warm mix asphalt technologies- an overview

21
1 Warm Mix Asphalt Technologies: An Overview By Prof. Prithvi Singh Kandhal* [This paper was published as Paper No. 561 in the Journal of the Indian Roads Congress, Volume 71-2, July-September 2010. Authors’ responses to comments received on this paper are also given at the end.] ABSTRACT Warm mix asphalt (WMA) technology allows the mixing, lay down, and compaction of asphalt mix at significantly lower temperatures compared to hot mix asphalt (HMA). WMA mixes can be produced at temperature of 120 C or lower. WMA offers many significant advantages such as (a) energy savings, (b) decreased emissions and fumes, (c) decreased binder aging, and (d) extended paving season into the cold winter months and at places located on high altitudes. The last advantage should be very useful to the Indian Border Roads Organization. India can also earn carbon credits under the Kyoto Protocol if WMA is implemented as a replacement for HMA wherever possible, thereby reducing greenhouse gas. The paper describes the WMA technologies developed to this date in Europe and the US, such as synthetic zeolite, Sasobit, Evotherm, WAM Foam, LEA, Rediset WMX, REVIX, and Double Green Barrel together with their laboratory evaluation, if available. It has been recommended to construct WMA demonstration projects in India soon. 1. INTRODUCTION Warm mix asphalt (WMA) is a fast emerging new technology which has a potential of revolutionizing the production of asphalt mixtures. WMA technology allows the mixing, lay down, and compaction of asphalt mixes at significantly lower temperatures compared to hot mix asphalt (HMA). The technology can reduce production temperatures by as much as 30 percent 1 . Asphalt mixes are generally produced at 150°C or greater temperatures depending mainly on the type of binder used. WMA mixes can be produced at temperatures of about 120°C or lower. The development of WMA was initiated in Europe in the late 1990s primarily in response to the need for greenhouse gas reduction under the Kyoto Protocol. WMA technologies such as Aspha-min, WAM Foam, and Sasobit were developed during that time. The US National Asphalt Pavement Association (NAPA) organized a European Scan tour in 2002 to examine these three WMA technologies. The National Center for Asphalt Technology (NCAT) initiated research 1,2,3,4 on WMA technologies in 2003. New WMA technologies such as Evotherm, Rediset WMX, REVIX, LEA (Low Energy Asphalt) and Double Barrel Green were later developed within the US during * Associate Director Emeritus, National Center for Asphalt Technology (NCAT), Auburn University, USA. Currently in Jaipur. e-mail: [email protected]

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Paper by P.S. Kandhal published in the Journal of the Indian Roads Congress Volume 71-2, 2010 gives an overview of current warm mix asphalt technologies.

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Page 1: Warm Mix Asphalt Technologies- An Overview

1

Warm Mix Asphalt Technologies: An Overview

By

Prof. Prithvi Singh Kandhal*

[This paper was published as Paper No. 561 in the Journal of the Indian Roads

Congress, Volume 71-2, July-September 2010. Authors’ responses to comments

received on this paper are also given at the end.]

ABSTRACT

Warm mix asphalt (WMA) technology allows the mixing, lay down, and compaction

of asphalt mix at significantly lower temperatures compared to hot mix asphalt

(HMA). WMA mixes can be produced at temperature of 120 C or lower.

WMA offers many significant advantages such as (a) energy savings, (b) decreased

emissions and fumes, (c) decreased binder aging, and (d) extended paving season into

the cold winter months and at places located on high altitudes. The last advantage

should be very useful to the Indian Border Roads Organization. India can also earn

carbon credits under the Kyoto Protocol if WMA is implemented as a replacement for

HMA wherever possible, thereby reducing greenhouse gas.

The paper describes the WMA technologies developed to this date in Europe and the

US, such as synthetic zeolite, Sasobit, Evotherm, WAM Foam, LEA, Rediset WMX,

REVIX, and Double Green Barrel together with their laboratory evaluation, if

available. It has been recommended to construct WMA demonstration projects in

India soon.

1. INTRODUCTION

Warm mix asphalt (WMA) is a fast emerging new technology which has a potential of

revolutionizing the production of asphalt mixtures. WMA technology allows the

mixing, lay down, and compaction of asphalt mixes at significantly lower

temperatures compared to hot mix asphalt (HMA). The technology can reduce

production temperatures by as much as 30 percent1. Asphalt mixes are generally

produced at 150°C or greater temperatures depending mainly on the type of binder

used. WMA mixes can be produced at temperatures of about 120°C or lower.

The development of WMA was initiated in Europe in the late 1990s primarily in

response to the need for greenhouse gas reduction under the Kyoto Protocol. WMA

technologies such as Aspha-min, WAM Foam, and Sasobit were developed during

that time. The US National Asphalt Pavement Association (NAPA) organized a

European Scan tour in 2002 to examine these three WMA technologies. The National

Center for Asphalt Technology (NCAT) initiated research1,2,3,4

on WMA technologies

in 2003.

New WMA technologies such as Evotherm, Rediset WMX, REVIX, LEA (Low

Energy Asphalt) and Double Barrel Green were later developed within the US during

* Associate Director Emeritus, National Center for Asphalt Technology (NCAT),

Auburn University, USA. Currently in Jaipur. e-mail: [email protected]

Page 2: Warm Mix Asphalt Technologies- An Overview

2

the 2004-2007 period11

. A second European scan tour5,6,7

was conducted in May 2007

by US engineers to gather additional information on WMA technologies in Europe.

Although WMA originated in Europe the scan team found that the US had already

made great progress in evaluating and implementing WMA technologies. This was

made possible by the public private partnerships.

2. ADVANTAGES OF WMA AND IMPORTANCE FOR INDIA

Warm mix asphalt offers the following significant advantages:

• Energy savings. The most obvious benefit of WMA is the reduction in fuel

consumption. Fuel is used to dry and heat the aggregate. Studies have shown that

lower plant mix temperatures associated with WMA can lead to as much as 30

percent reduction in energy consumption1.

