also lead to fuel savings and a reduction in carbon footprint, making it an environmentally friendly option and result in improving the mechanical performance of the mix²². Typically, studies have investigated how elastomers like polypropylene, vulcanized natural rubber, and epoxidized natural rubber^30–32. Other studies also show that with the inclusion of NBR, the density of modified mixed increased³³. Moreover, when the fines content percentage was at 40%, the minimal void ratio was at its lowest value³⁴.

Sudani et al.³⁵, presented the results of a laboratory experiment examining the effects of high temperatures on the properties of modified road bitumen, incorporating waste NBR from shoe soles. They experimented with different blends by varying the NBR content and adjusting the mixing time. The manufacturing characteristics, such as viscosity at 135 °C and storage stability, were evaluated. Additionally, rheological properties were analysed using dynamic mechanical methods with the Dynamic Shear Rheometer (DSR). The findings indicated good workability for all samples and showed improvements in key rheological parameters, including shear resistance and high-temperature rutting resistance, particularly in samples with a higher content of NBR waste. A study by³⁶ utilized NBR in combination with three different thermosets exploring a total of 81 different combinations of additive in bitumen. It was concluded that the indirect tensile strength and tensile strength ratio increased by about 60%. Moreover, the modified samples showed high stability and achieved average marshall stability of 25KN. The positive impact of NBR on the tensile strength of bituminous mixes was validated by another study³⁷, where mixes with NBR in combination with SBR showed high tensile strength.

Abedini et al.³⁸ explored the use of NBR as a bitumen modifier, focusing on the rheological behavior of bitumen emulsions modified with NBR latex. Various percentages of NBR latex were incorporated into bitumen emulsion samples, and both standard and modified residues were evaluated using conventional and DSR techniques. The addition of NBR latex led to an increase in the softening point of the bitumen emulsion residue, accompanied by a reduction in penetration grade and temperature sensitivity. Ultimately, it was found that bitumen emulsions containing more than 6% NBR showed improved rutting resistance at elevated temperatures, as demonstrated through DSR temperature sweep analysis.

A study by³⁹ evaluated the incorporation of NBR and SBS in bituminous mixes. Volumetric assessments and Marshall tests were performed on HMA samples with varying dosages of NBR and SBS to identify the optimal combination. The findings indicate that incorporating 6% NBR and 5% SBS provides the best results. This combination yielded the highest stability and the lowest flow, leading to an increased stiffness and a significant improvement in strength. Additionally, the temperature sweep curves from the DSR test demonstrated superior rutting resistance at elevated temperatures. These claims were supported by a study by⁴⁰. The study concluded that the addition of NBR to the modified mix leads to the highest level of stability and the lowest flow values when the concentration of NBR falls within the range of 5–10%. The sweep curves showed that within this specific concentration range, the material exhibits superior performance in terms of stability while maintaining minimal flow. These findings highlight the effectiveness of NBR in optimizing the mechanical properties of the mix under varying temperature conditions.

Research significance

This study’s primary goal was to create and characterize outcomes from NBR, which offers notable resistance to thermal and fatigue cracking and allowing it to better accommodate dynamic loads and vibrations under different climatic conditions. Other objectives of this study were to transform waste into a valuable resource with a novel approach, leading to more durable and eco-friendlier road infrastructure. Moreover, this study focused on evaluating and optimizing the bituminous mixes modified with NBR based on Marshall and rheological properties.

Materials and methods

Aggregate gradation

The coarse and fine aggregates used in the study were as per the standards of national highways authority (NHA), Pakistan. Some quality tests were performed to validate aggregate’s properties which can be seen in Table 1.

Table 1.

Aggregate’s properties.

