Bioasphalt Binders: Introducing Sustainability In A Non-Renewable Road Construction Material

Abhinay Kumar PhD Research Scholar, Department of Civil Engineering, Indian Institute of Technology Guwahati, Assam            Rajan Choudhary Professor, Department of Civil Engineering, Indian Institute of Technology Guwahati, Assam    Rupam Kataki Professor, Department of Energy, Tezpur University, Tezpur, Assam    Ankush Kumar PhD Research Scholar, Department of Civil Engineering, Indian Institute of Technology Guwahati, Assam

With a population of 1.4 billion and being one of the fastest growing world economies, India must ensure security and sustainability of energy systems to sustain the economic growth. India is the third largest consumer of petroleum crude after China and the US, and is dependent on foreign imports for about 83.8 percent of its crude oil demand (Sharma, 2020). Realizing the finite nature of fossil fuel reserves, there is a strong motivation to use alternative resources, particularly bio-based renewable sources for fulfilling the needs for energy and fuels. This is also apparent from the increased push by the government to use bio-ethanol as partial substitute for petrol and bio-diesel as partial substitute for petroleum-based diesel.

The road network in India is the world’s second largest with more than 5.89 million km of roads spanning the length and width of the country (MoRTH, 2020). However, the international standard highways (national highways and expressways) comprise only 2 percent of the total road length and carry more than 40 percent of road traffic (PIB, 2020). To meet the demand for high standard and durable highway network, many highway development projects aimed at construction/ expansion/ upgradation of the nation’s road infrastructure are under implementation. An amount of US $1.4 trillion has been allocated under the National Infrastructure Pipeline for 2019-25, aiming for the infrastructural development, and out of this the road sector accounts for about 18 percent of the capital expenditure (IBEF, 2020).

More than 95 percent of the road pavements in India are bituminous type (also called flexible pavements). Construction of pavements demands huge quantities of materials, primarily stone aggregates and the asphalt binder (bitumen). The bituminous binder (or asphalt binder) used for the construction of flexible pavements is primarily derived from fossil fuels (petroleum crude oil). Depletion of crude oil reserves also impact the supply and price of bitumen. It is thus very important to look for ways to partially substitute or replace the crude-based asphalt binder with alternative and renewable materials. Considering the strong push for road infrastructure development, the application of such renewable bio-based materials will positively impact the economy and energy security in regard to road infrastructure development.

The use of renewable biomaterials or products in road infrastructure development is a noteworthy emerging research domain. Biomaterials are receiving widespread interest primarily due to their renewability, lower price, environment friendliness, and less dependency on petroleum-based resources. Biomass may refer to any organic matter that stores solar energy and includes all plants, animals, microbes and the organic matter derived from these organisms. Plants produce biomass continuously through the process of photosynthesis. Biomass contains a significant amount of carbohydrates, e.g. cellulose and hemicellulose. Processing of biomass from wood industries, agriculture, forestry and other spheres results in the production of wastes. It has been estimated that India produces about 370 million tons of agricultural and forestry biomass wastes per year (Mary et al. 2016). Some examples of such wastes include saw dust, waste wood, seed cover, etc. These ‘bio’-wastes can function as feedstock for the production of sustainable materials for use in road construction.

