The problem with these methods is that the tests were not run under pavement conditions. The low-temperature test never reached the extremes observed in the field and no test existed for measuring rutting resistance in the upper pavement temperature region. The polymer-modified specifications have evolved through a series of steps. Originally, several test methods were developed to identify the presence of certain types of polymers. These methods were provided to state DOTs to assist them in writing specifications. These specification usually excluded other polymer modifiers from being used in asphalt pavements.
The next phase was the development of specifications which were unique to each type of polymer. Task Group 31 categorized tests which apply to styrene-butadiene-styrene (SBS), styrene-butadiene rubber (SBR), low density polyethylene (LDPE) and ethylene vinyl acetate (EVA). These specifications have subsequently become ASTM standards. The problem associated with these types of specifications is how to evaluate two different modified binders with different test methods. Most users want a level playing field in which to compare all the different modifiers. With the same testing criteria, performance can be directly related to cost. The performance based asphalts (PBA) by the West Coast User/Producer Group was the first performance-based specification. These specifications used conventional test methods like penetration, viscosity and ductility, and related them to different performance requirements like resistance to rutting, processability and low-temperature performance. The specifications are divided into different levels depending on the geographical location of the pavement. The next logical step was to develop performance-based specifications using methods which measure engineering properties rather than purely empirical values.
Superpave binder specs
Once the Strategic Highway Research Project ended there were several products which were ready for use. The Superpave Performance Based Binder Specifications received the most attention, while the mix design was being further refined. The components of Superpave consist of the Brookfield viscosity, Dynamic Shear Rheometer (DSR), Bending Beam Rheometer (BBR), the Direct Tension Test (DTT) as well as a set of mix design and analysis tools.
The performance grade (PG) is based on the high pavement design temperature at a depth of 20 mm, and the minimum one day minimum surface temperature for the location that the asphalt pavement is to be constructed in. A software database is available which provides PG grades based on historical weather data, geographic location and pavement depth. This can help to determine the PG for a specific region. The implementation of these specifications has resulted in many states having more grades of asphalts than before, requiring the contractor and supplier to acquire more tankage. Also, many user/producers have purchased the Superpave binder equipment. The creation of Superpave Centers which have the complete binder and mixture test equipment has helped to implement and transfer the technology. With all this effort the Superpave binder specifications are being refined to meet the needs of the various regions of the U.S. Several issues are on the forefront regarding the Superpave binder specifications.
The three areas of current study are the variation in the m-value between BBR instruments, the fatigue parameter and new DTT. There is a variation in the m-value (time dependent parameter measured at 60 seconds using the BBR) between the ATS and Cannon instruments. The variation is related to the time when data collection is initiated during a test and the geometry of loading of the sample. The difference between the ATS and Cannon instruments is typically 0.01. This makes a big difference between a passing 0.30 and a failing 0.29. Because most DOT's have the ATS instrument and the suppliers and refiners have the Cannon instrument, this could cause problems. Because of this difference the AASHTO Binder Expert Task Group has recommended that 0.01 be added to the m-value when the ATS instrument is used. Unlike high-temperature permanent deformation (more aggregate dependent) and low-temperature thermal cracking (more binder dependent), fatigue cracking is a combination of both. Poor correlation has been found between G*Sind and fatigue cracking in mixtures and pavements. A task group has been formed to study the relationship between fatigue of the mixture and binder properties. Data obtained from California and other sources will be used in the study.
Instron has developed a direct tension device which has improved the usability and precision for low-temperature tension testing. This system uses a liquid bath instead of the air-cooled system previously employed. Once the ruggedness testing is completed the FHWA plans on purchasing the system through the pooled fund. It was recommended by the Binder Expert Task Group that the DTT be operated at a lower strain rate and the samples undergo vacuum degassing prior to testing. The lower rate is used to simulate the cooling rate which occurs in the pavement. Vacuum degassing removes the small bubbles which could reduce the strain to failure, and increase the variability.
Although these issues exist, the development of performance-based specification have come a long way since viscosity-graded systems. The biggest problems lie with modified system which consist of a variety of modifiers which affect asphalts in different ways. The difficulties that exist when using Superpave for polymer-modified systems are: the rolling thin film oven test (RTFOT), low temperature testing, fibers, and compatibility/ separation tests.
When polymer-modified asphalts are tested in the RTFOT several types of problems can occur. The increased viscosity of the binder prevents the formation of a thin film, which is observed with unmodified binders. The thicker film on the bottle reduces the aging of the binder, suggesting that the polymer improves aging resistance. Furthermore, some polymer-modified asphalts "walk" out of the bottle during testing causing obvious problems.
One concern which is unique to polymer-modified asphalts is compatibility. Currently, there is no test method for compatibility in Superpave. Many methods have been used to test compatibility including fluorescence microscopy and the cigar tube test. In the past the softening point of the upper and lower section of the cigar tube have been used to determine if separation has occurred. The problem with this method is that the softening point does not correlate with Superpave parameters like G*/Sind. These methods give both qualitative or quantitative evaluation of compatibility, but have not received widespread acceptance. The importance of compatibility has prompted the development of commercially available systems which can stabilize polymer-modified asphalts. Two of the systems available are Polyphalt and Butophalt.
The low temperature performance of the asphalt pavements depends predominantly on the binder. Superpave was based on neat asphalts which display a limiting stiffness around 300 MPa. Polymer-modified asphalts behave differently and the specifications don't necessarily demonstrate the enhanced performance that some polymers give to asphalts.
Because a lot of these issues are being studied in the NCHRP 9-10 Superpave Protocols to Modified Binder, an immediate solution is not available. This is leaving many DOTs in a predicament regarding how to specify polymer-modified binders. The main problem occurs when a state DOT has successfully used polymer modifiers and the new specifications call for a binder which does not require modification. If they go back to using the unmodified asphalts the old problems may return. Some states are using a Superpave specification, but also are requiring a certain weight percent of polymer. This will continue to be a problem until Superpave has been amended to address modified systems.
The use of fibers in the binder has caused much debate. The Superpave specifications were based on the assumptions of linear viscoelasticity. When fibers are tested in the binder, the system does not display a linear viscoelastic region. This does not mean that fibers should not be used, but the criteria should be part of the mixture testing.
The future
The future holds a variety of events with respect to asphalt and polymer-modified asphalt specifications. This includes the widespread use by states, counties and cities of the Superpave binder specifications. The introduction of new types of modifiers and more combinations of modifiers being used in the binder. As Superpave is ushered in, the contractor will be increasingly more responsible for the quality control. The DOTs in general will rely more on independent laboratories to perform the quality assurance, resulting in the creation of more Superpave equipped testing laboratories. Suppliers who pre-blend may be required to increase their tankage. The point of sampling at the plant will be right before the asphalt is incorporated into the drum or pug mill. Allowances also will be made for in-line blending in the QC/QA requirements. With polymer modification growing, the leaders in the industry will continue to prosper, and Superpave specifications will be amended for polymer-modified asphalts. The specification will have an easy protocol so that as new materials enter the market they can be evaluated under the same criteria. Also, as a result of Superpave, the appearance of more engineered binders which consist of two or more modifiers will become common.
On the mixture side, Superpave volumetric mix design using the gyratory compactor will be adopted by most user agencies. The Superpave pavement performance models will be revised and enhanced, and used where necessary.
Superpave has changed the face of asphalt. Inevitably this effort will lead to better and longer lasting pavements.
