Evaluating Applications of Field Spectroscopy Devices to Fingerprint Commonly Used Construction Materials (2013)

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71 A p p e n d i x e This appendix presents the results of the literature search on the state-of-the-art theory and practice for “fingerprint- ing” the most common materials used in highway construc- tion. The appendix includes a review of the theory behind several spectroscopic techniques and a discussion on their applicability to highway construction materials. Finally, the most promising laboratory and field testing devices are com- pared in terms of their applicability, complexity of usage, sensitivity, and reliability. introduction The research team explored bibliographic databases using the Transportation Research Information Service (TRIS) and then expanded the search to include various Internet bib- liographic sources, such as the ASCE Civil Engineering database, ScienceDirect, and others. This approach did not limit the sources to those directly related to the fields of trans- portation or civil engineering but included various journals on chemistry and the petroleum industry where asphalt- related materials were widely discussed. On the basis of the results of the literature search, the research team created a computerized bibliographic database in specialized software called Bibus. The literature analysis was divided into the following topics: • Underlying principles of the most commonly used spec- troscopic methods of material analysis (i.e., infrared and Raman spectroscopy; gas, gel permeation, and liquid chro- matography; X-ray diffraction and fluorescence; and nuclear magnetic resonance); • Practical application of the above methods to identify sub- stances of interest (e.g., paints and coatings, antistripping agents, phosphoric acid additive, air oxidation, polymer additives, asphalt emulsions, water content, curing agents, portland cement content); • Evaluation of spectroscopic devices and their applicability to the on-site analysis of construction materials versus laboratory analysis in terms of availability, applicability, limitations, complexity of usage, sensitivity, and reliability; and • Review of the ASTM and AASHTO standards relevant to spectroscopic testing of the transportation construction materials. Overview of instrumental Techniques of Material Analysis Most spectroscopic techniques described here (with the exception of chromatography) rely on the interaction of light with matter. Electromagnetic waves of different wavelengths interact with atoms and molecules in different ways that are dictated by physical laws. Each material has a unique chemi- cal structure, consisting of chemical elements that combine in predictable amounts and spatial configurations. The type and chemical arrangement of elements in a compound dic- tate the outcome of its interaction with light of different wavelengths. Targeting a material with radiation of known wavelength and observing the outgoing signal can be used to infer the chemical properties of the material and potentially its amount within a mixture of different compounds. This section discusses theoretical aspects of chromatographic and spectroscopic techniques of interest for transportation materials. Chromatography Chromatographic methods physically separate the compo- nents of a mixture and distribute them between two phases. One phase remains stationary (a solid, a gel, or a liquid sol- vent), while the other phase (a liquid or a gas) moves in a defi- nite direction (1). The separated compounds can then be Literature Review

72 detected by coupling the chromatographic column with a detector that identifies each compound separately. The stationary phase can be placed in two shapes of appara- tus, the column and the planar chromatograph. In the column chromatograph, the stationary phase is placed in a cylindrical tube. In the planar chromatograph, a thin layer of solid parti- cles spread on a glass plate or a paper saturated by a liquid may serve as the stationary phase (1). The classification of the sepa- ration techniques according to the physical state of mobile and stationary phases include gas–liquid chromatography (GLC), gas–solid chromatography (GSC), liquid–liquid chromatog- raphy (LLC), and liquid–solid chromatography (LSC) (1). Finally, four different separation mechanisms can be used in chromatography: adsorption, partition (gas or liquid chroma- tography), ion-exchange chromatography, and size-exclusion chromatography (1). Each separation methods exploits a dif- ferent attribute of the mixture components: affinity to sorb on the stationary phase, affinity to dissolve in a particular medium, ability to occupy anionic or cationic sites, and size, respectively. Figure E.1 schematically summarizes the classification of chro- matographic methods. The separation results are typically pre- sented in the form of a chromatogram, where the concentration of each component in the mobile phase is graphed versus the mobile phase volume or over time. The detailed description of chromatographic techniques can be found elsewhere (1, 2). The research team identified the following methods as most commonly used for highway construction materials: (a) gel permeation chromatography (GPC), (b) size-exclusion chromatography (SEC), (c) GLC, and (d) high-performance liquid chromatography (HPLC). These methods are further discussed in the section on the current practice of material analysis in transportation, whereas the feasibility of using portable chromatographic devices is analyzed in the section on the overview of spectroscopic equipment. Infrared Spectroscopy Infrared (IR) spectroscopy is based on the energy absorption of IR electromagnetic waves by molecules. The IR radiation spectrum covers a range of wavelengths from 0.78 to 1,000 µm, which corresponds to the red end of the visible region at high frequencies and the microwave region at low frequencies (3). The IR range is usually subdivided into three regions: (a) near- infrared (NIR) spectra (0.78- to 2.5-µm wavelength), (b) mid- infrared (now referred to as IR) spectra (2.5-µm to 50-µm wavelength), and (c) far-infrared (50-µm to 1,000-µm wave- length) spectra (3). Photon energies associated with this part of the infrared spectrum (from 1 to 15 kcal/mole) are not large enough to excite electrons but may induce vibrational excitation of cova- lently bonded atoms and groups, for example –OH, C–C, or –COOH. Because organic compounds mostly comprise cova- lently bonded atoms, IR spectroscopy is primarily applied to fingerprint organic materials (even though inorganic com- pounds with covalent bonds are also amenable to IR analysis). IR spectrometers record the lattice’s absorption of electro- magnetic energy as a function of wavelength of specific groups of atoms in molecules. Each group has a unique absorption band at specific wavelengths regardless of the composition of the remaining molecular structure. The absorbance at these specific wavelengths can be used to quantify a particular functional group in the analyzed substance (4). The specific absorption peaks are easily identified on a spectrogram and can be used to identify a compound in a mixture once its IR peaks are known from the analysis in a pure state. Two types of IR spectrometers—dispersive and Fourier transform—are available for obtaining IR spectra. Dispersive IR spectrometers have been in use since the 1940s, whereas Fourier transform IR (FTIR) devices were introduced in the 1980s. Because FTIR spectrometers are faster and can analyze multiple spectra simultaneously, they gradually replaced dispersive spec- trometers. The FTIR devices employ an inert solid that is elec- trically heated to a temperature of 1,000°C to 1,800°C to produce infrared radiation. The other important component of FTIR spectrometers is the interferometer. The interferometer produces a unique type of signal, which has all of the infrared frequencies “encoded” into it. The signal can be measured quickly, usually on the order of 1 s or so. Thus, the time element per sample is reduced to a matter of a few seconds rather than several minutes. Most interferometers employ a beam splitter, which takes the incoming infrared beam and divides it into two optical beams. The two beams reflect off their respective mirrors CLASSIFICATION OF CHROMATOGRAPHY APPARATUS SHAPE • Planar (LC) • Columnar (GC, LC, HPLC) SEPARATION METHOD • Adsorption • Partition • Ion Exchange • Size Exclusion (GPC, HPLC) MOBILE PHASE STATE • Gas (GC) • Gel (GPC) • Liquid (HPLC) Figure E.1. Classifications of chromatographic techniques.

