1. Report No. 

FHWA/TX-07/0-5597-1 

 2. Government Accession No. 3. Recipient's Catalog No. 

 5. Report Date 

April 2007 

Published October 2007 

4. Title and Subtitle 

A SYNTHESIS OF WARM-MIX ASPHALT 

 6. Performing Organization Code 

 7. Author(s) 

Joe W. Button, Cindy Estakhri, and Andrew Wimsatt 

 8. Performing Organization Report No. 

Report 0-5597-1 

 9. Performing Organization Name and Address 10. Work Unit No. (TRAIS) 

Texas Transportation Institute 

The Texas A&M University System 

College Station, Texas 77843-3135 

11. Contract or Grant No. 

Project 0-5597 

13. Type of Report and Period Covered 

Technical Report: 

September 2006-February 2007 

12. Sponsoring Agency Name and Address 

Texas Department of Transportation 

Research and Technology Implementation Office 

P. O. Box 5080 

Austin, Texas 78763-5080 

14. Sponsoring Agency Code 

15. Supplementary Notes 

Project performed in cooperation with the Texas Department of Transportation and the Federal Highway 

Administration. 

Project Title: Evaluation of Warm-Mix Asphalt New Technology 

URL: http://tti.tamu.edu/documents/0-5597-1.pdf

16. Abstract

This synthesis documents the results of a comprehensive review of worldwide information dealing with the 

following issues as related to warm-mix asphalt (WMA) technology: 

• current state of the art/practice of WMA, 

• benefits and costs of WMA technology 

• plant modifications to accommodate certain WMA processes, 

• mixture design and analysis, 

• pavement structural design, 

• durability and performance, 

• performance-related testing, 

• quality control, 

• specifications, and 

• construction guidelines. 

A summary of findings and recommendations is provided. Also included in this synthesis is a complete 

documentation of the first warm-mix asphalt field trial conducted by the Texas Department of 

Transportation. 

17. Key Words 

Highway Construction, Warm-Mix Asphalt, Hot Mix 

Asphalt. 

18. Distribution Statement 

No restrictions. This document is available to the public 

through NTIS: 

National Technical Information Service 

Springfield, Virginia 22161 

http://www.ntis.gov

19. Security Classif. (of this report) 

Unclassified 

20. Security Classif. (of this page) 

Unclassified 

21. No. of Pages 

 94 

22. Price 

 Form DOT F1700.7 (8-72) Reproduction of completed page authorized 

A SYNTHESIS OF WARM-MIX ASPHALT

by 

Joe W. Button, P.E. 

Senior Research Fellow 

Texas Transportation Institute 

Cindy Estakhri, P.E. 

 Research Engineer 

Texas Transportation Institute 

and 

Andrew Wimsatt, P.E. 

Research Engineer 

Texas Transportation Institute 

Report 0-5597-1 

Project 0-5597 

Project Title: Evaluation of Warm-Mix Asphalt New Technology 

Performed in Cooperation with the 

Texas Department of Transportation 

and the 

Federal Highway Administration 

April 2007 

Published October 2007

TEXAS TRANSPORTATION INSTITUTE 

The Texas A&M University System 

College Station, Texas 77843-3135

v

DISCLAIMER

 The contents of this report reflect the views of the authors, who are responsible for 

the facts and the accuracy of the data presented herein. The contents do not necessarily 

reflect the official view or policies of the Texas Department of Transportation (TxDOT) or 

the Federal Highway Administration (FHWA). This report does not constitute a standard, 

specification, or regulation. The researcher in charge of the project was Joe W. Button, 

P.E. (Texas, # 40874) 

 The United States Government and the State of Texas do not endorse products or 

manufacturers. Trade or manufacturers’ names appear herein solely because they are 

considered essential to the objectives of this report. 
vi

ACKNOWLEDGMENTS

 Special appreciation is given to the TxDOT project director, Dr. Magdy Mikhail, 

P.E., for providing technical guidance and assistance. Acknowledgement is given to the 

project monitoring committee for this research project: Mr. David Kopp, Mr. Darwin 

Lankford, Mr. Dale Rand, Mr. Harry Bush, and Dr. German Claros. Thanks are also given 

to several TxDOT personnel in the San Antonio District and Materials and Construction 

Division, Dean Word Co., and Vulcan Materials Co. who provided information, laboratory 

testing, and field assistance in documenting the San Antonio warm mix asphalt field trial. 

This portion of the research project was completed in corporation with TxDOT and FHWA. 

.
vii

TABLE OF CONTENTS

Page 

List of Figures......................................................................................................................... ix 

List of Tables ........................................................................................................................... x 

Introduction.............................................................................................................................. 1 

 Background..................................................................................................................... 1 

 A Brief History Leading to Current WMA Technologies .............................................. 3 

Examples of Specific WMA Technologies.............................................................................. 5 

 Aspha-Min ...................................................................................................................... 6 

 Sasobit............................................................................................................................. 7 

 Evotherm......................................................................................................................... 9 

 WAM-Foam.................................................................................................................. 10 

 Low-Energy Asphalt..................................................................................................... 11 

 Asphaltan B................................................................................................................... 14 

 General Comments on Waxes....................................................................................... 15 

Mixture Design of Warm-Mix Asphalt.................................................................................. 16 

 Binder Grade Selection................................................................................................. 16 

 Selection of Optimum Binder Content ......................................................................... 16 

 Aggregate Gradation..................................................................................................... 17 

 Specimen Compaction .................................................................................................. 17 

 Specimen Cure Time before Testing ............................................................................ 18 

 Mixture Evaluation ....................................................................................................... 20 

Material Characterization for Structural Design with WMA ................................................ 20 

Construction with WMA........................................................................................................ 21

WMA Properties and Performance........................................................................................ 21 

 NCAT Laboratory Results for WMA ........................................................................... 21 

 Aspha-Min .................................................................................................................... 22 

 Evotherm....................................................................................................................... 24 

 Sasobit........................................................................................................................... 28 

 Low-Energy Asphalt..................................................................................................... 29 
viii

TABLE OF CONTENTS (continued)

Page 

WAM-Foam.................................................................................................................. 30 

General Discussion of WMA Properties....................................................................... 32 

Other Warm-Mix Asphalt Applications................................................................................. 32 

 Warm Open-Graded Mixes for Patching Permeable Friction Courses......................... 32 

 Warm In-Plant Recycling ............................................................................................. 33 

Current National Activities Related to Warm-Mix Asphalt .................................................. 34 

 National Asphalt Pavement Association (NAPA) Research ........................................ 34 

 Development of a Warm-Mix Asphalt Technical Working Group (TWG) ................. 40 

 National Cooperative Highway Research Program (NCHRP) Project 09-43 .............. 40 

Advantages and Disadvantages of Warm-Mix Asphalt......................................................... 41 

 Benefits of WMA as Compared to HMA ..................................................................... 41 

 Environmental Benefits of WMA................................................................................. 43 

 Benefits of WMA as Compared to Cold Mix ............................................................... 44 

 Hindrances to WMA Implementation........................................................................... 44 

Comparative Costs of Producing WMA................................................................................ 46 

Summary of Findings and Recommendations ....................................................................... 48 

 Findings......................................................................................................................... 48 

 Recommendations......................................................................................................... 50 

References.............................................................................................................................. 53 

Websites Listed ..................................................................................................................... 59 

Appendix A - TxDOT Evotherm Warm-Mix Trial—San Antonio District—2006 .............. 61 
ix

LIST OF FIGURES

Figure Page 

1 Functional Diagram Low-Energy Asphalt Technique............................................... 13 

2 Batch-Plant Equipped for Low-Energy Asphalt Processing...................................... 14 

3 Rut Depth Measurements on RV 120 in Norway ...................................................... 31 

4 Rut Depth Measurements on FV 82 in Norway......................................................... 31 

5 IRI Measurements on FV 82 in Norway.................................................................... 32 

6 Section View of Evotherm Layers at NCAT Test Track........................................... 39 
x

LIST OF TABLES

Table Page 

1 Volumetric Mix Design Data for Granite Aggregate Using the 

 Superpave Gyratory Compactor ................................................................................ 19 

2 Volumetric Mix Design Data for Limestone Aggregate Using the 

 Superpave Gyratory Compactor ................................................................................ 19 

3 Volumetric Properties of WMA and HMA Using Marshall Hammer and 

 Superpave Gyratory Compactor ................................................................................ 23 

4 Tensile Strength Results for Granite and Limestone ................................................. 35 

5 Results from Hamburg Wheel Tracking Test ............................................................ 36 

6 Cost Savings of WMA at Selected Sites.................................................................... 46 

7 Costs of Various WMA Technologies....................................................................... 47 
1 

A SYNTHESIS OF WARM-MIX ASPHALT 

“There is one thing stronger than all the armies in the world, 

and that is an idea whose time has come.” 

 - Victor Hugo - 

INTRODUCTION 

Background 

According to Dorchies et al. (2005), the United Nations conference on the environment 

and sustainable development that was held in Rio de Janeiro in 1992 marked the beginning of 

universal awareness of the risks of damage facing this planet. The destruction of natural 

resources and climate change are the main causes of damage and disruption of ecosystems. 

Industry, agriculture, and transport are blamed for being the main contributors. This awareness 

was formalized in 1997 by the Kyoto Protocol to the United Nations Framework Convention on 

Climate Change which featured, in particular, a commitment made by signatory states to bring 

greenhouse gas emission rates down to 1990 levels. This agreement came into force on February 

13, 2005. Warm-mix asphalt technology addresses this issue in a rather small but important way. 

Hot-mix asphalt (HMA) is typically produced in either batch or drum mix plants at a 

discharge temperature ranging from 280°F to 325°F. It has been necessary to use these elevated 

temperatures to dry the aggregates, coat them with the asphalt binder, achieve the desired 

workability, and provide sufficient time to compact the HMA mat. Researchers have been trying 

to reduce the mixing/compaction temperature of HMA since the 1970s (Zettler 2006) by utilizing 

the moisture in the aggregate, foaming the binder, and, of course, using emulsified asphalts. 

Reducing the HMA production temperature and the placement temperature could bring several 

economical, environmental, and even performance benefits. Certain members of the asphalt 

industry are studying technology for reducing production temperatures by 50°F or more within 

the next decade. One category of current promising products is termed warm-mix asphalt 

(WMA). Although WMA is in its infancy, the technology and its potential benefits are stirring 

significant interest in Europe (Els, 2004), North America, and other countries interested in 

economical, environmentally friendly paving materials. Without question, the need for an 
2 

environmentally friendly and economically attractive asphalt paving material is here, the product 

is here...the time has come. 

Reducing production (mixing) and paving (compaction) temperatures by using WMA in 

place of HMA will yield beneficial environmental effects: 

• decreased fuel or energy consumption (with consequential decreased cost); 

• reduced emissions and odors from plants; 

• reduced smoke and, thus, consternation from the public; and 

• improved working conditions at the paving site. 

These benefits will improve public perception of the asphalt paving process and vividly 

elucidate efforts by TxDOT and the industry to promote innovation and raise the standards. 

Technologies that allow lower HMA production temperatures may demonstrate positive 

impacts on pavement performance. Because the technologies improve the workability of the 

mix, they should reduce (or, at least, certainly not increase) compaction energy requirements 

and, thus, enhance in-place density (Hurley and Prowell, 2006). Enhanced compaction is, of 

course, a key parameter regarding performance. Further, the majority of aging of asphalt binder 

in HMA occurs in the plant when exposed to elevated temperatures. Lower mixing temperature 

will reduce oxidative hardening, which should reduce susceptibility to cracking by improving 

pavement flexibility and longevity. 

Today’s versions of WMA are the brainchild resulting from the 1997 German Bitumen 

Forum (Zettler, 2006). In fact, the Europeans are already using technologies to lower 

mixing/placement temperatures, and the results have been very promising (Barthel and Von 

Devivere, 2003). In the U.S., these technologies are being tried in an attempt to realize these 

potential benefits (Kuennen, 2004a). WMA technology has been successfully demonstrated for 

standard dense-graded mixtures as well as stone mastic asphalt (SMA) and offers potential for 

essentially any mix from interstates to parking lots (Zettler, 2006). 

Newcomb (2006) indicated that WMA is distinguished from other asphalt mixtures by 

the temperature regimes at which they are produced and the strength and durability of the final 

product. He holds that cold asphalt mixtures are manufactured at ambient temperatures (e.g., 

68°F to 120°F), while HMA is typically produced in the range of 285°F to 340°F. Warm mixes 

are those generally produced in the temperature range of 200°F to 275°F. HMA has higher 

stability and durability than cold-mix asphalt, which is why cold mix is used in the lower 
3 

pavement layers of low-volume roadways. The goal with warm mix is to obtain a level of 

strength and durability that is equivalent to HMA. 

There are far-reaching implications for WMA technology. Information available from 

manufacturers and materials suppliers indicates that, compared to standard HMA, reductions of 

about 30 percent in energy consumption (Asphalt Pavement Association of Oregon [APAO], 

2003; Stroup-Gardiner and Lange, 2002) and CO2 emissions are possible. According to 

Hampton (2005), 30 percent to 50 percent of an HMA producer’s overhead cost can be attributed 

to emissions control; thus by reducing CO2, significant cost reductions are possible. For WMA, 

the harmful fumes in the vicinity of workers are often below the detection limits, and there is a 

50 to 60 percent decrease in dust generation. 

The ability to improve the environmental friendliness of asphalt paving creates 

opportunities to expand production. For instance, by lowering the paving and compaction 

temperature, the ability to perform late/early-season paving is enhanced, thus extending the 

paving season. Because the mix starts at a lower temperature, it does not cool as rapidly, 

allowing longer haul distances and/or time periods to compact the mat. Other benefits include 

paving in ozone nonattainment areas and/or more plant operation during daylight hours in these 

regions. 

A Brief History Leading to Current WMA Technologies

Using lower temperatures to produce asphalt paving mixes is not a new concept. In 

1956, Prof. Ladis Csanyi, Iowa State University, realized the potential of foamed bitumen for use 

as a soil binder. Since then, foamed asphalt technology, which allows lower mixing 

temperatures, has been used successfully in many countries. The original process consisted of 

injecting steam into hot bitumen. In 1968, Mobil Oil Australia, which had acquired the patent 

rights for Csanyi’s invention, modified the original process by adding cold water rather than 

steam into the hot bitumen. The bitumen foaming process then became more practical 

(Kristjansdottir, 2006; Muthen, 1998). Conoco was later licensed to market foamed asphalt in 

the U.S. and further advanced the technology and evaluated the product as a base stabilizer both 

in the laboratory and in the field (Ruckel et al., 1980; Little et al., 1983). 

 In the early 1970s, Chevron developed mixture design and thickness design 

methodologies for paving mixtures (base, open-graded, and dense-graded) stabilized with 
4 

emulsified asphalt. In 1977, Chevron published their “Bitumuls Mix Manual” (Chevron, 1977) 

as a practical guideline, which contains much valuable information for specifying, designing, and 

producing emulsion-stabilized mixtures. Later, other similar guidelines followed (FHWA, 1979; 

Asphalt Emulsion Manufacturers Association [AEMA], 1981). Kuennen (2004b) reported that 

emulsified asphalt mixes are popular in rural settings where distances from HMA plants and 

lower traffic volumes may preclude HMA. Further, cold-mix plants have a lower initial cost 

than conventional HMA plants, are more easily transported, and may be situated anywhere 

without Environmental Protection Agency (EPA) permits due to their lack of emissions. 

Furthermore, they are amenable to mixes with high percentages of reclaimed asphalt pavement. 

In 1994, Maccarone et al. (1994) studied cold-mixed asphalt-based foamed bitumen and 

very high binder content emulsions and concluded that the use of cold mixes for use on roads 

was gaining acceptance worldwide due to energy efficiency and lower emissions. In fact, they 

stated that, “Cold technologies represent the future in road surfacing.” 

In 1995, Shell Bitumen filed a patent to cover a warm-mix asphalt technique that used a 

two-component technique (Harrison and Christodulaki, 2000). Koenders et al. (2000), of Shell 

Global Solutions, described an innovative WMA process that was tested in the laboratory and 

evaluated in large-scale field trials (in Norway, the United Kingdom, and the Netherlands) with 

particular reference to the production and placement of dense-graded wearing courses. Shell’s 

work resulted in the development of WAM-Foam®. 

In 1997, Sasobit® began to be marketed in Europe as an asphalt mixture compaction aid 

by Sasol Wax International AG (http://www.sasolwax.com/Applications.html). The technology 

later grew into the WMA process. 

Jenkins et al. (1999) introduced a new process involving a half-warm foamed bitumen 

treatment. They explored the concepts and possible benefits of heating a wide variety of 

aggregates to temperatures above ambient but below 212°F before the application of foamed 

bitumen. Preheating aggregates enhanced particle coating, mix cohesion, tensile strength, and 

compaction. This is particularly beneficial for mixes containing reclaimed asphalt pavement 

(RAP) or densely graded crushed aggregates. 

Probably due to their relatively higher prices of fossil fuels and asphalt, Europe 

(Koenders et al., 2000), South Africa (Jenkins et al., 1999; Jenkins et al., 2002), and Australian 
5 

Asphalt Pavement Association (AAPA, 2001) began early to examine the benefits and 

performance of WMA.

Cold/warm asphalt mixes occupy certain market areas in the paving industry, but to date, 

they have had no significant impact on HMA as the primary road surfacing material...but the 

potential for significant impact is now in view. Although WMAs have been used in the U.S. for 

less than 3 years, they are being evaluated at a rapidly increasing rate (Zettler, 2006). For 

example, WMA paving projects in North America have been or are being constructed in 

Alabama, Florida, Indiana, Kansas, Maryland, Missouri, Ohio, Vermont, North Carolina, New 

York, Tennessee, Texas (San Antonio), Wisconsin, Ontario, Alberta, and Washington, D.C. 

Additionally, several municipalities have conducted tests of WMA. Most, if not all of these 

tests, involve overlays on existing pavements. 

Most of the work on WMA has involved dense-graded mixtures; however, Koenders et 

al., 2000, stated that, in principle, WMA technology is equally applicable to other types of 

asphalt mixtures (e.g., open-graded, gap-graded, and stone mastic asphalts). He further stated 

that use can be made of conventional asphalt mixing plants as well as traditional paving 

equipment and techniques. 

EXAMPLES OF SPECIFIC WMA TECHNOLOGIES 

The three most widely used technologies in Europe for producing WMA (Aspha-Min®, 

WAM-Foam®, and Low-Energy Asphalt®) are all proprietary processes and are described 

below. A fourth proprietary product (Sasobit®) was developed in South Africa. A fifth nonproprietary product (Evotherm®) was developed by in the U.S. Another potential product, 

currently with little research, is Asphaltan B®. All six technologies allow the production of 

WMA by reducing the viscosity and/or expanding the volume of the asphalt binder at a given 

temperature. These technologies allow the aggregates to be fully coated at temperatures 

significantly lower than those traditionally required in HMA production. However, some of 

these technologies require significant equipment modifications at the asphalt mixing plant. 

