This knol is an original collection work for her seminar. This knol tells us the details about utilising plastic waste in the construction of bituminous pavements. This technology is not a new concept but rather not practised widely. These information makes us conversent with the technology and methods of this evolutionary method for reducing the plastic waste.
Plastics, a versatile material and a friend to common man become a problem to the environment after its use. Today, in India nearly 4 million tonnes of plastics are used and it is hoped to reach 12 million tonnes by 2010. Their visibility has been perceived as a serious problem and made plastics a target in the management of solid waste. Plastics are non-biodegradable. They also have very long lifetime and the burning of plastics waste under uncontrolled conditions could also lead to generation of many hazardous air pollutants (HAPs) depending upon the type of polymers and additives used. However, the end-of-life plastics can be recycled into a second life application but after every thermal treatment, degradation of plastics takes place to a certain extent.
Polymer modified bitumen is emerging as one of the important construction of flexible pavements. The polymer modified bitumen show better properties for road construction and plastics waste can find its use in this process and this can help solving problem of pollution. The studies on the thermal behavior and binding property of 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.
An alternate method to use higher percentage of plastic waste in flexible pavement is by using plastic coated aggregate (PCA). This method is known widely as dry process. The aggregate coated with plastic was used as the raw material. The bitumen was not blended with plastic waste.
2. CHARACTERISTICS OF PLASTIC WASTE
2.1 Thermal Study
A study of the thermal behavior of the polymers namely polyethylene, polypropylene, polystyrene, shows that those 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 at 7000C they undergo combustion, producing gases like CO and CO2.
2.2 Binding Property
The molten plastic waste inhibits good binding property. Following experiments were carried out to study the binding property.
- The aggregate was heated to around 1700C and the shredded plastic waste 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.
- The polymer coated aggregate was socked 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.
3. POLYMER MODIFIED BITUMEN
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
3.1.1 Preparation of blend
Polyethylene carry bags were cut into pieces using a shredding machine. It was sieved and the plastic 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.
3.1.2 Characterization of blend
- 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.
- Determination of Softening Point (IS:1205-1978)
The softening is the temperature at which the substance attains a particular degree of softening under specified condition of test. The softening point of bitumen is usually determined by Ring and Ball test. Generally higher softening point indicates lower temperature susceptibility and is preferred in warm climates. The blend of 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.
Table-1 Variation in softening point
|% of polymer in Bitumen||Softening Point 0C|
- Penetration Test (IS:1203-1978)
The penetration test determines the hardness or softness of bitumen by measuring the depth in tenths of a millimeter to which a standard loaded needle will penetrate vertically in 5 seconds. The bitumen grade is specified in terms of the penetration value. 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 the blends are decreasing depending upon the percentage of polymers and the type of polymer added. The increase in percentage of polymer decreases the penetration value. This shows that the addition of polymer increases the hardness of the bitumen.
|% polymer in Bitumen||Penetration value at 250C (1/10th of mm)|
Table-2 Variation in penetration value
It is important that the binders form ductile thin films around the aggregate. The ductility is expressed as the distance in centimeters to which a standard briquette of bitumen can be stretched before the thread breaks. Samples with different percentage of plastic waste in bitumen were prepared and ductility was checked. The 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.
Table-3 Variation in ductility
|% of polymer in bitumen||Ductility (cm)|
- 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 point “the flash point of a material is the lowest temperature at which the vapour of a substance momentarily takes fire in the form of a flash under specified condition of test.”Fire point “the fire point is the temperature at which the material gets ignited and burns under specified conditions of test.”Pensky –Martens closed cup apparatus or open cup are used for conducting the tests. Flash and 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.
3.2 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.
Table-4 Variation in flash and fire point (0C)
|% of polymer in bitumen||Polyethylene||Polypropylene||Polystyrene|
|Flash Point||Fire Point||Flash Point||Fire Point||Flash Point||Fire Point|
- Stripping test (IS :6241-971)
The stripping is due to the fact that some aggregates have greater affinity towards water than with bituminous binders and this displacement depends on the phsico-chemical forces acting on the system. Stripping is generally experienced only with bituminous mixtures which are permeable to water.
