We talk with leading educators and researchers to learn more about the people and topics of asphalt technology.
Dr. Cassie Castorena is an Associate Professor in the Department of Civil, Construction, and Environmental Engineering at North Carolina State University.
What drew you to transportation engineering?
My dad is a surveying engineering professor and encouraged me to follow a similar career path. I was very fortunate to obtain a research assistant position in Professor Hussain Bahia’s asphalt laboratory while I was a civil engineering undergraduate student at the University of Wisconsin-Madison. I enjoyed working with both asphalt and Professor Bahia and therefore, pursued a Ph.D. under his guidance.
What is an advancement you are seeing in chip seal quality measures?
Research is underway to develop a performance-related specification for asphalt emulsions used in chip seals. Current emulsion specifications rely on traditional residual binder viscosity, penetration and ductility measurements that do not relate directly to chip seal distress mechanisms. The future emulsion specifications will address bleeding and raveling with the incorporation of traffic and climate conditions, similar to the current Superpave performance-graded (PG) specifications (i.e., AASHTO M 320 and M 332). The Emulsion Task Force developed a draft emulsified asphalt PG specification based on two initial frameworks proposed by the Texas A&M Transportation Institute and North Carolina State University. NCHRP Project 09-63, led by the Asphalt Institute, is currently building on this draft specification to establish a national calibrated and validated PG specification for asphalt emulsions.
What research is needed when it comes to engineering asphalt mixtures with high recycled binder replacement (RBR) percentages for peak asphalt pavement performance?
A significant area of uncertainty in the engineering of asphalt mixtures with recycled materials is the recycled binder availability, that is, the percentage of total recycled binder that is available to blend with the virgin asphalt. It is generally accepted that complete availability is not achieved in practice, which we and several other groups have recently confirmed using advanced microscopy investigations of asphalt mixtures. Despite this, the majority of current mixture design procedures assume complete recycled binder availability. Research is needed to develop a practical method to quantify binder availability within mixture design procedures. The erroneous assumption of complete blending may lead to an underestimation of the required virgin asphalt binder during volumetric mixture design and consequently may yield mixtures with poor durability.
How important are thermal conditions when evaluating the effects of aging on asphalt pavements?
Asphalt binder oxidative aging rates vary exponentially with temperature and thus, considering thermal conditions is critically important when establishing laboratory aging protocols and pavement aging models. In NCHRP Project 09-54, we field-calibrated a loose mixture laboratory aging procedure that prescribes the duration of oven conditioning at 95ºC required to represent the long-term aged state of the asphalt mixture in a pavement as a function of climate and depth. The field-calibration of this procedure demonstrates that it requires two to four days of oven conditioning at 95ºC to simulate eight years of field aging near the pavement surface in much of the northern U.S. but eight to 10 days of oven conditioning to simulate the same extent of field aging in much of the southern U.S.
In what area should today’s students focus their asphalt research?
There are many important and exciting asphalt research topics for students to explore. Asphalt binders and mixtures are inherently very complicated materials. Increased variability in crude sources, the increased use of recycled materials in asphalt mixtures and the use of a wide range of additives have added to this complexity in recent years. Therefore, research is needed to support the development of more robust asphalt binder and mixture design specifications that accurately reflect performance. Specifications that accurately reflect performance can support material and recycling innovation without inadvertently compromising the long-term durability of asphalt mixtures. To explore these topics, it is important that students develop a fundamental understanding of asphalt binder and mixture composition as well as mechanical behavior.