Brief descriptions of these products are given below based on National Center for Asphalt 

Technology (NCAT), 2005; Hurley and Prowell, 2005a; Hurley and Prowell, 2005b; Corrigan, 

2006; and others indicated. 
6 

These WMA technologies are being continuously refined and improved (Els, 2004), and 

other WMA technologies are being developed and are appearing on the market. For example, 

WMA technology has recently been applied for patching open-graded asphalt pavements (Soto 

and Blanco, 2004a) and for recycling flexible pavements (Soto and Blanco, 2004b). The need 

for WMA to replace certain HMA projects has never been greater, and the concept is sensible. 

The authors expect increased interest and use of the products and processes worldwide and that, 

ultimately, WMA will become a standard product for routine paving in specific situations. 

The following subsections describe all known WMA products and/or processes. 

Aspha-Min 

Aspha-Min is supplied by Eurovia Services GmbH, Bottrop in Germany 

(http://www.aspha-min.com). It is a finely powdered synthetic zeolite (sodium aluminum 

silicate hydrate) that has been hydro-thermally crystallized. When Aspha-min® is added to the 

mix at the same time as the binder, water is released. This water release creates a foaming of the 

asphalt binder and, thereby, temporarily increases workability and enhances aggregate coating at 

lower temperatures. 

Aspha-Min is typically added at 0.3 percent by total weight of HMA mix. When it is 

heated above 185°F to 360°F, it gives up 21 percent water by mass, which microscopically 

foams the asphalt to aid coating of the aggregate. This foaming action of the liquid binder acts 

as a temporary asphalt volume extender and mixture lubricant, enabling the aggregate particles to 

be rapidly coated and the mix to be workable and compactable at temperatures significantly 

lower than those typically used for HMA. Barthel (2004) said it is important that the additive 

particles are losing their water in several steps and not all at once. He further stated that the mix 

can be compacted until the temperature drops below 212°F. 

In their promotional material, Eurovia indicates that Aspha-Min can yield a reduction in 

mixing temperature of more than 50°F, save 30 percent energy, and accommodate all commonly 

used asphalt and polymer-modified binders as well as the addition of RAP. Aspha-Min is 

available in a very fine white powdered form in 50-pound or 100-lb bags or in bulk for silos 

(Kuennen, 2004b). Currently, the process requires a specially built distributor to meter the 

Aspha-Min into the asphalt mixing plant. In a batch plant, it is introduced directly into the 
7 

pugmill; in a drum mix plant, it is pneumatically fed into the drum via the RAP collar (Barthel 

and Von Devivere, 2003). 

Corrigan (2006) indicated that zeolites are framework silicates that have large vacant 

spaces in their structures that allow space for large cations such as sodium, potassium, barium, 

and calcium and even relatively large molecules and cation groups such as water. In the more 

useful zeolites, the spaces are interconnected and form long, wide channels of varying sizes 

depending on the mineral. These channels allow the easy movement of the resident ions and 

molecules into and out of the zeolite structure. Zeolites are most used in water softeners. 

Zeolites are characterized by their ability to lose and absorb water without damage to their 

crystalline structures. They can have the water in their structures driven off by heat and other 

solutions pushed through the structure. They can then act as a delivery system for the new fluid. 

Sasobit 

Sasobit is a product of Sasol Wax (formerly Schumann Sasol) of South Africa. Sasobit 

is a Fischer-Tropsch (F-T) or synthetic wax that is created in the coal gasification process 

(http://www.sasolwax.com/www_sasobit_de.html). These organic waxes have longer chemical 

chain lengths and are different from petroleum or paraffin waxes (which are normally 

considered undesirable in asphalt) (Damm et al., 2002). The longer chains help keep the wax in 

solution, and it reduces binder viscosity at typical asphalt production and compaction 

temperatures. Sasobit has been used as a compaction aid and a temperature reducer. The

Sasobit process incorporates a low melting point organic additive that chemically changes the 

temperature-viscosity curve of the binder. Both of these additives melt at about 210°F and 

produce a reduction in the binder viscosity by providing liquids in the binder above their melting 

points. 

Blending 3 to 4 percent Sasobit by weight allows a reduction in production temperatures 

of 18°F to 54°F. The manufacturer anticipates that in-line blending of melted Sasobit with the 

asphalt binder stream at the plant will be finalized in the near future, thus eliminating the current 

use of the Sasobit distributor at the plant. Direct blending of solid Sasobit at the plant is not 

recommended because it will not give a homogeneous distribution of Sasobit in the asphalt. 

Further, Sasobit allows incorporation of Styrene Butadiene Styrene (SBS) modifier using a 

special cross-linking agent termed Sasoflex. Either Sasobit or Sasoflex can be blended into hot 
8 

binder at the blending plant without the need for high-shear blending. Sasobit can negatively 

impact low-temperature mix properties (Hurley and Prowell, 2005b). 

Corrigan (2006) explained that during coal gasification, the F-T process converts carbon 

monoxide and hydrogen into a mixture of hydrocarbons having molecular chain lengths of 1 to 

100 carbon atoms and greater. The process is important in the preparation of hydrogen and as a 

fuel in the making of steel and in other industrial processes. The synthesis gas is reacted 

exothermally in the presence of an iron or cobalt catalyst, and products such as methane, 

synthetic gasoline, waxes, and alcohols are made. 

Sasol emphasizes the difference between naturally occurring bituminous waxes and F-T 

waxes in terms of their structure and physical properties. The main difference is the much 

longer chain lengths and the fine crystalline structure of the F-T waxes. The predominant chain 

lengths of the hydrocarbons in Sasobit range from 40 to 115 carbon atoms; whereas, those in 

bituminous paraffin waxes range from about 25 to 50 carbon atoms (Butz et al., 2001), yielding 

lower melting points than F-T waxes. The longer carbon chains in the F-T wax yield a higher 

melting point. However, the smaller crystalline structure of the F-T wax, as compared to 

bitumen paraffin waxes, reduces the brittleness at low pavement service temperatures. Edwards 

et al. (2006) found that adding F-T paraffin decreased the physical hardening index for all 

bitumens tested. 

Sasol states that the melting point of Sasobit is approximately 210°F and that it is 

completely soluble in asphalt at temperatures above 248°F. It reduces the binder viscosity and, 

thus, reportedly enables mix production temperatures to be reduced by 18°F to 54°F and 

improves compactability. At temperatures below its melting point, Sasobit forms a lattice 

structure in the asphalt binder that is the basis for the reported stability of asphalts that contain 

Sasobit. At service temperatures, Sasobit-modified mixes exhibit increased resistance to rutting. 

Since 1997, more than 142 projects have been paved using Sasobit, which total more than 

2.7 million square yards of pavement (Hurley and Prowell, 2005b). Projects were constructed in 

Austria, Belgium, China, Czech Republic, Denmark, France, Germany, Hungary, Italy, Macau, 

Malaysia, The Netherlands, New Zealand, Norway, Russia, Slovenia, South Africa, Sweden, 

Switzerland, the United Kingdom, and the United States. A wide range of aggregate types and 

mix types were included (e.g., dense-graded mixes, stone mastic asphalt, and Gussaphalt). 

Sasobit addition rates ranged from 0.8 to 4 percent by mass of binder.
9 

Evotherm 

Evotherm was developed in the U.S. by MeadWestvaco Asphalt Innovations, 

Charleston, South Carolina (http://www.evotherm.com). Evotherm uses a non-proprietary 

technology that is based on a chemical package that includes cationic emulsification agents; 

additives to improve aggregate coating, mixture workability, and compaction as well as 

adhesion promoters (anti-stripping agents). MeadWestvaco states that they can deliver an 

emulsion with a unique chemistry customized for aggregate compatibility. Evotherm utilizes a 

high-residue emulsion (approximately 70 percent binder) that improves adhesion of the asphalt 

to the aggregate. The product enhances mixture workability, while lowering mixing 

temperatures to as low as 200°F. Some steam is normally liberated upon mixing. No plant 

modifications are required, the mix can be stored in silos, and Evotherm may be utilized with or 

without polymer modifier. 

According to Takamura (2005), thin water film between the aggregate and asphalt 

droplets improves workability of the mix even at temperatures below 195°F. The thin water 

film exerts up to 10 MPa of capillary pressure to promote quick coalescence of the asphalt 

droplets as it cures after compaction. He maintains that the cationic emulsifiers in the Evotherm 

emulsion adsorb onto the aggregate surface with their positively charged head groups and 

expose their hydrocarbon tails outward. This makes the aggregate surface oil-wet to promote 

strong asphalt adhesion for moisture resistance. 

Unlike traditional asphalt binders, Evotherm is stored at about 180°F. Temperatures of 

oil-jacketed lines should be reduced to about 200°F prior to pumping the Evotherm to prevent 

the emulsion from breaking in the lines (Hurley and Prowell, 2006b). 

During production, the Evotherm emulsion is simply used in place of the traditional 

asphalt binder. The emulsion is mixed with the aggregate in the HMA plant. Water in the 

emulsion is liberated in the form of steam when it is mixed with the hot aggregate. The resulting 

WMA has the appearance of HMA. MeadWestvaco reports that field testing has demonstrated a 

100°F reduction in production temperatures and that the decreased production temperatures can 

lead to plant energy savings of 55 percent, which results in a 45 percent reduction in CO2 and 

SO2 emissions, a 60 percent reduction in NOx, a 41 percent reduction in total organic material, 

and benzene soluble fractions below detectable limits. 
10 

At this writing, Evotherm has been successfully used on two projects in San Antonio, 

Texas, and several other projects in the U.S. 

WAM-Foam 

WAM-Foam is a product of a joint venture between Shell International Petroleum 

Company Ltd., London, United Kingdom, and Kolo-Veidekke, Olso, Norway 

(http://www.wamfoam.com) (Larsen et al., 2004). It is a two-component binder system that 

introduces a soft binder and a hard-foamed binder at different times in the mixing cycle during 

production. The extremely soft binder component is mixed with the aggregate in the first stage 

(210°F to 250°F) to fully coat the aggregate. In the second stage of production, an extremely 

hard binder component is then foamed into the pre-coated aggregate mixture. The harder binder 

is infused with a small quantity of cold water to induce foaming and enhance coating 

capabilities. This combination of soft binder and foaming of the hard binder acts to lower the 

mass viscosity of the mixture to provide the necessary workability, allowing the mixture to be 

placed and compacted at 175°F to 195°F. 

In a batch plant, 1 to 5 percent water is injected into the hot bitumen pipe using a special 

nozzle system just before the bitumen enters the pugmill to induce foaming. To minimize 

clogging, an air gun is then used to blow the foaming chamber and pipes clean after each foam 

injection (Kristjansdottir, 2006; Koenders et al., 2002). In their field trials in Norway, the dryer 

was operated at about 250°F to 265°F with no problems regarding burner instabilities or the dust 

collection system. At the lower temperatures, less dust is generated so there is lower loading of 

the filter system. 

Larsen et al. (2004) reports that, in a drum mixing plant, the foam can be produced 

continuously, which offers some operational advantages. That is, insulation of pipes and 

periodic cleaning is not necessary to the same degree as for the discontinuous process of the 

pugmill. 

Shell (Jenkins et al., 1999; Jenkins et al., 2002; Koenders et al., 2000) indicates that a 

successful WAM-Foam product depends on careful selection of the soft and hard components. 

They sometimes recommend addition of an adhesion improver in the first mixing stage. A vital 

element in the first mixing stage is to prevent water from reaching the binder/aggregate interface 

and entering the aggregate; finally, the water must be expelled from the asphalt mix to ensure a 
11 

high-quality end product. Shell reports that the decreased production temperatures of the 

WAM-Foam process can lead to plant fuel savings of 30 percent, which, consequently, yields a 

30 percent reduction in CO2 emissions. Independent testing has confirmed Shell’s earlier 

findings (Larsen et al., 2004). 

When RAP was used at levels from 15 to 25 percent in production of a wearing course in 

a drum mix plant, the amount of water coming from the RAP was about three times higher than 

the amount coming from the foam. Therefore, the temperature of the dryer was raised a little to 

accommodate the extra drying necessary. However, this would have also been necessary for 

HMA production (Larsen et al., 2004). 

WAM-Foam requires plant modification to accommodate foaming, which is estimated to 

cost $50,000 to $70,000. 

Low-Energy Asphalt 

Low-Energy Asphalt (LEA) was developed by Fairco of Zozay, France (Romier et al., 

2004; Romier et al., 2006). It is known in France as EBE (enrobage a basse energie). Because 

the LEA process differs substantially from typical HMA and WMA processes, Figures 1 and 2 

are included to illustrate the process. The process mixes hot asphalt cement (280°F to 350°F) 

with hot coarse aggregates (about 290°F) and then incorporates wet fine aggregates at ambient 

temperature. The moisture in the fine aggregates in combination with heat and certain additives 

causes the asphalt binder to foam, thus increasing its volume/surface area many-fold, such that it 

can rapidly coat the aggregates. It is well known that foamed asphalt favors coating the fine 

aggregates, which is desirable at this point since the coarse aggregates are already coated. The 

final temperature of the mixture should ideally be below 212°F (140°F to 180°F). This 

temperature (212°F) is critical, below which significant energy savings can be realized and above 

which they cannot (Romier et al., 2006). 

Romier et al. (2006) indicated that the LEA process is reaching the industrial stage. They 

have developed a LEA production kit that attaches to a batch plant. The kit includes a specific 

hopper that allows the metering of the amount of cold sand to be introduced into the mixer via a 

storage bin located over it. Also included is a device for adding water to the sand, if necessary. 

An asphalt binder metering device incorporates a surfactant addition system. The mixing phase 

of the process has been modified to implement the LEA sequential mixing of hot aggregate 
12 

followed by wet sand and bitumen and finally cold filler. This kit also provides for introduction 

of RAP directly into the mixer. 

The originality of the process lies in the best use of changes in the condition of the 

bitumen, fluid when it is hot and the ability to transform into foam or emulsion when in contact 

with water (Romier et al., 2006). It involves moderate heating of only the coarse aggregates, the 

rest of the aggregate skeleton being used at ambient temperature and wet. They reported that this 

technique yields significant savings in mixing energy while reducing the gas emissions. 

Interestingly, they also reported reduced soiling of production equipment (probably due to the 

presence of moisture), thus decreasing cleaning requirements and corresponding use of solvents. 

According to the developer, these mixtures offer performance equivalent to that of HMA. 

Minor plant modifications are required to accommodate this product. This process originated in 

France (Romier et al., 2004). As of January 2006, 6000 metric tons had been placed in France. 

It was tested in The Netherlands using a mix containing RAP in December 2005. This process 

has not been used in the U.S. After additional laboratory and field evaluations, Gaudefroy et al. 

(2007) still hold that this is a viable WMA process. 
13 

Figure 1. Functional Diagram Low-Energy Asphalt Technique 

(after Romier et al., 2006). 
14 

Figure 2. Batch-Plant Equipped for Low-Energy Asphalt Processing 

(after Romier et al., 2006). 

Asphaltan B 

Asphaltan B (Corrigan, 2006; Kristjansdottir, 2006) is a product of Romonta GmbH, 

Amsdorf, Germany. Asphaltan B is a mixture of substances based on montan wax constituents 

and higher molecular weight hydrocarbons. According to Corrigan (2006), it was created 

specifically for hot-rolled asphalt (a fine-grained HMA for pavement surfacing). 

Crude montan wax is found in Germany, Eastern Europe, and areas of the U.S. in certain 

types of lignite coal deposits that have formed over geologic time by the transformation of 

fossilized vegetation. Apparently, wax, which once protected the plant leaves from extremes of 

climate, did not decompose, but instead enriched the coal. Due to its high stability and 

insolubility in water, the wax survived over long time periods. After mining, the montan wax is 

extracted from the coal by means of a toluene solvent that is distilled from the wax solution and 

removed with super-heated steam. Romonta GmbH has a global market share of 80 percent in 

the crude mined wax products sector. 

Romonta recommends adding Asphaltan B to asphalt at 2 to 4 percent by weight. It can 

be added at the asphalt mixing plant or by the binder producer. It can be added to polymermodified binders. The melting point of Asphaltan B is approximately 210°F. Similar to F-T 
15 

waxes, it acts to improve asphalt flow at reduced temperatures. Romonta does not specify how 

much the production temperature can be lowered. Like F-T waxes, Romonta reports increased 

compactability, resistance to rutting, and moisture resistance of asphalt mixtures 

(http://www.romonta.de/ie4/english/romonta/i_wachse.htm). Work by Edwards et al. (2006) 

supports findings reported by Romonta. Asphaltan B is available in granular form in 25 kg 

bags. Little research has been performed with this product in asphalt. 

General Comments on Waxes

According to Edwards et al. (2006), natural wax in straight run bitumen today is low in 

content and of a type that should not be particularly harmful for binder or asphalt concrete 

properties. However, wax can unintentionally be produced through refining procedures like 

visbreaking or hydrocracking and, thus, affect asphalt binder properties. The most feared 

influence of wax in bitumen is the sudden decrease in viscosity due to the melting of crystallized 

wax, particularly, if this should occur within a temperature range affecting the resistance to 

permanent deformation of the asphalt pavement. 

She further reports that commercial wax, such as F-T paraffin and montan wax, is 

sometimes added to bitumen or asphalt concrete mixtures in order to obtain certain positive 

effects. These products, typically called “flow improvers,” are mainly used for reducing the 

asphalt mixing temperature in order to reduce energy consumption and emissions and to improve 

workability. However, other effects of different kinds may result from such wax modification. 

 The magnitude and type of the effect on bitumen rheology depend on the properties of 

bitumen as well as type and amount of additive (Edwards and Redelius, 2002). That is, bitumen 

composition is of significant importance. Additionally, incorporation of commercial waxes (i.e., 

F-T paraffin, montan wax, and polyethylene wax) showed no or marginally positive influence on 

aging properties for the asphalts and test conditions used by Edwards (2006). Edwards and 

Redelius (2002) demonstrated that the effects due to wax content shown in dynamic mechanical 

analyzer temperature sweeps (for HMA mixes) are related to the corresponding effects shown in 

differential scanning calorimetry thermograms (for asphalt binders). 

In the case of blown bitumens and/or wax-modified bitumens in road construction, which 

are frequently used in the U.S. and Canada in order to meet standard binder specifications, the 

effects on asphalt concrete properties may vary considerably (Hesp, 2004). In practice, 
16 

commercial wax, like F-T paraffin or montan wax, is added to bitumen in order to achieve 

certain preferred properties, such as reduced asphalt mixing temperature and higher stiffness of 

the pavement layer. 

