U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: FHWA-HRT-05-005 Date: May/June 2005|
Publication Number: FHWA-HRT-05-005
Issue No: Vol. 68 No. 6
Date: May/June 2005
Researchers demonstrate uses for a scanning electron microscope in highway-related applications.
Frequently associated with criminal forensics and investigations, scanning electron microscopes (SEM) also have a place in transportation analysis and research. With the help of this instrument, researchers studying natural and manmade materials can produce high-resolution images of surfaces and closely inspect objects that would be impossible to see with the naked eye or with a light microscope. In addition, when speed is essential, researchers can use an SEM for a chemical analysis of materials. How might this benefit the transportation community?
|A researcher is using the scanning electron microscope at the Turner-Fairbank Highway Research Center in McLean, VA.|
Resembling a topographic view of an alien landscape, an SEM image can show small particles or components of deterioration before they cause major safety risks, such as alkali-silica reactions on a bridge structure, which can cause failure if the condition worsens. The SEM also can help analyze crack failure, fatigue, and brittle fracture in steel structures, or provide quality assurance for paint applications. Researchers use the SEM for investigative transportation forensic analyses as well.
Unlike a light microscope, which uses light to illuminate and magnify an object, an SEM uses a beam of electrons to form a three-dimensional image of the object and project it on a cathode-ray tube or television monitor. An SEM offers low and high magnifications, enabling researchers to view a material's shape, size, and morphology (structure and form) via a three-dimensional visual of the surface of the material, with magnifications of up to 100,000 times.
|These scanning electron micrographs show (a) a virgin polypropylene fiber and (b) polypropylene fiber after aging in an oven. Transverse (horizontal) cracking is visible on the aged fiber.|
At the Federal Highway Administration's (FHWA) Turner-Fairbank Highway Research Center (TFHRC) Chemistry Laboratory, researchers use the SEM to conduct studies related to highway materials, structures, and pavements. Equipped with an energy dispersive x-ray spectrometer (EDS), TFHRC's SEM can perform elemental chemical analyses as well. Although an SEM does not replace a full chemical analysis, which may take hours or even several days, the SEM does provide preliminary chemical analysis results—almost instantaneously.
Several ongoing projects illustrate the variety of applications for the SEM as a research and transportation tool, including studies of geosynthetic materials, hazardous waste identifications, concrete durability, bridge paint compositions, and even ceramic archaeology.
In the 1990s, FHWA conducted a major study to evaluate geosynthetic material like liners, membranes, and geogrids manufactured from polymeric materials to be used with soil, rock, earth, or other geotechnical engineering materials in transportation projects. In particular, the study focused on polypropylene and polyolefins, high-density polyethylene (HDPE) and polyesters used for erosion control, filtration, soil stabilization, and reinforced earth structures. The purpose of the study was to test the materials for durability and the effects of aging on geosynthetics.
Using the SEM, researchers found that when polypropylene was subjected to thermal-oxidative treatments, pronounced transverse cracking developed and was clearly visible in the scanning electron micrographs.
|These scanning electron micrographs show (a) a virgin polypropylene fibers and (b) polypropylene fibers after alkaline hydrolysis, showing severe erosion and a reduced cross-sectional area in the fibers.|
Polyesters and HDPE, although relatively unaffected by neutral and acidic solutions, were severely eroded and "lost section" (that is, the geosynthetic fibers were reduced in a cross-sectional area), when subjected to highly alkaline solutions. Again, the effects were apparent when viewed through the SEM.
Researchers concluded from the test results that the SEM is effective for visually examining the surface morphology of geosynthetics for evidence of oxidation—typically circumferential cracking or hydrolysis—that is evidenced by erosion of the fiber surface. Scanning electron microscopy, therefore, proved useful for developing protocols to evaluate the thermal-oxidative and chemical durability of geosynthetics.
Researchers use a number of other tests to evaluate durability, including stress tests and oxidation induction time. The SEM complements those tests by offering graphic results of geosynthetic deterioration.
FHWA is using the SEM/EDS unit to answer questions regarding the durability of portland cement concrete (PCC). Delayed ettringite formation (DEF) and alkali-silica reaction (ASR) represent two areas of study.
