Transportation Research Record: Journal of the Transportation Research Board

This paper details the development of a framework for emulsion performance grade (EPG) specifications for microsurfacings. Microsurfacings are preservation surface treatments designed to improve the condition of the pavement surface while mitigating deterioration of the overall pavement structure. Asphalt emulsions used in microsurfacings are often selected on the basis of factors that are not necessarily related to performance. Rutting and thermal cracking have been identified as the most critical microsurfacing distresses related to binder performance. In the assessment of fresh emulsion properties, storage stability and mixability have been determined to be the most critical constructability concerns. For this study, binder and mixture test methods were identified to reflect the failure mechanisms for each critical distress type. The two emulsion residue test methods that were identified to capture microsurfacing performance were (a) the multiple stress creep and recovery test for rutting and (b) the dynamic shear rheometer frequency sweep test for thermal cracking. The identified critical fresh emulsion properties that related to constructability included storage stability and viscosity measured at a low shear rate. The proposed EPG specifications for fresh emulsion properties related to constructability were developed by using statistical analysis of the binder test results. The EPG specifications for residual binder were developed by defining the temperature-independent relationships between the emulsion residue properties and the mixture performance that corresponded to each critical distress. Preliminary specification limits were then established on the basis of the values of the binder properties that corresponded to the critical mixture performance thresholds.

This study evaluated the effects of the commercial product, AR 5, and two alternative products, waste cooking oil and castor oil, in the rejuvenation process of two asphalt binders. The analyses were carried out on microscopic and macroscopic scales on virgin, aged, and rejuvenated binders. Tests in a dynamic shear rheometer were performed to characterize rheological properties of the binders. Then, an atomic force microscope was used to identify microstructural changes in the materials. A digital image analysis technique enabled the quantification of key variables such as area fraction and spatial and size distributions of the binder constituents. Finally, the following chemical tests were performed to identify changes in the chemical composition of the binders resulting from the aging and rejuvenation processes: SARA (which measures saturates, asphaltenes, resins, and aromatics), gel permeation chromatography, and nuclear magnetic resonance. The results obtained from the tests were further compared to identify correlations between the properties evaluated in different scales. The results demonstrated the efficiency and potential of the rejuvenators evaluated in this study. The results also highlighted the importance of the use of advanced techniques to characterize and understand the material aging and rejuvenation processes.

Aggregate–binder adhesion is critical to the performance of asphalt mixtures. Recently, particle probe scanning force microscopes were created to measure the adhesion between aggregate minerals and various plain and modified binders at the microscale. In this study, statistical analyses were performed to evaluate comprehensively the effects of different factors on the aggregate–binder adhesion, including aggregate constituents, binder types, and modifier types. Average unit surface energy values were used to quantify the adhesion between aggregate minerals and various binders. Results showed that the alumina–binder pairs exhibited the greatest adhesion. For the control binder, the adhesion values increased as the weight percentages of styrene–butadiene–styrene, styrene–butadiene–rubber, and polyphosphoric acid increased. Adhesion differences between aggregate minerals and various plain and modified binders were observed by using particle probe scanning force microscopes.

Over the past 15 years, a surface performance-graded (SPG) specification for chip seal binders has been developed by the Texas Department of Transportation and has been validated with laboratory measurements and the visual field performance of 120 highway sections. The SPG specification was established in an effort to extend the service life of chip seals by providing a binder grading system and an associated selection method that (a) accounted for differences in climate and (b) used existing equipment and performance-based properties that precluded bleeding and aggregate loss in the critical first year of service after construction. A multiyear implementation effort of this specification is ongoing. This paper describes the motivation and evolution of the SPG specification, including a summary of the validation effort and a round robin testing program with the Texas Department of Transportation and suppliers of chip seal binders. Binder selection guidelines that use this specification are provided.

The search for a test and parameter that can effectively describe the fatigue cracking resistance of an asphalt binder has led to many approaches. Of these, researchers have used stiffness, strength, and ductility-based criteria to screen binders on the basis of inherent resistance to cracking. In this study, poker chip testing on thin films of asphalt binder was used at intermediate temperatures to obtain both stress and ductility-based properties of eight binders. In addition, a dynamic shear rheometer frequency sweep at an intermediate temperature was conducted to obtain stiffnesses of the binders and a surrogate parameter for ductility. The results showed no relationship between strength and stiffness. In most cases, binders that were rated to have high ductility on the basis of the dynamic shear rheometer parameter also had high toughness on the basis of the poker chip test. However, some binders clearly departed from this trend, with at least one binder exhibiting both ideally desired high stiffness and toughness. Examination of failure surfaces from the poker chip test provided additional information about the mechanisms that drove failure.

