Delta Tc (ΔTc) is a derived asphalt binder property that has been gaining attention for the last decade. It has become a topic of focus to both researchers in the asphalt binder technical community and user agencies seeking physical property parameters that will improve hot mix asphalt pavement performance. It is generally accepted that ΔTc targets cracking behavior that is affected by asphalt binder durability related to aging of the binder in an asphalt mixture. More specifically, ΔTc provides insight into the relaxation properties of a binder that can contribute to non-load related cracking or other age-related embrittlement distresses in an asphalt pavement. It is a calculated value using the results of the bending beam rheometer test determined on laboratory-aged asphalt samples or samples recovered from pavements. At the time this document was developed (mid-2019), ten user agencies have or soon will implement ΔTc as part of their purchase specification, with two more expecting to do so in the near future. In addition, several national level research projects are actively considering ΔTc as part of their studies. In fact, ΔTc was first hypothesized via a research project led by the Asphalt Institute.
Considering these factors, at the 2019 spring meeting of the Asphalt Institute, its Technical Advisory Committee decided that there was substantial need for a state-of-the-knowledge, engineering report to describe ΔTc and its relevance in characterizing the behavior of asphalt materials. It is intended that this document fulfill that purpose. Prior to this document, relevant information pertaining to ΔTc was scattered among research papers, presentations, various meeting minutes, and personal communications among individuals with detailed interest in ΔTc. Consequently, it was difficult to find and sort through relevant sources of information. Hopefully, this document provides a single, up-to-date reference on the topic of ΔTc. It is also hoped that this report will serve as a focal point for dialog among agency users, industry producers, academia, and others with a need to have a more detailed understanding of ΔTc.
- The second section describes the research that led to the development of ΔTc.
- The third section provides a detailed explanation of how ΔTc is computed along with a review of the testing involved to capture information needed to calculate ΔTc. Information is presented to help explain the physical meaning of ΔTc. To offer perspective on ΔTc, typical values are shown for a wide range of PG binders.
- The fourth section provides information pertaining to factors that affect ΔTc. Laboratory aging, both normal and extended, is discussed and data is offered that illustrates the change in ΔTc with various forms of laboratory aging. Information is presented describing the effect of other materials on ΔTc. These include materials such as recycled asphalt and elastomeric polymers.
- The fifth section presents practical considerations related to the measurement of ΔTc. Factors such as precision 1 and the effect of ΔTc on laboratory workflow are described. Because ΔTc is viewed as an asphalt binder property that is related to asphalt pavement performance, the estimated effect of ΔTc on various asphalt pavement distress types is also described.
- The sixth section describes ΔTc data derived from full scale projects.
- The seventh section describes the use of ΔTc in forensic and specification environments.
- Because there are numerous national-level research projects that are considering ΔTc, the eighth section provides a brief summary of that research that pertains to ΔTc.
- Summary and references used to produce this document are covered in the ninth and tenth sections.
- Lastly, a list of frequently asked questions (FAQ’s) pertaining to ΔTc is presented in Section 11.