The mineral ettringite (a complex hydrate of calcium, sulfur, and aluminum) is a reaction product formed in the setting of portland cement concrete. This "early ettringite" is beneficial in newly formed PCC because it helps regulate the setting. After some time, however, early ettringite may disappear and a new phase (delayed ettringite) may be formed in the mature concrete, possibly causing cracking, overall deterioration, or failure.
|This SEM micrograph shows an air void in a sample of portland cement concrete that was cured for 100 days in water with a high pH. The void is filled with "balls" of ettringite—a mineral formed during the curing process for PCC. The typical EDS spectrum for ettringite is shown on the right. The coordinates for the EDS spectrum are x-ray Fluorescence energy (x-axis) and x-ray fluorescence intensity (y-axis).|
This destructive DEF occurs months or years after placement in an environment where the concrete is frequently exposed to moisture. Ettringite in aged concrete typically is found in air and water voids, cracks, and rims (aggregate-paste interfaces). Researchers disagree on whether DEF involves heat curing, that is, when concrete is steam cured or cast in hot weather. "Even after decades of investigations, DEF is controversial with regard to whether it represents a major durability problem," says Dr. Richard Livingston, senior physical scientist with FHWA's Office of Infrastructure Research and Development (R&D).
In view of the uncertainties about DEF (including its prevalence and relevance), FHWA, the National Science Foundation, and the University of Maryland are collaborating on several studies designed to characterize DEF and factors affecting its occurrence, to mitigate the problems, and to develop a simple test for identifying DEF. In this collaborative effort, investigators evaluated numerous factors believed to influence the occurrence of DEF. The evaluations included examining PCC and mortar specimens that have been subjected to various curing regimes (time, temperature, and moisture conditions), treated with chemicals such as potassium and magnesium, evaluated with respect to fine aggregate type including reactive sands, and constituted using fly ash as a possible mitigating agent. The SEM/EDS unit was instrumental in evaluating the systems studied, and some of the results relate to the various theories on DEF.
|This micrograph of a PCC specimen shows a sand grain (A) with an ettringite deposit (B). The specimen experienced accelerated curing after treatment with potassium oxide.|
Using the SEM/EDS unit, investigators studied several laboratory-prepared PCC samples and identified the occurrence of air voids containing "balls" of ettringite, which look as if they are composed of a jumble of fine needles. The investigators hypothesize that this ettringite was formed by a through-solution mechanism in which critical components are first made soluble and then reprecipitated.
Ettringite also can occur in fairly massive deposits on aggregates, such as sand grains. One study involved accelerated curing of PCC specimens with potassium, which may catalyze the formation of ettringite. SEM micrographs showed individual sand grains containing "deposits" of ettringite needles. The researchers believe that this type of occurrence indicates that the ettringite was formed by a topochemical or solid-state reaction (that is, the ettringite formed in place on the sand grains).
Further evaluations of systems that develop DEF and how to mitigate the problem are underway. The evaluations include additional laboratory and collaborative field studies with the Maryland State Highway Administration.
In addition to studying DEF, the team of researchers is looking at alkali-silica reactions (ASR). ASR is a chemical reaction of water, the alkalis in portland cement, and the aggregate of PCC. The resulting reaction product is a hydrous alkali silicate gel of high volume, which can lead to expansion, causing cracks and joint failures in PCC pavements and severe misalignments in bridge elements such as beams and columns. Cases of this deterioration, especially involving ASR, are widespread and have been investigated for several decades.
At TFHRC, researchers conducted evaluations of concrete deterioration problems for a number of State departments of transportation (DOTs). Many of the problems were related to the formation of ASR gel. Recently, the Connecticut DOT submitted samples of a PCC bridge deck for examination. The samples contained a white deposit in some areas that encapsulated coarse aggregate particles. Using the SEM, researchers examined the material at the aggregate-paste interface and identified it as ASR gel. The micrograph showed cracking in the ASR gel, most likely due to the loss of moisture after the sample was removed from the deck and during sample preparation in the lab. Powder samples of the white material collected from the fractured PCC surface showed the same features and compositions as the interfacial material.
"The SEM results provide graphic evidence of the presence of deleterious ASR reaction product and also provide definitive morphological and compositional information," Livingston says. "These results help us to understand the cause of deterioration and to choose the right strategy for repair or replacement of the damaged structure."
|This SEM micrograph shows the cracking typical of alkali-silica reactions. The cracking is likely due to the loss of moisture after the sample was removed from the deck and during sample preparation in the lab. The corresponding EDS spectrum for ASR gel shows the presence of potassium and silicon, essential components of ASR.|
Moving now from the structures themselves to the coatings that cover and protect them, FHWA has found applications for the SEM in the evaluation of paints used on bridges. Early on, FHWA research and development efforts on paints focused on materials for delineating traffic lanes, especially those that improved durability and visibility at night and when wet.
