Postdoctoral associates push the boundaries of advanced highway research at FHWA.

"Fresh perspectives, that's what postdoctoral researchers offer," says Paul Teng, director of the Office of Infrastructure Research and Development (R&D) at the Federal Highway Administration (FHWA). The postdoctoral researchers are invited to FHWA's Turner-Fairbank Highway Research Center (TFHRC) in McLean, VA, through the Research Associateship Programs, administered by the National Research Council (NRC) of the National Academies. The program seeks researchers from across the country and around the world to engage their talents in new research applications.

"The research fellows who tenure at TFHRC," says Paul Zielinski, program administrator for the Research Associateship Programs, "are among the successful applicants in a worldwide competition to recruit the best young scientists and engineers to work at Federal laboratories." In 2003 TFHRC celebrated FHWA's 10th anniversary of participating in the program. During that time, eight postdoctoral research fellows have tenured with TFHRC, representing a range of disciplines from chemistry to nuclear engineering. They have contributed depth and breadth of perspective to TFHRC's traditional focus on civil engineering. Their cutting-edge research has ranged from controlling deterioration of reinforced concrete to using nuclear radiation as a nondestructive testing method and developing a new quality control parameter for pavement.

"The National Research Council's postdoctoral research fellowship program is an excellent way to get highly learned professionals to conduct highway infrastructure research and development activities," says Teng. "The Office of Infrastructure R&D has had several outstanding NRC postdoctoral fellows who did and are doing superb research for our program. Many times, the research fellows' exploratory work produces new ideas or tools for our applied research and technology program to put into practice."

Controlling Corrosion

In 1993, the first year that TFHRC participated in the program, postdoctoral researcher Ugo Bertocci conducted a study to evaluate the effectiveness of various chemical deicers in preventing corrosion in chloride-contaminated reinforced concrete.

Dr. Bertocci, an electrochemist formerly with the National Institute of Standards and Technology, performed research on the potential passivating effect of a new chemical deicer-calcium magnesium acetate-on the corrosion potential of rebars embedded in concrete containing sodium chloride (rock salt). Passivation, an inhibitor property of rendering steel rebars less susceptible or completely insusceptible to corrosion, is of particular interest to State highway agencies that manage older bridge decks that are contaminated with salt.

Using electrochemical impedance spectroscopy, Bertocci evaluated calcium magnesium acetate and other commercial passivators and sodium chloride solutions. During a simulation of field conditions where salt has contaminated reinforced concrete, Bertocci exposed embedded steel rods to various deicers and took measurements over a period of 11 months to determine the progress of corrosion.

Source: Impedance Spectroscopy for the Evaluation of Corrosion Inhibitors in Highway Deicers (FHWA-RD-96-178)

Bertocci's results showed that, as a corrosion inhibitor, calcium magnesium acetate proved superior to all other materials tested. In fact, it demonstrated a negative corrosion rate (a passivating effect). "Dr. Bertocci's results document that calcium magnesium acetate, in addition to its deicing ability, has the capacity to retard or inhibit corrosion of salt-contaminated bridges," says W. Clayton Ormsby, Ph.D., a geotechnical research specialist at Soil and Land Use Technology, Inc. "This is in sharp contrast to deicers Bertocci's results showed that, as a corrosion inhibitor, calcium magnesium acetate proved superior to all other materials tested. In fact, it demonstrated a negative corrosion rate (a passivating effect).

"Dr. Bertocci's results document that calcium magnesium acetate, in addition to its deicing ability, has the capacity to retard or inhibit corrosion of salt-contaminated bridges," says W. Clayton Ormsby, Ph.D., a geotechnical research specialist at Soil and Land Use Technology, Inc. "This is in sharp contrast to deicers such as sodium chloride, which promote corrosion of the reinforcing steel in bridge decks. This corrosion causes expansion, spalling, and failure of bridge decks."

FHWA published the results of Bertocci's research in a report, Impedance Spectroscopy for the Evaluation of Corrosion Inhibitors in Highway Deicers (FHWA-RD-96-178).

