Withstanding the Test of Time

Highway Construction Article December 28, 2000
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The first round of Federal Highway Administration (FHWA) accelerated pavement testing for ultrathin whitetopping is underway. A cooperative effort between the FHWA, American Concrete Pavement Association (ACPA) and the Virginia Ready-Mix Advisory Council, the ultrathin whitetopping (UTW) evaluation involves the testing of eight different pavement sections which are being tested with the FHWA’s accelerated loading facility (ALF) device at the Turner-Fairbank highway research facility in McLean, Va.

Ultrathin whitetopping is a concrete overlay, 2- to 4-in. thick, bonded to an existing asphalt pavement. Joints are spaced closely in the concrete overlay to reduce stresses from traffic loads and environmental conditions. Since its inception in 1991, over 200 UTW projects have been built across the U.S. to rehabilitate distressed intersections, municipal streets, general aviation airfields and other lower traffic-volume facilities.

While a number of these existing roadways are routinely inspected, FHWA’s ALF device gives additional information on the UTW.

ALF applies a constant rolling wheel load to the pavement. The wheel load is then rolled across the pavement in one direction, simulating a fully loaded truck traveling 12 mph. The loads can be carefully controlled and instruments can be placed within the pavement to measure strain and deflections.

Load testing

The UTW test sections were built in April 1998. Existing asphalt pavement, which had previously been tested with the ALF equipment, was milled to remove ruts and to enhance bonding between the asphalt and concrete overlay. Prior to milling, the asphalt was 8-in. thick while the eight test lanes of UTW were built with different overlay thickness, asphalt thickness, joint spacing and concrete mixtures.

Testing began in May 1998, with an average of 35,000 wheel loads applied weekly. For the first 300,000 load applications, a dual-tire load of 10,000 lb was applied, simulating a truck’s single axle load of 20,000 lb. The applied load was then increased to slightly over 12,000 lb for the remainder of the test, which was an additional 800,000 cycles.

“When we reanalyzed the UTW sections with the actual concrete strength and concrete thickness that were built, we realized the first test section was stronger than we anticipated,” explained the FHWA’s Jim Sherwood, project manager for the ALF-UTW evaluation. “So we increased the weight of the applied load to test the sections more rapidly.”

While the largest concrete flexural strength is 750 psi, the FHWA’s concrete test specimens showed the in-place

concrete flexural strength averaged over 950 psi. With some additional concrete thickness from deeper-than-expected milling, the load-carrying capacity of the UTW test sections was greater than originally planned.

“That’s one of the real advantages of the ALF device,” said Sherwood. “We can vary the weight of the loads as well as the number of load applications.”

With the increased load weight, the FHWA plans to finish testing all eight sections by the end of 1999.

No signs of change

With over one million cycles on the first UTW test section, a new challenge may be facing the research team. The UTW has performed well enough that the expected distress has not occurred.

“We’ve seen a slight increase in the measured strains and deflections from the wheel loads, but there’s no sign of other notable changes,” said Sherwood. “The UTW section just keeps taking load after load from the ALF device.”

“We’re very pleased with the results so far,” said Larry Cole, ACPA’s vice president of engineering and research. “The first UTW test section has exceeded our best expectations. But we’d really like to continue testing this first section until we see some significant cracking or other distress. Then we’ll know much more about the true capabilities of UTW.”

The FHWA and ACPA are considering their options, including increasing the applied wheel load or extending the duration of the ALF testing program.

Original research plans called for two thicknesses of concrete overlay, 2 1/2 and 3 1/2 in. The 8-in. thick existing asphalt was to have been milled 2 1/2 in. in four of the test sections, leaving 5 1/2 in. of asphalt under the 2 1/2-in. thick concrete overlay. The other four sections were planned as 3 1/2-in. UTW on 4 1/2-in. thick asphalt. Thickness measurements during construction shows that the actual concrete is somewhat thicker than planned, while the underlying asphalt base is slightly thinner.

Call in the reinforcements

Concrete in four of the test sections contains fiber and four other sections of similar thickness and joint spacing contain no fibers or other reinforcement. The fibrous overlays contain 3 lb of fibrillated polypropylene fibers per cubic yard. Most UTW projects built in the U.S. contain fibers, predominately polypropylene. The fibers increase the toughness of the overlay, adding residual strength should cracking occur.

A variety of instruments were carefully placed on the milled asphalt surface or imbedded in the plastic concrete during construction. As the ALF wheel rolls over the UTW, these instruments measure the minute deflections and strains in the pavement layers. Each measurement is instantly recorded into a computerized database. Measurements are made at different times during the ALF testing to determine if pavement properties change as the number of loads increase.

“These measurements are very important to our understanding of UTW,” explained Cole. “We’ve developed mathematical models to predict the stresses caused by wheel loads and temperature changes. These models allow us to predict the number of trucks a UTW section can carry. With the ALF-UTW measurements, we can verify our mathematical models and make adjustments to meet real-world conditions.”

Comparisons between the eight different UTW sections also will yield important information.

“For instance, we’ll be testing two sections with similar pavement thickness and material properties, but different joint spacing,” said Sherwood. “We’ll see how joint spacing affects performance. Then we’ll test pavements with different overlay and base thickness to see how these properties affect performance. Four sections have fibers in the concrete and four don’t. We’ll be able to compare fibrous concrete to plain concrete UTW.”

Quite a load

The first test has produced unexpected results. With over one million ALF wheel loadings, the first test lane has no significant distress. The one million ALF loadings are equal over 100 legally loaded trucks every day for 20 years. This represents over three million equivalent single axle loads (ESALs). ESALs are a measure of wheel loading used in the American Association of State Highway & Transportation Officials (AASHTO) pavement design procedure. Most state DOTs use the AASHTO design procedure to determine pavement thickness. It takes 10 to 20 years of service for most municipal streets to receive three million ESALs. Most residential streets never receive such a high level of truck traffic.

“That’s another advantage of ALF,” said Sherwood. “We can simulate a large number of trucks in a relatively short time.”

A joint effort

Industry and FHWA representatives also are pleased with the cooperation that has taken place. “This research project is a prime example of partnering between an industry and a public agency,” said Charlie Churilla, FHWA’s chief of pavement performance research. “We’ve worked together every step of the way, from planning through construction into testing and, soon, into data analysis. It’s been a very positive experience for us. We’re looking forward to cooperating with the concrete paving industry on more research and technology advances.”

FHWA and ACPA engineers jointly planned the test sections and testing procedures. ACPA contractor member Cherry Hill Construction, Jessop, Md., built the concrete overlay with materials provided by members of the Virginia Ready Mix Concrete Advisory Council. The FHWA instrumented the test sections and conducts the ALF operation and data collection. Additionally, FHWA and ACPA engineers will work together to analyze the results.

ACPA President and CEO, Val Riva, summed up the industry perspective, “For a number of year’s we’ve known that ultrathin whitetopping offers a viable new product to maintain and rehabilitate distressed asphalt pavements. The ALF-UTW testing program will give new insight into UTW, further expanding this technology. The FHWA stepped up to make this possible and we’re pleased to be working with them.”

Testing will continue throughout 1999 and into early 2000. The FHWA and ACPA won’t be waiting until then to start learning from the ALF-UTW testing. They’re now analyzing the measurements from the instruments in the first test section, comparing it to the theoretical values. Meanwhile, the ALF device continues to test the remaining UTW sections.

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