By Mark Buncher, Ph.D., P.E.
The Asphalt Institute (AI) recently released “IS-240 State-of-the-Knowledge: Use of the Delta Tc Parameter to Characterize Asphalt Binder Behavior.” This 64-page technical document is available as a free download on the AI website. Search under either Engineering or in AI’s store for the document labeled Informational Series (IS) 240.
What is Delta Tc (ΔTc)?
Delta Tc (ΔTc) is an asphalt binder parameter that has been gaining attention from both researchers and user agencies seeking physical property parameters that will help ensure long-term asphalt pavement durability performance.
There is no new equipment or even a separate test that is necessary. The ΔTc value is simply calculated by using the results (S and m) from the standard Bending Beam Rheometer (BBR) test that is necessary for determining the continuous low-temperature PG grade. It is intended to be used on binder that has been short- and long-term aged (RTFO plus PAV), but can also be used on binder recovered from asphalt pavements.
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. Block cracking is the most common distress of this type.
The words in the title, “state of the knowledge,” are key in understanding the reason the Asphalt Institute and its members decided in April 2019 to write this document.
Prior to this document, relevant information pertaining to ΔTc was scattered among research papers, presentations, various meeting minutes and personal communications among individuals with interest in ΔTc. Consequently, it was difficult to find and sort through relevant sources of information. This document provides a single, comprehensive and up-to-date reference on the topic of ΔTc, which can 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 and its relevance in characterizing the behavior of asphalt binder.
At the time the document was released in Oct 2019, ten U.S. user agencies had or soon will implement ΔTc as part of their purchase specification. Details around these cases are provided in the document, yet it is important to understand that the Asphalt Institute does not take a position on the adoption of ΔTc in a binder purchase specification.
The goal of the document is to describe the current state of the knowledge around ΔTc and is not intended to persuade or discourage an agency from using it. The document does, however, describe a number of key steps that should be carefully considered before a purchasing agency implements ΔTc as a specification requirement.
The document is not light reading. It summarizes a considerable amount of research over the past decade with an abundance of data in the form of tables, charts and graphs.
Chapter 1 is an introduction, with chapter 2 describing the research (led by Asphalt Institute’s Mike Anderson) that led to the development of ΔTc. Chapter 3 provides a detailed explanation of how ΔTc is computed along with a review of the testing involved. 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.
Chapter 4 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 lab aging. In addition, data is presented describing the effect of other materials on ΔTc, such as recycled asphalt and elastomeric polymers.
Chapter 5 presents practical considerations related to the measurement of ΔTc, such as precision and the effect of ΔTc on laboratory workflow. The estimated effect of ΔTc on various asphalt pavement distress types is also described. Chapter 6 discusses ΔTc data derived from full-scale projects.
Chapter 7 covers the use of ΔTc in forensic and specification environments, including a discussion of those agencies that have incorporated ΔTc in their binder specifications. A list of steps is described that agencies are encouraged to consider as they contemplate the use of Delta Tc in a purchase specification. Alternatives to ΔTc for addressing block cracking are also provided.
Chapter 8 is a summary of recent national research projects pertaining to ΔTc. Chapter 9 is a brief summary to the document while chapter 10 simply lists the numerous references used. Lastly, a list of twenty frequently asked questions (FAQs) pertaining to ΔTc is presented in Chapter 11, serving as a quasi-executive summary.
Developed from Chapter 11 are the following 10 questions that perhaps you wanted to ask about ΔTc, along with a brief answer.
What led to the development of Delta Tc?
Surprisingly, it was not how ΔTc is typically being used today. ΔTc was conceptualized in a research project sponsored by the Airfield Asphalt Pavement Technology Program (AAPTP), “Project 06-01, Techniques for Prevention and Remediation of Non-Load Distresses on HMA (Hot Mix Asphalt) Airport Pavements.” The goal of the study was to identify simple binder and/or mixture testing which could predict imminent block cracking or raveling so that pavement preservation strategies could be timed to delay or prevent damage to airport pavements. The study concluded that a new binder parameter called ΔTc had promise as a tool that could be used to analyze the durability-related properties of aged asphalt airfield pavements.