• Decreased emissions. WMA produces emissions (both visible and non visible)

from the burning of fossil fuels at a significantly reduced level compared to HMA.

This would permit asphalt plants to be located in and around non-attainment areas

such as large metropolitan areas that have air quality restrictions.

• Decreased fumes and odour. WMA produces lower fumes and odour both at the

plant and the paving site compared to HMA. This would also result in improved

working conditions at both places.

• Decreased binder aging. Short-term aging of liquid asphalt binder takes place

when it is mixed with hot aggregate in pug mill or mixing drum. This aging is

caused by the loss of lighter oils from the liquid asphalt binders during mixing at

high temperatures. It is believed that the short-term aging of the binder will be

reduced significantly because the loss of lighter oils will be less at relatively lower

mixing temperatures. This may enhance asphalt pavement durability.

• Extended paving season. By producing WMA at normal HMA temperatures, it

may be possible to extend the paving season into the colder months of the year or

in places located on high altitudes since the WMA additives or processes act as a

compaction aid. Further by narrowing the difference between compaction

temperature and ambient air temperature the rate of cooling is decreased. WMA

may also be transported over longer distances as compared to HMA with reduced

loss of mix temperature in the hauling units. This advantage should facilitate the

Indian Border Roads Organization (BRO) in constructing asphalt roads in high

altitude and/or remote areas far away from hot mix plants8.

• Compaction aid for stiffer mixes. WMA additives and processes may be used to

improve the compactibility of stiff mixes when mix is produced closer to typical

HMA production temperatures. Smaller reductions in temperature may also be

possible. There is extensive experience with the use of certain types of WMA with

SMA in Europe.

• Increased amount of RAP. Research9 has shown that the percentage of reclaimed

asphalt pavement (RAP) can be increased in WMA compared to HMA during hot

recycling.

• Generation of carbon credits for India. Developing countries like India can earn

CERs (Certified Emission Reductions) or popularly known as carbon credits

under the Kyoto Protocol if technologies such as WMA are introduced and

implemented.

Page 3: Warm Mix Asphalt Technologies- An Overview

3

Photo 1. Emission can be seen from the hot mix asphalt at 320 F (160 C) in left truck. No

emission is visible from the truck in right containing warm mix asphalt at 250 F (121 C).

(Photo courtesy: Matthew Corrigan, FHWA)

3. WARM MIX ASPHALT TECHNOLOGIES: FEATURES AND

EVALUATION

At the present time a mix is considered warm mix in the US if the mix produced at the

plant has temperature exceeding 100º C but significantly below that of a normal hot

mix. WMA has a wide range of production temperatures ranging from slightly over

100º C to about 20 to 30º C below typical HMA temperatures. WMA technologies are

also applicable to mixes made with polymer modified asphalt binders.

WMA technologies can be classified broadly5 as (a) those that use water, (b) those

that use some type of organic additive or wax, (c) those that use chemical additives or

surfactants.

Technologies which introduce small amounts of water to hot asphalt binder take

advantage of the phenomenon: when water turns into steam at atmospheric pressure it

expands in volume by a factor of 1,673. This causes tremendous increase in the

volume of asphalt binder which not only helps in coating the aggregate easily but also

lowers the mix apparent viscosity. Processes to introduce water into the asphalt binder

consist of foaming nozzles, use of hydrophilic material such as zeolite or use of damp

aggregate. Asphalt binder temperature typically is the same as that used for hot mix

asphalt.

Technologies that use organic additives or waxes lower the asphalt binder viscosity

above their respective melting points. It should be ensured that their melting points

Page 4: Warm Mix Asphalt Technologies- An Overview

4

are above the in-service pavement temperatures during hot summers so that

permanent deformation or rutting does not become a problem5.

Technologies that use some chemical additive and /or surfactants produce a variety of

different mechanisms to coat the aggregate at lower temperatures.

In 2003, the National Center for Asphalt Technology (NCAT) in the US was given

the task of evaluating some proprietary WMA technologies from Europe and one

WMA technology developed in the US1. The objective was to perform a laboratory

study to determine the applicability of WMA technologies/products to typical paving

operations and environmental conditions commonly found in the United States. The

studies were designed to answer the following questions:

(a) Does the WMA technology affect the compactibility of asphalt mix in the

Superpave gyratory compactor and, therefore, its optimum asphalt content?

(b) Does the WMA technology affect the structural strength of the asphalt mix in

terms of resilient modulus?

(c) Does the WMA technology increase the rutting potential of asphalt mix at high

temperatures?

(d) Does the WMA technology necessitate some cure time for the asphalt mix before

opening to traffic?

(e) Does the WMA technology increase the potential for moisture damage since the

mix is produced at significantly lower temperatures? Incomplete drying of the

aggregate may result from lower temperatures. Thus, the water trapped in the coated

aggregate may cause moisture damage10

.

As mentioned earlier, several WMA technologies have been developed in Europe and

the US. More are being developed. A general description of the existing WMA

technologies follows together with NCAT’s findings from their laboratory evaluation,

if it was conducted5.

3.1 Aspha-min®

Aspha-min is a product of Eurovia Services GmbH based in Germany. Aspha-

min is a manufactured synthetic sodium aluminium silicate, better known as zeolite.

The crystalline structure of zeolite has large interconnected spaces, which can hold

water molecules. Eurovia’s Aspha-min contains approximately 21 percent water by

mass, which is released in the temperature range of 85-185°C. When Aspha-min is

added to the mix at the same time as the liquid asphalt binder, water is released. This

water release causes the asphalt binder to microscopically foam, which allows

increased workability and aggregate coating at lower temperatures. According to

Eurovia’s recommendations, Aspha-min is added at a rate of 0.3 percent by mass of

the asphalt mix, which can result in a potential 28°C reduction in typical HMA

production temperatures5.

