Publication Number: FHWA-HRT-07-003

New Lime Test for Hot Mix Asphalt Unveiled

by Terry S. Arnold, Jenny Rozario, and Jack Youtcheff

FHWA researchers have devised a test to measure lime in asphalt, promising greater pavement longevity and lower costs to all concerned.

(Above) Fast, accurate detection of lime in asphalt can help ensure that pavements are constructed properly and meet their expected life cycles. Here, a road crew in Washington, DC, removes old asphalt in preparation for the new.

Some States require lime to be added to aggregates in hot mix asphalt to improve moisture resistance and extend the longevity of pavements. If the lime required by those States is left out of the hot mix asphalt, "stripping" may occur, defined by the National Lime Association (NLA) as "loss of adhesion between the aggregate surface and asphalt cement binder in the presence of moisture."

Early damage from omission of an additive such as lime curtails pavement life cycles, leading to substantially increased costs for State departments of transportation (DOTs) because the pavements have to be replaced. According to NLA, using lime results in savings of about $20 per 0.91 metric tons (1.0 ton) of hot mix asphalt, and an increase of 38 percent in pavement life expectancy.

Until now, there was no test to detect lime in hot mix asphalt. If a road builder is placing asphalt and fears a slip-up occurred at the manufacturing plant, nothing can be done to see whether lime was added. The choice is difficult: Remove what already has been applied and start over—likely at a high cost—or press on and complete what might be an inferior paving job. Similarly, DOTs or other authorities have no way to ensure the longevity of a pavement, or public safety and wise use of public funds.

The Federal Highway Administration (FHWA) may be rounding the corner on solving the problem. Researchers at FHWA's Turner-Fairbank Highway Research Center (TFHRC) in McLean, VA, recently applied a well-established technique, Fourier-Transform Infrared (FTIR) spectroscopy, to determine very rapidly whether the lime was added or not. Their novel use of the FTIR test detected even minute traces of lime in asphalt in as little as 30 seconds. In fact, the test is foolproof in detecting whether lime is present in hot mix asphalt. Further, the TFHRC researchers developed two methods for measuring the lime levels more accurately.

State Specs Most States' specifications for asphalt orders call for an ingredient that will improve moisture resistance. Some States have opted for lime. Georgia first instituted a lime specification in 1981; since then, at least 14 other States have followed suit. In this photo from Utah, a conveyor belt moves aggregate to the lime-addition process. Utah requires lime to be used in all its hot mix asphalts to protect against stripping. The specifications and procedures are tailored to local materials and the capabilities of construction firms and equipment. Nevertheless, all of the specifications have proven successful in producing asphalt mixes with enhanced performance. This success indicates that, regardless of the method used, hydrated lime effectively reacts with both the aggregate and the binder in asphalt mixes. Utah adopted a lime requirement in 1989. It requires lime to be added to all hot mix asphalt used on State roads, either by slurry or stockpile marination. In the slurry method, three parts water to one part lime are added to the mix at the cold feed. Marination is accomplished by sprinkling the slurry on the stockpile and storing it for 24 hours. The goal is to have 1.5 percent lime in the mix. "There is pretty good evidence from studies that lime is an effective anti-stripping agent, at least initially," says Kevin VanFrank, engineer for asphalt material at the Utah Department of Transportation. Utah prefers to employ the marination method, although the State has no requirement in its specifications for this as yet. Marination results in more intimate mixing than dry blending. The South Carolina Department of Transportation (SCDOT) specification calls for a water delivery system. All hydrated lime systems must be approved by SCDOT. Nevada also has embraced lime through the dry blending method. "Lime for us seems pretty universal—we put it in all our mixes," says Dean Weitzel, chief materials engineer for the Nevada Department of Transportation (NDOT). With lime mixed in, this Nevada aggregate must remain stockpiled for 48 hours to allow the mineral additive to take full effect. NDOT's specification calls for adding "mineral filler" (the lime) to moist aggregate, followed by 48 hours of marination. Marinated aggregate must be used within 60 days. At a typical plant, the lime is fed from a silo and onto a weigh belt by a rotary vane feeder. From there the lime is moved by a screw conveyor and discharged into a twin-shafted pug mill for mixing with the aggregate fraction (coarse or fine) of the intended mix. The required lime application rate in the NDOT specification is not less than 1 percent of the mass of dry coarse aggregate and 2.0 percent of the mass of dry fine aggregate. At the point of the lime's entry into the pug mill, water is added according to the stockpile moisture content of the aggregate. The intent is to ensure there is sufficient moisture to wet the aggregate thoroughly and activate the hydrated lime. The pug mill must be configured to alter the mixing pattern and rate of flow to ensure the aggregate is thoroughly coated with lime. The coated aggregate discharges from the pug mill onto a transfer conveyor for stockpiling.