Research Associateship Programs Celebrate 50th Anniversary The Albert Einstein Memorial Statue (copyright 1978 by Robert Berks) on the grounds of the National Academies headquarters. In November 2004, the National Research Council's Research Associateship Programs will celebrate 50 years of operation. Since 1954 the program has expanded to include 30 Federal agencies with research opportunities in virtually all disciplines in science and engineering. The Research Associateship Programs provide postdoctoral and senior scientists and engineers with an opportunity to perform research of their own choosing at a participating government laboratory. Applicants apply to the program by preparing a proposal, typically involving close collaboration with a research adviser. A list of participating laboratories and advisers is available on the Web at www4.nationalacademies.org/pga/rap.nsf. Nearly 4,000 research opportunities exist at participating laboratories. A panel of subject matter experts drawn from academia, industry, and government review the proposals and other application materials, including transcripts, letters of reference, and an evaluation by the prospective host laboratory. Due to the competitive nature of the program, for most agencies, only the highest ranked applicants are chosen. The Research Associateship Program is a continuation of an earlier program launched by the National Bureau of Standards in 1919 and funded by the Rockefeller Foundation. The earlier program awarded fellowships in mathematics and the biological, medical, and physical sciences. In 1954 the Rockefeller Foundation asked the National Research Council to identify a new sponsor, and the National Bureau of Standards (now the National Institute of Standards and Technology) took over. In November 1954, the National Bureau of Standards selected 6 research associates from a pool of 21 applicants. In 2002 NIST awarded 391 associateships from more than 900 applications. Paul Zielinski

Assessing Pavement Deterioration

Is there a structural feature in asphalt binders that causes pavements to age and deteriorate over time? If so, can researchers develop a costeffective chemical treatment to prevent deterioration caused by longterm exposure to heat, sunlight, water, salt, and ice? These are the questions that research fellow Rita Hessley sought to answer during her tenure at TFHRC from mid-1994 until July 1995.

A chemistry professor at Western Kentucky University at the time, Dr. Hessley focused her professional interests on how the chemical structures of materials affect their behavior in the environment in which they are used. Hessley says, "I had looked at how modifying the original structure through oxidation could alter the material's behavior. Having studied materials as different as coal and milkweed floss, turning to asphalt was a natural extension of my interests."

Previous FHWA studies led chemists to believe that particular portions of asphalt structures are uniquely susceptible to oxidative alteration. Hessley's goal was to determine cost-effective modifications that could protect the structures. Although Hessley's tenure expired before she could achieve substantial results, her work showed that a technique called attenuated total reflectance infrared spectroscopy (ATR) could be used to monitor a tag introduced into the asphalt. A tag is an atom, or group of atoms, that are not present naturally in asphalt. In this case, Hessley used bromine, which is known to attach to some of the same structural sites believed to react when asphalt is subjected to oxidative breakdown. Hessley theorized that once a tag was in place, controlled oxidation reactions could be carried out. The effect of oxidation could be monitored with ATR, and the effect on the performance of the oxidized material could be monitored using established mechanical tests. Depending on the nature of the correlation between the tagged site and the tests, preventative treatments could be designed to block the reactions leading to pavement loss.

"Coming to a research facility like TFHRC," Hessley says, "that is highly focused on one topic—improving highway pavements-after 20 years in academia was an exciting and challenging cultural and professional shift. The experience provided me with numerous insights to take back to the university setting. I try to convey such differences about professional settings to students as they develop their career ideas."

Nondestructive Testing for Concrete

Sodium and calcium chloride (from seawater and road deicing) can damage reinforced concrete structures by promoting corrosion of the steel reinforcements. The common methods for measuring chlorides in concrete involve drilling cores to perform chemical analyses. Habeeb Saleh, a research fellow from 1995 to 1998, sought to develop nondestructive methods for testing concrete using nuclear radiation.

Dr. Saleh, who holds a Ph.D. in nuclear engineering from Texas A&M University, used a weak source of neutrons-produced by the radioactive decay of the element californium- to perform a nondestructive test for measuring chlorides in hardened concrete. Saleh beamed neutrons into the hardened concrete where they were captured by atoms of chlorine or other elements. During the capture process, the atoms emit gamma rays with characteristic energies that travel out of the concrete, where researchers can measure them with detectors. Using advanced software developed at Los Alamos National Laboratory, he modeled the neutron- and gamma-ray characteristics and evaluated his results experimentally using test concrete slabs constructed in the laboratory. Saleh demonstrated that it was possible to devise a test method that met all the specifications, including radiation safety.

"Some of these methods have been used for field applications," Saleh says, "and some are in the developing stages for potential field applications."

After completing the research associateship, Saleh remained at TFHRC as a contractor working in the Nondestructive Evaluation Validation Center. He continues developing neutron-based testing methods and also helped establish an advanced facility for making threedimensional x-ray images, or computerized axial tomography (CAT) scans, of highway materials.

