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.

References

1. Al-Sabaeei, A. M., Napiah, M. B., Sutanto, M. H., Alaloul, W. S., and Usman, A. (2020). A systematic review of bio-asphalt for flexible pavement applications: Coherent taxonomy, motivations, challenges and future directions. Journal of Cleaner Production, 249, 119357.

2. Chebil, S., Chaala, A., and Roy, C. (2000). Use of softwood bark charcoal as a modifier for road bitumen. Fuel, 79(6), 671-683.

3. D’Angelo, J., Kluttz, R., Dongre, R. N., Stephens, K., and Zanzotto, L. (2007). Revision of the superpave high temperature binder specification: the multiple stress creep recovery test (with discussion). Journal of the Association of Asphalt Paving Technologists, 76, 123–162.

4. Dong, W., Ma, F., Li, C., Fu, Z., Huang, Y., and Liu, J. (2020). Evaluation of antiaging performance of biochar modified asphalt binder. Coatings, 10(11), 1037.

5. Golalipour, A., Bahia, H. U., and Tabatabaee, H. A. (2017). Critical considerations toward better implementation of the multiple stress creep and recovery test. Journal of Materials in Civil Engineering, 29(5), 04016295.

6. Hunter, R. N., Self, A., Read, J., and Hobson, E. (2015).  The Shell Bitumen Handbook (pp. 744-747). London, UK: ICE Publishing.

7. IBEF (2020). Road infrastructure in India, Indian Brand Equity foundation. Accessed from: https://www.ibef.org/industry/roads-india.aspx

8. Kumar, A., Choudhary, R., Narzari, R., Kataki, R., and Shukla, S. K. (2018). Evaluation of bio-asphalt binders modified with biochar: a pyrolysis by-product of Mesua ferrea seed cover waste. Cogent Engineering, 5(1), 1548534. 

9. Kumar, A., Choudhary, R., Nirmal, S.K., Pandey, I.K., and Kataki, R. (2019). Towards sustainable asphalt binders: Evaluation of bio-asphalt binders and mixes with biochar, Journal of the Indian Roads Congress, 80(3), 5-15.

10. Kumar, A., Choudhary, R., and Kumar, A. (2020a). Use of Products from Pyrolysis of Wastes in Asphalt Binder Modification: A Review, RECYCLE-2020: 3rd International Conference on Waste Management, February 13-14, 2020, IIT Guwahati, Assam. 11. Kumar, A., Choudhary, R., and Kumar, A. (2020b). Characterisation of asphalt binder modified with ethylene–propylene–diene–monomer (EPDM) rubber waste from automobile industry. Road Materials and Pavement Design, 1-25.

12. Liu, H., Zeiada, W., Al-Khateeb, G. G., Shanableh, A., and Samarai, M. (2020). Use of the multiple stress creep recovery (MSCR) test to characterize the rutting potential of asphalt binders: A literature review.  Construction and Building Materials, 121320. 13. Mary, G. S., Sugumaran, P., Niveditha, S., Ramalakshmi, B., Ravichandran, P., and Seshadri, S. (2016). Production, characterization and evaluation of biochar from pod (Pisum sativum), leaf (Brassica oleracea) and peel (Citrus sinensis) wastes.  International Journal of Recycling of Organic Waste in Agriculture, 5(1), 43-53.

14. Moreno-Navarro, F., Tauste, R., Sol-Sánchez, M., and Rubio-Gámez, M. C. (2019). New approach for characterising the performance of asphalt binders through the multiple stress creep and recovery test. Road Materials and Pavement Design, 20(sup1), S500-S520.

15. MoRTH (2020). Annual Report 2019-20, Ministry of Road Transport and Highways, Govt of India, New Delhi.

16. Pfeiffer, J. P. H. and Van Doormaal, P. M. (1936). The rheological properties of asphaltic bitumens. Journal of the Institute of Petroleum 22, 414–440.

17. PIB (2020). World Bank signs $500 million project to develop green, resilient and safe highways in India, Press Information Bureau, Govt of India, Release ID 1682770.

18. Puzinauskas, V. P. (1967). Evaluation of properties of asphalt cements with emphasis on consistencies at low temperatures. Journal of the Association of Asphalt Paving Technologists, 36, 489–540.

19. Roberts, F. L., Kandhal, P. S., Brown, E. R., Lee, D. Y., and Kennedy, T. W. (1996). Hot mix asphalt materials, mixture design and construction, 2nd Edition, National Asphalt Pavement Association Research and Education Foundation.

20. Sharma, S. (2020). Modi’s broken dream of cutting India’s dependence on oil imports; data reveals opposite  story. Accessed from: https://www. financialexpress.com/economy/modis-broken-dream-of-cutting-indiasdependence-on-oil-imports-data-reveals-opposite-story/1855834/

21. Su, N., Xiao, F., Wang, J., Cong, L., and Amirkhanian, S. (2018). Productions and applications of bio-asphalts–A review. Construction and Building Materials, 183, 578-591.

22. Walters, R., Begum, S. A., Fini, E. H., and Abu-Lebdeh, T. M. (2015). Investigating bio-char as flow modifier and water treatment agent for sustainable pavement design. American Journal of Engineering and Applied Sciences, 8(1), 138-146.