Test Description	Results (%)	Range Limit % (Max.)	Test Standards
Los Angeles Abrasion	22.6	45	ASTM C 131
Impact Value	15.8	3.0	BS 812
Flakiness Index	8.79	15	ASTM D 4791
Elongation Index	5.16	15	ASTM D 4791
Specific Gravity	2.70	-	ASTM C 127
Water Absorption (Coarse)	0.27	3.0	ASTM C 127
Water Absorption (Fine)	2.37	3.0	ASTM C 128

Aggregate gradation as per the standards of NHA was adopted to develop bituminous mixes. Final aggregate gradation was selected based on the defined limits. Summary of selected design and allowed limits can be seen in the Fig. 1 where upper limits defines the maximum amount of passing, lower limits defines the minimum amount of percent passing and the design line presents the schematics followed in this study against every size of aggregate.

Fig. 1.

Aggregate’s gradation.

Nitrile butadiene rubber

Polymer-based material used in the study consists of nitrile butadiene rubber (NBR) was exported from Malaysia and shredded into a grain size of 0.85 mm. NBR is an oil-resistant synthetic rubber produced from a copolymer of acrylonitrile and butadiene which possess properties like abrasion resistance, tear resistance, water and oil resistance, adequate resilience and tensile strength, and more^25,26. It is used in manufacturing fuel hoses, gaskets, rollers, and medical gloves etc. This study the acquired NBR in the form of disposable medical gloves were shredded into fine sizes and presented in Fig. 2. Different percentages of NBR were added into bitumen to assess its effect on penetration, ductility, softening points and flash/fire point of the bitumen. The effect of NBR on the properties of bitumen can be seen in Table 2.

Fig. 2.

Nitrile Butadiene Rubber.

Table 2.

Effect of NBR on Bitumen.

Consistency Tests	NR 0%	NR 2%	NR 4%	NR 6%	NR 8%
Penetration @ 25 °C	62	58	51	47	38
Softening Point (°C)	51	52	53	55	57
Ductility (cm)	> 100	14	16	14.7	13.8
Flash/Fire Point Test(°C)	263/278	269/281	271/283	267/276	262/270

Optimum binder content

Bitumen having penetration grade 60/70 was used in this study and procedure adopted by⁴¹ was employed for optimum binder content (OBC) calculation. Three trail samples with 75 number of blows on each side were prepared with bitumen content combinations of 3.5%, 4%, 4.5%, 5%, and 5.5% with the help of marshall compactor. The maximum specific gravity and bulk density were calculated using ASTM D2041⁴² and ASTM D2726⁴³, respectively. Optimum marshall parameters were found to be at 4.3% of bitumen content which was adopted as OBC for the rest of the study.

Marshall testing

Marshall samples with approximate diameter of 63.5 mm and thickness of 101.3 mm were prepared for five different combinations of NBR against OBC. Three samples for each combination of NBR varying from 0% NBR (control) until 8% with increment of 2% were tested for calculating the marshall stability and flow as per ASTM D113-17⁴⁴. An automatic marshall testing machine was used to assess the marshall stability for the prepared bituminous mixes. The specimen’s vertical deformation, represented as marshal flow is also measured using a dial gauge reading.

Sample performance test

Dynamic modulus and phase angle were determined for varying NBR percentage at different temperature and frequencies. Samples were tested at four different temperatures i.e. 4.4, 21.1, 37.8, and 54.4 °C and six different frequencies i.e. 25, 10, 5, 1, 0.5. and 0.1 Hz using “asphalt mixture performance tester (AMPT)” as shown in Fig. 3. AMPT comprises a trail axial cell, a hydraulic actuator with a pump, a temperature-controlled environmental chamber for specimen placement, refrigeration for temperature reduction, heating units for temperature increase, and a computer system for controlling and collecting data. It enables the performance evaluation of asphalt mixtures under various loading and temperature conditions.

Fig. 3.

Asphalt mixture performance tester.