Plastic waste was dissolved in bitumen and the blend was coated over aggregate. It was tested by immersing in water. Even after 72 hours, 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.
- Marshall Stability test
The specimen was prepared as per the IRC specification using plastic waste-blended bitumen. This shows that plastic waste-bitumen blended 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, increases Marshall Stability value and improve anti-stripping properties.
Yet the above process has its own limitations:-
- The preparation of such modified bitumen needs high power stirrer with thermostatic facilities to maintain the temperature between 160-1800C. Any increase in temperature could affect the properties of bitumen.
- 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 hours at a temperature of 1800C.
- 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.
4. 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, and cup etc with a maximum thickness of 60 microns. The bitumen was not blended with plastic waste.
4.1 Preparation of Plastic-Waste Coated Aggregate
The aggregate was heated to around 1700C; the plastic waste shredded to the size varying between 2.36mm and 4.75mm. This shredded plastic waste was added over hot aggregate with constant mixing to give a uniform distribution. The plastic got softened and coated over the aggregate. The hot plastic waste coated aggregate was mixed with hot bitumen 60/70 or 80/100 grade (1600C).
4.2 Characteristics of Plastic Coated Aggregate
- Moisture Absorption and Void Measurement
For the flexible pavement, hot stone aggregate (1700C) is mixed with hot bitumen (1600C) and the mix is used for road laying. The aggregate is chosen on the basis of its strength, porosity and moisture absorption capacity per IS code. The bitumen is chosen on the basis of its binding property, penetration value and viscoelastic property. The aggregate, when coated with plastics improve its quality with respect to voids, moisture absorption, soundness and other properties.
The coating of plastic decreases the porosity and helps to improve the quality of the aggregate and its performance in the flexible pavement (Table-5).
- Soundness Test
Soundness test is intended to study the resistance of aggregate to weathering action. The weight loss is attributed to the poor quality of the aggregate. The plastic coated aggregate did not show any weight loss, thus confirming the improvement in the quality of the aggregate (Table-5).
The porosity of the aggregate should be less than 2%. If pores are present, the air accumulated in the pores oxidizes the bitumen and the bitumen loses its viscoelastic property. The material becomes hard. By plastic coating, the pores are very much reduced. This is evidenced by the reduction in the moisture adsorption with the percentage increase in the plastic coated (Table-5).Moreover, during water stagnation; the pores accelerate the stripping of bitumen resulting in pothole formation. By coating with plastic the pores are reduced. Hence the quality of the aggregate is improved and there was no stripping of bitumen.
- Aggregate Impact Value
It is clearly observed that the coating of plastics improves Aggregate Impact Value, thus improving the quality of the aggregate. Moreover a poor quality of aggregate can be made useful by coating with polymers (Table-5).
- Los Angeles Abrasion Test
When the Los Angeles abrasion value of plain aggregate is compared with the Plastic coated aggregate; the values are less for polymer coated aggregate. Coating of waste plastics over aggregate will improve the Los Angeles abrasion value. Thus we can improve the Los Angeles abrasion value of soft aggregate to a better one. Similarly the coating of plastics reduces the crushing value (Table-5).
Table-5 Aggregate Qualities
|Stone aggregate||% of plastic||Moisture Absorption||Soundness||Aggregate Impact Value||Aggregate crushing value||Los Angeles Abrasion Value||Voids|
|Without plastic coating||
|With plastic coating||1%||2%||Nil||21.20%||21%||32%||2.2%|
4.3 Characteristics of Polymer Coated Aggregate with Bitumen
Over the hot polymer coated aggregate, the hot bitumen (1600C) was coated. The spreading was easy. The bitumen mixed polymer coated aggregate was subjected to testing like Stripping test, Bitumen extraction test and Marshall Value determination test.
