Ultrathin Whitetopping Gains Momentum

Concrete Roads Article December 28, 2000
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An emerging concrete pavement technology
is rapidly gaining a foothold across the nation. Known as
ultrathin whitetopping (UTW), this technology is used to pave
over deteriorated asphalt. The idea is to mill off the asphalt
surface and pave it with concrete 2 to 4 in. thick. The concrete
bonds to the underlying asphalt. Joints are closely spaced to
reduce pavement stresses and cracking.

Launched in 1991, UTW
is intended for use on low- to medium-volume roads, general
aviation airports and parking lots. Since 1992, well over 30
trial projects have sprung to life.

Results? To date they're
very promising, even exciting. Concrete pavement researchers are
making steady progress toward UTW design specifications that
soon will establish the concept as a cost-effective--and
structurally sound--rehabilitation technique.

At this
writing, some 100 ultrathin whitetopping projects, representing
more than 1 million sq yd, have been placed in North America in
the first five years since its launch. UTW has been used
throughout most regions in the U.S., as well as in Canada and
Mexico.

One of the key factors in this success has been a
strong and focused industry-wide marketing effort, led by the
American Concrete Pavement Association (ACPA) and the National
Ready Mix Concrete Association (NRMCA). Together, the two
associations have been working with regional promotional
councils, state associations and local promotional groups.

To date, the efforts have reached an audience of more than
10,000 specifiers. There also have been 15 demonstration open
houses across the country, a key factor in projects being placed
in 21 states, said Lon Hawbaker, ACPA director of local roads.
Hawbaker and NRMCA's Anne Ellis have served as stewards for UTW
technology.

A promotional campaign used to introduce UTW
featured a video, brochure and promotion kit (comprised of a
CD-Rom presentation, 60-page how-to guide and other background
information), as well as media support.

A concrete solution

"Before ultrathin concrete, there was only one way to do a
2- to 3-in. overlay, and that was with asphalt," said Gordon
Smith, executive vice president, Iowa Concrete Paving
Association. "Ultrathin whitetopping will open a lot of doors,
especially in the municipal market.

"Now we have a thin
section to use in matching pavements to bridge approaches and
streets to driveways."

"UTW is for low- to medium-volume
traffic, particularly where an asphalt section is in good
condition but the surface is rutted," said Larry Cole, vice
president of engineering and research at the ACPA. "UTW is
another solution that allows agencies to use concrete pavements
to solve pavement problems."

What promises to make UTW an
even stronger tool is that it adapts well to fast-track concrete
paving methods. With fast-track, special concrete mixes enable
the ultrathin whitetopping to carry traffic within 12 to 24
hours after placement.

"The majority of ultrathin projects
have employed fast-track paving and synthetic fibers in the
concrete," Cole said. Synthetic fibers add toughness, minimize
shrinkage and cracking, enhance impact resistance and improve
post-cracking integrity.

"The 2-in. section is a new animal,
and we think fibers add some insurance at that thickness," said
Dr. James Cable of Iowa State University. Fibers cannot stop
shrinkage cracks, Cable said, but they add considerable
toughness and "hold it together."

Cable is the researcher in
charge of monitoring and reporting on 41 variable UTW test
sections placed along a 7.2-mile stretch of Iowa Route 21 near
Belle Plaine, Iowa. Built in 1994 by Manatts Inc., Brooklyn,
Iowa, the project features test sections with concrete overlay
thicknesses of 2, 4 and 6 in.

The overlays were sawed into
squares to relieve stresses that can cause distortion in the
thinner sections. Sizes being tested are 2-, 4-, 6-ft squares
and 12-ft panels. Two types of synthetic fibers, monofilament
and fibrillated, are being tested.

Early test results
promising

The Iowa test sections will be monitored for five
years, Cable said. Performance of the ultrathin whitetopping
sections are being compared with a 4.5-in. asphalt overlay.

