According to a report, nearly 22% of GHG emission in India is from the construction sector. A number of studies are being carried out on sustainability using reused and recycled materials and alternate materials and technologies which are more eco-friendly. In Pavement Engineering, material characterisation of pavement materials plays an important role and replacing virgin aggregate with alternatives can contribute to the recycling industry and towards alleviating GHG emissions.
Sustainable pavement materials
The guiding principles for Sustainable Alternatives from the perspective of pavement can be summarised as follows:
- Energy conservation – minimising the use of high energy materials
- Minimising transportation and maximising the use of local skills
- Utilisation of industrial and mine wastes
- Recycling of used materials
- Renewable energy sources
Roads form an integral part of any country’s infrastructure framework. They require constant maintenance and will never cease to be constructed. Conventional pavement materials like asphalt and concrete consume high amount of energy for their production, thereby having a direct bearing on the consumed resources. A number of feasible alternatives to these materials exist. Fly-ash can be, and is, used as a substitute for concrete which has resulted in the improvement of resistance of soil. 10-15% of crushed waste glass of 4.75mm size is sometimes used as a portion of fine aggregate in asphalt paving mixes. Recycled rubber and plastics in bitumen as binders and modifiers are other alternatives. Use of rubber in asphalt has two distinct approaches – either the crumb rubber can be dissolved in the bitumen as a binder modifier or a portion of the fine aggregates can be replaced with ground rubber that has not fully reacted with the bitumen. Enzymes like Fujibeton (Japan), Terrazyme (USA) and Renolith (Thailand) have been developed which are environment-friendly and non-corrosive enzymes and can be mixed with the soil to enhance its properties in the construction of road infrastructure.
Using plastic as substitute for pavements
Recent experiments have shown that plastic need not be recycled to make only plastic; it can be used as substitutes for other materials too. In pavement engineering, plastic has been directly used as a reinforcement material for natural soil, as a bitumen modifier in the form of polymer in asphalt binders and as a replacement of the aggregate (plastiphalt).
Behaviour of plastic reinforced soils
Present design methods are generally empirical and often based on the use of California Bearing Ratio (CBR) test which was discarded in California over 50 years ago. The problem is one of understanding how soils and granular materials respond to repeated loading, and applying this knowledge to pavement design with the aid of appropriate theoretical analysis and an understanding of failure mechanism. Non-linear stress-strain characteristics of soil need to be accounted for in the design and evaluation of pavements. This constitutes the basis for Mechanistic-Empirical Design (MED) methods. To investigate the effects of plastic mixed waste on the strength of soils, a series of tests were performed for two different types of soils – red soil and sand. The tests included Unconfined Compression (UCC) test, Shear test and Compressibility test for different percentages of plastic waste in soil.
In the UCC test, it was observed that by increasing the percentage of plastic from 0-1, greater stress was required to produce the same strain. This implied that the soil samples mixed with plastic waste were able to resist compression better than the ordinary samples. Shear test was performed as Triaxial Compression test while compressibility test was performed in an oedometer as one dimensional compression test. Both showed promising results; there was an increase in the angle of internal friction φ and cohesion c by increasing the plastic content. This suggests that slope stability is increased in plastic mixed soils. Moreover, a significant decrease in compression and recompression index by increasing plastic fibre content signifies lesser settlement due to primary consolidation.
Recent experiments have shown that plastic need not be recycled to make only plastic; it can be used as a substitute for other materials too. In pavement engineering, plastic has been directly used as a reinforcement material for natural soil, as a bitumen modifier in the form of polymer in asphalt binders and as a replacement of the aggregate (plastiphalt).
Based on the results obtained, a model was proposed to calculate deviator stress under different loading conditions which incorporates the behaviour of plastic fibre. A constitutive model was developed and to validate the proposed analytical model, experimental results were compared with predicted results obtained from the model at a confining pressure of 100kPa. It was found that experimental results were more or less in accordance with theoretical values, thus validating the model.
When soft soil is mixed with 1% plastic waste, the maximum unconfined compression strength is 250kPa. This corresponds to a CBR of about 8.3 and for a cumulative load repetition of 1million msa, a pavement thickness of 250mm from IRC37:2001. On the other hand, significantly different values are observed for plain soil where the unconfined compression strength is obtained as 145kPa, CBR 4.83 and a corresponding pavement thickness of 400mm. There is nearly a reduction in pavement thickness of 37.5% with the addition of just 1% plastic waste. This shows that in terms of the enormous amount of plastic waste produced and its effective use in soil stabilisation, the potential savings in materials and cost is a highly significant amount. From a sustainable point of view, it leaves a lot of room for using the saved materials and capita elsewhere, thereby aiding in the economic growth of a country and reducing the amount of garbage.