Road Science: Low-Energy Asphalt Mixes: Part 5 in our 7-part series.
by Tom Kuennen, Contributing Editor
of Road Building
mixes reduce viscosity –
without the heat
n 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
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 Eu-
18 August 2012 Better Roads
rope, 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
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,