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www.acpa.org
Quarter 4, 2016
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W e s t D e s M o i n e s
When Microsoft announced
the construction
of a data center in West Des Moines, Iowa in
2014, the city committed to building new roads
to serve the new facility.
City Engineer Duane Wittstock saw the 3.5 mile
road leading to the data center as anopportunity to
test recent researchonnewconcretemixtures. “We
were seeing joint failures in concrete pavements
that were no more than 10 years old throughout
our city,” he said. “We have a lot of concrete pave-
ment in our city, but as we looked at theMicrosoft
project along with other future projects, we knew
we had to figure out why the pavement was failing.”
Wittstock attended a workshop at which Peter
Taylor, Ph.D., P.E., director of the National Con-
crete Pavement Technology Center at Iowa State
University, discussed the issue of premature joint
distress in some areas. “Our issues were similar
to other cities’ problems, but ours weremagnified
by the number of concrete roads in our city,” he
said. “Older pavements did not have the same
problems, just newer pavements.”
“We have been studying premature joint deteriora-
tion in nine states for about eight years,” Taylor
said. “Most of the states are in the Midwest, but
we also see the problem in parts of New York
and Washington.”
Research has identified two main causes of the
early deterioration:
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Freeze-thaw damage in saturated concrete
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Salt-related chemical reactions in pavements
treated with anti- or de-icing agents
While many areas of the United States routinely
treat roads to prevent icing in winter months,
pavements in areas that freeze and thaw repeat-
edly throughout the season—such as Midwest
states—are more likely to see early deterioration
compared to areas in which water trapped in
air voids in the pavement stays frozen for most
of the winter.
De-icing chemicals exacerbate damage as the
chemicals collect in the joints. Wittstock also
points to development of improved techniques
for salting roads as one reason newer pavements
exhibit damage earlier than older pavements. “We
used to treat roads with rock salt, much of which
was blown off the road by traffic,” he explains.
“Today, the liquid chemicals are uniformly placed
on the road and stay there.”
Although test sections of different concrete mix-
tures were constructed for research, Taylor points
toWest DesMoines willingness to commit to test
the findings in a real-life setting as a significant
step to evaluating solutions to the problem.
Specifications for themix were based on research
conducted by Oregon State University professor
Jason Weiss, Ph.D. and specifications used in
Minnesota due to that state’s experience paving
in cold weather, according to Jeremy Huntsman,
P.E., project manager at H.R. Green.
Based on a recommendation fromWeiss, cement
was replaced with Class C fly ash at a rate of 30 to
35 percent, which differs from the standard Iowa
DOTmix of 20 percent. Increased fly ash content
reduces the risk of damage fromde-icing salts by
converting calciumhydroxide (CH) into calcium
silicate hydrate (C-S-H), which results in a more
durable concrete with lower saturation rates and
greater long-term strength. Lower fly ash rates
enable the creation of calcium oxychloride, an
expansive compound that damages the cement
paste and reduces durability.
Other modifications to the mix included:
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Minimum of 6 percent air behind the
paver and a target water-to-cementitious-
materials (w/cm) ratio of 0.40 to a maxi-
mum 0.42 (the standard Iowa DOT mix is
about 0.45)—both strategies contribute to
reduced permeability, which reduces satu-
ration rates and the potential for freeze-
thaw damage.
Premature Joint Distress:
West Des Moines Tests Possible Solutions
Change in specs increases fly ash to minimize damage by de-icing chemicals
By Sheryl S. Jackson
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