Evaluating the effect of paper waste lignin in hot mix asphalt

Asphalt is a viscoelastic material which performs to resist rutting, fatigue cracking, and moisture susceptibility under different loading and temperature conditions. The use of innovative and renewable pavement construction materials is inevitable due to high axle loads, rapidly increasing traffic volumes, and varying climatic conditions. This study aims to assess the effect as well as the optimum dosage of paper waste lignin for use in hot mix asphalt (HMA). Lignin from the paper industry with dosage ratios of 5, 10, 15, and 20%, was utilized to study the effect of the addition of lignin to the asphalt binder. Virgin and lignin-modified binder samples, before and after the aging process, were subjected to physical testing through penetration, softening point, ductility, viscosity and specific gravity and rheological characteristics through dynamic shear rheometer (DSR), bending beam rheometer (BBR), and rational viscometer (RV). The fractional composition was assessed through saturates, aromatics, resins and asphaltenes (SARA) fractional composition technique. Statistical analysis was also performed to find correlation of different physical and rheological parameters. Furthermore, based on optimum dosage, the performance of asphalt mixtures was studied against rutting, fatigue cracking, and moisture susceptibility. The results indicated that the addition of lignin has improved the physical properties significantly. The amount of asphaltene decreased and aromatics increased in SARA fractional analysis. Moreover, the Colloidal Instability Index (CII) has also indicated a stable structure of the binder. The rheological characteristics are improved after modification. The asphalt mixture tests revealed that addition of lignin with optimum dosage (10%) has improved the performance against rutting, fatigue cracking and moisture susceptibility. Statistical analysis indicated good co-relation among different physical and rheological parameters. This study concludes that 10% dosage is the optimum dosage that can successfully replace the virgin asphalt binder for performance of hot mix asphalt.


