With environmental emissions laws forever being tightened and fuel prices at record highs the time may be right for cold and warm mix materials. Cold mixes using emulsions are well established the new class of mixes, warm mix asphalt was reported by Wayne Jones of the Asphalt Institute in ''Asphalt'', the magazine of the Asphalt Institute, Vol 19, No 4, Autumn 2004.

The following is a summary of his report.

What is WMA ?

In WMA the viscosity of the asphalt binder is lowered at an given temperature, thus, WMA technology allows the mixing, transporting and lay-down of mix much as 37° lower than traditional methods. The gives the benfit of a reduction in fuel consumption and corresponding savings to dry and heat the aggregate.

Along with lower mix temperatures, WMA offers several additional benefits. These benefits include lowered emissions from the burning of fossil fuels, lower fumes and odor generation - both at the plant and the jobsite, and easier laydown and compaction operations. By reducing the difference between compaction temperature and ambient air temperature, this would also allow laying at lower ambient temperatures and extension of the laying season.

WMA was developed in Europe based on foamed bitumen technology, In Europe there is a significant drive for the reduction of greenhouse gas emissions.

Technology comes to the United States from Europe . There are several competing processes.

WMA Processes

The three most widely used technologies in Europe for producing

WMA are based on foam technology and organic thixotropic additives. It was noted that foam mix could be stockpiled for several months in the 80s by Mobil technologists in Australia.

Aspha-Min®, marketed in Europe by Eurovia, is a synthetic zeolite. This material is added as a fine powder during the mixing cycle at the asphalt plant to create a foaming effect in the binder. Chemically, zeolite is an aluminum silicate mineral with 18, resulting in an almost continuous foaming action. This foaming action of the liquid binder acts as an extender and lubricant enabling the mix to be workable at lower temperatures. Production temperatures can be reduced dramatically, down to the 130 - 45°C (266-293T) range.

WAM-Foam® (Warm Asphalt Mix Foam) 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. WAM-Foam is a joint venture product between Shell International Petroleum Company Ltd., London , U.K. , and Kolo-Veidekke, Oslo , Norway . In this process, an extremely soft binder component is mixed with the aggregate in the first stage at 100-120°C (212-250T) 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. This combination of soft binder and foaming of the hard binder acts to lower the viscosity to provide the necessary workability. This allows the mixture to be placed and compacted at 80-90°C (175-195T).

Thixotropic Organic additives. A low melting point organic additive that chemically changes the temperature-viscosity curve is added, this is based on wax additive developments in the 90s.. The two types of organic additives used successfully to date are a synthetic paraffin wax and a low molecular weight ester compound. Both additives melt at approximately 990C (2100F) and produce a reduction in the binder viscosity by providing excess liquid above their melting points. Blending 3 to 4 percent by weight allows a reduction in production temperatures of 18 to 540F. The paraffin wax is Sasobit®, a product of Sasol Wax , South Africa , a long-chained aliphatic hydrocarbon derived from coal gasification. The ester compound is Asphaltan B®, a product of Romonta GmbH, Germany , and is a byproduct of the toluene extraction of brown coal.