Drawing on pavement cores from Colorado and Nevada, the researchers also found that lime can be detected even in 10-year-old pavements. This discovery adds credence to the new lime test.

The American Association of State Highway and Transportation Officials (AASHTO) Standing Committee on Highways' Subcommittee on Materials hopes to have an industry standard for the new test sometime in 2007. "It's a good detection tool," says Georgene Geary, member of the AASHTO committee and State materials and research engineer at the Georgia Department of Transportation.

The New Rapid Test

An ASTM International (originally known as the American Society for Testing and Materials) procedure existed for measuring lime purity (C25-99 Standard Test Methods for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime), but it is time consuming, requiring several hours, and does not measure the amount of lime in hot mix asphalt.

Shown here is the FTIR spectrometer used at TFHRC to determine the presence of lime in asphalt samples.

The small ATR device that holds the sample for testing is shown with a dime on top of it for scale.

The impetus for the new research work was the need to measure both the presence and extent of lime in test lanes at TFHRC's Accelerated Load Facility (ALF). Undispersed lime particles were found in the aggregate stockpiles at the facility, so a method was needed to measure how much lime was actually in the ALF test lanes. The TFHRC researchers identified a rapid analytical technique, FTIR spectroscopy, for unequivocal determination of the presence of lime and a semiquantitative determination of the amount of lime. For more accurate quantitative measurement, the researchers adopted a second procedure: They separated the lime from the aggregate by acid extraction, and they determined the calcium level by atomic absorption spectroscopy or ion exchange chromatography, and used this data to calculate the lime content.

FTIR spectroscopy is an analytical technique for identifying many organic and inorganic materials. Asymmetrical molecules have an uneven distribution of electrons; because electrons are negatively charged, this asymmetry causes the molecules to behave as tiny magnets. If the molecule is placed in a beam of infrared light, the movement of these tiny "magnets" absorbs infrared radiation at specific frequencies or wavenumbers. This can be measured as an infrared spectrum, which can be used as a fingerprint for the chemical being tested. Lime is calcium hydroxide and shows a very characteristic peak at a wavenumber of approximately 3,640-cm-1.

FTIR spectroscopy is convenient because it is rapid. There is practically no sample preparation and only a tiny sample, about the size of a grain of sugar, is needed. TFHRC used a Digilab Excalibur instrument equipped with a Golden Gate attenuated total reflectance (ATR) accessory to run the FTIR tests. The ATR accessory eliminates the need for sample preparation. It has a window about 2 millimeters (0.08 inch) square made of diamond. The mastic (mixture of asphalt and aggregate dust) contains the lime and is scraped directly from large stones in the hot mix and placed on the diamond window. A mouse click cues the device to begin testing, and 30 seconds later a result is delivered.

Drawing on core samples from the ALF pavements, researchers removed stones from the samples to expose fresh mastic. They scraped off a small amount of mastic, placed it on the ATR device, and measured the infrared spectrum. With each sample, a marked spike occurred at or very near the 3,640-cm-1 wavenumber, proving conclusively the presence of lime in the pavements.

Hydrated lime shows a sharp FTIR peak at a wavenumber of 3,640-cm-1 due to the presence of the hydroxyl group in calcium hydroxide. The spike is the tell-tale indication of lime in hot mix asphalt and can be used, though less accurately, to calculate the amount of lime. (The small peak at around 1,390 cm-1 indicates the presence of calcium carbonate impurity, stemming from the reaction of lime with carbon dioxide in the air.

Quantification

The TFHRC researchers began their approach to this analysis by removing the binder from the hot mix, using either combustion or solvent extraction; extracting the aggregate residue with acid; analyzing it for calcium either by atomic absorption spectroscopy or ion exchange chromatography; and back-calculating the lime content. (Later work demonstrated that removal of the binder was unnecessary.) Separating the lime from whole gyratory cores proved impractical and time consuming. Among other things, the volume quickly overloaded the available filtration equipment.

