Sustainable Pavement Engineering

The need for safe, durable and smooth riding pavements on highways and in airports cannot be contended. Construction and maintenance of pavements cost money - which is mostly provided through taxes that are collected from the citizens. The money is spent on employing workers, quarrying and manufacturing usable products, transporting them, and laying down and compacting the materials. In this whole process, which gets repeated to different extents during the maintenance and rehabilitation cycles, man-made material is laid down to cover virgin (most likely arable) ground (making it un-arable from that point onwards), a significant amount of natural resources is utilized, (non renewable) energy is consumed, landfills are created, and different types of chemicals (in the form of gases) are emitted to the environment (all of these processes are irreversible). Thus, pavement engineering is a destructive and high impact activity. But remember the first sentence of this paragraph - and herein lies the dichotomy of modern civilization - you build something that you need but in the process you affect the earth in an irreversible harmful way. So what is the solution?


There is most likely not "a" solution, but multiple ones. In fact, they are more like "tools" that exist in the pavement engineer's toolbox, which could be used, as the need arises. It is the intent of this article to describe a few of such important tools and their impacts.

First Consideration

First and foremost, the pavement infrastructure should be viewed as part of a larger "system," and not viewed alone. Buildings, pavements and natural ground/water bodies (nature) form the built environment in which we live and breathe. Hence, pavements need to be compatible and if possible, complementary to the other two components - buildings and nature. This means construction and maintenance of pavements should not be taxing the natural resources, energy utilization should be minimized, byproducts in the form of emissions should not be harmful to the environment, and the layout of the pavement should not be significantly impacting the natural environment. Broadly speaking, this includes layout and type, design and construction of pavement. It is only through this process we can ensure that our future generations will have no/less problem of continuing the work of building or maintaining/rehabilitating the pavement infrastructure. And, this is the very essence of sustainability. True sustainability consists of three aspects - social, economic and environmental sustainability. The discussions in the following paragraphs refer to either one or more of these three different types of sustainability.

Design of Layout of Pavements

Some of the more important issues with the layout/geometric design of a pavement can be summarized as follows:

1. Protection of forests: forests provide a home to many species of plants and animals and at the same time serve as a source of economic products such as timber and enjoyment for tourists. Roads should be preferably designed away from forests, or in a way that creates minimum impact on them.
2. Protection of wild life: Considerations must be made to provide features that would minimize the effect of road construction on animals and subsequent vehicle-animal collisions (such as protected cross overs).
3. Consideration of unique geological conditions, such as steep slopes with vegetation or caves/landforms should be made. Precaution should be taken to avoid such areas where a slight disturbance of the landscape would lead to a significant irreversible disturbance of the local environment.
4. Context-sensitive design should be adopted: the layout of the road should conform to the values of the communities through which it is passing, by making sure that it is integrated with the local character - such as flora and fauna in rural areas.
5. Access to critical areas, such as hospitals and police stations should be provided, such that the road helps the local communities and in return the people value the road.
6. Consideration of the impact of diversion routes on the local environment, during the construction of new pavement should be made. Proper forecasting methods and mitigation procedures should be adopted.
7. Consideration of traffic growth and providing adequate room for further growth should be made, such that traffic saturation does not lead to a drastic change in the character of the community and significant construction activity could be avoided for a longer period of time.

The issues listed above can be represented in terms of problems such as deforestation, noise pollution, crossing of habitat areas, flooding/storm water management, creation of wastage, urban heat island effect, drinking water pollution, air pollution, traffic jams, soil erosion, and use of natural resources and energy.

The above problems could be solved with the use of one or more of a variety of methods, which include habitat connection across roadways through underpasses or overpasses, fencing along the road for animal protection, amphibian rescue fences, storm water pollution prevention plan, pedestrian, bicycle, and transit access, noise barriers, buffer zones, or specially planted slopes/embankments to reduce noise pollution, intersections with interchanges, use of pervious surfaces in pavement or shoulders, runoff treatment practices to clean runoff water, avoiding sensitive aquifers, making the locations of service connections easily accessible, providing easy access to emergency services, and defining an implementation strategy to be followed during construction.

