Adding Waste Plastics in Bituminous Mixes for Road Construction

Dr. Ankit Gupta, Associate Professor and MoRTH Chair Professor, and Aakash Singh, Ph.D. Research Scholar, Department of Civil Engineering, IIT (BHU) Varanasi, present implementation, challenges, and limitations of converting waste plastic into a valuable resource and as a secondary raw material for road construction.

Raw material for road construction

 

Introduction

The increase in plastic production and use, and the resultant accumulation of plastic waste, has led to significant environmental challenges. The global recycling rate for plastics is currently estimated at approximately 9%, with the vast majority of over 80% being disposed of in landfills or released into the natural environment (Figure 1). Every year, an estimated 4 to 12 million metric tons of plastic waste enters the oceans (Geyer et al., 2017).

The current recycling methods are proving ineffective and non-sustainable for managing plastic waste. Consequently, innovative and sustainable solutions are necessary for effective plastic waste management. One promising avenue is the use of plastic waste in infrastructure, particularly in road construction. This not only addresses the disposal issues associated with plastic waste but also enhances the performance characteristics of pavements.

Raw material for road constructionFigure 1: Plastic waste management 1960-2017, EPA 2020


Recent advancements in materials science and engineering have enabled the integration of waste plastics, especially polyethylene (PE), into bitumen and mixtures. Researchers around the world have made significant advancements in converting waste plastic into valuable resource and secondary raw material for road construction, an important part of the circular economy. While much of the previous research was focused on integrating waste plastics into cement and concrete products, there has been comparatively less focus on their application in bituminous pavements.

Various studies have demonstrated that different forms of waste plastics can be effectively used as modifiers in both bitumen and bituminous mixtures. The inclusion of waste plastics has shown significant improvement in several performance parameters, such as thermal stability, marshall stability, resistance to permanent deformation, fatigue resistance, and resistance against moisture damage. These findings suggest that incorporating waste plastics can significantly improve the performance of bituminous mixtures. The overall effectiveness of these modified mixtures is influenced by both the method of incorporating the waste plastic and the specific types of plastic utilized.

Methods of addition of plastics

There are two primary methods for incorporating waste plastic into bituminous pavements, known as dry and wet processes. The dry process involves directly adding plastic to the bituminous mixture, either by replacing a portion of the aggregate or by modifying the mixture (Pasetto et al., 2022). When plastic is used as an aggregate substitute, it partially or entirely replaces aggregates of specific sizes, provided that its melting point exceeds the mixing and compaction temperatures of the bituminous mixture. Alternatively, in the mixture modification approach, plastic is applied to the hot aggregate. It is hypothesized that plastics forms a partial coating on the surface of aggregates (Singh and Gupta, 2024).

The wet process involves incorporating plastic into the bitumen by mixing it at a high temperature above its melting point, using a high shear mixer to prepare plastic-modified bitumen (Singh et al., 2023). Figure 2 illustrates the schematic representation of the wet and dry process.

Raw material for road constructionFigure 2: Schematic representation of the wet and dry process in batch mix plant.

 

Laboratory studies

Most of the studies on recycled plastics in bitumen modification focus on penetration, softening point, viscosity, ductility, and performance grade, with fewer using MSCR and DSR frequency sweep tests (Enfrin and Giustozzi, 2022). Generally, these studies show that recycled plastics reduce penetration, ductility, and non–recoverable creep compliance (Jnr) while increasing the viscosity, stiffness, high-temperature performance grade, and elasticity (as measured by the DSR-MSCR phase angle). This suggests improved rutting resistance for recycled plastic-modified bitumen.

Research on the impact of recycled plastics on fatigue and low-temperature cracking is limited. However, improvements in these properties were observed when additional additives such as crumb rubber, SBS, or waste vegetable and engine oils were used (Binti Joohari and Giustozzi, 2022). Few studies have assessed thermal properties using DSC, modulated DSC, TGA, and DTG, showing that blending temperatures of 170 to 200°C are generally suitable. Chemical properties were analyzed via FTIR and SEM, with some studies indicating physical interactions and others showing chemical interactions between recycled plastics and bitumen.

Phase separation due to differences in density and solubility is a common issue, making it challenging to achieve a homogeneous and stable blend of bitumen and plastic. To address this, researchers have used various stabilizers and compatibilizers, including EVA, maleic anhydride grafted LLDPE, nano-silica, organic montmorillonite, PPA, RET, SBS, crumb rubber, TPOR, waste vegetable oils, and sulfur. Chlorination and maleation of PE have improved compatibility, although chlorination has not been preferred due to its hazardous dioxin emissions. Elastomeric polymers, in particular, enhance the performance grade and elasticity of recycled plastic-modified bitumen. PE's chemical characterisation has been found complicated due to its insolubility in common solvents which also makes its extraction and recovery challenging.