Response surface methodology (RSM)

RSM was used to statistically analyze asphalt mix’s dynamic modulus and phase angle at different frequencies and temperature for bituminous mixes with varying NBR using design expert software. The design expert software is a powerful and versatile tool used in the field of experimental design and analysis. It offers a wide range of experimental design methodologies, including factorial designs and response surface methodology (RSM). Overall, design expert software is a valuable resource for experimental design and analysis, empowering users to make informed decisions, optimize processes, and enhance product development in a wide range of other industries than engineering. In this study modeling of dynamic modulus factors was performed on the basis of statistic-fit, diagnostic plots and statistical fit model. To predict the mechanical characteristics of asphalt concrete, linear models and quadratic polynomials were suggested based on the model importance. The responses were predicted using a second-degree polynomial Eq. 1. Performance of the developed model was assessed using various statistical measures shown Eqs. 2–4. The performance of the model based on statistical fit can be seen in Table 3.

1

2

3

4

Table 3.

Model’s performance in terms of statistical fit.

FIT STATISTICS
Std. Dev.	2039.26	R2	0.9085
Mean	6259.15	Adjusted R2	0.9011
C.V %	32.58	Predicted R2	0.8918
		Adeq. Precision	36.1984

Where:

DM = 

dynamic modulus.

t =

 temperature.

f = 

frequency.

Nr =

NBR percentage.

=

actual and predicted output

=

mean values of actual and predicted outputs and,

n, p  =

number of observation and input variables respectively.

Master curve analysis

The information is displayed on a single graph, usually on a logarithmic scale, with the temperature acting as the controlling parameter on the x-axis and a material quality or behavior, like toughness or strength, on the y-axis. The determined shift parameters and master curves can be utilized as inputs for the design guide and for the viscoelastic study of materials. The dynamic modulus data are shifted using a nonlinear optimization by concurrently solving shift parameters, and the parameters of the master curve model are then fitted using the least-squares method with the design expert program in this study, 21 °C was used as the reference temperature in this work. E master curves for every mixture were created using the time-temperature superposition approach at a reference temperature of 21 °C⁴⁵. The master curve was represented by shifting the data at different temperatures in accordance with frequency until the curve combined into a single sigmoidal function. This was accomplished by using a second-order polynomial relationship between the logarithm of the shift factors, The process of achieving time-temperature superposition involved concurrently computing the coefficients of the sigmoidal function and the Eq. 5 and Eq. 6.

5

Where:

• E*: The dynamic modulus of the asphalt mix at a given temperature and frequency.

• E_min: The minimum modulus, representing the long-term behavior of the asphalt at very low loading frequencies or high temperatures.

• E_max: The maximum modulus, representing the short-term behavior of the asphalt at very high loading frequencies or low temperatures.

• β (beta): A shape parameter that controls the steepness of the sigmoidal curve. It determines how rapidly the dynamic modulus changes from low to high values.

• γ (gamma): A shape parameter that controls the position of the curve along the log(ω_r) axis, affecting how the dynamic modulus behaves with changing frequency.

• ω_r (omega_r): The reduced frequency, which adjusts the measured frequency to a reference temperature using time-temperature superposition principles.

In addition, the shift factor (a(T)) is often incorporated to adjust the frequency data from different temperatures onto a single curve, allowing for a unified representation of dynamic modulus across temperatures. An alternative shift factor relationship based on the Arrhenius equation, which is presented in equation, can still be used to develop a dynamic modulus master curve (Bonaquist 2008)⁴⁶.

6

Where:

• ωr​: The reduced frequency, which is the equivalent frequency at a reference temperature (Tr). It allows data from different temperatures to be shifted to a reference temperature, creating a single curve.

• ω: The actual frequency at which the test or measurement is conducted, typically in Hertz (Hz).

• ΔEa​: The activation energy for the material, representing the energy required to overcome molecular resistance to deformation. This is usually specific to the material being tested (in this case, bitumen or asphalt mixtures). It’s typically measured in kilojoules per mole (kJ/mol).

• 19.14714: A constant, which is derived from the gas constant and a conversion factor. It is used to simplify the relationship between temperature and the shift factor in the context of bituminous materials.

• T: The absolute temperature (in kelvins, K) at which the measurement is conducted.