MIXTURE DESIGN OF WARM-MIX ASPHALT

An important issue regarding WMA is mixture design. One of the main issues is to 

determine if mixture design for WMA can be performed exactly like HMA. Based on findings 

published in current literature and discussions with WMA researchers, standard mix design 

procedures for HMA must be modified to accommodate WMA. The next section discusses 

specific elements in the mixture design process. 

Binder Grade Selection 

Romier et al. (2006) stated that LEA mixes use the same asphalt grades in the same 

proportions as HMA; this is generally true for other WMA products. However, there is limited 

evidence that, with certain WMA processes, it may be possible or even advisable to use one 

grade harder asphalt than that normally used with HMA (Newcomb, 2006). For example, Hurley 

and Prowell (2005a, 2005b) observed that Sasobit and Aspha-Min mixtures (two mixtures each 

made using granite and limestone), containing PG 64-22 binder and mixed/compacted at 

temperatures significantly lower than that of the corresponding control HMA mixtures which 

contained PG 58-28, had nearly the same air void level. This suggests that the lubricating action 

of Sasobit and Aspha-Min processes lowered the mixing and compaction temperature by 

approximately one asphalt grade. They recommended bumping one grade to counteract any 

tendency for increased rutting. Additional research is needed to verify this finding. One should 

not arbitrarily use an asphalt in WMA that is one grade higher than that typically used in HMA. 

Selection of Optimum Binder Content 

To date, NCAT has probably performed most of the WMA research in the U.S. In their 

laboratory studies, they used modified Superpave mixture design procedures including 125 

gyrations of the Superpave gyratory compactor and standard HMA mixing and compacting 

temperatures (Hurley and Prowell, 2006a). Until more research is completed, NCAT 

recommended determining the optimum asphalt content (OAC) without inclusion of the warm-
17 

mix additive using standard HMA design procedures. This is because the WMA additives 

enhance compaction so effectively that the OAC is reduced by about one-half a percentage point 

below that of an equivalent HMA. This brought about concerns regarding durability, 

permeability, and water susceptibility of the resulting paving mixture. 

When describing the LEA method, Romier et al. (2006) state that laboratory mix design 

methods for HMA apply to design of WMA paving mixtures. However, they further advise that 

laboratory procedures (mixing and compaction) must be adjusted to the temperature of the mixes 

resulting from the plant mixing process. 

Newcomb (2006) suggests that, if any modifications to the Superpave mixture technology 

are required for designing WMA, research will be needed to establish them. 

Aggregate Gradation 

It appears that most of those marketing WMA technologies and most highway agencies 

worldwide, who have evaluated any of the WMA technologies in the laboratory and in the field, 

have used conventional dense-graded mixtures identical to those they typically use in HMA 

(Hurley and Prowell, 2005a; Hurley and Prowell, 2005b; Kuennen, 2004a; Romier et al., 2006). 

This is true even for asphalt mixtures produced at ambient temperature using foamed or 

emulsified asphalt (Maccarone et al., 1994). There currently appears to be no reason to alter the 

gradation of typical dense-graded HMA mixes to accommodate WMA mixes. 

Koenders et al. (2000) and Romier et al. (2006) pointed out that WMA processes should 

be equally applicable to typical types of asphalt mixtures other than dense-graded mixes (i.e., 

SMA, open-graded, stone-filled, coarse-base mixtures). Personal communication with Mr. Larry 

Michaels (2007) indicates that he agrees with this assessment. Kristjansdottir (2006) reported 

that Sasobit has not only been used for dense-graded mixtures in Germany but also in SMA and 

gussasphalt. 

 Several WMA processes (e.g., WAM-Foam, LEA) have demonstrated success in 

mixtures containing RAP (Romier et al., 2006; Jenkins et al., 1999). 

Specimen Compaction 

As with HMA, laboratory compaction of WMA must simulate the density that will 

ultimately be achieved in the field. Standard HMA laboratory compaction procedures (i.e., 125 
18 

gyrations of the Superpave gyratory compactor) have proven to be acceptable for WMA 

mixtures (Hurley and Prowell, 2005a, 2005b, 2006a, 2006b). However, it currently appears that 

the compaction temperature must be reduced to simulate plant production temperatures. 

Hurley and Prowell (2006a) demonstrated clearly that Aspha-Min, Sasobit, and Evotherm 

significantly lowered the required compaction temperature to achieve essentially equivalent air 

voids as an HMA mixture using the same aggregate type and gradation. They pointed out that 

earlier work (Bahia and Hanson, 2000; Huner and Brown, 2001) indicated that the Superpave 

gyratory compactor was rather insensitive to compaction temperature. Their work provided 

further verification of this finding (Tables 1 and 2). Incidentally, they also pointed out that the 

Marshall hammer had been found to be quite sensitive to compaction temperature. 

Specimen Cure Time before Testing 

For testing HMA in the laboratory, there are essentially no cure time requirements for 

compacted specimens. They are often tested as soon as they reach the specified test temperature. 

This is probably acceptable for those WMA products that do not depend on moisture to enhance 

workability and compaction (e.g., Sasobit and Asphaltan B). However, for those products that 

incorporate moisture to promote aggregate coating, workability, and compaction (e.g., Evotherm, 

Aspha-Min, and WAM-Foam), some cure time may be needed to expel the moisture and yield 

realistic predictions of performance. If this moisture is not expelled, laboratory tests to evaluate 

long-term performance may be negatively impacted (i.e., falsely predict unacceptable 

performance). 

When using foamed asphalt, Maccarrone et al. (1994) stated that 3 days of curing at 

140°F appeared relevant for 12 months of field curing for the binder systems he studied. That is, 

their oven-cured specimens yielded similar moduli as pavement cores taken 12 months after 

construction. 
19 

Table 1. Volumetric Mix Design Data for Granite Aggregate Using the Superpave 

Gyratory Compactor (after Hurley and Prowell, 2006a). 

Additive Compaction 

Temp., °F 

AC, 

% 

Gmm %Gmm 

@ Ni

Gmb

Air 

Voids, 

% 

VMA 

Voids 

Filled w/ 

Asphalt 

(VFA) 

Control 300 5.1 2.467 88.0 2.365 4.1 13.6 69.6 

Control 265 5.1 2.467 88.2 2.371 3.9 13.3 71.0 

Control 230 5.1 2.467 87.7 2.360 4.4 13.8 68.4 

Control 190 5.1 2.467 87.5 2.356 4.5 13.9 67.6 

Zeolite 300 5.1 2.457 88.8 2.376 3.3 13.9 76.4 

Zeolite 265 5.1 2.457 88.9 2.382 3.0 13.6 77.7 

Zeolite 230 5.1 2.457 88.7 2.378 3.2 13.1 75.5 

Zeolite 190 5.1 2.457 88.3 2.368 3.6 13.5 73.2 

Sasobit 300 5.1 2.461 88.4 2.375 3.5 13.9 74.8 

Sasobit 265 5.1 2.461 88.0 2.377 3.4 13.8 75.5 

Sasobit 230 5.1 2.461 88.0 2.360 4.1 14.4 71.7 

Sasobit 190 5.1 2.461 NA NA NA NA NA 

Evotherm 300 5.1 2.465 88.7 2.389 3.1 12.7 75.7 

Evotherm 265 5.1 2.465 88.5 2.387 3.2 12.8 75.2 

Evotherm 230 5.1 2.465 88.4 2.384 3.3 12.9 74.5 

Evotherm 190 5.1 2.465 88.6 2.390 3.0 12.7 76.0 

Table 2. Volumetric Mix Design Data for Limestone Aggregate Using the Superpave 

Gyratory Compactor (after Hurley and Prowell, 2006a). 

Additive 

Compaction 

Temp., °F 

AC, 

% Gmm

%Gmm 

@ Ni Gmb

Air 

Voids, % VMA VFA

Control 300 4.8 2.544 85.4 2.433 4.4 15.0 70.8 

Control 265 4.8 2.544 85.1 2.430 4.5 15.1 70.3 

Control 230 4.8 2.544 85.3 2.435 4.3 14.9 71.3 

Control 190 4.8 2.544 85.5 2.439 4.1 14.8 72.1 

Zeolite 300 4.8 2.544 85.8 2.442 4.0 14.7 72.8 

Zeolite 265 4.8 2.544 85.8 2.449 3.7 14.4 74.3 

Zeolite 230 4.8 2.544 85.7 2.444 3.9 14.6 73.2 

Zeolite 190 4.8 2.544 84.8 2.418 4.9 15.5 68.2 

Sasobit 300 4.8 2.545 86.1 2.459 3.4 14.1 76.1 

Sasobit 265 4.8 2.545 86.3 2.463 3.2 14.0 76.7 

Sasobit 230 4.8 2.545 86.3 2.465 3.1 13.9 77.4 

Sasobit 190 4.8 2.545 NA NA NA NA NA 

Evotherm 300 4.8 2.547 86.0 2.472 3.0 13.6 78.4 

Evotherm 265 4.8 2.547 85.6 2.458 3.5 14.1 75.3 

Evotherm 230 4.8 2.547 86.2 2.477 2.8 13.5 79.6 

Evotherm 190 4.8 2.547 85.2 2.451 3.8 14.4 73.9 
20 

Mixture Evaluation 

Mechanical characterization of mixtures should be performed using standard volumetric 

analyses and laboratory specimen testing. State departments of transportation (DOTs) are 

familiar with their analysis procedures and what the results mean regarding pavement 

performance. WMA mixtures should be held to the same standards as HMA mixtures; however, 

proper curing methods for those WMA specimens that initially incorporate water must be 

determined. 

A routine analysis of WMA should include those few tests used in routine analysis of 

HMA (e.g., volumetrics, modulus, rut resistance, moisture susceptibility, and dust proportion) 

and the same testing conditions. Similarly, a complete analysis of WMA should include those 

tests typically performed on HMA specimens (e.g., all of the above plus creep and fatigue). This 

is even true for cold (foam) mix, after proper curing (Maccarrone et al., 1994). 

MATERIAL CHARACTERIZATION FOR STRUCTURAL DESIGN WITH WMA

Hurley and Prowell (2006a) studied Sasobit, Aspha-Min, and Evotherm in the laboratory 

and field and concluded that all products yielded resilient moduli similar to those of 

corresponding HMA mixtures. Others (Mallick et al., 2007; Kanitpong et al., 2007; and 

Wasiuddin et al., 2007) have reported similar findings. Therefore, the structural value of WMA 

using these products can be considered equivalent to that of typical HMA. 

Hurley and Prowell (2006a) further concluded that Sasobit, Aspha-Min, and Evotherm 

did not increase rutting potential, as measured by the asphalt pavement analyzer (APA). In fact, 

the WMA product increased rutting resistance in some instances. However, rutting potential did 

increase with decreased mixing and compaction temperatures; they attributed this phenomenon 

to decreased aging of the binder. Furthermore, they reported no evidence of differing strength 

gain with time (up to 5 days) for the two WMA products when compared to similar HMA 

mixtures. 

Based on findings during this review of current literature, the authors currently believe 

that WMA should provide the same structural values as HMA. 
21 

CONSTRUCTION WITH WMA

Newcomb (2006) pointed out that, since certain WMA technologies require a mixing 

process that is different from conventional HMA, new guidelines need to be developed for 

proper Quality Control/Quality Assurance (QC/QA) of the mix. He further stated that the 

suitability of WMA for the high production rates of asphalt plants in the U.S. needs to be 

examined. There is concern by some engineers that those WMA products that utilize moisture 

may induce some clogging in bag houses; however, no such problems have been reported in the 

literature. 

Except for the temperature of the mat, there are generally no differences in construction 

activities whether using HMA or WMA, after the product leaves the plant. Experience of the 

authors has shown that Evotherm can be stored in a silo in a manner similar to HMA. Romier et 

al. (2006) indicate this is also true for LEA. 

When studying WAM-Foam, Koenders et al. (2000) recommended keeping the 

breakdown roller directly behind the paver; otherwise, the compaction effort required seemed to 

be considerably increased. This recommendation is likely to maximize the advantages of the 

lubricating effects of the moisture in the mat before it dissipates. Compaction temperature of a 

WMA mat is apparently less critical than that of HMA, but it is still important to complete 

compaction quickly and efficiently while the mat temperature is within the appropriate window 

for the specific WMA product. 

WMA PROPERTIES AND PERFORMANCE 

NCAT Laboratory Results for WMA 

NCAT has probably conducted more studies of WMA than any other agency in the U.S. 

(Hurley and Prowell, 2005a, 2005b, 2006a, 2006b; NCAT, 2005). They have demonstrated that 

Aspha-Min, Sasobit, and Evotherm improve compactability in the Superpave gyratory compactor 

and reduce air voids by an average of 0.65, 0.87, and 1.5 percent, respectively, over that of their 

corresponding control HMA mix. Improved compaction was noted at temperatures as low as 

190°F. Addition of Aspha-Min, Sasobit, and Evotherm did not affect the resilient modulus of 

the mixes. Decreased aging due to lower mixing and compacting temperatures may have 

contributed to lower indirect tensile strengths of the WMA mixes. Aspha-Min did not increase 

rutting potential of the mix, while Sasobit and Evotherm generally decreased the rutting potential 
22 

of the mixes evaluated. The rutting potential increased with decreasing mixing and compaction 

temperatures, which may be related to the decreased aging of the binder. NCAT recommended 

bumping the high-temperature grade of the asphalt by one grade to offset any slight increase in 

rutting potential that may occur when using a WMA product. The reports also indicated that 

lower mixing and compaction temperatures can result in incomplete drying of the aggregate and 

that water trapped in the coated aggregate may cause moisture damage. However, they indicated 

that anti-stripping agents were effective in addressing this problem in the laboratory. Their 

results suggested that, when hydrated lime is used as anti-strip, it will stiffen the WMA and assist 

rutting resistance. 

For all three WMA products, NCAT personnel aged the densification samples for 2 hours 

at the compaction temperature prior to compaction. These samples were tested for resilient 

modulus and APA rut depth. 

Aspha-Min 

Tennessee. A warm-mix asphalt paving demonstration was conducted at the World of 

Asphalt 2004 Conference in Nashville, Tennessee, using the Aspha-Min process (Jones, 2004). 

In this 3-hour demonstration, conventional HMA and WMA were placed side by side. 

Temperature readings taken behind the screed on the two different materials showed about 80°F 

difference. Workability and compaction are concerns with conventional mixes as they cool, but 

the WMA technology performed as predicted. Despite the 80°F temperature difference, the 

same density was achieved on both the HMA and WMA mat. Comments made by the 

construction crew were that the WMA was easier to handle and place than the HMA. 

Florida. In February 2004, a control mix and a WMA using Aspha-Min were placed on 

a parking lot in Orlando, Florida, for a demonstration project. NCAT personnel were present at 

this project and obtained samples from the plant for both mixes (Hurley and Prowell, 2005a). 

The control mix consisted of a fine-graded Superpave mix with crushed granite aggregate and 20 

percent RAP; it was designed for 3 to 10 million (equivalent single axle loads (ESALs) with an 

Ndesign of 75 gyrations. The WMA was produced by adding 0.3 percent Aspha-Min by weight to 

the control mix; the additive was introduced to the mix using a specially built feeder. The WMA 

mix discharge temperature was 36 degrees lower than the control mix (336°F for the control and 
23 

300°F for the WMA). The mat temperature behind the screed ranged from 293°F to 315°F for 

the control mix and from 256°F to 260°F for the WMA. 

NCAT personnel compacted samples at the plant using both the Superpave gyratory 

compactor (SGC) and the Marshall method. They compacted the control mix at 310°F and the 

warm mix at 270°F. 

In their report, Hurley and Prowell (2005a) state that additional warm-mix samples were 

compacted after a 1-hour oven-aging period at the compaction temperature and that oven aging 

was performed to assess the ability to store the WMA in a silo. 

Table 3 shows volumetric properties. 

Table 3. Volumetric Properties of WMA and HMA Using Marshall Hammer and 

Superpave Gyratory Compactor (Hurley and Prowell, 2005a). 

Volumetric Property Control Mix 

Unaged 

Aspha-Min Warm Mix 

Unaged, Aged 

SGC Air Voids, % 4.96 5.24 5.04 

SGC VMA, % 14.8 15.0 14.8 

Marshall Air Voids, % 6.43 6.50 6.95 

Marshall Stability (lb) 2930 2853 2733 

Marshall Flow 12.5 13.0 12.0 

The NCAT laboratory test trends of the samples obtained from the Florida project 

matched the trends that NCAT observed in their previous laboratory study. Hurley and Prowell 

(2005a) deduced that the average air voids of both mixes were essentially identical even though 

the compaction temperature of the warm mix was 40°F lower. The gyratory air voids for the 

unaged warm mix was slightly higher than the control, but this also corresponds to a slightly 

higher voids in the mineral aggregate (VMA). All Marshall specimens were compacted using 75 

blows per face. They further stated that the Marshall method air voids of the two unaged mixes 

were identical even though the Marshall hammer has historically been very sensitive to 

compaction temperature. The Marshall method air voids for the aged specimens were higher, 

which could be due to aging of the binder or a dissipation of the moisture released by the zeolite. 
24 

Furthermore, 75 blows with the Marshall hammer produced significantly less compaction than 

75 gyrations with the SGC. 

According to their report, the paving crew at the site noted that the WMA mix was more 

workable than the control mix; in addition, the densities were the same for both sections. 

NCAT personnel visited the site again in March 2005 to conduct a pavement condition 

assessment of the WMA and control sections and to obtain cores from both sections for moisture 

damage testing. According to Hurley and Prowell (2005a), no distress was observed in either 

section. In addition, the testing of the cores indicated that there was no difference in moisture 

damage resistance between the WMA and the control mix. However, the report states, “it should 

be noted that these sections do not receive regular traffic. It is believed that traffic contributes to 

the occurrence of moisture damage.” 

Europe. Eurovia constructed test sections with WMA mixes using Aspha-Min and 

HMA control sections in Europe. Barthel et al. (2004) reported that 3 years after construction of 

the first WMA test section using zeolite measurements indicated no significant changes in the 

surface characteristics. When pavement cores were taken from the road surface and the 

connection between the several layers was tested, no changes were found when compared to 

traditionally constructed pavement layers. 

Evotherm

NCAT Facility—Auburn, Alabama. NCAT conducted laboratory studies of WMA 

using Evotherm and installed test sections on their test track in late October to early November 

2005. In the June 2006 issue of Better Roads, Brian Prowell, Assistant Director at NCAT, 

stated, “After 500,000 ESALS, we had only 1 mm of rutting in the two Evotherm test sections 

and an HMA control section.” The article indicated that the daily high temperatures during the 

first part of the test were initially in the 70°F to 80°F range but then “lowered somewhat after 

that.” Prowell also said, “The traffic was returned within an hour after construction—and few 

sites in the U.S. receive as high a loading rate as the NCAT test track, but just because the 

sections gave an excellent performance in that weather doesn’t guarantee that they would 

perform as well in the heat of the summer” (McKenzie, 2006). 