73 and are recombined when they meet back at the beam splitter. Because the path that one beam travels is a fixed length and the other is constantly changing as its mirror moves, the signal, which exits the interferometer, is the result of these two beams “interfering” with each other. The resulting signal is called an interferogram. An interferogram has the unique property that every data point (a function of the moving mirror position) making up the signal has information about every infrared fre- quency, which comes from the source. FTIR is the most com- monly used IR method. Attenuated total reflectance (ATR) accessories have recently become available for obtaining IR spectra of highly absorbing samples that cannot be readily examined by the normal transmission method (3). X-Ray Fluorescence Spectroscopy X-ray fluorescence spectroscopy (XRF) is the emission of char- acteristic “secondary” (or fluorescent) X-rays from a material that has been excited by a bombardment of high-energy X-rays or gamma rays (5). When materials are exposed to short- wavelength X-rays or gamma rays, their component atoms may ionize. As a result, the material emits radiation at energy characteristic of the atoms present. The term fluorescence is applied to phenomena in which the absorption of higher- energy radiation results in the re-emission of lower-energy radiation (5). The wavelength of this fluorescent radiation can be calculated and analyzed either by sorting the energies of the photons (energy-dispersive analysis) or by separating the wavelengths of the radiation (wavelength-dispersive analysis). Once sorted, the intensity of each characteristic radiation is directly related to the amount of each element in the material. The XRF method is widely used to determine the elemen- tal composition of materials. Because this method is fast and nondestructive to the sample, it is often used in field and industrial applications for quality control of materials. X-Ray Diffraction Spectroscopy X-ray diffraction (XRD) spectrometry is one of the most power- ful analytical tools available for identifying crystalline sub- stances in complex mixtures (6). All crystals are composed of regular, repeating planes of atoms that form a lattice. When coherent X-rays are directed at a crystal, the X-rays interact with each atom in the crystal, exciting their electrons and causing them to vibrate with the frequency of the incoming radiation. The electrons become secondary sources of X-rays, reradiating this energy and creating interference patterns that depend on the distance between atomic layers, chemical composition, and the angle between the X-ray beam and the atomic plane (6). The diffraction pattern created by constructive interfer- ence is recorded by a beam detector as the X-ray tube and the detector are rotated around the sample. The relationship between the angle at which diffraction peaks occur (2q) and the interatomic spacing of a crystalline lattice (d-spacing) is expressed by Bragg’s law: nl = 2dsin u. Traditionally XRD traces, or diffractograms, are expressed in units of two theta degrees (2q). Because each crystalline structure is unique, the angles of constructive interference form a unique pattern. By comparing the positions and intensities of the diffraction peaks against a library of known crystalline materials, sam- ples of unknown composition can be identified (6). Raman Spectroscopy Raman spectroscopy is named after C. V. Raman, who dis- covered the photon-scattering effect. The Raman method exploits the effect of the inelastic scattering of light from laser in NIR visible or ultraviolet (UV) range. Light interacts with phonons (quantized modes of vibration) or other excitations in the system, resulting in a shift of the laser photons’ energy (7). The difference between the incident and scattered frequencies corresponds to an excitation of the molecular system. A Raman spectrum is obtained by measuring the intensity of the scattered light as a function of the frequency difference. This spectrum reveals information about the chemical structure and physical state of the sample (8). A Raman instrument comprises three basic components: (a) a laser to excite the sample, (b) a spectrograph that chan- nels the light and separates the component frequencies, and (c) a detector that measures the energy, or intensity, of each component frequency. Many complete Raman systems also include operating software for instrument control, data acqui- sition, data processing, and analysis (8). The Raman method can be applied to solid, liquid, or gas- eous samples (transparent or nontransparent) of a wide size range (from 1 m2 to a few square centimeters) with different surface textures (i.e., Raman spectroscopy can be applied to any optically accessible sample, where a pretreatment of the sample is not necessary) (9). The main drawback of Raman reported in the literature is its poor sensitivity, which pre- cludes the detection of substances at very low concentrations (9). However, two special techniques—the resonance Raman effect and surface-enhanced Raman scattering—can be used to enhance the intensity of the inherently weak Raman sig- nals by several orders of magnitude (9). A detailed description of Raman spectroscopy can be found in the literature (7–10). Applications of Raman spectroscopy to highway materials and specific Raman devices are discussed later in this report. Nuclear Magnetic Resonance Nuclear magnetic resonance (NMR) is a phenomenon that occurs when the nuclei of certain atoms are immersed in a static