Research on the impact of recycled plastics in bituminous mixtures has traditionally focused on assessing marshall properties, with findings generally showing that plastics contribute to improved marshall stability. More recent studies have expanded the scope of testing to include advanced characterization methods, such as wheel tracking, indirect tensile strength, dynamic modulus, and various fatigue assessments. The common consensus observed was that recycled plastics tend to improve rutting resistance and increase the stiffness of the mixture.

However, there is less agreement on how these plastics influence fatigue resistance, moisture susceptibility, and indirect tensile strength. These inconsistencies can be attributed to factors such as the type of plastics used, the method of incorporation (whether wet or dry), and variations in sample preparation, testing methods, and test temperatures. Specifically, several studies from India that applied the dry process found that the improvement in rutting resistance was primarily due to the plastic coating on the aggregates, which improved internal friction within the aggregate structure. These studies also reported that plastic-coated aggregates showed higher abrasion resistance, bond strength, toughness, and reduced optimum bitumen content.

Field studies

Although numerous studies around the world have highlighted the benefits of using plastic-modified mixtures, detailed field performance data for these pavements is still lacking. Recently, several pilot projects using proprietary recycled plastic products have been initiated in countries like Australia, Canada, China, Colombia, Indonesia, Mexico, the Netherlands, New Zealand, South Africa, the United Kingdom, and the United States. As many of these projects are relatively new, the long-term durability of these pavements is yet to be fully assessed. India has over 20 years of experience in repurposing waste plastics into bituminous pavements, resulting in the construction of more than 13,000 kilometers of roads using the dry process. In India, the National Rural Infrastructure Development Authority (NRIDA) has undertaken an extensive evaluation project encompassing 20,491 kilometers of rural roads, of which 13,139 kilometers incorporated plastic waste. The project aims to assess the life cycle and performance of PMGSY roads built with this technology. These roads are strategically located across the length and breadth of country and exposed to diverse loading and climatic conditions.

The study produced mixed results. In many instances, roads containing waste plastics outperformed conventional roads, though the opposite was also observed in some cases. The incorporation of waste plastic was found to extend pavement life by approximately 1 to 1.5 years. Additionally, these roads exhibited improved riding quality. A Life Cycle Cost Analysis (LCCA) showed overall cost savings, particularly through reduced haulage expenses. It was reported that over 60% of roads containing plastic waste outperformed conventional roads. The study concluded that roads containing waste plastic show a slower rate of deterioration than conventional pavements.

The primary forms of distress observed in roads with waste plastic were ravelling and cracking. Ravelling was the most common, constituting approximately 55-60% of all distress, whereas conventional roads showed around 40%. Cracking was also significant, accounting for about 25-30% of distress, followed by shoving at approximately 15%. A significant proportion of roads with waste plastic have higher International Roughness Index (IRI) values compared to conventional roads, although the difference was not significant. The specific reason for this variation is unclear based on current studies, but it is plausible that defects such as ravelling contribute to this variation. In summary, roads built with waste plastic exhibit performance comparable to conventional roads. Considering the various benefits, including the recycling of plastic waste, constructing roads with waste plastic is a viable option for low-volume road projects.

Limitations of wet and dry process

Both methods have been used to add plastics in bituminous pavements, but there are no specific guidelines for the wet process. Whereas, dry process has been described in IRC SP 98: 2020, but the majority of challenges associated with dry process has not been addressed. Research indicates that the wet process could offer economic benefits for recycling plastics compared to traditional polymer-modified bitumen, primarily because the cost of recycled plastics is fractional compared to elastomeric polymers.

However, the wet process presents certain handling difficulties, such as the risk of plastic separation within the bitumen (as shown in Figure 3). This issue may be resolved by high shear mixing of the plastic-modified bitumen, incorporating the compatibilizer and stabilizers which reduces the phase separation, or immediately using the plastic-modified bitumen just after modification without giving any chance for phase separation. Several research studies suggest that the wet process has the potential to outperform the dry process, due to its stringent quality control, better mechanical performance of pavements, low environmental risks, and improved safety of working personnel.

the plastic-modified bitumenFigure 3: 3% (left) and 2% HDPE (right) modified bitumen right after blending and cooling


The dry process adds recycled plastics directly into the pugmill at the hot-mix plant. This method allows recycled plastics to function as bitumen modifiers, aggregate replacement, aggregate coating, or a combination. Due to the absence of high shear blending and shorter mixing times, plastics in this process do not fully dissolve into the bitumen, creating a multi-phase system. Plastics partially melt and mix with bitumen, whereas a proportion of aggregates coats the aggregates, and the remaining solid part remains in the aggregate matrix either in burnt form or in clusters.