23. Zhao, S., Huang, B., Ye, X. P., Shu, X., and Jia, X. (2014). Utilizing bio-char as a bio-modifier for asphalt cement: A sustainable application of bio-fuel by-product. Fuel, 133, 52-62

Building road infrastructure is an intensive process involving land-use, human resource, materials, and energy. The predominant goal of conventional road building with a typical design life of 15-30 years is minimizing the upfront construction costs. This is unlike some critical national infrastructure like nuclear or hydropower plants, which are designed atleast for 100 years. As such, roads undergo a faster life-cycle resulting into less sensitive long-term co-existence with environment and society. However,roads built with long-term, harmonious environmental-social-economic considerations are much more sustainable. 

To support her economic and social development, India has set herself on an ambitious journey of building her National infrastructure. Road building is at the forefront of this diversified plan. From expressways to rural roads, the speed of road construction is also accelerating the speed of consuming limited resources.Withfinite land, natural resource, and energy availability, it is increasingly becoming critical to effectively manage these resources for the envisioned growth. In addition, existing ways and methods need a critical review for their future applicability. This brings a chance to learn from other Nations that have walked a similar path. Hidden within these lessons are opportunities,when explored could lead to wholesome, stable, and resilient developments.The more learned Nations are implementing the solutions with a single motif of making earth more sustainable than before. One such recent concept, albeit in its infancy and yet to be characterized adequately is that of circular economy (CE). This article explores the necessity and urgency of implementing CE for the Indian road sector.

Grey Versus Green (Circular) Economy

Grey infrastructure is built without concern for sustainability, guided instead by upfront costs and conventional, prescriptive design standards and construction specifications. These include projects that have little concern for alternate modes of transportation, ecologically sensitive areas, wildlife, the wider community, and other issues of sustainability. On the other hand, Green infrastructure considers social and environmental issues, alongside economic concerns. These projects attempt to address environmental issues such as habitat fragmentation, deforestation and the destruction of wetlands, and social concerns such as alternative transport and public health. (1)

Grey infrastructure utilizes principles of linear economy and is often characterized by one-way, linear, eco-efficient use of materials using “take-make-dispose” approach. This product-focused economy is shorttermed and often leads to substantial waste generation, pollution, and down-cycling. In this economic systemvalue is created by producing and selling as many products as possible. Green infrastructure on the contrary, is based on CE characterized by eco-effective, “reduce-reuserecycle” plans. Focusing on perpetual, long-term, multi-cycle use, this economy often leads to creative upcycling that focuses on services. Refer Figure 1. The World is gradually getting convinced that CE is a viable, sustainable, and unavoidable alternative. Besides, it is compatible with the inherent interests of the corporations, as it is aligned with the competitive and the strategic frameworks and it is capable to enrich the contract between the consumers and the producers (2).

Fig. 1: (Left): Linear-Recycling-Circular Economies (2) (Right): Eco-Effectiveness and Eco-Efficiency (3)

Circular Economy, Defined

ACE aims to redefine growth, focusing on positive society-wide benefits. It entails gradually decoupling economic activity from the consumption of finite resources, and designing waste out of the system, while being ecologically intelligent (5). Underpinned by a transition to renewable energy sources, the circular model builds economic, natural, and social capital. It is based on three principles viz.
– Design out waste and pollution;
– Keep products and materials in use;
– Regenerate natural systems (6) 

In an analysis of 114 definitions of CE, one of the comprehensive definitions that emerge is the following. An economic system that replaces the ‘end-of-life’ concept with reducing, alternatively reusing, recycling, and recovering materials in production/distribution and consumption processes. It operates at the micro level (products, companies, consumers), meso level (eco-industrial parks) and macro level (city, region, nation and beyond), with the aim to accomplish sustainable development, thus simultaneously creating environmental quality, economic prosperity and social equity, to the benefit of current and future generations. Novel business models and responsible consumers enable it. (7)

Fig. 2: Circularity Strategies within the Production Chain, in the Order of Priority (8)

Strategies

A higher level of circularity of materials in a product chain means that those materials remain in the chain for a longer period, and can be applied again after a product is discarded, preferably retaining their original quality. Several circularity strategies exist to reduce the consumption of natural resources and materials, and minimise the production of waste. They can be ordered for priority according to their levels of circularity as summarized in Figure 2 (8). As per these hierarchies, it may be noted that recycling is not far from linear economy. 

It may be noted that while various strategies exist for the transitioning from linear to circular economies, the key however is in having an integrated approach. The solution will be realized when Economic and Social models are closely synced with Ecological model and intelligence. A study in the Netherlands has enumerated the benefits of transitioning to CE as shown in Figure 3. It has also been cautioned that realising these opportunities is no easy feat. Investments and new alliances between companies will be required, and the traditional, already established companies will likely slow the transition down. Government policy will often be needed to overcome barriers and to change the perception of the importance of natural resources. (9)

Fig. 3: Benefits of CE for the Netherlands

Initiatives, Challenges And Enablers For CE

It is vital to note that the business community popularized CE, which currently to a large degree is legislation driven. There are many listed at the European Circular Economy Stakeholder Platform (10), while other examples of such legislation include,

i). Closed Substance Cycle and Waste Management Act in Germany (1996);
ii). The Recycling-Based-Society in Japan (2002);
iii). Circular Economy Promotion Law in China (2009);
iv). EU Resource Efficiency Scoreboard (2015);
v). The Raw Materials Scoreboard (2016) (9)

The findings of an industry-wide survey conducted in Europe summarized following as the key challenges to the implementation of CE:

i). Lack of awareness at Industry-wide level;
ii). The absence of a broad consensus of what the CE looks like in the built environment;
iii). The parts of the supply chain, such as clients and designers, have little knowledge on how to adopt CE principles is likely to impede uptake of circularity in the short term;
iv). Lack of incentive to design for end-of-life issues;
v). Lack of market mechanisms to aid greater recovery and an unclear financial case;
vi). Fragmented nature of the construction industry;
vii). Existing stock of buildings and infrastructure where circularity principles have not been adopted.

