Sustainable Pavements in India The Time To Start Is Now

Prof. Rajib Basu Mallick, Worcester Polytechnic Institute, MA, USA Prof. A. Veeraragavan, Indian Institute of Technology, Chennai.

Pavements – A Costly Necessity

The quality of roads dictate the economy of a country and hence the quality of our lives. In India, road transport handles more than 60% of the freight and more than 80% of the passenger traffic. Roads are vital for the transport of goods and passengers. Village roads are critical for the basic minimum connectivity for the upliftment of the social and economical condition of the rural people. Such roads provide access to employment, means of transporting agricultural produce and access to health care and social services.

Realizing the need for good road infrastructure, the Indian government has embarked on a vigorous road building effort – and investing `1,20,000 crore per annum. In the next five years, $60 billion will be invested to build 35,000 km of roads. The roads that are being built now will be ready for maintenance and rehabilitation in the next five to ten years. Some lessons can be learnt from the US highway system, which is a mature system, and most of the work in the recent past and at present is on maintenance and rehabilitation, rather than on new construction. Because of soaring material costs and budget shortfalls, there is a huge backlog in maintenance and rehabilitation of pavements, leading to the issue of sustainability of the maintenance of road assets that have been created by huge investments. It should be noted that in addition to the ongoing new construction, in the next five years, a significant amount of work will also be needed for the maintenance and rehabilitation of the new pavements. The combined work will need (in addition to money) a massive amount of materials and energy.

Where will all these materials and energy come from? This article tries to present a clear picture of the future – the challenges and the solutions.

Impact of Infrastructure Improvement on the Environment

It is an accepted fact that because of human activities and resultant global warming. Global mean temperature and average sea level have risen, whereas the snow cover both in Northern Hemisphere and the Himalayas have been reduced. The loss of glaciers in the Himalayas is of great relevance, because of the dependence of crores of people on water from the rivers that are fed by these glaciers. Overall, in the world transportation represents 10% of the world’s gross domestc product, and has been reported to be responsible for 22% of the global energy consumption, 25% of fossil fuel burning across the world and 30% of global air pollution and greenhouse gasses.

On top of this, consider the amount of natural aggregates (mostly from quarries) that are being used - 1,300 million tons of virgin aggregates are used in the US every year for pavement construction. In India, about 15,000 tonnes of aggregates are required per kilometer of highway. A typical project of National Highway Development Project (NHDP) of 60 km road improvement requires 20 lakh ton of material. In India, in most cases, old pavement materials are dumped into landfills. Note that landfills have been identified as the largest source of methane caused by humans .

The production of pavement mixes such as hot mix asphalt (HMA) requires a significant amount of energy, such as those needed for production of bituminous binder from crude petroleum and drying aggregates. For example, approximately 1.5 gallons of fuel is used for drying and heating one ton of aggregates – consider that figure in terms of lakhs of tons of aggregates that is used. The heating of bituminous binder and HMA releases a significant amount of green house gases such as carbon dioxide, and harmful pollutants such as sulphur and nitrogen oxides. Construction people are exposed to such gases on a regular basis. The amount of emission doubles for every 10oC increase in production temperature, and increasingly, higher temperature is actually being used for the production of HMA with modified binders.

The scarcity of aggregates now forces truck delivery of materials from great distance – the use of diesel for running these trucks leads to emission of pollutants such as particulate matter, nitrogen oxides and sulfur dioxides. Many of the diesel engine emissions have been identified as carcinogenic, and harmful to the human health, even at occupational and environmental levels of exposure. The problem of availability of aggregate is acute in the Northern Part of the country. For a lead of 200 km (which is common in north India) it will require 180 lakh liter of diesel in transportation alone. We need to initiate applied research on large scale to find alternate crust composition so that material consumption in road construction gets reduced and we are able to save diesel also. A serious attempt has to be made to improve engineering properties of traditional construction material by suitable additives so that they are able to effectively distribute load and thereby reduce consumption.

Pavement materials have very low thermal conductivity but relatively high heat capacity. As a result, roads and parking lots retain a huge amount of heat, which is released back into the air, and which leads to the rise in temperature, especially in urban areas. This phenomenon is known as urban heat island effect. This high temperature leads to a greater use of polluting equipment such as air conditioning, and increases the ozone concentration and air pollution. The net effect is more pollution and adverse health effect of the inhabitants of the urban areas.

Finally, the effect of paving on the recharge of groundwater is disastrous. A paved surface significantly reduces the amount of water that infiltrates into the ground – most of the water flows away as storm water. This lowers the groundwater level, leading to scarcity of water, pollution of water (the water now comes from levels which have harmful minerals), and storm flooding.