Re-refined engine oil bottom (REOB) has been used to modify asphalt binders for a long time. Recent studies revealed that pavements constructed with REOB-modified binders cracked much faster than those constructed with binders containing no REOB. Therefore, some state transportation agencies enforced a complete ban while others set an upper limit for its use. It is imperative to quantify REOB content in binders. This paper presents a systematic method to address this need. Researchers prepared 414 asphalt binder–REOB blends using 16 base binders and six REOBs at five dosages, and measured intensities of 11 key elements using wavelength dispersive X-ray fluorescence (XRF) spectroscopy. XRF tests showed that element intensities depended directly on (a) binder source, (b) binder performance grade, (c) REOB source, and (d) REOB content. This method has three major steps. In the first step, potential sources and performance-grade levels of binders that were used to prepare an unknown sample on the basis of its closeness to calibration standards in an intensity plot of sulfur versus vanadium were identified. In the second step, potential sources of REOBs were determined on the basis of calibration curves of the ratio of calcium and potassium intensities. In the final step, the REOB content in an unknown sample was estimated by using the calibration curve of the zinc intensity of the selected binder source, binder performance grade, and REOB source. The reasonableness of this method was verified by preparing and testing some additional blends. These tests showed that the new method can quantify REOB content in asphalt binders with reasonable accuracy.

The use of re-refined engine oil bottoms (REOBs) in asphalt pavements is a topic of much debate because the aging susceptibility and quantity of the additive in the binder may affect field performance. In this study, four mixtures with varying REOB contents (up to 15%) were long-term oven aged by using a high-temperature, short-duration method (135°C, 24 h) and compared with existing data on mixtures aged with more conventional short-term and long-term oven-aging procedures. The recovered binders underwent Superpave^® grading, double-edge notched tension, and extended bending beam rheometer testing, and the binders were subjected to dynamic modulus, cyclic fatigue testing in the asphalt mixture performance tester, and thermal stress restrained specimen testing. Results showed a general insensitivity when short-term oven-aged (135°C, 4 h) materials were compared with long-term oven-aged (85°C, 5 days) materials, whereas the aging process at 135°C over 24 h resulted in substantial changes to the stiffness and relaxation properties across high-, intermediate-, and low-temperature modes. The difference between the stiffness-based and slope-based low-temperature performance grades appears to be an indicator for REOB modification, with the test for physical hardening of the binder being more sensitive than standard bending beam rheometer testing. The details in this paper provide information for the asphalt pavement community to consider as specifications. Standard practices are developed to address REOB modification.

A coupled diffusion–flow model by phase-field method is proposed in this paper with the goal of predicting the storage stability of polymer-modified bitumen (PMB). In this study, the incompressible Navier–Stokes equations were coupled with a previously developed phase-field model for PMB phase separation. The coupled model was implemented in a finite element software package with experimentally calibrated parameters and reported data in the literature. Effects of the parameters (bitumen density and dynamic viscosity) that affected the gravity-driven flow and phase separation in PMB were evaluated at 180°C with the simulation results. The results indicate that the coupled diffusion–flow model can predict the storage stability (and instability) of PMBs. A good correlation between the simulation results and the previously reported experimental results (storage stability tube test) was observed. The different gravity-driven phase separation behaviors of PMBs might have resulted from the different composition of the equilibrium phases in the PMBs as well as the different densities and dynamic viscosities of the individual components (polymer and bitumen). A bigger polymer–bitumen density difference, a lower bitumen dynamic viscosity, or both caused a faster flow and separation in the PMB at storage temperature. The investigated variation of bitumen dynamic viscosity had a more significant influence than the investigated variation of bitumen density in this study, but this finding might depend on the specific values of the model parameters. With this study as a foundation, further experimental and numerical studies will be conducted to increase understanding of storage-stable PMB binders and to develop a more efficient test method for determining PMB storage stability.