74 magnetic field and exposed to a second oscillating magnetic field. Only the nuclei with spin (a fundamental particle prop- erty produced by the rotation of nucleons, protons, or electrons around their own axis) exhibit the NMR phenomenon (11). Time-domain NMR is used to probe molecular dynamics in solutions, whereas solid-state NMR is used to determine the molecular structure of solids in the frequency domain (11). NMR spectroscopy employs the radio frequency (RF) range (60 to 800 MHz for hydrogen nuclei) to measure the energy of photons (11). The NMR signal results from the dif- ference between the energy absorbed by the spins making a transition from a lower to a higher energy state and the energy emitted by the spins simultaneously making a transition from a higher to a lower energy state (11). The signal is thus pro- portional to the population difference between the states. NMR is a rather sensitive spectroscopy because it is capable of detecting these very small population differences by mea- suring resonance, or exchange of energy, at a specific fre- quency between the spins and the spectrometer (11). The difference between the resonance frequency of the nucleus and a standard (usually, carbon-13 or hydrogen-1) is called the chemical shift of a nucleus, which is measured in parts per million (ppm) and denoted by d (11). Measuring the chemi- cal shift enables the identification of chemical elements. Figure E.2 shows a schematic diagram of the NMR hard- ware (11). Within the bore of the magnet are the shim coils for homogenizing the produced magnetic field. The probe placed within the shim coils contains the RF coils, which in turn produce the magnetic field necessary to rotate the spins by 90° or 180° and detect the signal from the spins (11). A com- puter controls all of the spectrometer components, including the RF frequency source and pulse programmer, which control the pulse sequences and frequencies. Current Practice of Material Analysis in Transportation This section presents a review of current applications of spec- troscopic techniques for identifying transportation-related materials and their components. The review includes the techniques applied by researchers and material engineers in the laboratory as well as in the field conditions. Table E.1 pro- vides a summary of references grouped by materials and methods. Applications of Chromatography For the past two decades, material engineers have used differ- ent chromatographic methods to study the properties of vari- ous asphalt additives and, recently, of portland cement. The most commonly used methods are SEC/GPC, liquid chro- matography (LC), ion-exchange chromatography (IEC), and gas chromatography (GC). Below is a brief review of the rel- evant studies. Size Exclusion and Gel Permeation Chromatography This method showed success in characterizing asphalt com- ponents and predicting asphalt behavior in the field (12–14). The aromatic fractions of asphalt were analyzed by GPC to study the process of crystallization (12, 13). In the early 1990s, a strong correlation was found between GPC profiles and rheological properties of asphalt, such as viscosity and resis- tance to oxidation (14, 15). The U.S. Army Corps of Engineers used GPC to investigate the aging of asphalt- and coal tar-based joint sealants (16, 17). Figure E.2. Diagram of the major systems of an NMR spectrometer.

Table E.1. Summary of References by Materials and Methods Methods NMR Raman FTIR NIR XRD XRF GC LC/HPLC SEC/GPC IEC Reference 13, 55, 70, 79, 88–92 50, 63–73 19, 24, 34–59 60–62 69, 70, 74–83 81, 84–87 33–35 20, 29, 30 12–25,26–28 28, 31, 32 Materials Coatings Epoxy coatings 59 Asphalt- Related Materials and Properties Hydrated lime 32 Antistripping agents 45, 46 Asphalt contaminants 34, 35 34, 35 Phosphoric acid additives 27 Aging and antioxidants 91 19, 38–44 20 18–20, 22–25 Polymer additives 71 24, 47–49 33 24–26, 31 Asphalt emulsions 42, 43 Air-blown asphalt 21 Physical properties 13, 88–90, 92 72,73 60 74 12–14, 28 Sealants Joint sealants 16, 17 Portland Cement Concrete- Related Materials Sulfate– magnesium reactions 54, 55 Fly ash and aggregates 67 81, 82 Portland cement 55,70, 79 50, 63–66, 68–70 50–53 69, 70, 75–80 Admixtures 56–58 29 Reinforcement/corrosion 83 Soil stabilization/compaction 62 Wood structures 61 Detection of metals in air and soils 78, 83 84–87 75

76 The study showed that GPC could be successfully used to iden- tify sealants that have been exposed to prolonged heating in the laboratory or to natural weathering in the field. However, the researchers failed to correlate the changes in the molecular size distribution of the sealants to their physical properties (16, 17). The GPC method was also successfully used to study the effect of aging on asphalt behavior (18–20). Using GPC allowed for the identification of air-blown asphalts, which are prohibited for use in some states because of preoxidation that occurs during production (21). GPC results also reflected well the compositional changes in rejuvenated binders (22, 23). The foregoing studies agreed that the distribution of molecu- lar sizes was affected by the age of asphalt (i.e., the amount of large-sized molecules increased with the time as the amount of the medium- and small-sized molecules decreased). Additionally, a number of studies used GPC to evaluate polymer-modified binders and emulsions (24–26). For exam- ple, the GPC chromatograms of styrene–butadiene–styrene (SBS) polymer indicated that the considerable degradation of polymer over time contributed to the binder aging (24, 25). However, others reported that GPC merely indicated the pres- ence of a polymer in the binder, but failed to identify its chem- ical structure (26). Similarly, a study investigating the effect of polyphosphoric acid (PPA) addition in binders succeeded in identifying the presence of PPA in the binder using GPC (27). To facilitate the process of defining the asphalt fractions, a multiple-column SEC/GPC was developed under a FHWA study in 2001 (28). SEC/GPC was employed not only for asphalt but also for portland cement concrete-related research, such as the evaluation of the performance of a plasticizer– water reducer (29). GPC is a reliable technique to determine molecular weight of different components in a mixture. Liquid Chromatography When using a high-performance liquid (e.g., helium-sparged n-hexan, methanol, and toluene), the separation technique is referred to as high-performance liquid chromatography (20, 28, 30). HPLC was employed to quantify the aromatic frac- tion of aged recycled asphalts using the refractive index response factor (20). An FHWA study indicated that exces- sively large molecules can be slowed inside the column dur- ing the SEC/GPC separation and may thus be unaccounted for (28). Using HPLC also allowed for the prediction of pave- ment service life based on differences in the molecular size distribution of individual asphalts (28). Ion-Exchange Chromatography The IEC separation method uses an activated anion or cation resin as a mobile phase. A detailed description of the method can be found elsewhere (1, 28, 31). This method is less com- monly used in the asphalt industry because of the high costs and the complexity of sample preparation (32). Nevertheless, it was effectively used for determining the concentration of hydrated lime in asphalt (32). Gas Chromatography GC was successfully used in Canada to measure the initial boil- ing point of low-fuming SBS-modified asphalt (33). It allowed hot-mix plant operators and paving contractors to minimize fumes and odors from mix plants and paving job sites (33). Purdue University researchers used GC to detect the presence of contamination (alternative fuel) in asphalt (34, 35). Chromatographic methods are particularly applicable to fractionate asphalt binders in order to identify individual chemicals. Most of the chromatographic separation methods require stationary laboratory equipment, which may not be readily available to departments of transportation and con- tractors. However, attempts have been made recently to develop portable chromatographic devices (e.g., handheld GC and table-size HPLC equipment). The feasibility of using those devices for transportation-related materials analysis is discussed later in this report. Applications of Infrared Spectroscopy IR spectroscopy is the most commonly used method to deter- mine the chemical composition of various construction materials, including portland cement, asphalt, and epoxy coatings. Below is a review of the research conducted and documented during the past 15 years. Fourier Transform Infrared Spectroscopy Various studies employed FTIR to investigate aging of asphalt. For instance, FTIR was used to quantify the extent of oxidation based on the area under the carbonyl absorption band (19, 36–44). The mechanism of rejuvenator diffusion into the re- cycled binder was also qualitatively evaluated using FTIR (39). The researchers found FTIR suitable for monitoring the diffu- sion process involved in bituminous binder mixing if a differ- ence between IR absorption exists between the two components (39). Contaminant residues (decomposed tars and fuel) were effectively identified by FTIR in South Dakota (34, 35). FTIR has also been used to detect the presence of a stripping agent (45, 46). AASHTO T302-05 standardized the FTIR method to iden- tify polymer-modified binders (PMB) procedures (47). The Virginia Department of Transportation established this test as a first-level indicator for quality assurance of the PMBs (48). SBS polymers were successfully identified using FTIR spectra (peaks at 966 cm-1 and 700 cm-1) instead of a tradi- tionally used softening point test (24, 49). Similar results were obtained in studies of emulsified binders (42, 43). The pres- ence of antioxidants, such as zinc dialkyldithiophosphate and

