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https://nap.nationalacademies.org/catalog/27432/critical-issues-in-transportation-for-2024-and-beyond

ACID/BASE CHEMISTRY FOR ASPHALT MODIFICATION (WITH DISCUSSION)

The advent of Superpave heightened the need for asphalts with an extended range between the temperature at which the asphalt has sufficient modulus to resist rutting (T sub h) and the temperature limit for critical stiffness to prevent thermal cracking (T sub l). Acidic and basic modifiers have been shown to improve asphalt stiffness, elevating T sub h without unduly harming T sub l. The low cost of these materials relative to polymers and other modifiers that improve asphalt temperature susceptibility makes their use compelling in a competitive environment focused upon manufacturing Performance-Graded (PG) asphalt binders. Amine antistripping agents (ASAs) have long been added to asphalt mixtures to reduce deleterious effects of moisture. The antistripping activity of the amine is typically attributed to the nitrogen lone pair of electrons, which show strong affinity for the aggregate surface in the presence of water. What happens, then, when such basic compounds are added to mixtures containing acid-modified asphalt? Fundamental laws of chemistry would dictate that the acid modifier and the basic ASA may react with each other to form an amine salt. This raises several questions: Will T sub h drop if the acid modifier is neutralized? Can the ASA continue to resist moisture if the active electron pairs have been blocked by the acidic protons (H+)? Will the product salt contribute to moisture damage in the pavement, either by leaving the asphalt film more permeable to water, or by emulsifying the asphalt in the presence of heat, moisture, and traffic? To address these questions, a research study was initiated by the Kansas Department of Transportation (KDOT). Phase 1 was a binder study, with dynamic shear rheometry (DSR) measurements of T sub h on asphalt binders modified with varying concentrations of phosphoric acid, two different modified fatty imidazoline antistripping agents, and blends of acid and imidazoline. These same binders were then dissolved in toluene and extracted with water to determine extracted pH and to monitor for possible emulsification of asphalt into the aqueous phase. Phase 2 was a mixture stripping study. A moisture sensitive aggregate was blended with PG 58-22 that had been modified with various acid/base combinations. The resulting mixtures were evaluated for moisture damage. Hamburg and Asphalt Pavement Analyzer wheel-tracking tests were run alongside KDOT's retained tensile strength test (KT-56), since it was theorized that mechanical loading might be necessary to simulate damage from in-situ asphalt emulsification. Key findings were: phosphoric acid additives significantly increased asphalt stiffness at high temperatures and moisture damage may occur if acid is used at elevated concentrations; ASAs consistently reduced moisture damage in both KT-56 and wheel-tracking tests; phosphoric acid reacts with ASA, resulting in significant moisture damage; and binder T sub h should be specified or confirmed following amine addition when a mix requires ASA.

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  • English

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  • Accession Number: 00970360
  • Record Type: Publication
  • Files: TRIS
  • Created Date: Mar 1 2004 12:00AM