By Michael T. Beavin and R. Michael Anderson
Chocolate pudding, synthetic rubber, toothpaste, and asphalt. At first glance these items appear to have been selected at random. While perhaps not apparent, each of these products shares a common, fundamental characteristic—their rheological, or flow and deformation, properties can be measured by one instrument.
A rheometer measures flow and deformation characteristics and can take the form of any instrument that has the ability to apply either a prescribed stress (force required to create flow and deformation) or strain (deformation as a result of flow) and then measure the resulting stress or strain. If the stress and strain associated with an arm-wrestling match were electronically monitored, the opponents and monitoring system would constitute a crude rheometer. Fortunately, there exists a much more precise means of measuring flow and deformation for materials.
Some of the early users of a piece of equipment known as the Dynamic Shear Rheometer, or DSR, were scientists in the food industry. Consistency in food products like ketchup could be monitored by studying the change in rheological properties from one production batch to the next. Designed to precisely measure stresses and strains of various materials under oscillatory, steady shear, and repeated loads at various temperatures, the DSR became a useful scientific tool for industry.
The Asphalt Industry Begins to “Ketchup”
In the 1980s, the asphalt industry began to focus on the increasing need for a better understanding of the rheological properties of asphalt binders. Up until then, fundamental flow properties were principally measured using the viscosity test. For conventional, unmodified asphalt binders, the viscosity test was quite suitable in characterizing high temperature flow properties. At high temperatures like 60°C (140°F), the elastic component of a typical paving-grade asphalt binder was virtually nonexistent. As such, the measured viscosity of the asphalt binder was directly related to its stiffness.
Unfortunately the viscosity test could not provide all the information needed to understand the rheology of a typical paving-grade asphalt binder at intermediate to low temperature. This is true because asphalt changes properties as the test temperature changes. At lower temperatures, an asphalt binder acts more like an elastic solid than a viscous fluid. As such, the elastic part of an asphalt binder starts to become important compared to its viscous part. Since the viscosity test only measures the viscous part of the asphalt binder, the test could not adequately describe the flow properties of an asphalt binder at intermediate to low temperatures.
In addition to lacking good fundamental measurements at intermediate and low temperatures, another major factor affected the use of the viscosity test as the preferred specification property. During the 1980s, many users and producers began increasingly working with modified asphalt binders to address some of the concerns about the performance of asphalt pavements. Since an elastomer, such as styrenebutadiene (SB), was the main method of modification used by many suppliers, the “new” asphalt binders had a significant elastic part, even at high temperatures, making the viscosity test less useful for characterizing its flow properties.
SHRP: Using “Cutting Edge” Technology for Asphalt
Recognizing the need for improved tests and specifications to assure the performance of asphalt materials, in the 1980s Congress initiated the Strategic Highway Research Program, or SHRP. Among other research tracks, SHRP included a five-year, $50 million research program studying asphalt materials. As part of this research program, asphalt researchers began working to refine the tests and specifications needed to better characterize the physical properties of asphalt binders at a wide range of operating temperatures. The production temperatures ranged from as high as 135°C (275°F) to in-service winter temperatures as low as -40°C (-40°F).
Additionally, the researchers needed to consider the fact that aging, whether short term aging such as the type that happens during construction or long-term aging such as the type that happens in the pavement after years of exposure to the elements, increases the stiffness of asphalt binders. It was also expected that the use of modified asphalt binders would increase as the demands on asphalt pavements, in the forms of increased traffic loading and higher performance expectations, continued to increase. Recent surveys have shown that this is indeed the case.
To handle this wide range of testing needs, the SHRP researchers focused on the DSR as the main tool in the toolbox that would be used by future technicians in characterizing the rheological properties of asphalt binders. While it was possible to identify a DSR that could adequately handle measurements at a wide range of temperatures and stiffness, it became clear that, with the chosen parallel plate geometry, torque capacity could likely drive the complexity and expense of the equipment. As a compromise, the researchers elected to use a DSR with sufficient torque capacity to handle intermediate to high temperature testing for specification purposes. Today’s specification-grade DSRs are more than capable of performing at temperatures from 0°C (32°F) to 100°C (212°F), depending on the stiffness of the asphalt binder.