New Specification Parameter for Pavement

Aroon Shenoy, a research fellow from February 1998 to August 2001, determined that a paving material's volumetric flow rate could be used to determine the performance-grade specification of asphalt-thus identifying a new parameter for quality control and quality assurance as well as for the fundamental specification. Prior to his associateship, Dr. Shenoy served as a consultant in Pune, India. With a Ph.D. in chemical engineering, Shenoy authored a number of technical papers and books on topics such as thermoplastic melt rheology and processing, filled-polymer rheology, non-Newtonian fluid mechanics, and heat transfer. At TFHRC, he extended the ideas and principles of unifying rheological data that he developed earlier to the specific case of paving asphalts.

Shenoy correlated fundamental rheological data from a dynamic shear rheometer for a number of unmodified asphalt binders. He showed that one curve describes all unmodified asphalts for each rheological parameter, such as the complex modulus and phase angle. The unified curve enables researchers to predict the behavior of unmodified asphalt at any temperature by determining the volumetric flow rate at a particular load condition. Shenoy extended the idea of unification to polymer-modified asphalts and showed that unified curves could be obtained for polymer-modified asphalts using the same method.

The approach offered a simple means to determine the high-temperature specification performance grade of asphalt using an inexpensive and portable flow-measuring device. Unlike other rheological parameters that must be measured in the lab, the volumetric flow rate can be assessed at paving sites or in refineries, making it a more versatile parameter that can be used for routine quality control. Eleven technical papers and one internal report have been published on this project, and they are available at https://www.fhwa.dot.gov/pavement/asphalt/labs/binder/brlpubs.cfm.

Following his tenure as a research fellow, Shenoy also remained at TFHRC, accepting a position as a senior research rheologist. "The associateship gave me an opportunity to change my research career path from polymer to asphalt rheology," Shenoy says. "The most enjoyable part of the position was that it provided a platform to bring new ideas into asphalt research."

Monitoring Structural Health

During his tenure as a research fellow from 1999 to 2003, Shuang Jin, a mechanical engineer from China, applied nonlinear dynamics (chaos theory) to monitoring the deterioration of highway structures. After working for several years in ship design, Dr. Jin came to the United States and earned a Ph.D. in mechanical engineering from The George Washington University. In real structures, the stress-strain relationship may be nonlinear at some points because of cracking or in a complicated three-dimensional (3-D) structure, even though the individual members behave linearly, their interactions may lead to nonlinear dynamics for the overall structure.

Jin's approach used chaos theory to refine the analysis of structural vibrations. Earlier approaches such as global monitoring typically examined a structure's fundamental modes of vibration-or resonant frequencies. Significant changes in the resonant frequency, however, usually were not detected until the structure was severely damaged. In chaos theory, the resonant frequencies are not fixed but instead wander in time in a characteristic pattern around a central value, called an attractor. Using data obtained from a computer simulation, Jin plotted a strain in the lower flange of a standard bridge girder under random loading from trucks.

In a chaotic system, a set of parameters called Lyapunov exponents takes the place of fundamental frequencies. The chaos theory approach to structural health monitoring, therefore, involves determining the Lyapunov exponents of a structure and then observing how they change with time in service. Jin demonstrated that chaos theory can describe bridge vibrations and help engineers monitor structural deterioration. Ultimately, his research will lead to more efficient monitoring systems and more effective maintenance and repair programs.

Jin currently serves as an onsite contractor at TFHRC, where he continues to develop algorithms for applying chaos analysis. His most recent work involves analyzing data from actual bridge-monitoring systems, such as one on the Commodore Barry Bridge in Philadelphia, PA, where researchers at Drexel University have been collecting data.

3-D Plot of Data on Bridge Vibrations

Modeling Aggregate Structures in Asphalt Concrete

Xiaoxiong Zhong, a research fellow from June 1998 through May 2000, modeled asphalt concrete pavements using particulate mechanics. Dr. Zhong earned one Ph.D. in rock mechanics from the Institute of Rock and Soil Mechanics at the Chinese Academy of Sciences in 1991, and another in civil engineering from the University of Massachusetts Amherst in 1998.

The goal of Zhong's research at TFHRC was to calculate the performance properties of an asphalt mixture by applying the principles of mechanics to the detailed mesostructure of a heterogeneous mixture. The micromechanics calculation explicitly utilizes the locations, sizes, and orientations of the aggregate particles. With this type of calculation, researchers can predict pavement performance and create virtual mix designs on the computer using only a few basic measured parameters.

