Jean-Pascal Planche

Evolution of the Crossover Modulus with Oxidative Aging: Method to Estimate Change in Viscoelastic Properties of Asphalt Binder with Time and Depth on the Road

This study is based on recovered binders from a hot-mix asphalt comparative test site constructed in Arizona in 2001 with mix collected during construction and cores collected in 2005 and 2010. The intent of the study is to evaluate the potential of using the evolution of the binder crossover modulus with time and depth in the pavement to determine the change in linear viscoelastic properties of the binder during the life of the pavement. The crossover modulus is one of three parameters in the Christensen–Anderson (CA) complex shear modulus model. The CA model is used in this study to develop complex modulus and phase angle master curves of the binder as a function of field time, depth, and temperature. The master curves generated demonstrate the change in SHRP specification parameters, such as m-value and creep stiffness, as well as in the mix dynamic modulus with time and depth on the road. In addition, although asphalt dependent, a linear relationship between oxygen uptake and the log of the crossover modulus is reported.

Low-Temperature Oxidation Kinetics of Asphalt Binders

A simple dual-mechanism model successfully fits the oxidation of 12 unmodified asphalt binders originating from a wide variety of sources. The kinetic formulation includes fast and slow reaction paths in parallel with free radical interactions between the two reaction pathways. The same Arrhenius parameters are used for all 12 binders studied. The differences in asphalt binder oxidation rates can be explained with the use of only one adjustable parameter, the amount of reactive material available for the fast reaction. This result suggests that unmodified asphalt binders oxidize with essentially the same chemical mechanisms. Because the Arrhenius parameters apply universally, a simple test may be performed to characterize the oxidation kinetics for unmodified binders without expensive, long-term oxidation experiments at multiple temperatures. A rheological study of the materials generated in the aging of the 12 binders using dynamic shear rheometry was also performed to investigate the relationship of rheological changes with chemical changes as binders oxidize. The rheometry consisted of the generation of a series of isothermal frequency sweeps, followed by fitting the resulting master curve with the Christensen–Anderson model. Simple shifting cannot account for the master curve changes, but changes in the model parameters follow a log-linear relationship for oxidation chemical changes. These fits appear to be source dependent, suggesting that a method with two aging time conditions would be required to characterize the rheological property changes in an unmodified asphalt binder as it ages. Such a method would produce a complete master curve–shift function set at any extent of aging, suitable for input into rational performance models.

Recovery and Laboratory Testing of Asphalt Emulsion Residue: Application of Simple Aging Test and 4-mm Dynamic Shear Rheometer Test

This study involves application of two new test methods to the recovery of asphalt emulsion residue, long-term oxidative aging of the residue, and mechanical testing of the unaged and aged residue to determine low- and intermediate-temperature rheological properties. The new methods are the simple aging test (SAT), which is a thin-film (300 μm) oxidative aging test, and a dynamic shear rheometry technique (commonly referred to as 4-mm DSR), which allows testing to −40°C and requires only 25 mg of material. Three polymer-modified emulsions are recovered by two methods: AASHTO PP 72-11 Method B and use of an SAT plate. Both methods use the same evaporative technique to recover the residue (6 h at 60°C in a forced draft oven). The rheological properties (m-value, G* sin d, and ductility) of the recovered emulsion residues by both methods are reported as well as the rheological properties of the aged emulsion using the SAT. The rheological properties of the unaged emulsion residues recovered are roughly equivalent. The proposed SAT recovery method and application of 4-mm DSR offer significant improvements over Method B and current DSR methodology. The recovery process is simpler with the SAT plate than with Method B, and the SAT plate has been designed so that it can be placed directly in a standard pressure aging vessel (PAV) for long-term aging, and the time required for standard PAV aging is reduced from 20 to 8 h. There is no extrapolation of intermediate-temperature DSR data to low temperature. The error from instrument compliance at low temperature is corrected in the 4-mm DSR procedure.