• Tr​: The reference temperature (also in kelvins, K), at which the data from other temperatures are shifted to align with a master curve. Typically, this is chosen as a central or moderate temperature for the specific material.

Numerical optimization

Numerical optimization was done to assess different combinations of NBR at different temperatures and frequency to optimize the mix for highest dynamic modulus and lowest phase angle. Six different frequencies and four different temperatures at maximum dynamic modulus and lowest phase were evaluated to calculate the NBR%. Based on dynamic modulus and phase angle, the best combination of NBR with temperature and frequency was recommended as optimum combination. Figure 4 presents the overall methodology followed in this study.

Fig. 4.

Methodological flowchart.

Results and discussions

Marshal testing

The Marshall stability test, based on ASTM D6927-15⁴⁷ demonstrated that the inclusion of NBR in the bituminous mix as a modifier significantly influenced the mechanical performance of the asphalt. Various proportions of NBR, ranging from 0 to 8%, were added at 2% increments to assess the effect on Marshall stability and flow, critical indicators of the material’s ability to resist deformation under loading. The results showed that the maximum load-bearing capacity, or Marshall stability, was 12.1 kN at 6% NBR, which marked a significant improvement compared to the virgin bitumen mixture. This increase in stability can be attributed to the interaction between the NBR polymer and the bitumen matrix, where the similar chemical structure of NBR and bitumen allows for better compatibility. The rubber’s elastomeric properties enhance the stiffness and durability of the mix by reinforcing the bitumen and increasing its resistance to external stresses such as heavy traffic loads and environmental conditions.

The improvement in stability at 6% NBR suggests that this proportion of the modifier optimally balances flexibility and stiffness, resulting in a mixture that can better withstand rutting and deformation. However, as the NBR content increased beyond 6%, the stability decreased, dropping to 11.23 kN at 8% NBR. This decline indicates that excess NBR may lead to over-stiffening, reducing the mix’s ability to absorb stresses and leading to potential brittleness. Over-modification can create an overly rigid structure, making the pavement more susceptible to cracking under cyclic loading or extreme temperature variations, where elasticity is crucial for performance. In terms of flow values, the modified bituminous mix containing 6% NBR exhibited a minimum deformation of 2.63 mm, which is lower than that of the virgin bitumen. The reduction in flow values correlates with the increased stiffness imparted by the NBR. A lower flow value suggests that the mixture is more resistant to deformation, a key characteristic in preventing rutting under heavy traffic.

The elastomeric NBR acts as a reinforcing agent within the bitumen, enabling the mix to recover more efficiently from loading deformations. This is particularly advantageous in regions subjected to high traffic volumes and heavy loading, as it minimizes the risk of permanent deformations such as rutting. The chemical affinity between NBR and bitumen plays a central role in these performance enhancements. NBR, a synthetic rubber known for its resistance to oils and heat, complements the viscoelastic nature of bitumen. This synergy improves the rheological properties of the mixture, allowing it to perform better under both high and low temperatures. The increase in stability and the reduction in flow values further highlight the importance of optimizing NBR content to balance stiffness and flexibility. The results from the 6% NBR modification point to the most favorable combination of stability and deformation resistance, making it the ideal quantity for improving the overall durability of the asphalt mixture. A Summary of corrected stability and flow can be seen in Table 4; Fig. 5.

Table 4.

Effect of NBR on Marshall parameters at optimum asphalt content of 4.3%.

NBR %	Air Voids %	VMA %	VFA%	Stability (KN)	Flow (mm)
0	4	15.10	71.00	10.17	2.63
2	4	14.11	72.13	10.37	2.71
4	4	13.23	71.31	11.87	2.82
6	4	13.00	71.22	12.10	2.93
8	4	13.13	70.87	11.23	3.01

Fig. 5.

Effect of NBR on marshall properties.