Introduction
Hot mix asphalt (HMA) or asphalt mixture is composed of bitumen binder and a combination of fine and coarse aggregates along with a certain amount of air voids with or without admixtures (Al-Bayati Tighe and Achebe 2018).The bounded asphalt binder and mineral aggregate form a stone-like structure that gives strength and toughness to the pavement system (Saha et al 2017).Due to rapidly growing urbanization, depletion of natural resources, greenhouse gases, lack of energy resources, and increase in oil prices throughout the world, the pavement industry is facing shortage and unavailability of energy resources and adhesive materials (Hasaninia and Haddadi 2017).Furthermore, the pavement construction industry is facing two major problems related to flexible pavements; rutting and fatigue cracking due to the viscoelastic behavior of bitumen in response to temperature and heavy loading.Research on sustainable and green pavements is underway throughout the world due to the greenhouse effect, depleting natural resources, premature pavement failures and bio-waste issues (Borghi et al 2017, Angius 2018).Advancement in pavement structure designs, materials production technologies and innovation in building materials is inevitable due to high axle loads, rapidly increasing traffic volumes and varying climatic conditions (Saha et al 2017).Such scenarios have compelled the researchers produce to develop innovative binding materials from renewable energy resources for use in HMA to resist early failures (Menglan et al 2017), (Wang et al 2020).
The innovative materials including recycled asphalt shingle (RAS), warm mix asphalt (WMA) and bio-asphalt from bio-waste can lead to sustainable and greener pavements along with dealing with above-stated performance issues (Zhou et al 2020, Ragab andEltawab 2018.The modifier is meant to modify certain properties of asphalt binder; the rejuvenator is to improve, enhance, or recover certain properties like aging while binder is meant to perform as a binding material like bitumen.The characteristics of bio-oils or bio-additives are greatly dependent upon the source and way of their production but their composition is very close to that of asphalt bitumen in terms of SARA fractions, physical and rheological properties (Fini et al 2011,Yang et al 2014, Zahedi et al 2020a, Bo et al 2021, Flavia Justino Uchoa et al 2021, Wu et al 2021, Zhang et al 2022, and Uchoa 2021).
Lignin is eco-friendly, biocompatible, low toxic, and sensitive against enzymatic degradation (Luo et al 2019) and is being applied not only in the field of chemical engineering but also in pavement engineering through modification of asphalt binder.Lignin modified binder was found to improve hardness, viscosity as well as performance at high temperatures (Yao et al 2022a).Different level improvement has been observed against stability at high temperature, low temperature cracking and drainage performance of asphalt mixtures (Yao et al 2022b).The addition of lignin to asphalt binder can lead to improved resistance aging and has diverse effects on crack and fatigue resistance at low-temperature and high-temperature performance rheological property (Yao et al 2022b), (Xiu et al 2011).It can be applied for modification as a modifier coupling agent in combination with asphalt and modifiers to improve asphalt properties (Yao et al 2022a).
Research work conducted by Zahedi et al 2020b reported that adding 3 to 6% of lignin to the asphalt mixture improved rutting and fatigue life, but by increasing the amount of lignin to 9%, the fatigue life was reduced (Zahedi et al 2020b).The viscosity of asphalt binder increases with increasing the lignin content.The lignin modified asphalt binder showed higher rutting resistance at high temperatures and higher resistance to thermal cracking at low temperatures, also demonstrating a higher photodegradation resistance than the conventional binders mainly for the 6 wt% of lignin addition (Batista et al 2018).The addition of lignin tends to improve performance against rutting but has no significant performance against low temperature.The addition of waste engine oil in combination with lignin content leads to improved performance at low temperatures (Fakhri and Norouzi 2022).
The lignin fiber modified asphalt mixture (LFMAM) produced the highest negative environmental impacts in all impact categories, followed by the diatomite-lignin fiber composite modified asphalt mixture (DLFMAM) as compared to the control asphalt mixture (Yue et al 2022).Lignin combined with glass fiber has a good impact on water stability and fatigue performance of asphalt mixture but it is not possible to improve the overall performance of a mixture by using only a single admixture.This study further added that the optimum amount of lignin fiber is 0.2%-0.4% of asphalt mixtures, and the amount of glass fiber is 0.2%-0.6% of asphalt mixtures (Luo et al 2019).The presence of lignin does not compromise bond strength between bio-binder and natural aggregate, even better it seems to guarantee and reduce moisture susceptibility as compared to reference bitumen, especially in the case of hydrophilic aggregate (Gaudenzi et al 2022).
Although, research work by different researchers has concluded that lignin can be used in the modification of asphalt binder, but the optimum dosage ratio that can be successfully added or replaced with asphalt binder still needs to be studied in detail and finalized.Moreover, different versions have been reported on performance of lignin modified binder against low temperature and fatigue cracking.Very limited work has been carried out to check its performance against moisture susceptibility.Therefore, detailed study based on the lignin from waste of local paper industry has been framed to study the effect of lignin in modification of asphalt binder as well as to find its percentage that can successfully replace asphalt binder for sustainable and green pavements.
This research work is aimed to investigate the effect of paper industry-based lignin for use in HMA.It intends to modify virgin asphalt binder with different dosage ratios of lignin, in light of the literature review, to develop lignin modified binder.Overall performance of lignin modified binders, with different dosage ratios, through basic physical testing, rheological testing, and mixture performance testing has to be carried out to study the effect of lignin in HMA as well as the optimum dosage ratio that can successfully replace the typical asphalt binder.It also intends to find the effect of lignin on the SARA fractions and colloidal instability index of the modified binder.The statistical analysis performed to develop a co-relation between physical and rheological properties.The optimum dosage of lignin has been added in asphalt mixture to investigate the effect on rutting, fatigue and moisture susceptibility.

