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|Federal Highway Administration > Publications > Public Roads > Vol. 75 · No. 4 > Why Does FHWA Have a Chemistry Lab?|
Publication Number: FHWA-HRT-12-002
Why Does FHWA Have a Chemistry Lab?
by Terry Arnold and Gretchen Stoeltje
A state-of-the-art renovation in 2011 increased the laboratory's utility and effectiveness. Find out how the laboratory can work for you.
On the face of it, paving a highway with hot-mix asphalt is a straightforward process. In reality, it is very complex. Many decisions, based on a number of variables, need to be made at every stage of the process -- from selecting the raw materials, proper blending of the aggregate sizes, and laying the pavement itself to create a sound structure. At any stage, something can go wrong that may not be manifested until long after the pavement is in service, possibly resulting in a drastically shortened service life. Extensive repairs or replacement of pavements can be extremely costly.
The focus of the Pavement Materials Team in the Federal Highway Administration's (FHWA) Office of Infrastructure Research & Development is to evaluate pavement and materials to optimize their use, extend pavement life, and reduce costs. The chemistry laboratory at FHWA's Turner-Fairbank Highway Research Center (TFHRC) in McLean, VA, is part of that effort. One of the lab's key objectives is to provide Federal researchers, State agencies, and industry partners with a forensic toolbox to assist them with controlling the quality of materials and investigating premature failures. In addition, the TFHRC research facility sometimes evaluates commercial materials that are widely used in the paving industry as part of a research study to determine their potential efficacy and longevity in pavement structures.
A chemistry laboratory has existed within the Federal highway system for more than 100 years. Prevost Hubbard was chief of the Physical and Chemical Labs, Bureau of Public Roads, U.S. Department of Agriculture from 1905-1919. The laboratory moved to its present location in 1950. In 2011 the lab underwent a major renovation to make it a state-of-the-art facility for research. Today, it is housed in three rooms: One consists of a wet chemistry laboratory, the second contains various chemical and spectroscopic instruments, and the third houses a scanning electron microscope.
"Fundamental challenges we need to address with regard to engineering problems are related to the chemistry of the component materials and their interactions," says Jorge E. Pagán-Ortiz, director of FHWA's Office of Infrastructure Research and Development. "To enable us to resolve these complex challenges we need a sophisticated and well-equipped chemistry laboratory."
Chemistry: The Molecular Perspective
Traditionally, some people think of chemistry labs as dark, smelly places containing lots of glassware. Strange liquids bubbling away, producing ominous odors. This is traditional "wet chemistry" where many analyses and chemical reactions are carried out with liquids in glass flasks. Typically, these analyses and reactions involved processes like distillation, titration, filtration, and chemical reactions. These types of chemistry laboratories still exist, including one at FHWA where the staff does sample preparation.
The world has changed. Chemistry has changed with it. Analyses that used to take hours or days now can be completed in minutes with the aid of new electronic analysis equipment. The main instrumentation lab at FHWA clearly shows the difference. This room houses most of the electronic analysis equipment. Another room houses a powerful scanning electron microscope that enables the staff to visually examine samples of aggregate, concrete, and other materials in great detail.
Paving and bridge engineers concern themselves mostly with the bulk physical properties of materials. Yet, steel and concrete, like everything else, are composed of atoms and molecules. It is the bonding between these atoms and molecules that give strength to structures like bridges. If engineers know more of what is happening at the atomic and molecular levels, they are in a better position to judge the condition of a bridge or pavement.
Atoms and molecules are extremely small. For example, a piece 0.66 inch by 0.66 in. (17 millimeters, mm, by 17 mm) of 1-in. (25.4-mm)-thick steel gusset plate would contain 6.0221415 x1023 atoms of iron (Avogadro's number). This is such a huge number that it is difficult to conceive just how large. Bill Bryson provides an interesting way to comprehend this number in his book, A Short History of Nearly Everything: If the atoms in that small sample of gusset plate were the size of popcorn kernels, they would cover the entire United States to a depth of 9 miles (15 kilometers).
While atoms and molecules cannot be seen with the naked eye, they can be excited in various ways using heat, light, or x-rays, for example, and their responses to those stimuli can be measured. From this, researchers can deduce information about the chemical structures. The response generally is plotted against a variable like wavelength of light or x-ray energy. These plots are known as spectra, which are produced using machines called spectrometers. To make a spectrum easier to read, the usual practice is to show the wavelength as a wavenumber, the reciprocal of the wavelength.