Plastics with melting temperatures above the production temperature can replace fine or medium-sized aggregate particles without compromising shear strength. Given these varied roles, further research is needed to understand the aggregate coating and replacement concepts and to evaluate the dispersion of plastics in lab and plant-produced samples, as dispersion affects the consistency and performance of plastic-modified bituminous mixtures. Both methods have unique requirements and implications, necessitating detailed guidelines and further research to optimize the use of recycled plastics in bituminous pavements.

Implementation challenges and way forward

Incorporating plastic waste in bituminous pavements seems a promising avenue for the management of plastic waste. There are two methods of utilizing the waste plastics i.e., wet and dry process. Proper implementation of these methods necessitates clear guidelines. Currently, no guidelines exist for the wet process, whereas the Indian Road Congress (IRC) has published standards for the dry process.

  • IRC SP 98 published in 2013 and revised in 2020 provides guidelines for the dry process. However, the widespread application of the dry process on major highways and expressways is limited due to a lack of quality control during construction and other issues. Also, the problem of a non-homogeneous mixture arises because the plastic doesn’t melt properly and does not coat the aggregates during the mixing.
  • Although IRC SP 98 (2020) suggests a mixing time (25 to 30 seconds) for plastics and aggregates, this time has been found insufficient in many instances. There is uncertainty about how the dry process improves mechanical performance, as plastics may only partially coat aggregates, leading to a multi-phase system i.e., integration with bitumen or mastic phase or solid particles within the aggregate matrix in burnt form.
  • Key issues with the dry process include whether low melting point recycled plastics can uniformly coat aggregates and if mixing time and temperature has been increased to improve viscosity and coating. Plant operations face additional challenges, such as higher energy consumption, fine particles of recycled plastics potentially clog filter bags in the baghouse which can compromise efficiency, and increase fire hazards.
  • The wet process, while having its challenges, can be more easily applied in batch mix plants due to better control and lower associated handling hazards. This process improves the rheological properties of bitumen, which improves bituminous mixture performance. However, determining the appropriate percentage of waste plastic depends on factors like the type of base bitumen and plastic, with storage stability and dispersion characteristics also being significant challenges.
  • Bitumen modification with plastics increases bitumen stiffness and viscosity, reducing the workability and compactability of asphalt mixtures. While increasing production temperature could potentially solve this issue, although it is not recommended due to higher emissions and energy use. Alternatives such as using softer base bitumen or incorporating warm mix asphalt (WMA) additives shall be considered, though their effectiveness requires a thorough evaluation.

Recycling waste plastics for use in bituminous pavements presents a sustainable yet valuable opportunity to repurpose waste plastics. However, this process introduces technical challenges and constraints that can increase processing costs. To address these issues effectively, collaborative efforts are required from the agencies and institutions involved. Researchers and highway engineers have started realizing the economic and environmental benefits of recycling plastic waste into bituminous pavement construction.

This approach not only aids in managing plastic waste but also offers a cost-effective and sustainable alternative to virgin materials. It aligns with the principles of the circular economy and supports the Indian government's "waste to wealth" initiative. A higher amount of plastic waste can be incorporated into bituminous pavements, citing the high material demand in pavement construction. This also can reduce reliance on virgin materials while simultaneously improving the strength of bituminous pavements. Although there might be a slight reduction in certain performance aspects of the pavement, incorporating an optimal amount of recycled plastic waste can be achieved without significant negative impacts.

References:

  • Binti Joohari, I., Giustozzi, F., 2022. Oscillatory shear rheometry of hybrid polymer-modified bitumen using multiple stress creep and recovery and linear amplitude sweep tests. Constr Build Mater 315, 125791. https://doi.org/10.1016/J.CONBUILDMAT.2021.125791
  • Enfrin, M., Giustozzi, F., 2022. Recent advances in the construction of sustainable asphalt roads with recycled plastic. Polym Int. https://doi.org/10.1002/PI.6405
  • Geyer, R., Jambeck, J.R., Law, K.L., 2017. Production, use, and fate of all plastics ever made. Sci Adv 3, e1700782. https://doi.org/10.1126/SCIADV.1700782
  • Pasetto, M., Baliello, A., Pasquini, E., Poulikakos, L., 2022. Dry Addition of Recycled Waste Polyethylene in Asphalt Mixtures: A Laboratory Study. Materials 15, 4739. https://doi.org/10.3390/ma15144739
  • Singh, A., Gupta, A., 2024. Upcycling of plastic waste in bituminous mixes using dry process: Review of laboratory to field performance. Constr Build Mater 425, 136005. https://doi.org/10.1016/J.CONBUILDMAT.2024.136005
  • Singh, A., Gupta, A., Miljković, M., 2023. Intermediate- and high-temperature damage of bitumen modified by HDPE from various sources. Road Materials and Pavement Design 1–14. https://doi.org/10.1080/14680629.2023.2181017.
NBM&CW - November 2024

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