77 zinc dibutyl dithiocarbamate, was effectively tracked using FTIR even at very low concentrations (41). FTIR has been successfully used in the portland cement industry for more than three decades. Since the 1970s, clinker constituents (alite, belite, tricalcium aluminate, and brown- millerite) have been quantified with FTIR (50–53). The behav- ior of PCC exposed to magnesium- and sulfate-aggressive environments has also been studied (54, 55). FTIR studies were recently conducted on the effect of different admixtures (pozzolanic, polymeric, and air-entraining additives) on port- land cement properties (56–58). Another example of a material successfully studied with FTIR is protective coating of steel structures (59). Analysis of FTIR spectra identified a low degree of curing as the reason for premature failure of the coating. Near-Infrared Spectroscopy Although most studies found in the literature were conducted in the mid-infrared region (2.5-µm to 25-µm wavelength), some researchers exploited the near-infrared region (0.75-µm to 2.5-µm wavelength) to investigate a range of construction materials. For example, a good correlation was reported between NIR spectra and asphalt properties, such as penetra- tion, viscosity, and flash point (60). The NIR absorbance spectra showed very strong correlation (R2 = 0.9) with the weathering behavior of in-service wood structures (61). The NIR method, though, was not recommended for use in moni- toring compaction of soils because of high sensitivity to non- moisture factors that harm the accuracy of results (62). FTIR technology became the most common method of material analysis because of the equipment’s compact size and the relatively easy interpretation of absorbance spectra. This makes the FTIR spectroscopy a good candidate for further evaluation because of its applicability to on-site identification of different chemicals. A detailed discussion of the FTIR equip- ment as compared to other spectroscopic methods is provided later in this report. Applications of Raman Spectrometry Raman has been used primarily for the analysis of substances with low light absorptivity, such as white portland cement and its supplements. Additionally, the hard X-ray Raman spectra have been used to characterize light elements in petroleum products. Infrared Raman Spectroscopy Raman was first used in the late 1970s for the chemical analy- sis of portland cement (50, 63). At that time, the application of Raman to gray cement was limited by the low laser power, though it was successful in studying white cement (64). Recently, researchers conducted a comprehensive Raman analysis of portland cement, slag, and fly ash, and they con- cluded that structural fluorescence can be overcome by using a 785-nm NIR laser (65, 66). In another study, the chemical composition of fly ash was established using Raman (67). Raman also appeared to be useful to characterize hydraulic lime (68, 69) as well as various forms of gypsum using a 514.5-nm excitation wavelength (66). Despite these success- ful applications, some researchers still question the usefulness of IR Raman, compared to other spectroscopic methods (e.g., XRD and NMR), to analyze cement mineralogy (70). Over the past decade, multiple attempts were made to develop reliable portable Raman devices. For instance, the pro- totype of a portable FTIR Raman spectrometer was reported to be used successfully to correlate Raman kinetic data with rheo- logical properties of cured polymers (71). X-Ray Raman Spectroscopy The literature search yielded few studies on the use of Raman for asphalt analysis. One study found that X-ray Raman energy peaks were correlated with the macroscopic proper- ties of polycyclic aromatic hydrocarbons and asphaltenes (72, 73). In conclusion, while some promising results have been shown and documented in applying Raman technology to the analysis of cement-based materials, further research is needed to establish the feasibility of using Raman for finger- printing asphalt-based products. Applications of X-Ray Spectroscopy X-ray spectroscopy is the oldest spectroscopic method for material analysis. Over the past two decades, it has been fre- quently used to study physical material properties and to iden- tify the chemical components of many transportation-related materials, including aggregates, asphalt binders, emulsions, portland cements, and coatings. The three most commonly used X-ray methods are X-ray tomography (imaging), XRD, and XRF. Traditionally, X-ray technology required room-size equipment to generate X-rays, which made it inapplicable for field use. Recently, however, portable XRF and XRD devices have become available. Because on-site fingerprinting of tar- get chemicals concerns this project, the XRD and XRF spec- troscopic methods are discussed below. X-Ray Diffraction XRD has been rarely employed for the analysis of asphalt materials as compared with chromatography and FTIR spec- troscopy (Table E.1). Only one study employed XRD in con- junction with GPC and NMR to compare asphalt binders with different rheological properties (74). On the contrary, numerous studies present XRD analysis as a good quantitative

