Unravelling the efficient use of waste lignin as a bitumen modifier for sustainable roads

Construction and Building Materials 230 (2020) 116957

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Unravelling the efficient use of waste lignin as a bitumen modifier for sustainable roads Eyram Norgbey a,b,f,⇑, Jingyu Huang a,⇑, Volker Hirsch c, Wen Jie Liu a,⇑, Meng Wang a, Oliver Ripke c, Yiping Li b, Georgina Esi Takyi Annan d, David Ewusi-Mensah b, Xiaohui Wang e, Gabriela Treib c, Adrian Rink c, Amechi S. Nwankwegu b, Prince Atta Opoku b, Philip Nti Nkrumah b a

School of Ecology and Environment, Hainan University, Haikou 570228, Hainan, China College of Environment, Key Laboratory of Integrated Regulation and Resources Development on Shallow Lakes, Ministry of Education, Hohai University, Nanjing 210098, China Federal Highway Research Institute (BAST), Chemistry, Environmental Protection Issues, Laboratory Services, Bergisch Gladbach, Germany d School of Architecture, South East University, Nanjing 210096, China e State Key Laboratory of Pulp & Paper Engineering, South China University of Technology, Guangzhou 510640, China f Department of Civil Engineering, Kwame Nkrumah University of Science and Technology, PMB, Kumasi, Ghana b c

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Lignin increased the viscosity of the

binder at higher and lower temperature.
Lignin increases the rutting resistance and recovery component of bitumen at high temperature.
Lignin had insignificant effect on low temperature cracking behavior of the binder.
Lignin reduced the damage tolerance level of the binder.
LBC binder had a better adhesion with aggregate in the presence of water.

a r t i c l e

i n f o

Article history: Received 12 March 2019 Received in revised form 27 August 2019 Accepted 13 September 2019

Keywords: Waste reuse Lignin bitumen composite Sustainability Rheology Fatigue Recovery component

a b s t r a c t The high cost and the environmental impact associated with using petroleum bitumen in pavement construction is a problem facing the asphalt industry. The study analyzes the effects of waste lignin on the properties (characterization, morphology, decomposition behavior, low and high-temperature behavior, fatigue resistance, deformation, adhesion bonding with aggregate in the presence of water and storage stability) of bitumen binders. The results show that increasing the lignin content, increased the cohesion and stiffening of the binder. Adding 10% lignin had a negligible influence on the workability and compaction characteristics of the asphalt mixture. The flow characteristics, of the lignin bitumen composite (LBC), at low and high temperature, decreased with increasing lignin content. Furthermore, the lignin fibers decreased the decomposition rate of the binder thus reducing the volatility of the binder (C02 emissions). Dynamic Shear Rheometer test results revealed that adding lignin fibers to bitumen increased the rutting resistance of the binder while the linear amplitude sweep (LAS) results showed a decrease in the fatigue resistance. The multiple stress creep recovery (MSCR) results showed that LBC binder recovered

⇑ Corresponding authors at: School of Ecology and Environment, Hainan University, Haikou 570228, Hainan, China (E. Norgbey, J. Huang, W. J. Liu). E-mail addresses: [email protected] (E. Norgbey), [email protected] (J. Huang), [email protected] (W.J. Liu). https://doi.org/10.1016/j.conbuildmat.2019.116957 0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.

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better than the pure bitumen. In addition, LBC binder deformed less at different temperatures and stress levels. The LBC binder also had a better adhesion with aggregate as compared to bitumen. The study provides baseline information that complements past studies and can be useful to all stakeholders in the asphalt industry. Ó 2019 Elsevier Ltd. All rights reserved.

1. Introduction In 2050, the global energy demand is expected to double. Thus, researchers are developing renewable materials that will act as an alternative source to fossil materials in order to meet the world’s increasing energy demand while putting less stress on the environment [1–3]. Bitumen, a fossil material obtained from petroleum oil, is the world’s most important binder for road construction worldwide. Based on the current demand for petroleum products, it is estimated that the world will run out of petroleum oil in 2063 [4]. From this perspective, scientists have resorted to the partial replacement of bitumen with renewable biomass to produce cheaper, more sustainable and readily available binding material, which is environment friendly. Thus, this study focuses on the partial replacement of bitumen (fossil material) with a sustainable and renewable material, lignin, to help reduce the high dependence on the petroleum binder and in turn contribute to sustainable construction [5,6]. Lignin, a form of wood waste, is unwanted during the production of paper or ethanol and often used as a source of fuel through combustion to power industrial plants. Every year, more than 25 million tons of Kraft lignin are produced from the pulping of wood chips in the United States [2,3]. It is worth noting that lignin has become very popular among researchers because of the nature of the material and its availability. The high-energy density and intrinsic aromatic based structure of lignin make it possible to

