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Superpave Future Bright as Chosen Method of Mix Design

Wed September 13, 2006 - National Edition
Dan Brown

The future of binders and asphalt mixtures is a bright one indeed. Increasingly, state transportation departments and local agencies will adopt Superpave — and will improve it as their chosen method of mix design. As state testing staffs become smaller relative to their workloads, more states will move to statistically-based quality assurance programs.

The use of stone matrix asphalt or SMA will increase. And if the cost of fuel continues to rise and air quality regulations get tougher, we will see more warm mix asphalt. Full scale tests on warm mix are expected this year. Someday, researchers hope to be able to automate quality control testing so that samples are never touched by human hands.

A range of tests will be developed for both binders and mixtures that will enable technicians to predict more accurately the mechanistic behavior of asphalt pavements under given loadings and temperature conditions.

“We’re planning to convert from a sawed-off shotgun to a deer rifle with a scope,” said Gary Fitts, senior field engineer, Asphalt Institute, San Antonio, TX. “We want to reduce the scatter in our predictions. We can improve our ability to predict performance if we use properties that have applied physical characteristics.”

The future will bring greater emphasis on using reclaimed asphalt pavement in mixtures, Fitts said. The industry will improve the effectiveness of using recycled material.

The next five years should see more emphasis on mixture research than binder research, said Rick Holmgreen, asphalt technical support manager, ConocoPhillips Co., Houston, TX. A number of research programs across the country focus on modeling an asphalt pavement’s behavior — such as reflective cracking or permanent deformation — based upon certain properties measured in the laboratory. Holmgreen ticks off a list of universities doing such research including: Arizona State, Texas A&M and the University of Illinois.

Lowered Design Compaction

Most experts agree that Superpave’s design level of laboratory compaction, as measured by N-design, will be adjusted downward over the next few years. At the National Center for Asphalt Technology (NCAT), research will wind up this summer on NCHRP 9-9(1). Ray Brown, director of NCAT said, “The bottom line is the recommendation that N-design will be reduced by a certain amount.”

“I do not think we have a good sound basis for N-design numbers as they exist in Superpave,” said Gerry Huber, associate director of Research, Heritage Research Group, Indianapolis, IN. “The numbers are reasonable but I think they are too high.”

High levels of compaction in the gyratory can lead to problems when designing mixes with soft aggregates, Huber said. The aggregate skeleton gets crushed in a gyratory mold, yet rollers on the pavement do not compact the mix to that degree.

For example, you can design a mixture in the gyratory to have 4 percent air voids, but the aggregate is crushed. The same mix can have 6 or 6.5 percent air voids under field compaction — which then makes it very difficult to get 93 percent of maximum theoretical density in the field, said Huber.

High values for N-design limit the ability of states to use local aggregates that may be more marginal in quality, according to Mark Blow, senior field engineer, Asphalt Institute, Sioux Falls, SD. He said many states have already reduced N-design, because high costs of transportation and aggregates force them to use local aggregates.

“High values for N-design, utilized on historically acceptable local materials, reduces the VMA, leaves less room for asphalt, and results in drier mixes,” said Blow.

Blow cited Iowa as an example. State mix designers adapted local, existing mixtures to the gyratory system of design, so that local aggregates could be used. N-design was reduced from recommended Superpave levels.

“So they use a gyratory all the time but they’re not 100 percent Superpave,” said Blow.

New Stiffness Test

In summer 2006, the Federal Highway Administration (FHWA) hopes to complete preliminary tests on the Dynamic Modulus Test, which involves new equipment to measure asphalt mixture stiffness.

“Some states have already bought the equipment, and it will be available from several manufacturers right away,” said John D’Angelo, asphalt materials engineer of FHWA.

He said the test determines asphalt stiffness at varying temperatures and different loading rates. Dynamic Modulus equipment has a hydraulic actuator that applies an axial load to a cylindrical specimen. NCHRP 9-19, the study that developed the Superpave performance test, is complete, D’Angelo said. The next step is to complete NCHRP 9-29, which will finalize test equipment and develop testing procedures for the Dynamic Modulus test.

FHWA has been running Dynamic Modulus tests across the country in its mobile laboratory, and the results are all positive.

“The equipment is working extremely well, we get good repeatability, the test is easy to run, and it does a good job of distinguishing one mix from another,” D’Angelo said.

“The Dynamic Modulus Test can be used in a couple of ways,” said Mike Anderson, director of Research and Laboratory Services, Asphalt Institute.

“For one, you can use the data generated in asphalt pavement thickness design software. A second way is that you can use the test results to gauge which mixture is appropriate for a given application. You can use a softer mix for a subdivision than for an interstate, and this test can measure the different,” said Anderson.

Anderson has received his new Dynamic Modulus equipment and is gaining experience with it. There is a draft procedure for using it, but matters such as test temperatures, and what is a good number and what is a bad number, remain to be decided. “Eventually the Dynamic Modulus test will be adopted,” said Anderson.