All stakeholders ranked a clear business case as the most important enabler, with commercial viability identified in the breakout sessions as fundamental to shift current practices. Technical challenges including the lack of recovery routes and the complex design of buildings, whilst significant, are likely to be overcome to some extent through further research on enabling technologies and sharing of knowledge. (10)

CE And India

The challenges listed above are applicable to Indian economy as well. For India to achieve continued economic growth, poverty alleviation, hunger elimination, human development, andenvironmental improvement, new transformative and radical solutions are needed rather than incremental improvements. While India has committed to the UN Sustainable Development Goals (SDGs), termed the Agenda 2030, progress is hampered by haphazard urban development and ineffective regulatory controls. Moreover, the focus on long-term sustainability is often trumped by social and political turbulence, as well as unexpected disruptions such as terrorism, industrial accidents and extreme weather events. Despite all these, a recent FICCI-Accenture study suggested that India through various strategies could unlock approximately half-a-trillion dollars of economic value by 2030 through adoption of CE business models (12). A report based on high-level economic analysis of three focus areas key to the Indian economy and society viz. cities and construction, food and agriculture, and mobility and vehicle manufacturing, shows that a CE trajectory could bring India annual benefits of INR 40 lakh crore (USD 624 billion) in 2050, and would in addition reduce negative externalities (13).

Some of the measures that could help transitioning to CE include:

i). Product eco-labelling schemes;
ii). Tax incentives; 
iii). Design standards that include resource efficiency and use of secondary raw materials;
iv). Standard business models and financial mechanisms for end-of-life product recovery;
v). Social innovations;
vi). Special Economic Zones for establishment of recycling capabilities; Preferential “green” procurement schemes;
vii). Raising consumer awareness and stakeholder consultations on policy implementation experiences and possible improvements.

Stimulation of voluntary compliance and innovation will be critical to the future success of waste management and CE programs. Moreover, enlightened policies provide a foundation for change, but successful policy implementation will require stimulus for innovative demonstration programs at the local and regional level. (12)

Fig. 4: Value Realization Potential from Circular Business Models by 2030 (13)

The Case Of Ambuja Cement

Responsively responding and embarking her journey, Ambuja Cement has started working towards The Sustainable Development Ambition 2030, which provides a broad framework for the company’s strategies to meet the challenges in four broad thematic areas: Climate, CE, Water & Nature, and People & Communities. Refer Figure 5.

Indian Road Sector And Circularity

Indian economy boasts of 62,15,797 km (16) of roads constructed on the foundations of robust demand, attractive opportunities, higher investments and conducive policies. While this appears attractive, it must also be appreciated that most of these constructions are based on linear economy or at the most and partly recycling economy. This brings a cautious two-fold opportunity for the upcoming road construction projects. The first is in regard to dealing with the existing road network and second one to transition to an integrated, rapid and effective CE. 

This is a unique opportunity for India to define its CE driven road infrastructure. The new concept of sustainable road (18) is characterized by the following:

i). constructed to reduce environmental impacts;
ii). designed to optimise the alignment (vertical and horizontal including considerations of ecological constraints and operational use by vehicles);
iii). resilient to future environmental and economic pressures (e.g. climate change and resource scarcity);
iv). adaptable to changing uses including increased travel volumes, greater demand for public and active (cycling and walking) transport; and
v). able to harvest the energy to power itself

Fig. 5: A Case for CE in Construction Materials (15)

Awareness – An Essential First Step

One of the first steps for transitioning to CE is to educate and create awareness about its urgency and need. Efforts from governments, industry, the research community, and society will be needed to overcome these challenges and accelerate the efficient use of materials. Policy and action priorities could include the following:

i). Train, build capacity and share best practices;
ii). Develop regulatory frameworks and incentives to support material efficiency
iii). Adopt business models and practices that advance CE objectives 
iv). Shift behaviour towards material efficiency 
v). Increase data collection, life-cycle assessment and benchmarking
vi). Improve consideration of the life-cycle impact at the design stage and in climate regulations 
vii). Increase end-of-life repurposing, reuse and recycling (19)

Favourable Policy Framework

Road construction is largely done using public finance. Encouraging sustainable road construction with public funds can be done by regulating and implementing favourable policies, a sampling of which is summarized in Table 1.

Innovation Is The Key

Any road’s lifecycle starts from its concept and takes the path of materials, design, construction, use, maintenance and preservation and end of the life-cycle (19). In each of these aspects, innovation is the key. Figure 6 summarizes a broad list of such initiatives that need “all-stakeholder integration”

Fig. 7: Key Initiatives Required for Sustainable Pavements and CE

Summary

Transitioning to CE for sustainable road infrastructure is viable, sustainable, and unavoidable. India has a two-fold challenge-cum-opportunity viz. dealing with a 6.22 Million km of existing roads mostly built with the principles of linear economy and effectively yet swiftly transitioning to a CE. Such transitioning needs to be founded on very strong principles of integration from all stakeholders supported adequately with commensurate patience and resource engagements. Creating awareness, educating, deploying effective vision, having an efficient and workable policy framework, regulated implementation, and radical innovation could be some of the initial steps. 