‘Bioasphalt binder’ is a term used to denote asphalt binders or bitumen modified/partially substituted/replaced with renewable bio-based or bio-derived materials. Review of international literature in this field shows that a variety of materials have been investigated to yield bioasphalt binders (Su et al. 2018; Al-Sabaeei et al. 2020; Kumar et al. 2020a). One such category is materials derived from pyrolysis of different types of biomasses. The term ‘pyrolysis’ originates from two Greek words: ‘pyro’ meaning fire and ‘lysis’ which means separation. Pyrolysis is a mainstream thermochemical technology that converts biomass into liquid (biooil), solid (biochar), and gaseous (syngas or producer gas) products. Pyrolysis of biomass refers to thermochemical decomposition of the biomass in an oxygen deficient atmosphere at temperatures of 300-1000 °C. Biochars and biooils derived from pyrolysis of waste biomasses are of significant interest as they have shown good potential when used/replaced with conventional bituminous binders, while their use also contributes to the disposal and management of biomass waste. Another beneficial aspect of pyrolysis-derived biomaterials is that the biomasses considered do not compete with the demands of either food or fodder. While many bioasphalt studies have utilized biooils from different biomasses as partial substitute to petroleum-based asphalt binder, the use of biochar for bioasphalt has started gaining attention recently. Biochar being a carbon-rich material shows good possibility as other carbonaceous additives (examples include carbon black, carbon fibers, waste toner ink, activated carbon, and coke dust) that have already been used for asphalt modification. Moreover, biooil which is the primary product of the pyrolysis process, can be later upgraded to alternative fuels, whereas biochar is generally considered a pyrolysis by-product and therefore its use in bioasphalt also constitutes a channel for its large-scale utilization.

When alternative materials are added to asphalt binders or asphalt mixtures, it is quite important to characterize the composite through evaluation of rheology of the bio-binder/bitumen under different temperature and aging conditions to assess the performance towards various distresses. This requires dedicated and focused research efforts given the wide variety of bio-based materials having potential for use in bioasphalt. The following discussion presents some insights into some previous and recent research work on bioasphalt binders by our research group using biochar obtained from pyrolysis of two different biomasses, seed cover waste of Mesua ferrea tree and bamboo chips, abundantly available in North eastern regions of country.

Use Of Biochar In Bioasphalt Binders And Their Rheological Characterization

Biochar obtained as a by-product from the pyrolysis of seed cover waste of Mesua ferrea tree (rose chestnut and locally known as Nahor in the North-Eastern part of India) and bamboo chips are studied for production of bioasphalt binders. Pyrolysis of seed cover waste of M. ferrea and bamboo chips is mainly targeted to get biofuels and the biochar is generated as a by-product of the process. Details of pyrolysis process can be referred elsewhere (Kumar et al. 2018, 2019). The physical appearance of both M. ferrea seed cover and bamboo biochar is like a black powder. The scanning electron microscopy (SEM) images of both biochars are shown in Fig.1, which reveal rough, irregular and porous surface attributes of the biochars.

 
Fig. 1: SEM Images of Biochar: (a) Mesua Ferrea Seed Cover (Kumar et al. 2018), (b) Bamboo (Kumar et al. 2019)

The bioasphalt binders were prepared by blending different contents of biochar with a conventionally used viscosity grade (VG 30) asphalt binder using a high shear mixer. Fig. 2 shows the schematic flowchart of the preparation process of bioasphalt binders. Dosage for preparation of the blends was chosen as 5%, 10%, 15%, and 20% by neat binder weight for both biochars. Under rheological investigation, the Superpave rutting parameter (G*/sin δ, where G*: complex shear modulus; δ: phase angle) of bioasphalt showed an enhanced performance with increasing modifier dosages which implied increased stiffness of the bioasphalt binders (Kumar et al. 2018). Further, bioasphalts showed enhanced persistence towards rutting as the multiple stress creep and recovery (MSCR) results yielded lower non-recoverable compliance (Jnr) and accumulated strain. Also, the stress-sensitivity of the binders was found to decrease with increasing biochar dosages (Kumar et al. 2018; 2019). In terms of asphalt mix properties, both static creep and dynamic creep test outputs indicated improved rutting resistance of bioasphalt mixes with bioasphalt binders from bamboo biochar when compared to the control mix. Additionally, bioasphalt binders from bamboo biochar aided to improved resistance of bioasphalt mixes against moisture-induced damages when assessed in terms of tensile strength ratio (TSR), which also increased with increment in biochar dosages (Kumar et al. 2019). Bioasphalt with M. ferrea seed cover biochar also improved binder’s aging resistance, which was determined through the rheological aging index (RAI) at 64°C based on G*/sin δ result values for original (unaged) and short-term aged binders. The reduction in aging susceptibility of bioasphalt from the M. ferrea seed cover biochar was designated to the presence of phenolic compounds that offered an anti-oxidant characteristic to the bioasphalt (Kumar et al. 2018).