obtain various valuable materials from lignin such as polymer and aromatic rich pyrolysis oil [7]. The chemical compounds and hydrocarbons present in lignin are similar to those of bitumen. Furthermore, the cementitious nature of lignin makes it a good candidate material for sustainable roads construction [8]. On these notes, laboratory analyses were performed on bitumen and lignin in order to give a better understanding of the basic characteristics, morphology, decomposition behavior, low and high-temperature behavior, fatigue resistance, deformation, storage stability and adhesion of the binder to aggregate (Fig. 1). The efficient use of waste lignin (WL) in making bitumen pavements will not only reduce environmental pollution but will also reduce the dependence on petroleum bitumen and improve efficiency in the wood industry by making use of unwanted waste byproduct. This will improve the global economy in both the wood and bitumen industries. Past studies on using lignin fibers in road construction have shown satisfactory coating, workability, compaction [9], fatigue resistance [10] an increase in pavement lifespan [11–14] and an improvement in the rutting resistance of bitumen binder at warm temperatures [20]. In addition, studies by Pérez et al. (2019) showed that 20% of liquid industrial waste containing lignin can be used successfully as a bitumen extender [8]. In this study, the lignin was obtained from the corncob industry in south China. Despite the potential applicability of lignin as a bitumen modifier, there are still some questions that need to be addressed.

Fig. 1. Experiment plan.

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1. Does the addition of 10% of waste lignin obtained from the corncob industry have an insignificant influence on the workability and compaction characteristics of the asphalt mixture? 2. How does the LBC binder behave (flow characteristics) at extremely low temperature? 3. What are the damage tolerance and the non-recoverable creep compliance (recovery rate) of the LBC binder at warm temperature? 4. How well does the composite binder cover aggregate in the presence of water? 5. What is the storage stability behavior of the LBC binder? It is interesting to note that depending on the biomass source and its industrial processing, different types of lignin can be obtained. For the low-temperature properties, previous works only focused on using bending beam rheometer (BBR) test in studying the behavior of the binder under cold temperature [10,15]. No studies have been done to analyze in details the flow characteristics of the composite binder at a very low temperature (25 °C). In view of this, our study aims to introduce a novel approach, the tension retardation test (TRT), in determining the behavior of the binder under cold temperature conditions giving a better understanding of how the LBC binder behaves under low temperature. For the high-temperature properties, previous studies used the temperature sweep curve with the help of the dynamic shear rheometer to describe the rutting resistance of the binder [15– 17]. However, there have not been enough studies on the damage tolerance and non-recoverable creep compliance (recovery rate) of the binder. The linear amplitude sweep (LAS) and multiple stress creep recovery (MSCR) tests were conducted to complement past studies and to provide a deeper knowledge of the damage tolerance and non-recoverable creep compliance of the LBC binder. The tube-test was conducted to provide information on the storage stability behavior of the LBC binder. Finally, for the adhesion test between the LBC binder and aggregates, no study has been done to analyze the composite binder coverage over aggregate in the presence of water in details. The rolling bottle test (RBT) was conducted to give further understanding of the affinity of LBC binder on aggregate. 1.1. Objective and scope The main objective of this study was to perform different laboratory tests (Fig. 1), to understand the basic characteristics, morphology, decomposition behavior, low and high-temperature behavior, rheological properties, fatigue resistance, deformation, adhesion bonding with aggregate in the presence of water and the storage stability characteristics of the LBC binder. The laboratory tests results were compared between the pure bitumen and the lignin bitumen composite (containing 5% and 10% lignin). The basic characterization (penetration, softening point and force ductility test) and the morphology with the help of the digital microscope were analyzed. Also, the behavior of the binder with

increasing temperature was studied using the thermogravimetric analysis (TGA) equipment while the low-temperature behavior of the binder was analyzed using the BBR and TRT machines. The rheological properties, damage tolerance, recovery (deformation) rate were measured using the dynamic shear rheometer (DSR) machine. The adhesion of the binder on aggregate was carried out using the RBT method. Finally, the storage stability of the binder was analyzed using the tube test method. The results of this study will complement past studies on the potential use of WL as a sustainable bitumen modifier and will be useful for all the stakeholders in the road and wood industry. 2. Materials The bitumen grade of 50/70 and 160/ 220 used in this study was obtained from the Federal Highway Research Institute (BAST) in Germany. The physical properties of the binders are shown in Table 1. The lignin is a waste by product that was obtained directly during the production of bio-ethanol from corncobs. The WL was obtained through Shandong Longlive Bio-technology Co., Ltd in China. The corncob was treated hydrothermally to break down the hemicelluloses and obtain xylo-oligosaccharides. The residue was collected and treated with an alkaline solution, to release lignin and produce a cellulose-rich residue. The solid cellulose-rich residue served as feedstock for bio-ethanol production and effluent (after alkaline treatment) had its pH adjusted (acidic conditions) to precipitate the lignin. The chemical analysis of WL was conducted according to laboratory standards [36] (Table 2). The particle size results of the WL are presented in Table 3. The lignin had a moisture content of 3.6%.