78 method for analyzing portland cement and its supplements (69, 70, 75–79). In general, XRD is more appropriate for crys- talline organic compounds and only finds application in organics analysis in the pharmaceutical industry. Recently, an ASTM method was developed for quantitative X-ray diffrac- tion testing of portland cement using the Rietveld method (80). However, the technique is only used to quantify clinker minerals in dry cement, which are well-defined crystalline compounds. Testing the composition of hydrated portland cement and concrete is a more complicated task. The NCHRP Research Results Digest 281 pointed at XRD as a suitable technique for identifying the mineral components of aggregates (81). In India, an XRD test was conducted to study the crystalline phases present in fly ash (82). In a study of potential soil stabilization options, XRD analysis yielded the quantitative composition of the soil with reasonable accuracy (78). Because X-rays are extremely sensitive to the presence of heavy metals, XRD was successfully used to study corroded reinforcement (83). X-Ray Fluorescence XRF was used to assess the durability of carbonate aggregates (81). Portable XRF devices have been available since the early 1990s, which, in conjunction with their ability to quantify the concentration of heavy metals, facilitates their use in air and soil analysis (84–87). This also makes XRF a good candidate for asphalt applications concerning metals. Application of Nuclear Magnetic Resonance Among various spectroscopic techniques, NMR appears to be the most powerful tool to quantitatively describe the atomic and molecular structure of materials. In the transportation field, researchers have been successfully using NMR spectros- copy and NMR imaging to analyze asphalt- and portland cement-related materials since the 1990s. The carboxylic acid and phenol content of a variety of asphalts (first fractionated by SEC) were measured using carbon-13 and hydrogen-1 NMR spectra (13, 88–90). In another study, NMR imaging facilitated assessing the degree of aging and asphalt compat- ibility in aged asphalt (91). NMR imaging was recently used to evaluate moisture-induced damage in asphalt layers by measuring the interfacial properties of asphalt components (92). For cement-based materials, NMR was used to investi- gate the structural organization of portland cement and its phases using silica-29 spectra (55, 70, 79). NMR has traditionally required a laboratory environment because of the high magnetic fields required. Recently, how- ever, portable NMR devices have become available (93), cre- ating an excellent opportunity to explore their application in field conditions as part of this study. Summary of Applications Review Table E.1 summarizes the references used in this section in the form of a material–method matrix. On the basis of the number of references related to a specific method or material, one can reasonably conclude that some techniques were used for the analysis of particular materials more often than others. For example, it appears that FTIR has been successfully used for the analysis of fundamental properties of asphalt and its additives as well as for the analysis of portland cement and its admixtures. XRD has been used more often to determine the composition of portland cement with almost no asphalt- related applications. All types of chromatography appeared necessary to separate asphalt components to enable the inter- pretation of FTIR or Raman spectra. NMR spectroscopy and imaging have been reported to be robust quantitative meth- ods for both asphalt and portland cement. The suitability of Raman technology for asphalt analysis should be evaluated further in this study. Another important observation from the literature review is that researchers have been more successful in the qualitative analysis of chemicals components compared to quantitative analysis. Quantification of the components or phases in con- struction materials requires comprehensive spectra libraries and precise analytical procedures. These issues, along with the availability of the portable equipment, the limitations, and complexity of device usage, analysis time, sensitivity, and reli- ability are discussed in the next section. Overview of Spectroscopic equipment Spectroscopic methods are compared in terms of the size of the apparatus (i.e., room size, table size, portable, or hand- held), sample preparation requirements, limitations and complexity of usage, analysis time, sensitivity, and reliability. The following types of equipment are discussed: SEC/GPC, HPLC, GC, FTIR, Raman, XRD, and NMR. Typical features and images are provided for each device type. Size-Exclusion Chromatography and Gel Permeation Chromatography The SEC/GPC apparatus developed in the 2001 FHWA study includes the following main components: a refraction col- umn (usually 5 cm in diameter and 100 cm long) filled with reagent (e.g., toluene, methanol, or dichloromethane), two flasks filled with sample of asphalt solution, several graduated cylinders, about 10 m of tubing, and a laboratory pump (28). A sample of asphalt to be separated is relatively small (16 g) (28). FHWA reported that the precision of the method depends on the sharpness of the onset of fluorescence in the

79 SEC Fraction-II eluates. The method is very precise when the same asphalt dependent cut point for SEC Fraction-I is used for each run (28). One drawback of the setup is that a large amount of toluene is necessary for the separation, although the toluene is typically distilled and is reused (28). For porta- bility of the proposed setup laboratory, room conditions are essential for the process, making on-site use difficult. An image of a Waters GPC system can be found elsewhere (94). Size-Exclusion Chromatography and High-Performance Liquid Chromatography Chromatographs Concurrently with the GPC setup, the fast SEC/HPLC experi- mental protocol was developed in the FHWA study (28). Although the main components of the apparatus remained the same, a modern and compact device was added, the Hewlett Packard (HP) Series II 1090 liquid chromatograph (95), along with a laptop computer with the HPLC analysis software (28). This HPLC device uses liquid-grade toluene as a reagent. The sample size (weighed, dissolved, and centrifuged) is about 24 mg contained in 220-mL solution (28). A very high level of precision (1% standard deviation from the average for the larg- est fractions) was reported by the developers of this experi- mental setup, though some calibration was still required (28). Another example of a portable HPLC chromatograph is the SRI Model 210D with specifications similar to those of HP Series II 1090 liquid chromatograph (96). The portability of HPLC devices can be described as low, because they cannot be carried by the operator. However, installation of such devices in a mobile laboratory (e.g., truck-based) is possible. Gas Chromatographs GC was reported to be successful for detection of asphalt con- taminants and polymer additives. Therefore, the use of porta- ble GC devices could facilitate on-site quality control. One example of such a device is a Photovac’s Voyager portable chro- matograph (97). According to its manufacturer, the device is capable of detecting a range of chemicals, including petro- chemicals, latex polymers, and volatile organic compounds when an appropriate compound library is used with the device. The Voyager portable GC is 39 cm long, 27 cm wide, and 15 cm high and weighs about 7.2 kg. It is also equipped with a NiCd battery that has an 8-h life (97). These characteristics make the Voyager a good candidate for a feasibility study. Fourier Transform Infrared Spectrometers FTIR spectrometers have been used in the analysis of a wide range of transportation-related materials. These devices have seen dramatic improvements in portability, precision, reliability, and time efficiency in the past two decades. Only the most recently developed devices are compared here as relevant to this project and available to the research team. ALPHA FTIR device by Bruker Optics is 22 × 30 × 25 cm in dimensions and weighs about 7 kg (98). It has a spectral range from 7,500 cm-1 to 375 cm-1 with a resolution of at least 2 cm-1 (0.9 cm-1 is optional) and an accuracy of 0.01 cm-1 (98). The device is designed to operate at 18°C to 35°C and is powered by 100 to 240 volts AC power or by a high-capacity battery. Software, including a comprehensive library of chemical components, is provided with this device (98). The exchangeable measurement modules include transmission, ATR, and reflection, making it possible to analyze a range of materials (liquid, solid, or gas) (98). The research team has already acquired an ALPHA FTIR device that will be used in the experimental phase of the project in both laboratory and field conditions. A handheld Exoscan ATR FTIR spectrometer has been developed by A2 Technologies (99). It has dimensions of 17 × 12 × 23 cm and weighs about 3 kg (99). The spectral range of Exoscan (4,000 cm-1 to 650 cm-1) is narrower than that of Bruker’s ALPHA. The device operates at any positive outside temperature (0°C to 50°C) and has a lithium battery life of 3.2 h, which makes it valuable for on-site analyses of large sam- ples that cannot be brought to the laboratory (99). A personal digital assistant attached to the Exoscan device facilitates the analysis of the sample, practically, on the spot. The operational modes include ATR and reflection, which, according to the manufacturer, make Exoscan useful in a range of applications, such as evaluation of coatings, glues, and cured polymers, and capable of sampling liquids, solids, gels, and gases (99). A simi- lar device, the HazMatID Ranger, uses ATR FTIR principles of operations (100). It is able to operate at a slightly wider range of temperatures (-7°C to 50°C). The library of identifiable chemicals includes up to 32,000 materials (100). Raman Analyzers One example of briefcase-size Raman spectrometers is the Real-Time Analyzers’ RamanID (101). The analyzer measures the entire Raman spectrum in each scan, from 150 to 3,350 cm-1, with a spectral resolution selectable from 2 cm-1 to 32 cm-1. Both 785- and 1,064-nm laser excitation are offered (Class 3B lasers). Fiber optic probes and enclosed sample compartments (for added eye safety) are also avail- able. According to the manufacturer’s specifications, the device is capable of analyzing any solid or liquid in a very short (about 10 s) time and is equipped with software installed on a laptop computer (101). Additionally, it does not require any sample preparation or calibration, although sam- ple preparation is a function of the measurement objective (for example, asphalt binders may be too complex to analyze

