Better Roads

RoadScience 10 September 2014 Better Roads CRM and other modifiers Because of the prevalence of polymer modified asphalt in the age of Superpave's performance graded (PG) asphalt binders, researchers have worked to quantify the relationship between CRM and polymer modifiers in binders. There are two classic polymer types, elastomers and plas- tomers. Elastomers increase modulus (stiffness) and also give the asphalt elasticity and the ability to stretch. Under load they can provide recovery under deflection. Plastomers are more limited in that the just stiffen the asphalt, so they don't provide the kind of recovery you get with an elastic material, but still may be best suited for a particular application. A very common elastomer for asphalt modification is styrene-butadiene-styrene (SBS), in which the butadiene molecule actually is a kind of synthetic rubber. The Novem- ber 2012 publication, Performance Testing for Superpave and Structural Validation (Google FHWA-HRT-11-045), says explicitly that "based on the full-scale performance and laboratory tests, crumb rubber (recycled tires) modified asphalt (Arizona wet process) was shown to significantly slow or stop the growth of fatigue cracks in a composite asphalt pavement structure." But what of rubber and polymer modifiers? "A hybrid technique to modify asphalt with a combination of crumb rubber and conventional polymers (terminally blended) ex- hibited good fatigue cracking resistance relative to the control binder," the report says. That SBS-type polymer modifiers are "friendly" to CRM is borne out in recent Canadian research, which investigated performance-based asphalt cement (AC), AC modified with warm mix technology additives (warm AC), rubberized asphalt cement (RAC), and warm RAC. The 2013 TRB paper, Quality and Durability of Warm Rubberized Asphalt Cement in Ontario, by Hattie Xu, Andrew McIntyre, Tham Adhikari and Simon A.M. Hesp, Queen's University, Kings- ton, Ontario, and Pamela Marks and Seyed Tabib, Bituminous Section, Ontario Ministry of Transportation, describes how control, warm and rubberized asphalt cement (RAC) binders from Ontario construction contracts were investigated. "One warm AC and two field-blended RAC samples showed high levels of physical hardening, which can lead to early cracking," they write. "In an effort to formulate warm RAC with improved properties, a number of compositions were prepared with soft Cold Lake AC and a small quantity of naphthenic oil. These binders showed little chemical and physical hardening and reasonable critical crack tip open- ing displacements. Strain tolerance was much improved by co-blending with a high vinyl type styrene-butadiene-styrene (SBS) polymer and a small amount of sulfur." Replace SBS Completely? A white paper issued recently by RAF concludes that recycled tire rubber binders can be used in place of polymer modified binders such as SBS and achieve the same performance-graded (PG) results. The white paper was authored for RAF by advisory board member Dr. John D'Angelo, P.E., principal of D'Angelo Consulting, LLC, and a long-time FHWA expert on asphalt. "Polymer modified binders such as Superpave PG 76-22 have been used extensively on high volume highways to im- prove rutting and cracking performance," said RAF chair Way. "Given the current economics and higher costs for materials, highway agencies are looking for alternatives to the typical polymer modified binder systems." Recycled Tire Rubber (RTR) binders have been extensively used to provide the same type of improved performance as SBS, RAF says. The issue with polymer modifiers 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 that is attractive for use to produce improved binders. "History has demonstrated recycled tire rubber binders will perform well in rutting and cracking," D'Angelo says. "Using the new testing techniques, RTR binders can be com- pared directly to the polymer modified binders. This clearly demonstrates that RTR can be used in place of or in combi- nation with polymer to provide a high quality performance graded, (PG) binder." Asphalt rubber pavements are created as granulated tire rubber reacts with hot liquid asphalt to modify asphalt performance, as here in Arizona. Photo courtesy of Rubber Pavements Association

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• ROAD SCIENCE: Rubber Expands: After years of promotion, pavements using asphalts rubber, or rubberized asphalt (there's a difference), are spreading far and wide in United States
• HIGHWAY CONTRACTOR: Recycled Roads: New York City's Department of Transportation is now allowing its city streets to have mixes made completely of recycled asphalt pavement.
• EQUIPMENT: bauma China 2014: Registration is open for the November Chinese equipment show.
• SPECIAL REPORT: The Essential Highway Trust Fund Timeline: Here's a look at the Highway Trust Fund that is notoriously in danger of going in the red. For now, there's a short-term fix to keep it afloat.