Polymer Modified Bitumen (PMB)
The properties of bitumen can be enhanced by the addition of certain polymers. PMBs have been used to create different Superpave mixes used extensively in the United States. Polymers change the physical nature of bitumen such as softening point and brittleness. This, in turn, affects the bituminous mix in which the PMB is used. Polymers spread out their long chain molecules to create an interconnecting matrix of the polymer throughout the bitumen. It produces many desirable properties like increased stiffness at high temperatures to avoid wheel rutting and increased softness at low temperatures to avoid fatigue cracking due to the brittle nature of the bituminous mix. Common polymers used in bitumen are Thermoplastic Rubbers, Styrene Butadiene Styrene and Ethylene Vinyl Acetate.
There are a few drawbacks associated with using PMBs. Separation of polymers and storage becomes an issue when an excess of polymer is added. The polymers start separating themselves from the bitumen when stored for a long time which mandates their immediate usage. Moreover, the production of PMBs requires a higher powered stirrer which goes against the entire concept of using lesser energy materials for sustainable development.
Plastiphalt is a conjunction of two words – plastic and asphalt. It involves replacing a portion of the aggregate used in bituminous mixes by plastic polymers. It signifies a great step towards sustainable use of materials since it is a solution to the two-fold problem of using lesser natural materials and disposing non-biodegradable waste. Experiments have showed that plastiphalt mixes have superior properties over conventional bituminous mixes, notable being the Marshall stability at failure. Also, the percentage of creep recovery after unloading is significantly higher for plastiphalt mixes. Other desirable properties like lesser porosity of the mix, increased flow value, higher indirect tensile strength, etc., has proved to be significantly more for plastiphalt mixes than the ordinary bituminous mixes.
Recycling of pavements
Pavement recycling, particularly, HMA pavement recycling is a well-established and proven technology. It involves recycling the bituminous material from deteriorated pavements called Reclaimed Asphalt Pavement (RAP), and using it in the construction of new pavements. The advantages include (i) less user delay, (ii) conservation of energy, (iii) preservation of environment, (iv) reduced cost of construction and (v) preservation of existing pavement geometrics. Recycled mix is supposed to have higher resistance to shearing and scuffing which in turn increases the rutting resistance.
Owing to the rapid modernisation of road network in India, sustainable practices need to be used. It is never too late to start experimenting with sustainable technology and its feasibility for developing nations like India.
Pavement recycling is not popular in India. However, its potential benefits can be seen by the fact that 33 million tons of RAP is used per year for recycling purposes in USA, 0.84 million tonnes in Sweden and 7.3 million tonnes in Germany (Das & Aravind). A host of different techniques exist for different situations – hot mix recycling, hot-in-situ recycling, cold mix recycling, cold-in-situ recycling and full depth reclamation. India’s contribution to sustainable development in Pavement Engineering can start by adopting any of the above measures.
Perpetual Pavements constitute an important aspect of sustainable development. They incorporate mechanistic-empirical design (MED) philosophy which evaluates a pavement based on its responses under traffic loads. Threshold limits are set and if the responses are below them, the design is considered feasible. Life Cycle Cost Analysis (LCCA) of Perpetual Pavements has shown that they are economically far more advantageous than pavements with successive overlays. By reducing the maintenance costs and other overheads and increasing the design life to 50 years instead of 20 years, Perpetual Pavements require a judicious and arduous investigation for Indian conditions.
Use of geosynthetics
Geosynthetics in the form of geotextiles and geogrids made of polymeric materials are being used to improve the bearing capacity of pavements. These materials provide confinement effect through friction. Geosynthetics improve the bearing capacity by three possible mechanisms.
In order to provide confinement, in the recent years, the use of geosynthetic reinforcement in the form of geocells has emerged as an attractive alternative. Among various stabilisation techniques available for improving the bearing capacity of pavements, providing layers of geocells at the base of the foundation is simple, fast and cost-effective. Geocells are three dimensional forms of geosynthetic materials with interconnected cells filled with soil.
All the above technologies lead to saving of soil, aggregate and bitumen in road construction and result in reducing carbon foot print and need to be implemented in practice in India.
Challenges in material characterisation
While new additives and alternatives may be proposed to design different kinds of pavements, it must be kept in mind that the evaluation of the proposed design needs to follow a rigorous pattern before field implementation. The structural performance of the pavement will have to be closely monitored. Some additives may have rate effects and hence, they may influence the implications in mix design and structural performance. Optimum combination of addition of the materials as well as State parameters needs to be obtained considering variability in the laboratory and fields for specifications of performance.G L Sivakumar Babu Professor GeotechnicalEngineering Division Indian Institute of Science
Praveen Ravi Junior REsearch Fellow Department of Civil Engineering Indian Institute of Science
Pawan Kumar Junior REsearch Fellow Department of Civil Engineering Indian Institute of Science