It can be clearly seen that the RTR PG 76-22 can meet all the binder requirements of the PG specification. These studies did show the increase in modulus of the binder with the addition of the RTR and that the size, percentage of rubber and base asphalt all had an effect on the binder properties.
However, to address RTR in general use, which comes in many different sizes, the most typical size is larger than 30 mesh, thus new approaches to testing are needed. This requires testing of the binder with larger particle sizes using geometries with larger gaps.
Testing systems available
Many studies have shown that the RTR size, shape, mixing temperature and asphalt binder will all affect the final properties of the RTR binder. Without a well-defined binder specification adoption of the use of RTR binder by the U.S. highway agencies will be almost impossible to achieve. Test procedures that can evaluate the performance characteristics of RTR binder are crucially needed.
Performing PG testing on RTR binders with larger particles will require using new geometries that will provide larger gap sizes that can accommodate those particle sizes. Rubber particles may range in size from 0.5 mm up to over 1 mm in size.
A 1 mm particle tested in a DSR with 1 mm gap parallel plate geometry would be touching both top and bottom plates at the same time so that test results would represent the rubber particle not a rubber modified binder. One approach that has been used in the food industries has been testing with concentric cylinder geometries. DSR’s currently used for asphalt testing can be adapted to use a Searle system. This system is one where the center cylinder or bob rotates and the outside cylinder or cup is stationary.
This type of system can perform all the same type of testing that is currently used for asphalt binder grading. The advantage is that the cup and bob geometry can easily handle larger gaps up to 4 to 7 mm and therefore larger RTR particles. One disadvantage of the system is that it does require a much larger sample for testing. Graphics and pictures of the geometry are shown in Figure 2.
The Bending Beam Rheometer (BBR) is another primary piece of testing equipment used in the Superpave PG grading system. The BBR is used to measure the low temperature stiffness and relaxation properties of the binder. The testing is done on a beam of asphalt binder 6.4 X 12.7 X 127 mm. Since the beam has a cross section of 6.4 X 12.7 mm it can actually accommodate RTR particles of about 1 mm. Because of this size, no changes should be needed to test RTR binder in the BBR with the larger particle sizes.
Initial testing of the new DSR testing geometry to the existing 1 and 2 mm gape parallel plate geometry has shown that equivalent results can be obtained. The testing was done on both neat and RTR binders. PG testing of the RTR binders clearly shows the changes that occur to the base 64-22 with RTR size and percentage.
Figure 3 shows the comparison using the different geometries of the complete continuous grading of a base PG 64-22 binder to the base plus 10% 60 mesh blend. The 10% 60 mesh RTR increases the high temperature stiffness of the PG 64 to a PG 70. The addition of the RTR also lowers the intermediate DSR stiffness and BBR low temperature properties.
The 10 % 60 mesh RTR changed the PAV DSR continuous grade from 24.6?C down to 20.4?C. The addition of the RTR also lowered the low temperature continuous determined from the BBR from -22 to -25.5?C. The testing demonstrated that results are equivalent for the different geometries and the improvement in properties with the addition of RTR.
Preliminary testing with the new geometry has shown that it will provide the same results as the standard parallel plate geometry in the DSR. Both course and fine ground RTR were evaluated and provide improvements in the properties of the binder.
Using the new geometry for the DSR and the BBR, RTR binders even with larger crumb size can be evaluated using the Superpave binder specification. This allows for direct comparisons of polymer modified binders to RTR binders.
History has demonstrated RTR binders will perform well in rutting and cracking. Using the new testing techniques, RTR binders can be compared directly to the polymer modified binders. This clearly demonstrates that RTR can be used in place of or in combination with polymer to provide a high-quality performance graded (PG) binder.