Some of these desirable features are shown in Figure 1.

Construction of Pavements

For construction of asphalt pavements the key factors that need to be considered are using local materials (to reduce transportation related energy expenses), lowering the need for new materials, reducing production and construction temperature, using more environment-friendly products (such as bio-asphalt), and environment-friendly layers/mixes (such as those that would allow the percolation of water into the soil layers and hence facilitate recharging of aquifer, and those that have reduced noise levels), utilizing mixes that would allow recycling or reuse of discarded materials, and longer lasting pavements, and conducting a proper life cycle analysis.


The favorable impact of using sustainable pavements is far reaching. For example, if a recycling procedure is used to rehabilitate an existing pavement, then the following benefits can be ensured:

1. Less use of new natural resources such as mineral aggregates
2. Significant reduction in transportation cost of new materials
3. Less amount of energy use in obtaining and processing new materials (aggregates)
4. Less amount of energy use in manufacturing new materials (asphalt binder, cement)
5. Less emission and pollution of the environment
6. Reduction or avoiding of filling up precious landfill space
7. Significant reduction in overall transportation energy, specifically if an in-place recycling process is utilized.

There is a variety of recycling techniques that are available today, each having its applicability under specific conditions, as shown in Table 1.


A pervious layer in roads would allow the percolation of rain water to the lower layers, and offer the following benefits, that are related to both groundwater as well as storm water/flood control:

1. Recharge of ground water, and hence a reduction in the potential of lowering of groundwater and the related problems.
2. Detention of rain water and hence a lowering of the storm water volume that ends up in road sewer system;
3. Filtering of storm water that is contaminated by different methods on the road; studies show that a significant amount of suspended solids, phosphorous, zinc and hydrocarbons are removed.

For asphalt pavements, a typical pervious pavement is obtained by using asphalt treated permeable base over a layer of pervious stone layer over the subgrade (with filter material) as shown in Figure 2. For concrete pervious pavements, there is the option of using interlocking/segmented concrete pavers. Even if not used in mainline, storm water tree trenches can allow the detention of a significant amount of storm water for sufficient time before releasing it into the sewer system (Figure 3).


Low energy mixes, particularly warm mix asphalt are becoming more common these days, because of their inherent advantage of lower energy cost and lower emission potential. Another key factor in considering warm mix asphalt is the fact that because of lower temperature, the asphalt binder undergoes less aging, and hence a better pavement mix is obtained during construction - which translates to longer life and hence less frequent rehabilitation cycles. There are different types of WMA techniques available, such as zeolites, wax, emulsion and foam - with newer products and techniques being developed, and one or more suitable methods can easily be adopted for regular use by the industry. The selection of the specific method should be made on the basis of the availability of the material, expertise and experience with that specific technology.

The use of binders manufactured from bio-resources is being researched and conducted in different parts of the world. These binders are essentially derived from biomass - agricultural and forest residue with the help of biochemical or thermochemical processes. One such example of bio binder which is fully derived from vegetable is Vegecol from COLAS. Such binders can be utilized for modifying asphalt (< 10% replacement), extending asphalt binder (>25% replacement) or totally replacing asphalt. Different grades of such binders could also be used for cleaning road equipment or partially replacing fluxing agents. The advantages of using such binders are significant reduction in pollution - no SOx and major reduction in NOx emissions, and manufacturing from 100% renewable agricultural products. Furthermore, the temperature of mix production could be reduced significantly.


One of the most exciting technologies that are being developed in pavement engineering is the combined use of the recycling and warm mix concepts to produce good performing and long lasting pavements. Binder from renewable source could also be used in this process. Using sophisticated materials and plant/process control technology, it is now possible to effectively recycle close to 100% reclaimed asphalt pavement (RAP) material, at a reduced temperature (for example below 130oC as opposed to 150oC) to meet emission requirements, and still obtain a pavement which has excellent performance under both high and low temperatures, and which can be opened to traffic quickly after construction, even at a temperature which is higher than that at which conventional pavements could be opened. Overall, this translates to savings in money and time, materials and natural resources and energy, and significant reduction in emission. Example of such a project (in Germany) is shown in Figure 4.

