From Hot to Warm

Warm-mix asphalt (WMA) technologies offer the promise of enhancing the performance, construction efficiency, resource conservation, and environmental stewardship of asphalt mixtures. This article provides an overview of the history, related research activities, and benefits and concerns associated with the use of WMA. WMA processes and products use various mechanical and chemical means to either reduce binder viscosity at lower temperatures or reduce the shear resistance of the mixture at construction temperatures while maintaining or improving pavement performance. Manufacturers typically produce WMA in the 220-275 °F (104-135 °C) range, which is as much as 100° F (38°C) under hot mix asphalt (HMA) production temperatures. WMA technologies can improve construction quality, extend the construction season, and minimize negative impacts to the environment through lower energy consumption. Development of WMA technologies began in Europe in the late 1990s, and U.S. government agencies and asphalt-related associations have been conducting research on its potential since 2004. FHWA is working closely with State and industry partners to develop and monitor demonstration projects and research, and to advance the knowledge and state of practice of WMA materials and technologies. Several issues related to WMA need to resolved. Reducing the production temperature of HMA without the additional implementation of materials handling and production best practices might lead to incomplete drying of the aggregate, which could have negative implications for pavement performance. For WMA to succeed in the United States, engineers must be satisfied that WMA mixtures will be as strong and durable as current pavements. In addition, further research is needed to measure the degree of environmental improvement, fundamental mix characteristics, and impact on performance of the new technologies.