IntroductionThe rst national workshop on Warm Mix Asphalt (WMA) in India was held on August 8th and 9th at IIT Madras (IITM), under the sponsorship of several major industries, and direction of the faculty members of the Civil Engineering department at IITM. More than 200 representatives from the academia, government and industry actively participated in this workshop, which consisted of lectures, videos and vigorous discussion on the subject of WMA. Judging from the participation, the workshop was a grand success! But one wonders, what is it that made the workshop a success? Why did 200+ people (even through a holiday) took the pain to attend it and actively participate in it? The answer is, more than anything else, the topic itself! WMA is a smart technology that can help engineers realize their dream of constructing good performing and long lasting pavements, and at the same time, decrease energy consumption, emissions and increase the use of

reclaimed asphalt pavement (RAP) materials signicantly. Too good to be true? Not quite! Every bit about WMA is true, and the rest of the article shows why it is so good.

WMAAs we all know, because of high viscosity of asphalt binders (bitumen), both aggregates and bitumen need to be heated to high temperatures (generally > 150C) for proper mixing, laydown and compaction of asphalt mixes, commonly known as Hot Mix Asphalt (HMA). The high temperature requires the use of signicant amount of fuel, and also result in fumes and emissions in the plant and at site. Although well below dangerous levels, these emissions are hazardous and anything that could be done to reduce them, is a step towards the right direction. The rate of emission actually doubles for every 10C rise in temperature.

So, what is Warm Mix Asphalt? Warm Mix Asphalt, or WMA, as it is popularly called, is a collection of

technologies that allow a reduction in the temperatures at which asphalt paving mixes are produced, placed and compacted. The reduction in temperature reduces a number of problems, as noted in the above paragraph, and actually helps in producing a better mix. How so? Because, it decreases the aging (such as through oxidation and volatilization) of the bitumen during production by reducing the temperature (higher the temperature, higher the oxidation) and helps to retain some of the good properties of the bitumen in the asphalt mix, and thus decrease the rate of deterioration (such as cracking and the moisture damage) over time. Furthermore, WMA technologies expand the time window of compaction, allowing rolling and compaction at lower than conventional temperatures (such as 90C), and improves the uniformity of density and thus helps in improving density of the pavements as we all know, density is the key property that governs strength, stiffness, durability and resistance

Warm Mix Asphalt A Smart Solution for Building

Sustainable Pavements in IndiaRajib B. Mallick, PhD, PE, Ralph White Family Distinguished Professor, Civil and Environmental Engineering Department, Worcester Polytechnic Institute (WPI), Worcester, MA 01609. A.Veeraragavan, Professor of Civil Engineering, Indian Institute of Technology Madras, Chennai.

106 NBM&CW SEPTEMBER 2013

WMA

108 NBM&CW SEPTEMBER 2013

WMA

against moisture damage, and hence longevity of the pavement.

Popularity and UseBecause of the signicant advantages over HMA, WMA has become signicantly popular in Europe and US, among other pats of the world, very rapidly in less than two decades. Although started in Europe in the mid nineties, WMA has been adopted at a very rapid rate in the last few years in the US, and there is now more WMA pavements in the US than in Europe, and in the near future, most of not all of asphalt mixes will be produced with some WMA technology.

The reasons are obvious it is a technology that helps in improving the product, construction, and at the same time cuts down energy use and emissions. Under the leadership of the Federal Highway Administration (FHWA) in the US, the use of WMA has increased from a mere 5% (of asphalt mixes) to more than 30% in the last three years! A brief timeline of WMA is shown in Table 1.

So, what exactly is WMA and how does it work?Actually, WMA consists of several technologies, which can be broadly classied into four different types.

1. Organic additives, 2. Chemical additive, 3. Foaming processes, and 4. Hybrid technologies. The foaming technology can be further subdivided into two classes, foaming additives and water injection system. Currently there are altogether more than 30 different WMA technologies.

Although the end effect of reduction of mixing, laydown and compaction temperatures are the same, the different technologies work in different ways. The additives, which are either waxes or other hydrocarbon modiers improve lubrication by reducing the viscosity of bitumen and allow a reduction of 28-40C in mixing and compaction temperature. Examples of such additives are Sasobit, SonneWarmix, ECOBIT, LEADCAP and BituTech. Typical dosage amounts are 0.5 to 1.5% by weight of bitumen. Sometimes these additives are also added as modiers for increasing the stiffness of asphalt mixes, for specialty applications, such as in racing tracks.