3. Methodology 3.1. Sample preparation The bitumen binder was uniformly mixed with different amounts of lignin at a temperature of 150 °C using a high shear mixer (IKA PROCESS-PILOT 2000/4 mixer) at a revolution of 6000 rounds per minute (rpm) for 1 h. The composite samples were allowed to cool down for further testing. The experiment plan has been outlined in Fig. 1. 3.2. Basic characterization The penetration test was done in accordance with EN 1426. 100 g of needle penetrated into the bitumen binder for 5 s at 25 °C [18]. The softening point was found using EN 1427 via the ring and ball method [19]. The force retardation test was done in accordance with BS EN 13589:2018 (EN 1426, 2015; EN 1427, 2015) [20]. The penetration and softening point data were analyzed using descriptive statistics and one-way ANOVA in the Excel Analysis ToolPak. A confidence level of 95% was used during the descriptive statistics analysis while an alpha value of 0.05 was used in the one-way ANOVA [21]. The statistical analysis was conducted to evaluate the effect (significance) of lignin content on the workability and compaction during pavement construction. If the P-value is less than 0.05, the effect is statistically significant.

Table 1 Properties on bitumen binder B50/70 and B160/220. Test

Units

Standard

Penetration at 25 °C 0.1 mm DIN Softening °C DIN Flash point °C DIN Solubility % DIN Fras breaking point °C DIN Resistance to hardening under the influence of heat and air – RTFOT (DIN EN 12607-1) Penetration retained % DIN Increase of softening point °C DIN a Mass variation % DIN

EN EN EN EN EN

1426 1427 ISO 2592 12592 12593

EN 1426 EN 1427 EN 1427

B50/70

B160/220

65.0 49.5 230 99 8

190 39.8 220 99 15

50 9 0.5

37 11 1

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Table 2 Chemical composition and molecular weight of wood waste lignin (WWL). Composition (%)

Wood waste lignin

Total lignin Klason lignin Acid-soluble lignin Carbohydrate content Arabinose Galactose Glucose Xylose Mannose Glucuronic acid Galacturonic acid Ash Others Weight-average, (Mw g mol1) Molecular number (Mn g mol1) Polydispersity (Mw/Mn)

94.42 90.81 3.61 0.63 0.13 0.02 0.32 0.13 0.03 ND ND 2.16 2.16 3258 2260 1.44

Table 3 Particle size results. Parameter

Lignin

Detected largest grain size HS circularity Mean Aspect ratio mean Elongation Mean

186 mm 0.879 0.785 0.215

3.3. Morphology The distribution of 10% lignin in the bitumen binder at the particle level in the composite was studied using the digital microscope (Keyence VH-Z500). The images were captured at 50 lm. The mixing temperature of the composite before analysis was 150 °C. The slides were heated to 140 °C. The composite material was then placed on the slides, allowed to cool down and was later photographed for analysis.

3.4. Decomposition analysis The Mettler Toledo instrument was used for the Thermogravimetric analysis (TGA) analysis. TGA measured the weight gained or lost of LBC samples as a function of temperature. About 10 mg of the sample was weighed into a crucible and analyzed by thermogravimetric analysis. The test was done in an atmosphere of nitrogen between 25 and 500 °C. The atmosphere was changed to oxygen from 500 to 900 °C. The gases were injected at a flow rate of 50 mL min1. A curve of weight loss versus temperature was constructed from the data obtained by the instrument [22,23].

3.6. Low-temperature behavior 3.6.1. BBR test The BBR test measures two parameters namely the creep stiffness (S value) and creep rate (m-value) of a binder at a low temperature. The m-value describes changes of bitumen stiffness under applied load whiles the S value measures bitumen binder’s ability to resist constant loading. In this analysis, the binders were first subjected to short-term (rolling thin-film oven method) aging and later to long-term (pressure aging vessel) aging. Furthermore, a creep load is applied to a small bitumen beam that mimics the stress building in pavements at low temperature. Because of the low temperature recorded in north China (20 to 30 °C), the fluid bath temperature was set to 22 and 28 °C. The data were obtained via a computerized system. Three replicas of each sample were tested and the average was recorded [24]. 3.6.2. Tensile retardation test (TRT) The TRT measures the tensile viscosity (characteristic of material flow behavior) of a specimen at a very low temperature of 25 °C with high precision. During the TRT analysis, a constant uniaxial tensile stress is applied as a static load on the sample at a constant temperature. The axial expansion of the sample is then measured as a function of time. The testing analysis was done in accordance with works by Bommert (2016) [29]. Fig. 2 shows the specimen with dimensions. The specimen has a volume of 104 cm3. The prepared composite binder is heated gently to a temperature that the binder can easily stir while avoiding producing bubbles in the binder. Paraffin is used to grease the inner sides of the mold. The mixed heated binder is poured into the mold in excess and allowed to cool down. The cooled specimen is stored in a cooling device at a temperature of 5 °C overnight. A heated spatula is used to level the surface of the specimen for analysis. The tensile load is applied to the specimen and the results are measured Bommert [29]. Eq. (1) shows the tensile viscosity of the binder sample.

k¼

r r ¼ Dl ½MPa  s e Dtl

where: k – Binder’s tensile viscosity [MPas]; r – Uniaxial tensile stress constant [MPas]; έ – Linear range gradient of time/elongation curve [1/s]; Dl – Change in length over observed period [mm]; Dt –time recorded during elongation observation [s]; l – The specimen’s effective length. The various test results are converted into a logarithmic scale (Eq. (2)).

logðkÞ ¼ c þ mðTÞ 3.5. Aging analysis The binder was subjected short-term aging using the rolling thin-film oven (RTFO) method according AASHTO T-24. The short-term aging was done to replicate the thermal stress on the bitumen during production and paving of the asphalt mix. In addtion, the pressure aging vessel (PAV) method was used to artificially age the binder (long-term aging) to mimic the state of the binder after 10 years’ of service life. The long-term aging was done in accordance to AASHTO R-28. The unaged binder was used in the TRT and damage tolerance analysis whiles the aged binder (shortterm and long-term aging) was used in the BBR, TSC, TGA and MSCR analysis (Fig. 1).