Materials and methods
Penetration Grade 60/70 bitumen samples were arranged from Attock Oil Refinery, Morgaah Rawalpindi as per requirements of local climatic conditions.Lignin samples were collected from the waste of the paper industry from Bhullay Shah paper Mills, Kasur.The residue left after the extraction of paper called black liquor was then treated with sulphuric acid to obtain concentrated lignin.Black liquor has not been used due to excessive moisture content and performance issues (Kalampokis et al 2022).Aggregates were collected from Margalla quarries, located near Islamabad, the capital of Pakistan.Virgin binder was initially heated to a temperature of about 135 °C and the lignin sample was gradually added along with mixing by a shear mixer at 3000 rpm for 30 min to obtain a homogeneous mixture (Zahedi et al 2020b).Blends were prepared by replacing virgin bitumen with lignin in the dosage ratios of 5, 10, 15 and 20% in light of literature review and earlier research work conducted by various researchers (Zahedi et al 2020a), (Kalampokis et al 2022).A graphical representation of the research methodology is illustrated in figure 1.

Asphalt binder characterization
After the addition of lignin, virgin binder (VB) and lignin modified binder (VBL) samples were subjected to physical tests including softening point, penetration, viscosity, ductility, and specific gravity.The samples were then conditioned to artificial aging through a rolling thin film oven (RTFO) and pressure aging vessel (PAV) to assess the effect of aging.Physical properties were again investigated to check their properties after aging.
Based on the results of physical tests, samples were prioritized for the study of rheological properties of virgin binder (VB), lignin-modified binder (VBL) through Dynamic shear rheometer (DSR), bending beam rheometer (BBR), and Viscosity, through rational viscometer (RV).The performance of virgin, as well as modified binders, was studied against a wide range of temperatures to assess their performance against rutting, fatigue cracking, and moisture susceptibility.

Basic physical properties
The penetration test was carried out to check the hardness or softness of the bitumen binder ASTM D36/D36M-12 both for virgin (VB) as well as lignin modified (VBL) samples.Ductility tests were carried out as per ASTM D113-99 to assess the elongation or deformation of VB & VBL samples.A specific gravity test of VB and VBL samples was carried out as per ASTM D70-97.This value varies between 0.97 and 1.02.The specific gravity of bio binders is very close to that of asphalt binders (Yang et al 2014).The viscosity of VB & VBL samples was carried out in accordance with ASTM D4402 to measure their resistance against the flow.Bituminous materials undergo oxidative aging during mixing, construction, and service life.The performance of asphalt pavement is greatly dependent upon the aging of the asphalt binder.Aging was measured by rolling thin film oven test (RTFOT) as short-term aging and Pressure aging vessel as long-term aging.Samples were prepared and subjected to short term aging in RTFO at 163 °C temperature for 85 min as per ASTM D2872.These samples were then subjected to PAV as per ASTM D6521-22 under 2.10 MPa pressure and 100 °C temperature for 20 h to simulate in service life long-term aging.

SARA fractions of asphalt binder
The asphalt binder, according to molecular weight, is divided into four fractions called SARA fractions namely Saturates (A), Aromatics (A), Resins ®, and Asphaltenes (As).The analysis for the fractional composition of virgin (VB) and lignin modified (VBL) samples was performed in accordance with ASTM D4124-09 to make a comparison and assess the effect of lignin on the properties of the virgin binder.

Rheological properties of asphalt binder
Performance Grade (PG) covers a broad range of temperatures from freezing to optimal temperatures.Furthermore, it relates different aspects of diverse asphalt binders to field performance.It usually characterizes binders on the basis of regions of low and moderate to high temperatures.The failure mechanism of asphalt binder in flexible pavements comprises the thermal shrinkage that starts and propagates cracks due to low temperature.The bending beam rheometer (BBR) test was performed according to AASHTO M320 at RTFOT and PAV aged binder samples both for virgin as well as lignin-modified samples.The samples were subjected to testing at different temperature ranges of −4 °C, −10 °C, −16 °C and −22 °C.The Dynamic Shear Rheometer (DSR) test was carried out in accordance with AASHTO M320 to evaluate effect on asphalt binders against variations of time and temperature simultaneously.It was performed on virgin as well as lignin-modified samples.DSR was used in a strain-controlled mode (10% strain) accompanying 10 radian/second frequency level.Output parameters for every sample were determined in response to a sinusoidal stress value.