- Stripping test
The stripping of polymer coated aggregate bitumen mix was nil even after 92 hrs whereas the non-coated showed 8% stripping even after 24 hrs.
- Extraction test
The sample was also submitted to bitumen extraction using benzene. The result is tabulated (Table-6). The results show that the removal of bitumen is slow in the case of plastic waste-coated aggregate compared to plain bitumen coated aggregate. This may help to conclude that a portion of the bitumen was held by the polymer whose release was difficult. When the residue was treated with decaline, organic solvent both polymer and bitumen were removed. This shows that there is an inter mixing surface layer of polymer and bitumen at the surface with gives strength and improvement in the bonding. The bonding strength of bitumen with the polymer coated aggregate has increased due to polymer bitumen interaction at the surface.
Table-6 Extraction Characteristics
|% of Plastics added||Bitumen removed after 5 min.
|Bitumen removed after 10 min.
|Bitumen removed after 15 min.
- Marshall Stability Value determination
In Marshall Stability test; the resistance to plastic deformation of cylindrical specimen of bituminous mixture is measured when the same is loaded at periphery at 5 cm per min. This test procedure is used in designing and evaluating bituminous paving mixes. The Marshall stability of mix is defined as a maximum load carried by a compacted specimen at a standard test temperature of 60ºC. The flow value is deformation the Marshall Test specimen under goes during the loading upto the maximum load, 0.25 mm units. In this test and attempt is made to determine optimum binder content for the type of aggregate mix and traffic intensity.
Marshall Stability Value for the different mix prepared with polymer coated aggregate were determined, as per IS Code (Table-7). This shows that plastic coated aggregate bitumen mix has high load withstanding strength. Further an optimum of 10% polymers can be used and the addition can be extended to 15% under specified conditions.
Table-7 Effect of Variation of Bitumen/ Plastic Content on Marshall Stability Value
|% of bitumen||% of plastic w.r.t wt of bitumen||Type of Plastic||PCA||Marshall Stability Value
Marshall Stability Values were calculated with different percentage of polymer and by varying the polymer nature. This shows that stability value increases with increase in the percentage of plastics. It is also observed that the polymer modified bitumen mix has a lower Marshall Stability Value when compared to Polymer coated aggregate bitumen mix (Table-8).
Table-8 Comparative Study on MSV for PCA and PMB
|% of Bitumen||% of Plastic w.r.t wt of Bitumen||Type of Plastic||PCA/ PMB||Marshall Stability Value
4.4 Mixing by Mini Hot Mix Plant
Step I: Plastic waste made out of PE, PP and PS cut into a size between 2.36mm and 4.75mm using shredding machine.
Step II: Similarly the bitumen is to be heated to a maximum of 1600C to have good binding and to prevent weak bonding. (Monitoring the temperature is very important)
Step III: At the mixing chamber the shredded plastic waste is to be added to the hot aggregate. It gets coated uniformly over the aggregate within 30 Secs, giving an oily look Plastic coated aggregate is obtained.
Step IV: Hot bitumen is then added over the plastic coated aggregate and the resulting mix is used for road construction. The road laying temperature is between 1100C to 1200C. The roller used is 8-ton capacity.
4.5 Mixing by Central Mixing Plant (CMP)
The dry process can also be carried out using central mixing plant. The shredded plastic is added along with the aggregate in the conveyor belt. This is transferred into the hot cylinder. There aggregate is coated with plastic first and then with the bitumen. The mixer so prepared is then loaded in the dipper lorry and transported for road laying. CMP helps to have better control of temperature and better mixing of this material thus helping to have a uniform coating.
5. FIELD STUDY
More than 200km length of plastic tar road has been laid in India at different states from 2002. These roads are functioning well without pothole, raveling and rutting.
5.1 Testing on Roads
As per the above discussion, the load withstanding capacity of the road is increased by a large amount. This is proved by conducting various tests on the built plastic tar roads at different places at different times. This test has been
carried out with the help of National Transport Planning and Research Centre, Trivandrum, Kerala, India. Tests were carried out to measure the following:-
- The roughness of the pavement surface.