At the 2007 Annual Meeting of the Transportation Research Board, Prowell et al. (2007) 

reported, “The WMA produced using the Evotherm process was successfully compacted after 
25 

being stored in a silo for 17 hours. In-place densities of the WMA surface layers were equal to 

or better than the HMA surface layers even when compaction temperatures were reduced by 

15°F to 75°F. Significantly improved in-place densities were observed for the WMA base and 

binder layer; however, the asphalt contents were also higher than expected for these sections. 

Laboratory rutting susceptibility tests conducted in the APA indicated similar performance for 

the WMA and HMA surface mixes using the PG 67-22 base asphalt. The two WMA sections 

and HMA section showed excellent field performance in terms of rutting after the application of 

515,333 ESALs in a 43-day period. The WMA in section N1 showed good rutting performance 

even though traffic was returned to the sections 1.75 hours after paving commenced. Laboratory 

tests indicated increased potential for moisture damage with the WMA mixes. Field cores have 

been taken to corroborate this result after trafficking.” The report indicated that the measured rut 

depths after 513,333 ESALs ranged from 0.8 mm to 1.1 mm. 

Indiana. In July 2005, Evotherm was first used in the U.S. to produce 660 tons of WMA 

for a county road near Indianapolis, Indiana. The mixture was produced in a hybrid batch/drum 

plant and operated in the drum plant mode. Evotherm was used with a 12.5 mm nominal 

maximum aggregate size coarse-graded Superpave mix with crushed dolomite and 15 percent 

RAP. Discharge temperature from the drum was approximately 200°F. The mix was placed at 

an average depth of 50 mm and temperatures behind the screed were between 160°F and 200°F. 

The mix did not appear to be tender (Prowell and Hurley, 2005) 

Canada. J.K. Davidson with McAsphalt (based in Toronto, Ontario) produced three 

reports concerning Evotherm trials in Canada. 

The first trial involved paving a parking lot and a mainlane truck exit for the Miller 

Paving marketing office in Aurora in August 2005. Two different mixes were used—a 19 mm 

maximum aggregate size base course compacted to a thickness of 60 mm and a 13.2 mm 

maximum aggregate size surface course compacted to a thickness of 40 mm (Davidson, 2005a). 

The second trial involved paving residential streets in Northeast Calgary in September 

2005 using a 16 mm maximum aggregate size surface course compacted to a thickness of 

approximately 50 mm (Davidson, 2006). 

The third trial involved overlaying Road #46 in the Ramara Township in October 2005 

using a 16 mm maximum aggregate size surface course compacted to a thickness of 60 mm. An 

HMA control section was also placed in October (Davidson, 2005b). 
26 

Davidson reported that for the Aurora and Calgary projects the mix appeared to stay 

tender for an extended period of time. He reported the following for all three trials: 

• Evotherm emulsion was slightly slower to pump up to the asphalt weight hopper than 

normal asphalt cement, which slightly slowed production. 

• Batch size had to be reduced due to limitations in the capacity of the asphalt weigh 

hopper. 

• Because the Evotherm emulsion is only 68 to 70 percent residue, the quantity of 

emulsion needed for the mix is 45 percent higher. 

• Evotherm mix flowed under the paver screed without any evidence of tearing behind 

the screed. 

• The breakdown roller could travel right up to the back of the paving machine with no 

evidence of pushing or shoving of the pavement mat. 

• Using the vibratory mode on the breakdown roller showed no evidence of cracking in 

the mat. 

• The longitudinal joint between lanes appeared to be very tight. 

• The Evotherm mix generally had the appearance of HMA. 

 San Antonio, Texas. TxDOT placed their first warm-mix asphalt trial using the 

Evotherm process on Loop 368 in the San Antonio District in August/September 2006. All test 

sections are performing well at this time. TxDOT is still in the process of evaluating the shortand long-term performance of this field trial through field cores and performance monitoring. 

Documentation of this field trial is presented in Appendix A. Preliminary findings based on 

information documented thus far include the following: 

Mixture Design

Both warm mix and control mixes were designed according to Item 341, Type C densegraded mixes, which employ the use of the Texas gyratory compactor. Both mixes were 

designed to a target density of 96.5 percent. The control hot mix asphalt concrete 

(HMAC) had an optimum asphalt content of 4.8 percent, and the warm-mix optimum 

asphalt content (residual binder) was 4.2 percent. The control mix was produced with a 

PG 76-22 binder, and the warm mix (after the Evotherm modification) was also a PG 76-

22. Both warm mix and control mix used the same aggregate source (predominately 
27 

crushed limestone) and gradation. Also, both warm and control mixes were produced in 

the same asphalt plant. 

Production

• The warm mix was produced at a temperature of 220°F, and the control was produced 

at 320°F. 

• No reduction in fuel consumption was observed for the warm-mix production, and 

this observation is attributed to a heavy rain prior to production, which caused the 

aggregate stockpiles to be excessively wet, requiring more energy for plant operation. 

• Warm mix was stored in silos for a maximum of 2 hours prior to load out. 

Quality Control

WMA samples were compacted in the field laboratory to densities averaging 97 percent, 

which was the same as the control HMAC compacted densities. To evaluate the effect of 

laboratory curing on the warm mix, samples were compacted after three curing 

conditions: no cure, cure for 2 hours at 200°F, and cure for 2 hours at 240°F. The 

different curing conditions had no effect on compacted density. 

Placement and Compaction 

• The warm mix and control mixes were placed over the course of three nights. The 

warm mix was placed at a temperature ranging from 170°F to 210°F. Nuclear density 

tests on the warm mix ranged from 92.1 to 95 percent. The control mix was placed at 

305°F, and nuclear density tests averaged 94.2. 

• The same rolling patterns were used for both control and warm mixes. 

• No problems were observed with the placement and compaction operation. 

• Traffic was allowed on to the warm mix in some areas as soon as 2 hours after 

placement. 

Laboratory Testing on Lab-Molded Samples and Roadway Cores 

• Density of roadway cores taken after 1 month of traffic showed an overall average 

density of the warm mix (based on 26 cores) of 93.3 percent. Average density of the 

roadway cores from the control mix (based on four cores) was 92.6 percent. 

• Density of warm-mix roadway cores taken in the wheel paths were compared with 

those taken between the wheel paths. There is no indication that the warm mix is 

densifying further under the action of traffic (after 1 month). 

• For laboratory testing in the Hamburg and Texas Transportation Institute (TTI) 

Overlay Tester, warm-mix samples were compacted at two different temperatures: 

240°F and 300°F. The warm-mix samples compacted at 300°F performed better in 

the Hamburg than those compacted at 240°F. All of the warm-mix samples failed the 
28 

Hamburg test requirements (of no more than 12.5 mm rut depth at 20,000 passes) 

with the exception of the samples compacted at 300°F from the second night of 

warm-mix production. The warm-mix cores taken at 1 month also failed the 

Hamburg requirements. The control mix generally passed the Hamburg test with the 

exception of one set of field cores. 

• Indirect tensile strength tests were performed during the mix design and on roadway 

cores for both warm and control mixes. The control mix had tensile strengths of 

around 170 psi (both mix design and roadway cores). During the mix design process, 

the warm mix only had a tensile strength of 60 psi; however, the warm-mix roadway 

core tensile strengths ranged from 121 to 178 psi. 

• Overlay test results for the roadway cores were compared to the lab-molded plant mix 

samples. All of the lab-molded warm-mix and control HMAC specimens performed 

poorly in the overlay test. However, there was a significant improvement seen in 

some of the cores taken at 1 month from the warm-mix sections. 

Laboratory Study. Wasiuddin et al. (2007) conducted a laboratory study on Aspha-Min 

and reported that the rheological properties of two commonly used binders (PG 64-22 and PG 

70-28) were evaluated, with and without Sasobit and Aspha-Min. No significant decrease in 

mixing temperature due to Aspha-Min was observed when using the rotational viscometer. No 

significant changes in binder grading were observed with the addition of Aspha-Min. 

Sasobit 

Sasol Wax maintains a list of projects on its web site that utilize Sasobit in asphalt paving 

(Sasol Wax, 2005). As of October 2005, Sasol Wax listed 235 projects and trials in many 

countries, including Germany, Denmark, France, the Czech Republic, Hungary, Italy, the 

Netherlands, New Zealand, Norway, Russia, the United Kingdom, South Africa, Sweden, and 

Switzerland. The projects and trials mainly involved various dense-graded mixes, SMA, and 

gussasphalt. For the projects on the list where Sasobit addition rates were indicated, many 

projects reported using a 3 percent rate, but the rates on other projects varied from 1 to 4.5 

percent. 

The authors could not find any reports on the field performance of WMA mixtures 

containing Sasobit. Findings of two laboratory studies are discussed below. 

Kanitpong et al. (2007) conducted a laboratory study on Sasobit and reported that Sasobit 

modification can improve the workability and the fundamental properties of asphalt binders as 

indicated by improved rutting and fatigue resistance and higher complex shear modulus. 
29 

Measurement of compactability showed that less energy is required to compact the Sasobitmodified mixes to the desired density even at 68°F to 104°F below compaction temperature. 

Sasobit-modified mixtures demonstrate greater resistance to densification under traffic or higher 

potential to resist permanent deformation. However, there is no effect of Sasobit on the 

resistance of asphalt mixtures to moisture damage, but the reduction of mixing and compaction 

temperatures can cause detrimental effects on moisture sensitivity. 

Wasiuddin et al. (2007) conducted a laboratory study on Sasobit and reported that 

rheological properties of two commonly used binders (PG 64-22 and PG 70-28) were evaluated, 

with and without additives (Sasobit and Aspha-Min). For PG 64-22, 2, 3, and 4 percent Sasobit 

reduced the mixing temperature of the pure binder from 325°F to 297°F (i.e., by 28oF). For the 

PG 70-28, the reductions are 50oF, 53oF, and 55oF, respectively, for 2, 3, and 4 percent Sasobit. 

Addition of 3 percent Sasobit increased the high-temperature binder grading of PG 64 (actually 

PG 65) to PG 68, while 4 percent Sasobit improves the PG 70 (actually PG 75) to PG 80. 

Reduction in binder viscosity and improvement in binder grading without increasing the 

viscosity indicated a two-way reduction (both direct and indirect) in production temperatures by 

Sasobit. Finally, Sasobit decreased the APA rut depths significantly, and these rut depths 

correlate well with the rutting factor G*/sin (δ). Wasiuddin et al. (2007) observed that rutting 

potential decreased with decreasing mixing and compaction temperatures. 

Low-Energy Asphalt 

Romier et al. (2006) reported the following concerning the production and placement of 

an LEA mix in November 2003: A total of 150 tons of LEA was produced using a batch plant 

modified for the sequential introduction of the two parts of the aggregates into the weighing 

system ahead of the mixer. The mixes were stored in the finished product storage hopper for 

more than 1 hour. They were placed using a paver-finisher and compacted using a double-drum 

vibrating compactor following the usual application procedures. The climatic conditions 

corresponded to normal seasonal values, with air temperatures a few degrees above freezing in 

the morning. Placement temperature was of the order of 140°F to 160°F. Residual water content 

in the mixes was near 0.5 percent. As the difference with the ambient temperature is smaller for 

LEA mixes than for their high-temperature counterparts, the rate of drop in temperature with 
30 

time is less significant. The LEA mix exhibited a surface appearance comparable to that of 

HMA, including at joint locations. 

WAM-Foam

Figure 3, from Larsen et al. (2004), shows rut depth measurements on a roadway project 

(RV 120) in Norway where WAM-Foam WMA was placed in September 2000 along with a 

regular HMA section. The report did not state the compacted thickness of the WMA mat. Rut 

depth measurements for the WMA and HMA are similar, as shown in Figure 3. The report 

states, “The mixture was a dense asphalt concrete Ab11 with an 85 pen (final) binder. The 

asphalt concrete was produced in a batch plant (7260 lb/batch). The first set of data for 

September 5, 2000 was the condition of the road before the paving of the new one-lift wearing 

course. The large increase after the first winter period is due to studded tires that wear off the 

mortar on the surface. This is typically observed on all Norwegian roads where the percentage 

of studded tires on cars in the winter period is close to 60 percent. The road has now gone 

through three winters.” 

Figures 4 and 5, also from Larsen et al. (2004), show rut depths and international 

roughness index (IRI) measurements on FV 82 in Norway that involved the placement of WAMFoam mix and an HMA control section. Again, the report did not state the compacted thickness 

of the WMA mat. Rut depths and IRI measurements for the WMA and HMA are similar as 

shown in Figures 4 and 5. The report concludes that the FV 82 road provides another example of 

a WMA wearing course for which it is possible to show the behavior of the road after a number 

of years. A section was placed on this road with a dense asphalt concrete WAgb11 using 180-

pen binder (160/220 grade). The asphalt concrete mixture was produced in a batch plant (7260 

lb/batch). Rut depths were measured on both the WAM-Foam and reference hot mix sections. 

These measurements cover a period from October 2000 to June 2003. The results associated 

with October 2000 were obtained before placing the new layers. The results for September 2001 

were obtained after placing a leveling course (mixture prepared using cold recycling foam 

technique with 100 percent RAP). The results obtained in October 2001 were after paving with 

the WAM-Foam mixture (WAgb11) and the corresponding hot mixture (Agb11). Subsequent 

measurements were recorded to monitor the development of the rut depths. 
31 

Figure 3. Rut Depth Measurements on RV 120 in Norway (after Larsen et al., 2004). 

Figure 4. Rut Depth Measurements on FV 82 in Norway (after Larsen et al., 2004). 
32 

Figure 5. IRI Measurements on FV 82 in Norway (after Larsen et al., 2004). 

General Discussion of WMA Properties 

Roadway sections using WMA appear to be performing well in the field according to the 

limited number of reports that address field performance, but in general, those sections have been 

in place for less than 5 years. In the authors’ experience, asphalt pavement sections that fail 

prematurely will typically exhibit signs of distress within less than 5 years after construction. In 

addition, only a few studies appear to be addressing laboratory testing and applying the results to 

field performance, and these are mainly those studies conducted by NCAT. 

OTHER WARM-MIX ASPHALT APPLICATIONS 

Warm Open-Graded Mixes for Patching Permeable Friction Courses 

Patching a permeable friction course (PFC) to maintain its permeability and noise 

abatement qualities has proven to be difficult. The use of hot PFC for patching often results in 

wasting large quantities of expensive material because the open-graded mix cools so rapidly. 

Patching with standard dense-graded cold mix destroys permeability and, thus, creates 

impediments to the designed flow patterns. 
33 

Soto and Blanco (2004a) have proposed a solution to this problem. This potential 

solution is a mixture combining the cohesion of HMA with the sustained workability of cold 

mix. They used a polymer-modified cationic medium setting emulsion as binder in a mix 

manufactured at a hot-mix plant but at temperatures not higher than 175°F. The mix can easily 

be stored for 24 hours. Several successful applications of this product have been demonstrated in 

Spain. 

Why warm mixtures? They report that warm emulsion allows the use of aggregates that, 

considering their quality, gradation, and moisture conditions, could not be utilized in a 

continuous mixing system. The polymer-modified emulsion provides total aggregate coating and 

film uniformity. Placement and compaction is enhanced while the mix is warm. No blinding 

(sanding) is necessary before trafficking, which would be obligatory with cold mixes. Curing of 

the mix is fast, as the warm emulsion cures rapidly. 

Warm In-Plant Recycling

Soto and Blanco (2004b) report that warm recycling can be used to produce and place up 

to 100 percent RAP using either a batch or drum mixing plant. The process heats the RAP to 

about 195°F and mixes it with an appropriate grade of asphalt emulsion. The mix can be stored 

in a silo for 24 hours to accommodate placing at above 140°F. This process allows the resulting 

pavement to be opened to traffic immediately, thus eliminating the curing period necessary in 

cold recycling. 

Their laboratory investigation revealed the following advantages/improvements over cold 

recycled mixes: improved resistance to moisture damage (based on immersion-compression 

tests), increased density under similar compaction (3 percent lower air voids), and dynamic 

modulus was similar to that for conventional HMA. 

Soto and Blanco (2004b) reported on field trials conducted in Spain and Portugal using 

conventional methods and equipment. Workability of the mixture was reportedly similar to that 

for HMA. They reported the following two advantages over cold in-place recycled RAP. 

Quality control is significantly improved since the warm mix is produced in a plant. Since most 

of the water was eliminated during production, a curing period is not necessary for the warm 

recycled mix before trafficking. 
34 

Mallick et al. (2007) evaluated two methods for using 100 percent RAP in warm 

applications for construction of pavement base layers: They heated the RAP to 230°F 

• then added PG 64-28 at 300°F, 

• then added 140°F emulsified asphalt (MS-2) in an asphalt plant, and 

• then added Sasobit (at 1 and 1.5 percent) with PG 64-28 to permit a lower mixing 

temperature (255°F) than a typical hot plant operation. 

Technicians compared workability, compactability, stiffness, and resistance to moisture damage 

of several types of mixes. They prepared slabs approximately 3 feet by 3 feet by 5 inches thick 

and tested cores from the slabs. Heating the RAP significantly improved dispersion of the binder 

as well as slab densification. Sasobit seemed to improve dispersion of the asphalt binder at the 

lower temperature. The use of Sasobit achieved almost similar workabilities, compactabilities, 

and moduli as the HMA specimens but at a lower temperature. For this RAP mix, 1 percent 

Sasobit yielded better results than 1.5 percent Sasobit. 

CURRENT NATIONAL ACTIVITIES RELATED TO WARM-MIX ASPHALT

National Asphalt Pavement Association (NAPA) Research 

According to Corrigan (2006), NAPA in cooperation with the FHWA proposed a 

research program to investigate the performance of WMA products. Initial research was 

conducted on the feasibility of using these technologies in the U.S. through a cooperative 

agreement between NCAT and FHWA. These studies included additional monetary support 

from NAPA and the individual WMA technology providers. NCAT has completed the research 

and published the findings in three reports that are available on their website 

(http://www.eng.auburn.edu/center/ncat/): 

• Report 05-04—“Evaluation of Aspha-Min Zeolite for Use in Warm-Mix Asphalt,” 

Hurley and Prowell (2005a) 

• Report 05-06—“Evaluation of Sasobit for Use in Warm-Mix Asphalt,” Hurley and 

Prowell (2005b) 

• Report 06-02—“Evaluation of Evotherm for Use in Warm-Mix Asphalt,” Hurley and 

Prowell (2006b) 

NCAT Laboratory Findings. Although the initial research focused on Aspha-Min, 

Sasobit, and Evotherm, future research in this area is rapidly expanding to include other WMA 

technologies. The objectives of these laboratory studies were to determine the applicability of 
35 

the three products to paving operations and environmental conditions commonly found in the 

United States, including the performance of the mixes in high-temperature conditions and 

situations that must be quickly returned to traffic. The findings of these three studies at NCAT 

are summarized in Hurley and Prowell (2006a). Tables 1, 2, 4, and 5 highlight their experiment 

and selected findings. 

