Dr. Ambika Behl (Pr. Scientist), Prof Satish Chandra (Director), CSIR-Central Road Research Institute, Delhi-Mathura Road, New Delhi
India generates nearly 26,000 tonnes of plastic waste every day, making it the 15th biggest plastic polluter globally. Discarded plastic waste litter the country’s roads, rivers and also form huge mounds in garbage dumps across the country. According to CPCB, of this 26,000 tonnes of plastic waste 94% is thermoplastic, or recyclable materials such as PET (polyethylene terephthalate), and PVC (polyvinyl chloride). Yet, these materials can be recycled utmost 7-9 times, after which they have to be disposed off. Plastic accounts for 8% of the total solid waste generated in the country annually, with Delhi producing the biggest quantity, followed by Kolkata and Ahmedabad, said a 2018 report by the Delhi-based The Energy and Resources Institute (TERI), citing data from India’s central pollution control board (CPCB). It is a common sight in India to find empty plastic bags and other type of plastic packing material littered on the roads and choking the drains. Due to its impermeability clubbed with non-biodegradability, it creates stagnation of water and associated hygiene problems besides reducing the fertility of the land. Waste plastic, mainly used for packing are made up of PE, PP, PS. In India almost 16.5 million tons of plastic is being consumed annually, in which 60% is recycled and remaining 40% is littering the environment. It is necessary to utilize the plastic waste effectively with technical development in each field.
Bitumen has been known to be used as a binder for road construction. However, the performance of these bituminous binders is questioned time and again, given that they are brittle and hard in cold environments and soft in hot environments. The steady increase in high traffic intensity in terms of commercial vehicles and the significant variation in daily and seasonal temperature has led to increased demand for improved road characteristics. Any improvement in the property of the binder is the need of the hour. It has been recognised that the deficiencies of bitumen can be overcome by the addition of polymers for improving visco-elastic behaviour besides maintaining its own advantages. The addition of polymers typically increases the stiffness of the bitumen and improves its temperature susceptibility, but the addition of fresh/virgin polymers doubles up the bitumen cost and increase the total cost of road construction. Use of waste plastic in bitumen has revealed improved performance, stability, strength and fatigue life, reduction in overall rutting. Apart from solving the problem of waste disposal, addition of waste plastics in bituminous mix results in reduction in consumption of bitumen thereby resulting in overall cost reduction.
Plastic A Boon or Bane?
The present age, in particular the last 30 years can be easily named as The Plastic Age. The invention of plastic was acclaimed as man’s brilliance – a boon. It was light in weight and easy to mould. Within a very short time it replaced metal for items of daily use. The markets were flooded with colourful plastic goods that were cheap and easy to maintain. Being cheap, people began to look at it as a dispensable commodity and a new term ‘use and throw’ became a way of life. Plastic bags and bottles became the most common items. Plastics took over each industry. Packaging industry is the biggest contributor to plastic waste.
Waste Plastic Roads
In order to reduce the accumulation of waste plastic on mother earth and also encourage its use for improvement of pavements, research has been carried out in recent years. The related experimentation has indicated that the waste plastic, when added to hot aggregate will form a fine coat of plastic over the aggregate and then when mix is produced with the binder, it is found to give higher strength, higher resistance to water and better performance over a period of time. Therefore, it is pertinent to use waste plastic in the construction of roads. Plastic increases the melting point of the bitumen and makes the road retain its flexibility during winters resulting in its long life. Shredded plastic waste acts as a strong binding agent between bitumen and aggregates. CSIR-Central Road Research Institute has been promoting the use of plastic waste to construct asphalt roads. Plastic roads mainly use plastic carry bags, disposable cups, and bottles that are collected from garbage dumps as an essential ingredient of the construction material. When mixed with hot bitumen, plastics melt to form an oily coat over the aggregate and the mixture is laid on the road surface like a normal bitumen road.
Advantages of using waste plastic in making roads
- Stronger road with increased Marshall Stability Value.
- Better resistance towards moisture susceptibility.
- Increased binding and better bonding of the mix.
- Reduction in pores in aggregate and hence less rutting and ravelling.
- High resistance to temperature susceptibility.
- The strength of the road is increased by 100%.
- For 1km X 3.75m road, 1 ton of plastic (10 lakh carry bags) is used.
- Value addition to the waste plastics.