The researchers responded by developing a technique using a hammer drill with a 9.5-millimeter (0.37-inch) tungsten carbide bit to drill through the core. Each drilling produced 15-20 grams (0.53-0.71 ounce) of dust. Calculating the binder content of each dust sample showed good agreement with the known binder contents, suggesting the quantity of drilling dust used for analysis was large enough to be representative of the whole core.

How well the lime was actually distributed in the hot mix is not known; consequently, the researchers needed to ensure the test samples were as homogenous as possible. To achieve this, many drillings of approximately 300-350 grams (10.6-12.4 ounces) of dust were made from each core. These samples then were quartered twice to end up with 15-20 grams (0.53-0.71 ounce) of dust for each extraction.

Next came the chemical separation of the lime, which was fairly straightforward. Lime is converted to a water-soluble salt (calcium chloride or calcium acetate) by treating it with dilute hydrochloric acid or acetic acid. The calcium content is measured, and the amount of lime present is back calculated. If the aggregate contains calcium, as would be the case with limestone (calcium carbonate) aggregates, then this too will be dissolved by the acid and give an incorrect (high) result for lime content.

Hydrochloric acid is somewhat hazardous to handle and proved to offer no real advantage over acetic acid. Therefore, the researchers adopted as the standard technique the method of boiling the drilling dust for 30 minutes with 4 percent acetic acid (vinegar is 4-8 percent acetic acid) without removing the binder first. The mixture is filtered and made up to a known volume, and the calcium content measured by either ion exchange chromatography or atomic absorption spectroscopy.

Ion exchange chromatography, widely used in analysis of water, separates ions from each other and accurately determines the concentration of each element.

A second technique takes advantage of the characteristic of certain elements, which when heated in a flame impart very specific colors to the flame. By measuring the intensity of the specific color, the amount of the element present can be determined. This technique is known as atomic absorption spectroscopy.

Atomic absorption spectroscopy can measure only one element at a time—in this case calcium—whereas ion exchange chromatography can measure many elements at the same time. In this analysis, atomic absorption spectroscopy and ion exchange chromatography gave the same result and were used only as a confirmation of the analysis.

Because the sample preparation is more complex, both ion exchange chromatography and atomic absorption spectroscopy techniques take much longer—2 to 3 hours—than the FTIR technique.

Researchers acknowledged concern that core samples from the dry climates of Colorado and Nevada might not show the effects of rain, snow, and other weathering on pavement. Mountain and high desert areas are known for their stripping problems, caused by the Solar Still Effect (atmospheric moisture condensed on pavement during the night when temperatures drop rapidly), up to 230 freeze-thaw cycles annually, and aggregates that have poor adhesion to asphalt in the presence of moisture. In this photo, a western road crew paves the shoulder of a two-lane highway.

The three tests may vary in terms of their feasibility for use by States. FTIR spectrometers cost about $40,000 to $50,000, and ATR devices are another $10,000 approximately. Still, they are basic pieces of equipment in chemistry laboratories, and State DOTs with paint testing laboratories likely have FTIR spectrometers because they are central to analyzing paint films. The technology involved in the ion exchange chromatography and atomic absorption spectroscopy approaches is also expensive and is less common in labs. Most universities have the equipment, but that may not be the case with State DOTs.

Colorado Samples

TFHRC received significant help from the Colorado Department of Transportation (CDOT) and Nevada Department of Transportation (NDOT), which provided pavement cores from existing highways. The purpose of testing these cores was to determine whether TFHRC's new-found method would still detect lime after a road has been in service for several years.

"In Colorado we require 1 percent [by weight] hydrated lime. This is an expensive ingredient and we really have no accurate way to tell if we are getting it," says Tim Aschenbrener, materials and geotechnical branch manager at CDOT. "An accurate and timely test will help us make sure the contractor is meeting the requirements of the specification, which will result in the longer performing pavement we want."

In all, five cores were received from CDOT in June 2005. Two cores were from the Limon Bypass, which was completed in 1999, and the other three were from U.S. 40, completed in 2004. All the cores should have contained approximately 1 percent lime at the time they were laid down, according to Colorado State specifications.

To retrieve a sample for the FTIR test, the TFHRC researchers removed a sample of mastic from the core and measured the FTIR spectrum.

The samples from both U.S. 40 and the Limon Bypass showed the characteristic calcium hydroxide peak at around 3,640-cm-1, clearly demonstrating the presence of lime in the mix, 1 year or so after the road was paved. (The test detected lime even 6 years after pavement application, in the 1999 Limon Bypass work.)