80 without any sample preparation). The RamanID operates at 2°C to 37°C and has a 5-h rechargeable battery (101). The analyzer employs interferometry to generate the Raman spec- trum, which eliminates need for recalibrations (101). The handheld lightweight (about 2 kg) Raman detectors have been reported to be used for verifying chemical identities of pharmaceutical and other industrial materials (e.g., raw materials, oils, coating) (102, 103). The two examples of such equipment are Ahura Scientific’s TruScan (102) and DeltaNu’s Inspector Raman (103). Both devices provide a common, but truncated, spectral range (about 250 cm-1 to 2,000 cm-1) with resolution of 7 cm-1 to 10 cm-1 across the range (102, 103). X-Ray Diffraction and X-Ray Fluorescence Systems High-resolution laboratory X-ray equipment occupies a space comparable to a medium-size room. Because this proj- ect is focused on the devices suitable for field operations, only portable X-ray equipment is discussed in this section. The portable XRD/XRF Terra analyzer (104) is briefcase size (about 49 × 40 × 20 cm in dimensions and 14.5 kg by weight). This device was developed by inXitu (104) for the identification of minerals and aggregate mixtures. According to the manufac- turer, the system requires only minimum sample preparation and, within minutes, detects single minerals or simple mixtures. The Terra analyzer provides XRD resolution of 0.25° 2q over the range of 5° to 55° 2q and XRF resolution of 230 eV (at 5.9 keV) over the range of 2 to 25 keV (104). The device can operate in autonomic mode for at least 4 h and a laptop com- puter allows the user to configure the analysis, preview live data, explore archive files, and download data for pattern matching with a mineral database using commercial software (104). Handheld XRF devices are available for the analysis of metals in soils and lead in air, such as Innov-X Alpha (105), Thermo Scientific NITON XL3t, and Skyray Instruments EDX Pocket-II analyzers (106, 107). They usually contain a miniature X-ray tube and a built-in screen (106, 107). As reported by the manufacturers, the portable X-ray devices are capable of identifying a wide range of metallic elements with varied detection limits (e.g., 15 ppm for Cd; 8 ppm for Pb, Hg, and Br; and 25 ppm for Cr) (106, 107). The research team already owns an Innov-X portable XRF analyzer. Nuclear Magnetic Resonance Analyzers Since the 1980s, laboratory-based NMR systems have been used for the analysis of petroleum products, mostly by university-based researchers. For example, a Bruker AV300/1 NMR spectrometer uses time-domain analysis of solutions or frequency-domain analysis of solids in 1H, 13C, 31P, or 19F envi- ronments (108). Such equipment may weigh up to 1,500 to 2,000 lb. This time-domain or low-resolution NMR (TD- NMR) has been used for quality assurance/quality control in the petrochemical and polymer industry (93). A portable high- resolution spin track NMR spectrometer has been developed by Process NMR Associates LLC (109). A laptop computer with software containing many standard NMR relaxation routines and applications is provided with this device (109). Summary of Spectroscopic Devices Overview The spectroscopic devices discussed above were identified as potential candidates for a feasibility study for on-site finger- printing of transportation-related construction materials. Table E.2 summarizes the specifications of these devices. On Table E.2. Summary of Spectroscopic Equipment Specifications Devices Feature SEC/GPC SEC/HPLC GC FTIR Raman XRD XRF NMR Sample type Liquid Liquid Liquid Solid, liquid, gas Solid, liquid Solid Solid, gas Solid, liquid Detection range 100–250,000 Da (28) 300–100,000 Da (28) 5–50 ppb (97) 7,500–375 cm-1 (98) 4,000–600 cm-1 (99) 3,350– 150 cm-1 (101) 2,000– 250 cm-1 (103) 5°–55° 2q (104) 3–25 keV (104) 100–800 MHz (Stationary) (93,108) 5–60 MHz (Spin Track) (109) Precision 1%–3% (113) 0.40‰-1.90‰ (114) 0.20‰- 1.00‰ (116) 0.01 cm1 (98) 0.05 cm1 (101) 0.25 2q (104) 230 eV (104) ±2% (110) Accuracy ±3% (113) 3%–7% (115) -1.11 ± 3.16‰ (116) 2–0.9 cm-1 (98) 32–2 cm-1 (101) ±3% (112) 1%–20% (111) 2%–9% (110)