Aspha-min zeolite is approximately a 50-mesh material, which may be added directly

to the pug mill of a batch plant. It can be added to a drum plant through RAP collar or

pneumatically fed using a specially built feeder. Aspha-min is available as a fine

white powder in 25- or 50-kilogram bags or in bulk for silos5.

Page 5: Warm Mix Asphalt Technologies- An Overview

5

Photo 2. Specially built feeder (right) was used in Florida to blow Asphalt-min

into the drum using an existing fiber addition line (Photo courtesy: NCAT)

Photo 3. Placement of warm mix asphalt made with Aspha-min at the 2004 World of

Asphalt Conference in Nashville, Tennessee (Photo courtesy: Matthew Corrigan, FHWA)

Page 6: Warm Mix Asphalt Technologies- An Overview

6

More than 300,000 tons of WMA have been produced worldwide with zeolite mainly

in Germany, France and the US5.

NCAT has completed the laboratory evaluation of Aspha-min. The final report, 05-04,

can be accessed at NCAT’s website, www.ncat.us

The following conclusions were drawn from this study1:

• Addition of Aspha-min zeolite lowered the measured air voids in the Superpave

gyratory compactor (SGC). While this may indicate a reduction in the optimum

asphalt content, at this time it is believed that additional research is required and

that the optimum asphalt content of the mixture determined without the zeolite

should be used. It should be noted that the optimum asphalt content of the mixture

without the addition of the zeolite was used for all of the testing (with and without

zeolite) completed in this study.

• Aspha-min zeolite improved the compactability of the mixtures in both the SGC

and vibratory compactor. Statistics indicated an average reduction in air voids of

0.65 percent using the vibratory compactor. Improved compaction was noted at

temperatures as low as 88°C.

• Addition of zeolite does not affect the resilient modulus of an asphalt mix.

Improved density improves the measured resilient modulus. Therefore, there

would not be any effect on pavement thickness design when using WMA

produced with Aspha-min zeolite, all other things considered equal.

• Addition of zeolite does not increase the rutting potential of an asphalt mix. The

rutting potential increased with decreasing mixing and compaction temperatures,

which may be related to the decreased aging of the binder.

• There was no evidence of differing strength gain with time for the mixes

containing zeolite as compared to the control mixes. The addition of Aspha-min

may not require a cure time for the asphalt mixture prior to opening to traffic.

• The lower compaction temperature used when producing WMA with Aspha-min

may increase the potential for moisture damage. As mentioned earlier, lower

mixing temperatures may result in incomplete drying of the aggregate and the

resulting water trapped in the coated aggregate may cause moisture damage.

Reduced tensile strength and visual stripping were observed in both the control

and Aspha-min zeolite mixes produced at 121°C.

• Various anti-stripping agents were evaluated to mitigate the potential for moisture

damage. Hydrated lime appeared to be effective with the granite aggregate. The

addition of 1.5 percent hydrated lime resulted in acceptable performance in terms

of both cohesion and moisture resistance over the warm mixtures without

hydrated lime.

• Hamburg wheel tracking results confirmed the test results produced by the TSR

testing (AASHTO T 283), as well as suggesting the lime will also assist in the

rutting resistance of warm mixtures compacted at lower temperatures due to the

lime stiffening the asphalt binder.

• More research is needed to further evaluate field performance, the selection of the

optimum asphalt content, and the selection of binder grades for lower production

temperatures.

Page 7: Warm Mix Asphalt Technologies- An Overview

7

3.2 Sasobit®

Sasobit is a product of Sasol International, Hamburg, Germany. Unlike Aspha-

min, which relies on foam to enhance mix workability at lower temperatures, Sasobit

is a paraffin-wax compound derived from coal gasification using the Fischer-Tropsch

(FT) process. The smaller crystalline structure of the FT wax is believed to reduce

brittleness at low temperatures as compared to bitumen paraffin waxes. Sasobit is

designed as an “asphalt flow improver,” both during the asphalt mixing process and

during lay down operations, due to its ability to lower the viscosity of the liquid

asphalt binder. This decrease in viscosity allows working temperatures to be

decreased by 17 to 54º C degrees5.

Sasol recommends that Sasobit be added at a rate of 0.8 percent or more by mass of

the binder, but not to exceed 3 percent. Sasobit can be blended into hot liquid asphalt

binder at the blending plant (terminal) without the need for high shear mixing. Sasobit

is available in two forms: flakes for molten additions or prills (small pellets) for direct

addition to the mix.

Photo 4. Sasobit flakes for melting into asphalt binder (Photo courtesy: NCAT)

In the United States, Sasobit has been blended with the liquid asphalt binder at the

terminal or blown directly into the mixing chamber at the same point cellulose fibers

were being added to an SMA. Commercial supplies of Sasobit are available in 25 kg

bags and 600 kg super-sacks.

Page 8: Warm Mix Asphalt Technologies- An Overview

8

Photo 5. Sasobit prills for adding directly into pug mill or mixing drum

(Photo courtesy: NCAT)

Photo 6. Feeder for Sasobit prills (Photo courtesy: Matthew Corrigan, FHWA)

Over 10 million tons of asphalt mix has been produced worldwide with Sasobit, most

of it was used to produce WMA. In the US over 250,000 tons of WMA have been

produced with Sasobit5.

The following conclusions which are generally similar to Aspha-min, were

drawn from NCAT’s evaluation of Sasobit1:

Page 9: Warm Mix Asphalt Technologies- An Overview

9

• The modified asphalt binder including Sasobit needs to be engineered to meet the

desired performance grade. As an example in NCAT’s study a PG 58-28 binder

(approximately equivalent of VG-10 in India) was used as the base asphalt binder

with the addition of 2.5 percent Sasobit to produce a PG 64-22 binder

(approximately equivalent to VG-20 in India).