Table 4. Tensile Strengths for Granite and Limestone (after Hurley and Prowell, 2006a). 

Aggregate Mix Type Indirect Tensile Strength 

Unsaturated, psi Saturated, psi 

TSR, % 

Granite Control 126.6 123.4 0.97* 

Granite Zeolite 155.0 126.3 0.81* 

Granite Sasobit 59.5 40.6 0.68** 

Granite Zeolite 67.2 40.4 0.60** 

Granite Control 75.9 88.3 1.16 

Granite Zeolite 72.5 48.7 0.67 

Granite Sasobit 53.2 38.0 0.71 

Granite Evotherm 70.8 67.7 0.96 

Limestone Control 109.5 71.2 0.65 

Limestone Zeolite 86.6 44.2 0.51 

Limestone Sasobit 53.9 49.1 0.91 

Limestone Evotherm 75.0 46.8 0.62 

Note: * Indicates samples were prepared in SGC in accordance with American Society for 

 Testing and Materials (ASTM) D 4867. 

Note: ** Indicates samples were prepared in SGC in accordance with ASTM D 4867 

 with 250°F compaction temperature. 
36 

Table 5. Results from Hamburg Test (modified after Hurley and Prowell, 2006a). 

1

 Only one of two specimens exhibited a stripping inflection point; reported value is average of 

10,000 cycles and recorded stripping inflection point of second specimen.

2

 NA = No stripping inflection point observed. 

The bulleted list below summarizes their conclusions and recommendations. 

• The gyratory compactor was not sensitive to the reduction in compaction temperature. 

Therefore, all test samples designed to simulate field compactability were compacted in 

the vibratory compactor. 

• Use of any of the three warm-mix processes lowers the measured air voids in the gyratory 

compactor. 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 any additives included should be used. The 

optimum asphalt content of the mixture without the addition of any additive was used for 

all of the testing in this project. 

• All processes improved the compactability of the mixtures in both the SGC and vibratory 

compactor. Statistics indicated an average reduction in air voids up to 0.77 percent for 

the Aspha-Min, 0.89 percent for the Sasobit, and up to 1.53 percent for the Evotherm in 

the vibratory compactor. Improved compaction was noted at temperatures as low as 

190°F for all three additives. 

Aggregate Mix Type Treatment 

Stripping 

Inflection 

Point, cycles 

Rutting 

Rate, 

mm/hr 

Tensile 

Strength 

Ratio 

(TSR) 

Total 

Rutting 

@ 

10,000 

cycles 

Granite Control None 65001 1.841 1.16 7.31 

Granite Sasobit None 3975 2.961 0.71 11.75 

Granite Aspha-Min None 3450 5.139 0.67 20.39 

Granite Evotherm None > 10,000 1.708 0.96 6.78 

Granite 

Aspha-Min 1.5% Hydrated 

Lime 2-Stage 

Addition 85001

 1.912 0.87 7.59 

Granite 

Aspha-Min 1.5% Hydrated 

Lime - Added Dry > 10,000 0.687 0.75 2.73 

Granite Sasobit 0.4% Magnabond > 10,000 0.164 0.94 0.65 

Limestone Control None 2500 4.284 0.65 17.00 

Limestone Aspha-Min None 1700 2.835 0.51 11.25 

Limestone Sasobit None 2900 3.976 0.91 15.78 

Limestone Evotherm None 2550 3.178 0.62 12.61 
37 

• None of the warm-mix processes affected resilient modulus of an asphalt mix when 

compared to control mixtures having the same Performance Grade (PG) binder. Higher 

density increased resilient modulus. Therefore, there is no effect on pavement thickness 

design when using these warm-mix processes. 

• Addition of Sasobit, Aspha-Min, or Evotherm did not increase the rutting potential of the 

mix. Rutting potential increased with decreasing mixing and compaction temperatures; 

this may be related to the decreased aging of the binder. However, the mixes containing 

Sasobit were less sensitive (in terms of rutting) to the decreased production temperatures 

than the control mixes. 

• Indirect tensile strengths for mixes containing Sasobit were lower, in some cases, as 

compared to the control mixes. This reduction in tensile strength is believed to be related 

to the ability of Sasobit to reduce binder aging. 

• APA and Hamburg tests indicated good rutting resistance for the mixes containing 

Sasobit. 

• Mixes containing WMA additives exhibited no difference in strength gain with time as 

compared to the control mixes. Addition of Aspha-Min or Evotherm may not require a 

cure time for the asphalt mixture prior to opening to traffic. Field data from Europe 

support this conclusion that addition of Sasobit does not require a cure time before 

opening trafficking.

• Lower mixing and compaction temperatures for warm mixes may increase the potential 

for moisture damage. Lower mixing/compaction temperatures can result in incomplete 

drying of the aggregate. Water trapped in the coated aggregate may enhance moisture 

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

and warm asphalt mixes produced at 250°F. 

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

damage. Hydrated lime with Aspha-Min was effective with the granite aggregate. 

Hydrated lime (1.5 percent) in the WMAs resulted in improved cohesion and moisture 

resistance. Addition of AKZO Nobel Magnabond (Kling Beta 2912) improved the tensile 

strength ratio values to acceptable levels for the Sasobit. An alternate Evotherm 

formulation provided good moisture resistance for the limestone aggregate. 

• Hamburg wheel-tracking tests indicated good performance in terms of moisture 

susceptibility and rutting for the mixtures containing Sasobit and Magnabond. 

• Hamburg results suggested that lime will augment rutting resistance of mixtures 

containing Aspha-Min compacted at lower temperatures due to the stiffening effects of 

lime. 

• A binder modified with Sasobit needs to be engineered to meet the desired performance 

grade. For example, a PG 58-28 was used as the base asphalt with the addition of 2.5 

percent Sasobit to produce a PG 64-22. 

• Optimum asphalt content should be determined without the addition of any warm-mix 

additive. Additional samples should then be produced so the field target density can be 
38 

adjusted (e.g., if the laboratory air void content with any additive included was decreased 

in the lab by 0.5 percent, then the field target density should be increased by 0.5 percent). 

• Based on the compaction and rutting results, a minimum field mixing temperature of 

275°F and a minimum field compaction temperature of 250°F is recommended. If the 

mixing temperature is below 275°F, then the high-temperature grade should be bumped 

upward by one grade. Standard HMA performance testing should be conducted. Field 

compaction will dictate the true minimum compaction temperature depending on a 

number of factors. 

• Tensile strength ratio testing should be conducted at the anticipated field production 

temperatures. If results are unfavorable, an anti-stripping agent should be incorporated to 

yield an acceptable tensile strength ratio. 

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

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

production temperatures.

During the discussion of Hurley and Prowell (2006a) at the conference of the Association 

of Asphalt Paving Technologists, Mr. Gerald Reinke indicated a potential problem for WMA. 

He pointed out that, in Wisconsin, the climate calls for a PG 58-28. Further, if one uses a PG 64-

22 in WMA, with the reduced binder aging, it would provide essentially the same rutting results 

as a PG 58-28 in HMA. This is fine for the high-temperature performance. The potential 

problem arises when the low-temperature specification properties of the WMA binder are 

measured. If one extracts and recovers the PG 64-22 from the WMA and performs the pressure 

aging vessel test, experience indicates that the resulting low-temperature properties typically 

miss the -28 grade by about 2 degrees. A modifier could be used to address this issue, but its 

cost would offset some of the economic benefits of the WMA. This needs to be addressed 

during future studies. 

NCAT Test Track Findings. Prowell et al. (2007) reported findings from accelerated 

loading tests on WMA sections containing Evotherm at the NCAT Test Track. Evotherm was 

placed on three 200-foot test sections as rehabilitation layers in early November 2005. Figure 6

shows a section view of the layers. Sections N1 and N2 were milled to a depth of 5 inches to 

complete the structural rehabilitation of those sections. Two lifts of 19.0 mm nominal maximum 

aggregate size (NMAS) WMA were placed in Sections N1 and N2. The original track structure, 

20 inches of HMA, 5 inches of asphalt-treated drainable base, and 6 inches of aggregate underlay 

the 1-inch Evotherm inlay placed in Section E9. The binder for the 9.5 mm NMAS surface mix 
39 

was varied for each section (Figure 6). An HMA control containing a PG 67-22 binder (similar 

to AC-30) was placed in Section N2. All mixtures contained a blend of granite, limestone, and 

coarse sand. 

WMA Test Sections 

 N2 N1 E9 

←9.5 mm NMAS→

1 inch HMA Control 

PG 67-22 

Evotherm 

PG 67-22 + 3% Latex 

Evotherm 

PG 67-22 

2 inch 19.0 mm NMAS w/ 

Evotherm PG 67-22

2 inch 19.0 mm NMAS w/ 

Evotherm PG 67-22

Figure 6. Section View of Evotherm Layers at NCAT Test Track 

(after Prowell et al., 2007). 

In-place densities of the WMA surface layers were equal to or better than the 

corresponding HMA test sections, even though compaction temperatures were reduced by 15°F 

to 75°F. In fact, the Evotherm mixture was successfully compacted after storage in a silo for 17 

hours. Laboratory rutting susceptibility tests conducted in the APA indicated 

similar performance for the WMA and HMA surface mixtures that contained PG 67-22 base 

asphalt. However, laboratory tests indicated an increased potential for moisture damage with the 

WMA mixtures. The two WMA sections and corresponding HMA section demonstrated 

excellent field performance in terms of rutting following application of >500,000 ESALs in a 43-

day period. They turned over one of the WMA sections to traffic less than 2 hours after paving 

commenced and showed good results with pavement surface temperatures reaching about 90°F 

for the first 10 days after construction. 

Prowell et al. (2007) used an infrared camera to monitor the thermal consistency of the 

mat behind the paving machine. Infrared images clearly demonstrated minimal thermal 

differential in the mat indicating no appreciable thermal segregation. 
40 

Development of a Warm-Mix Asphalt Technical Working Group (TWG) 

Corrigan (2006) notes that a WMA TWG was initiated by NAPA and FHWA with the 

mission to evaluate and validate WMA technologies and to implement proactive WMA policies, 

practices, and procedures that contribute to a high-quality, cost-effective transportation 

infrastructure. The WMA TWG will foster an environment where transportation officials from 

government and industry share information on new, innovative, or proven WMA technologies 

and validate those technologies, which will safeguard the transportation infrastructure of the 

U.S. and ensure that transportation funding and programs are efficiently and effectively utilized. 

The WMA TWG was developed to: 

• provide national guidance in the investigation and implementation of WMA 

technologies; 

• identify, review, validate, and provide technical guidance that will provide a WMA 

product with quality, cost-effectiveness, and performance equivalent to conventional 

HMA with the additional benefit of reduced emissions during production and placement; 

and 

• discuss problems with WMA technology and develop solutions to such problems. 

The WMA TWG is made up of representatives from the FHWA, NAPA, state highway 

agencies, state asphalt pavement associations, American Association of State Highway and 

Transportation Officials (AASHTO), NCAT, the hot mix asphalt industry, labor, and National 

Institute for Occupational Safety and Health (NIOSH). Further information on the WMA TWG 

is found at NAPA’s webpage (http://www.warmmixasphalt.com/). 

National Cooperative Highway Research Program (NCHRP) Project 09-43 

NCHRP has requested proposals on Project 09-43, “Mix Design Practices for Warm-Mix 

Asphalt Technologies,” for fiscal year 2007. The objective of this research is to develop a mix 

design method for WMA in the form of an AASHTO-style manual of practice. This method 

shall: 

• be based on Superpave mixture design methodology, 

• include a suite of performance tests to assess whether a WMA mixture design will 

provide satisfactory field service, and 

• apply to any WMA technology used to lower mixing and compaction temperatures. 
41 

Additional information on this project is available at the NCHRP webpage 

(http://www4.trb.org/trb/crp.nsf/All+Projects/NCHRP+9-43). 

ADVANTAGES AND DISADVANTAGES OF WARM-MIX ASPHALT

The benefits and limitations of WMA that were identified during review of the literature 

and through discussions with individuals having experience with WMA are listed in this 

subsection. The specific advantages and disadvantages of WMA are rather dependent on the 

specific WMA process being considered. Therefore, it may be somewhat misleading to 

assemble all the WMA processes into one group and elucidate their features that are superior or 

inferior to HMA. Nevertheless, the authors have attempted this in the following two subsections. 

Benefits of WMA as Compared to HMA 

Some of the benefits of WMA have been discussed above where the individual WMA 

processes are discussed. The potential advantages of WMA products/processes, in general, over 

HMA are tremendous. The specific benefits and the degree of the benefits depend, of course, 

upon which WMA process is used. However, potential benefits include: 

• significantly lower production and placement temperatures; 

• less aging of binder during plant mixing and placement, thus improving longevity of 

pavement service life; 

• reduced thermal segregation in the mat; 

• less fuel/energy consumption, thus lowering fuel/energy costs; 

• decreased emissions/odors from mixing plant and during placement; 

• decreased dust production due to lower temperatures and shorter heating time; 

• extended paving season (i.e., paving during cooler weather); 

• extended mix haul distance (due to less difference between ambient temperature and mix 

temperature) and, thus providing expanded market areas; 

• paving in non-attainment areas; 

• more daylight paving (i.e., reduced requirements for night paving); 

• facilitates compaction, which is beneficial for stiff mixes, RAP mixes, low-temperature 

paving, and reducing compaction effort; 
42 

• faster construction of pavements made of deep lifts of asphalt (e.g., intersections, which 

need to be opened as soon as possible; less time is required to cool the mix before the 

next lift is placed); 

• improved working conditions for plant/paving crew; 

• improved thin-lift capabilities (i.e., lower cooling rate from maximum temperature or 

lower compaction cessation temperature); 

• quicker opening to traffic for some WMA products (a particularly important factor for 

some airports); 

• diminished consternation of public over emissions; and 

• easier permitting for plant site in urban areas. 

A study of materials and pavements in Canada (Warm-Foam, similar to WAM-Foam) 

(Johnston et al., 2006) and Iceland (WAM-Foam, Aspha-Min, and Sasobit) (Kristjansdottir, 

2006) indicated that WMA offers specific advantages for paving during cold weather when the 

WMA is placed at or near temperatures typically used for HMA. This offers a greater 

temperature gap between production and compaction cessation of the more easily compacted 

WMA and, incidentally, increases the allowable haul distance. This process is particularly 

beneficial for stiff mixes such as those containing hard binder and/or RAP. 

Kristjansdottir et al. (2007) deduced that while lower emissions and reduced energy 

consumption are admirable benefits by themselves, they do not make a strong business case for 

adoption of WMA technology. Reduced viscosity by itself or in combination with other benefits 

appears to offer more potential for widespread WMA adoption, because these could allow owner 

agencies and contractors to: 

• reduce compaction risks associated with cold weather, 

• reduce compaction equipment needed on the jobsite, and 

• lower the risk of poor compaction when working with stiff mixtures. 

The extent of these potential benefits and how to optimize them needs to be studied in a 

strategic, nationally coordinated research program. In the U.S., several state DOTs, 

municipalities, and the NCHRP have initiated studies of WMA, but these are not coordinated and 

do not address all of the aspects of WMA. Studies should include long-term performance, lifecycle cost analyses, and a commitment by owner agencies (Kristjansdottir et al., 2007). 
43 

Environmental Benefits of WMA

 To maximize our survival time on this planet, society must develop ways to reduce 

consumption of energy and fossil fuel, generation of heat, and greenhouse gas emissions. 

Scientists must conduct the required research, and then agencies must enact incentives, when 

necessary, to implement those ideas that make sense. Research needs to determine if WMA is 

logical, environmentally beneficial, and cost effective, and if so, engineers must develop the 

tools that will bring it into common use. 

Jenkins et al. (1999) stated that the WAM-Foam process can be performed with 40 

percent less energy consumption during plant processing than HMA. McKenzie (2006) 

describes how lower WMA production temperatures could lead to fuel savings that outweigh the 

additional cost of $3.60/ton of mix for the Aspha-Min additive. 

According to Barthel and Von Devivere (2003) when using Aspha-Min, measurements 

conducted for Eurovia indicated a 30 percent reduction in energy consumption because of a 54°F 

to 63°F reduction in mix temperature and a 75 percent reduction in fume emissions resulting 

from a 47°F reduction in production temperature. Measurements at the application site indicate a 

reduction in fume emissions of more than 90 percent, when the mix temperature was reduced 

from 345°F to 285°F and, in all cases, when Aspha-Min has been added and temperatures 

reduced, odor has decreased, and crew members have confirmed improved working conditions. 

When using WAM-Foam, Shell reported plant fuel savings and CO2 reductions of 30 

percent. Measurements from a drum plant in Norway showed that WAM-Foam production 

yielded the following reductions, as compared to HMA at identical production rates: 40 percent 

in diesel consumption, 31 percent in CO2 emissions, 29 percent in CO emissions, and 62 percent 

in NOX emissions (Larsen et al., 2004). 

Based on simple heat balance calculations, Romier et al. (2006) report that the heating 

energy required for LEA is less than 50 percent of a similar HMA. Based on those calculations, 

they further report that LEA can reduce greenhouse gas (CO2, N2O, and CH4) emissions by 50 

percent. 

If WMA technologies, in general, decrease heat of the mixture and emissions of volatile 

organic compounds (VOCs) and polycyclic aromatic hydrocarbons (PAHs), they will certainly 

improve safety and working conditions for production workers and paving crews. 
44 

Benefits of WMA as Compared to Cold Mix 

In summary, Soto and Blanco (2004a) and Els (2004) offer the following advantages of 

WMA over cold mixtures for paving and patching, particularly for patching permeable friction 

courses. 

• For WMA, essentially no curing time is necessary before trafficking. 

• WMA allows use of higher quality aggregates that cannot be used in cold mixes. 

• WMA provides better quality mixes due to the total coating of aggregates and binder film 

uniformity. 

• WMA offers improved handling and compaction over cold mix. 

Hindrances to WMA Implementation

Even with the demonstrated benefits of WMA, particularly for large urban 

(non-attainment) areas along with and encouraging performance results, the new product still has 

significant hurdles to overcome before widespread acceptance in the U.S. (Zettler, 2006). 

Generally speaking, the paving industry, as a whole, is slow to accept new technologies. For 

example, Superpave has been around for 11 years, and it is still not universally accepted by all 

state DOTs. 

Since WMA technologies are new, and most are proprietary, they will likely increase 

initial production costs. For mix production in Iceland, Kristjansdottir (2006) estimated per-ton 

premiums of $0.30 for WAM-Foam, $3.50 for Sasobit, and $4.00 for Aspha-Min. These costs 

were offset by reductions in fuel costs such that the percentage increase in mix production were -

1.5 percent, 2.1 percent, and +2.7 percent, respectively. Some WMA additives can be added to 

the pugmill or blown into the mixing plant at the same point where the asphalt is injected into the 

drum (or the RAP collar), while others require additional equipment at the plant. 