Note: The views expressed in this paper are those of the author.

References

1. Bassi, A M, McDougal, K and Uzoki, D. Sustainable asset valuation tool: Roads. Winnipeg, Manitoba, Canada : International institute for sustainable development, 2017.
2. Linear Economy Versus Circular Economy: A Comparative and Analyzer Study for Optimization of Economy for Sustainability. Sariatli, F. 1, 2017, Visegrad Journal on Bioeconomy and Sustainable Development, Vol. 6, pp. 31-34.
3. [Online] https://thercollective.com/blogs/r-stories/circular-economy-vs-linear-economy.
4. EPEA. Cradle to cradle: Beyond sustainability. [Online] https:// epea.com/en/about-us/cradle-to-cradle.
5. Crade to cradle: remaking the way we make things. Braungart, Micheal and McDonough, William. s.l. : vintage Classics, 2008.
6. Foundation, Ellen Macarhur. What is a circular economy? A framework for an economy that is restorative and regenerative by design. [Online] https://www.ellenmacarthurfoundation.org/ circular-economy/concept.
7. Conceptualizing the circular economy: An analysis of 114 definitions. Kirchherr, J, Reike, D and Hekkert, M. 2017, Resources, conservations & recycling, Vol. 127, pp. 221-232.
8. Potting, J, et al., et al. Circular economy: measuring innovation in the product chain. s.l. : Universiteit Utrecht, 2017.
9. Bastein, T, et al., et al. Kansen voor de circulaire economie in Nederland, TNO-rapport. s.l. : Ministerie van Infrastructuur en Milieu, 2013.
10. European Union. European circular economy stakeholder platform. [Online] https://circulareconomy.europa.eu/platform/en.
11. Measuring the circular economy – A multiple correspondence analysis of 63 metrics. Parchomenko, A, et al., et al. 2019, Journal of cleaner production, Vol. 210, pp. 200-216.
12. Circular economy in construction: current awareness, challenes and enablers. Adams, K T, et al., et al. s.l.  : Institution of civil engineers, 2017, Waste and resource management, pp. 1-11.
13. Steps toward a resilient circular economy in India. Fiksel, J, Sanjay, P and Raman, k. 2021, Clean technologies and environmental policy, Vol. 23, pp. 203-218.
14. Ellen MacArthur Foundation. Circular Economy in India: Rethinking growth for long-term prosperity. 2016.
15. Accenture & FICCI. Accelerating India’s circular economy shift, A half-trillion USD opportunity. New Delhi, India : s.n., 2018.
16. Ambuja Cement. Sustainable Development Ambition 2030. [Online] https:// www.ambujacement.com/Sustainability/Sustainable-Development-Ambition-2030.
17. Mnistry of Road transport and highways, GoI. Annual Report 2020-21. MoRTH. [Online] https://morth.nic.in/annual-report. 18. Newman, P, hargroves, C and Desha, C. Reducing the environemtnal impact of road construction. Australia  : Sustainable Built Environment National Research Centre (SBEnrc), 2021.
19. International Energy Agency. Material efficiency in clean energy transition. International energy agency. [Online] 2019. https://www.iea.org/reports/ material-efficiency-in-clean-energy-transitions.
20. FHWA. What can we do to improve pavement sustainability? Federal Highway Adminstration. [Online] https://www.fhwa.dot.gov/pavement/ sustainability/what.cfm.

As construction of roads requires lot of money which can be saved considerably by appropriate engineering design principles and effective use of waste materials in the pavements. Binder modification (blending bitumen with polymers like polyethene, crumb rubber, zycosoil, sulphur, carbon black, SBS, etc.) proved fruitful and the continuing study in this line (Use of discarded waste plastics as modifier can reduce the need of bitumen by about 10%-15%, reduces the cost by (35,000-40,000)Rs per km and increases the melting point of bitumen making it suitable for application in warmer regions (Vasudevan et. al. ,2012) focuses on developing binders with better rheological and mechanical properties.

1. Professor IIT Roorkee and ex Director, NIT Delhi praveenaeron@ gmail.com
2. PG Student, Transportation Engg Group, Civil Engg Deptt, IIT Roorkee

Plastics are user friendly but not eco-friendly as they are nonbiodegradable generally, it is disposed by way of land filling or incineration of materials which are hazardous. Also, use of plastic bags in road help in many ways like easy disposal of waste, better road, prevention of pollution and so on. The better binding property of plastics in its molten state has helped in finding out a method of safe disposal of waste plastics. The coating of plastics reduces the porosity, absorption of moisture and improves soundness.

Need of the Study. Road surface with neat bitumen can cause bleeding in hot climate; may develops cracks in cold climate; possesses fewer load bearing capacity and can cause serious damages because of higher axle load in present exposure conditions due to climate, temperature, boom in traffic growth and rapid infrastructure development. So, in order to achieve longer service life and enhance bitumen pavement performance under strenuous conditions of huge growth in traffic volume; heavy axle loads; adverse varied climatic conditions(less than 00 C to 500 C); insufficient degree of maintenance; demand of bitumen more but indigenously available less; neat bitumen lose their elasticity at 700 C as their phase angle is greater than 900.