Recycling of discarded tires in the form of crumb rubber in asphalt pavement mixes is now an accepted practice in many parts of the world. Apart from allowing the user to avoid the filling up of precious landfill space with hazardous waste, this process allows the user to:

1. Incorporate more asphalt binder (thicker film) in mixes to make, them longer lasting and durable
2. Use a coarser gradation for specific uses such as drainage of water.
3. Reduce noise levels from pavements significantly.

The key to successful use of rubber modified asphalt binder is successful incorporation and retention of the rubber into the binder/mix. As is the case for many technologies, this is heavily dependent on available plant and construction equipment. For example, appropriate technology must exist to uniformly blend the crumb rubber mechanically or chemically with the binder, ensure that the binder remains in that state during transportation, and for proper quality control test methods to test and prevent the separation of the rubber and the binder prior to laydown and compaction. In terms of recycling of other materials, artificial aggregates such as blast furnace slag, generated from steel and iron production could be successfully incorporated in paving mixes.

The production of Portland cement, the key constituent in concrete pavements and clinker required for manufacturing of cement, are a major source of CO₂ emission in the modern world. The use of limestone and supplemental cementitious materials (SCMs, such as fly ash from thermal power plants using coal, and rice husk ash) provide attractive options of reducing the use of cement and thereby cutting down its CO₂ footprint. Blended cement, consisting of Portland cement and SCMs are becoming popular. For maintenance of concrete pavements, modern techniques such as dowel bar retrofits, cross stitching, partial-depth repairs, joint and crack resealing, slab stabilization, diamond grinding could be used to improve the conditions of concrete roads and at the same time avoid or reduce the need for energy intensive major repairs/rehabilitation work.

The concept of perpetual asphalt pavement has been recently proposed as a sustainable technology. The objective of this technique is to provide thick and appropriately constructed (with specific mixes for different layers) layers such that the critical responses (such as tensile strain) in the pavement are kept very low, and as a result no major structural failure could be expected for a long period of time. The only deterioration that could be expected will be in the form of surface distresses, which could be relatively easily and economically addressed by less involved rehabilitation procedure, such as in place surface recycling. Obviously, these pavements would cost more than conventional pavements initially but if constructed properly, they allow the users and the highway agencies to enjoy a better quality and durable pavement with significantly reduced cycle of rehabilitation (the pavements need to be maintained properly), and hence significant reduction in activities that could negatively impact the environment.

In terms of longevity, proponents of the concrete pavement industry argue that the longer life of concrete pavements translates to reduced needs for new natural resources, energy and waste disposal. Another important factor is that because of lower deflection in concrete pavements and hence lower rolling resistance, the mileage is increased by about 3.85% in concrete pavements over asphalt pavements (with similar smoothness) - which translates to lower amount of fuel spent and hence lower amount of greenhouse gas emission. Furthermore, because the surface of a concrete pavement reflects a higher amount of solar energy than asphalt pavement, concrete pavement helps in reducing the indirect contribution of pavements towards the urban heat island effect. And at nighttime, the lighter color of the concrete pavement surface helps in a significant reduction in the amount of energy that is needed for illumination. However, note that smoothness has a significant positive effect on gas mileage, asphalt pavements are in general smoother than concrete pavements, and the reflectivity of asphalt pavements can be improved by using surface gritting or chip seals with light colored aggregates, colorless synthetic binders and surface paints, and the surface temperature could be reduced with the help of open graded friction course.

Life cycle analysis (LCA) in pavement engineering is an established concept, and is utilized in many, if not most, high cost projects. However, traditionally, the cost is evaluated in terms of materials/construction/salvage cost only. For sustainable pavement engineering, the environmental and social cost must also be evaluated. Furthermore, the cost of extending the life of a pavement (and thus lowering the life cycle cost) must be evaluated in the context of the amount of natural resources and energy that are spent on producing the premium materials that are used to extend its life. The impact of the pavement construction/maintenance and rehabilitation should be evaluated in terms of all three components of sustainability - social, economic and environmental. Examples of such framework and components of framework are the Sustainability Life Cycle Assessment (SLCA), the IRF Greenhouse Gas Calculator, and PaLATE (Pavement Life-cycle Assessment Tool for Environmental and Economic Effects).