Master curve analysis

This study demonstrates that the impact of varying NBR modifier percentages on the dynamic modulus (∣E*∣) of the asphalt mixtures was highly dependent on the combination of NBR content, temperature, and frequency. The dynamic modulus, which reflects the stiffness of the mixture, varied significantly with different NBR percentages, as illustrated in Fig. 6. The results indicate that the mixture containing 4% NBR exhibited the highest dynamic modulus (|E*|) across the tested frequencies, particularly at mid-range frequencies. This increase in stiffness is attributed to the polymer’s ability to interact with the bituminous matrix, enhancing its viscoelastic properties and improving its resistance to deformation under load. The 4% NBR content appears to strike an optimal balance between flexibility and stiffness, enabling the mixture to absorb and distribute stresses effectively while maintaining its structural integrity. This is particularly important for applications where rutting resistance is a priority, as stiffer materials are less likely to deform permanently under traffic loads.

Fig. 6.

Master curve analysis.

When the NBR content was increased to 6%, the dynamic modulus decreased slightly at middle frequencies compared to the 4% mixture but showed better performance at lower and higher frequencies. This behavior suggests that the 6% NBR modifier provides a more uniform response across a broader range of loading conditions. The reduction in ∣E*∣ at mid-frequencies can be explained by the increased elastomeric content, which introduces more flexibility into the mixture. While this may slightly reduce stiffness at certain frequencies, the flexibility conferred by the NBR enhances the mixture’s ability to recover from deformations, improving its fatigue resistance over time. Additionally, the 6% NBR mix performed well at lower and higher frequencies, indicating that it may be better suited for a wider range of operating conditions, particularly in environments with variable loading patterns. In contrast, the mixture containing 8% NBR exhibited lower dynamic modulus values at intermediate and high temperatures compared to the 4% and 6% mixtures. While higher NBR content typically increases the elasticity of the mix, excessive amounts can lead to a reduction in stiffness, especially at elevated temperatures. This is due to the over-softening of the binder, which can diminish the material’s ability to resist deformation under sustained loads. The behavior of the 8% NBR mix suggests that while it provides greater flexibility, it may not offer the same level of structural support as lower NBR percentages, particularly in warmer climates where temperatures can cause the binder to soften excessively.

The observed performance of the NBR-modified mixtures can be explained through the time-temperature superposition principle, which indicates that materials behave differently under varying temperature and loading conditions. As temperatures increase, the stiffness of the asphalt mixture tends to decrease, especially for mixtures with higher NBR content. The 4% and 6% NBR mixtures, however, maintained higher dynamic moduli at lower temperatures, making them more resistant to low-temperature cracking and deformation. This is a critical advantage for regions where pavements are exposed to both low and high-temperature extremes. The NBR polymer modifies the base asphalt by providing several key benefits: it enhances stiffness through a reinforcing mechanism, fills voids within the aggregate structure, and enables the mixture to recover elastically after loading. The elastomeric nature of NBR allows the mixture to deform temporarily under stress and then return to its original shape, which is particularly beneficial for preventing permanent deformations such as rutting. At the same time, the increase in stiffness conferred by NBR helps the pavement resist dynamic loads, improving overall durability.

Response surface modeling (RSM)

This study reveals that the effects of varying NBR modifier percentages on the stiffness of asphalt mixtures are not uniform across all combinations of temperature and frequency. The dynamic modulus (|E*|), which is a critical indicator of the mixture’s ability to resist deformation under loading, was significantly influenced by the percentage of NBR, as shown in Fig. 6. The mixture containing 4% NBR exhibited the highest dynamic modulus across most frequencies, indicating that this NBR content maximizes stiffness while maintaining flexibility. At mid-range frequencies, the 4% NBR modifier provided the best balance between elasticity and resistance to deformation, making it highly effective in enhancing the stiffness of the mixture. This suggests that at 4%, NBR optimally interacts with the bitumen matrix, reinforcing the asphalt without causing excessive rigidity. This enhanced stiffness is particularly beneficial for increasing rutting resistance, as the material can better withstand the heavy loads typically experienced by road surfaces. When the NBR content was increased to 6%, a different pattern emerged. While the 6% NBR mixture exhibited a slightly lower dynamic modulus at mid-range frequencies compared to the 4% mix, it showed better performance at lower and higher frequencies.