Conventional bitumen has to be modified using suitable additives [27] such as polymers, fibers, plastic, anti-stripping agent etc. There are several major industries in the country which produces a large amount of waste materials (waste plastic bags) in the form of by products, whose disposal is an alarming concern not only to regain the valuable space occupied but also to diminish the pollution and other hazards on ecology. Plastic carry bags in one form or another has penetrated into the houses of commonly in such a way that it is impractical to impose a complete ban on the use of plastic present across the nation. However by improving upon the plastic waste management system this harm can be lowered. One possible solution can be to use the waste by products effectively in the pavement layers.

So, in order to improve the properties of bituminous mixtures (healthier roads) and to reduce the negative impact of the waste materials on ecology as a whole, it seems to be practical to re-use by products from industries (through complying with the design principles) in engineering and industrial construction projects such as road pavements.

Research significance. A lot of studies are available to evaluate the effect of inclusion of plastic waste on bituminous properties. However, a comprehensive study on Indian conditions wherein a large waste is being generated annually has not been conducted.

Objective of the Study. The main objective of the study is to compare the fatigue and rutting response of conventional and waste plastic modified binders and mixtures at different stress and strain levels using LAS, MSCR, 4PBB and Wheel Rut Tester. The effect of pre-compressed waste plastic boards on fatigue and rutting responses was also being considered during the study.

Materials

1. Modifiers. Plastic fibers obtained from waste plastic carry bags and waste plastic cement carry bags were used as modifiers in this study. (Tab 1)

2. Binders. VG 10 grade bitumen was modified with waste plastic fibres though ascertaining the required specifications; properties of different binders considered in present study are tabulated below:

Experimental Investigation

The waste plastic modification can affect the rheological properties of the binders as well as bituminous mixtures such as viscosity, fatigue and rutting performances. Therefore, the present study aimed at evaluating the various performance parameters of control and waste plastic modified binders and mixtures. Firstly, control binder (VG-10) was modified with different doses of modifier. Then, the viscosity of all the binders prepared was measured at 1500 C using Rotational viscometer. Further, Superpave high temperature PG of binders was determined. The rutting performance of binders was evaluated using MSCR test parameters while that of mixes was done with wheel rut tester test procedure. Also, the fatigue performance of the binders was evaluated using LAS test parameters while for bituminous mixtures four point bending beam test procedure was followed. In middle of the study, requirements to be fulfilled for designing bituminous mixtures were checked by performing Marshall method of mix design on all the prepared samples. Lastly, plate load test was performed on the four different types of variants over a rigid CC pavement slab as a flexible overlay to rigid pavement.

1. Linear Amplitude Sweep (LAS). As per AASHTO designation: TP 101-14, it is a collaboration of test results of amplitude sweep test and frequency sweep test. Data obtained from these two tests is incorporated into VECD analysis, to determine bitumen’s resistance to damage by means of cyclic loading by introducing linearly increasing load amplitudes. During present study, a sample was prepared consistent with DSR using 8mm parallel plate geometry and 2mm gap setting. Test protocol follows two testing in successions- frequency sweep for undamaged properties and amplitude sweep for evaluating the damaged properties.

VECD analysis given by Kim et.al. (2006) was employed in order to observe the rate of damage accumulation in the binder specimens and for determining various parameters as discussed below

Determination of parameter ‘α’

Using the data for frequency sweep test results, a best fit straight line is applied on a plot of storage modulus [G′(ω)] and frequency (ω) on a log-log scale.

log G′(ω) = m(log ω) + b  (1)
using this equation, ‘α’ can be obtained as , α=1/m Determination of parameters A, B and N_f

Using the data for amplitude sweep test results, damage accumulation in the specimen at any testing time t, can be calculated as:

D_t = ∑ⁿ_i=1 [π² (C_i-1−C_i )]^α/(1+α) (t_i -t_i-1) ^1/(1+α)       (2)

D_t ≈ ∑ (π² (C_i-1-C_i ))^α/(1+α) (t_i -t_i-1) ^1/(1+α)   (3)

Where, Ct = G*(t) / G*(initial)   (4)

0= applied strain for the given data point, percent G*(t) =complex modulus at any time t, MPa It is assumed that at testing time t=0, corresponding values of C and D are 1 and 0 respectively and a relation between C_(t) and D_(t) can be established using a power law:

Where, C1 and C2 are curve coefficients which can be determined by plotting curve between (C0 -Ct ) and Dt on a log-log scale. The value of Dt at failure, Df which corresponds to reduction in initial G* at peak stress situation can be calculated as:

C_t = C₀ -C₁ (D_t ) C ^{2 (5)}       (5)

It can be linearized as,

log(C₀ -C_t )=log(C₁ )+C₂ .log(D_t )          (6)

D_f = (C₀ – C at peak stress)/ C₁ (7)

Now fatigue parameters,

A = f× (D_f)^k/k(πC₁C₂)^α (8)

B = 2α (9)

N_f = A× (_max) ^-B  (10)

Where, f is loading frequency (10 Hz), k=1+ (1- C₂) ×α and _max = maximum expected strain in the bitumen.