Asphalt mixture performance testing
The optimum dosages of lignin were used to develop lignin modified samples were binder utilized to assess the performance of asphalt mixtures against rutting, fatigue cracking, and moisture susceptibility.

Physical characterization
The penetration values of virgin (VB) and lignin-modified binders (VBL) before and after artificial aging are presented in figure 2. It can be noted from figure 2 that the penetration value of virgin binders is higher than lignin modified binders.The same trend is observed in RTFO-aged as well PAV-aged binder samples.Although upon aging, values decreased but modified binders still have lower values than virgin binders.It indicates that penetration values decrease with increasing the dosage of lignin.This implies that the addition of lignin to the virgin binder improves its stiffness which ultimately leads to improved rutting resistance.On the other side, it indicates that increasing the percentage of bio-additives beyond 10% tends to reduce low temperature performance.The same trend has been in an earlier research work well (Kalampokis et al 2022).The softening point of virgin and modified binder samples is presented in figure 3. From figure 3, it can be seen that the softening point of the virgin binders is lower than that of modified binders.Results indicate that the addition of lignin to the virgin binder tends to increase the softening point and keeps increasing with increasing dosage ratio which implies that the addition of lignin tends to improve performance at high temperatures.But at the same time, it indicates that increasing the dosage ratio tends to reduce performance against low temperatures.Hui Yao et al 2022, have also reported the negative effect of lignin over fatigue resistance of asphalt binder but have not mentioned the limits over which lignin modified binder can perform satisfactorily overall (Yao et al 2022b).The ductility of different samples is presented in figure 4.
It can be noted from the figure 4 that the addition of lignin to base binder tends to increase the values which indicates that lignin modified samples show improved performance against rising temperature.But on the other hand, increasing dosages tend to reduce performance at low temperature due to improved stiffness.The values of specific gravity recorded for lignin modified binders fall between 1.01 to 1.02 which are very close to that of virgin binder.

Flow characteristics
The values of viscosity of different samples are presented in figure 5.It can be noted from figure 5 that viscosity value increases with the aging of binder and upon the addition of lignin, it again rises upon aging but still has lower values than that of aged virgin binder.Increasing the dosage ratio tends to increase the viscosity value which indicates an improvement against rutting but at the same time it also indicates a reduction in performance against low temperature especially when percentage of lignin is beyond 10%.

SARA fractions
The results of SARA fractions analysis of different virgin and lignin-modified samples are presented in figure 6.It can be noted from figure 6 that SARA fractions change with the addition of lignin.Saturates content decreases slightly while Asphaltenes content increases due to the loss of volatile compounds during aging that makes the binder stiffer and brittle.Upon aging, the effect on the saturates content is very small but asphaltenes content keeps increasing even after aging as well.On the other hand, resins content tends to decrease with the addition of lignin content.The same trend is observed even after aging.The similar type of trend is observed in physical tests as well.

Colloidal instability index
The colloidal instability index (CII) represents the colloidal model of asphalt binder to indicate a sole-gel type of structure (Jamal et al 2020).Furthermore, the variation of fractional contents through CII and SARA analysis is graphically represented below in figure 7.

Saturates Asphaltenes Resins Aromatics ( )
It can be noted in figure 7 that asphalt binder indicates an unstable colloidal structure with long-term aging.Through the addition of lignin, it indicates a stable structure that can perform better in the service life of the pavement.

