Now That’s Cool

Low-energy asphalt mixes reduce viscosity – without the heat

In the last two decades, improvements in the physiochemistry of asphalt mixes have sparked a revolution in their production and placement.

Today’s warm and cold asphalt mixes – which to minimize confusion might be better termed “low energy” mixes – are stealing the spotlight from classic hot mix asphalt.

Warm mix asphalt (WMA) is created by mixing one of a variety of solid or liquid chemical compound additives with asphalt mix in the plant, or by foaming the mix with water in the plant. WMA processes generally reduce the viscosity of the liquid asphalt through a variety of means, and enable the complete coating of aggregates at temperatures 35 to 100 degrees F lower than conventional hot mix asphalt.

As with many trends, low energy mixes began in Europe, where warm mixes were popularized in the 1990s. In 2002, the National Asphalt Pavement Association led a study tour to Europe to examine WMA technologies, and the association began exploring their attributes, most recently, at the 2nd International Warm-Mix Conference in October 2011 in St. Louis.

In 2007 a Federal Highway Administration (FHWA) international scanning tour investigated warm mix practices in Europe to gather information on technologies used to produce WMA, with particular emphasis on long-term field performance. Its report – Warm-Mix Asphalt: European Practice – was published in February 2008 (download the report by Googling its title).

In the meantime, another type of low energy asphalt mix – a warm mix created by foaming hot mix asphalt with water in a drum plant – began growing in popularity in the 1990s. And yet another foamed asphalt – a cold mix created in a portable plant, or in situ in the field, using as much as 100-percent reclaimed asphalt pavement – also is called foamed asphalt or foamed bitumen mix.

The one thing all of these components have in common is that they are “green” technologies that reduce plant emissions, lower fuel consumption at the plant, and create a better environment for workers in the field. Their future is so established that in the past year NAPA changed its web site URL from hotmix.org to asphaltpavement.org, and changed the name of its magazine from HMAT (for Hot Mix Asphalt Technology) to Asphalt Pavement.

Data indicate plant emissions are significantly reduced with low energy mixes. “Typical expected reductions are 30 to 40 percent for CO2 and sulfur dioxide (SO2), 50 percent for volatile organic compounds (VOCs), 10 to 30 percent for carbon monoxide (CO2), 60 to 70 percent for nitrous oxides (NOX), and 20 to 25 percent for dust,” according to Warm-Mix Asphalt: European Practice in 2008. “Actual reductions vary based on a number of factors. Technologies that result in greater temperature reductions are expected to have greater emission reductions.”

Burner fuel savings with WMA typically range from 11 to 35 percent, FHWA’s report states, with fuel savings possibly higher (possibly 50 percent or more) with other processes.

And workers will benefit as well, according to the scanning tour report. “Tests for asphalt aerosols/fumes and polycyclic aromatic hydrocarbons (PAHs) indicated significant reductions compared to HMA, with results showing a 30- to 50- percent reduction,” the report says, but with a caveat: “It should be noted that all of the exposure data for conventional HMA were below the current acceptable exposure limits.”

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From Near-Solid to Liquid

At ambient temperatures, “liquid” asphalt or bitumen from the refinery is a near-solid material. To lower its viscosity and make it workable, it is kept hot, and classic hot mix asphalt production – in which liquid asphalt coats aggregate – takes place at 275 to more than 330 degrees F, and compaction between 260 and 300 degrees F.