Why Plate Size “Torques” So Many Technicians
After reviewing the equipment specs, the SHRP management saw that the need to determine rheological properties at intermediate temperatures (after simulating long-term aging) was important. The problem was that the asphalt binder stiffness was usually so high that by using the same parallel plate geometry that was used at high temperature, a 25-mm diameter plate, the torque capacity of the DSR would have to be greatly increased. With everything else being equal, the torque required to determine an asphalt binder’s stiffness at intermediate temperature may be 100 to 1,000 times the torque required at a high temperature!
To keep the DSR as an affordable, specification-grade piece of equipment, the designers either needed to “supersize” the torque or find a way to lower the demand. Since the measured complex modulus, G*, is a function of the specimen radius raised to the fourth power, small changes in the size of the parallel plates could yield big reductions in torque requirements. By cutting the plate size to a third of its original size (25 mm), the 8-mm diameter plate resulted in a required torque that was approximately 100 times lower than the torque required by the 25-mm diameter plate.
Although the torque issue seemed to be resolved, technicians found that using the smaller plate at intermediate temperature yielded more variable test results than they were used to seeing at high temperatures with the larger parallel plate geometry. Some of this variability can be attributed to the increased handling due to the asphalt binder aging procedures. With two aging procedures, Rolling Thin Film Oven and Pressure Aging Vessel, performed on the asphalt binder before testing at intermediate temperature, the opportunity for errors increased. Murphy’s Law tells us that much. The other piece of the puzzle for increased variability lies with testing variables. The smaller plate means that trimming is even more important. Errors in trimming creep in and are rapidly multiplied.
Recent studies conducted by the AASHTO Materials Reference Laboratory (AMRL) indicate that the acceptable range for intermediate temperature DSR test results from two different labs is 40 percent. That means if one lab tests an asphalt binder and gets a specification value of 3700 kPa at 22°C, then it is assumed that the material is well within specification limits and life is great. If that same asphalt binder is supplied to a user, and his lab tests a sample of that asphalt binder, he can get a specification value at 22°C that is as high as 5500 kPa and still be considered within the “normal” variability of the test. The problem is in the Performance Graded (PG) Asphalt Binder Specification, a spec value of 5500 kPa fails while the other result (3700 kPa) easily passes.
Put another way—suppose you buy a car and the listed gas mileage is 30 MPG. After driving it awhile you find that it does not seem to get anywhere near 30 MPG; it gets more like 20 MPG. So you go online, do some research and find that the manufacturer reported a test result of 30 MPG, while an independent lab reported a test result of 20 MPG. If the test variability was as high as our intermediate DSR value of 40 percent, then those two gas mileage tests would be considered the same statistically speaking.
Towards a Reduction in Variability
As with all things, education and experience, or practice, improves the ability to produce consistent results. To consistently shoot free throws in a basketball game, you have to know the proper form that works for you and practice it repeatedly. You still will not be perfect because errors happen, but your shooting percentage ought to increase.
The analogy works for asphalt binder testing. The better you understand the tests and the key factors affecting test results, the better prepared you will be to produce consistent results. Then it is simply a matter of practicing good technique repeatedly.
It is with this variability in mind that the Asphalt Institute recently developed a new manual entitled Asphalt Binder Testing: Technician’s Manual for Specification Testing of Asphalt Binders. This manual, principally authored by Dr. Dave Anderson (one of the architects of the Superpave PG asphalt binder system), covers the details needed for an asphalt binder technician to achieve consistent results in testing. The manual will be a useful tool for technicians to hone their skills and cut down on the high variability in some tests.
|Mike Beavin is the Technical Training Coordinator for the Asphalt Institute.
Mike Anderson is the Director of Research and Laboratory Services at the Asphalt Institute.