• Sasobit improved the compactibility of the mixtures in both the SGC and

vibratory compactor. Statistics indicated that an average reduction in air voids up

to 0.9 percent was obtained.

• Addition of Sasobit does not affect the resilient modulus of an asphalt mix

compared to mixtures having the same PG binder.

• Addition of Sasobit generally decreased the rutting potential of the asphalt mixes

evaluated.

• Lower compaction temperature used in producing WMA may increase the

potential for moisture damage. TSR tests (AASHTO T 283) indicated moisture

damage in both the control and Sasobit mixes produced at 121°C. However, the

addition of a liquid anti stripping agent improved the TSR values to acceptable

levels.

• More research is needed to evaluate field performance, the selection of optimum

asphalt content, and the selection of binder grades for lower production

temperatures.

3.3 WAM-Foam® (Warm Asphalt Mix Foam)

WAM-Foam is a two component binder system: a soft binder and a hard

foamed binder, which are introduced at different times in the mixing cycle during the

production of asphalt mix. WAM-Foam is a joint venture product of Shell

International Petroleum Company Ltd., U.K. and Kolo-Veidekke, Norway. First, a

soft asphalt binder is mixed with the aggregate at 100-120°C temperatures to coat the

aggregate fully. Next, a hard asphalt binder in the form of foam is mixed with the pre-

coated aggregate. Cold water is injected into the hard asphalt binder to produce a

large volume of foam. This combination of soft binder and foamed hard binder acts to

lower the viscosity to provide increased workability. The resulting asphalt mixture

can be placed and compacted in the temperature range of 79-91°C. The hard binder

combines with the soft binder to achieve the desired PG grade in the mix5.

Over 60,000 tons of WMA have been produced with WAM-Foam primarily in

Europe. NCAT has not performed any laboratory evaluation of WAM-Foam.

3.4 Evotherm

Evotherm technology based on chemical additives was developed in 2004 by

MeadWestvaco Asphalt Innovations in the United States. It is based on a chemistry

package that includes additives to improve coating and workability, adhesion

promoters, and emulsification agents. The chemistry is delivered in an emulsion with

a relatively high asphalt residue (approximately 70 percent). In the Evotherm

Emulsion Technology (ET) the water in the emulsion is liberated in the form of steam

when it is mixed with hot aggregate. The resulting WMA appears like HMA in

appearance and has a mix temperature ranging from 85 to 115º C1. No plant

modifications are required for using Evotherm. The produced asphalt mix can be

Page 10: Warm Mix Asphalt Technologies- An Overview

10

stored in silos.

A newer process, Evotherm Dispersed Asphalt Technology (DAT) was developed in

2005 and introduced in the field in 2007. Evotherm DAT uses the same chemical

additives as Evotherm ET but it is injected directly into the asphalt binder line just

before it enters the mixing zone of the drum plant. In case of batch plant, the chemical

additive can be fed directly into the pug mill. No substantial modifications to the plant

are required except a separate pipe line to inject the additive.

Over 100,000 tons of WMA have been produced worldwide using Evotherm.

Evotherm has been evaluated in 17 states of the US5.

Photo 7. Placement and compaction of warm mix asphalt made with Evotherm

near Indianapolis, Indiana (Photo courtesy: NCAT)

The following conclusions, which are generally similar to those for Aspha-min and

Sasobit, were drawn from the laboratory evaluation of Evotherm ET Process by

NCAT1:

• Evotherm improved the compactibility of the mixtures in the vibratory compactor.

Statistics indicated an average reduction in air voids up to 1.5 percent.

• Addition of Evotherm does not affect the resilient modulus of an asphalt mix

compared to mixtures having the same PG binder.

• Addition of Evotherm generally decreased the rutting potential of the asphalt

mixes evaluated.

• Lower compaction temperature used in producing WMA may increase the

potential for moisture damage.

• More research is needed to evaluate field performance, the selection of optimum

Page 11: Warm Mix Asphalt Technologies- An Overview

11

asphalt content, and the selection of binder grades for lower production

temperatures.

3.5 LEA (Low Energy Asphalt)

Low energy Asphalt (LEA) WMA technology based on foaming process was

developed by LEA-CO, a French firm and is distributed in the US by McConnaughay

Technologies. A coating and adhesive agent is added to the hot asphalt binder at the

rate of 0.5 percent by weight of the binder. The treated total binder maintained at

normal binder temperature is mixed with hot coarse aggregate heated to about 150º C.

After the coarse aggregate is coated, it is mixed with cold, wet fine aggregate (about

3-4 percent moisture). The moisture in the fine aggregate turns into steam and causes

the asphalt binder around the coarse aggregate to foam and coat the fine aggregate.

The resulting mix temperature is usually about 100º C5.

In case of a batch plant, a separate cold feed bin is required to add the cold, wet fine

aggregate. In case of a drum plant, the cold, wet fine aggregate has to be introduced in

the middle of the drum similar to reclaimed asphalt pavement (RAP).

At least eight projects totalling over 38,000 tons of WMA have been produced with

LEA so far in the US with over 100,000 total tons produced worldwide. The first field

test section was constructed in the US in 20065.

3.6 Rediset WMX

Rediset WMX technology based on chemical additives was developed in 2007 by

Akzo Nobel Surfactants. The Rediset WMX additive is in solid form and consists of

cationic surface-active agents (surfactants) and rheology modifiers (organic

additives). The surface-active agents (a) improve the wetting of the aggregate surface

by the binder, (b) provide good aggregate-binder adhesion, and (c) enable coating of

damp aggregate which could be encountered at lower drying temperatures. The

rheology modifiers reduce the asphalt binder viscosity at mix temperatures and also

provide lubricating effect in the asphalt mix. Both of these effects allow mixing and

compaction temperatures to be reduced by up to 33º C5.

