Can Chemistry Improve the Nation’s Roadways?
The Federal Highway Administration’s Turner-Fairbank Highway Research Center (TFHRC) has had the Chemistry Laboratory for more than 100 years. That may seem strange in a highway-oriented world of civil engineers, program managers, construction inspectors, environmental and transportation specialists, planners, and other corresponding personnel; however, the scientists and researchers in the Chemistry Laboratory have been able to earn their keep, helping to foster innovation in support of FHWA’s long-standing mission of increasing safety on the Nation’s roadways.
From performing research on concrete and asphalt to the development of standards to conducting forensic investigations on pavements and other highway structures, the FHWA Chemistry Laboratory plays a key role behind the scenes to:
- Advance the understanding of chemical changes that contribute to road failure or damage.
- Advance the understanding of chemical changes that can contribute to potential performance enhancements.
- Develop state-of-the-art characterization tools.
- Test and foster new materials development.
“Chemistry provides an understanding of interactions at a molecular level and provides new solutions to pavement/material performance,” says Jack Youtcheff, pavement materials team leader at FHWA.
Throughout the years, the Chemistry Laboratory has developed several testing standards that have been submitted to the American Association of State Highway and Transportation Officials (AASHTO)—as part of its “forensic toolbox” research—to improve and facilitate the chemical analysis of highway materials and to characterize and quantify new or alternative sustainable materials. For example, AASHTO recently designated provisional Standard TP144-21, as described below.
In FHWA’s Chemistry Laboratory, scientists have worked diligently for the past 12 years to develop a test that detects the quality of aggregates (a group of raw materials including sand and stone used in the construction of roads) more accurately and quickly than ever before. In 1935, a distress mechanism in concrete was discovered on a bridge built in France; the first paper documenting the mechanism was published in the United States in November 1940 by Thomas Edison Stanton of the California State Highway Division. The paper detailed the chemical compound alkalis found in cement that can react with some aggregates in concrete to form a gel resulting from an alkali-silica reaction (ASR). ASR, also referred to as concrete cancer, is a swelling reaction. The resulting gel can absorb water and swell causing concrete to crack. Even after all these years, this type of swelling is still a major issue in the concrete industry.
Aggregate tests first appeared in 1947. Generally, all aggregate tests rely on measuring the expansion of concrete or mortar samples placed in a sodium hydroxide solution at an elevated temperature. However, none of these tests work particularly well. Typically, they show about 65 to 70 percent accuracy.
Currently, the best engineering test for aggregates is ASTM International’s ASTM C1293: Standard Test Method for Determination of Length Change of Concrete Due to Alkali-Silica Reaction, which takes nearly one year to garner results. By contrast, the scientists in the Chemistry Laboratory developed a test—dubbed T-FAST—which takes 21 days. It does not measure physical expansion, rather it predicts from chemical methods the potential of an aggregate or combination of aggregates to expand deleteriously due to any form of alkali-silica reactivity.
Several rounds of testing with T-FAST have produced results 100 percent in agreement with other field tests measured on concrete blocks exposed to the weather for many years. The Chemistry Laboratory is currently working with laboratories at 23 State agencies and universities on a round-robin testing schedule to bring awareness to and ensure proper usage of T-FAST. The T-FAST test has been approved by AASHTO as a provisional test method (TP 144-21: Determining the Potential Alkali-Silica Reactivity of Coarse Aggregates (TFHRC-TFAST)).
Improving Pavement Traction on Gatlinburg Bypass, Tennessee
The Gatlinburg Bypass in Smoky Mountains National Park is a 5.8-kilometer (3.6-mile) scenic roadway that connects Pigeon Forge, TN, to Gatlinburg, TN, circumventing the heavily trafficked commercial strip of Pigeon Forge. In 2013, the Bypass was repaved as part of normal maintenance with a 25.4-millimeter (1-inch) ultra-thin bonded wearing course (the upper layer in the roadway.) To ensure the wearing course had good adhesion to the old roadway, an asphalt emulsion tack coat was sprayed onto its surface. Within a short time, the pavement began exhibiting poor skid resistance and the bleeding of asphalt to the surface caused black tracking marks on the white concrete bridges. However, it was not known at the time whether the asphalt bleeding to the surface was caused by too much asphalt in the hot-mix (a mixture of asphalt binder and graded mineral aggregate) or if excessive tack coat had been used.