Research on the Dynamic Modulus test also is ongoing at NCAT, said Brown. He said applied loads for the Dynamic Modulus are relatively lower than for the Repeated Load Deformation Test, another mixture test the NCAT is working on. With the Repeated Load Deformation Test, a loading head applies a load of approximately 500 lbs. to a cylindrical specimen at a rate of 10 times per second.

“We look at the deformation after several thousand loading cycles,” said Brown.

Moreover, NCAT is researching a Wheel Tracking Test for rutting, and a Creep Recovery Test on mixtures. With the Creep Recovery test, a load is applied to a specimen, then removed, and the resulting creep is measured.

“The Wheel Tracking Test is more empirical,” said Brown. A wheel is run back and forth over a sample for typically thousands of cycles, and the resulting rutting is measured.

“We’re continuing to look at the ability of this test to predict performance over a wide range of mix designs,” said Brown. “We’re looking at ways to improve its accuracy.”

Laboratory rutting test results can be compacted to results on the 1.7-mi. NCAT Test Track — and the laboratory test can be refined as a result.

Binder Research

Holmgreen said more research is needed to address the high-temperature behavior of binders.

“The industry’s goal is to identify a fundamental engineering property that would better address high-temperature distress,” he said. “You can have a high void content and still get rutting with a good binder. Binder won’t fix bad mix design, but we’re trying to identify binders that will work to resist high-temperature deformation.”

Currently one measure of binder stiffness is G*— divided-by-sine-delta, which is a value determined by the Dynamic Shear Rheometer (DSR), a common binder testing device. “G*— divided-by-sine-delta has done an adequate job, but it needs to do a better job at the higher end of the temperature scale,” said Holmgreen.

Superpave’s Performance Grade (PG) system was developed around unmodified asphalts, but a better, more standardized system is needed for modified asphalts at higher temperatures, said Holmgreen.

The property being looked at currently is called creep recovery, which measures the degree to which a binder recovers its original shape after being loaded. The corresponding test is called a Multiple Stress Creep Recovery (MSCR) test. It uses the Dynamic Shear Rheometer.

“If you add a very low level of stress [load] you may get 98 percent of the original shape back,” said Holmgreen. “The amount of recovery that you get depends on the stress applied. MSCR looks like it has the potential to be a good test.”

He said ConocoPhillips has submitted a half a dozen or more binders to the Asphalt Institute for testing with MSCR — and other binder suppliers have done the same. The Institute needs to test a large number of binders.

“We need to see if we have improved our ability to predict rutting in the mixture,” Holmgreen said.

Anderson said the Asphalt Institute is doing a MSCR study in cooperation with FHWA. The effort seeks to evaluate MSCR and compare it to some of the more common binder tests such as Elastic Recovery, which is used to detect the presence of a modifier.

“If we can show that this [MSCR] test is comparable to the Elastic Recovery test, then the states can replace Elastic Recovery with this new one,” said Anderson. “MSCR is quicker to run than Elastic Recovery. But some questions remain. We have to refine some of the test procedures. We are evaluating what the specification limits should be, and determining the representability of the procedure.”

Anderson said that the MSCR will need more work before it is ready for use by the states.

“We probably will not see widespread use for a few more years,” he said. “In all likelihood it will end up replacing Elastic Recovery, Force Ductility and some of the PG-Plus tests, but I will say it’s a little early to predict that for sure.”

Creep Test for Mixtures

D’Angelo said FHWA is working on a creep recovery test for both binders and mixtures. For the binder test, a small sample is placed in the Dynamic Shear Rheometer and a torsional load is applied.

“Existing SHRP tests for binder are better than they used to be but they don’t identify key performance characteristics of modified binders,” said D’Angelo. “We need to do a much better job of describing binder behavior at higher temperatures, and the MSCR test will do that.”

FHWA probably will complete work this fall on the MSCR binder test, he said.

Meanwhile FHWA is continuing work on a creep recovery test for mixtures. It’s called a “Flow Number” test, and it uses the Dynamic Modulus equipment.

“You apply the loads differently and measure the response differently than for the Dynamic Modulus Test,” D’Angelo said. “For high temperatures, the Flow Number test will be an improvement.”

The test will require two to three more years of work, he said.

At NCAT, Brown said researchers are working to automate aggregate and asphalt testing procedures so that humans never need to touch the sample. Already, he said, “We can take an aggregate sample off the belt and run a gradation test on it.” That procedure works with automated sieves.

What’s more, the potential exists for lasers to detect aggregate gradations without running sieve tests.

“We’re trying to reduce the test time so that we get results quicker and can adjust to any problems quicker,” he said.

Fitts predicts a rapid rate of change in the industry.

“The polymers we have today and the developments in the asphalt industry were just a pipe dream 30 years ago,” he said. “Today, it’s everyday practice. One assumes that this pace of change will continue.”

(Dan Brown is the principal of Technicomm.)

(This article appears courtesy of “Asphalt“ magazine.)

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