ð1Þ

ð2Þ

c – is the tensile viscosity at 0 °C (abscissa);m – regression line’s gradient;T – testing temperature. The gradient m of the graph measures the temperature sensitivity of the lignin composite binder. 3.7. High-temperature behavior 3.7.1. Temperature sweep curve (TSC) The temperature sweep curve was obtained during the DSR test with the help of the Malvern Kinexus rheometer. The test characterizes the viscoelastic behavior of bitumen binder at high temperatures. The DSR test evaluates the bitumen specimen’s response to the sinusoidal stresses and calculates the complex shear modulus

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Fig. 2. Specimen for testing.

(G*) and phase angle (d). The binder was aged using RTFO and PAV method to simulate short-term and long-term aging. The magnitude of the complex modulus and the degree of phase angle are used to determine the binder’s stiffness and rutting resistance [26,27]. 3.8. Multiple stress creep recovery (MSCR) The MSCR analysis was done using the Malvern Kinexus Rheometer in accordance with EN 16659 using stress levels of 0.1 kPa and 3.2 KPa. The LBC samples were aged (RTFO and PAV method) and were subjected to a creep load for one second. The load was then removed to allow the sample to recover under zero stress for nine seconds. The percent recovery (R) shows the ability of the binder return to its initial state whiles the non-recoverable creep compliance (Jnr) refers to the non-recoverable strain of the binder after the initial load is taken. The deformation of the binder with time was also analyzed. R and Jnr provides a better understanding of the binders’ resistance to permanent deformation at high temperatures, which is a well-known problem in pavements. R and Jnr were calculated according to Eqs. (3) and (4) [28] (EN 16659, 2016).

R¼

rp  rn rp  r0

Jnr ¼

ð3Þ

rn  r0 s

ð4Þ

rp – Peak strain after one-second creep duration;rn – Nonrecoverable strain after nine-second recovery;r0 – shear strain at the beginning of the cycle;s – creep loading stress. 3.9. Damage tolerance – linear amplitude sweep (LAS) The linear amplitude sweep (LAS) test measures the fatigue performance of the binder through the application of cyclic loading with increasing linear amplitudes. In the LAS test, the standard parallel plate of 8 mm in diameter with a 2-mm gap between parallel plates was used. The test method was done in accordance with AASHTO TP 101-14 [30]. The fatigue resistance and the damage accumulation in bitumen binder with time were also calculated according to the Eqs. (5)–(7).

DðtÞ ffi

XN  i¼1

a a ½pc0 2 ðCi1  Ci Þ 1þa ðti  ti1 Þ1þa

ð5Þ

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CðtÞ ¼

jG jðtÞ jG jinitial

ð6Þ

statistical analysis was performed on the penetration test values to compare the difference between the top and bottom samples.

where: 4. Results and discussion D(t) – damage accumulation with time t; jG jðtÞ – Complex shear modulus at time t, MPa; jG jinitial – is the initial undamaged value of jG j; / – m1, in which m is the slope of a best-fit line in the logarithmic scale plot relating storage modulus to frequency. T – testing time, seconds; c – strain (%). The values of the integrity parameter (C) and Damage accumulation (D) are obtained at each data point at a given time t (Eq. (7)).

CðtÞA ¼ C0  C1 DðtÞC2

ð7Þ

C – 1, the initial value of C;C1 and C2 – curve-fit parameters derived from linearization power law [31]. 3.10. Adhesion test between LBC and aggregate – rolling bottle test (RBT) The RBT was done in accordance with EN 12697-11B [32]. The bitumen binder used in this test was grade B50/70. The aggregates, diabase (dolerite), were first washed and dried. The aggregates were sieved to obtain a diameter of 8–11 mm. The rolling speed was set at 60 rpm and the temperature was set at 20 °C ± 1. The rolling time of the bottles was done at 6 h, 24 h, 48 h, and 72 h. At every stage, two experts observed the samples and the final image was captured after 72 h (EN 12697-11, 2012). 3.11. Storage stability of LBC binder In order to study the storage stability of the LBC binder, the tube test was conducted. The test gives a better understanding on how the LBC binder will behave in a heated tank without circulation during road construction. 50 g of LBC binder was placed in an aluminum tube (25 mm in diameter and 140 mm in height). The sample was placed vertically in an oven at 163 ± 5 °C for 48 ± 1 h. After, the samples were then frozen at 6.7 ± 5 °C for 4 h. The frozen tube was divided in three parts (top, middle and bottom) [33]. The top and bottom are taken for further analyzed with penetration test. Three replicates are conducted for each test and the average has been reported as the final test result. In addition, temperature sweep curve was conducted top and bottom samples. Finally,