The diminished tensile strength reported by NCAT (Hurley and Prowell, 2005a, 2005b) is 

an indicator that more water remains in WMA than in HMA. However, to date, researchers are 

getting mixed signals on this issue, since field cores obtained from WMA test pavements 

indicate no damage. If findings from the laboratory and the field conflict, then, clearly, the 

laboratory procedures for evaluating WMA will need to be modified. 

Koenders et al. (2000) asserted that WMA products that use emulsions (and possibly 

others that use water) might cause problems in the weighing system and the dust collection 
45 

system of the mixing plant. They further indicated that, in their trials, this was not a major 

problem, but in routine operations, one should be aware of this possibility. 

Hadley et al. (1969) associated lower HMA mixing temperatures with decreased tensile 

strength. Later, Kennedy et al. (1984) showed that increasing mixture temperature will enhance 

moisture susceptibility of HMA. They both associated higher mixing temperatures with lower 

viscosity of the asphalt and, thus, better wetting of the aggregate surface along with slightly more 

asphalt absorption into the aggregate surface which results in maximizing adhesion at the 

asphalt-aggregate interface. WMA research should include comparative evaluations of moisture 

susceptibility of similar WMA and HMA paving mixtures. 

Research is needed to answer questions that remain when one carefully considers the 

WMA mixture design, mixture evaluation, construction, and performance issues as compared to 

traditional HMA. 

• Because of the lower mixing temperature, does WMA yield less binder absorption into 

the aggregates? If so, how will lower absorption affect mixture design (optimum asphalt 

content) and long-term performance (e.g., moisture susceptibility)? 

• Some WMA products have significantly lowered air voids during standard Superpave 

gyratory compaction as compared to similar HMA, which indicates reduced optimum 

asphalt content. Will the reduced asphalt content lead to problems related to durability 

(e.g., cracking, oxidative aging, and/or moisture susceptibility)? 

• For testing HMA in the laboratory, there are essentially no cure time requirements for 

specimens. Are these procedures acceptable for WMA specimens, or is some cure time 

needed to expel the moisture for certain WMA products to yield realistic predictions of 

performance? 

• Less heat energy for WMA processes likely leaves more moisture in the aggregates and, 

thus, in the compacted mat. How will this potential moisture affect short-term pavement 

performance? 

• With less oxidative aging in the plant, should one start with a harder asphalt binder than 

typically used for HMA? If not, will the less aged, softer binder combined with the 

potential moisture in some WMA mixtures lead to premature permanent deformation? 

• Will some cure time be required for WMA products containing moisture before traffic 

can be allowed on it without concern about permanent deformation? 

In the survey conducted by Kristjansdottir et al. (2007), several respondents indicated that 

WMA will only be an option if it cost the same or less than HMA, unless (1) environmental 

regulations are made stricter or (2) it provides some quality or construction benefit (then a 

moderate increase in cost would be acceptable). 
46 

COMPARATIVE COSTS OF PRODUCING WMA

Kristjansdottir et al. (2007) reported that, on the few WMA jobs where energy 

consumption was measured, there was typically a 20 to 75 percent reduction as compared to 

HMA, depending on how much the production temperature was lowered. The level of this 

benefit depends on the type and cost of energy. For example, if energy cost is high, the benefit, 

of course, is greater. Whether the power comes from fossil fuel or electricity, it requires roughly 

300,000 BTUs to produce a ton of HMA, which is equivalent to about 2 to 3 gallons of fuel oil or 

diesel or about 2.5 to 3.5 therms of natural gas. Table 6 shows general costs for producing HMA 

and estimated savings from WMA technologies based on selected local energy costs. Costs for 

specific plants and materials (e.g., aggregate moisture content) will vary. 

Table 6. Cost Savings of WMA at Selected sites (after Kristjansdottir et al., 2007). 

Location Iceland Honolulu, HI Joliet, IL 

Fuel Source No. 2 fuel oil Diesel Natural gas 

Amount to make 1 ton of HMAa 2 - 3 gallons 

(7.6 - 11.4 L) 

2 - 3 gallons 

(7.6 - 11.4 L) 

2.5 - 3.5 therms 

Fuel costb $2.50/gallon 

($0.66/L) 

$2.20 - $3.00/gallon 

($0.58 - $0.79/L) 

$0.70 - $0.80/therm 

Fuel cost to make 1 ton of HMAc $5.00 - $7.50 $4.40 - $9.00 $1.75 - $2.80 

Electricity to make 1 ton of HMAd 8 - 14 kWh 8 - 14 kWh 8 - 14 kWh 

Industrial electricity coste $0.02/kWh $0.1805/kWh $0.0445/kWh 

Electricity cost to make 1 ton of HMAf $0.16 - $0.28 $1.44 - $2.53 $0.36 - $0.64 

Total energy cost to make 1 ton of HMAg $5.16 - $7.78 $5.84 - $11.53 $2.11 - $3.44 

20% savings with WMAh $1.00 - $1.50 $0.88 - $1.80 $0.35 - $0.56 

50% savings with WMAi $2.50 - $3.75 $2.20 - $4.50 $0.88 - $1.40 

a. Aggregate moisture content assumed typical at 2 – 4 percent. Amounts of fuel are general averages. 

b. Numbers taken from personal correspondence with a producer in each area. 

c. Range shown is the low end amount of fuel multiplied by the low end fuel cost and the high end 

 amount of fuel multiplied by the high end fuel cost. In general, this constitutes the cost to dry and 

 heat aggregate. 

d. Taken as the average of 8 to 14 kWh range obtained from (EFAI 2006). This constitutes other power 

 requirements not furnished by the aggregate dryer or drum plant burner. 

e. Taken as the average industrial retail price for the particular region either from the web page of 

 Reykjavik Energy, www.or.is (Iceland) or from Table 5.6A of the Energy Information 

 Administration’s July 2006 Electric Power Monthly. 

f. Range shown is the low electricity requirement multiplied by the low end electricity cost and the 

 high end electricity requirement multiplied by the high end electricity cost. 

g. Fuel cost added to electricity cost. 

h. A rough estimate of the low end of expected savings from WMA technology. Range shown is the 

 low end and high end of the fuel cost each multiplied by 20 percent. 

i. A rough estimate of the low end of expected savings from WMA technology. Range shown is the 

 low end and high end of the fuel cost each multiplied by 50 percent. 
47 

Use of WMA increases costs associated with various aspects of the technology. 

Kristjansdottir et al. (2007) tabulated costs for some of the leading WMA technologies (Table 7). 

They added that the purpose of Table 7 is not to compare costs of WMA technologies, but rather 

to show that WMA technologies have associated costs that must be at least matched by their 

perceived benefits. Since these technologies are relatively new, their costs fluctuate and will, 

with increasing use, likely decrease with time. 

Table 7. Costs of Various WMA Technologies (after Kristjansdottir et al., 2007). 

WMA Technology WAM-Foama Aspha-Min Sasobit Evothermb 

Equipment modification 

or installation costs 

$30,000-$70,000 $0-$40,000 $0-$40,000 minimal 

Royalties $15,000 first yr 

$5,000/plant/yr 

$0.30/ton 

None None None 

Cost of material NA $0.60/lbc $0.80/lbd 7%-10% more than 

asphalt binder 

Recommended dosage 

rate 

NA 0.3% by 

weight of 

mix 

1.5% to 3% by 

weight of 

binder 

Use in place of asphalt 

binder 

Approximate cost per ton 

of mix (per tonne of mix) 

$0.30e 

($0.33) 

$3.60e 

($3.96) 

$1.30 - $2.60 

($1.43 - $2.86) 

$3.50 - $4.00 

($3.85 - $4.40) 

a. According to electronic mail from Øyvind Moen, Kolo Veidekke, Norway. February 10, and January 

19, 2006. The high end estimate of equipment cost comes from a Prowell and Hurley presentation 

accessible at: http://www.pavementpreservation.org/library/getfile.php?journal_id=735. 

b. According to phone conversation with Johathan MacIver, Business Development Manager, Asphalt 

Innovations, MeadWestvaco. July 28, 2006. 

c. According to electronic mail from Barry McKeon, Technical Manager at Hubbard Construction 

Company in Orlando, Florida. February 6, 2006. 

d. According to electronic mail from Matthias Nolting, Business Unit Manager, Sasol Wax. January 19, 

2006. 

e. Not including first-year and royalty costs. 

f. From Brown (2006). 

Contractors perceive risk with new processes and materials and, as a result, increase their 

bid prices. Kristjansdottir et al. (2007) suggested that risks associated with WMA can be broadly 

classified into long-term performance and uncertainty. WMA is relatively new with the oldest 

sections being about 10 years old. While performance has generally been good, substantial 

empirical evidence of pavement life equivalent to HMA is needed to reduce perceived risk. 
48 

SUMMARY OF FINDINGS AND RECOMMENDATIONS

WMA is distinguished from other asphalt mixtures by the temperature regimes at which 

it is produced along with the strength and durability of the final product. Cold asphalt mixtures 

are typically manufactured at ambient temperature (e.g., 68°F to 120°F), while hot mix is 

typically produced in the range of 285°F to 340°F. Warm mixes are those generally produced in 

the temperature range of 200°F to 275°F. The goal with warm mix is to obtain a level of 

strength and durability that is equivalent to HMA. 

Findings 

Based on a review and synthesis of all known literature from worldwide sources, the 

findings are highlighted. 

• WMA is not a new process; however, there are significant new technologies which 

deserve exploration and are being studied by a number of state agencies and 

municipalities. 

• Six new WMA technologies appear to deserve further investigation: Aspha-Min, WAMFoam, Low-Energy Asphalt, Sasobit, Evotherm, and Asphaltan B. Aspha-Min, Sasobit, 

and Evotherm are most likely to be available in Texas. Only Evotherm requires no plant 

modifications. A brief history leading up to the current technologies and detailed 

descriptions of each are provided. 

• Benefits and limitations of WMA as compared to HMA are listed in this synthesis in a 

section dedicated to these issues. Specific advantages and disadvantages of WMA are 

dependent on the specific WMA process being utilized. 

• Advantages of WMA over cold asphalt mixtures for paving and patching, particularly for 

patching permeable friction courses, may include: no curing time before trafficking, use 

of higher quality aggregates that cannot be used in cold mixes, better quality mixes due to 

the total coating of aggregates and binder film uniformity, and improved handling and 

compaction. 

• Reducing production (mixing) and paving (compaction) temperatures by using WMA in 

place of HMA will yield beneficial environmental effects: decreased fuel or energy 

consumption (with consequential decreased cost); reduced emissions and odors from 

plants; reduced smoke and, thus, consternation from the public; and improved working 

conditions at the paving site. Generally, findings indicate the potential for 30 to 50 

percent reduction in energy/fuel, 30 percent reduction in CO2, 40 percent less fumes at 

the paving machine, and 50 to 60 percent reduction in dust generation. 

• On the few WMA jobs where energy consumption has been measured, a 20 to 75 percent 

reduction was shown as compared to HMA. This depended on how much the production 

temperature was lowered, as well as the type and cost of the fuel used (higher energy cost 
49 

yields greater savings). Contractors perceive risk with new processes and materials and, 

as a result, increase their bid prices. 

• The National Center for Asphalt Technology has conducted more research on WMA than 

anyone else in the U.S. Their findings from several reports are summarized herein. They 

recommended that the optimum asphalt content of WMA should be determined in the 

usual fashion for HMA and then be used in the WMA. Laboratory and field performance 

of WMA was generally similar to that for HMA. 

• Lower mixing and compaction temperatures for WMA as compared to HMA may 

contribute to incomplete drying of the aggregate and thus increase the potential for 

moisture damage in the resultant pavement layer. 

• Standard mix design procedures for HMA must be modified to accommodate WMA. 

Aggregate gradations typically used for HMA are acceptable for WMA. WMA 

technologies aid compaction, thus compactive effort must be reduced to prepare realistic 

laboratory specimens. 

• For testing HMA in the laboratory, there is no cure time requirement for compacted 

specimens. They are often tested as soon as they reach the specified test temperature. 

This is probably acceptable for those WMA products that do not depend on moisture to 

enhance workability and compaction. However, for those products that incorporate 

moisture to promote aggregate coating, workability, and compaction, some cure time may 

be needed to expel the moisture and yield realistic predictions of performance. If this 

moisture is not expelled, laboratory tests to evaluate long-term performance may be 

negatively impacted (i.e., falsely predict unacceptable performance). 

• Mechanical characterization of mixtures should be performed using standard volumetric 

analyses and laboratory specimen testing. WMA mixtures should be held to the same 

standards as HMA mixtures; however, proper curing methods for those WMA specimens 

that initially incorporate water must be determined. 

• Regarding structural design, WMA should be given the same value as HMA. 

• Compaction temperature of a WMA mat is apparently less critical than that of HMA, but 

it is still important to complete compaction while the mat temperature is within the 

appropriate window for the specific WMA product. 

• Time to traffic after placing WMA is not a significant issue. 

• Based on a limited number of reports that address field performance of WMA, pavements 

are performing well, but, typically, the sections reported had been in place for less than 5 

years. 

• WMA processes can incorporate RAP. In fact, some have reported the use of 100 

percent RAP in certain WMA processes. 

• NCHRP has implemented Project 09-43, “Mix Design Practices for Warm-Mix Asphalt 

Technologies.” The objective is to develop a mix design method for WMA which shall 

be based on Superpave mixture design methodology, include a suite of performance tests, 

and apply to any WMA technology. 
50 

• A WMA Technical Working Group was initiated by NAPA and FHWA with the 

missions to provide national guidance; evaluate and validate WMA technologies; and 

implement proactive WMA policies, practices, and procedures that contribute to a highquality, cost-effective transportation infrastructure. 

Recommendations

The preceding findings appear to support these recommendations. 

• Research is needed to answer questions that remain when one carefully considers the 

WMA mixture design, mixture evaluation, construction, and performance issues as 

compared to traditional HMA. 

3⁄4 Because of the lower mixing temperature, does WMA yield less binder absorption 

into the aggregates? If so, how will lower absorption affect mixture design 

(optimum asphalt content) and long-term performance (e.g., moisture 

susceptibility)? 

3⁄4 Some WMA products have significantly lowered air voids during standard 

Superpave gyratory compaction as compared to similar HMA, which indicates 

reduced optimum asphalt content. Will the reduced asphalt content lead to 

problems related to durability (e.g., cracking, oxidative aging, and/or moisture 

susceptibility)? 

3⁄4 For testing HMA in the laboratory, there are essentially no cure time requirements 

for specimens. Are these procedures acceptable for WMA specimens, or is some 

cure time needed to expel the moisture for certain WMA products to yield 

realistic predictions of performance? 

3⁄4 Less heat energy for WMA processes likely leaves more moisture in the 

aggregates and, thus, in the compacted mat. How will this potential moisture 

affect short-term pavement performance? 

3⁄4 With less oxidative aging in the plant, should one start with a harder asphalt 

binder than typically used for HMA? If not, will the less aged, softer binder 

combined with the potential moisture in some WMA mixtures lead to premature 

permanent deformation? 

3⁄4 Will some cure time be required for WMA products containing moisture before 

traffic can be allowed on it without concern about permanent deformation? 

• If a northern climate calls for a PG 58-28 and, because of reduced binder aging in WMA, 

one decides to use a PG 64-22, it would provide essentially the same high-temperature 

performance (rutting) as a PG 58-28 in HMA. However, if one extracts and recovers the 

PG 64-22 from the WMA and performs the pressure aging vessel test, one may find that 

the resulting low-temperature properties will be inadequate to meet specified values. A 

modifier could be used to address this issue, but its cost would offset some of the 

economic benefits of the WMA. This issue needs to be addressed during future studies. 
51 

• Nationally coordinated research is needed to further evaluate field performance, the 

selection of the optimum asphalt content, and the selection of appropriate binder grades 

for lower production temperatures. 

53 

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Koenders, B.G., D.A. Stoker, C. Bowen, P. de Groot, O. Larsen, D. Hardy, and K.P. Wilms, 

“Innovative Processes in Asphalt Production and Application to Obtain Lower Operating 

Temperatures,” 2nd Eurasphalt & Eurobitume Congress, Barcelona, Spain, 2000. 
56 

Koenders, B.G., D.A. Stoker, C. Robertus, O. Larsen, and J. Johansen, “WAM-Foam, 

Asphalt Production at Lower Operating Temperatures,” Proceedings, 9th Conference of 

International Society for Asphalt Pavements, Copenhagen, Denmark, 2002. 

Kristjansdottir, O., “Warm Mix Asphalt for Cold Weather Paving,” Report No. WA-RD 

650.1, A thesis for partial fulfillment of the degree of Master of Science in Civil Engineering, 

University of Washington, Seattle, Washington, 2006. 

Kristjansdottir, O., L. Michael, S.T. Muench, and G. Burke, “Assessing the Potential for 

Warm Mix Asphalt Technology Adoption,” Proceedings (CD), 86th Annual Meeting of the 

Transportation Research Board, National Academy of Sciences, Washington, D.C., 2007. 

Kuennen, T., “Paths to Progress—2005,” Better Roads, James Informational Media, Inc., 

Des Plaines, Illinois, December 2004a. 

Kuennen, T., “Warm Mixes are a Hot Topic,” Better Roads, James Informational Media, 

Inc., Des Plaines, Illinois, June 2004b. 

Larry Michaels, personal communication, engineering consultant, formerly state materials 

engineer for Maryland DOT, January 2007. 

Larsen, O.R., O. Moen, C. Robertus, and B.G., Koenders, “WAM-Foam Asphalt Production 

at Lower Operating Temperatures as an Environmental Friendly Alternative to HMA,” 

Proceedings, 3rd Eurasphalt & Eurobitume Congress, Vienna, Austria 2004. 

Little, D.N., J.W. Button, and J.A. Epps, “Structural Properties of Laboratory Mixtures 

Containing Foamed Asphalt and Marginal Aggregates,” Transportation Research Record 

911, Transportation Research Board, National Academy of Sciences, Washington, D.C., 

1983. 

Maccarrone, S., G. Holleran, and A. Ky, “Cold Asphalt Systems as an Alternative to Hot 

Mix,” Proceedings, 9th International AAPA Conference, Surfers Paradise, Queensland, 

Australia, 1994. (http://asphalt.csir.co.za/FArefs/) 

Mallick, R.B., J.E. Bradley, and R.L. Bradbury, “An Evaluation of Heated RAP Material and 

Wax Modified Asphalt for Use in Recycled Hot Mix Asphalt,” Proceedings (CD), 86th

Annual Meeting of the Transportation Research Board, National Academy of Sciences, 

Washington, D.C., 2007. 

