Classification of Asphalt Binders Into Simple and Complex Binders

Classification of Asphalt Binders Into Simple and Complex Binders

Hussain U. Bahia, Walter P. Hislop, Huachun Zhai, and Andres Rangel

Abstract

The objective of the NCHRP 9-10 Project is to evaluate the suitability of using the Superpave binder and mixture test system for modified asphalt binders. As part of this project, a survey of users and producers of modified asphalts was conducted. Also, a detailed analysis and review of published literature was carried out. These efforts resulted in identifying the asphalt additives most commonly used, the concerns that users and producers have with the application of Superpave binder specification to modified binders, and the projected change in the use of modified binders based on their experience and the future needs for high quality pavements. The results indicate that there are few additives that are widely used in the current practice of asphalt modification in the United States and Canada. They also indicate that modified binders will be used more in the future and that there are major concerns regarding the applicability of the Superpave PG-grading system to modified binders.

The analysis of information collected from the surveys and the literature search had also resulted in identification of important gaps in the PG-grading system. It is evident that the Superpave specification is based on assumptions that apply only to binders that exhibit simple rheological behavior, such as conventional unmodified asphalts. There is, therefore, a need for a new classification system that differentiates between binders that can be considered as part of the PG-grading system and others that cannot. Binders that satisfy the assumption made in the PG-grading system can be considered simple binders. The other binders should be classified as complex binders because they exhibit complex behavior that does not satisfy the assumptions made in the PG-grading system.
In addition to the need for a new classification system, a set of new or revised tests are needed to address characteristics that are typical of modified binders and that are not included in the current PG-grading. As part of the NCHRP Project 9-10, the Particulate Additive Test (PAT) has been developed to determine the effective volume concentration of particulate additives. Also a new test to measure storage stability and potential for thermal degradations called the Laboratory Asphalt Stability Test (LAST) has been introduced. A modification of the RTFOT procedure to handle modified asphalts has also been developed and used with a wide variety of asphalts.

This paper covers a brief summary of the survey results and the literature review. It includes the background and the rationale of the new classification system of simple and complex binders, and also describes the new and revised testing being proposed to apply Superpave binder specification to all simple binders, modified and unmodified. Typical results for a wide variety of modified asphalts are presented to show the utility of the new tests and the proposed method for interpretation.