Current Superpave^® PG specification uses parameter |G*|sin(δ) to quantify asphalt binder fatigue resistance. The parameter’s effectiveness has been debated for a long time. AASHTO recently adopted the linear amplitude sweep test as a provisional standard, AASHTO TP 101-12. The authors evaluated the sensitivity of this standard to different aging conditions: unaged original binders, rolling thin-film oven-aged binders, and 20- to 80-h pressure aging vessel–aged binders. Test results showed, in many cases, longer predicted fatigue lives for more-aged binders. Thus this study developed a simple fatigue cracking test for asphalt binders. In this new test, the pure linear amplitude sweep (PLAS) test, peak shear strain was increased linearly from 0% to 30% over a course of 3,000 oscillatory cycles. A new fatigue parameter, the fatigue resistance energy index (FREI), was derived with fracture mechanics. The PLAS test and FREI parameter were sensitive to both binder aging conditions and rejuvenator type and dosage. Four laboratory mixtures were employed to evaluate the correlation between this new binder fatigue test and the two mixture cracking tests: the Texas overlay test and the Illinois flexibility index test. The results showed that the PLAS and FREI correlated well with the mixture cracking tests. Additionally, the proposed method was preliminarily verified with the FHWA accelerated loading facility test, and a fair relationship with the full-scale fatigue test data was observed. It is obvious that the PLAS and associated FREI need further validation through more field test sections.

Moisture damage is a prominent problem of asphalt pavements. The bond strength between asphalt and aggregates is a crucial factor that influences the capability of asphalt to resist moisture-induced damage. In this study, a binder bond strength (BBS) test was conducted to evaluate the effects of various modifiers and additives of different amounts on bond strength between asphalt and aggregates. Furthermore, the influence of styrene–butadiene–styrene (SBS) on adhesion behavior of asphalt binder was investigated through a gel permeation chromatography (GPC) test. Finally, the results of the BBS test were compared with the findings obtained from a Hamburg wheel-tracking device (HWTD) test, which reflected the moisture sensitivity of mixtures under wet conditions. Results indicated that gilsonite, high-density polyethylene, and polyphosphoric acid improved the bond strength of the base asphalt; SBS had no positive effects on asphalt adhesion properties; and SBS at a low amount reduced the bond strength. Ethylene bis-stearamide wax, crumb rubber, terminal blend (TB) rubber powder, and compound modifier TB rubber powder plus SBS decreased the bond strength. The GPC test results showed that SBS possibly did not actively contribute to the formation of bond strength between asphalt and aggregates. Test data for BBS and HWTD tests under wet conditions confirmed that there was no discernible correlation between these two tests when adhesion properties of modified asphalts were evaluated. However, the results of the BBS test were in accordance with those of the HWTD test when the adhesion of asphalt with different amounts of the same modifier and the mixture resistance to water damage were evaluated.

This paper summarizes the results of a laboratory testing program conducted to evaluate the microstructure, adhesion, and other mechanical properties of asphalt binders meeting the same performance grade (PG) but produced by using different processes and modifiers. To that end, atomic force microscope (AFM) tapping mode imaging and force spectroscopy experiments were conducted on different straight-run and modified asphalt binders meeting the same PG. In addition, bitumen bond strength and semicircular beam (SCB) tests were conducted on the different binders evaluated and on the mixes prepared by using those binders, respectively, for comparison. The AFM images indicated that the microstructure of the modified binders was different from those of the straight-run binders. The AFM force spectroscopy test results showed that binders with the same PG could have significantly different adhesion properties. The results of the SCB tests indicated that the fatigue performance was affected by the adhesion properties of the binders evaluated. The AFM bonding energy had a very good correlation with the flexibility index parameter obtained from SCB test results. The results of this paper suggest that the adhesion properties of asphalt binders should be included in their evaluation process and specifications.

This paper reports on experiments aimed at validating a model for predicting the rate of oxidation of bitumen, in asphalt mix and chip seal road surfacings, over time periods of practical significance. Experiments were conducted in which bitumen films of varying thickness were oxidized in the field for 20 years and in the laboratory for up to 5.5 years. Bitumen oxidation in the laboratory was carried out at 50°C in air at atmospheric pressure and resulted in an extent of oxidation approximately equivalent to 11.5 years in the field. Oxidation of bitumen in the films was followed for changes in carbonyl, sulfoxide, and hydroxyl group concentrations measured by infrared spectroscopy, weight change, and viscosity. The data were fitted to an earlier derived rate equation based on a model in which the overall reaction was approximated as the sum of a fast and slow reaction. The model had been found to describe oxidation product formation very well over short time frames. The current work has shown that the model also applies over time frames (extents of oxidation) relevant to actual surfacing lives. A key assumption in the model that depletion of bitumen reactants in the slow reaction phase was negligible was also supported over long time frames. Both fast and slow phases of the reaction were found to be dependent on oxygen concentration, as assumed in the model. The oxygen dependence of the rate is discussed as a two-stage reaction mechanism previously described in the literature.