The Rediset WMX solid additives can be blended in liquid asphalt binder without

high-shear blending at the refinery, terminal, or HMA plant. The recommended

dosage is 1.5 to 2.5 percent by weight of asphalt binder. Liquid anti-stripping agent or

hydrated lime is not normally required.

Laboratory evaluation of Rediset WMX has been conducted by NCAT using one

asphalt binder grade and one aggregate type (granite). Compaction tests conducted

with a vibratory compactor indicated that a reduction in mix temperature of 40º C did

not reduce the density compared with control HMA specimen. The mix also had

adequate resistance to moisture induced damage when tested in Hamburg Wheel

Tracker and by AASHTO T 283 (TSR testing).

A plant scale (230 tons) demonstration project of Rediset WMX was completed in

California in November 2007.

Page 12: Warm Mix Asphalt Technologies- An Overview

12

3.7 REVIX

REVIX technology based on chemical additive was developed in 2007 by Mathy

Technology and Engineering Services and Paragon Technical Services in the US. The

technology is not based on the principle of foaming or viscosity reduction. Rather, a

combination of surfactants, waxes, processing aids, polymers, etc. is used to reduce

the internal friction between aggregate particles in the asphalt mixture. This

phenomenon takes place when the asphalt mix is subjected to high shear rates (during

mixing) and high shear stresses (during compaction). Mixing and compaction

temperatures for WMA produced with REVIX technology are generally between 33

to 44º C lower than those typically used in HMA mixtures5.

Plant modifications are not required when REVIX technology is used. Liquid or dry

REVIX additive can be added to the liquid asphalt binder at the refinery or at the hot

mix plant.

At least nine projects totalling over 5,600 tons of WMA have been constructed with

REVIX technology in the US5.

3.8 Double Barrel Green WMA System

The Double Barrel Green WMA System was developed in 2007 by Astec Industries

of US. It utilizes the Astec’s double barrel drum mix plant in conjunction with a

multi-nozzle foaming device to foam the hot asphalt binder with water. In a double

barrel asphalt plant, aggregate is dried and heated in the inner drum. The foamed

asphalt is introduced in the outer drum (space between the two drums). The resulting

WMA production temperature ranges from 120 to 135º C with compaction

temperature as low as 104º C. A number of demonstration projects using this WMA

technology have been constructed in 2007 in the US5.

3.9 Economic Aspects of WMA

Cost reductions primarily result because less fuel is needed to dry and heat the

aggregate for producing WMA. Cost increases are likely from the additional cost of

additives (if they are used) and technology licensing costs, if any. Based on these

factors, the cost of WMA is likely to be similar to or slightly higher than that of

normal hot mix asphalt.

3.10 Additional Research

Additional research of WMA technologies is needed in the following areas:

• Mix designs. Modifications to standard Superpave or Marshall Mix Design

Methods11

, if required, for designing WMA, need to be established. Selection

of binder grades for lower production temperatures needs to be examined.

• Long-term performance. Durability of asphalt pavements constructed with

WMA needs to be investigated in terms of binder effects (binder is either

foamed or chemically modified) and increased potential for moisture damage.

• Cost benefits. Reduction in fuel consumption and emissions need to be

quantified to ascertain cost benefits.

Page 13: Warm Mix Asphalt Technologies- An Overview

13

• Control of mixing process. Since WMA has a different mixing process than

the conventional HMA, new guidelines need to be developed for proper

quality control/quality assurance (QC/QA) of the produced mix.

• Workability at the paving site. Although the WMA may appear workable

and easily compactable when produced, it should remain workable at the

paving site as well. This needs to be investigated on field demonstration

projects.

• Quick turnover to traffic. More field demonstration projects are needed to

verify that the WMA pavements can be opened to traffic as soon as possible

after construction similar to or earlier than the conventional HMA pavements.

4. FIELD PERFORMANCE OF WMA

Early test sections of WMA have been in service in Germany and Norway for over 10

years. Seven test sections of WMA were constructed in Germany between 1998 and

2001. Six of the seven test projects were stone matrix asphalt (SMA) and one was

dense graded mix. All projects have the same or better performance than the HMA

control sections12

.

The oldest test sections were constructed in Norway in 1999 with WAM-Foam

technology. These test sections are also performing similar to previous dense-graded

HMA overlays12

.

Although numerous WMA projects using different technologies have been

constructed in the US since 2004, field performance data is limited because the

projects are not old enough. However, no major performance problems have been

reported up to this date on most research projects in the US.

WMA using Evotherm has been evaluated in three test sections on the NCAT Test

Track4. Two test sections in which WMA was used in the surface course have shown

excellent field performance in terms of rutting after the application of over 515,000

ESALs in a 43-day period.

Field performance of several WMA test sections consisting of Aspha-min, Sasobit,

and Evotherm in Missouri and Wisconsin has been evaluated2. Very good rut

resistance has been reported after two years in service.

A major 3-year research project has been undertaken to evaluate the field performance

of WMA technologies across the US. This National Cooperative Highway Research

Program (NCHRP) Project 09-47, “Engineering Properties, Emissions, and Field

Performance of Warm Asphalt Mix Technologies” is slated to be completed in March

2011.