- The resistance offered by the pavement surface against skidding of vehicles.
- The pavement macro texture for the geometrical deposition.
- The structural evaluation of flexible pavement for the strength of the pavement.
- The field density of the road.
- The gradation of the laid road.
- To carry out different tests on recovered bitumen.
- To examine the condition of the road (cracks, raveling, potholes, rutting, corrugation edge break).
The test results values are tabulated in Table-9.
- From the skid resistance studies of the five stretches it has been proved that the road was having good skid resistance values.
- From the surface studies of the five stretches it has been proved that the roads have good texture values.
- From the deflection studies of the five stretches it has been proved that all the stretches are reasonably strong.
- From the bump integrator studies of the five stretches it has been proved that the unevenness index value of these three these three road sections are nearly 3000 mm/km, which indicates a good surface condition.
Table-9 Summery of Test Results
|Skid number||Texture Depth||Field density (mm)||Rebound Deflection
|Canteen road, TCE||2006||3100||45||0.65||2.86||0.86|
|Plain bitumen road||2002||5200||76||0.83||2.33||1.55|
5.2 Economy of the Process and the Plastic Waste Available
A comparative study for 25mm thickness SDBC (Semi-Dense Bituminous Concrete) – 10m2 has been made. Three kilogram of bitumen is saved and three kilogram of waste plastic is used. The coast of bitumen is much higher than that of plastics and this process also helps to save the natural resources. There is no maintenance cost for a minimum of five years. Hence the process is cheap and eco-friendly.
6. ADVANTAGES AND DISADVANTAGES
- Better binding property
- Higher Softening point; withstands high temp.
- Lower penetration value; withstands higher load.
- No stripping – Resists the permeation of water.
- Higher Marshall Stability–increased strength of road.
- Operation temperature range: 160-1700C.
- Cost less compared to bitumen road. Use of waste Plastics (MSW, road side etc.).
- Better disposal of waste plastics.
- Ten lakhs or one ton carry bags in one kilometre road.
- The burning of plastic waste creates air pollution and also health hazards.
The generation of waste plastics is increasing day by day. The major polymers namely polyethylene, polypropylene, polystyrene show adhesion property in their molten state. The plastic coated aggregate bitumen mix and plastic modified bitumen forms better materials for flexible pavement construction as the mixes shows higher Marshall Stability value and suitable Marshall Coefficient. Hence the use of waste plastics for flexible pavement is one of the best methods of easy disposal of waste plastics. The use of polymer coated aggregate is better than the use of polymer modified bitumen in many aspects. Foe example if all the roads in India (3.3 million km) are converted into plastic tar road, all the waste plastic available will be used on the road and the disposal of waste plastics will no longer be a problem.
But as the burning of plastic waste creates health hazards and also air pollution, the use if this should be restricted to open areas or in lesser populated areas. Also; the workers involved should be provided with proper safety devices.
These processes are socially highly relevant, giving better infrastructure. Let us grow with these newer technologies.
- Aslam and Shahan-ur-Rahman (2009), “Use of Waste Plastic in Construction of Flexible Pavement”, New Building Materials & Construction World.
- Khanna, S.K. and Justo, C.E.G., “Highway Engineering”, 8th edn, Nem Chand & Bros, Roorkee (U.A), 2001
- Vasudevan,R. (2006), “Utilization of waste plastics for flexible pavement”, Indian High Ways (Indian Road Congress), vol. 34, no.7, pp.105-111.
- IRC, “Guidelines for the Design of flexible pavements,” IRC: 37-1970, Indian Roads Congress.
- IRC, “Tentative Specifications for Bituminous Surface dressing Using Pre-coated Aggregates,” IRC: 48-1972, Indian Road Congress.
- A Support Manual for Municipal Solid Waste, Central Pollution Control Board, July 2003.