Fig. 2: Schematic Flowchart of the Bioasphalt Preparation Process

Further, results of recent study conducted to characterize temperature susceptibility (based on three approaches) and rutting susceptibility (based on MSCR test) of bioasphalt binders with biochars derived from the pyrolysis of Mesua ferrea seed cover (abbreviated as BCMF) and bamboo chips (abbreviated as BCB) are presented in this article. The bioasphalts with BCMF biochar at 5%, 10%, and 15% dosages are respectively denoted as BCMF-5, BCMF10, and BCMF-15. Corresponding notations for BCB bioasphalts are BCB-5, BCB-10, and BC-15. Being a thermoplastic material, the consistency of an asphalt binder changes considerably with temperature. Temperature susceptibility of the binder indicates the rate at which a property of the binder changes with temperature. Highly temperature susceptible binders can result in tender mix problems during mix compaction and/or shrinkage cracking at low service temperatures (Roberts et al. 1996). Therefore, asphalt binders with a lower temperature susceptibility are desirable. In this study, temperature susceptibility was determined for control and bioasphalt binders using three approaches: (1) penetration index (PI), (2) viscosity-temperature susceptibility (VTS) based on viscometry, and (3) VTS based on rheometry. The biochar dosages used are 5%, 10%, and 15% by weight of the neat VG 30 grade binder. PI was first proposed by Pfeiffer and Van Doormaal (1936) and is calculated based on penetration and softening point of the binder using Equation 1 (Hunter et al. 2015):

where, pen = penetration (1/10 mm) and SP = softening point (°C). Equation 1 assumes that penetration of the bitumen at softening point is 800. Fig. 3 shows the results of PI values of control and bioasphalt binders. It has been reported that most paving bitumen have PI in the range of –1 to +1 whereas bitumen with PI lower than –2 are highly temperature susceptible and may be very brittle at low temperatures (Roberts et al. 1996). In Fig. 3, the PI values of binders vary in the range –0.25 to –0.16 and the bioasphalt binders have higher PI than the control asphalt indicating that the inclusion of biochars reduces the binder temperature susceptibility. Further, a higher biochar concentration leads to a binder with lower susceptibility to temperature, which is seen for both BCB and BCMF modified binders. 

The temperature susceptibility of a binder can also be determined using the slope of the viscosity-temperature profile obtained through viscosity measurement at two or more temperatures. The parameter thus defined is referred as the viscosity-temperature susceptibility (VTS). Two approaches were followed in the study for VTS determination, based on temperature ranges and instruments used for viscosity measurements. In the first approach, VTS was derived based on viscosity measurements at 60 °C and 135 °C using viscometers. VTS was calculated as per Equation 2: 

where vis(T1 ) and vis(T2 ) are viscosities (cSt) at temperatures T1 (60 °C) and T2 (135 °C), respectively. In the second approach, VTS was calculated as the absolute value of the slope of double logarithm of viscosity and logarithm of temperature. The viscosity was determined on a dynamic shear rheometer (DSR) and the temperatures used were 40, 50, 60 and 70 °C. Fig. 4 presents the VTS results obtained based on both approaches. A higher slope of viscosity-temperature profile would indicate a rapid viscosity change with temperature and hence a lower VTS is desirable. As Fig. 4 shows, the addition of both biochars (BCB and BCMF) reduces the VTS and therefore reduces the susceptibility of viscosity to changes in the temperature. In the first approach, the typical VTS values observed for paving bitumen range from 3.36 to 3.98 (Puzinaukas, 1967). The VTS values for the bioasphalt binders prepared are found to range from 3.48 to 3.69, which lies in the typical range reported. The VTS results agree with PI values and indicate that bioasphalt binder systems have a lower temperature susceptibility than the control binder, and the improvements become significant at higher biochar contents. The reduction in temperature susceptibility of binders on addition of biochar is indicative of changes in the colloidal structure of the modified binder (Zhao et al. 2014). 