This behavior suggests that the 6% NBR mixture is more versatile across a wider range of loading conditions, offering a broader dynamic response. The increase in NBR content introduces more elasticity into the mixture, which enhances the mixture’s ability to recover from deformations. This is crucial for long-term pavement durability, as it helps mitigate fatigue cracking, especially under cyclic loading conditions. However, the results also show that at high frequencies or low temperatures, the 6% NBR content delivers stiffness levels comparable to those of the 4% mix, demonstrating that it is highly adaptable to different stress environments. In terms of temperature susceptibility, the 6% NBR mixture maintained its dynamic modulus relatively well at both lower and higher temperatures, though the modulus decreased slightly as temperatures rose, following the time-temperature superposition principle. In contrast, the mixture with 8% NBR exhibited a lower dynamic modulus at intermediate and high temperatures compared to the 4% and 6% mixtures. While higher NBR content typically enhances flexibility, excessive amounts can lead to a reduction in stiffness. This is because at 8%, the elastomeric content becomes too dominant, resulting in an overly soft mixture that lacks the necessary stiffness to resist deformation at high temperatures.

The reduced stiffness at 8% NBR, especially at higher temperatures, suggests that over-modification can weaken the structural integrity of the mixture, making it more prone to permanent deformation under sustained loads. The relationship between NBR content, temperature, and frequency is further illustrated in Fig. 7, where the dynamic modulus increases with NBR content and frequency but decreases beyond 6% NBR at 4.4 °C. The maximum dynamic modulus was observed at 6% NBR and 25 Hz at 4.4 °C, highlighting this combination as the optimal point for enhancing stiffness and performance. As the temperature increased, the overall effect of NBR and frequency on dynamic modulus became less pronounced, with the lowest modulus observed at 54 °C, 8% NBR, and 0.1 Hz. This finding underscores the importance of temperature control in ensuring that NBR-modified mixtures perform optimally, as higher temperatures tend to soften the binder and reduce the mixture’s stiffness.

Fig. 7.

Dynamic modulus vs. (a) NBR, (b) Temperature, (c) Frequency.

The Response Surface Methodology (RSM) provided a deeper understanding of how NBR percentage, frequency, and temperature interact to influence dynamic modulus. One-to-one relationships revealed that the dynamic modulus increased with NBR content up to 6%, after which it began to decline, indicating that 6% NBR is the optimal level for maximizing stiffness without sacrificing flexibility. Similar trends were observed for frequency, where an increase led to a gradual rise in dynamic modulus, though the effect of frequency was less significant than that of NBR content. Conversely, temperature showed an inverse relationship with dynamic modulus, with increasing temperatures leading to a decline in modulus values. The NBR-modified mixtures showed significant improvement in performance with up to 6% NBR, beyond which the benefits diminished due to over-softening. The study highlights that careful optimization of NBR content is essential for achieving the desired balance of stiffness, flexibility, and durability. The 4% and 6% NBR mixtures emerged as the most effective in enhancing rutting resistance and stiffness across a range of temperatures and frequencies, while higher NBR content (8%) may lead to reduced performance under certain conditions. This information is crucial for tailoring asphalt mixtures to specific environmental and loading conditions to improve the longevity and resilience of road surfaces.

Figure 8 presents relationship between dynamic modulus with NBR and frequency with varying temperature. From Fig. 8a, it can be seen that the dynamic modulus increases with increase in NBR and frequency and shows a decline upon increase in NBR beyond 6% at 4.4 °C. Maximum dynamic modulus was observed at 6% NBR and 25 Hz frequency at 4.4 °C. The overall relation of frequency and NBR with dynamic modulus was the same but the intensity of its effect decreased with increase in temperature. The lowest dynamic modulus was found at 54 °C, 8% NBR and 0.1 Hz. Frequency as shown in Fig. 8d.