McKenzie, P. “Taking a Closer Look at Warm Mix,” Better Roads, Volume 76, Number 6, 

June 2006, pp. 64-69. 

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Ltd. and CSIR Transportek, South Africa, 1998. 

(http://asphalt.csir.co.za/foamasph/foamasph.pdf) 

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Number 2, Fall, 2005. 
57 

Newcomb, D., “An Introduction to Warm Mix Asphalt,” National Asphalt Pavement 

Association, Lanham, Maryland, 2006. (http://fs1.hotmix.org/mbc/Introduction_to_Warmmix_Asphalt.pdf) 

Prowell, B., and G. Hurley, “Expect Cooler Temperatures: Ongoing Research at NCAT Is 

Proving Warm-Mix Asphalt Is a Clear and Ongoing Option in the U.S.,” Roads and Bridges, 

Volume 43, Number 2, February 2005, pp. 4. 

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NCAT Test Track,” Proceedings (CD), 86th Annual Meeting of the Transportation Research 

Board, National Academy of Sciences, Washington, D.C., 2007. 

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9, 2005 (http://www.roadsbridges.com/Starting-to-Warm-article6274). 

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Performance of Hot Mix Asphalt,” European Roads Review, ISSN 1763-3087, Paris, France, 

2004, pp. 20-29 (http://cat.inist.fr/?aModele=afficheN&cpsidt=17303194). 

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Performance of Hot-Mix Asphalt,” Transportation Research Record 1962, Journal of the 

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als.pdf), accessed January 2007. 

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Open-Graded Mixes with Polymer-Modified Medium Setting Emulsions,” Proceedings, 3rd

Eurasphalt & Eurobitume Congress, Vienna, Austria, 2004a, pp. 560-567. 

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Proceedings, 9th International Conference on Asphalt Pavements, Copenhagen, Denmark, 

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58 

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D.C., 2007. 

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Inc., Des Plaines, Illinois, February 2006. 
59 

WEBSITES LISTED

http://www.sasolwax.com/Applications.html 

http://www.aspha-min.com 

http://www.sasolwax.com/www_sasobit_de.html 

http://www.evotherm.com 

http://www.wamfoam.com 

http://www.romonta.de/ie4/english/romonta/i_wachse.htm 

http://www.eng.auburn.edu/center/ncat/ 

http://www.warmmixasphalt.com/ 

http://www4.trb.org/trb/crp.nsf/All+Projects/NCHRP+9-43 

http://www.or.is 

http://www.pavementpreservation.org/library/getfile.php?journal_id=735 

http://www.aapa.asn.au/content/aapa/download/AdvisoryNote17.pdf 

http://www.fhwa.dot.gov/pavement/asphalt/wma.cfm 

http://www.ust.is/Mengunarvarnir/Graent_Bokhald/ 

http://www.enr.construction.com/products/newproducts/archives/030428.asp 

https://www.sasolwax.com/data/sasolwax_/Bitumen%20Modification/Roads%20and%20trial

s.pdf 

http://www.roadsbridges.com/Starting-to-Warm-article6274

61 

APPENDIX A 

TxDOT EVOTHERM WARM-MIX TRIAL 

SAN ANTONIO DISTRICT—2006 

63 

TxDOT EVOTHERM WARM-MIX TRIAL ON LOOP 368, 

SAN ANTONIO DISTRICT—2006 

Background

This project represents the first warm-mix asphalt trial placed by the Texas 

Department of Transportation. Evotherm, developed by MeadWestvaco Asphalt 

Innovations, Charleston, South Carolina, uses a non-proprietary technology that is based on a 

chemical package that includes emulsification agents; additives to improve aggregate 

coating, mixture workability, and compaction; as well as adhesion promoters (anti-stripping 

agents). The product enhances mixture workability, while lowering mixing temperatures to 

as low as 200°F. No plant modifications are required; the mix can be stored in silos and may 

be utilized with or without polymer modifier. 

Objectives

The objectives of TxDOT in conducting this field trial include the following: 

• to evaluate the production, placement, and compaction of warm mix as compared 

with a conventional hot mix control using a standard TxDOT mixture design, and 

• to evaluate the short- and long-term performance of the warm mix versus a control 

hot mix. 

Project Description

This project was in Bexar County within the city limits of San Antonio as shown in 

Figure A1. The project is located on Loop 368 (Old Austin Highway) and is a four-lane 

roadway divided by a median with curb and gutter and many businesses along each side. 

The existing pavement (prior to placement of the warm mix and control) consisted of 

a cold-milled asphalt surface that had been seal coated with AC-15P and a Grade 4 precoated 

aggregate. The seal coat had been under traffic for about a month prior to the overlay. 

All of the paving for this project was conducted at night, and a description of the 

paving sequence and locations is described in the following sections. 
64 

 

Figure A1. San Antonio Evotherm Field Trial Location and Layout. 

Day 1 of Field Trial (Warm Mix) 

Mixture: Warm Mix (produced by Vulcan Materials) 

Placed: Began PM of Aug 30, 2006, and ended in the AM of Aug 31, 2006 

Quantity: 1202.81 tons 

Limits: Northbound and Southbound, 2 lanes in each direction, 

 Station 10+50 to Station 33+00 

Day 2 of Field Trial (Control) 

Mixture: Control Mix (produced by Vulcan Materials) 

Placed: Began and ended in the PM of Aug 31, 2006 

Limits: Northbound and Southbound, 2 lanes in each direction, 

 Station 33+00 to Station 52+00 

 

Day 3 of Field Trial (Warm Mix) 

Mixture: Warm Mix (produced by Vulcan Materials) 

Placed: Began PM of Aug 30, 2006, and ended in the AM of Aug 31, 2006 

Quantity: 697.81 tons 

Limits: Southbound Inside Lane, Begin Station 73+00, End Station 52+00 

Northbound Inside Lane, Begin Station 52+00, End Station 70+00 
65 

The warm and control mixes were produced by Vulcan Materials of San Antonio and 

placed by Dean Word Company of New Braunfels. Researchers also noted that the field 

trials were placed within the limits of a much larger HMAC paving project (CSJ 0016-08-

027) that was both produced and placed by Dean Word Company. 

Mixture Design 

The control and warm mixtures met the gradation requirements of a TxDOT Item 

341, Type C, dense-graded HMAC. The mixture designs were performed by Vulcan 

Materials laboratory. The asphalt used for the control HMAC was Valero PG 76-22. The 

base asphalt for the warm mix started as a Valero PG 64-22 prior to modification. Ergon 

modified and emulsified the asphalt using chemistry supplied by MeadWestvaco. Once 

modified, the warm-mix binder met the specifications of PG 76-22 (see Table A1). The 

modified asphalt was then emulsified and provided to Vulcan Materials laboratory to perform 

the mixture design. Two aggregate sources were used for the mixtures: Vulcan’s Helotes Pit 

limestone and the Harris Pit field sand. Aggregate properties are as shown in Table A2. Note 

that the same aggregate sources and gradations were used for both the warm mix and the 

control. 

Both warm and control mixtures were designed using a Texas gyratory compactor 

with a target density of 96.5 percent. Tables A3 and A4 present mixture design information. 
66 

Table A1. Valero PG 76-22 Base Asphalt Superpave Grading Information. 

SUPERPAVE BINDER SPECIFICATION M-320

Customer Information: Paragon Information:

 

Company Name: Mead Westvaco Project Number: 2006-21

Address: Sample Identification:

City/State/Zip: Technician Identification: AGW

Contact Person: TOM GIRARDEAU Report Number:

Sample Description: 2% NX-1181 LATEX Date Tested: 5/11/06

Sample Identification: ZW361095 Date Issued: 

Sample Condition: Notes: 24 hour 80C Evap. Residue

Date Received: 

 ORIGINAL BINDER T240 

RESIDUE R 28 RESIDUE

Phase Angle:

T 28 Flash: 

T 316 @135C 1460.00

T 316 @163C Wt. Loss, % 

T 316 @190C

 T 315 T 315 T 315 T 313 T 313 T 314

Temp. o

C

G*/sin δ 

1.00 kPa 

min.

G*/sin δ 

2.20 kPa min. Temp. o

C

G* sin δ 

5.00 MPa 

max.

Temp. o

C

Stiffness, S 

300 MPa 

max

Slope, m 

0.300 min.

Strain, % 

1.0% min. 

40.00 37.00 12.00

46.00 34.00 6.00

52.00 31.00 1.843 0.00

58.00 28.00 -6.00

64.00 26.50 -12.00 126.0 0.312 

67.00 25.00 -18.00 

70.00 22.00 -24.00

76.00 1.210 2.735 19.00 -30.00

82.00 0.638 16.00 -36.00

88.00 13.00 -42.00

P/F 77.80 P/F P/F

TG TG TG

Perf. Grade: 76-22 Certified by Andrew Menapace, Group Leader, Asphalt and Testing

True Grade:
67 

Table A2. Aggregate Properties (from Mix Design Reports). 

VMC - 

Helotes 

VMC – 

Helotes 

VMC – 

Helotes 

VMC – 

Helotes 

F. 

Harris 

Property Test Spec 

Reqmt. 

“C” 

Rock 

“D” Rock “F” Rock Mfg 

Sand 

Field 

Sand 

Decantation, % Tex-217-F 1.5 max 0.2 0.4 0.1 0.3 0.2 

Deleterious 

Material, % Tex-217-F 1.5 max 0.0 0.0 0.0 0.0 0.0 

Surface Agg. 

Class 

Tex-438-A 

Tex-612-J B min B B B B n/a 

Magnesium 

Sulfate 

Soundness, % 

Tex-411-A 30 max 7.7 10.2 10.2 n/a n/a 

LA Abrasion, 

% 

Tex-410-A 40 max 25.3 26.8 26.8 n/a n/a 

Crushed Face 

Count Tex-460-A 85 min 100 100 100 100 n/a 

Micro Deval, 

% 

 Information 

Only 19.6 18.7 19.6 29.9 n/a 

Combined Aggregate 

Sand 

Equivalent, % Tex-203-F 45 min 91 91 91 80 82 
68 

Table A3. Mix Design Information for Evotherm Warm Mix. 

Bin No. 1 Bin No. 2 Bin No. 3 Bin No. 4 Bin No. 5 

Aggregate 

Source 

VMC - 

Helotes 

VMC – 

Helotes 

VMC – 

Helotes 

VMC – 

Helotes 

F. Harris 

Aggregate 

Description “C” Rock “D” Rock “F” Rock 

Manufactured 

Sand 

Field 

Sand 

Individual 

Bin 

Percentage 

21% 11.0% 28.0% 28.0% 12.0% Total 

Sieve Size % Passing % Passing % Passing % Passing % Passing % Passing 

1” 100.0 100.0 100.0 100.0 100.0 100.0 

3⁄4” 99.9 100.0 100.0 100.0 100.0 100.0 

3/8” 20.6 64.6 99.9 100.0 100.0 79.4 

No. 4 6.5 26.0 44.6 99.0 100.0 56.4 

No. 8 0.9 6.3 10.3 79.0 99.9 37.9 

No. 30 0.7 0.5 0.4 44.2 95.1 24.1 

No. 50 0.6 0.5 0.3 20.5 71.4 14.6 

No. 200 0.3 0.4 0.3 2.9 8.5 2.0 

Asphalt Source & 

Grade: Valero PG 76-22 * Residual 

Binder Percent, (%): 4.2 Asphalt Spec. Grav.: 1.043 

*Note the base asphalt was Valero PG 64-22 but was modified by 

MeadWestvaco to a PG 76-22 prior to emulsification. 

Antistripping Agent: - Percent, (%): - 

 

VMA at Optimum 13.0 

Mixture Evaluation @ Optimum Asphalt Content 

Hamburg Wheel Tracking Test 

Indirect Tensile 

Strength (psi) Number of 

Cycles Rut Depth (mm)

60.4 

5000 

6577 

7.86 

12.5 

Remarks: Mixed at 240°F. Cured for 2 hours and compacted at 240°F. Hamburg specimens 

remained at room temperature overnight prior to testing. 
69 

Table A4. Mix Design Information for Control Hot Mix. 

Bin No. 1 Bin No. 2 Bin No. 3 Bin No. 4 Bin No. 5 

Aggregate 

Source 

VMC - 

Helotes 

VMC – 

Helotes 

VMC – 

Helotes 

VMC – 

Helotes 

F. Harris 

Aggregate 

Description “C” Rock “D” Rock “F” Rock 

Manufactured 

Sand 

Field 

Sand 

Individual 

Bin 

Percentage 

21% 11.0% 28.0 28.0% 12.0% Total 

Sieve Size % Passing % Passing % Passing % Passing % Passing % Passing 

1” 100.0 100.0 100.0 100.0 100.0 100.0 

3⁄4” 99.9 100.0 100.0 100.0 100.0 100.0 

3/8” 20.6 64.6 99.9 100.0 100.0 79.4 

No. 4 6.5 26.0 44.6 99.0 100.0 56.4 

No. 8 0.9 6.3 10.3 79.0 99.9 37.9 

No. 30 0.7 0.5 0.4 44.2 95.1 24.1 

No. 50 0.6 0.5 0.3 20.5 71.4 14.6 

No. 200 0.3 0.4 0.3 2.9 8.5 2.0 

Asphalt Source & 

Grade: Valero 76-22 Binder Percent, (%): 4.8 Asphalt Spec. 

Grav.: 1.052 

 

Anti-stripping Agent: Pre-Tech Pave Grip 

400 Percent, (%): 0.75 

 

VMA at Optimum 14.2% 

Mixture Evaluation @ Optimum Asphalt Content 

Hamburg Wheel Tracking Test 

Indirect Tensile 

Strength (psi) Number of 

Cycles Rut Depth (mm)

173 20,000 12.5 
70 

Mixture Production 

Both the warm mix and control were produced at Vulcan Materials plant located in 

San Antonio on Loop 1604. The plant was a parallel flow Astec with external coater. 

The emulsion was pumped from the tanker trucks into the end of the drum through 

the regular plant metering system. 

Production rate for the warm mix was about 190 tons per hour (conventional hot mix 

for this plant is around 250 tons per hour). The production rate was less than expected and 

was due to high moisture content in the aggregate stockpiles (primarily the field sand) from a 

rain that occurred the day prior to the first night of WMA production. The combined 

stockpile moisture content ranged from 4.8 to 5.2 percent. (Normal stockpile moisture 

content for these aggregates was between 3 and 4 percent). The limit on the production rate 

was due to problems with the external coater motor. It would have been desirable to have a 

mix discharge temperature of 200°F, but at temperatures below 220°F, the plant started 

having trouble with the coater (asphalt too viscous), causing the motor to trip. Amperages 

were about 20 percent higher with the warm mix. The drag chain, which transfers the mix up 

into the silos, was operating normally. 

On the average, the fuel consumed was the same for the warm mix as for the hot mix. 

No reduction in fuel use was observed for the warm mix because of the high moisture content 

in the aggregates. There were no moisture problems in the baghouse—lots of steam was 

observed, but bags were not plugged up or caked over. 

For the three nights that the test mixes were produced, the plant started producing mix 

around 7:00 pm and began shipping mix out at around 9:00 pm. About 180 tons of mix were 

stored during this first 2 hours of production but after the first 180 tons of mix, there was no 

need to silo mix. 

Temperature of the warm mix at the time of loading into the haul trucks was 220°F. 

Warm-mix plant samples were compacted in the laboratory under three different 

curing conditions: 

• no cure, 

• curing for 2 hours at 200°F, and 

• curing for 2 hours at 240°F. 
71 

Table A5 shows these results. Averages of all sublots are summarized in Figure A2 for 

each curing condition. Based on these data, the density of the samples which were cured, 

whether at 200°F or 240°F, were no different than those that were not cured. The density of 

the compacted warm-mix specimens were comparable to the control. The control HMAC 

was compacted at 300°F, and all samples were compacted in the Texas gyratory compactor. 

Hamburg and Overlay Tests on Plant Produced/Lab Compacted Specimens 

Samples of the loose warm mix and hot mix were also sent to TxDOT’s Construction 

Division Laboratory in Austin. These warm-mix samples were reheated and compacted to 93 

percent density using the SGC at two different temperatures: 240°F and 300°F. The control 

samples were compacted to 93 percent density in the SGC at 300°F. These samples were 

subjected to Hamburg and overlay testing, and the results are shown in Tables A6 and A7. 

The Hamburg test is used by TxDOT to measure the moisture susceptibility and 

rutting potential of HMA layers in Texas. During the test, two 2.5 inch high by 6-inch 

diameter HMA specimens are loaded at 122°F to characterize their rutting properties. The 

samples are submerged in a water bath and loaded with steel wheels. 

The test loading parameters for the Hamburg test were as follows: 

• Load: 705 N (158-lb force) 

• Number of passes: 20,000 

• Test condition/temperature: Under water at 122°F (50°C) 

• Terminal rutting failure criterion: 0.5 inch (12.5 mm) 

• HMAC specimen size: 6 inch diameter by 2.5 inch high
72 

Table A5. Plant-Produced Warm-Mix Samples Compacted after Different Curing 

Conditions Compared to the Plant-Produced Hot-Mix Compacted Specimens. 

Mixture Sample Description Asphalt 

Content 

(Ignition 

Method), 

% 

Maximum 

Specific 

Gravity 

(Gr) 

Bulk 

Specific 

Gravity 

(Ga) 

Voids in 

Mineral 

Aggregate 

(VMA) 

Density, 

% 

Start up sample 

(Day 1) 

4.6 2.408 2.359 12.5 98.0 

Sampled at 250 tons 

(Day 1, Lot 1, Sublot 1) 

4.6 2.441 2.377 13.1 97.4 

Sampled at 750 tons 

(Day 1, Lot 1, Sublot 2) 

4.6 2.423 2.368 13.3 97.7 

Day 3, Lot 2, Sublot 1 4.6 2.439 2.321 15.1 95.1 

Warm Mix: 

No Cure 

Average 4.6 2.428 2.356 13.5 97.1 

Start up sample 

(Day 1) 

4.6 2.419 2.349 13.3 97.1 

Sampled at 250 tons 

(Day 1, Lot 1 Sublot 1) 

4.6 2.438 2.375 13.1 97.4 

Sampled at 750 tons 

(Day 1, Lot 1 Sublot 2) 

4.6 2.430 2.371 12.9 97.6 

Day 3, Lot 2 Sublot 1 4.6 2.442 2.325 15.0 95.2 

Warm Mix: 

Cured for 

2 hours at 

200°F 

Average 4.6 2.432 2.355 13.6 96.8 

Start up sample 

(Day 1) 

4.6 2.423 2.346 13.5 96.8 

Sampled at 250 tons 

(Day 1, Lot 1, Sublot 1) 

4.6 2.458 2.390 13.3 97.3 

Sampled at 750 tons 

(Day 1, Lot 1, Sublot 2) 

4.6 2.432 2.365 13.2 97.2 

Day 3, Lot 2, Sublot 1 4.6 2.440 2.334 14.6 95.7 

Warm Mix: 

Cured for 

2 hours at 

240°F 

Average 4.6 2.438 2.359 13.7 96.8 

Start up sample 4.6 2.424 2.370 12.6 97.8 

Day 2, Sublot 1 4.7 2.460 2.390 13.5 97.2 

Day 2, Sublot 2 4.5 2.471 2.371 14.2 96.0 

Control 

Hot Mix 

Average 4.6 2.452 2.377 13.4 97.0 
73 

Lab-Compacted Specimens

90

91

92

93

94

95

96

97

98

99

100

Warm Mix - No

Cure

Warm Mix - 200°F

Cure

Warm Mix - 240°F

Cure

Control HMAC

Density, Percent

Figure A2. Compacted Laboratory Density after Different Curing Conditions for 

Warm Mix Compared to HMAC (Compacted in Texas Gyratory Compactor). 