4.1. Basic characterization The effect of the lignin on the physical properties of bitumen (B160/220 and B50/70) was determined using the penetration and softening point test as shown in Fig. 3 and Table 1. The tests compare both aged and unaged bitumen to the LBC binder. Table 4 shows the peak force obtained from the force ductility test results. The penetration test measures the sink depths of the samples. The sink depths of LBC binder decreased as the concentration of lignin in the composite increased for both the aged and unaged samples. The lower penetration indicates higher stiffness within the composite. The results from the study support past works that show that fibers absorb the light components of bitumen and increase the viscosity and stiffness of bitumen [27]. The lignin have had a great effect on reducing the sink depth and improved the shear stress within the bitumen binder. Softening point tests were applied to both aged and unaged samples. The LBC binder showed a higher softening point than bitumen (Fig. 3). Our results are similar to past studies by Wang and Kristen Derewecki [20], the lignin increased the softening point of the bitumen. It is obvious that regardless of binder type, the softening point increases as the lignin content increases whiles the penetration value decreases as the lignin content increases in the binder. It was noted that the difference in penetration and softening point values of the base binder and after addition of lignin was significant with a P-value < 0.05. Thus, the change in the base binder was significant after adding lignin. However, since the base bitumen (B160/220) has a very low-viscosity, after the addition of lignin (5, 10%), the influence on the workability and compaction characteristics of an asphalt mixture produced is negligible. Although, it can be assumed that when using a higher viscosity base bitumen, the modification will change the workability.

Table 4 Force ductility test-maximum force on samples. Sample

Maximum force/Fpeak (N)

B160/220 B160/220 + 5% Lignin B160/220 + 10% Lignin

15.4 19.2 23.7

Fig. 3. Basic characterization of the bitumen and lignin bitumen composite.

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The results of the measured force ductility curves (Table 4) indicate that the LBC composite is non-modified since non-modified bitumen have only a single maximum [34]. The peak force (Fpeak) for bitumen, LBC-5%, and LBC-10% are shown in Table 4. It was observed that as the lignin content increased, the Fpeak values also increased. The Fpeak values correlate with the penetration test values. An increase in the Fpeak values corresponds to lower penetration test values. Therefore, the increase in Fpeak with increasing lignin content can be interpreted as a stiffening or increase in cohesion within the binder. 4.2. Morphology In this study, the morphology of the bitumen, lignin, and LBC binder were studied using the digital microscope (Fig. 4). The lignin sizes were non-uniform with a large surface area with rough edges. Furthermore, Fig. 4 shows a bee-like image when the lignin was uniformly mixed in the bitumen binder. Studies by Smaranda & Tucu [37] and Yin et al. (2012) [38] show that fibers with a large surface area and rough surface edges play a positive role in bitumen absorption and adhesion. The authors showed that fibers with rough edges form a three-dimensional multi-directional spatial network with the binder thus improving the binder properties by resisting cracking propagation and aggregate sliding within the binder[26,27]. The results from our study (Fig. 4) show a uniform distribution of lignin in the binder. Furthermore, more tests such as the use of a scanning electronic microscope will be conducted in future studies to investigate the detailed bonding between lignin and bitumen at the microscopic level. 4.3. Decomposition analysis Thermal decomposition measures the loss of the weight of material as the temperature changes. The loss of weight may be a physical process such as drying or a chemical reaction taking place within the material [35]. Fig. 5 shows the thermal decomposition curve of unaged bitumen and LBC samples. Initial decomposition temperature (IDT) is the temperature at which a material loses 5% of its weight due to heat [36] while the decomposition temperature after 95% mass loss is denoted as D95. For the unaged samples, pure bitumen had IDT and D95 values of 376.33 °C and 722.5 °C respectively. For LBC-5% and LBC 10%, the IDT values were 371.33 °C and 361.17 °C while the D95 values were 795.33 °C and 825.5 °C respectively. From Fig. 5, adding lignin to bitumen affected the decomposition rate of the binder. The rate of decomposition based on IDT values shows the decomposition in this order, LBC-10% > LBC-5% > LBC-0%, indicating that LBC started decomposing faster than bitumen. However, after IDT, bitumen with higher lignin content decomposed slower than the binder with lower amounts of lignin as seen in Fig. 5. The temperatures at D95 shows the decomposition in this order, LBC-0% > LBC-5%

7

> LBC-10%. Thus, it can be concluded that bitumen with high amounts of lignin shows more resistance against decomposition. Fig. 5 buttresses the fact that increasing the lignin content in the bitumen increases the resistance of the sample to decomposition. In other words, lignin decreased the rate of decomposition in the composite. For the aged samples (Fig. 5), the IDT for bitumen, LBC-5% and LBC-10% were 344.66, 370.10 and 361.97 whiles the D95 values were 704.39, 853.32 and 920.88 °C respectively. Generally, after aging the samples, the decomposition rate of the samples was similar to that of unaged samples with LBC binder exhibiting higher resistance to decomposition when compared to pure bitumen. Our study supports past studies by Batista et al. (2018) [43]. Thus, lignin increases the thermal stability of the LBC binder. So, during the heating of the binder to high temperatures, the volatility of the LBC binder (C02 emissions) will be reduced thus protecting the environment. Furthermore, future test on volatility will be conducted on the LBC binder at the temperature range of mixing and production of asphalt pavement, to measure in details to what extent the lignin reduce the volatility of the composite binder.