In visiting the site, researchers from the Chemistry Laboratory were able to gather a sample of the emulsion tack coat from an off-ramp to the Bypass. Forensic studies were performed and showed that it contained high levels of vanadium which is typical of asphalt from Venezuelan crude oil. The high level of vanadium present allowed researchers to identify the source of the asphalt used in the tack coat of the pavement and was key to solving why the asphalt bled to the surface. By extracting additional thin slices of the wearing course, recovering the binder, and analyzing for vanadium, researchers were able to see that the high vanadium content of the asphalt was bleeding up to the surface of the pavement. The researchers found that too much tack coat had been used.
Identifying Rod Failures in Wilson Tunnel, Hawaii
When it became federally mandated to test tunnels as well as bridges in the State, the Hawaii Department of Transportation (HDOT) inspected the John H. Wilson Tunnel—a pair of tunnels built in the 1950s—and found that some of the stainless-steel rods supporting the roof had broken and shifted next to the concrete. In this type of tunnel, rods extend from the very top, which is the actual boring, through the mountain, into the roof. Tunnels have a flat ceiling. Between the roof and the circular concrete of the tunnel itself there is an air space used for ventilation of the tunnel. The rods broke at the surface of the ceiling. Faced with the prospect of pouring another tunnel, HDOT was anxious to find what caused these rods to break.
Forensics performed on concrete taken from the tunnel’s roof, via x-ray fluorescence (XRF), showed that the concrete contained both chlorine and bromine—two elements commonly found in seawater. Analysis showed the chloride content to be about 0.2 percent; this percentage was compared with the chloride content of sample concrete taken from the Arlington Memorial Bridge in Washington, D.C., which had salt placed on it every year since it was built in 1932. Analysis returned a chloride level of only 0.03 percent, an amount far less than the concrete in Hawaii’s tunnel. Originally, a reason for how chlorine and bromine penetrated the tunnel’s concrete could not be found; though, many surmised it was due to absorption from atmospheric moisture. It was later discovered that concrete in Hawaii was made using beach sand through the 1970s.
Science Pinpoints Cause of Bus Crash
A chartered bus lost traction on a wet slippery interstate highway where the speed limit was 75 miles per hour (121 kilometers per hour). The bus skidded sideways, found traction in the grass, and turned over; nine passengers were killed
Samples or cores from the pavement were provided to the Chemistry Laboratory to pinpoint a reason for the pavement’s poor skid resistance. The Chemistry Laboratory found that the surface of the cores contained a layer of neat (or unmodified) asphalt. Infrared analysis also showed this asphalt contained a styrene-butadiene polymer—a hard rubber used for products like tire threads and soles of footwear. Though no polymer was found in the core itself, styrene-butadiene polymers are often added to strengthen asphalt tack coats, which are used as glue to bind stones to the surface in chip seal applications. The Chemistry Laboratory’s researchers determined that the chip seal surfacing of the pavement was incomplete as the tack coat was applied but the stones were never actually added to the surface. This determination turned out to be the case. Unfortunately, due to poor recordkeeping by personnel working on the project, the chips were omitted; good recordkeeping would have indicated that stones had not been added onto the asphalt surface.
Note: The above content provided for laboratory context only. Specific details of the bus incident, beyond what is shown, are not permitted to be released as directed by the investigating agency.
The Chemistry Laboratory’s staff is continually working to develop new testing standards, field procedures, and recommended practices for the safer design and construction of highways, bridges, and other transportation infrastructures. Currently, their research is delving further into the mechanism of ASR, with specific emphasis on the role of aluminum and identifying important parameters for a reliable test method to predict the reactivity of aggregates. The Chemistry Laboratory is also working to develop additional test methods for the use of handheld XRF, a Fourier transform infrared spectrometer, and Raman spectrometers for the field analysis of highway materials (versus their uses within a laboratory). For a look at the original Chemistry Laboratory article, visit the January/February 2012 issue of Public Roads: https://highways.dot.gov/public-roads/januaryfebruary-2012/why-does-fhwa-have-chemistry-lab.
Terry Arnold is a manager at TFHRC’s Chemistry Laboratory and a member of the pavement materials team. He is a Fellow of the Royal Society of Chemistry chartered chemist.
For more information, visit https://highways.dot.gov/research/laboratories/chemistry-laboratory/chemistry-laboratory-overview or contact Terry Arnold at 202-493-3305 or email@example.com.