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Asphalt Rubber Myths

MYTH: The Contractor's Equipment Will Be Damaged
TRUTH: Crumb rubber modified (CRM) hot mix asphalt (HMA) will not harm any of the contractor's storage, production, placement, or compaction equipment. However, agitated storage tanks like those used to store PG 76-22 (SBS polymer-modified) asphalt binder are also recommended for storage of CRM asphalt binder.

MYTH: The Contractor Needs Completely Different Equipment
TRUTH: No additional equipment is required for terminal blending, and only one additional piece of equipment is required for on-site blending. For terminal blending, the CRM asphalt binder is blended at the asphalt binder supplier's terminal and shipped to the contractor's HMA plant on heated tankers. An agitated storage tank at the HMA plant is required for re-agitation of the binder before use. All other production, placement, and compaction equipment are the same as those used for conventional HMA.

For on-site blending, a blending unit is the only extra equipment required to blend the CRM asphalt binder at the contractor's HMA plant. The blending unit connects directly to the contractor's asphalt binder storage tank (an agitated tank is recommended). All other production , placement, and compaction equipment are the same as those used for conventional HMA.

ARTS has a blending unit available to contractors for use on CRM HMA projects. Click here to see how the blending unit works. Click here to contact ARTS for information about equipment availability and lease conditions.

MYTH: The Aggregate Blend and Mix Design are Different
TRUTH: All ARTS CRM HMA mix designs use the same Superpave gradations and mix design procedures as those currently required by the South Carolina Department of Transportation (SCDOT). However, the mixing and compaction temperatures that are used are slightly higher than those required for conventional unmodified PG 64-22 asphalt binders but slightly lower than those required for polymer-modified PG 76-22 asphalt binders.

MYTH: All CRM Binders are the Same
TRUTH: Different rubber particle sizes and different amounts of rubber can create vastly different CRM binders. Different agencies require the use of different rubber particle sizes for CRM asphalt binders. ARTS projects specify the use of minus #40 mesh crumb rubber particles because CRM binders made with this size of rubber can flow through the same pipes, valves, and pumps that are used with conventional asphalt binders, while CRM binders made with larger particle sizes may require specialized equipment (larger pipes, valves, and pumps) at the HMA plant.

In addition, different applications need different amounts of rubber. For example, when using minus #40 mesh rubber, gap-graded mixes require 10% rubber by weight of the virgin liquid asphalt binder, while open-graded friction course (OGFC) needs 18%, and stress absorbing membranes (SAMs) require at least 20%.

MYTH: The Cost is Much Higher
TRUTH: The cost of CRM HMA is very similar to the cost of polymer modified PG 76-22 asphalt binder, which is now used on all Interstate HMA paving projects in South Carolina. Both of these high-performance binders cost approximately 20% to 30% more than conventional unmodified PG 64-22 binder. Although the initial cost of CRM HMA is slightly higher than that of conventional HMA, it is an enhanced product that last longer and performs better than conventional HMA. Click here to access reports from several life-cycle cost analysis (LCCA) studies of conventional, CRM, and polymer-modified asphalt applications.

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