- The cost of road construction is also decreased.
- The maintenance cost of the plastic roads is also less in comparison to control roads.
- Disposal of waste plastic will no longer be a problem.
- Films (Carry Bags, Cups) thickness up to 60micron (PE, PP and PS).
- Hard foams (PS) any thickness.
- Soft Foams (PE and PP) any thickness.
- Laminated Plastics thickness up to 60 microns (Aluminium coated also) used as packing materials.
The Process of Using Waste Plastic in Road Construction
There are two main processes namely: Dry process and Wet process.
Dry Process: In Dry process waste plastics are used as coating materials by softening the plastic and not by burning. For a flexible pavement heated stone aggregate (170oC) is mixed with hot bitumen (160oC) and the mix is used for road laying. The aggregate when coated with plastics improved its quality with respect to voids, soundness and moisture absorption and decreases porosity and thus the performance of the pavement is increased.
Wet Process: Plastic waste is ground and made into powder. Powdered plastic waste is added to the bitumen at 160oC. High shear blender is required to prepare plastic modified bitumen and specific chemical treatment may be required to restrict the phase separation. The process does not yield a homogenous mix with prominent separated solid deposits of mix therefore wet process was not adopted.
Dry process is the most widely adopted method for using waste plastic in road construction.
Properties of Waste Plastic-Asphalt Mix
To evaluate the effect of waste plastic content on properties of asphalt mix, a study was carried out in CSIR-CRRI. Asphalt mixes were prepared in laboratory with varying dose of waste plastic, it was found that the asphalt mix shows the best Marshall stability at 8% waste plastic content. Waste plastic content higher than 8% by weight of bitumen, makes the mix too stiff and prone to crack. Figure 3 shows the variation in Marshall stability and bulk density with varying waste plastic content in asphalt mix.
The waste plastic-asphalt mix was also evaluated for its beam fatigue and rutting strength characteristics and resilient modulus in comparison to conventional control mix. Figure 4 shows that when tested for flexural beam test, the conventional mix failed in 8650 cycles whereas the waste plastic modified asphalt mix could sustain till 17554 cycles.
Figure 5 shows rut depth versus number of passes for conventional and waste plastic modified asphalt mix, it was observed that waste plastic modified mix performed better than the conventional asphalt mix and showed less rutting.
When waste plastic-asphalt mix was tested for resilient modulus test, it was found that the resilient modulus of the control asphalt mix (without waste plastic) was in the range of 3600 MPa whereas waste plastic asphalt mix (8% waste plastic) gave resilient modulus value of 6600 MPa at 25°C.
Construction of Waste Plastic Road in Shimla, H.P
CSIR-CRRI along with Department of Environment, Science & Technology & PWD Shimla laid a road in Shimla using waste plastic. CSIR-CRRI provided the technical guidance during the laying of the road. Photos 1 to 9 shows the construction process.
The municipal waste is first segregated and the plastic waste is separated from the rest of the waste. The segregated waste plastic is then washed, dried and shredded in small pieces as shown in photo 3. The shredded waste plastic is added into the mixing plant by putting it manually on the conveyor belt along with the aggregates. The aggregates and waste plastic are heated in the rotating drum so that the plastic melts and coat the aggregates, after which the bitumen is mixed with the plastic coated aggregates. As the waste plastic passes through the heat chamber in the mixing plant along with the aggregates, it is required to maintain the temperature of the heating chamber to 165-170oC to avoid burning of waste plastic. The waste plastic modified asphalt mix was then transported to site and was laid using conventional procedure and machinery. Photo 9 shows the final look of the waste plastic road.
The generation of waste plastics is increasing day by day. The major polymers, namely polyethylene, polypropylene, and polystyrene show adhesion property in their molten state. Plastics will increase the melting point of the bitumen. Hence, the use of waste plastics for pavement is one of the best methods for easy disposal of waste plastics. The use of the innovative technology not only strengthened the road construction but also increased the road life as well as creating a source of income. Plastic roads would be a boon for India’s hot and extremely humid climate, where temperatures frequently cross 50°C, and torrential rains create havoc, leaving most of the roads with big potholes. With the use of waste plastic in roads, it is expected that in near future India can have strong, durable and eco-friendly roads that will relieve the country from all type of plastic waste. For wide and effective use of this technology there is an urgent need for efficient planning and effective waste management system in the country.