Use of Waste Plastic in Construction of Flexible Pavement

Polymer modified bitumen is emerging as one of the important construction materials for flexible pavements. Use of plastic waste in the construction of flexible pavement is gaining importance because of the several reasons. The polymer modified bitumen show better properties for road construction & plastics waste, otherwise considered to be a pollution menace, can find its use in this process and this can help solving the problem of pollution because most of the plastic waste is polymers.

Various studies (1-5) are being carried out to improve the quality of bitumen used in bituminous road construction. One of the results of such studies is to use polymer-modified bitumen. Use of disposed plastic waste (specially plastic bags) is the need of the hour. The studies on the thermal behavior and binding property of the molten plastics promoted a study on the preparation of plastic waste-bitumen blend and its properties to find the suitability of the blend for road construction.

Polymer Modified Bitumen

In the construction of flexible pavements, bitumen plays the role of binding the aggregate together by coating over the aggregate. It also helps to improve the strength of the road. But its resistance towards water is poor. Anti-stripping agents are being used. A common method to improve the quality of bitumen is by modifying the rheological properties of bitumen by blending with organic synthetic polymers like rubber and plastics. Studies on this subject are going on both at national and international level.

Plastic Waste Scenario

The use of plastic materials such as carry bags, cups, etc. is constantly increasing. The consumption of plastics have increased from 4000 tons/annum (1990) to 4 million tons/annum (2001) and it is expected to rise 8 million tons/annum during the year 2009. Nearly 50 to 60% of the total plastics are consumed for packing. Once used plastic materials are thrown out. They do not undergo bio-decomposition. Hence, they are either land filled or incinerated. Both are not eco-friendly processes as they pollute the land and the air. Any method that can use this plastic waste for the purpose of construction is always welcomed.

Characteristics of Plastic Waste

Thermal study

A study of the thermal behavior of the polymers namely polyethylene, polypropylene, polystyrene shows that these polymers get softened easily without any evolution of gas around 130-140⁰C, this has been scientifically verified.

At around 350⁰C, they get decomposed releasing gases like methane, ethane etc. and above 700⁰C, they undergo combustion producing gases like CO and CO₂.

Binding property

The molten plastic waste inhibits good binding property. Following experiments were carried out to study the binding property.

1. The aggregate was heated to around 170⁰C and the shredded plastic waste (size between 2.36mm and 4.75 mm) was added. Plastics got softened and coated over the aggregate. The mix of aggregate and plastic was compacted and cooled. The block was very hard and showed compressive strength not less than 130 MPa and binding strength of 500 kg/cm². This shows that the binding strength of the polymer is good.
2. The polymer coated aggregate was soaked in water for 72 hours. There was no stripping at all. This shows that the coated plastic material sticks well with the surface of the aggregate.

Preliminary Studies

The studies on the thermal behavior and binding property promoted a study on the preparation of plastic waste-bitumen blend and its properties to find the suitability of the blend for road construction.

Plastic Waste Blending Materials

1. Preparation of blend
   Polyethylene carry bags were cut into pieces using a shredding machine. It was sieved and the plastics pieces passing through 4.75mm sieve and retaining at 2.36mm sieve were collected. These plastic pieces were added slowly to the hot bitumen of temperature around 170-180⁰C. The mixture was stirred well using mechanical stirrer for about 20-30 minutes. Polymer-bitumen mixtures of different compositions were prepared and used for carrying out various tests.
2. Characterization of Blend

1. Separation test (IRC-SP: 53-1999)
   Samples of different composition were prepared and subjected to the separation test. The following observations were made on the basis of their softening point values. Homogeneity was obtained approximately up to 1.5% blend. Beyond this composition, the variation of softening point was much higher for the top and bottom layer of the test samples showing that there is a separation of polymer from bitumen on standing.
2. Determination of softening point (IS: 1205-1978)
   The blend of different composition with different percentage of plastic waste has been prepared and their softening points were determined as given in Table-1. It is observed that the softening point increases by the addition of plastic waste to the bitumen. Higher the percentage of plastic waste added, higher is the softening point. The influence over the softening point may be due to the chemical nature of polymers added.
3. Penetration Test: (IS: 1203-1978)
   