Performance grading of asphalt binder
The performance grading for low and high temperature was performed through BBR and DSR respectively.. Different values of PG grading for low temperature and high temperature are shown in figure 8.
It can be noted from figure 8 that PG grading is changing at low and high temperatures.The effect of lignin indicates a decrease in low temperature and an increase in high temperature.However, slight changes are observed with the addition of optimum dosages.The master curve is plotted in figure 9 for virgin and modified binders.
It can be noted from figure 9 that initially, there is a linear trend that indicates higher stiffness in upper, medium and lower temperatures in different samples.At the higher frequency, the trend is changing minorly.Same trend is observed in the SARA analysis as well.Extension of aging period from laboratory to field leads to viscous and stiffer asphalt binder due to change in their fractional components.A significant decrease in stiffness at low frequency is observed that reflects the regions of moderate to high temperatures.Based on the results of asphalt binder testing, it can be summarized that the optimum lignin binder content for addition the of virgin binder is 10%, as the higher dosage ratio is leading towards stiffer and brittle binder.

Statistical analysis and co-relation
Statistical analysis was conducted using IBM SPSS Statistics 22 on test data.The main aim of this analysis was to detect any suitable correlation between parameters like penetration, ductility, softening point and viscosity from physical and asphaltene content and, colloidal instability index from the SARA fraction.Initially, parameters were tabulated, and descriptive statistics were applied to calculate the mean value and standard deviation as shown in table 1.The total ten variables were chosen from different properties of asphalt binder with a confidence level of 95%.All the possible correlations were calculated without applying the selection of independent and dependent variables.Then, the correlation matrix was applied between different parameters as shown in table 2.
It indicates that a linear correlation exists between individual pairs of variables.This coefficient is a dimensionless parameter that can vary between −1.0 and 1.0 and designates the scale of expected correlation between each pair of data.It indicates the positive trend when pair of data increases at the same time, whereas a negative value represents the possible opposite trends.The null correlation depicts that the pair of data are not linearly dependent on each other as discussed in the literature.It indicates the various medium and high significance type correlations between different parameters for different samples of asphalt binders.
It can be noted that most parameters are directly related as one increases, the other also increases.However, a few parameters are inversely correlated.In addition, analysis of variance (ANOVA) was conducted for different   regression equations, and significant values (Sig.) for every linear regression model were established.The basic purpose of these regression models is how much change in one affects the reduction or change in the other parameters which shows that the correlations are statistically valid.

Asphalt mixture performance testing
Asphalt mixture tests with optimum dosage were conducted to evaluate the performance of lignin modified binders against rutting, fatigue cracking, and moisture susceptibility.Based on the Marshal mix design, dosage ratios of 4.52% and 4.50% have been used for 5% (VBL5) and 10% (VBL10% lignin dosage ratios respectively.Single aggregate source and gradation were selected.The rolling bottle, wheel tracker, and four-point beam fatigue tests were conducted at standard temperatures.

Wheel tracking test
In asphalt pavements, rutting is defined as the progressive accumulation of longitudinal depressions in a wheel path under repetitive loading.The majority of rutting problems result from plastic deformation of the surface course.It is characterized by shear deformation inside the asphalt mixture (Sandoval et al 2008).Wheel tracking test was used to assess rutting susceptibility in the laboratory under specified conditions of loading, speed and temperature that simulate the effect of traffic in the field.Samples were subjected to repetitive loading under the moving wheel to estimate permanent deformation characteristics of the pavement.Wheel tracking test for rutting resistance was performed in accordance with AASHTO T324.In this study, the rutting resistance of virgin, and lignin modified mixes was determined with the help of the wheel tracking test in accordance with AASHTO T 324.The results of asphalt mixtures are presented in figures 10-12.
It can be noted from figure 10 that the addition of lignin to the virgin binder improves performance against rutting.In case of VBL5, rutting is improved but with increasing number of passes, the effect is very low.But in case of VBL10, rutting is overall improved.Zahedi et al 2020b, aas stated in the literature review portion, have reported that addition of 6% lignin as optimum dosage gives better performance (Zahedi et al 2020b).But on the basis of these results, as an overall, it can be concluded that addition of lignin with 10% dosage has better performance against rutting without effecting other performance characteristics.were checked and noted at certain intervals of the load cycle and the test was terminated when the beam reached 50% stiffness reduction.The fatigue test results are presented in figure 11.
It can be noted from figure 11 that through the addition of lignin, the fatigue life of asphalt mixes improves which further increases with increasing the dosage ratio of lignin.The mixture and binder characteristics concluded that the modified binder with 10% dosage performs well.The same results have been reported by A H Bradley et al 2021, in the TAC conference & Exhibition session.