5. IMPLEMENTATION OF WMA IN INDIA

Large scale implementation of technology such as WMA directly from Europe is not

easy according to the author’s experience13

when stone matrix asphalt (SMA)

Page 14: Warm Mix Asphalt Technologies- An Overview

14

technology was imported in the US from Europe in the early 1990s. In European

countries, technologies are usually proprietary; recipe type mixes are used with

limited experience with the locally available aggregates only; generic specifications or

mix design methods are not easily available; long range performance history is not

documented well; and language/translation is a handicap. When SMA was imported

in the US from Europe demonstration projects were constructed based on Europe’s

recipe specifications. However, extensive research was undertaken simultaneously by

a number of agencies to develop SMA Mix design procedures and specification which

can be used in a large country like the US with extreme climate ranging from hot and

dry to cold (including freezing) and wet. Early construction of demonstration projects

also provided field performance data. Once such documents13

are developed in the US

using a wide variety of binders and aggregates, it becomes easy for most countries in

the world to adapt the technology to their specific environment and needs. A major

NCHRP Project 09-43 is under way in the US which will develop mix design

practices for WMA technologies.

Since quite much practical and research work has been done and documented on

WMA in the US, some demonstration WMA projects can be constructed now in

India. The Central Road Research Institute (CRRI) and the Border Roads

Organization (BRO) should take a lead in trying different WMA technologies similar

to what is being done in the US. It is quite common and desirable for the concerned

technology provider to furnish both laboratory and field technical expertise to make

the demonstration project a success. Such demonstration projects should be initiated

as soon as possible so that India gets ready for eventual widespread use of WMA to

combat global warming by reducing greenhouse gas. India can also earn carbon

credits with the use of WMA as mentioned earlier.

It is very difficult to state at this time which WMA technologies are most suitable for

India. The situation is very dynamic as more and more new technologies are being

developed at the time of this writing. If the field performance is equal, the

technologies which are economically competitive are likely to be acceptable

ultimately. That is why, gathering field performance data from demonstration projects

using different WMA technologies is very important. In India it is important to

evaluate the potential stripping problem associated with the WMA produced at lower

temperatures.

Many states in the US are maintaining list of approved WMA technologies based on

their satisfactory field performance. On major WMA projects, these approved

technologies are encouraged to compete by using a generic specification which

requires specific end test results only. It appears this approach will be suited for India

after gaining experience from field demonstration projects. Right now, there is no

AASHTO or ASTM standard for WMA technologies which are still evolving. The

WMA Expert Task Group in the US is formulating a generic specification for WMA

which can be considered for use in India after modifications to suit Indian conditions.

6. SUMMARY, CONCLUSIONS AND RECOMMENDATIONS

Warm mix asphalt (WMA) is a fast emerging new technology, which allows the

mixing, lay down, and compaction of asphalt mixtures at significantly lower

temperatures compared to hot mix asphalt (HMA). WMA offers many significant

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15

advantages such as (a) energy savings in producing asphalt mix, (b) decreased

emissions from asphalt plants, (c) potential of decreased asphalt binder aging during

production, (d) extended paving season especially in colder winter months and/or in

places located on high altitudes, and (e) compaction aid for stiffer mixes.

A number of WMA technologies have been developed in recent years in Europe and

the US. These technologies can be classified broadly as follows.

• Those that use water which causes the hot asphalt binder to foam (examples:

zeolite, WAM Foam, Low Energy Asphalt, and Double Barrel Green)

• Those that use some type of organic additive or wax (example: Sasobit)

• Those that use chemical additives or surfactants (examples: Evotherm, Rediset

WMX and REVIX)

All preceding WMA technologies have been described in the paper together with the

findings of their laboratory evaluation by the National Center for Asphalt Technology.

It has been recommended to conduct field trials of WMA in India especially by the

Border Roads Organization in remote places far away from hot mix plant and/or

located on high altitude with colder climate. Introduction of WMA in lieu of HMA

should also be encouraged in response to the need for greenhouse gas reduction and

earn carbon credits for India under the Kyoto Protocol.

7. ACKNOWLEDGEMENTS

The use of photographs from Matthew Corrigan of the US Federal Highway

Administration and the Asphalt Technology News of the National Center for Asphalt

Technology is hereby acknowledged. The opinions expressed in this paper are solely

of the author.

The Indian Roads Congress and the author do not endorse any proprietary products or

technologies mentioned in this paper. These appear herein only because they are

considered essential to the objective of this paper.

8. REFERENCES

1. Kandhal, P. S. NCAT Evaluates Warm Mix Asphalt. National Center for Asphalt

Technology, Asphalt Technology News, Vol. 17, Number 2, Fall 2005.

2. Hurley, G. C., and B. D. Prowell. Field Performance of Warm Mix Asphalt. CD-

ROM, 87th Annual Meeting of the Transportation Research Board, Washington, DC,

January 2008.

3. Hurley, G. and Prowell, B. Evaluation of Potential Processes for Use in Warm

Asphalt Mixes. Journal of the Association of Asphalt Paving Technologists, Vol. 75,

2006.

Page 16: Warm Mix Asphalt Technologies- An Overview

16

4. Prowell, B., Hurley, G., and Crews, E. Field Performance of Warm Mix Asphalt at

the NCAT Test Track. Paper No. 07-2514, Presented at the 86th Annual Meeting

of the Transportation Research Board, Washington, DC, January 2007.

5. Prowell, B. D. and G. C. Hurley. Warm Mix Asphalt: Best Practices. Quality

Improvement Series 125, National Asphalt Pavement Association, 2007.

6. D’Angelo, J., E. Harm, J. Bartoszek, G. Baumgardner, M. Corrigan, J. Cowsert, T.

Harman, M. Jamshidi, W. Jones, D. Newcomb, B. Prowell, R. Sines, and B. Yeaton.

Warm-Mix Asphalt: European Practice. International Technology Scanning Program,

Federal Highway Administration, December 2007.

7. Prowell, B. D. Warm Mix Asphalt: The International Technology Scanning

Program Summary Report. Federal Highway Administration, Washington, DC, June

2007.

8. Kandhal, P. S. Evaluation of Warm Mix Asphalt. Paper prepared for Indian Border

Roads Organization Workshop conducted at IIT, Madras on 31 May 2007.