Workers

Hundreds of thousands of human beings - men, women and in many cases even children work in pavement related projects all over the world. In many cases, this work goes on under extreme conditions of weather, in areas that are remote from basic facilities such as school or hospitals, and under conditions of emissions/pollution from construction conditions (smoke/gas/particulate emission). This makes the entire realm of pavement engineering socially unsustainable. Conditions must be improved such that environmental and economic justice is prevalent among the pavement workers. While there are many things that need to be done, first and foremost, it is the responsibility of pavement engineers to select materials and methods such that working conditions are improved. For example, switching to warm mix asphalt instead of hot mix asphalt will reduce harmful emissions significantly (Figure 5). Secondly, engineers and authorities must enforce strict safety regulations and provide for the basic necessities such as protective body and eye wears.

Pavement-Building-Nature-Symbiosis

As pointed out earlier, pavements need to be considered as part of the built environment - going one step forward, in the future; pavements need to be considered as part of a symbiotic system that also includes buildings and the nature. Imagine a day when we will have smart pavements, that will be able to self control temperatures and adapt to changing environmental conditions to sustain loads without deforming or cracking, will be able to harvest heat energy from the solar radiation falling on them, and supply this to buildings for various uses, and in return, the buildings will be keeping the pavements below a critical temperature, thus extending their lives, and also reducing the amount of heat that is radiated back into the environment, and hence indirectly reducing the urban heat island effect. Multidisciplinary research, as indicated in Figure 6, needs to be conducted to make this vision a reality.

Regulatory Bodies and Impetus for Sustainability

Adopting sustainable practices can reduce many of the environmental impacts of pavement construction. However, both impetus and regulations ("carrots and stick") need to be in place to accelerate the adoption of sustainable practices. Impetus in the form of prestigious certification, first voluntary, and then gradually, mandatory, should be introduced. Example of such certification is the US Green Building Council's Leadership in Energy and Environmental Design (LEED) in the United States, which awards silver, gold and platinum certifications for projects in increasing order of contribution towards "improving performance across all the metrics that matter most: energy savings, water efficiency, CO₂ emissions reduction, improved indoor environmental quality, and stewardship of resources and sensitivity to their impacts." Regulatory bodies such as the Environmental Protection Agency (EPA) in the US enforce strict emission standards to prevent the emission of harmful gases and particulates into the atmosphere.

Human Factor

Pavements are engineered systems that need to be planned, designed, constructed, maintained and rehabilitated by knowledgeable and skilled people. However, for achieving sustainable pavement engineering, we need socially responsible engineers, with broad vision and compassion for humanity and respect for nature.

Therefore, the preparation for sustainability starts today in our Colleges and Universities with the students who will be joining the profession tomorrow. Professors, administrators and the tax paying general public need to be educated on the impacts of pavement engineering and the different ways to minimize such impacts. This can be achieved by establishing multidisciplinary centers of excellence in sustainable pavement engineering in major colleges and Universities in conjunction with the industry and the government in the different parts of the country. While education on existing and new concepts of sustainability can be transmitted by this method, the centers should also focus on developing, through research, new sustainable concepts and practices. And in that research, it is vital that engineers look at the future - innovative, sophisticated materials and techniques, as well as into the past - to determine what has survived over centuries without harmfully affecting our environment.

This has been a fairly long article, and it is time for the authors (and perhaps the readers as well) to relax and sip a cup of tea- preferably in an Indian clay cup- have you ever wondered what a marvel of sustainable engineering the Indian clay cup is? It comes from the earth, serves its purpose in the most efficient way - even adds flavor to the tea, and then goes back completely into the earth, without leaving any lasting impact on the environment. We, pavement engineers, have a lot to learn from it!