Fig. 8.

Dynamic modulus vs. NBR and Frequency at (a) 4.4 C, (b) 21 C, (c) 37.8°C, (d) 54°C.

Numerical optimization of data

Numerical optimization was carried out to determine the optimal quantity of NBR for varying frequencies and temperatures, based on the Response Surface Methodology (RSM) model developed during the study. The optimization aimed to maximize the dynamic modulus and minimize the phase angle, two critical parameters for assessing the stiffness and flexibility of asphalt mixtures. This approach allowed for a systematic evaluation of the most effective NBR content under different environmental and loading conditions. The results from the optimization process shown in Table 5 indicated that at the lowest temperature (4.4 °C) and lowest frequency, the optimal NBR content was 4.4% by weight. This combination provided the best balance between stiffness and elasticity, particularly at low temperatures where the pavement is prone to cracking. The ability of the NBR-modified asphalt to maintain a high dynamic modulus at lower temperatures suggests that NBR enhances the material’s resistance to low-temperature thermal cracking, a common issue in cold climates. The flexibility provided by NBR at this content allows the pavement to withstand contraction and expansion cycles without cracking, improving overall durability. At higher temperatures, the optimal NBR content increased, reaching a maximum of 5.9% at 21 °C and a frequency of 10 Hz. This finding highlights the need for increased NBR content as the temperature rises to maintain sufficient stiffness. At 21 °C, the bitumen becomes more fluid, and the higher NBR content helps counteract this by providing additional rigidity and resistance to deformation. The increase in frequency also necessitates a higher NBR percentage, as the asphalt is subjected to more frequent loading cycles, which can lead to permanent deformation (rutting) if the material is not stiff enough.

Table 5.

Numerical optimization based on max. Dynamic modulus and min. Phase angle.

Numerical Optimization
Temp °C	Freq Hz	N.R%	Temp °C	Freq Hz	N.R%
4.40	0.1	4.40	37.8	0.1	4.19
4.40	0.5	4.42	37.8	0.5	4.18
4.40	1.0	4.42	37.8	1.0	4.20
4.40	5.0	4.45	37.8	5.0	4.20
4.40	10	4.45	37.8	10	4.20
4.40	25	4.67	37.8	25	3.69
21.0	0.1	4.48	54.4	0.1	3.80
21.0	0.5	4.54	54.4	0.5	3.60
21.0	1.0	5.48	54.4	1.0	3.50
21.0	5.0	5.49	54.4	5.0	3.48
21.0	10	5.90	54.4	10	3.42
21.0	25	5.80	54.4	25	3.40

The optimization also revealed that the relationship between NBR content, frequency, and temperature is non-linear. At higher temperatures, such as 37.8 °C and 54 °C, the optimal NBR content showed a different trend. While higher NBR percentages were needed at moderate temperatures to balance flexibility and stiffness, the maximum NBR content decreased as both frequency and temperature reached their upper limits. This suggests that beyond a certain point, adding more NBR becomes counterproductive, as the mixture may become too flexible, reducing its ability to maintain the required stiffness at elevated temperatures. The lowest proportion of NBR was recommended at the highest frequency and temperature, further emphasizing that excessive NBR can negatively affect the mixture’s performance under these conditions. The trade-off between dynamic modulus and phase angle is central to understanding these results. The phase angle represents the lag between the applied stress and the strain in a viscoelastic material, with lower values indicating better elasticity and quicker recovery after deformation. The optimization process found that increasing the NBR content generally reduced the phase angle, improving the mixture’s elasticity. However, beyond 5.9% NBR, the phase angle reduction was offset by a decline in dynamic modulus, indicating that excessive NBR could lead to over-softening of the binder, reducing the material’s overall stiffness.