FHWA researchers at the TFHRC chemistry lab have an array of rapid spectroscopic, optical, and analytical tools at their disposal that enable them to study pavement materials at the atomic and molecular levels. These techniques, along with more traditional wet chemistry methods, offer a powerful combination that FHWA researchers can use to investigate paving phenomena and assist other researchers in examining pavement structures.
By studying molecules and atoms, chemists look at things with a different perspective than engineers do. For instance, pavement contractors have used phosphoric acid for many years as a low-cost way to stiffen asphalt and help it resist rutting caused by traffic. A premature pavement failure in Nebraska, at first blamed on the use of phosphoric acid, led to unsubstantiated fears in the industry concerning phosphoric acid. The FHWA chemistry laboratory carried out a research program to investigate these concerns. Mostly, the fears had no technical merit and were unfounded.
The chemistry lab researchers learned: Yes, you can safely use phosphoric acid as an additive. No, it won't cause the asphalt to age more rapidly. Yes, you can use it with limestone aggregates. Yes, you can use it with some antistrip additives. Yes, it might lower the moisture resistance of the pavement. Although the exact cause of the pavement failure in Nebraska is still under investigation, the lab researchers determined that the appropriate use of phosphoric acid can be suitable for modifying asphalt mixtures to improve rutting resistance and pavement life. These findings reassured some State departments of transportation (DOTs), although some still do not allow use of phosphoric acid.
During this 6-year research program, the team developed a quick and simple test method requiring no specialized knowledge or equipment that State DOTs could use to detect the presence of phosphoric acid in asphalt binders. The American Association of State Highway and Transportation Officials (AASHTO) has adopted this test: Detecting the Presence of Phosphorous in Asphalt Binder AASHTO Designation TP 78-09.
Putting Chemistry To Work
Hot-mix asphalt pavements contain approximately 95 percent aggregate and 5 percent asphalt binder, the black sticky residue left at the end of the refining process after all the fuels and oils have been removed. Its chemical composition and properties are dependent on the source of the crude oil from which it came. Many materials can be added either to the asphalt or the aggregate to improve performance and extend the life of the pavement.
When water penetrates asphalt pavements, some asphalts can lose their adhesion to the aggregate, resulting in the demise of the pavement via stripping (separation of the asphalt film from the aggregate in the presence of water). Pavement engineers can use additives to prevent this adverse condition from occurring. One approach is to treat moisture-sensitive aggregate with lime (calcium hydroxide). Since no direct test method existed for determining the presence of lime, it was impossible for DOTs to determine if an aggregate had been treated with lime.
To address this gap, the FHWA researchers at TFHRC developed a test method that is now an AASHTO provisional method. Part of the method, which answers the question of whether lime is present in the pavement, uses a technique called Fourier Transform Infrared Spectro-scopy (FTIR).
How is FTIR applied to determine the presence of lime in asphalt? Well, molecules are always in motion, vibrating and flexing in different ways. When they vibrate, they absorb energy at different wavelengths of the electromagnetic spectrum. Sunglasses, for instance, absorb in the visible region. Carbon dioxide, the greenhouse gas, absorbs in the infrared region. By measuring the absorption at different wavelengths, chemists can tell a lot about the molecular structure of the material. The plot of absorption against wavelength (more commonly, wavenumber) is an FTIR spectrum, which contains a number of peaks. The positions of the peaks provide information about the kinds of chemical groups in the sample, while the area under the peaks is indicative of the amount present.
Application of the technique is simple. A small sample is placed on the diamond window of an accessory called an Attenuated Total Reflectance bridge, and the spectrum collected. The test typically takes less than a minute. Asphalt produces a very characteristic spectrum, whereas the FTIR spectrum of lime is completely different. Lime has a very sharp peak at 3,600 wavenumbers that can be used as a marker. If the asphalt contains lime, the distinctive marker is clearly visible.
The FTIR spectrum will show whether lime is present. Determining exactly how much lime is contained in the sample is more complicated, but this part of the test can be completed in a few hours. The FHWA test method is now an AASHTO provisional test method (AASHTO TP 72-08 ) that the industry can use to ensure that lime has been added to the mix.