MSCR test is gaining wide popularity for rutting characterization of asphalt binders with demonstrated correlations to field rutting performance (Liu et al. 2020). The test was originally developed as research output of the US Federal Highway Administration (FHWA) to overcome deficiencies associated with the Superpave rutting parameter (G*/sin δ) (D’Angelo et al. 2007). The test has also been included in the latest revision of the Indian Standard (IS 15462-2019: Polymer Modified Bitumen (PMB) - Specification). The MSCR test was performed on control and bioasphalt binders using a DSR. Four test temperatures were used: 40, 50, 60 and 70 °C, representing a range of high pavement service temperatures at which rutting resistance of the binder becomes critical. The stress levels used were 0.1 and 3.2 kPa on short-term aged binders (all binders were short-term aged using a rolling thin film oven (RTFO) at 163 °C for 85 min). During each creep-recovery cycle, the creep stress was applied for 1 s followed by recovery for 9 s (i.e. time for one cycle = 10 s). This pattern of load-recovery is expected to better simulate the actual traffic loading conditions wherein an element in the bituminous mix is deformed by the traffic load and the deformation is recovered (partially) as the vehicle travels away from the element. Further, some recent research studies have shown that the application of thirty (instead of conventionally used ten) cycles in the MSCR test provides more reliable/homogeneous response due to the attainment of a stable steady-state creep behavior of the asphalt binder (Golalipour et al. 2016; MorenoNavarro et al. 2019). Thirty MSCR cycles were therefore used in this study and the last five cycles were used for the determination of results. The stress was immediately increased from 0.1 kPa to 3.2 kPa at the conclusion of 30 MSCR cycles at 0.1 kPa. Non-recoverable creep compliance (Jnr) obtained from the MSCR test is generally used as a rutting potential index. Jnr is calculated as the ratio of non-recovered strain at the end of a creep-recovery cycle and the stress level (Equation 3):

J_nr = ε_{nr / σ  (3)}

where, Jnr = non-recoverable creep compliance (kPa–1); εnr = nonrecovered strain at the end of a creep-recovery cycle, and σ = stress level (kPa).

 
Fig. 5: Strain-Time Response of Bioasphalt (a) BCB Binders (b) BCMF Binders

Fig. 5 shows the strain-time plot of the last five cycles of all binders at 60 °C at 3.2 kPa stress levels (similar results were found at other temperatures and are omitted for brevity). The addition of biochars reduces the strains that the binders are subjected to at high pavement service temperature and high stress level, which is beneficial for the resistance against permanent deformation. Fig. 6 displays the MSCR Jnr results of the control and bioasphalt binders at shear stress levels of 0.1 and 3.2 kPa, respectively at all four test temperatures. Higher test temperatures and stress levels mobilize more permanent deformation in the asphalt binders, which is also seen from increase in Jnr with increase in temperature and stress level in Fig 6.