9. Mallick, R. B., P. S. Kandhal, and R. L. Bradbury. Using Warm Mix technology to

Incorporate High Percentage of Reclaimed Asphalt Pavement Material in Asphalt

Mixtures. Transportation Research Board, Transportation Research Record 2051,

2008.

10. Kandhal, P.S. Moisture Susceptibility of HMA Mixes: Identification of Problem

and Recommended Solutions. National Asphalt Pavement Association, Quality

Improvement Publication (QIP) No. 119, December 1992.

11. Roberts, F.L., P.S. Kandhal, E.R. Brown, D.Y. Lee, and T.W. Kennedy. Hot Mix

Asphalt Materials, Mixture Design and Construction. Textbook. NAPA Education

Foundation, Lanham, Maryland, Second Edition, 1996.

12. European Asphalt Pavement Association. The Use of Warm Mix Asphalt..

Position Paper, EAPA, Brussels, June 2009.

13. Kandhal, P.S. Designing and Constructing Stone Matrix Asphalt Mixtures—State-

of-the-Practice. National Asphalt Pavement Association Quality Improvement

Publication QIP-122 (Revised Edition), March 2002.

Comments on IRC Paper No. 561

“Warm Mix Asphalt Technologies: An Overview”

Prof. Prithvi Singh Kandhal

Shri Raj Kumar Verma

In the context of Hill States especially cold mix technology using bitumen emulsion

vis-à-vis warm mix technology will be more useful. Further, the cold mix technology

altogether avoids the emission of harmful emissions. Relative

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17

advantages/disadvantages, cold mix vis-à-vis warm mix technology may kindly be

highlighted. Why we are not using cold mix technology in large scale in India?

Dr. Sunil Bose

The review paper on warm mix asphalt by Prof. Kandhal explains the alternate

technologies available in USA using warm mix asphalt. As a matter of fact CRRI has

been working in the area since last five years. We have worked with SASO B II,

Evotherm and Sulphur Extended Asphalt Modifier. The laboratory evaluation using

warm asphalt has already been successfully concluded. The final trials using these

products have been completed by CRRI in DSIDC Bawana and on NHAI section near

Nasik. Our observations are very interesting. The Crumb Rubber Modified Bitumen

was brought out of the site at 1200C and compacted between 100 and 105

0C. A

density of 99% was achieved and the section is under observation since last one year.

Now, locally and CRRI we have developed a warm mix additive and it is about to be

patented. Two papers are pending in IRC so we hope we will be able to present the

results in the forthcoming publication.

Shri R.K. Jain

The plant is more than 30 km away from the city. How much temperature margin you

suggest so that the mix does not cool to non-rollable levels.

Shri Sunil George

In comparison with other mixes such as DBM/BC what are the reasons of WMA in

arresting moisture damage. Please explain the moisture damage in detail and the

advantages of WMA. Congratulations for introducing new methods for asphalt mixes

which reduces global warming.

Smt. M. Pennamma

What about the use in water logged areas and in rainy seasons.

Shri A. Shanmugha Velayudam

a. Is this new technology now exists in India?

b. IRC has brought out any code.

c. Please explain moisture damage

d. How many kilometers this mix can be taken to site with the same temperature.

Shri Vanlal Duhsaka

How well it works with marginal aggregates – WMA

Dr. B.R. Tyagi

1. Can modified bitumen be used for warm mix technology?

2. Effect of SASOBIT on the performance of modified bitumen.

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18

Col. A.K. Tiwari, VSM

How will the temperature regime change for WMA done with polymer modified

bitumen? What are the working temperatures in such case?

Shri Vishal R. Thombare

Can we replace PMB with WMA and what will be the cost difference?

Shri G.C. Tallur

Has any national agency like AASHTO certified WMA? How does it compare cost-

wise with other mixes?

Shri B.V. Hari Prasad

What about the bleeding problems, which normally occur if the air voids are less.

Shri C.S. Bhagawan Raju

What about the cost benefit analysis as compared to WMA pavements in terms of

construction and moisture?

Shri S. Bhowmik

a. What is the cost variation between conventional system and in alternative

WMA.

b. Durability comparison between conventional and WMA.

c. MTC methodology.

Shri Rajib Chattaraj

1. There are broadly three aspects on which WMA may be judged —

i) environmental ii) economic iii) long-term durability/serviceability. Can

different WMA technologies be rated in the perspective of the above three

aspects?

2. It is mentioned in the paper, in USA, the guidelines of WMA is under

formulation. But in Europe, WMA was applied earlier than USA, is there any

design guideline for WMA available in Europe?

3. In the WMA technology where water is used (like in WAM foam, LEA,

Zeolite etc.), is there any chance of moisture damage?

4. In the WMA technology where wax is used, will there be any apprehension for

lesser durability?

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19

Dr. M.C. Jain

Can you provide comparative performance of reduction in temperature by using

Aspha min, SASOBIT and Evotherm

Dr. Sukumar Saha

By compacting the bituminous layer densely, is it possible that the failures in the

pavement due to drainage can be controlled in the layer beneath the bituminous layer?

Replies by the author

Shri Raj kumar Sharma has posed a very good question as to why cold mix asphalt

technology using bitumen emulsion cannot be used in lieu of warm mix asphalt

(WMA). It must be realized that WMA is more or less equivalent to hot mix asphalt

(HMA) except that it can be produced at significantly lower temperatures although

both use the same type of hot liquid asphalt binders. Therefore, the question becomes

why we are not using cold mix technology with bitumen emulsions in place of hot

mix. Cold mix has the following major advantages: (a) there is considerable savings

in fuel costs because these mixes are produced and placed at ambient temperatures,

and (b) decreased emissions. However, cold mixes are not very commonly used

throughout India including India compared to HMA. This is because of the following

disadvantages:

• Unlike HMA, cold mixes need some curing before opening to traffic. Open

graded cold mixes use medium setting emulsion which takes some time to

cure or set. Dense graded cold mixes which are more desirable in pavements

have to use slow setting emulsion, which take very long time to cure.