Tracking of traditional tack coat materials is a common concern during hot-mix asphalt overlay construction. This problem can be avoided by using trackless tacks, which are recently developed tack products that resist sticking to tires. Thus loss of tack materials from the paving surface is preventable. Various trackless tack products have been introduced to the market; however, there is still a lack of evaluation on their tracking resistance. The objective of this study was to measure the rheological and tack properties of trackless tack materials through the dynamic shear rheometer. Six trackless tacks and a traditional tack were evaluated. To identify rheological characteristics, the dynamic shear rheometer frequency sweep test was performed on the tack residues. Also, the modified dynamic shear rheometer tackiness test was conducted on both tack emulsions and residue at different temperatures. The emulsion samples were tested throughout the curing period. The tack samples were categorized into soft and stiff binder groups with respect to complex shear modulus obtained from the frequency sweep test. The tack energy was estimated to quantify the stickiness in the tackiness test. A significant difference in tack energy was observed between soft and stiff group binders. The stiffness of the investigated tack materials with respect to the complex shear modulus is well correlated with the tack material properties.

The main challenge of asphalt modified with styrene–butadiene–styrene (SBS) during storage and application is the phase separation of SBS-rich phases and asphaltene-rich phases. These phases have a close relationship with the morphology of SBS-modified asphalt [SBS–polymer-modified binders (PMBs)]. Carbon nanotubes (CNTs) were selected in this paper to provide different morphology of SBS-PMBs with CNTs. The morphology was composed of SBS-rich phases, asphaltene-rich phases, and their interphase, captured from fluorescence microscopy (FM), atomic force microscopy (AFM), and scanning electron microscopy (SEM). The components and morphologies of the interface on a molecular scale were obtained by molecular dynamic simulation. The results from FM images showed an increase in the relative content of SBS-rich phases as the amount of CNTs increased, highlighting CNTs’ positive role in the compatibility of SBS and asphalt. Mixing CNTs with a mechanical stirrer provided a larger size of SBS-rich phases than that of SBS-PMBs in AFM images using an ultrasound performed on an interpenetration network of AFM and FM images, whereas mixing with an extruding machine resulted in an orange-skin morphology in FM images. The results from SEM demonstrated that CNTs changed the interface of SBS-PMBs by pullout behaviors. The results from the molecular dynamic simulation showed that SBS intertwined or surrounded CNTs to improve their interaction. Saturate, aromatic, and resin were absorbed into the tube of CNTs. Therefore, CNTs had a positive action on the interaction between SBS and asphalt because they acted as a bridge to link the different phases, finally reinforcing the interface of SBS-PMBs.

Limited field studies have considered the aging of warm-mix asphalt (WMA) binders, especially from the perspective of long-term aging. This paper quantifies the long-term (10 to 82 months) field aging properties of WMA binders and the control hot-mix asphalt (HMA) binders and identifies the key factors associated with long-term field aging of asphalt. Asphalt binders from 23 field projects consisting of 65 HMA and WMA pavements were recovered, and high-temperature performance grade (PG) was tested. The effects of climate, month of aging, WMA technologies, and original binder high-temperature PG on field asphalt aging were analyzed. A prediction model that included multiple parameters was developed and validated. Results indicated that Evotherm WMA binder had a lower high-temperature PG than HMA binder shortly after construction, but this difference reduced with time. No statistical difference of field aging between HMA and WMA binders was observed. The climate effect on asphalt field aging was apparent within dry areas or freeze areas, whereas the aging difference between dry and freeze areas was inconclusive and requires further research. Other conclusions were that (a) the field asphalt aging affects more of top-down longitudinal wheelpath crack than transverse crack, (b) foaming WMA binder ages slowest of all binders considered, (c) PG 64-XX and PG 70-XX binders aged more than PG 58-XX and PG 76-XX binders, and (d) the prediction model had good agreement with test results and was well validated. The identified factors that affected field asphalt aging were overlay thickness, in-place air voids, effective binder content, complex shear modulus, and solar radiation.