A similar approach can be applied to asphalt binders modified with polymers. To improve the damage resistance (that is, resistance to rutting and cracking) of asphalt pavements, common practice is to add polymers to the asphalt binder. These materials are expensive compared to the other components in the mix. The most common polymer used in the United States is SBS, a rubbery polymer made from styrene and butadiene. This polymer confers some elastic properties on the binder. Many State DOTs use time-consuming methods to measure the elastic properties of the binder to ensure that they are obtaining the materials for which they are paying. Determining the presence of these polymers can be achieved rapidly and simply by taking an FTIR spectrum of the material in question. Some DOTs use this method, and the FHWA researchers at TFHRC have the capability of running the test in the chemistry lab. Two peaks in the spectrum indicate the presence of the styrene and butadiene, and the size of the peaks can be used to calculate the quantities present.
FTIR Spectra: Asphalt and Lime
A third application of FTIR spectroscopy is to monitor the aging of an asphalt binder. Asphalt is a black sticky solid that reacts slowly with oxygen in the air and oxidizes. With time, it becomes brittle and may begin to crack and fall apart. This aging is a major factor limiting the life of an asphalt pavement. Researchers study asphalt aging in order to monitor the degradation of a pavement with a view to predicting and extending pavement life. By using FTIR to measure the amount of carbonyl and sulfoxide, two of the oxidation products that contribute to the pavement's embrittlement, researchers can study the rate of aging to determine its extent or to find ways of slowing it down.
Over the years, a number of materials have been marketed as additives for asphalt binders with the promise of extending pavement life. Some of these additives can be detected using x-rays. While FTIR spectroscopy provides information about molecular environments, x-ray fluorescence spectroscopy (XRF) can provide information about the elements themselves. With XRF, samples are irradiated with x-rays, and a complete analysis of all the elements from sodium to uranium in the Periodic Table is provided in just a few minutes. FHWA initially purchased the XRF unit for the chemistry lab to analyze cement and concrete, but the team has found it useful for investigating asphalt binders as well.
Examination of the trace metals in asphalt binders recovered from pavements can provide insights into what materials have been added. These binders can be helpful in forensic investigations.
Sample AAK-1 is a reference asphalt used in the Strategic Highway Research Program. The crude oil source from which the sample was derived came from Venezuela. All crude oil contains vanadium, but this particular one from Venezuela contains an unusually high level of this element.
Sample B6286 had been modified by a special process in which rubber ground from used tires was digested into the asphalt. The high level of zinc in this asphalt came from the tire rubber.
Sample B6269 also contained high levels of zinc as well as iron and copper. This too had been blended with ground tire rubber by a simple thermal shearing process used in Arizona.
Sample XRF1-75-2 was from a forensic study in cooperation with the Maine Department of Transportation. The sample had been modified with phosphoric acid.
Sample AS1-134-2, the last sample, had been modified with a residue obtained from waste engine oil. Calcium and zinc found in the sample came from additives used in the manufacture of the engine oil; iron and copper were metals worn from the engines.
FTIR Spectrum of Asphalt Binder Containing SBS Polymer
Another application is illustrated by a recent forensic study in which the FHWA researchers at TFHRC were asked to help identify the root cause of a premature pavement failure in Nevada. The distress mechanism was described as top-down stripping (moisture damage) and fatigue cracking. Federal Lands personnel who submitted the request suspected that lime had been omitted from the mix and that the asphalt binder had been contaminated with heating oil or diesel fuel. By using the two techniques together (FTIR and XRF), FHWA was able to show that lime had indeed been added to the mix and that the asphalt binder had been modified with waste engine oil residues. Characterization of the cores turned up nothing out of the ordinary except that the effective asphalt film thickness was found to be below specification and was the most likely cause of failure.
The FHWA chemistry lab also is involved with research into liquid antistrip additives sometimes used in asphalt binders. Liquid antistrip additives are used to improve the moisture resistance of asphalt pavements and function in a way similar to the addition of lime. The major differences are that the liquid additives containing amines or phosphate esters are added to the asphalt binder and not to the aggregate as is the case for lime usage. Since many liquid antistrips are available and customers have their own preferences, asphalt producers usually meter them into the truck while it is being loaded at the asphalt terminal. No method exists, however, to determine accurately whether the correct quantity of the specified material was added and ended up in the asphalt mixture. If the pavement shows signs of moisture damage early in its life, this sometimes leads to disputes between DOTs and contractors. The FHWA chemistry lab is developing a method to identify and accurately determine the quantity of liquid antistrip in asphalt binders.