Delta Tc measures the relaxation properties of a binder. What exactly is relaxation and how does it affect mixture performance?
When thermal stresses build up as a pavement gets colder, the asphalt binder will gradually experience viscous flow and the stresses will greatly reduce. This reduction of stresses over time is what is known as relaxation. In general, as a binder ages, its relaxation properties are diminished. An asphalt pavement that has a binder with good relaxation properties will be less likely to have durability-related cracking than a pavement containing a binder with poor relaxation properties.
Can Delta Tc be used to predict cracking?
Yes. ΔTc is thought to be principally related to block cracking. However, fatigue, edge, longitudinal, reflection and transverse cracking may indirectly be related to ΔTc of the binder. These distress types are typically caused by other factors, yet ΔTc can play a supporting role in their development.
How is Delta Tc calculated?
Using the BBR test (AASHTO T313) results, the critical (or continuous) temperature for both creep stiffness (S) and creep rate (m) at the AASHTO 320 limiting values of 300 MPa and 0.300 respectively are calculated. ΔTc is simply the mathematical difference between these two critical temperatures, expressed in degrees C and typically reported to one decimal point. The equation is:
ΔTc = Tc,S – Tc,m.
Are there any AASHTO or ASTM standards that discuss ΔTc?
ASTM D7643 Section 6.3 “Calculation of ΔTc”, and AASHTO PP 78-17 Section 7 “Binder Quality Requirements for Binder Embrittlement”, both discuss how to calculate ΔTc.
What does the binder’s ΔTc value tell me?
The sign on ΔTc, either positive or negative, indicates whether the binder’s PG low-temperature grade is governed by its creep stiffness S (+ ΔTc) or governed by its creep rate m (-ΔTc). When ΔTc is positive, the binder is referred to as being “S-controlled” (failing the S criterion at a warmer temperature than the m criterion), while a negative ΔTc value indicates the binder is “m-controlled” (fails m criterion at a warmer temperature than the S criterion). The absolute magnitude of the ΔTc value indicates the degree to which the binder is S- or m-controlled.
As a binder ages, how does it affect ΔTc? How do RAP and RAS binders affect ΔTc?
As a binder ages, either in the lab or the field, ΔTc will decrease. When testing the recovered binder that contains RAP and/or RAS, the aged RAP and/or RAS binder will cause the ΔTc of the total binder blend to decrease, which is an unfavorable trend.
Why use 40 instead of 20 hours of PAV aging?
Some believe 40 hours of aging better reflects the aging that occurs in real pavements that have been in service for longer periods of time. Using 40 hours, the agency has less risk of accepting material that may be subject to premature binder embrittlement than if it used 20 hours. However, at 40 hours binder suppliers may have more risk of having an acceptable binder rejected.
Can I use ΔTc to evaluate an asphalt binder that contains a polymer modifier?
Yes. However, there remains dialog in the asphalt technology community concerning the validity of characterizing polymer modified binders using ΔTc. Certain features of polymer modification could possibly have a worsening effect on ΔTc and therefore make it appear as if polymer modified binders will exhibit diminished durability.
Are agencies using ΔTc in their specifications?
At this time, approximately ten agencies in North America have or soon will adopt ΔTc as a specification parameter. There is about an even split between agencies using 20- and 40-hour PAV aging protocols. Most (but not all) agencies have adopted a minimum limit for ΔTc of -5.0°C.
If you want to learn more about the binder parameter ΔTc, it’s simple one-stop shopping by downloading and reading the free Asphalt Institute’s comprehensive state-of-the-knowledge document on ΔTc.
Buncher is the Asphalt Institute Director of Engineering.