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."