Using the acetic acid extraction technique, the researchers found the U.S. 40 cores to hold about 7 percent lime, far above the expected level. This result suggested that the aggregate contained some limestone. Examination of the aggregate taken from the core, by using an x-ray fluorescence attachment on the scanning electron microscope, indicated that the aggregate was not limestone but did contain a significant amount of calcium (20-30 percent).

For the Limon Bypass cores, the acid extraction method yielded a lime content of 1.91 percent for one core and 1.62 percent for the other. No investigation of the aggregate was done.

Nevada Samples

More extensive and conclusive, the tests on the Nevada pavement cores may give even greater support for the new test. Importantly, NDOT also provided cores from a 10-year-old paving project to determine whether the lime could still be detected after long outdoor exposure. This test would gauge whether the lime was converted to calcium carbonate by absorbing carbon dioxide from the air or was leached out by rain.

An array of samples was provided, from mix designs performed in the current construction season, from projects under construction, and from old projects. A group of samples came from the southern part of the State and another from the northern part. (One sample carried an unknown date—it was simply termed "old"—and an unknown location and mix design.)

For the FTIR testing, as with the ALF and Colorado samples, a stone was pried from the side of each core with a screwdriver and a spatula used to scrape a small amount of mastic and the spectrum measured. In every case the peak at approximately 3,640-cm-1 indicated the presence of lime in the cores.

Some of the Nevada cores showed the presence of a high and unrealistic amount of lime. In each case, the aggregate was tested and found to contain limestone. Clearly, the acid extraction test cannot be used to determine the lime content of mixes containing limestone aggregates. The FTIR method, although only a semiquantitative approach, does yield a result closer to what might be expected. For example, cores BF05-31 and BF95-167, both of which originally were supposed to contain 1.5 percent lime, were shown to contain 34.85 percent and 31.18 percent by acid extraction and 0.96 percent and 1.1 percent by FTIR.

Setting the Standard

As mentioned, AASHTO currently is weighing the lime tests for adoption as industry standards, with a decision expected sometime in 2007. The procedures also have been submitted to ASTM International for similar consideration.

Given the results of the ALF tests, bolstered by experiments with the Colorado and Nevada cores, the new test holds promise for State DOTs and other entities to gauge the presence of lime quickly in existing and future pavements. FTIR spectroscopy for lime detection provides a reasonably accurate indication. The biggest advantage of this method is its simplicity and speed: The FTIR spectrum can be obtained in just 30 seconds.

Chemical analysis was found to be more accurate for measuring lime content, and the TFHRC researchers developed an original, easily replicable process: Pavement cores are drilled with a hammer drill, the drilling dust collected and boiled for 30 minutes with a 4 percent acetic acid solution, and the resulting extract analyzed using either atomic absorption spectroscopy or ion exchange chromatography to determine accurately the amount of lime present.

The State cores further helped by raising flags about some aspects of the TFHRC test. For example, the Colorado samples indicated that calcium-containing aggregates may interfere with the acid extraction analysis and give an artificially high result. To be more accurate, a background analysis should be done with the virgin aggregate. The Nevada samples suggested the acid extraction technique cannot be used for mixes containing limestone aggregates, although the FTIR method appears to give more realistic answers. It will be one of the tasks of the AASHTO team to determine the accuracy of the method.

Both States' samples showed that lime was still present in pavement some time after it was laid-for Nevada, even 10 years later. In that case, the lime level was found to be 1.1 percent, little different from the 0.96 percent level found in a lab-compacted field sample that was not subject to weathering. The lime was not changed to calcium carbonate by absorbing carbon dioxide from the air, nor was it leached out by rain.

An AASHTO industry standard that could come as early as 2007 could help govern the content of asphalt that is placed on the Nation's roads by workers such as these.

Terry S. Arnold is from the United Kingdom and manages the chemistry laboratory at TFHRC. He was granted an honors degree in chemistry in 1971 by the Royal Society of Chemistry and is a fellow of that society.

Jenny Rozario, from Dhaka, Bangladesh, is a researcher at TFHRC's chemistry lab. She holds a bachelor's degree in biology from Iowa's Luther College and has worked at the Mayo Clinic in Rochester, MN. She has been with FHWA since 2001.

Jack Youtcheff is the team leader of the materials and construction team at TFHRC. He received a bachelor's degree in chemistry from The Pennsylvania State University in 1977 and a doctorate, also from Penn State, in materials science and engineering in 1983.

For more information, contact Terry Arnold at 202-493-3305 or terry.arnold@fhwa.dot.gov.