Moisture susceptibility
The presence of moisture leads to adhesive failure (Stripping) at the bitumen-aggregate interface and cohesive failure within the binder.For this, a rolling bottle test was used according to BS EN 12697.Moisture sensitivity testing (LC 26-001) was conducted on asphalt with virgin binder (VB) bitumen, 5% lignin, and 10% lignin.The water resistance (WR) was determined by performing Marshall tests on dry and wet specimens.Testing found that the moisture sensitivity was 82.3%, 90.8%, and 86.5% for the 0%, 5%, and 10% lignin samples.All of these exceeded the performance threshold of 70% and were, therefore, considered satisfactory.This result agrees with the bitumen moisture sensitivity results and indicates that stripping of lignin-modified asphalt is not likely to be an issue.
It can be seen from figure 12 that loss of coating i.e. performance against moisture susceptibility improves upon the addition of lignin to the virgin asphalt binder.

Conclusions and recommendations
Lignin from the waste of the paper industry was utilized in different dosage ration of 5%.10%, 15%, and 20%, and blends of lignin modified binders were prepared accordingly.The physical and rheological properties of virgin and lignin modified binders were studied before artificial aging.SARA analysis and performance grading tests were also conducted.Based on the results of binder testing, asphalt mixtures were developed and subjected to testing against rutting, low temperature fatigue cracking, and moisture susceptibility.Conclusions of this research work can be summarized as follows: • The physical properties of lignin modified binder are improved.The addition of lignin improves aging and reflects better indication with a 10% dosage of lignin.This implies that lignin can act as a rejuvenator as well.
• SARA fractions are improved with the addition of lignin.This indicates a stable colloidal structure.However, during long-term aging, this structure turns out to be hard but still indicates improved performance at the optimum dosage of lignin.
• The rheological properties indicate that lignin modified binders perform better in moderate to high temperature regions.
• The mixture testing indicates that optimum dosage can perform better to resist rutting, fatigue and moisture susceptibility.
• Statistical analysis indicates good co-relation among different physical and rheological parameters for different samples.
• This research concludes 10% dosage ratio of lignin is optimum against which the overall performance of lignin modified asphalt binders is satisfactory.This implies that lignin can be used as a binder and can replace asphalt bitumen up to 10% satisfactorily.
• Based on the findings, this study recommends to investigate properties of lignin modified binder in combination with other adhesive materials so as to improve the dosage ratio by complying the overall performance parameters.

Figure 2 .
Figure 2. Penetration values of virgin and lignin modified binders.

Figure 3 .
Figure 3. Softening Point values of Virgin and lignin modified binders.

Figure 4 .
Figure 4. Ductility values for Virgin and lignin modified binders.

Figure 5 .
Figure 5. Viscosity of Virgin and lignin modified binders.

Figure 8 .
Figure 8. PG Grading of virgin and lignin modified binder.

Figure 9 .
Figure 9. Master Curve of Virgin and Modified Binders.

Table 2 .
Co-relations of different parameters of asphalt binders.
3.6.2.Four point beam fatigue testThis four-point beam fatigue test was applied to assess the fatigue life of HMA at operating temperatures of intermediate level in accordance with AASHTO T 321 at a specified strain level.Flexural fatigue test.Test results