Significance – It is carried out in order to study the fatigue properties of the bitumen by employing cyclic loading at low temperatures. – It proves effective over the conventional procedure (Time sweep test) of finding bitumen’s fatigue properties.

2. Multiple Stress Creep and Recovery (MSCR) Test. As per AASHTO Designation D7405, sample is prepared in accordance with D7175 using 25mm parallel plate geometry with a gap of 1 mm. The sample is loaded at a constant stress for 1s and is then allowed to recover for 9s. Twenty creep and recovery cycles were run at 0.1kPa creep stress and were followed by ten creep and recovery cycles at 3.2kPa creep stress. Here the cycle of creep and recovery follows one after the other in order to evaluate out the elastic response and stress dependency of bitumen in terms of- the percent recovery and non-recoverable creep compliance (Jnr).

Creep and recovery. A specimen is subjected to a constant load for a fixed time period and is then allowed to recover at zero load for a fixed time period.

Non-recoverable creep compliance (Jnr). It is defined as the amount of residual strain left in the bitumen after application of each creep and recovery cycle divided by original stress applied to the specimen, kPa. For checking out the elastic response of binder specimens following steps were being followed.

For each of the last 10 cycles at the 0.1 kPa stress level and the 10 cycles at the 3.2 kPa stress level following observations were recorded:
₀ = initial strain value at the beginning of creep portion of each cycle
_c =strain value at the end of creep portion of each cycle (that is, after 1.0 second)
₁ =adjusted strain at the end of creep portion= _c – ₀  (11)
 _r = strain value at the end of recovery portion of each cycle (that is, after 10.0 second)

₁₀= adjusted strain at the end of creep portion= _r – ₀  (12)

Percent recovery

Percent recovery at two stress levels of 0.1 kPa and 3.2 kPa is given by:

_r (0.1, N) = 100(1- ₁₀/ ₁ ), N =11-20  (13)
 _r (3.2, N) = 100(1- ₁₀/ ₁ ), N =1-10  (14)

If some reading comes out to be negative, then a value of 0 is to be noted for that particular reading. Average percent recovery at two stress levels is given by:

R _0.1=(∑ _r (0.1, N))/10  (15)
R  _3.2=(∑ _r (3.2, N))/10  (16) 

While the percent difference in recovery between two stress levels was calculated as:

R _diff =100(1- R _3.2/ R _0.1)  (17)

Non-recoverable creep compliance (J_nr)

The value of J_nr at two stress levels of 0.1 kPa and 3.2 kPa is given by: J_nr (0.1, N) = ₁₀/0.1  (18)

But when r (0.1, N) comes to be negative, in that case J_nr (0.1, N) = ₁ /0.1  (19)

because in this situation adjusted creep strain at 1.0 second is more appropriate strain value to be used as there is no recovery.

J_nr (3.2, N) = ₁₀/3.2  (20)

But when r (3.2, N) comes to be negative, in that case J_nr (3.2, N) = ₁ /3.2  (21)

Average Jnr at two stress levels is given by: 
J_nr(0.1) =(∑ J_nr (0.1, N))/10  (22)
J _nr(3.2) =(∑ J_nr (3.2, N))/10  (23)

While the percent difference in non-recoverable creep compliance between two stress levels was calculated as:

J_{nr diff} = 100(J_nr(3.2)/ J_nr(0.1) -1)  (24)

Values of percent recovery were noted to the nearest of 0.1%, while the values of Jnr were noted to three significant figures. Significance

– It is used to identify the change in elastic response of the bitumen at two different stress levels.
– It is used to study the rutting properties of bitumen samples.
– J_nr is found to be better correlated with the rutting phenomena of pavements than the rutting factor (G*/sinδ) of PG test because in PG test system, oscillatory load to bitumen is applied at a very low strain which may not simulate better to the actual field conditions but while calculating Jnr values high levels of strains are applied.

3. Marshall Stability and Flow. About 1200 g of aggregate (of the desired gradation BC Grade 1) was taken and mixed with the different percentages of bitumen for preparing different samples. The aggregate and bitumen is heated to the required temperature for the preparation of Marshall sample. The mixture is then transferred to a pre-heated Marshall mould having a height of 63.5 mm and diameter of 101.6 mm. A mechanical hammer of standard weight is used to compact the sample. The preheated hammer was placed in position and the mix was compacted by applying 75 blows on each face of samples. Samples were prepared at four different binder content for each type of mix. Three identical samples were prepared at each binder content. The compacted samples were allowed to cool at room temperature overnight.

The extracted samples were used for the determination of the bulk specific gravity. The samples were then transferred to a pre-heated water bath having a temperature of 60 °C for 30 to 40 minutes. Test was performed on these samples following the specification laid out in ASTM D6927 and required Marshall stability and flow values were noted down. The tested sample was loosened by application of heat in the oven and is used for the determination of the theoretical maximum specific gravity (G_mm) as per ASTM D2041 using the vacuum flask method. Similarly the whole procedure is repeated at other binder contents and a series of Marshall stability, flow, G_mm, G_mb, volumetric properties values were obtained.