   
   Samples having different percentage of plastic waste in bitumen were prepared and their penetration values determined as per the IS Code given in Table-2. The penetration values of blends are decreasing depending upon the percentage of polymers and the type of polymer added the increase in the percentage of polymer decreases the penetration value. This shows that the addition of polymer increases the hardness of the bitumen.
4. Ductility
   Table-3 shows that the ductility is decreasing by the addition of plastic waste to Bitumen. The decrease in the ductility value may be due to interlocking of polymer molecules with bitumen.
5. Flash and fire point (IS: 1209-1978)
   The studies of flash and fire points of the plastic waste-bitumen blend helps to understand the inflammability nature of the blend. Flash & fire point of plain Bitumen is 175-210⁰C. From the experimental results it is observed that the inflammability of the blend is decreasing as the percentage of polymer increases. The blend has developed better resistance to burning. The polymer bitumen blend road surfaces will be less affected by fire hazards.

Characterization of Plastic Waste-Bitumen Blend For Flexible Pavement

The utility of the plastic waste blended bitumen-aggregate mix for flexible pavement construction was characterized by studying stripping value and Marshall Stability value of the mix for the blends having a maximum of 1.5% plastic waste.

1. Stripping Test (IS :6241-971)
   Plastic waste was dissolved in bitumen and the blend was coated over aggregate. It was tested by immersing in water. Even after 72 hrs, there was no stripping. This shows that the blend has better resistance towards water. This may be due to better binding property of the plastic waste-bitumen blend.
2. Marshall Stability Test
   The specimen was prepared as per the IRC specification using plastic waste-blended bitumen. This shows that plastic waste-bitumen blend has higher strength compared to pure bitumen, whose value is approx. 1200Kg
   
   Moreover, the Marshall Quotient is also within the range of tolerance, thus showing that the plastic waste (polyethylene) blended bitumen mix is better and more suitable for flexible pavement construction.

Results of Preliminary Studies

The studies of properties of the plastic waste-blended bitumen show that the addition of plastic waste to bitumen increases softening point, decreases penetration value and ductility, increases flash point and fire point, increase Marshall stability value and improve anti-stripping properties. Yet the above process has its own limitations

1. The preparation of such modified bitumen needs high power stirrer with thermostatic facilities to maintain the temperature between160-180⁰C. Any increase in the temperature could affect the properties of bitumen.
2. The proper storage of such polymer-blended bitumen is very important. It should be stored in a freezer and it is also referred that it is stable for 6 hrs at a temperature of 180⁰C
3. It was also observed from the separation test that when the plastic was mixed beyond the soluble range (from 2% and above) the excess plastic material got separated on cooling.

These limitations necessitated developing an alternate method for using higher percentage of plastic waste for flexible pavement.

Modified Process (Dry Process)

An alternate method was innovated to find an effective way of using higher percentage of plastic waste in the flexible pavement. The aggregate coated with plastic was used as the raw material. The plastic used were the disposed carry bags, films, cups was etc. with a maximum thickness of 60 microns. The bitumen was not blended with plastic waste.

Preparation of Plastic-Waste Coated Aggregate

The aggregate was heated to around 170⁰C; the plastic waste was shredded to the size varying between 2.36mm and 4.75mm. This shredded plastic-waste was added over hot aggregate with constant mixing to gave a uniform distribution. The plastic got softened and coated over the aggregate. The hot plastic waste coated aggregate was mixed with the hot bitumen 60/70 or 80/100 grade (160⁰C).

Characterization of PCA Bitumen Mix

1. Stripping Test (IS: 6241-1971)
   
   The plastic waste coated aggregate bitumen mix prepared by the above process was immersed in water. Even after 96 hours there was no stripping. This shows that the plastic waste coated aggregate-bitumen mix has good resistance towards water.
2. Marshall Stability Test
   
   It is observed that the Marshall stability values obtained for were generally much higher than the Marshall stability value obtained for pure bitumen min.
3. Field study
   
   Using this dry process technique, road length of more than 1200 km were laid at different places in Tamil Nadu both by the department of Rural Development Agency and by Highways at Cochin, Mumbai, and Pondicherry the corporation laid test roads using this technology. The roads are exposed to heavy traffic, monsoonal change, heavy rain, hot summer etc. The roads are functioning well without potholes, raveling and rutting. Expert’s opinions are also in agreement.
4. Water absorption test
   