"This is the third winter [1996-97] for the project," said Brian
McWaters, pavement engineer with the Iowa DOT. "After five
years, we hope to have data to use for writing UTW design
procedures."

Judging by early indications, however, insiders
say the best balance of strength and construction efficiency
comes from the 6-ft square panels built of 4-in.-thick concrete
with fibers. That way, longitudinal joints on a 24-ft slab
consist of one cut down the centerline and two others one in the
center of each driving lane. Transverse joints are sawed at 6 ft
intervals.

When building UTW projects, milling the asphalt
ahead of concrete placement exposes the aggregate and helps
achieve a better bond, Cole said. The milled surface should be
broomed and cleaned prior to paving.

A systematic study

Based on results from a Louisville, Ky., project where 2- and
3.5-in. UTW pavements stood up well to accelerated truck
loadings, Cole said the concrete-to-asphalt bond "significantly
reduces the calculated stresses in the concrete overlay."

Researchers have found that UTW pavements require a thick
asphalt section, at least 3 in., to provide an adequate
composite concrete-on-asphalt pavement section. "The more sound
the base, the better the ultrathin will perform," Cable said.

Using the PAVER system protocol, ACPA's Cole recently
performed a pavement condition survey on 11 UTW projects. The
PAVER system was selected because it is an objective evaluation
of 19 possible concrete pavement distresses. The type, amount
and severity of distress were observed and reported, and a
current pavement condition index (PCI) was determined for each.

Most of the UTW projects were placed in 1992 or 1993.
Observations were made in the summers of 1995 and 1996.

The
results are very positive. With no unexpected change in traffic
loadings, projections of the PCI show that 10 sections will
serve eight to 12 years before the condition index drops below
55 (Good). In the summer of 1996, nine of those 10 sections
rated "Excellent" by PAVER system criteria, and the tenth rated
"Very Good."

The vast majority--some 94%--of cracks on the
10 sections were of low severity; most cracks were less than a
half-millimeter (0.02 in.) wide. Only one of the 11 projects
shows unacceptable damage, and is being studied to discover the
reasons for its unusual condition.

What are the common
characteristics of the best-performing UTW projects in the ACPA
survey? While it is still a bit early to draw solid, scientific
conclusions, it appears that panel size and asphalt thickness
are key factors. The UTW sections with panels 16 sq ft (4 ft x 4
ft) or less, as well as those on substantial asphalt base, are
performing best.

UTW for airports

In addition to its
potential for streets and low- to medium-volume highways,
ultrathin concrete holds a great deal of promise for general
aviation airports, according to the ACPA.

Already, UTW has
been placed at two general aviation airports, one near St.
Louis, Mo., and the other at New Smyrna Beach, Fla. As one of
the Midwest's largest general aviation airports, the Spirit of
St. Louis Airport relieves traffic from Lambert Airport, St.
Louis' main airfield. Through the early '90s, the airport's
asphalt airport parking areas had deteriorated severely.

In
1994, CRD Campbell, a local engineering firm, persuaded the
Missouri Highway Transportation Department (MHTD) that UTW could
serve as a bonded overlay for apron sections carrying light
aircraft. CRD Campbell, working with ACPA and its own lab
findings, designed a 3.5-in.-thick concrete overlay to cover
14,000 sq yd of light-load areas that support aircraft weighing
up to 12,500 lb.

Paving started on Dec. 22, 1994. The
contractor, Vee-Jay Cement Contractors, St. Louis, used cold
weather concreting practices, and included 3 lb/cu yd of
synthetic fibers in the mix. Joints were cut, often at night, at
intervals of 50 in. The airport opened in February 1995.

Airport officials say the whitetopping has tripled the life of
the apron pavement. "An ultrathin overlay saves the costs of
asphalt removal, new aggregate base, compaction, grade
trimming--all that work," said ACPA's Cole. "After minimal
asphalt preparation, you just pave a concrete overlay on it and
gain many years of additional life. The idea is a winner."

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