4.4. Low-Temperature behavior 4.4.1. BBR test The low-temperature behavior was analyzed using the BBR test. The results from the BBR test are shown in Fig. 6. The BBR method is performed to analyze the low-temperature properties of the bitumen material. Fig. 6 presents the BBR test results of the unaged samples blended with different lignin content. The values of creep stiffness (S) and creep rate (m) were calculated at the 60th second of loading. Bitumen with lower creep stiffness value (S) and higher m-value has a good low-temperature performance [37]. High mvalue shows that the binder is flexible and has the ability to relax quickly under thermal stress while low S value means lower thermal stresses on the bitumen [15]. It is observed from Fig. 6 that at a temperature of 22 °C, both bitumen and LBC binder had m-values greater than 0.3 and creep stiffness less than 300 MPa whiles at a temperature of 28 °C, both bitumen and LBC binder had mvalues less than 0.3 and creep stiffness more than 300 MPa. After RTFO and PAV aging of the samples (Fig. 6), both bitumen and LBC binder had m-values less than 0.3 at 22 °C and 28 °C. In addition, creep stiffness was less than 300 MPa at 22 °C and creep stiffness was more than 300 MPa at 28 °C. It is worth noting that the values of bitumen, LBC-5% and LBC-10% at 22 °C met the requirements of creep stiffness (maximum 300 MPa) and mvalue (minimum 0.3) stated in the Superpave specification to prevent thermal cracking [17]. From our study, adding 5% and 10% lignin to bitumen did not show any significant change in m-value and S value in all cases. The statistical analysis on data shows a p-value greater than 0.05 indicates that the difference in values for pure bitumen and LBC

Fig. 4. Microscopy of A. Bitumen (B160/220) B. Lignin C. LBC binder with 10% Lignin.

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Fig. 5. Thermal decomposition of: (A) & (B)-unaged samples (C) & (D)-aged samples (E) lignin sample only at temperature 0–1000 °C.

was insignificant. From this, we can infer that the influence of lignin on the low-temperature behavior of the binder is negligible. Although the change is negligible, LBC-5% and LBC-10% showed a slightly lower m-value, as well as a slightly greater S value compared to pure bitumen. This indicates that the LBC binder is less flexible and has higher thermal stress than pure bitumen. This makes it

slightly susceptible to thermal cracking. Thus, pure bitumen showed a slightly better low-temperature property than LBC binder. 4.4.2. Tension retardation test (TRT) The TRT test has the ability to measures the flow characteristics of the new binder especially at a very low temperature of 25 °C

E. Norgbey et al. / Construction and Building Materials 230 (2020) 116957

Fig. 6. BBR Test results (m-value and creep stiffness) at 22 and 28 °C for A. unaged samples and B. aged samples.

Fig. 7. Tension retardation test result on samples.

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with high precision [25]. The tension retardation test uses a principle similar to the tensile creep test (EN12697-46). Fig. 7 shows the comparison of different viscosities of the different binders containing bitumen combined with 0%, 5% and 10% lignin. It was observed that low viscosity values were recorded for the bitumen binder grade (160/220) while bitumen binder grade (70/100) showed the highest viscosity values. The paving grade (160/220) was blended with lignin. It was noted that the addition of 5% lignin increased the viscosity of bitumen binder grade (160/220). A further increase in the lignin content by 10% further increased the viscosity of the binder.

From Fig. 7, it is seen that the TRT provides a clear picture of the viscosity properties of the different binders. For all the binder samples tested, the viscosities at the 5, 15, 25 °C could be clearly distinguished. The test results clearly show that different binders have different viscosities. Furthermore, the test results show that the same binder with different concentration of lignin (5%, 10%) also have different viscosity. The logarithm viscosity values of bitumen binder grade (160/220) at 5, 5, 15, 25 °C were 1.5192, 3.0182, 4.5172, 6.0162 MPas respectively while that of B70/100 at 5, 5, 15, 25 °C were 2.1057, 3.4637, 4.8217, 6.1797 MPas respectively showing the maximum and minimum viscosities

Fig. 8. Temperature sweep curve for unaged and aged samples of (A) complex shear modulus against temperature and (B) phase angle against temperature.

Fig. 9. Rutting factor for (A) unaged sample at 40 and 60 °C; (B) unaged sample at 70 and 80 °C; (C) aged sample at 40 and 50 °C and (D) aged samples at 70 and 80 °C.

