Warm Mix Asphalt

Warm mix asphalt (WMA) is asphalt mixture produced and placed at lower temperatures than conventional hot mix asphalt (HMA), using water-based foaming or additives that let the binder coat the aggregate and the mix compact at reduced temperature. It uses less fuel and produces fewer emissions than HMA.

How Warm Mix Differs from Hot Mix

In 2010, warm mix was typically produced at about 212 to 275°F (100 to 135°C), compared with about 275 to 350°F (135 to 175°C) for hot mix. The lower temperature is made possible by changing the production process, either by foaming the binder or by adding an additive; both reduce the temperature needed to mix and compact the material.

Production Methods

Foaming

A small amount of water is added to the hot binder. The water turns to steam and expands greatly in volume, foaming the binder and temporarily lowering the viscosity of the mix. Foaming can be produced in several ways; the most widely adopted is to inject water through a pump or nozzle system at the plant. How foaming works is described in Foamed Asphalt.

Additives

• Organic additives are typically waxes added to the binder to lower its viscosity (see Asphalt Additives).
• Chemical additives are typically surfactants, which improve adhesion between the binder and the aggregate.

Some additives are supplied as an emulsion in water and so also have a foaming effect on the binder.

Benefits

• Lower fuel use. Plants burn less fuel to heat the aggregate and binder. Since energy is a significant part of production cost, this reduces cost.
• Lower emissions. Lower mixing temperature reduces stack emissions at the plant as well as fugitive emissions and odor at the plant and paving site, which can help projects meet air quality standards.
• Longer working time. Warm mix cools more slowly than hot mix because there is a smaller difference between mix and air temperature. It stays workable longer in cool weather or with delays, can be hauled farther, and can extend the paving season.
• Compaction. Warm mix technologies reduce the mix's resistance to flow at a given temperature, so adequate compaction can be achieved at a lower temperature. In other words, warm mix has a lower cessation temperature than hot mix (see HMA Compaction and Cold Weather Paving).

Concerns and Production Issues

Engineers comparing warm mix with hot mix have looked at several possible issues:

• Less binder aging. Lower production temperatures mean less oxidation ("aging") of the binder during mixing. This could leave the binder softer and the pavement more prone to early rutting.
• Residual moisture. Aggregate may not dry completely at lower temperatures, leaving moisture in the mix.
• Consolidation. Questions have been raised about further consolidation of warm mix under traffic after construction, and how it affects density.
• Compactability. Whether warm mix reliably compacts at its intended temperatures continues to be monitored.

At the plant, burners can be harder to hold at a steady rate at reduced temperatures, and binder may not flow as readily. These problems are addressed with good plant maintenance, keeping production at normal rates, and raising the temperature slightly where needed (see HMA Plant Operations).

The same quality construction practices that apply to hot mix apply equally to warm mix.

Performance Evidence

Warm mix was first used in Europe; the first warm mix pavements there were built in the late 1990s. As of 2011, US use was too recent for full life-cycle evaluations, and agencies were relying on laboratory testing, test sections and accelerated loading. Findings reported at that time included:

• Virginia. A two-year comparison of several WMA technologies with an HMA control found the WMA sections performing comparably to HMA, and suggested that some WMA technologies might slow binder aging.
• Nebraska. A combined laboratory and field study found WMA rutting at the same rate as HMA, and predicted from laboratory data no failures over a 20-year service life.
• California. Accelerated loading with a heavy vehicle simulator showed somewhat higher moisture content and higher initial rutting in WMA than HMA. Moisture remained within reasonable limits, and rutting evened out over the longer term.
• Ohio. Accelerated loading showed WMA more prone to consolidate under heavy loads, though the effect on total consolidation was relatively small.
• Germany. Seven WMA test sections built between 1998 and 2001 were performing as well as HMA would be expected to.
• Norway. Rutting from studded tires is widespread, but there was no indication it affected WMA more than HMA.

Alabama and Missouri had also built WMA test strips and were following up with investigations of pavement quality. In both the California and Ohio accelerated loading studies, the WMA sections were of similar quality to the HMA controls. Overall, early findings indicated that warm mix performs much like hot mix. Work was continuing on WMA mix design, the use of recycled materials in WMA, and which technologies perform best.

Summary

Warm mix asphalt lowers production and placement temperatures through foaming or additives, cutting fuel use and emissions and extending working time. Early field and accelerated-loading studies found it performing much like hot mix, provided the same good production and construction practices are followed.

@misc{pavint-warm-mix-asphalt,
  author = {{Pavement Interactive}},
  title = {Warm Mix Asphalt},
  year = {2026},
  publisher = {HeadLight},
  howpublished = {\url{https://pavementinteractive.org/reference-desk/pavement-types-and-history/pavement-types/warm-mix-asphalt/}},
  note = {Accessed [date]}
}