On July 17, Doug Carlson, vice president of asphalt products for Liberty Tire Recycling and a RAF and Rubber Pavements Association member company representative, gave an update of rubberized asphalt technologies at an open house hosted by the MDOT Grand Rapids District. The open house was organized to bring attention to the recent demonstration project using a performance graded rubberized asphalt binder. The prime contractor was Michigan Paving and the binder supplier was Seneca Petroleum.
Due to the high cost of virgin asphalt and virgin material asphalt modifiers (tied to high crude oil prices), the engineering community is evaluating the use of recovered and recycled materials for use in highway maintenance and construction to save money. Recycled tire rubber (RTR) has a second opportunity for implementation in asphalt paving applications due to the fact that rubber costs less than asphalt pound for pound. RTR used to cost twice as much as asphalt back in the 1990s, but now it is about half as much, and unfortunately many people still think of rubberized asphalt in 1990s terms.
There are three major categories for RTR modified asphalt: the plant mix, PG rubberized asphalt, and asphalt-rubber (a highly modified binder).
The plant mix system refers to where the rubber and additives are added to the asphalt mix in the drum. It is all done at the mixing plant.
PG rubberized asphalt is a “wet system” where the rubber and additives (if any) are added to the liquid first. This can be done off site, at a terminal, temporary terminal, or on site. Both of these systems use about 8-12% rubber by weight of the liquid. In practice they use about 11 pounds of rubber per ton of mix. The PG rubberized asphalt complies with the current PG QA/QC systems and does have the most similarity to PMA. These both use a 30 minus rubber particle that can fit into a dense graded mix.
The third category is a wet process for asphalt-rubber binder and uses about 30 pounds of rubber in a ton of SMA or open graded friction course mix. It requires about 60 minutes of reaction (saturation) time after the rubber is added to the asphalt and is done at 350F. This requires extra tanks that have the heating and agitation capacity and is usually done on site at the plant with a mobile (temporary) blending unit that ties into the plant’s asphalt feed line for the mixing drum. The RTR in asphalt rubber can be more coarse, more on the 20 mesh side, but uses anywhere from 10-50 mesh material. Due to the very high rubber content, the viscosity of the binder can be high so liquid temperature management and larger pipes and pumps (that come with the blender unit) are used.
Presently the biggest value to the asphalt community is the ability to replace virgin SBS polymer with RTR. Polymer modified asphalt typically costs $125 more per ton than the base asphalt, an example from present market condition is base (un-modified) asphalt at $550 and $675 for polymer modified.
When RTR is used as an alternative polymer, some other additives may be required to get polymer like properties, if the agency requires it. However, all of the physical performance related properties of modified asphalt can usually be met with RTR alone. A generic target is 3% RTR for every 1% SBS to achieve the same properties. If an agency requires the classic polymer tests and an additive other than rubber is used, the amount of additive will be around 0.5% to 1.5% of the rubber content (and can even be SBS). The additives when blended with the asphalt can make the rubber blend go up to ~$0.50/pound, but will stay much lower than the current ~$2.00/pound price for SBS. It is reported in Louisiana that the State DOTD is realizing a 15% savings with rubberized asphalt compared to polymer modified asphalt.
Two DOTs have specifications for the use of plant mix rubber. The Georgia DOT uses the Special Provision 820 for their plant mix materials and Pennsylvania has some special provisions for dry systems, including the use of rubber as a stabilizer in stone matrix asphalt mixes to prevent binder draindown. The plant mix system will typically use a fiber feeding system that is available in most plants to introduce the rubber materials into asphalt mix. Ideally, any and all additives should not be referred to by name, but by the property they provide to the mix. Generic terms such as suspending agent, or rubber and asphalt compatibility agent should be used.
The LA DOT has adopted a complete substitution of rubber for polymer and they use a PG 82-22 grade for rubber to replace a PG 76-22 for polymer. Since they allow an onsite blending process, the higher grade for rubber provides insurance that a PG 76 material can be easily delivered. A PG 82-22 rubber modified asphalt can be used in the place of any other grade of asphalt cement. Blending can be done at the asphalt terminal or the asphalt plant or anywhere in between. The Texas DOT provision for plant mixing of RTR binders is provided for reference. A suspension agent or processing aid is commonly used when material is delivered from a terminal and the liquid is stored in a polymer modified tank with agitation. On site blending usually requires additional equipment at the plant which provides an aggressive agitation or a mechanical suspension.
Asphalt rubber is the technology that took off in the Southwest before Superpave and has continued use in California, Arizona, Texas and Florida. Agencies that have recently adopted this technology include New Jersey, Massachusetts, Nebraska, some Canadian provinces, Sweden, a few European countries, and China. It is considered a highly modified binder with typcal rubber contents between 17-22%. Due to the high rubber concentration in the paving material, either a doubling of the pavement life OR a reduction in overlay thickness can be obtained. Many agencies are adopting asphalt rubber for the reduced thickness design and thin lift pavement preservation values it provides. The AR open graded friction course is the material of choice for quieter pavement designs such as the thin overlays on the concrete pavements in the Phoenix metropolitan area, Las Vegas area and in New Jersey.