Lately, the agency has placed more emphasis on bridge paints, developing test methods for durability and corrosion as well as evaluating commercial products. Recent staff and contract studies have addressed the development and evaluation of test protocols for durability and corrosion, as well as environmental factors and materials that are environmentally acceptable. Materials tested thus far include lead- and chromium-free paints whose emissions of volatile organic compounds (VOCs) are below the limits set by the U.S. Environmental Protection Agency (EPA). FHWA's coatings laboratory, for example, has evaluated commercially available waterborne and low-VOC, solvent-based paints. In addition, the lab investigated the performance of rapid-deployment systems—those that involve two instead of three paint layers.
"The SEM facility is helping characterize a wide variety of paint materials, especially pigments [inorganic paint components]," says Dr. Shuang-Ling Chong, a research chemist with FHWA's Office of Infrastructure R&D. "The SEM can readily identify and evaluate the elemental chemical composition and particle size and shape of these materials."
Based on a scanning electron micrograph and EDS readout of pigment from a rapid-deployment paint system, researchers can gather both qualitative and semiquantitative chemical analysis data, which are useful in many of the studies conducted by FHWA staff.
As a result of these and other FHWA studies on commercial paints, researchers have developed new and improved procedures to evaluate paint performance and have helped to promote the more efficient and economical use of environmentally acceptable bridge paint systems. Evaluations of the particle size and chemical composition of zinc pigment conducted using the SEM/EDS unit helped characterize, for example, the performance of zinc-rich, moisture-cured urethane bridge paints. The results were summarized in the report Laboratory and Test-Site Testing of Moisture-Cured Urethanes on Steel in Salt-Rich Environment (FHWA-RD-00-156).
In another application, TFHRC's Fairbank Building underwent extensive renovations in the mid 1990s, and the operation involved recovery and disposal of hazardous materials like asbestos and lead-based paint. Because manmade materials such as fiberglass and glass wool may have morphologies similar to those of asbestos, the challenge is to identify the more hazardous elements and dispose of them as quickly as possible.
|This scanning electron micrograph shows a zinc-rich pigment from a rapid-development paint system. The spheres are metallic zinc particles, and the different sizes are characteristic of the particular sample evaluated.|
Researchers studied a sample of insulating material taken from a fixture in the chemistry laboratory in the Fairbank Building during the renovation. After producing a micrograph and an EDS spectrum for the sample, they identified the material as chrysotile asbestos. A comparison of the insulating material with a known sample of chrysotile asbestos from a mine in Thetford, Quebec, Canada showed similarities in morphology and composition, verifying the identification.
FHWA staff also used the SEM to determine the composition of a pipe-insulating material. The morphology and chemical compositions revealed through the micrograph and EDS spectrum identified the sample as amosite, a particularly harmful type of asbestos.
|This micrograph shows a sample of insulating material taken from a fixture in the Fairbank Building during a renovation. Using the SEM, FHWA researchers determined that the material (magnified 46 times) was chrysotile asbestos, as confirmed by the EDS spectrum (right) generated for the sample.|
Because asbestos is a hazardous material, the agency then followed the regulations prescribed by EPA and the Occupational Safety and Health Administration (OSHA) to remove and dispose of the wastes.
In another circumstance, researchers identified a sample of pipe-insulating material as fiberglass, based on morphology and composition. Although the sample has a similar shape (needles) to amosite, the chemical composition (sodium, calcium, aluminum, and silicon), determined through the EDS spectrum, differentiates the fiberglass sample from asbestos. After determining that the material was not hazardous, the work crews recovered and disposed of the waste less expensively, and under less stringent rules than those for asbestos disposal.
As with the insulation wastes, FHWA researchers also used the SEM and EDS unit to determine the chemical composition of numerous paint samples. When the Fairbank Building was built in the 1950s, various sections were painted using lead-based paint. Renovations therefore required that workers handle paint debris as potentially hazardous wastes. Those samples found to contain lead, such as a pigment of lead silicochromate, were therefore handled as a hazardous waste.