Quantification of Water in Asphalt by Karl Fischer Titration and Its Application to Emulsion Recovery

In this study, the Karl Fischer (KF) titration method for quantification of water was successfully applied to asphalt binder. Previously, observed side reactions between the titration solvent and certain functional groups present in asphalt prevented accurate KF-type titration. However, recent advances in KF solvent formulation have allowed for rapid titration of water in asphalt with minimal interference reactions. Repeatable results were obtained with moisture-sensitive techniques such as drying glassware, using only anhydrous solvents, and performing all sample preparation in a dry atmosphere. The developed KF titration method was implemented in the evaluation of three competing laboratory asphalt emulsion residue recovery procedures. All three methods showed sufficient removal of water for rheologic testing of the eight recovered residues. Measuring the water content in recovered residues by the mass lost at 135°C was not suitable for some emulsion residues because of the evaporation of nonwater solvent or distillate at the elevated temperature.

Chemo-mechanical Characterization of Bitumen Binders with the Same Continuous PG–Grade

Chemo-mechanical analysis tools were used to provide plausible reasons behind different binder properties that are not well captured by the conventional Superpave PG-grading system. In this study, six binders were divided in two groups based on their continuous PG-grades. The binder matrix includes: two SBS modified binders, one air blown bitumen, one bitumen with high wax content and two other unique binder blends. Although the binders in each group have the same continuous PG-grades based on AASHTO M320, they exhibit very different low-temperature performance based on the binder relaxation ΔTc index, and they have different upper PG performance according to MSCR testing and AASHTO MP19, which takes into account both traffic load and climate conditions. Based on the results, high apparent molecular weight waxes appear to lead to poor low-temperature cracking and lower molecular weight waxes lead to poor rutting performance. Meanwhile incompatible polymer modification seems to lead to poor rutting and cracking performance relative to unmodified binders or even air blown binder.

New bitumen performance indicators - A feasibility study

Since the 1999 Eurobitume Workshop, the search and validation of performance related bituminous binder properties continues to be a key issue for the paving industry in Europe, as well as in the rest of the world (especially for binders with complex rheological behavior). Important progress has been made and concepts such as the complex modulus measured with Dynamic Shear Rheometers (DSR) and low temperature stiffness and relaxation behavior measured with the Bending Beam Rheometer (BBR) have become familiar, along the lines of the Superpave system implementation in the US. Measurements in the linear domain of viscoelasticity have however also shown their limits and a new generation of binder tests, which try to directly address failure behavior, is now developing. Along with “conventional” rheology, three such test methods have been contemplated in this paper. In the low temperature domain, the ABCD (Asphalt Binder Cracking Device) test mimics the TSRST (Tensile Strength Restrained Specimen Test) performed on bituminous mixes. The LAS (Linear Amplitude Sweep) test is expected to be related to fatigue whereas the MSCRT (Multiple Stress Creep & Recovery Test) addresses high temperature performance. To evaluate the ability of each test method to differentiate between binders, the investigations have been based on three bitumen of the same penetration grade (70/100) but very different in chemical structure, varying from a “gel” to a “sol” structure (colloidal index of 0.24 to 0.09). Full rheology (DSR, BBR), MSCRT, ABCD and LAS tests have been conducted on all three bitumen, as well as on the corresponding RTFOT hardened and RTFOT+PAV aged binders. Further developments on the LAS test and its relation to fatigue performance are discussed in a separate paper by WRI.

Effect of polyphosphoric acid on bituminous binder oxidation

Polyphosphoric acid (PPA) has been successfully used for years in the US and other parts of the world to enhance high service temperature stiffness of paving grade bitumen. Several researches show evidences of PPA interaction with asphaltenes and resins. These interactions typically lead to an increase of the asphaltene content concurrently with a decrease of the resins content. The mechanism describes a dispersion of the asphaltene-resins phase leading to the increase in viscosity of the bituminous binder. Whereas the impact of PPA onto the physical properties of the asphalt at high temperature is reasonably well described, the favorable effect reported by some authors, on bitumen aging remains unclear. In order to study the effect of PPA on bitumen oxidation, two bituminous binders from two different sources were treated with PPA and oxidized as thin films at 70 °C for up to 8 weeks along with the untreated control materials. Oxygen uptake was determined by transmission Fouriertransform infrared spectroscopy for each material, and the rates of reaction of the treated and untreated bitumens were compared. Oxygen diffusivity differences were investigated under the same conditions with thicker films (1000 micron). Molecular weight change differences were measured with size exclusion chromatography. The changes in rheological properties were examined using dynamic shear rheometry. The overall results indicate that PPA alters the oxidation reaction mechanism compared to reference bitumen by slightly reducing the oxygen uptake.