XRF Elemental Analysis of Asphalt Samples
Another part of the lab's current research is to find a rapid method to identify the presence of alkali-silica reaction (ASR) gels in concrete. The presence of ASR gels causes a destructive expansion that takes place in some concrete structures. No reliable field test to detect the presence of ASR gels exists. The chemistry laboratory is using a technique called Raman spectroscopy to detect the presence of ASR gels. Raman spectro-scopy is a technique widely used by the FBI to investigate forgeries and by the art world to examine paintings. The sample is irradiated with a powerful laser light, which polarizes the electrons around the molecule and results in a Raman spectrum, similar to the FTIR spectrum described earlier. The reason for this research is not only to develop a field test, but also to come up with a rapid test to determine the potential of an aggregate to form ASR gels when the aggregate is used in concrete. This test would replace the mortar bar test ASTM 1260, which takes 16 days, and ASTM1293, which takes 1-2 years.
The analytical techniques discussed so far deal with atoms and molecules. The lab also can look at larger entities, namely crystals that have a uniform chemical packing. The atoms in crystals arrange themselves in a very precise way, giving the crystal a characteristic shape that depends on the material. Table salt, as an example, under a microscope appears as tiny cubes. Many materials exhibit characteristic crystalline structures and have the ability to diffract x‑rays in characteristic patterns.
FTIR Spectrum of an Aged Asphalt Binder
These diffraction patterns are obtained using an x-ray diffractometer. The sample is exposed to a narrow x-ray beam. The detector measures the response from the sample, and the signal strength then is plotted against the angle of the x-ray beam to the sample. The characteristics of these patterns facilitate the identification of the various crystals present in a sample, and the lab's researchers use them to study cement hydration and fly ash being used as a substitute for cement in concrete.
The manufacture of cement produces carbon dioxide, which is considered a greenhouse gas. The industry can reduce the amount of cement manufactured and the greenhouse gas emissions produced by substituting fly ash for the cement used in concrete. Fly ash is the dust collected from the stacks of coal-burning power plants. In the United States, coal plants produce more than 100 million tons (90.7 million metric tons) of fly ash per year, of which 30 million (27.2 million metric tons) are utilized, some of it to replace cement, and 70 million (63.5 million metric tons) are landfilled. The problem is that concrete made with high amounts of fly ash takes longer to set than regular concrete, although its ultimate strength might be higher. A concrete pavement usually can be opened to traffic a week or so after construction. High levels of fly ash probably could double this time.
Cement contains materials with delightful names like alite, belite, aluminate, and ferrite. When concrete is made by mixing cement with water, sand, and aggregate, chemical reactions take place and these materials change. They have a characteristic x-ray diffraction pattern that changes as new substances are formed. These changes can be measured using an x-ray diffractometer. By placing a small sample of wet cement in the machine, researchers can accurately measure the rate at which these materials form, indicating how rapidly the concrete will set. When fly ash is introduced, the reaction products and the rate at which they are formed change. Researchers can use these changes to explore ways of increasing reaction rate so that the concrete sets more rapidly and can accommodate highway traffic sooner.
Summary of Major Points
This snapshot of some of the chemistry lab's activities perhaps can suggest to readers how their agencies might make use of its capabilities. Though the lab's researchers use complex equipment to conduct much of their work, they have an eye toward developing simpler devices for use in the field.
Forensic investigations are far from trivial exercises. For readers who find themselves scratching their heads over tough problems, the lab's team urges them to call -- and adds, "Even if you don't have a problem, come and see the lab. Visitors are always welcome!"
Raman Spectrum of an ASR Gel
Terry Arnold manages the chemistry research complex at TFHRC. A native of England, he has a bachelor's degree in chemistry from the Royal Institute of Chemistry and is a fellow of the Royal Society of Chemistry.
Gretchen Stoeltje works in the Texas Department of Transportation's Office of Strategic Policy and Performance Management. She researches, writes, and makes films about transportation and its connection to other areas of public policy. She earned a bachelor's degree in film theory and a graduate certificate in film production from the University of California Santa Cruz. She earned a law degree from Santa Clara University.
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