To summarize, the total effect of paving for roads and airports include depletion of natural resources and energy, rise in temperature and air pollution and lowering of groundwater and scarcity of drinking water. This essentially means that we will be leaving a much less greener environment, much less natural resources and a much less comfortable earth for our next generation than what we began with. Since the success or failure of a generation is not to be judged by the people of that generation, but by future generations, are we not failing magnimously? This observation is tied to the very basic idea of sustainable development – “development that meets the needs of the present without compromising the ability of future generations to meet their own needs”.

One important point should be remembered: pavements are for the infrastructure and improving the quality of life, and hence the quality of life should not be sacrificed for the sake of pavements. Therefore, at the beginning of this Herculean task of building a massive transportation system in India, it is imperative that we acknowledge the fact that even though roads are needed for developing the economy and improving the quality of our lives, they adversely affect our environment. The next step is to embrace, wholeheartedly, every available green technology for pavement design and construction. The following paragraphs summarize a few such technologies.

Recycling

Pavement recycling, particularly, HMA pavement recycling (HMA is the most widely used pavement material in India) is a well established proven technology. A host of different techniques exist for different situations – hot mix recycling, hot in-place recycling, cold mix and cold in-place recycling and full depth reclamation. The past few decades have seen tremendous growth in equipment and techniques in recycling, from milling machines to mixing plants and in-place trains. Plants capable of recycling in high percentages, and materials that are capable of restoring correct asphaltenes-maltene proportions in aged asphalt are available. The literature is replete with good practices and manuals for effective and successful recycling operations and guidelines (Table 1 gives applications and considerations for the different methods).

Hot Mix Recycling

Hot mix recycling is accomplished by incorporating milled and processed reclaimed asphalt pavement material (RAP) in new mixes in the hot mix plant. RAP can be added at different percentages, and depending on the percentage, a specific grade of asphalt binder or recycling agent could be used. The milled RAP needs to be regraded before combining with the new aggregates to ensure conformity to the specified gradation. Mix design using the Marshall method or the Superpave method can be done. Various forms of drum and batch plant modifications exist, which allow the use of RAP in HMA production on a regular basis. The primary benefit of hot mix recycling is the control of the quality of the finished product, and the ability to process the milled RAP. In drum plants, the RAP is introduced in that part which is not exposed to the burner flame – it is critical that the RAP is not overheated, and is heated by the superheated aggregates.

Hot In-place Recycling

Hot in-place recycling (HIR) involves the use of a single or more commonly, a train of equipment that heat the existing pavement and mill/scarify it, mix the softened RAP in-place with rejuvenating agent, compacts it in one single pass. Sometimes remixing with new materials and/or repaving with new layer are also done. The selection of the type and amount of the rejuvenating agent could be done using mix design principles, and the new aggregates could be selected to obtain the desired gradation when combined with the existing scarified mix aggregates. The benefit of this process includes those that are related to in-place recycling – avoiding transportation and related fuel use and emissions. The end product is also similar to HMA, and hence of good quality. However, the depth to which HIR could be conducted effectively is limited to about 38 mm, and the use of large trains is only possible in long and straight stretches of roads.

Cold Mix Recycling and Full Depth Reclamation

Cold mix recycling could be done both in-place (cold in-place recycling, CIR) or in the plant. In CIR, either a specialized single equipment or a train of equipment is utilized. The primary steps are milling of part or all of the HMA layer, and/or processing (for example, to get rid of plus 50 mm particles), mixing with emulsion and/or other additive such as cement, and compaction, and curing. It is important to make sure that the mix is cured before the next layer is put down, to avoid entrapment of moisture and subsequent deterioration. CIR is generally used to obtain good quality base course, with the utilization of the existing (aged and with distresses, such as with cracks). In cold central plant recycling, the milled RAP is mixed with emulsion or foamed asphalt, and transported back to the site and laid down with a paver and compacted. In this case, it is possible to have a better control over the finished product, and fines could be added to ensure the conformity to a specific gradation, such as that needed for the successful use of foamed asphalt.

Full depth reclamation (FDR) is similar to CIR, except that in this case, milling is extended below the depth of the HMA layer to include part of the granular layer. Obtaining a good base course is the objective in this case.

Cold recycling, as discussed above, ensures economy and benefit to the environment by avoiding heat and energy, and emissions. In-place recycling allows further economy and other benefits by avoiding transportation. While CIR allows the reuse of old and aged asphalt pavements as base course in new pavements, FDR allows the modification to solve the base related problems.