4. Retained Marshall Stability Test. For each type of mix six different specimens were prepared and divided into two groups, each having three specimens. Group 1 specimens were subjected to conditioning by immersing them in a water bath maintained at 60 °C for a period of 24 hours. On the other hand, specimens of group two were kept unconditioned following the normal immersing of specimens for 30 minutes at a temperature of 60 °C. All the samples were tested in a Marshall stability testing machine until failure. The average stability values for each group was calculated and the retained Marshall stability (RMS) was determined using the following equations.

Significance

– It is used to evaluate the susceptibility of the asphalt mixes to moisture which represents the durability of the mix. The higher the RMS value, lower will be its susceptibility to moisture.

5. Indirect Tensile Strength (ITS) Test. Indirect Tensile Test (ASTM D 6931-12) involves the application of load to a cylindrical specimen along its vertical diametrical plane. A nearly uniform tensile stress is developed normal to the direction of the applied load along the same vertical plane causing the specimen to fail by splitting along the vertical diameter as shown below:

Fig. 1: Load Configurations (a) and Failure of the Specimen (b) in Indirect Tensile Strength Test

Indirect tensile strength test procedure consists of applying a load along cylindrical specimen’s diametrical axis at a fixed deformation rate of 51mm per minute until failure and determining the total vertical load at failure of the specimen. Failure is defined as the point after which there is no increase in load. The maximum load sustained by the specimen is used to calculate the indirect tensile strength with the help of the following expression. 

Indirect tensile strength (MPa) = 2P/πDH  (26)

Where, P -the load till failure of the specimen (newtons), D -average diameter of the Marshall specimen (mm) and H -average height of the Marshall specimen (mm).

A high value of indirect tensile strength is an indication of higher resistance to low temperature cracking and the capability of mix to withstand larger tensile strains prior to cracking.

Tensile strength ratio (TSR) is the average indirect tensile strength of the conditioned specimens expressed as percentage of the average indirect tensile strength of unconditioned specimens. Conditioning was done by keeping the specimens in water maintained at 60 °C for 24 h and by curing at 25 °C for 2 h before commencing the test. Mixes for which the minimum specification criteria of 80% TSR was not satisfied, anti-stripping agent should be used to protect them from being vulnerable to moisture effects.

Significance
– It is useful in assessing the tensile properties of the asphalt mixes which can be correlated with the cracking of the pavement.
– It is also significant to evaluate the sensitivity of the mixture to moisture damage.

6. Wheel Rut Testing. As per the rutting testing protocol of AASHTO T-324, a small loaded wheel is rolled repeatedly across a prepared HMA specimen to measure parameters like rut depth, dynamic stability, etc. The test can be done both in air and water control modes. In this study, air control mode was used to simulate the effect of air temperature on the pavement in terms of rut displacement. As the height of beam was fixed, the weight of the mixture required to achieve the target air void was pre-calculated. The aggregates and bitumen were mixed at the required mixing temperature and were placed in the pre-heated mould. A compression testing machine was used for applying load till the desired height was achieved. After compaction the specimen was allowed to cool for 24 hours. The sample was extracted from the mould and the air void content was measured using the saturated surface-dry procedure (AASHTO T166). As the height of the sample is fixed, it might happen that due to different orientation of aggregate particles within the mix for different specimens slight variation in the fixed air void content of 4% may result. So an allowance of ±0.4% was given to the required air void content. The testing protocol mentioned below was adopted for conducting the wheel rut tester test.

Significance
– It is used to measure the pavement quality in respect of its susceptibility to rutting distress.
– It also evaluates the moisture susceptibility effects in flexible pavements under water control mode during its operation.

7. Four Point Bending Beam Test (4PBBT). The flexural fatigue testing protocol of AASHTO T321-2003 requires dimensions which are 380 ± 6 mm in length, 50 ± 6 in height, and 63 ± 6 mm in width. All the specimens were prepared to achieve a target air void content of 4% by weight of the total mix through similar procedure as was applicable for rutting specimens. The testing protocol mentioned below was adopted for conducting the four point beam bending (4PBB) test.

Significance – It is used to evaluate the performance of the bituminous mixtures for their susceptibility to load associated cracking in low-temperature conditions.

8. Plate Load Test. Plate load test is used to evaluate the support capability of subgrade, base and in some cases complete pavement. It involves measurement of modulus of subgrade reaction for in situ material through analysing a load settlement curve. Plate bearing assembly comprises of a bearing plate which is pressed using a hydraulic jack onto the surface to be evaluated and surface deflections are being noted down corresponding to the increased load values from the jack.

The modulus of subgrade reaction is determined by- 

k = P/∆ (27)

Where,
k, modulus of subgrade reaction p, applied pressure ∆, measured deflection

Significance – As per AASHTO T 222, plate bearing procedure is useful for evaluation and design of airport and flexible pavements.

9. Proportioning of Bituminous Mixtures Ingredients. Rothfutch procedure was adopted for blending aggregates used in bituminous mixtures whose results are tabulated below-

Results and Analysis

1. Multiple Stress Creep and Recovery Test Results. The results obtained for binder specimens using MSCR test procedures, compared at different test conditions are tabulated below – (Table 8)

Since, % recovery is an important consideration while evaluating the performance of binders under high temperature rutting deformation criteria.
Higher is the % recovery better will be the resistance of binder to the rutting susceptibility. At lower creep stress level of 0.1 kPa for temperatures of 400 C and 700 C, values for recovery % are
more-less same but at 500 C and 600 C there is an increase in the values by about 1.44 times and 2.18 timesrespectively. At higher creep stress level of 3.2 kPa, modified bitumen showed a
throughout improvement across the considered temperature range (Fig-2).