81 the basis of the review of the spectroscopic equipment, the following preliminary conclusions can be drawn: • The GPC, HLPC, and GC chromatographs are useful for separation and qualitative analysis before further identifi- cation by other spectroscopic methods. Portable chro- matographs are available for use in the field and mobile laboratories. • Infrared and Raman analyzers can be used for the analysis of a wide range of materials in the liquid, solid, or gas phase (mid-IR only). Several field-portable systems are available. • The handheld XRD and XRF systems are available for the identification of a wide range of metals. They can be oper- ated in outdoor conditions at temperatures above freezing. • Most of the NMR equipment for analyzing solid-state matter is laboratory-based and cannot be used in the field. However, bench-top (or semiportable) time-domain NMR analyzers are available that appear to be useful for the analy- sis of liquid-state substances, such as fractionated asphalts. Spectroscopy Standards in State Transportation Agencies in the United States This part of the literature review covers the ASTM/AASHTO standards relevant to spectroscopic testing of transportation construction materials. Additionally, it describes local speci- fications developed by the state highway agencies (SHAs) throughout the United States. ASTM/AASHTO Standards Twenty-six national standards were found to be relevant to the objective of this study (Table E.3). Three spectroscopic meth- ods appear to be the best documented: (a) FTIR, (b) XRF, and (c) chromatography (liquid, gas, and ion). Out of 26 standards summarized in Table E.3, only seven were found to be used by SHAs for testing highway construction materials, primarily portland cement and its products and paint coatings. Two procedures were developed by FHWA for asphalt-related materials. State Highway Agencies’ Spectroscopic Testing Procedures A search of all 50 SHAs’ websites revealed that nine states share their locally developed (or modified ASTM/AASHTO) standards, including Arizona, California, Kentucky, Louisiana, Maryland, Washington, and West Virginia. Table E.4 contains the spectroscopic procedures available for download from each department of transportation website. Summary of Literature Review The objective of this project is to evaluate the suitability of vari- ous spectroscopic techniques for fingerprinting transportation construction materials in the field. Task 1 of this study focused on the comprehensive search of literature available from transportation-related sources (e.g., TRIS), as well as from multi disciplinary bibliographic databases, such as the ASCE Civil Engineering Database and ScienceDirect. The literature review covered the underlying principles of the most com- monly used spectroscopic methods, as well as the current prac- tice of their application to the analysis of asphalt, portland cement, and other construction materials. Finally, the research team prepared an overview of the available equipment with emphasis on portable devices for the feasibility study. Tables E.5 and E.6 show universality rankings of methods most applicable to transportation construction materials. On the basis of the quantitative comparison, it appears that the methods that can be applied to most materials are FTIR, XRF, Raman, NMR, and SEC/HPLC. This does not exclude though other discussed methods that can be very productive in some particular applications. Table E.3. ASTM/AASHTO Spectroscopic Testing Standards Designation Title Most Relevant Standards ASTM C25-06 Standard Test Methods for Chemical Analysis of Limestone, Quicklime, and Hydrated Lime AASHTO T260 Standard Method of Test for Sampling and Testing for Chloride Ion in Concrete and Concrete Raw Materials ASTM C114-09 Standard Test Methods for Chemical Analysis of Hydraulic Cement ASTM C311-07 Standard Test Methods for Sampling and Testing Fly Ash or Natural Pozzolans for Use in Portland-Cement Concrete ASTM D2621-87 (2005) Standard Test Method for Infrared Identification of Vehicle Solids from Solvent-Reducible Paints ASTM D4326-04 Standard Test Method for Major and Minor Elements in Coal and Coke Ash by X-Ray Fluorescence ASTM D5380 93 (2009) Standard Test Method for Identification of Crystalline Pigments and Extenders in Paint by X-Ray Diffraction Analysis (continued on next page)

82 Table E.4. SHA Spectroscopic Testing Procedures State Material Spectroscopic Method Arizona Exchangeable sodium in topsoil Spectrophotometry Chloride in concrete admixtures Ion meter California Portland cement, fly ash, pozzolan XRF Chloride in soils and waters Ion chromatography Portland cement concrete admixtures FTIR Pigments and extenders in paints and coatings XRD Kentucky Hydrated lime, fly ash, portland cement XRF Louisiana Portland cement concrete admixtures FTIR Maryland Sulfur fungicide products FTIR Washington State Coatings (pigmented sealers) on concrete structures FTIR West Virginia Hydraulic cement XRF Table E.3. ASTM/AASHTO Spectroscopic Testing Standards Designation Title Other Standards ASTM E1151 93 (2006) Standard Practice for Ion Chromatography Terms and Relationships ASTM E682–92 (2006) Standard Practice for Liquid Chromatography Terms and Relationships ASTM D3016–97 (2003) Standard Practice for Use of Liquid Exclusion Chromatography Terms and Relationships ASTM E355–96 (2007) Standard Practice for Gas Chromatography Terms and Relationships ASTM E1840–6 (2007) Standard Guide for Raman Shift Standards for Spectrometer Calibration ASTM E1683–2 (2007) Standard Practice for Testing the Performance of Scanning Raman Spectrometers ASTM D2124–99 (2004) Standard Test Method for Analysis of Components in Poly(Vinyl Chloride) Compounds Using an Infrared Spectrophotometric Technique ASTM D3124–8 (2003) Standard Test Method for Vinylidene Unsaturation in Polyethylene by Infrared Spectrophotometry ASTM E275-08 Standard Practice for Describing and Measuring Performance of Ultraviolet, Visible, and Near-Infrared Spectrophotometers ASTM D5477-02 Standard Practice for Identification of Polymer Layers or Inclusions by Fourier Transform Infrared Microspectroscopy (FTIR) ASTM D5576–00 (2006) Standard Practice for Determination of Structural Features in Polyolefins and Polyolefin Copolymers by Infrared Spectrophotometry (FTIR) ASTM D5594–98 (2004) Standard Test Method for Determination of the Vinyl Acetate Content of Ethylene-Vinyl Acetate (EVA) Copolymers by Fourier Transform Infrared Spectroscopy (FTIR) ASTM D6247–8 (2004) Standard Test Method for Analysis of Elemental Content in Polyolefins by X-Ray Fluorescence Spectrometry ASTM E1621-05 Standard Guide for X-Ray Emission Spectrometric Analysis ASTM D6248–8 (2004) Standard Test Method for Vinyl and Trans Unsaturation in Polyethylene by Infrared Spectrophotometry ASTM C1118-07 Standard Guide for Selecting Components for Wavelength-Dispersive X-Ray Fluorescence (XRF) Systems ASTM D6645-01 Standard Test Method for Methyl (Comonomer) Content in Polyethylene by Infrared Spectrophotometry ASTM E1421–99 (2004) Standard Practice for Describing and Measuring Performance of Fourier Transform Mid-Infrared (FTMIR) Spectrometers: Level Zero and Level One Tests ASTM E1944–98 (2007) Standard Practice for Describing and Measuring Performance of Laboratory Fourier Transform Near-Infrared (FTNIR) Spectrometers: Level Zero and Level One Tests FHWA FHWA Susan P. Needham Test Method for Detecting the Presence of Phosphoric Acid in Asphalt FHWA Laboratory Procedure for the Determination of Lime in Hot-Mix Asphalt (continued)

