Hot mix asphalt (HMA), the most prevalent paving product, is generally fabricated by heating typically 5 % bituminous binder up to around 160°C (320°F) in order to decrease its viscosity and then blending it with 95 % of hot aggregate. The specifications for paving asphalt generally follow this application (see RoadMat Note 2). As a consequence, there is a significant environmental footprint deriving from the heating of the bitumen combined to the heating and drying of the aggregate. In addition, fumes are emitted generating the recognizable odour associated with the asphalt laying operations.

Even if worker exposures are likely to be within regulatory limits and are unlikely to be hazardous to health when processed at these temperatures [1], the industry decided to lower fume emissions and started in the late 1990s to investigate ways to reduce manufacturing temperature while maintaining performance. Given that additional benefits were rapidly identified, in particular a reduced environmental footprint, several additives or innovative processes rapidly developed and a new type of mixtures emerged, now called Warm Mix Asphalt (WMA). WMA are generally manufactured 30°C-50°C (80-120°F) lower than HMA. They are sometimes called half-warm mixes when the temperature falls below 100°C (212°F). As a result, almost 20% of the asphalt mixtures now produced in the USA are indeed WMA, when the figures are close to 10% in Europe.

There are several ways to reduce the manufacturing temperature (Table 1). Basically, there are two main routes, one based on adding water and the other based on adding an appropriate additive. Combinations of both also exist, with some technologies being based on a water-releasing agent such as a zeolite or using a bituminous emulsion. Some technologies that were proposed in the early stages of development, have also essentially disappeared such as the use of two grades of binder (sequential coating), the soft one acting as a lubricant for the harder one.