In the 1990s, the new Superpave binder specification was introduced. The Superpave binder specification is based on the rheological properties of the asphalt binder measured over a wide range of temperatures and aging conditions.
Various pieces of equipment are used to measure stress strain relationships in the binder at the specified test temperatures. This equipment includes the Dynamic Shear Rheometer (DSR) and Bending Beam Rheometer (BBR). Measuring the binders' rheological properties over a wide range of temperatures, loading conditions, and aging conditions allows performance relationships to be established between the test results and the pavement. The details of this asphalt binder testing are described in the American Association State Highway and Transportation Officials AASHTO Specification (AASHTO) M 320.
Polymer modified binders growing in the U.S.
The use of polymer modified asphalt binders has grown tremendously in the United States. This is due primarily to the increased stress on the highways from higher traffic volumes and heavier loads. The growth can also be attributed to the new Superpave specifications, which provide a procedure to evaluate the performance characteristics of the polymer modified binder. This allows the highway agencies some assurance in the quality and consistency of the binder. Currently almost 20% of the asphalt binder sold in the United Stateves for paving is polymer modified.
Scrap tire rubber, also known as recycled tire rubber (RTR), has been used since the 1960s to modify asphalt binder. Uses have included stress absorbing membranes, inter-layers, crack seals, hot mix asphalt, and open graded friction courses. RTR binder has also been used to address the issue of increased traffic and heaver loading.
Historically the specifications for RTR binder in most of these applications have been recipe or method type. Method specifications describe very specific processes and amounts of material to produce a specific product. In many cases where contractors have experience with these specifications good performance is achieved. However, this makes transfer of these processes and specifications difficult from one location to another and increases the potential for failures. These issues make highway agencies very reluctant to try RTR technology.
Alternatives to polymer modified binders
Given the current economics with higher costs for materials highway agencies are looking for alternatives to the typical polymer modified binder systems such as Styrene Butadiene Styrene (SBS). Polymer modified binders, such as Superpave PG 76-22, have been used extensively on high volume highways to improve rutting and cracking performance. RTR binders have been used to provide this same type of improved performance.
The issue with polymer modifiers such as SBS is that they are subject to supply demands and chemical production variations that can lead to supply shortages and higher costs. Scrap tire rubber for RTR is in plentiful supply with a relatively stable cost which is attractive for use to produce improved binders. The biggest question is performance testing of the RTR binders to evaluate its properties.
Performance testing RTR binders
Test procedures of a somewhat crude nature have been used to provide for field quality control for the various RTR binder processes. The primary device is the hand held rotational viscometer. This can provide some indication of viscosity increase from the addition and blending of rubber into the binder but has high variability.
Some preliminary binder testing has been done using the Superpave binder tests on RTR binder but this has been limited to RTR sizes that can be handled in the 1 or 2 mm gap using DSR parallel plate geometries typically 30 mesh material or smaller. Figure 1 shows a comparison of a typical SBS modified PG 76-22 compared to a RTR PG 76-22.