83 Table E.5. Universality Rank of Spectroscopic Devices Material Category Method ObjectiveSEC/GPC SEC/HPLC GC FTIR Raman XRD XRF NMR Structural coatings (epoxy and poly- urethane based) 0 0 0 2 1 1 1 1 Formula verification Presence of solvents/diluents Pavement markings (such as epoxy markings) 0 2 1 2 2 1 0 1 Formula verification Presence of solvents/diluents Epoxies (for concrete repair) 0 2 0 2 1 0 0 1 Formula verification (proper propor- tioning for two components) Portland cement 0 0 0 2 2 2 2 1 Cement quality and type Portland cement concrete (PCC) 0 0 0 0 0 2 2 0 Cement/water/additives content in PCC Detection of prohibited chemicals/ modifiers Admixtures for PCC 1 0 0 2 2 2 0 1 Formula and type verification Curing Compounds for PCC 0 0 0 2 2 1 1 1 Formula and type verification Modified asphalt (SBS, Elvaloy, EVA, PPA) 1 2 1 2 1 1 0 1 Type and content verification Asphalt concrete 0 2 0 1 1 0 1 0 Detection of prohibited chemical/ modifiers (such as motor oil, diesel fuel) Asphalt emulsions 0 0 0 1 1 0 0 1 Type and water content Antistripping agents in asphalt concrete 0 0 0 2 1 0 0 1 Type and content verification Oxidation in asphalt concrete 2 2 0 2 1 0 0 1 Presence and amount verification Structural steel 0 0 0 0 0 0 2 1 Quality and type verification Aggregate minerals 0 0 0 1 1 1 0 0 Amount of noncarbonate aggregate Evaluation of the resistance to a hydrochloric acid solution Universality score (total sum) 4 10 2 21 16 11 9 11 Universality rank 1 2 1 4 3 2 2 2 Note: 0 = will not work. There is no evidence in the literature review; it is not possible according to trial testing and the experience of the research team. 1 = could work. There is some evidence in the literature, based on the research team’s experience. 2 = very promising. This is based on an example found in the literature.

84 Table E.6. General Ranking of Spectroscopic Devices Feature Weighting Factor SEC/GPC SEC/ HPLC GC FTIR Raman XRD XRF NMR Analysis time 0.8 8–12 h (29) 0.5–1 h (29) 0.5–1 h (29) 1 min (5 min pretest) (97) 10 s (2 min warm- up) (100) 0.5–2 h 1–5 s (105) 10 min 1 3 3 4 4 2 4 4 Portability 0.5 No Low Low Medium Medium Medium High Medium 0 1 1 2–3a 2–3a 2 3 2 Ambient temperature range 0.5 Room (29) 0°–55°C (94) 5°–40°C (96) 18°–35°C Benchtop (97) 0°–50°C- Handheld (99) 2°–37°C (100) 0°–50°C (103) -30°– 40°C (103) Room (107) 1 1 1 1 1 1 1 1 Ease of use 0.8 Medium to difficult Difficult Difficult Easy to medium Easy to medium Medium to difficult Easy Medium 1 1 1 2–3 2–3 1–2 3 1 Price 0.8 $30,000 $30,000 $27,000 <$20,000 $60,000 $65,000 $35,000– 40,000 <$16,000 2 2 2 2 1 1 2 3 On-site calibra- tion required 0.8 No No No No 1 1 1 1 1 1 1 1 Sample prepa- ration (As it is, solution, separation, enrichment, heated/cooled) 0.8 None to complex None to complex None to complex None to complex None to complex Crushing None None to complex 1–3 1–3 1–3 1–3 1–3 2 3 1–3 Universality rank (From Table E.5) 1 1 2 1 4 3 2 2 2 Final rank 7.1 10.2 9.2 14.7 12.9 9.1 14.4 12.3 Note: Analysis time: >5 h = 1, 1–5 h = 2, <1 h = 3, <10 min = 4. Portability: No = 0 (no portable device available), Low = 1 (heavy tabletop device), Medium = 2 (light tabletop device), High = 3 (handheld). Temperature range: Does not work in common field conditions = 0, works in common field conditions = 1. Ease of use: Difficult with extensive training = 1, medium with light training = 2, easy with minimal training = 3. Price: >$50,000 = 1, $30,000–50,000 = 2, $15,000–30,000 = 3, <$15,000 = 4. On-site calibration required: 0 = Yes, 1 = No. Sample preparation: Complex = 1 (enrichment, separation required), simple = 2 (crushing), none = 3. This preparation may vary by material. Universality ranking: 1–6 = 1, 7–12 = 2, 13–18 = 3, 19–23 = 4, 24–28 = 5. Weighting factors: Established based on the responses to the workshop questionnaire (see Chapter 3). a Both FTIR and Raman have handheld and light tabletop equipment available, but the latter equipment is more suitable for most materials in this project.

85 The literature search indicated that, because of the com- plexity of asphalt-related materials, chromatography is typi- cally used first to separate the components of interest, and then a spectroscopic analysis is performed to verify the iden- tity and quantity of the components. The literature also indi- cates that some techniques are more favorable for the analysis of particular materials than others are. FTIR was successfully used to determine fundamental properties of both asphaltic materials and portland cement. XRD has been traditionally used to investigate the portland cement composition rather than for the analysis of asphalt components. The suitability of Raman technology for the asphalt analysis should be eval- uated further in this study, because literature references were not adequate to establish this. In the majority of published studies, the researchers succeeded in the qualitative rather than quantitative analysis of chemical compounds. 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