This Circular is a synopsis of current physicochemical understanding of the chemistry, kinetics, and mechanisms of asphalt oxidation and its influence on asphalt durability. The oxidative behavior of asphalt is thought to be one of the critical factors contributing to the performance of hot-mix asphalt (HMA) pavements. The content of this review reflects more than half a century of research work on the physicochemical consequences of oxidative aging and its effects on the fundamental rheological properties of the oxidized asphalt. Emphasis is made on how the component fractions of asphalt behave under oxidation, including a consideration of the effects of the mineral aggregate and metals contained in asphalts. A section of the Circular reviews recent data and offers further insight on pavement aging as a function of air-void content of the mixture, temperature, and pavement depth and how oxidative hardening contributes to decreasing pavement service life. Civil engineers, chemists, and asphalt technologists developing fundamental pavement performance models will find this Circular to be of interest. Professionals in owner agencies will have at their fingertips a useful handbook to understand the subtleties of asphalt oxidation and how it contributes to distresses in an HMA pavement. Beginning students of asphalt technology will find this a welcome review with many references to original work.

A Review of the Fundamentals of Asphalt Oxidation: Chemical, Physicochemical, Physical Property, and Durability Relationships

Molecular dynamics study of oxidative aging effect on asphalt binder properties

The correlation between asphalt viscosity increase on oxidative aging and the carbonyl compounds formed (almost exclusively ketones) has been well established; however, the effect of sulfoxide formation on physical properties during age hardening has received little attention. Evidence is presented in this paper that shows that the alcohols, which are formed concurrently with the sulfoxides from the same hydroperoxide precursors as the ketones, have a similar effect on asphalt viscosity increase as does the ketones. These alcohols are the main contributors to oxidative age hardening of high sulfur asphalts. Analysis of the kinetic data for ketone and sulfoxide formation during asphalt oxidation also provides additional evidence for the validity of the dual asphalt oxidation mechanism previously reported.

Asphalt Oxidation Mechanisms and the Role of Oxidation Products on Age Hardening Revisited

Field aging effect on chemistry and rheology of asphalt binders and rheological predictions for field aging

The use of reclaimed asphalt pavement (RAP) and recycled asphalt shingles (RAS) in asphalt concrete (AC) mixtures can reduce demand for virgin aggregates and asphalt binder, bringing environmental and economic benefits. However, replacing virgin asphalt binder in AC mixtures poses challenges in terms of mixture volumetrics and low-temperature cracking, fatigue cracking, and other deterioration related to cracking. To counter these effects, softer virgin asphalt binder grades or modifying agents are used to improve production consistency. The purpose of this study was to develop and evaluate protocols, procedures, and specifications for testing engineering properties and performance of AC mixtures with high amounts (up to 60%) of RAP and RAS. The criteria for selection of the test method were (1) statistically significant spread in test outcome, representing a mix’s cracking resistance; (2) applicability and seamless implementation; (3) correlation to other independent test methods and engineering intuition; and (4) correlation to field performance. A practical test method, the Illinois modified semi-circular bending test (IL-SCB) was developed that can be readily implemented to quantify AC mixture’s cracking potential. In addition, a flexibility index (FI) was introduced to determine cracking resistance in a consistent way. The IL-SCB showed consistent and repeatable trends for changes in AC mix design properties. The developed FI was shown to provide greater separation between AC mixes to capture some of the changes that could not be captured by fracture energy alone. This approach does not enforce any limits on any AC mixture design characteristics, such as RAP and RAS content, or other alternative materials or technologies. Instead, it categorizes AC mixes based on their performance index. Hence, this approach encourages innovation and sustainability at the same time.

Testing Protocols to Ensure Performance of High Asphalt Binder Replacement Mixes Using RAP and RAS

The kinetic data and chemistry of asphalt oxidative age hardening suggested a sequential, dual mechanism for asphalt oxidation. The dual mechanism rationalizes conflicts between earlier mechanistic investigations and explains the hyperbolic-like, time-versus-property plots characteristic of asphalt oxidative aging. The oxidation kinetics provide further confirmation of the asphalt microstructural model. It is proposed that the rapid initial oxidation rate of asphalt results from reaction of oxygen with limited amounts of highly reactive hydrocarbons. Final oxidation products of this initial reaction are sulfoxides and, most likely, ring aromatization. During this initial reaction, a slower oxidation reaction of asphalt benzylic carbons is initiated; final products are ketones and sulfoxides. The ratio of ketones to sulfoxides formed and the rate of age hardening were found to be dependent on temperature and oxygen pressure. Low-temperature oxidative aging, as occurs in pavements, was found significantly m...

P. M Harnsberger

Papers

Asphalt aging: dual oxidation mechanism and its interrelationships with asphalt composition and oxidative age hardening