Chemical additives are Surfactants (surface active agents) that reduce surface tension between the polar aggregates and non-polar bitumen, improve wetting and reduces internal friction, and allows a reduction of 28-50C in mixing and compaction temperatures. Evotherm, CECABASE

RT, QualiTherm, Rediset L, LEA Lite are examples of chemical additives. Typically they are added at the rate of 0.20 to 0.75 percent by weight of bitumen.

The foaming process works by creating foamed asphalt that improves coating and compaction at lower temperature. Water expands 1,600 times when converted into steam at atmospheric pressure, and the steam is encapsulated by viscous bitumen producing foam, which occupies a much greater volume compared to the original bitumen. The water for creating the foam is either added as water through a water injections stem in a specialized equipment, or from zeolites (which contain about 20% water). Water is added at a rate of 1.25 to 2.0% by weight of bitumen (about 1 lb. of water per ton of mix), whereas the zeolites are added a rate of 0.1 to 0.3% by weight of the mix. Foaming by water allows 18-30C reduction in temperature whereas foaming by zeolites allow a reduction of 30-40C. Examples of water injections systems are AESCO/Madsen, Astec, Gencor, Herman Grant, Maxam, Meeker, Stansteel, Tarmac and Terex; examples of zeolites are Advera and Aspha-min.

Hybrid technologies utilize a combination of two or more WMA technologies to achieve the reduction in temperature. For example, Low Energy Asphalt (LEA), utilizes a chemical additive with a water injection system to improve coating at lower temperatures.

Finally, there are products that were originally developed for other uses, but do incorporate the WMA technology for reducing temperature and hence better utilization of the product. Examples are Thiopave (sulfur and WMA) and TLAX (Trinidad lake asphalt and WMA technology).

!!"

!

112 NBM&CW SEPTEMBER 2013

WMA

BenetsThe most obvious monetary benet of using WMA is the savings in fuel cost because of reduced mixing temperature in the plant. Theoretical calculations show a savings of 11% for a 28oC reduction in temperature, and reported savings in the US eld trials range from 15 to 77%. Reported reduction in emissions range from 15-40% for CO2, 18-35% for SO2 and 18-70% for NOX. A signicant improvement in work environment has been noted, with a reduction in total organic matter by 33-61%. Cessation temperatures for compaction has been reported to be as low as 60oC, resulting in signicantly enhanced compactibilty of mixes. Improvement in workability has also been reported, and in the case of certain WMA technologies the use of anti-stripping agent for resisting moisture damage have been completely avoided. Reported haul time for WMA from different parts of the world range from 1-3 hours a signicant fact that could be utilized for hauling mixes and still ensuring good compaction in rural areas, where plants are few and far between. Uniformity of density has been reported to improve signicantly (for example only 24oF difference within mat temperature for WMA compared to 62oF for HMA), leading to reduction in distress and improvement in ride quality. Since WMA is produced

at a lower temperature, the rate of cooling is lower, and hence allows better cold weather paving. Because of lower temperature, an additional amount of RAP could also be utilized in these mixes, and the use of 10% more RAP can result in a savings of about $4 per ton of the mix. Finally, bitumen recovered from WMA mixes (86% of original penetration) have shown a signicant reduction in aging compared to that in bitumen recovered from HMA (63% of original penetration). This translates to a signicant improvement in durability of the paving mixes.

Are there any production concerns?As with any new technology, there are a few concerns about the production of WMA, specically because of the lower temperatures that are utilized during production. Fortunately, all of these problems are expected and solvable, in many cases through the adoption of techniques that could also be utilized for improving conventional HMA production.

The rst concern is about incomplete drying of aggregates (specically the internal moisture) at the reduced temperatures. It has been seen that for aggregates with an absorption value of less than 1 %, drying of aggregate has not been reported to be a problem at WMA temperatures. To prevent the incomplete drying of aggregates, it is suggested that stockpiles be kept as dry as possible by sloping sides, paving surrounding areas, and keeping them under cover. To dry aggregates with high moisture content the retention time in the dryer drum could be increased and the dryer shell should be insulated properly. Ways to detect incomplete drying include a >20oF fall in temperature in mix between discharge and loading, dripping water from silos and excessive steam from slat conveyors and a loss

