Rubberized Asphalt Concrete—Available Options
This article briefly describes the options available to the contractor who is considering Rubberized Asphalt Concrete (RAC). This is not an endorsement of rubberized asphalt concrete and is for information only.
BINDERS CONTRIBUTION TO PAVEMENT PERFORMANCE
Asphalt concrete is comprised mainly of aggregate and asphalt binder (sometimes referred to as “oil”). Binders play a very important role in the performance of a pavement during production as well as during the service life of the pavement. The binder is heated to allow for flow and mixing in the asphalt plant and to allow for proper placement and compaction. When the binder cools, it becomes the glue that holds the aggregate particles together creating hot mix asphalt. The properties of the binder are affected by temperature, rate of loading and aging. Aging occurs both during mixing at the plant and also when the pavement has been subjected to oxidation, which in turn causes the pavement to become brittle and more susceptible to cracking. Rutting, fatigue cracking, cold temperature cracking, placement as well as aging can be influenced by the binder properties. Therefore, using the correct binder type for a given project can make a substantial difference in performance.
Historically, Caltrans has used the AR grading system for binders. Typically in the central coast region, an AR4000 or AR8000 is specified. AR stands for aged residue. The binder sampled at the refinery is subjected to an aging process to simulate the aging that takes place in the asphalt plant. The number designation (i.e. 4000 or 8000) stands for the viscosity measured on the aged binder or residue. The higher the number, the more viscous the binder is. Viscosity is a measurement of a fluids resistance to flow. Syrup would therefore be more viscous than water.
The current AR grading system does not adequately address all of the important characteristics needed for a binder to perform well. Therefore, if a pavement is to perform well and resist rutting and cracking, modifications can be made to influence the binder properties for the better. Some of the available alternatives to AR binders are described hereafter.
RUBBERIZED ASPHALT CONCRETE (RAC)
Rubberized Asphalt Concrete (RAC) incorporates ground tire (crumb) rubber into the hot mix. Crumb rubber is ground or granulated rubber particles, which are derived from auto, light truck or other sources that use a high content of natural rubber. The wet process is typically used in rubberized asphalt within the State of California. The wet process mixes the rubber with the binder (80% asphalt cement / 20% crumb rubber) in a field blending unit prior to introduction at the hot mix plant. Caltrans began experimenting with rubberized asphalt produced by the “wet process” in the 1970s and developed design criteria using roadway deflection testing that resulted in thinner overlay courses than conventional asphalt concrete. Research has shown that 4″ of conventional asphalt can be replaced with 2″ of rubberized asphalt to achieve the same fatigue life. In the early 1990s, Caltrans testing confirmed the reduced thickness design of a gap-graded mix and received approval from the Federal Highway Administration. The stated advantages of Rubberized Asphalt Concrete are as follows:
- Rubberized Asphalt Concrete is highly skid-resistant, quieter and resists shoving and rutting if a gap-graded mix is used.
- Rubberized asphalt concrete is environmentally friendly. A two-inch resurfacing project can use approximately 2,000 waste tires per lane mile.
- Rubberized asphalt concrete provides excellent long-lasting color contrast for striping and marking.
- Rubberized asphalt concrete provides a long-lasting, durable pavement that resists reflective cracking.
- Rubberized asphalt can be placed using conventional paving equipment and methods.
Although research has shown that a thinner section can be placed and may ultimately last longer than conventional asphalt concrete, rubberized asphalt concrete does cost more per ton to produce due to the ground rubber and other ingredients as well as the cost of bringing in a portable blending unit to the hot mix plant. Rubberized asphalt can also be produced using a terminal blended process where the crumb rubber is added at the refinery or asphalt cement terminal. The advantage being that no specialized costly rubberized blending plant is required by the hot mix producer. The binder can be shipped to the hot mix plant just as a typical binder would be. This method offers a substantial cost savings over the wet process.
Graniterock recently produced hot mix asphalt using a terminal blended rubber for a running track at a local high school in Aptos, CA. This binder, supplied by the refinery, was the first ever use of a RAC-15 product conforming to ASTM-D6114 Asphalt-Rubber Binder specifications within the State of California. ASTM stands for American Society of Testing and Materials. The school applied for and was awarded a monetary grant by the California Integrated Waste Management Board for using recycled materials (i.e. rubberized asphalt concrete).
Graniterock has a proven track record of using Rubberized Asphalt Concrete to enhance the performance of asphalt pavements in certain applications. If you would like to learn more about your binder choices, please call Graniterock’s sales office at 831.768.2380.