   A known quantity of aggregate was taken dried at 110⁰C and cooled. The weight of aggregate was determined. It was then immersed in water for 24 hrs. Then the aggregate was dried using dry clothes and the weight was determined. The water absorbed by the aggregate was determined from weight difference. 500gms of the aggregate was taken and heated to around 170⁰C. It was then coated with plastic at that temperature. The plastic coated aggregate was cooled to room temperature. It was immersed in water for 24 hrs. Then it was removed dried and the weight of the aggregate was determined. It is observed that the absorption of water had decreased with the increase in the percentage coating of plastic over the aggregate. This shows that the coating of plastic reduces the voids. Hence, coating of plastic over aggregate helps to improve the quality of the aggregate.

Results and Discussion

Preliminary studies on the use of plastic-waste as a blending material with bitumen, suggest that the blends behave similar to PMB, thus having improved properties compared to plain bitumen. It is also observed that this process of blending has limitation. At high percentage of blending there is separation of plastic. Hence, process modification was needed and a new product namely plastic waste coated aggregate was developed. This product is not only easy to prepare but also helps to use higher percentage of plastic-waste for coating without much of difficulty.

The coating of molten-plastic over the aggregate will reduce water absorption. This shows that the voids at the surface were reduced. Lesser the voids better the quality of the aggregate. Otherwise, the air entrapped in the voids would cause oxidation of bitumen resulting in stripping, pothole formation etc. Moreover, the presence of water in the voids is detrimental to adhesion between aggregate and the binder namely bitumen. Hence the aggregate with lesser voids is considered to be good for better road construction. These observations help to conclude that plastic-waste coated aggregate can be considered as more suitable material for flexible pavement construction.

Conclusion

Polymer Modified Bitumen is used due to its better performance. But in the case of higher percentage of polymer bitumen blend, the blend is a more polymer dispersion in bitumen, which get separated on cooling. This may affect the properties and quality of the blend and also the road laid using such blend.

In the modified process (dry process) plastics-waste is coated over aggregate. This helps to have better binding of bitumen with the plastic-waste coated aggregate due to increased bonding and increased area of contact between polymer and bitumen. The polymer coating also reduces the voids. This prevents the moisture absorption and oxidation of bitumen by entrapped air. This has resulted in reduced rutting, raveling, and there is not pothole formation. The road can withstand heavy traffic and show better durability.

• The dry process thus helps to:
• Use higher percentage of plastic waste
• Reduce the need of bitumen by around 10%
• Increase the strength and performance of the road
• Avoid the use of anti-stripping agents.
• Reduce the cost to around Rs. 5000/Km. of single lane road
• Carry the process in situ
• Avoid disposal of plastic waste by incineration and land filling
• Add value to plastic waste
• Generate jobs for rag pickers
• Develop a technology, which is eco-friendly

References

• Appea, A. K., Al-Qadi, I. L., Bhutta, S. A., and Coree, B. J., “Quantitative Assessment of Transmission Layer in Flexible Pavements,” 77th Transportation Research Board, Paper No. 980994, Washington, DC, 1998.
• IRC, “Guidelines for the Design of flexible pavements,” IRC: 37-1970, Indian Roads Congress.
• Battiato, G., and Verga, C., “The AGIP Visco elastic Method For Asphalt Pavement Design,” Proceedings Of The Fifth International Conference On The Structural Design Of Asphalt Pavements, Ba Arnhem; Netherlands, August 23-26 1982, pp. 59-66.
• Al-Qadi, I. L., Brandon, T. L., Smith, T., and Lacina, B. A., “How Do Geosynthetics Improve Pavement’s Performance,” Proceedings of Material Engineering Conference, San Diego, CA. 1996, pp. 606-616.
• Barksdale, R. D., Brown, S. F., and Francis, C., “Potential Benefits of Geosynthetics in Flexible Pavement Systems,” National Cooperative Highway Research Program, Report No. 315, Transportation Research Board, Washington, D. C. 1989.
• ISI, “Indian Standards Specifications for Roads Tar”, IS: 215, Indian standard Institution.
• IRC, “Tentative Specifications for Bituminous Surface dressing Using Pre-coated Aggregates,” IRC: 48-1972, Indian Roads Congress.