Los Angeles Accelerates Superpave Field Performance Tests

The standards division of the city of Los Angeles developed a fast, simple, repeatable test with the Rutwheeler from Pine Instruments, Grove City, Pennsylvania, to predict permanent deformation and moisture susceptibility in hot-mix asphalt Superpave designs.

The sprawling city of Los Angeles has approximately 6,500 centerline miles of roads and 800 centerline miles of alleys to maintain—a daunting responsibility by any measure. Add to that 1,205 miles of bus lanes, according to MTA service performance analysis department, and the challenge for the Bureau of Street Services to keep the roads up to standard is enormous.

The city’s standards division testing laboratory provides full testing support (QC/QA) for the Bureau of Street Services operations, resulting in more than 130,000 tests annually for the pavement preservation program alone. According to Richard Villacorta, the Bureau’s Roads and Highways Section Head, “Proper design and research have been fundamental pillars as the city of LA faces the mounting challenges presented by the overwhelming traffic loads prevalent today.”

Accelerated street deterioration has become problematic with the increased volume and weight of vehicles, particularly when heavy, articulated buses came into service. “Rear axles in those buses represent 7 ESALS^[1] per rear single axle/pass or a total of 9 to 10 ESALS per bus. This has caused the accelerated rutting of bus lanes, which leads to a huge deterioration of the pavement structure and the reduction of pavement life,” Villacorta said.

The established empirical laboratory design procedures were not enough to keep pace with the new types of vehicles on the road. Asphalt binder testing was limited. New performance based asphalt materials characterizations were needed.

The Superpave system, of which the city of LA was an early adopter in 1996, offered new tools to fight permanent pavement deformation (rutting), fatigue cracking and low temperature cracking.

Leading Lab Technician Jorge Esparza loads a gyratory compacted specimen into the Pine Rutmeter (wheel tester). All photos are courtesy the city of Los Angeles, Roads & Highways section.

To counter these problems, high rutting resistant HMA mixes using high viscosity engineered modified binders had to be designed and evaluated. As a result of intensive research conducted by the city’s standards division, Pine Instrument’s AFW1A Rotary Asphalt Wheel Tester (which the LA spec refers to as the Pine Rutmeter) was employed to evaluate samples, which were prepared in accordance with Superpave specifications as to their resistance to permanent deformation and moisture susceptibility.

“Basically, the asphalt concrete cylindrical specimens are prepared and compacted in a Superpave gyratory compactor, and then after cooling, the specimen is preconditioned at 60⁰C for two hours and placed in the Rutmeter to be confined between three metal wheels in continuous synchronized rotation.^[2] Each wheel applies a fixed load around the periphery of the specimen while it’s immersed in water at 60⁰C. This intense pressure allows us to evaluate rutting resistance and moisture susceptibility.”

One of the most valuable advantages the Superpave system provided, Villacorta pointed out, was the opportunity to predict and deal with rutting, which was the number one cause of pavement distress nationwide.

“While Superpave gives us a set of valuable tools, agencies must perform their own research to ensure the new tools are applicable to their weather, local materials and topography,” he added.

Asphalt Lab Supervisor Jesus Barajas demonstrates a typical graph showing the rutting depth of a gyratory compacted specimen.

By 2003, the city of LA’s testing laboratory already had extensive experience in evaluating permanent deformation in several thousand HMA mixes using a unidirectional CPN Rutmeter. That year the standards division was offered the opportunity to evaluate the Pine’s Rutmeter test and its effect on HMA mixes.

“We spent the next couple years correlating data from both machines and on April 12, 2006, released a report detailing our findings,” Villacorta said. “What we liked most about the Pine system was that it’s fast, easy to use and accurately predicts field performance of the HMA. In addition, it correlates with two other tests we use to evaluate HMA, the Modified Marshall Stability and Indirect Tensile Strength Tests.”

Today, the city’s lab uses the system for both design and QC/QA testing. According to Villacorta, it has helped them learn about the rutting values of failed mixes and the values of the new mixes that will perform much better under heavy loads.

“Now we are able to evaluate engineered binders, structured or polymer modified, and correlate their field and laboratory performance accurately,” he added.

During the Rutmeter test, the instrument panel displays the specimen’s progress. Lab Tech Jorge Esparza monitors results on the computer screen.

In 2012, Villacorta’s department, the Department of General Services, earned MVP awards from the city of LA for its accomplishments, a record-breaking 747 miles of successful pavement preservation work. In a 2013 to 2014 project, the city’s standards division also successfully used its Rutmeter test to complete the Wilshire Blvd. Middle Lane Federal Project, which helped the lab monitor the performance of the components of the mixes on a continuing basis, especially in a heavily traversed boulevard. The Bureau received a federal grant in combination with the transit authority; they put between four and six inches of Superpave mix with a polymer-modified PG76-10 binder in the center lane of Wilshire Blvd. Pine’s Dave Savage explained that’s a “pretty expensive” mix, but in certain sections of Wilshire, that strong mix is exactly what they needed. And it’s exactly what the Rutmeter told them they would get: a strong pavement.

The city of LA’s standards division has performed more than 11,000 tests with the Pine Rutmeter and has established its own design and QC specifications. “We use the system daily in our pavement preservation program, which has been instrumental in predicting pavement performance,” Villacorta said.

“This simple and accurate test can represent millions of dollars in savings to any resurfacing program at the state or municipal government level,” he added.

^[1]An ESAL is a concept developed from data collected at the American Association of State Highway Officials (AASHO) Road Test to establish a damage relationship for comparing the effects of axles carrying different loads. The reference axle load is an 18,000-pound single axle with dual tires.

^[2] The specimen continuously rotates between 3 Hamburg-style wheels with each rotation of the specimen providing three load cycles. The path around the outside of the specimen is infinite length, eliminating the need to prepare and join together multiple specimens. Wheel velocity is constant, unidirectional and can be adjusted from the front panel.