Aggregate structures (the locations and orientations of all the stones in the asphalt mixture) play a major role in supporting traffic loads and can have a significant impact on the performance of asphalt concrete. Conventional means of determining the structure, however, have been limited to empirical methods, which neglect the mesolevel details of stone-on-stone interactions. Zhong studied the structures that enhance performance (to avoid rutting or fatigue) and how to tweak the mixdesign and compaction procedures to obtain those structures. His research provided an analytical connection between the observed global properties and the local geometry and material parameters.

Zhong modeled a collection of aggregates without binder using a 2-D discrete-element method, a computer calculation particularly useful for this type of situation. He displayed the values and propagation of force within aggregate packing and obtained a preliminary contact model that enabled him to calculate stonestone interactions in the asphalt more precisely. He validated the model by using the force distribution in a 2-D representation of asphalt concrete that his colleagues developed using photo-elastic imaging. Photo-elastic imaging enables researchers to obtain optical images of stress-strain paths using an assemblage of glass discs under load as a representation of the aggregate structure.

Zhong developed a simulation model for 3-D aggregate packing that shows graphically the stress distributions from different directions with light intensity proportional to the magnitude of the stress. Researchers now are using more advanced forms of 3-D models to distinguish between various aggregate gradations used in the asphalt concrete industry.

Classifying Fly Ash

From 2000 to 2003, Walairat ("May") Bumrongjaroen used advanced characterization methods to classify fly ashes used in mortar. A native of Thailand, Dr. Bumrongjaroen earned a bachelor's degree in environmental engineering from Chulalongkorn University in Bangkok, followed by a master's and Ph.D. from the New Jersey Institute of Technology.

Pavement designers often specify mixes of portland cement, fly ash,and silica fume to create high-performance concretes that offer increased compressive strength and improved durability. The existing classification method for fly ashes-Class C or Class F-is based on chemical composition, depending on the sum of the chemical constituents silicon dioxide (SiO2), aluminum oxide, and iron oxide. This method, however, is insufficient for predicting mix performance because the reactivity of fly ash depends significantly on its mineralogical composition, especially on the amount of glassy or amorphous material. Silicon dioxide, for example, in the form of mineral quartz is relatively unreactive compared with SiO2 glass. Moreover, fly ash is not a homogeneous material but rather a variable mixture of minerals and glass phases with a range of particle sizes and densities.

Bumrongjaroen's research involved advanced centrifuge methods, automated elemental analysis with a scanning electron microscope, Rietveld X-ray diffraction, and automated glass refractometry. In collaboration with the National Institute of Standards and Technology, she measured fly ash reactivity nondestructively, using inelastic and small-angle neutron scattering. She also investigated the application of glass durability theory to model the reactivity of fly ash Finally, to present the information in a format that would be easy to understand, Bumrongjaroen developed software for displaying several independent variables in an appropriate ternary diagram.

The standard method for characterizing fly ash mineralogy is x-ray diffraction. In a typical spectrum, minerals show up as individual peaks. The glassy mineral phases, which do not have a crystal structure, appear as a broad hump. Bumrongjaroen applied an advanced mathematical method called Rietveld analysis to quantify the glassy region in the spectrum. Since the glassy phase represents the reactive component of fly ash, researchers can deduce that the less durable the glass, the higher is its reactivity. Bumrongjaroen plotted the durability index of individual fly ash particles based on analysis of data generated using an automated scanning electron microscope.

Bumrongjaroen's research demonstrated the feasibility of developing an improved classification system for fly ash. The improved classification in turn will enable pavement engineers to optimize concrete mixes for better performance in bridges and pavements-ultimately leading to significant cost savings.

Bumrongjaroen presently works at the Hawaii Institute of Geophysics and Planetology at the University of Hawaii, where she is preparing the results of her research for publication and continuing to develop advanced methods for characterizing fly ash.

Triaxial Plot of Fly Ash Particles

Understanding Alkali-Silica Reactions

John Phair, a chemistry Ph.D. from the University of Melbourne in Australia, is the current resident associate at TFHRC. Having arrived in November 2001, he recently completed his second year of tenure with FHWA. Dr. Phair's postdoctoral work focuses on characterizing the damaging gels that form due to a reaction between the alkalis (potassium and sodium) in cement and certain types of silicate rocks used as aggregates in concrete. The gel created during the alkali-silica reaction swells and exerts expansive stresses that can crack concrete in highways, pavements, bridges, and dam walls.