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values in the TRT test. Furthermore, the logarithm viscosity values of the bitumen binder grade (160/220) combined with 5% were 1.6509, 3.1479, 4.6449, 6.1419 MPas at 5, 5, 15, 25 °C were respectively. When the bitumen binder grade (160/220) was doped with 10% lignin, the logarithm viscosity values of 1.8622, 3.3152, 4.7682, 6.2212 MPas were obtained at 5, 5, 15, 25 °C respectively. From the graph, the distinct logarithm viscosity values of the various binders at the temperature range signify that the respective binders could be clearly distinguished using the tension retardation test. We also observed a decreasing trend for the logarithmic viscosities values as the temperature kept on increasing. This clearly shows that for all the binders observed, an increase in temperature reduces the viscosity of the binder. We observed from Fig. 7 the high correlation values of all binders with R2 > 0.9991 for all the binders under study. The high correlation seen in the graph shows that the tension retardation is highly accurate without giving ambiguous results. Thus, the TRT can be depended on to provide good results. The TRT accurately measures the viscosities of binders at low temperature. The regression line helps indicate the temperature sensitivity of the binder. With the equations obtained for each binder, the viscosity of each binder can easily be determined in the service temperature range. This is possible due to the high correlation obtained from the regression lines. The test can easily determine the changes in viscosities of a binder doped with lignin as low as 5% addition.

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4.5. High-temperature behavior 4.5.1. Temperature sweep curve (TSC) Fig. 8 shows the relationship between the phase angle (d), complex shear modulus (G*), and rutting factor (G*/sin d) of the samples with changes in temperature. The complex modulus measures the total resistance of the binder to deformation (rutting) when repeatedly sheared. The rutting factor is used by Superpave to grade bitumen binders according to their resistance to rutting at high-pavement temperatures. It must be noted that bitumen binders with the high G*/sin d have more resistance to rutting [6]. For both unaged and aged binder, the complex shear modulus value of the LBC binder at each temperature was higher than pure bitumen. As the lignin content increased from 5% to 10%, the complex shear modulus further increased indicating that the addition of lignin to the binder made the binder stiffer. For the phase angle (Fig. 8), at each temperature, adding lignin to the bitumen causes a decrease in the phase angle value. It must be noted that LBC-10% had the lowest phase angle value at each temperature. In general (Fig. 8), the complex shear modulus of the binders (unaged and aged) decreases with the increase of temperature while the phase angles increase with the increase of temperature. The rutting factor (G*/sin d) is the indicator of bitumen’s ability to resist rutting. Higher G*/sin d means better rutting resistance. G*/sin d values of the bitumen and the LBC binder were calculated and shown as a function of test temperatures in Fig. 9. G*/sin d increased for all test temperatures when lignin was added to the

Fig. 10. MSCR results for unaged samples showing (A) non-recovery at stress level of 0.1 kPa; (B) recovery at stress level of 0.1 kPa; (C) non-recovery at stress level of 3.2 kPa; (D) recovery at stress level of 3.2 kPa.

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Fig. 11. MSCR results for aged samples showing (A) non-recovery at stress level of 0.1 kPa; (B) recovery at stress level of 0.1 kPa; (C) non-recovery at stress level of 3.2 kPa; (D) recovery at stress level of 3.2 kPa.

bitumen indicating better rutting resistance of the LBC binder compared to unmodified bitumen. 4.6. Multiple stress creep recovery (MSCR) The MSCR test was used to analyze the recovery and nonrecovery properties of the LBC binder with regards to recovery (R) and non-recoverable creep compliance (Jnr). The stress levels of 0.1 kPa and 3.2 kPa measurements were performed with ten cycles each. The analysis was carried at 58 °C, 64 °C and 70 °C. The Jnr and R results of both unaged and aged binders are shown in Fig. 10. Changes in the testing temperature and applied stress levels had a significant change in the Jnr and R values for both unaged and aged samples. The R values decreased with the increase in temperature and stress levels while Jnr values increased with the increase of temperature and stress levels. The pure bitumen binder had the greatest Jnr value while LBC-10% had the lowest Jnr value at each test temperature and stress level. The same can be said of the aged samples showing that as lignin content increases, the Jnr value decreases at each test temperature and stress level. The Jnr refers to the non-recoverable strain of the binder after the initial load is taken. This indicates that the LBC binder improves the rutting resistance supporting TSC results and past research works [17]. The R results showed negative values for the unaged binder (Fig. 10). This is due to the soft nature of the bitumen binder used during the test. However, the aged binder (Fig. 11) showed a clear

correlation with the clear difference between the pure bitumen and the LBC binder. The pure bitumen binder had the lowest R-value while LBC-10% had the greatest R-value at each test temperature and stress level. It is worth noting that the greater the R-value, the higher the tendency of the binder return to its initial state. Thus, this indicates that LBC binder recovers more than the pure bitumen during the recovery period. This is buttressed by the fact that as the lignin content increased from 5% to 10% the R value increased at each temperature and stress levels (Figs. 10 and 11). The results from our studies were similar to past results by Xu et al. [22] and support the fact that lignin increases the rutting resistance and recovery component of the bitumen binder under high-temperature conditions. Furthermore, Fig. 12 shows the deformation of unaged and aged binder at different temperature and stress levels. In general, the deformation percentage decreased as the lignin content was increased from 5 to 10% under different stress levels and temperatures. A similar trend was observed with the aged samples, thus indicating that the presence of lignin in the bitumen binder decreases the deformation rate of the base binder. 4.7. Damage tolerance – linear amplitude sweep (LAS) The damage tolerance and the fatigue resistance of the binder were analyzed through the LAS test. Fig. 13 shows the relationship between integrity parameter (C) and the damage intensity (D) for the unaged samples, In general, it is observed that C decreases as

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Fig. 12. MSCR of deformation with time at stress levels and temperature of: (A) 0.1 kPa & 58 °C (B) 3.2 kPa 58 °C (C) 0.1 kPa 64 °C (D) 3.2 kPa 64 °C (E) 0.1 kPa 70 °C (F) 3.2 kPa 70 °C.