Low Energy Mixes

Use of huge amount of energy and emissions have led to the development of many types of cold and warm mix asphalt (WMA) technologies in the last decade. Cold mixes have been used for a long time for various applications such as full depth reclamation of cold in-place recycling for obtaining base courses. WMA, in various forms, have been tried successfully in Europe as well as in the US (See tables 2, 3 and 4 for different methods). The main principle behind WMA is the use of a material and/or technique to lower the viscosity of the asphalt to the range of that required for mixing and compaction, at lower than conventional temperature. The benefits of WMA include reduced Emissions (Table 5) and fuel usage, sufficient workability at long haul distances, improved compaction as well as the ability to use more recycled asphalt pavement material.

There are four competing WMA processes that are widely used: A two-component binder system called WAM-Foam® (Warm Asphalt Mix Foam) that introduces a soft and hard foamed binder at different stages during plant production; The addition of a synthetic zeolite called Aspha-Min® during mixing at the plant to create a foaming effect in the binder; The use of thixotropic additives such as Sasobit®, a Fischer-Tropsch paraffin wax and Asphaltan B®, a low molecular weight esterified wax; Evotherm is a non-proprietary product that includes additives to improve coating and workability.

There are several benefits of the WMA in comparison to HMA. They include reduced emissions and worker exposure due to a lowering of temperature, reduced fuel use, ability to allow longer haul distances without losing workability and extension of paving seasons in cooler climate areas.

Porous Pavements

Pavements with high porosity have been developed, that can allow a significant amount of infiltration of water. Such pavements are well suited for parking as well a slow speed areas. In principle, such a pavement consists of an open graded HMA course over an uniformly graded crushed aggregate source with approximately 40% voids over the subgrade (Figure 1). A 25 to 50 mm thick aggregate 12.5 mm down size aggregate layer is used between the open graded and the aggregate courses, and a geotextile is used between the aggregate and the subgrade layer. Test sections have proven the success of such pavements, and research studies have shown that most of the pollutants are removed by filtration through the layers.

Cool Pavements

Although different techniques could be adopted for reducing the surface temperature of pavements, the use of materials with a higher albedo has been so far the most common. Albedo, the ratio of reflected to the incident power, is different for different colored materials – it is much higher for concrete (0.5) and asphalt pavement with a white painted surface (0.25) compared to conventional new HMA (0.05). Specialized pavement colors are available for obtaining pavements with relatively high albedo, to reduce the amount of absorbed heat, and thus to reduce the urban heat island effect. A good summary of the problem as well as the different approaches is given by the EPA (EPA, 2005). Table 6 shows a comparison of costs of cool pavements constructed with different techniques. Another approach is the use of an energy harvesting system installed within a pavement to extract heat energy for useful purposes and reduce the surface temperature of pavements.

Critical Need

As the above paragraphs indicate, sufficient number of “green” technologies exists for immediate adoption in India. Since every technology has its own niche place, sufficient opportunities exist for practical research that could prepare pavement designers in India to specify the right technology for the right case, that is, to answer the question, which sustainable technology is best suited for every pavement construction? Such research can easily be conducted by the premier institutions in India, such as the IITs, which have excellent facilities and brainpower. Centers of excellence for sustainable pavement construction must be set up to facilitate such activity. In addition, such centers will become hubs for training and technology transfer in future.

It is of critical importance that investment should be made now on equipment and training to adopt field-ready sustainable practices for pavement construction, and equipment and software for research for developing guidelines to adopt such techniques for Indian conditions, and developing indigenous techniques. The results of this research must be ready within five-ten years, when the huge roadway network that is being built now, will be due for maintenance and rehabilitation.

India cannot be, and should not be termed as a “poor” country any more as the latest computers and cell phones are available here. India launches its own spaceships with satellites. It is hoped that the leaders of this great country would acknowledge the great environmental challenges of pavement construction, and invest wisely in technologies that would make road building sustainable. The time to start this is NOW.

References

• International Road Federation. Innovative Practices for Greener Roads.www.irfnet.org
• Soderlund, Martina. Sustainable Roadway Design- A Model For An Environmental Rating System. Master of Science in Civil Engineering Thesis. University of Washington, 2007.

Sources:

• Moen, Ø. “Warm-Mix Asphalt (WMA) International Scanning Tour.” Norwegian Public Roads Administration, Norway, Presentation to WMA Scan Team, May 2007.
• Brosseaud, Y. “Warm Asphalt-Overview in France.” LCPC, France, Presentation to WMA Scan Team, May 2007.
• Andersen, E.O. “WAM-Foam-An Environmentally-Friendly Alternative to Hot-Mix Asphalt.” Norwegian Public Roads Administration, Oslo, Norway, Presentation to WMA Scan Team, May 2007.