Generally, the warm-mix samples compacted at 300°F performed better in the 

Hamburg than those compacted at 240°F. All of the warm-mix samples failed the Hamburg 

test with the exception of the samples compacted at 300°F from the second night of 

warm-mix production. 

The TTI overlay tester shown in Figure A3 is used to measure the reflection cracking 

potential of HMA surface layers in Texas. The test loading parameters for the overlay tester 

were as follows: 

• Loading: Cyclic triangular displacement-controlled 

waveform at 0.025 in (0.63 mm) 

• Loading rate: 10 seconds per cycle 

• Test temperature: 77°F (25°C) 

• Terminal cracking failure criterion: 300 load cycles (for surface mixes) 

• HMAC specimen size: 6 inch total length by 3 inch width by 1.5 inch 
74 

Table A6. Hamburg Test Results from Laboratory-Molded Samples. 

Hamburg Results, Lab Molded Samples 

Plant Mix 

Description Sample 

Molding 

Temp, 

°F 

Rut at 

5K, mm 

Rut at 

10K, 

mm 

Rut 

at 

15K, 

mm 

Rut at 

20K, 

mm Passes 

1 

2 

7.8 7400 

3 

4 

240 

6.4 9700 

1 

2 5.9 12.2 10,500 

3 

Day 1 Warm Mix 

Sampled at 750 

Tons, Lot 1, 

Sublot 2 

4 

300 

9.1 8700 

1 

3 

9.6 6500 

2 

4 

240 

9.7 6600 

1 

2 

4.2 10.7 11,300 

3 

Day 1 Warm Mix 

Sampled at 250 

Tons, Lot 1, 

Sublot 1 

4 

300 

4.3 8.1 14,700 

2 

3 

2.3 3.6 5.1 7.0 20,000 

4 Day 2 Control Mix 

5 

300 

3.3 5.5 7.8 10.3 20,000 

1 

2 

3.7 9.2 13,001 

3 

7 

240 

5.2 10.7 10,701 

6 

7 

2.6 3.2 4.2 5.6 20,000 

4 

Day 3 Warm Mix 

Sampled at 250 

Tons, Lot 2 

5 

300 

1.5 2.8 4.1 5.2 20,000 
75 

Table A7. Overlay Test Results on Laboratory-Compacted Specimens. 

Max load Final**

(lbs)* Load (lbs)

1 582.9 39.8 93.2 12

2 577.2 30.9 94.6 21

1 646.4 34.6 94.6 3

2 600.9 38.8 93.5 7

1 575 38.1 93.4 21

2 631.3 43.5 93.1 21

1 838.2 58.4 93 77

2 795.7 54.8 93.1 7

1 877.1 59.7 93.2 8

2 851.6 57.4 93.3 12

1 627.7 43.5 93.1 11

2 646.2 44.6 93.1 36

1 650.9 44.1 93.2 6

2 661.6 41.4 93.7 2

3 628.4 42.4 93.3 6

Day 3 Warm Mix 

Sampled at 250 

Tons, Lot 2 

Day 2 Control Mix 300

240

300

% Decline Cycles

240

Overlay Results

Mix Description Sample

300

Molding Temp

Day 1 Warm Mix 

Sampled at 250 

Tons, Lot 1 

Sublot 1

240

300

Day 1 Warm Mix 

Sampled at 750 

Tons, Lot 1 

Sublot 2

* Max Load is the load associated with the initial test cycle. 

** Final Load is the load associated with the last test cycle. 

Figure A3. TTI Overlay Tester. 

The overlay tester was developed to judge a mixture’s resistance to thermally induced 

reflection cracking. However, mixes that pass this test will also have good fatigue resistance. 

The warm-mix laboratory-compacted mixes did poorly in this test, as did the control mix. 
76 

Newly developed criteria for TxDOT mixes recommend that standard mixes should last a 

minimum of 300 cycles, whereas crack-resistant overlays should last more than 750 cycles. 

The overlay test results seen here for both the warm mix and the control mix are 

typical of many current TxDOT dense-graded mixes. This has been a result of several 

factors including: 

• the move to stiffer binders, and 

• general reduction in asphalt concrete (AC) content. 

This has been recognized by TxDOT, and the overlay tester described above has been 

proposed to address this issue. 

Placement and Compaction

The mix was loaded into belly dump trucks, which were all tarped and insulated. The 

mix was hauled a distance of 20 miles (about 25 minutes) to the jobsite. A remixing 

windrow elevator (Lincoln model 660 AXL) was used to transfer the mix into a Barber 

Greene 260B paver. 

The paver had a vibratory screed, which was on during the paving. One observation 

by the paving contractor was that “we normally heat the screed once about 20 minutes before 

starting, but for the warm mix, we had to relight the burners on the screed every 8 to 10 loads 

because the mat started to tear.” Also, he observed that the angle of attack on the screed was 

doubled what is typically seen on hot mix. 

The compacted mat thickness was about 2 inches. Both the control and warm mix 

were compacted using the same roller pattern: two passes with a vibratory roller, one pass in 

static mode, then between four and six passes with pneumatic roller. 

The vibratory roller was a Hamm HD120 operated at low amplitude and a frequency 

of about 2500 Hz (midrange). 

The pneumatic roller was Ingersall Rand, PT 240N, (24,000 lb, 8 wheels). The tires 

were bias ply with a tire pressure of 50 psi. 

Portions of the overlay were opened to traffic within 2 hours of placement. 

Asphalt concrete placement testing as reported by Arias and Associates is shown in 

Table A8. 
77 

Table A8. Asphalt Concrete Placement Data. 

Mixture Mix Delivery 

Temperature 

Placement 

Temperature 

Average Nuclear 

Density, % 

Warm Mix (Day 1 – 

Aug. 30, 2006 

180°F to 200°F 170°F to 180°F 93.5 to 95.0 

Control Mix (Day 2 – 

Aug. 31, 2006) 

315°F 305°F 94.2 

Warm Mix (Day 3 – 

Sept. 7, 2006) 

220°F 205°F to 210°F 92.1 to 93.8 

Evaluation of 1-Month Road Cores 

Cores were obtained 1 month after the warm and control mixes were placed and were 

sent to TxDOT’s Construction Division in Austin for testing. Table A9 tabulates the results 

of these tests. 

The Ga in Table A9 represents the bulk specific gravity of the mix, and the Gr 

represents the maximum specific gravity. The maximum specific gravity values shown in 

Table A9 are based on averages values associated with the respective production lot shown in 

Table A5. 

Core samples taken for the indirect tensile strength tests included three samples taken 

from the wheel path and three samples taken from between the wheel paths for each lot. 

Averages of the densities of these cores are summarized in Figure A4. Indirect tensile 

strengths are summarized in Figure A5. Any mix tenderness, binder softening, or insufficient 

curing that one may expect to be associated with the warm mix could be reflected with 

increased densities in the wheel path after 1 month of trafficking. However, the densities in 

the wheel path are less than the densities between the wheel paths for both the warm mix and 

the control mix. Note in Table A9 (Indirect Tensile Strength Cores) that the cores taken in 

the wheel paths are generally thicker than the cores taken between the wheel path. This may 

be an indication that the wheel paths may have been rutted prior to overlay (although the 

surface had been cold milled). This variation in mat thickness will lead to a differential 

compaction effort resulting in the roller applying more effort on the higher points of the 

pavement or, in this case, between the wheel paths. 
78 

Table A9. Test Results from Roadway Cores. 

Core Description Rut at 5K, mm Rut at 10K, mm Rut at 15K, mm Rut at 20K, mm Total Passes Ga Gr Density, %

1a 2.307 94.3

1b 2.313 94.6

2a 2.236 92.1

2b 2.272 93.6

1a1 2.306 93.5

1a2 2.275 92.3

1b1 2.265 91.8

1b2 2.287 92.7

Max load Final

(lbs) Load (lbs) Original Trimmed

2c 626.4 43.6 93.0 638 2.252 2.243 92.4

2d 554.8 38.8 93.0 224 2.242 2.240 92.3

1c 696.0 47.9 93.1 41 2.317 2.317 94.7

1d 783.0 52.6 93.3 118 2.312 2.313 94.6

2.446

2.428

2.466

2.428

Gr Density, %

2.446

Ga

Overlay Results

Warm Mix, Lot 1 sublot 2 

(15+60 OL) (7'Offset South Bond)

Warm Mix, Lot 1 sublot 1 , 1st 

night 

Core % Decline Cycles

Control Mix

Control Mix 4.5 5.7 7.5

Description

6.8 9.1

Hamburg Results

Warm Mix, Lot 1 sublot 1, 1st 

night (15+00 CL 6'offset CL)

Warm Mix, Lot 1 sublot 2 

(15+60 OL 7' Offset South Bond)

8.2

7.3

6401

10.7

7901

20000

14601

Core Description Diam (in) Ht (in) Load (lb) Strength (psi) Ga Gr Density, %

1a 5.9 1.9 2254 128.1 2.248 92.2

1b 5.9 1.8 2269 136.1 2.248 92.2

1c 6.0 1.8 2085 123.0 2.255 92.5

1a(wp) 6.0 2.0 2285 121.3 2.240 91.9

1b(wp) 5.9 2.0 2247 122.5 2.247 92.2

1c(wp) 5.9 2.0 2260 122.6 2.234 91.7

2a1d 5.9 2.0 2842 153.4 2.331 95.5

2b1e 5.9 2.0 3293 177.8 2.320 95.1

2c1f 6.0 2.0 2874 152.5 2.331 95.5

2a(wp) 6.0 2.4 3422 151.4 2.287 93.7

2b(wp) 5.9 2.4 3433 154.4 2.284 93.6

2c(wp) 5.9 2.5 3607 155.8 2.279 93.4

3a 5.9 1.7 2266 143.9 2.258 91.6

3b 5.9 1.7 2420 153.7 2.273 92.2

3c 5.9 1.7 2552 162.1 2.255 91.4

3a(wp) 5.9 1.7 3079 195.5 2.322 94.2

3b(wp) 5.9 1.8 3178 190.6 2.310 93.7

3c(wp) 5.9 1.8 3031 181.8 2.319 94.0

2.437

2.440

2.466

Day 1 Warm Mix 

Day 3 Warm Mix 

Day 2 Control Mix

Indirect Tensile Strength
79 

One Month Cores

90

91

92

93

94

95

96

97

98

99

100

Warm Mix Lot 1 Warm Mix Lot 2 Control HMAC

Average Core Density, %

Wheel Path Between Wheel Path

Figure A4. Average Core Densities in the Wheel Paths and between the 

Wheel Paths for Warm and Control Mixes. 

0

20

40

60

80

100

120

140

160

180

200

Warm Mix

(Lab Mix

Design)

Warm Mix 

Lot 1 - 1 Month

Cores

Warm Mix 

Lot 2 - 1 Month

Cores

Control (Lab

Mix Design)

Control HMAC

- 1 Month

Cores

Indirect Tensile Strength, psi

Figure A5. Indirect Tensile Strength of Road Cores for Warm Mix and 

Control Mix Compared to Values Obtained from the Mixture Design. 
80 

Indirect Tensile Strength Test Results for Road Cores

Results from the indirect tensile strength tests are shown in Table A9. Comparing the 

tensile strength of the cores in the wheel paths to those between the wheel paths indicate no 

significant difference. Average tensile strengths for each lot are shown in Figure A5 and are 

compared to the tensile strength of the lab-molded sample tested during the mix design 

process. The tensile strengths of the warm-mix cores taken at 1 month show a significant 

improvement over the tensile strength of the warm mix during the mix design process. 

Hamburg Test Results for Road Cores

Hamburg test results for the roadway cores are compared to the lab-molded plant mix 

samples in Figure A6. As mentioned previously, some improvement is observed in the 

warm-mix samples compacted at 300°F versus those compacted at 240°F. However, the 

warm-mix cores taken at 1 month did not indicate that the mix improved with time (in terms 

of rut or moisture susceptibility resistance) compared to the lab-molded samples. 

0

2,000

4,000

6,000

8,000

10,000

12,000

14,000

16,000

18,000

20,000

Warm Mix (Lab Mix Design)

Warm Mix, Lot 1, Sublot 1

Warm Mix, Lot 1, Sublot 2

Warm Mix, Lot 2

Warm Mix, 1-Month Cores

Warm Mix, 1-Month Cores

Control (Lab Mix Design)

Control Plant Mix

Control Plant Mix

Control Mix, 1-Month Cores

Control Mix, 1-Month Cores

Hamburg Passes

240°F Molding Temp 300°F Molding Temp

Warm Mix Control HMAC

Figure A6. Hamburg Wheel Tracking Test Results for Lab Molded Warm Mix 

and Control Mixes Compared to 1-Month Roadway Cores. 
81 

Overlay Test Results for Road Cores

Overlay test results for the roadway cores are compared to the lab-molded plant mix 

samples in Figure A7. All of the lab-molded warm-mix and control HMAC specimens 

performed poorly in the overlay test. However, there was a significant improvement seen in 

some of the cores taken at 1 month from the warm-mix sections. There may be a difference 

in density associated with the cores as compared with the lab-compacted specimens. The lab 

specimens were compacted to 93 percent density, and the warm-mix road cores ranged from 

92.3 to 94.6 percent density. This difference in density would not account for the 

improvement seen in the overlay test road cores. This difference would indicate there is a 

“curing” effect that is occurring with time, providing for improved cracking resistance. 

0

50

100

150

200

250

300

350

400

450

500

550

600

650

Warm Mix, 

Lot 1, Sublot 1

Warm Mix, 

Lot 1, Sublot 2

Warm Mix, Lot

2

Warm Mix 1-

Month Cores

Control HMAC

Overlay Test Cycles to Failure

240°F Molding Temp 300°F Molding Temp

Figure A7. Overlay Test Results for Lab-Molded Warm-Mix and 

Control Mixes Compared to 1-Month Roadway Cores. 

Summary 

 TxDOT placed their first warm-mix asphalt trial using the Evotherm process on Loop 

368 in the San Antonio District in August/September 2006. All test sections are performing 

well at this time. TxDOT is still in the process of evaluating the short- and long-term 

performance of this field trial through field cores and performance monitoring. Preliminary 

findings based on information documented thus far include the following: 
82 

Mix Design

Both warm and control mixes were designed according to Item 341, Type C 

dense-graded mixes, which employ the use of the Texas gyratory compactor. Both 

mixes were designed to a target density of 96.5 percent. The control HMAC had an 

optimum asphalt content of 4.8 percent, and the warm-mix optimum asphalt content 

(residual binder) was 4.2 percent. The control mix was produced with a PG 76-22 

binder, and the warm mix (after the Evotherm modification) was also a PG 76-22. 

The same aggregate source (predominantly crushed limestone) and gradation was 

used for both warm and control mixes. Also, both warm and control mixes were 

produced in the same asphalt plant. 

Production

• The warm mix was produced at a temperature of 220°F, and the control mix was 

produced at 320°F. 

• No reduction in fuel consumption was observed for the warm-mix production, 

which is attributed to a heavy rain prior to production that caused the aggregate 

stockpiles to be excessively wet requiring more energy for plant operation. 

• Warm mix was stored in silos for a maximum of 2 hours prior to load out. 

Quality Control

WMA samples were compacted in the field laboratory to densities averaging 97 

percent which was the same as the control HMAC compacted densities. To evaluate 

the effect of laboratory curing on the warm mix, samples were compacted after three 

curing conditions: no cure, cure for 2 hours at 200°F, and cure for 2 hours at 240°F. 

The different curing conditions had no effect on compacted density. 

Placement and Compaction 

• The warm and control mixes were placed over the course of three nights. The 

warm mix was placed at a temperature ranging from 170°F to 210°F. Nuclear 

density tests on the warm mix ranged from 92.1 to 95 percent. The control mix 

was placed at 305°F, and nuclear density tests averaged 94.2 percent. 

• The same roller pattern was used for both control and warm mixes. 

• No problems were observed with the placement and compaction operation. 
83 

• Traffic was allowed onto the warm mix in some areas as soon as 2 hours after 

placement. 

Laboratory Testing on Lab-Molded Samples and Roadway Cores 

• Density of roadway cores taken after 1 month of traffic showed an overall average 

density of the warm mix (based on 26 cores) of 93.3 percent. Average density of 

the roadway cores from the control mix (based on four cores) was 92.6 percent. 

• Density of WMA roadway cores taken in the wheel paths was compared with 

those taken between the wheel paths. There is no indication that the warm mix is 

densifying further under the action of traffic (after 1 month). 

• For laboratory testing in the Hamburg and overlay tester, warm-mix samples were 

compacted at two different temperatures: 240°F and 300°F. The warm-mix 

samples compacted at 300°F performed better in the Hamburg than those 

compacted at 240°F. All of the warm-mix samples failed the Hamburg test 

requirements (of no more than 12.5 mm rut depth at 20,000 passes) with the 

exception of the samples compacted at 300°F from the second night of warm-mix 

production. The warm-mix cores taken at 1 month also failed the Hamburg 

requirements. The control mix generally passed the Hamburg test with the 

exception of one set of field cores. Although the test results indicate the potential 

for rutting or stripping, these problems have not yet been evident in the field. 

• Indirect tensile strength tests were performed during the mix design and on 

roadway cores for both warm and control mixes. The control mix had tensile 

strengths of around 170 psi (both mix design and roadway cores). During the mix 

design process, the warm mix only had a tensile strength of 60 psi; however, the 

warm-mix roadway core tensile strengths ranged from 121 to 178 psi. 

• Overlay test results for the roadway cores were compared to the lab-molded plant 

mix samples. All of the lab-molded warm-mix and control HMAC specimens 

performed poorly in the overlay test. However, there was a significant 

improvement seen in the some of the cores taken at 1 month from the warm-mix 

sections. 

Future Work

• Cores were taken at 3 months and are in the process of being tested at TTI. 

• The district also plans to take cores at 1 year. 

• Performance monitoring will continue over the next year.