"By characterizing the swelling properties of these gels at the nanoscale level as a function of composition," Phair says, "it will be possible to determine under what circumstances the gels swell and why. This provides invaluable information for the development of predictive tools and standard tests to monitor and prevent the occurrence of alkali-silica reactions in concrete structures."

Although previous researchers identified alkali-silica reactions as a major problem, many questions remain. What proportions of alkalis cause gel formation? Why do certain gels swell while others do not? What are the physical and mineralogical characteristics that make silicate rocks reactive?

The extremely fine colloidal particles that make up the gels have dimensions of around 10 nanometers. Because of the difficulty of observing and measuring the particles, much of Phair's research deals with applying advanced methods for materials characterization. Using neutron scattering and x-ray diffraction, he measured the rate of reaction of certain types of silicate materials with solutions of different chemical compositions. The results show that under certain conditions the alkali-silica reaction gel transforms from an amorphous material into a microcrystalline, layered material that may be more prone to swelling. These findings are consistent with research conducted at the University of Illinois at Urbana-Champaign using other instrumental methods.

Phair also investigated the poresize distribution of these gels using a positron annihilation facility at Lawrence Livermore National Laboratory. At the mineral physics laboratory at the University of Hawaii, he studied the gels' elastic properties using Brillouin scattering. In addition, he is developing a laboratory method to measure the swelling pressure of the gels based on dialysis membrane technology.

After completing his tenure at TFHRC, Phair plans to continue research in either industry or academia and eventually pursue a career in academia. "The most enjoyable aspect of conducting research at TFHRC," Phair says, "has been the helpful people and the camaraderie. My colleagues and advisor constantly present me with new challenges, suggesting new approaches to tackling old problems. This has pushed me to develop a more critical understanding of the subject matter."

Each year, FHWA staff members propose new ideas for potential research opportunities at TFHRC. The National Research Council advertises the opportunities in printed booklets and, more recently, online at www4.nationalacademies.org/pga/rap.nsf.

"All the previous fellows have focused on asphalts and concrete," says FHWA's Paul Teng, "because the research ideas were generated in the Office of Infrastructure. But there are plenty of things that could be done in other focus areas like traffic control, bridges, and intelligent transportation systems. The sky is the limit."

Research fellows may be true postdoctoral students-that is, scientists and engineers who recently received their doctorates-or senior associates who are at a later stage in their careers. For recent doctoral graduates, the program provides an opportunity for concentrated research in collaboration with selected members of the permanent professional laboratory staff. For established scientists and engineers, the participation offers the chance to conduct research without making long-term commitments and without the interruptions and distracting assignments typical of permanent career positions. TFHRC benefits from the new ideas, techniques, and approaches offered by intelligent, highly motivated, recent doctoral graduates and senior investigators with established records of research productivity.

John Phair describes his experience as a research fellow at TFHRC as an enriching one. "Not only have I been able to collaborate with and learn from world leaders in cement research," he says, "but I have had the opportunity to broaden my experimental expertise to include a range of cutting-edge techniques. I also have been given numerous opportunities to interact with colleagues in my field of research and to present my work at international conferences."

Who should apply to become the next research fellow at TFHRC? According to Aroon Shenoy, "Any researcher who believes that he or she has ideas to push the frontiers of road research in a new direction." The invitation is on the table.

Richard A. Livingston, Ph.D., is the TFHRC liaison for the Research Associateship Program. He is a senior physical scientist in FHWA's Office of Infrastructure Research and Development. Livingston's professional interests concern the materials science and nondestructive testing of construction materials. He has a bachelor's in medieval history from Dartmouth College, a bachelor of engineering degree from the Thayer School of Engineering at Dartmouth College, a master's in nuclear engineering from Stanford University, and a Ph.D. in geology from the University of Maryland.

Ernest J. Bastian, Jr., Ph.D., is a senior research chemist at TFHRC. He joined FHWA in 1986 to work in the areas of asphalt chemistry and pavement science. His assignments have included managing the Binder Rheology Laboratory, reviewing contracts for the Strategic Highway Research Program, and serving as the technical representative for several congressionally mandated research projects with the Western Research Institute. Bastian earned his Ph.D. in physical chemistry in 1969 from Carnegie Mellon University.

For more information about postdoctoral research opportunities at TFHRC, visit www4.nationalacademies.org/pga/rap.nsf or contact Dick Livingston at 202-493-3063 or dick.livingston@fhwa.dot.gov.