Using the SEM to test specimens for hazardous materials, therefore, helped make recovery and disposal operations more efficient, and more economical.
|This SEM micrograph shows a sample of fiberglass insulation material. The image typifies fiberglass with mostly long and largely smoot fibers, irregularly arranged and held together with a binder material. To the right is the sample's EDS spectrum. The fiberglass has an essential composition of sodium, calcium, silicon, and aluminum (typical glass composition).|
|This SEM micrograph shows a pipe-insulation material identified as amosite asbestos. The long, narrow fibers are characteristic of amosite, and the EDS spectrum (to the right) is an exact match for the mineral, which is composed of magnesium, silicon, and iron.|
Finally, through an innovative partnership, researchers at TFHRC used the SEM to help staff from the Smithsonian Institution's Freer Gallery of Art to study ancient ceramic materials excavated in the Hebei Province of northern China.
Ceramic archeology is the study of ancient ceramic products, usually from excavated sites, to learn more about the cultures, lifestyles, and technologies of past civilizations. Ceramic shards excavated from an ancient kiln were examined with regard to glaze composition and structure. Tests to establish firing temperatures and to determine glaze oxide compositions were performed at the Freer Gallery of Art. TFHRC staff used the SEM facility to determine glaze thickness, texture, and qualitative composition. Individual features of the glazes were pinpointed using EDS, and these features were subsequently used for quantitative microprobe analyses at the Smithsonian Institution's National Museum of Natural History.
|Using the SEM/EDS unit, researchers at TFHRC can quickly identify hazardous paint materials. This EDS spectrum, typical of lead-based paints, was accomplished in less than 1 minute, demonstrating the power of the EDS accessory in rapid chemical analyses.|
The tests involved reconstructing the production process by "firing" or heating samples of the ceramic coating to establish the temperature of heat treatment. These data, along with information on viscosity, composition, and thermal expansion, helped to characterize the coating materials. Scanning electron microscopy revealed structural and textural features of the coating materials.
The heating experiments led the researchers to conclude that the coating system was fired in three steps: firing the body (clay) with slip coating, firing the glaze at a lower temperature, and firing the enamel (overglaze) at a still lower temperature.
Results from the tests revealed that ancient artisans used a process known as overglazing in the manufacture of some of the ceramic artifacts. According to researchers at the Freer Gallery of Art, this application of overglaze enameling may represent the first time that this process was used.
The researchers concluded that the "enameled ware, the first known example in China of the application of low temperature enamel over high fired glaze, is believed to be an integral step in the development of the Chinese overglazing enameling techniques that were perfected in southern China in the later Yuan and Ming dynasties." (Liu, Wei, and McCarthy, Blythe, "Analysis of Cizhou Monochrome Green Enamels and Lead Glazes from Guantai Kiln in Northern China, Song to Jin Dynasty," Materials Issues in Art and Archeology, Proceedings of the Symposia from the 2001 fall meeting of the Materials Research Society.)
"The ability of SEM and EDS to link compositional and structural information at the microscale makes them invaluable tools for understanding the production processes and life history of ancient ceramics," says Blythe McCarthy, conservation scientist with the Freer Gallery of Art and Arthur M. Sackler Gallery, Smithsonian Institution. "Our recent research on Chinese enameled wares would not have been possible without the access to this instrumentation provided by the researchers at TFHRC."
|This SEM micrograph shows the different layers of material—enamel, glaze, slip, and clay—that comprise an enameled ceramic specimen, analyzed by TFHRC researchers on behalf of the Freer Gallery of Art in Washington, DC.|
These synopses of the SEM research represent just a handful of the numerous current and potential applications for the use of scanning electron microscopy in highway research and development. The benefits of applying SEM techniques to relevant research projects reach beyond improved understanding of the physical and chemical processes that affect natural and manmade materials. They include enhancing the safety and durability of the Nation's transportation infrastructure.
Clay Ormsby is a researcher in geotechnology and chemistry. After 40 years of Government service, he retired from FHWA in 1992. Currently, he is employed by Engineering and Software Consultants, Inc., and works onsite at FHWA's TFHRC.
Rongtang Liu is a materials engineer and concrete petrographer employed by SaLUT, Inc. He works onsite at FHWA's TFHRC. Liu has a Ph.D. in civil engineering from Purdue University.
In addition to supporting transportation research, the SEM is available to other government agencies and others who participate in cooperative studies. For more information, contact W.C. Ormsby at 202–493–3057 or firstname.lastname@example.org.