Conclusions

This study comprehensively evaluated the use of Nitrile Butadiene Rubber (NBR) as a modifier for flexible pavement, intending to improve structural performance and durability. Various NBR proportions (2%, 4%, 6%, and 8%) were systematically incorporated into the asphalt mix, and their effects on key performance indicators like Marshall stability, dynamic modulus, and phase angle were examined across a range of temperatures and frequencies.

• The experimental results showed that adding NBR up to 6% significantly increased Marshall stability, peaking at 12.1 kN. This optimal NBR content provided a balance between stiffness and flexibility, crucial for resisting deformation and enhancing rutting resistance. Beyond 6% NBR, the stability declined, indicating potential over-stiffening and brittleness.

• In terms of dynamic modulus, the 6% NBR mixture demonstrated the best performance across all tested frequencies and temperatures, making it highly resistant to fatigue and deformation, particularly under low-temperature conditions. The dynamic modulus decreased with increasing temperatures, reinforcing the importance of temperature control in ensuring optimal performance.

• Numerical optimization using Response Surface Methodology (RSM) identified the ideal NBR content for maximizing dynamic modulus and minimizing phase angle. The model suggested 5.9% NBR as optimal for moderate temperatures (21 °C) and frequencies (10 Hz), while lower temperatures (4.4 °C) required 4.4% NBR to balance stiffness and elasticity. At higher temperatures, excessive NBR content reduced performance, indicating the need for precise calibration of NBR content to avoid over-softening.

• Qualitatively, the study highlighted NBR’s ability to enhance pavement durability by improving resistance to rutting, fatigue, and thermal cracking, all while contributing to sustainability through the use of recycled materials. The research provides a clear framework for optimizing NBR-modified asphalt mixtures, making them ideal for various environmental and traffic conditions.

• From the numerical optimization, it was concluded that a maximum of 5.9% NBR could be added if it is intended to achieve the highest dynamic modulus and lowest phase angle. Future studies should focus on long-term aging, durability, and environmental impacts to fully unlock the potential of NBR in creating more resilient and sustainable road infrastructure.

Recommendations

While this study demonstrated the potential of Nitrile Butadiene Rubber (NBR) as a modifier for asphalt mixtures, certain areas require further investigation to fully understand its long-term benefits and limitations. Specifically, future research could explore the following aspects:

1. Aging and Durability: Although this study focused on short-term performance metrics, understanding the long-term aging and durability of NBR-modified asphalt under real-world conditions is crucial. Long-term testing would provide insights into how the material’s properties evolve over time, especially when subjected to environmental stressors.

2. Performance Under Varied Load Conditions: Extending the scope of performance testing to include different traffic loading scenarios, such as high traffic volumes or heavy loads, would help assess the full potential of NBR-modified asphalt in diverse road conditions. Tests such as the HAMBURG WHEEL TRACKER and flow time tests could be valuable for evaluating fatigue resistance and deformation.

3. Environmental and Sustainability Impact: A deeper analysis of the environmental impact of using NBR, especially in terms of waste reduction and sustainability, would be beneficial. Comparing the environmental footprint of NBR-modified asphalt to conventional mixes could provide valuable insights into its role in promoting eco-friendly pavement solutions.

4. Interaction with Aggregates: Further exploration of the interaction between NBR and different types of aggregates commonly used in asphalt mixtures could help optimize the mixture design. Examining how NBR-modified asphalt performs with varying aggregate gradations and surface properties may reveal opportunities for improving the material’s overall structural integrity.

Author contributions

The authors confirm their contribution to the paper as follows: study conception, design and paper writing: Inamullah Khan and Zahoor; data collection: Zahoor; analysis and interpretation of results: Muhammad Imran Khan, Mujahid Ali, Nasir Khan, Manidurai Paulraj, Siva Avudaiappan, Nasir Khan, Manidurai Paulraj, Siva Avudaiappan review draft manuscript preparation. All authors reviewed the results and approved the final version of the manuscript.

Data availability

The datasets used and/or analysed during the current study are available from the corresponding author on request.

Declarations

Competing interests

The authors declare no competing interests.