• Structural strength of cold mixes is usually lower than that of HMA and,

therefore, cold mixes have to be placed in thicker courses compared to HMA

to get the equivalent pavement structural strength. Therefore, cold mixes may

not be cost effective.

• Mix design technology for cold mixes is not well established compared to that

for HMA.

• It has not been well established whether cold mixes are as durable as hot

mixes.

In response to Dr. Sunil Bose’s comments, the author is glad CRRI is involved in

evaluating WMA technologies. If CRRI has developed a WMA additive which is

about to be patented, it is hoped CRRI will bring the patent in public domain so the

entire country can benefit from it.

Responding to Shri R.K. Jain, it is not easy to give a definitive temperature margin

even for HMA because it depends on many factors such as mix temperature, ambient

temperature, insulation of truck, and time taken to traverse 30 km distance. However,

the temperature margin would be definitely lower in case of WMA because the major

factor affecting cooling is the difference between mix temperature and ambient

temperature.

Shri Sunil George has asked about moisture damage in WMA. It has been mentioned

in the paper that some WMA technologies do increase the potential for moisture

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damage because the mix is produced at significantly lower temperatures which may

result in incomplete drying of aggregate prior to mixing with bitumen. That is why:

most WMA technologies require some kind of antistripping agent in the bitumen or

the mix to minimize moisture damage.

In response to Smt. M. Pennamma, WMA can be used anywhere where HMA is

used including water logged areas and in rainy season. However, similar to HMA it

should be ensured by testing that the WMA is resistant to moisture damage.

Antistripping agent may have to be used, if needed.

In response to Shri Shanmugha Velayudam, some WMA technologies are already

available in India and have been evaluated by CRRI. More field experiments are

expected in the near future. The author has submitted a generic specification for

WMA to the Flexible Pavement Committee of the Indian Roads Congress in

September 2010. It is hoped it would be approved and adopted as soon as possible to

facilitate more field trials and wide usage.

Concerning moisture damage, when a pavement is exposed to moisture/moisture

vapour/water there is a potential for stripping (loss of adhesion between bitumen film

and aggregate surface) which in turn can cause disintegration of the bituminous

pavement. This is termed as moisture damage.

Concerning the distance the WMA can be hauled, Shri Velayudam is referred to

author’s response to Shri R.K. Jain.

In response to Shri Duhsaka, the performance of WMA with marginal aggregate

would be similar to that of HMA with marginal aggregate.

In response to Shri Tyagi, it has been mentioned in the paper that WMA technology

including Sasobit can also be used for mixes made with modified bitumen.

In response to Col. Tiwari WMA containing PMB would also have lower mixing and

compaction temperatures compared to HMA containing PMB. The working

temperature is different for different WMA technologies. The WMA technology

provider should be contacted for this information.

In response to Shri Thombare it should be noted that WMA is a bituminous mix

(similar to HMA but with significantly lower mixing and laying temperatures)

whereas PMB is polymer modified bitumen which is just a binder constituent of both

WMA and HMA. So there is no question of replacing PMB with WMA.

In response to Shri Tallur, AASHTO does not certify any specific WMA technology.

Many states in the US conduct field trials using different WMA technologies and

based on satisfactory field performance a certified list of WMA technologies is

maintained. Many states have a generic specification for WMA (like the one author

has submitted to IRC for use in India) which is used for bidding by the certified

WMA technology providers. Now, the National Center for Asphalt Technology

(NCAT) where the author is Associate Director Emeritus, has started a national

certification program based on laboratory and field evaluation (on NCAT test track)

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of different WMA technologies. Many states are willing to accept NCAT’s

certification.

In response to Shri Hari Prasad, WMA is no different than HMA concerning

bleeding problem if air voids are less.

Concerning the cost benefit analysis of WMA pavement as asked by Shri Raju, it is

mentioned in the paper in Section 3.9 Economic Aspects of WMA. The cost of WMA

is likely to be similar to or slightly higher than that of normal HMA.

In response to Shri Bhowmik, he is referred to the preceding reply given to Shri Raju

concerning cost variation. Section 4 of the paper on field performance of WMA

provides details on durability of WMA in Europe and the US. So far most WMA test

sections are performing similar to HMA. As mentioned in the paper a major 3-year

research project has been undertaken to evaluate the field performance of WMA

technologies across the US. This National Cooperative Highway Research Program

(NCHRP) Project 09-47, “Engineering Properties, Emissions, and Field Performance

of Warm Asphalt Mix Technologies” was slated to be completed in March 2011.

Response to Shri Chattaraj’s queries is as follows:

1. WMA technologies are relatively new and some are still evolving. No

definitive, comparative information is available at the present time

about different WMA technologies in terms of three aspects:

environmental; economical; and long-term durability.

2. Most WMA projects in Europe are based on recipe which varies from

country to country and general guidelines are not readily available.

3. Concerning potential for moisture damage, please refer to the reply

given earlier to Shri Sunil George.

4. As mentioned in Section 3.2 of the paper, Sasobit is a paraffin-wax

compound derived from coal gasification using the Fischer-Tropsch

(FT) process. The smaller crystalline structure of the FT wax is

believed to reduce brittleness at low temperatures as compared to

bitumen paraffin waxes.

Dr. M.C. Jain is advised to obtain the latest information on reduction in temperatures

from the specific WMA technology providers.

It appears the query from Dr. Saha does not relate to WMA. However, the response is

as follows. Compacting the bituminous layers densely ensures that surface water does

not enter the underlying courses from top. However, unless adequate subsurface

drainage is also provided, the underlying courses can fail from the moisture

underneath.

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