Fig. 6: MSCR Jnr Results of Bioasphalts (a) 0.1 kPa Stress (b) 3.2 kPa Stress 

In terms of the effect of biochars, it is observed that the addition of biochars improves the rutting resistance as non-recoverable creep compliance (Jnr) decreases for the bioasphalt binders at all four test temperatures. Further, higher the biochar content, lower is the Jnr at all test temperatures and stress levels. For example, at 50 °C temperature and 0.1 kPa creep loading level, the J_nr of BCB-5, BCB-10, BCB-15, BCMF-5, BCMF-10, and BCMF-15 binders are 26%, 37%, 54%, 19%, 32%, and 38% lower than the control binder. The corresponding reduction percentages at 3.2 kPa are 24%, 35%, 53%, 16%, 32%, and 37%, which are almost quite close to those observed at 0.1 kPa. Therefore, biochar modification has about the same contribution to rutting resistance improvement at both levels of creep loading. Previous studies on biochar modification of asphalt have shown that that effect of biochar was predominant at high temperatures (or low frequencies) while the effect was somewhat less significant at low temperatures (or high frequencies), and therefore biochar particles have different interactions with the asphalt binder compared to inactive solid fillers (Zhao et al. 2014). Due to its hollow fibrous and porous morphology, and ionic adsorption characteristics, biochar particles are reported to adsorb high molecular weight bitumen fractions such as asphaltenes and their derivatives leading to the presence of a thin film of asphaltenes on biochar particles (Chebil et al. 2000; Zhao et al. 2014; Walters et al. 2015). The blending of biochar with the asphalt binder causes inter-particle and inter-molecule interactions in the bulk of the blend that leads to improvement in the binder stiffness as seen by increase in viscosity, and reduction in MSCR Jnr (Chebil et al. 2000; Dong et al. 2020). It is also observed that the BCB biochar leads to higher reductions in Jnr than BCMF biochar, which shows that the source of biochar does affect the performance properties of the bioasphalt binders. In addition to the biomass source, pyrolysis conditions may also affect the biochar physical and surface properties such as morphology, specific surface area, pore size distribution and volume, surface chemical functional groups, etc. More detailed investigations are needed to gain insight on how differences in biochar physical and surface properties affect the properties of the resulting bioasphalt binders.

The strain-time response of the binders during one MSCR creeprecovery cycle can be observed from Fig. 7 for control, BCB-10, and BCMF-10 binders. In the creep phase (one second at the beginning of the cycle), the strain increases sharply as the load is applied. In the recovery phase (nine seconds after the creep phase), some strain is recovered immediately while some viscous strain recovers gradually with time. This response is typical of a viscoelastic material such as asphalt binder. The same response is also exhibited by the bioasphalt binders. Using the theory of linear viscoelasticity, a four element Burger’s model can mathematically represent the creep-recovery response of the binders. Burger’s model essentially consists of a series arrangement of a Maxwell element (series arrangement of a spring and a dashpot) and a Kelvin element (parallel arrangement of a spring and a dashpot). Details of the model constitutive equation and fitting methodology can be found in results published elsewhere (Kumar et al. 2020b). Fig. 7 also shows the Burger’s model fit (shown by solid lines) to the strain-time profile of control, BCB-10, and BCMF-10 binders at 60 °C temperature and 3.2 kPa stress level. The model fits the data very well with coefficient of determination (R2 ) of 0.99. The parameter ηM represents the coefficients of viscosity in the Maxwell element in the Burger’s model, and characterizes the strain due to viscous flow; a higher value corresponding to a lower permanent strain and therefore a better rutting resistance. Fig. 8 shows the ηM values of the binders at 60 °C and 3.2 kPa obtained from Burger’s model fit. The values of ηM are found higher for the bioasphalt binders with both biochars than the control binder. The values further increase with increase in the dosage of biochar, which is also in congruence with the trends of MSCR Jnr implying an improved resistance of bioasphalt binders against permanent deformation. 

Concluding Remarks And Perspectives For Future Research

Results of the recent studies showed a good potential for use of biochar from pyrolysis of waste biomass in production of sustainable/ bioasphalt binders. Biochar dosages showed a significant impact on the ability of the binders to resist stresses at high service temperatures (40–70 °C). An interaction of biochar with the asphalt binder helped to lower the temperature susceptibility and improve the rutting resistance across different temperatures and stress levels. The use of products derived from pyrolysis of waste biomasses in bioasphalt presents a unique opportunity as an avenue for large-scale application of the products while enabling partial substitution of asphalt binders with renewable non-crude based materials.

More investigations are certainly needed to understand the chemistry of bioasphalt binders to understand the modified asphalt microstructure, modification mechanisms, and their relationship to rheology. Studies on performance of the bioasphalt binders should take into consideration aspects such as thermal storage stability, aging resistance, and performance at high, intermediate and low pavement service temperatures. Characterization of bioasphalt mixes in view of critical pavement distresses including rutting, moisture-induced damage, fatigue cracking, thermal cracking, is required for large scale use and adoption. It is envisaged that continued research efforts on bioasphalt binders and mixes will be significant in realizing engineering practices leading it to become a mainstream pavement construction technology.

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