Since, lower values for non-recoverable creep compliance Jnr are proven favorable while checking the rutting deformation resistance of binders under high temperature climatic regions. So, by modifying base bitumen with waste plastic fibres the values of Jnr at creep stress of 0.1 kPa get reduced approximately to 2.27 times, 4.69 times, 5.18 times and 3.5 times the corresponding values of unmodified bitumen at 400 C, 50⁰, C, 60⁰, C, and 70⁰, C respectively (Fig-3.While, at higher creep stress of 3.2 kPa, the reduction in values of Jnr achieved were 3 times, 3.8 times, 3.95 times and 3 times at temperatures of 40⁰C, 50⁰C, 60⁰C and 70⁰C respectively. It proves the suitability of waste plastic modified binders under warmer conditions for laying flexible pavements.

2. Linear Amplitude Sweep Test Results. The results obtained for binder specimens under study using LAS test procedures at varied temperatures are tabulated below (Table 9):

At lower strain levels of 2.5% when the impact of vehicles is not so significant regarding tensile strain at the bottom of top layer of flexible pavements, the improvements obtained in terms of fatigue

life of pavements by modifying the bitumen were better by 2.1times, 2.3 times and 1.8 times at 100 C, 200 C and 300 C respectively (Fig-4). Under higher strain conditions of the level of 5 %, modifier showed improvement approximately of the order of 1.46 times, 1.87 times and 1.37 times the base bitumen performance at 100 C, 200 C and 300 C respectively.

3. Four Point Bending Beam Test Results. At low temperature, cracking performance of bituminous mixtures was found to be 2.1 times and 7.4 times (compared to the performance of control mix) with the modification via dry process of mixing and introducing waste plastic boards respectively(Fig-5). Whereas mixing of ingredients via wet process shows lesser fatigue life cycles than that for control mix, which may be attributed to the formation of more organized network structure of the binder with the modification .

4. Wheel Rut Tester Test Results. Under high temperature condition the rutting susceptibility increases which results in poor resistance of bituminous mixtures to the rutting deformation. In the present study, when compared with the results of control mix, the bituminous mixtures prepared with modifications in one way or other showed an improvement of around 1.27 times using wet process of modification. On the other hand, with the use of dry process of modification or waste plastic boards as modifiers to the base condition, the results were on the negative side because of the poor interlinking between the various ingredients of the bituminous mixtures.

5. Moisture Susceptibility Tests Results. Since, the durability of the mix design is an important consideration while looking at the susceptibility of mixes to moisture and climatic conditions. The results obtained (Fig-7) after conducting the standard test procedure of Retained Marshall Stability and Indirect Tensile Strength for moisture susceptibility were found satisfying the minimum criteria for the same. Dry procedure of mixing the waste plastic shredded fibres reflects the resistance against the moisture better to wet procedure of mixing which may be attributed to the formation of plastic coating over the aggregates resulting in the reduction in the penetration of water which in turn increasing the service life of pavements.

6. Plate Load Test Results. The results obtained for bituminous mixtures after performing plate load assembly test procedure, for checking out the deflection regarding ability of storing the strain energy in the respective mixtures are tabulated below (Table 10-14.)

Since, area under the load deflection curve represents the estimation of energy absorbing capacity or toughness of the mix materials which in other words meant for the improved performance regarding resistance of the mix materials to fatigue. Dry procedure of mixing the various ingredients of the bituminous mixtures was found to give best results (Table 14) in this context attributed to the increased load carrying capacity of ingredient aggregates and to flexibility imparted by the way of modification.

The structural evaluation of pavements can be looked in terms of the support stability of the underlying pavement layers through modulus of subgrade reaction, k values. The use of binders modified by waste plastic fibres showed higher results (Table 14) as compared to control mix but the mix with waste plastic boards wherein the deformation was highest with a little load to bear provided least results.

Conclusions

Based upon the study performed the following conclusions were drawn
1). Addition of waste plastic shredded fibres at optimum content of 4% by weight of bitumen to the base binder has improved its conventional properties namely – penetration value, softening point and viscosity.
2). Modified binder prepared using waste plastic fibres was found suitable for its application in warmer areas as shown by PG test results in terms of rutting factor (G*/sinδ) and MSCR test results in terms of non recoverable creep compliance (Jnr).
3). With the modification, improvement in conventional and rheological parameters was significant but within the modification 4% CB as modifier was found superior to 4% CCB.
4). Wet procedure of mixing the waste plastic shredded fibres for preparing the bituminous mixtures resulted in lowest fatigue life as shown by 4PBB test results but was found the best mix regarding the rutting resistance shown by Wheel Rut Tester test results.
5). Dry procedure of mixing the waste plastic shredded fibres for preparing the bituminous mixtures reflected best results regarding the performance of mixtures for durability aspects as shown by RMS and TSR test procedures.
6). With the modification, improvement was observed in the support capability (modulus of subgrade reaction, k value) of pavement layer as observed by the Plate Load test results.
7). Waste plastic boards as a way of modifying the bituminous mixtures were found better in fatigue performance and shear cracking considering the results obtained through 4PBB and ITS test procedures, but were not found advantageous regarding rutting performance as depicted by the results obtained from Wheel rut tester and Plate load test procedures.