So, how much does it cost?Before answering this question it must be pointed out that the benets of using WMA could easily offset the added investment cost that one needs to incur, as well the cost of the use of additive, if any. The cost of using WMA technology can be reduced or avoided completely depending on the volume of mix the contractor is producing with WMA technology. Popular WMA additives in the US increase the cost by about $2-$3 per ton of the mix, whereas a water injections system would cost $30K to $100K for the installation of the system, which translates to about 7c per ton, if 100% f the mix that is produced is WMA. The additives that are mostly added to the bitumen can be added at the terminal, or in the asphalt plant, in which case, a separate line for blending and metering is necessary. Blending in the tank with agitation is also possible. For solid additives, a vane feeder to blow additive into bitumen stream in drum or a RAP collar could be used, or can be fed in bulk into a pugmill in the case of a batch plant. The type of plant modication that is needed depends entirely on the type of WMA technology that one decides to use, and the cost decreases as more and more WMA is produced (instead of HMA).

114 NBM&CW SEPTEMBER 2013

WMA

of >0.5% of the weight of mix during moisture content test.

The second concern is regarding incomplete combustion of fuel at the reduced temperature and the resulting risk of getting unburnt fuel in the mix. Evidence of such a problem include brownish color of mix and higher than normal emissions. Proper maintenance and tuning of burner, and preheating of burner fuel are recommended solutions to this problem.

The last but not least problem is the potential of condensation of baghouse nes, leading to the clogging and decreased efciency of the emission control system. Recommended solutions include proper preheating of baghouse, sealing of leaks, adjusting ights and slopes of the dryer to increase baghouse exhaust temperature, insulation of baghouse and ductwork and addition of duct heaters to increase baghouse temperatures, if needed. A high (>4-5 psi) pressure drop across bags is an indicator of caking due to condensation.

How do I design and evaluate WMA?So far, most WMA are designed exactly the same way as the corresponding HMA mixes, the only difference being the lower temperature of mixing and compaction; however some additional tests could be required to ensure that the reduced temperatures are adequate for the proper production of mixes. For example, the American Association of State Highway and Transportation Ofcials (AASHTO) specications (appendix to R35) include a coating test (> 95% coating) and a compactability test (ratio of number of gyrations to 92% of theoretical maximum density, TMD at 30C below planned temperature at start of temperature to that at the planned temperature at start of

compaction should be less than 1.25). Extensive laboratory studies,

and eld trials in the US, Europe and Australia have conrmed equal or even better performance of WMA (compared to HMA). However, authorities are encouraged to evaluate each and every WMA technology before adoption, and the recently released Research Digest 374 from the NCHRP provides helpful guidelines for independent evaluation of WMA products. In the US, most states maintain a list of approved WMA technology, and many of them now allow the use of WMA wherever HMA is specied. Generally, the cost of evaluation is borne by the supplier and the product is evaluated in the laboratory and through accelerated eld trails within a time span of 18 months.

Selection of Best WMA TechnologyThe selection of the best WMA technology depends on many factors, and in most cases is dependent on the monetary incentives and benets of using WMA. Important factors to consider include the reduction in temperature that is desired, the tonnage of mix that is anticipated and whether or not to invest in plant technology that are needed for certain additives. It should also be pointed out that the green benets of adopting WMA technologies should not be overlooked, and that a reduction in emission through a reduction of temperature can help contractors/agencies receive signicant amount of carbon credits.

The way forward..Four critical things need careful attention at this stage. First, the need for plant modication should be carefully considered before adopting any specic WMA product, in conjunction with the expected

volume of WMA. For long term and high volume commitment to WMA it makes sense to invest in permanent plant modications, such as water injection system or in-line blending and metering unit, whereas for experimental work addition in tank with agitation may be considered. Secondly, a preliminary but proper specications for the use of WMA in India should be completed and published by the responsible authorities, such as the Indian Road Congress (IRC) as soon as possible to encourage the adoption of this technology. All stakeholders, such as the industry, government and the academia should be involved in this process, and specications that have been successfully adopted by other countries should be consulted. The specications should be tight but permissive enough to encourage innovations. Thirdly, there is an immense scope of research and development in this technology, and premier institutes such as the IITs and the NITs must be encouraged and supported by the government and the industries to pursue research. This will not only help the country to reap the benets of this smart technology to the maximum extent, but also introduce the subject to the undergraduate and post-graduate students, who will be joining the workforce in the near future. Finally, an annual get-together, such as the one that was recently held at IIT Madras should be held to bring all stakeholders together to present, debate and discuss practical topics for the successful implantation of WMA in India.

AcknowledgementsThe authors gratefully acknowledge the help of US Department of Transportation Federal Highway Administration (FHWA) in collecting much of the data and information that have been used in this paper.