D increases. It is worth noting that the C value of one shows the highest level of integrity of the binder indicating that the binder is in an undamaged state. C value of zero indicates a complete failure of the binder [31].

From our results, at each damage level, pure bitumen exhibited the highest integrity value followed by LBC-5%. LBC-10% had the lowest integrity value indicating that as the lignin content increases in the binder, the integrity value (fatigue resistance)

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Fig. 13. Relationship between (A) integrity parameter and the damage intensity; (B) G*sin (d) and damage intensity.

decreases. This result supports past works by Xu et al. [22], and clearly shows that the presence of lignin in the binder will reduce the damage tolerance level of the binder. 4.8. Adhesion test between LBC and aggregate rolling bottle test (RBT) Fig. 14 shows the degree of binder coverage on the aggregates versus the rolling time. The results clearly show that LBC-10% lignin had a higher degree of coverage in aggregate than unmodified bitumen at different rolling time. After 6 h, the lignin in LBC binder does not seem to affect the adhesion of the bitumen to the aggregates. After 6 h of rolling, the percentage of coverage of unmodified bitumen to aggregate was 85% while that of LBC binder to aggregate was 90%, showing less difference in the degree of coverage. However, after 24 h of the testing, it was obvious that LBC samples had a higher affinity for the aggregate compared to unmodified bitumen binder. This observation was even more significant as the testing time in the rolling bottle test increased to 72 h. After

a prolonged rolling time of 72 h, LBC showed a higher adhesion with aggregate as compared to the unmodified binder (Fig. 15). The presence of the lignin in bitumen exhibit a greater adhesion with the aggregate compared to the base binder. Future tests are needed to complement our preliminary study using RBT method due to its shortcomings on poor reproducibility. 4.9. Storage stability of LBC binder The penetration test and temperature sweep curve on the top and bottom sections of the samples after the tube test are shown in the Table 5 and Fig. 16 respectively. The tube test simulated the worst-case scenario of a binder over time period without mixing in a construction plant. The results (Table 5 and Fig. 16) show that the degree of separation varies among the different lignin content in the samples. The temperature sweep curve (Fig. 16) shows a disparity between the top and bottom sections of the samples with different lignin content. Furthermore, statistical analysis showed a

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Fig. 14. Degree of bitumen coverage for LBC binder and unmodified binder on aggregate.

Fig. 15. Degree of coverage of LBC and unmodified binder on aggregate after testing.

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Table 5 Penetration test after tube test. Penetration test

B160/220

B160/220 + 5% lignin

B160/220 + 10% lignin

Top Bottom

196.0 196.0

169.0 155.0

164.0 138.0

3.

4. significant difference on the penetration test values. It was observed that difference between the top and the bottom layer for both LBC binder with 5% and 10% lignin was content was statistically significant showing a p-value of less than 0.05. Further studies need to be done on how to better add lignin to the bitumen binder. 5. Conclusion

5.

6. 7.

The aim of this study was to evaluate the feasibility of using WL as a bitumen modifier and extender. For this, the characterization, morphology, decomposition behavior, low and high-temperature behavior, fatigue resistance, storage stability as well as the composite binder’s adhesion with aggregate were tested according to standards. The various tests results were analyzed and compared with past studies. The study showed that: 1. The increase in the Fpeak values (force ductility test) due to an increase in the lignin content correlates with a decrease in the penetration test values, which can be interpreted as a stiffening or increase in cohesion. 2. The flow characteristics of the binder at low temperature decreased with increasing lignin content. The high correlation (R2 > 0.9991) in the TRT shows that TRT is highly accurate in

8.

describing the flow characteristics of the binder without giving ambiguous results. Lignin did not have any significant change in the binder’s mvalue and S value (P value > 0.05) in the BBR test. Thus, lignin did not have a significant influence on the low temperature cracking behavior of the binder. At higher temperatures, the addition of lignin to the bitumen binder increased the rutting resistance of the base binder; this was observed from the DSR test results. The MSCR results showed that during the recovery period, the LBC binder recovered better and deformed less at different temperature and stress levels than pure bitumen. In addition, the presence of lignin significantly increased bitumen binder’s viscosity. The LAS results showed that adding lignin to bitumen decreased the fatigue resistance of the composite binder. In terms of bonding with aggregate, the LBC binder had a better adhesion with aggregate as compared to pure bitumen during the rolling bottle test. Due to the soft nature of the base binder, the addition of 10% lignin would have a negligible influence on the workability and compaction characteristics on the asphalt mixture produced with it.

The result from our study points to the potential of lignin as a suitable sustainable bitumen extender and will serve as a baseline for future research works. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 16. Temperature sweep curve on tube test samples showing the complex modulus against temperature.

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Acknowledgements This research was funded by the Federal Ministry of Education and Research, Germany through the Green Talent Program and the National Natural Science Foundation of China (No. 41663010).

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