Hassan A. Tabatabaee

Multiple Stress Creep and Recovery and Time Sweep Fatigue Tests

Crumb rubber modification (CRM) is an effective means of improving neat asphalt performance. Numerous studies have investigated the effects of CRM using the current Superpave® laboratory tests. Recent studies have shown that the assumption of linear viscoelastic behavior or employing insufficient stress or strain levels makes current tests inadequate for properly assessing the performance of binders, especially when modified. This problem has resulted in the development of improved test methods such as the time sweep binder fatigue test and the multiple stress creep and recovery (MSCR) test for permanent deformation. The present study assesses the performance of binders modified with a wide range of crumb rubber content using these newly developed test methods. Additional binder and mixture performance tests were used to compare and validate the results. The results showed that the new tests performed well in predicting performance. Current Superpave criteria also showed good conformance with mixture test results for CRM binders. Tests results suggest that the MSCR can complement current rutting prediction testing. Also, the time sweep test produces results similar in accuracy to current Superpave tests, but it requires longer testing time and is thus not recommended for performance grading.

Modeling Thermal Stress in Asphalt Mixtures Undergoing Glass Transition and Physical Hardening

Asphalt binders have been shown to undergo significant time-dependent stiffening when stored at low temperatures. This physical hardening has a significant effect on the laboratory performance of asphalt binders. However, the importance of isothermal conditioning for asphalt mixtures and its effect on thermal cracking performance have been a subject of significant debate. A theoretical approach accounting for the glass transition and physical hardening in the thermal stress buildup in mixtures was derived from relaxation modulus master curves, the William–Landel–Ferry equation, Boltzmanns superposition principle, and a model describing the isothermal contraction of asphalt as a continuous function of conditioning time and temperature. With the model predictions, it is shown that thermal stress relaxation and stress buildup induced by physical hardening can continuously affect thermal stress throughout the cooling process. The cooling rate also affected the amount of delayed stress buildup that occurred after the temperature had stabilized at isothermal conditions as a result of physical hardening. A relatively simple device was developed and used for verification and support of the thermal stress model. Mixture measurements performed at different cooling rates and isothermal conditions supported the theoretical predictions. The findings clearly show that the effect of physical hardening on stress buildup in mixtures is measurable and important. Therefore, the glass transition of asphalts and their behavior under isothermal conditioning needs to be measured to better predict thermal cracking.

Asphalt thermal cracking analyser (ATCA)

The Asphalt Thermal Cracking Analyser (ATCA) is a device that can simultaneously test two asphalt mixture beams while undergoing selected thermal history. The first beam is unrestrained and thus the change of its length with temperature can be used to obtain glass transition temperature (Tg) and coefficients of thermal expansion or contraction. The second beam is restrained at the ends and can be used to measure the thermal stress build-up as a function of time and temperature. The measures of length change and stress in beams can be used to get a comprehensive evaluation of the low temperature performance including change in strain, stress as a function of time and temperature.

Critical Problems with Using the Asphalt Ductility Test as a Performance Index for Modified Binders

Despite the adaptation of advanced binder rheology characterization methods by many agencies, the asphalt ductility test is still being used in some specifications in the United States and a few other countries as a performance indicator for asphalt modification. In this study, binder properties known to reflect rutting and fatigue resistance were examined in a set of binders modified with two types of commonly used elastomeric polymer modifiers. The results showed no correlations with binder ductility. Additionally, a test procedure was developed by using a dynamic shear rheometer (DSR) as a surrogate to the conventional ductility test, results of which showed that both elastomeric modified binders are much more ductile than conventional binders, even when conventional ductility showed a loss of ductility. Finite element modeling was used to show the significant effects of decreasing true strain rates with elongation on the samples stress and strain state attributable to the constant crosshead speed in the conventional ductility test. Because of the well-known dependency of failure stress and strain of viscoelastic material on strain rate and temperature, comparing binders with varying ductility values measured in the conventional test is essentially flawed because it is equivalent to comparing them at different temperatures and is thus fundamentally unreliable as an indicator of the asphalts performance in the pavement. It is therefore strongly recommended that the practice of using low-temperature conventional ductility be removed from modified binder specifications, or the DSR procedure proposed in this paper be used to evaluate modified asphalts as a preferable test method.

Establishing use of asphalt binder cracking tests for prevention of pavement cracking

Premature loss of serviceability due to cracking remains one of the most prominent distress modes afflicting asphalt pavements today. Although there has been significant progress in developing binder fracture tests, concerns exist about a number of the assumptions used in current analysis methods. In particular are three related main issues: (1) sorting the total failure energy into time-dependent flow and fracture energy; (2) defining the role of binder fracture in pavement cracking; and (3) selecting relevant loading rates such as to correspond to binder thermal strain during pavement cooling. This study aims to investigate and address all three of the aforementioned issues to establish the use of binder crack resistance tests as an effective tool for preventing pavement cracking. Pseudo-strain analysis showed that separation of viscous dissipation and time-independent fracture energy is unnecessary to relate binder failure energy to mixture fracture. Finite element analysis showed that even in the presence of aggregates, crack initiation in fine-aggregate mixtures is directly related to the experimentally derived binder fracture properties. Finally, multi-scale analysis was used to derive the binder strain rates in the pavement during cooling, and successfully used to develop a practical method for selection of binder fracture test loading rates that are comparable to field cooling rates and conditions.

Analytical investigation of the impact of a novel bio-based recycling agent on the colloidal stability of aged bitumen

While many additives have been used to modify the properties of aged bituminous materials to ensure long-term durability in highly recycled pavements, consensus on the definition of “rejuvenation” and the associated mechanisms does not exist. Thus, further clarification and assessment of impacts on ageing and of various approaches to “rejuvenation” are imperative. The present study utilises chemical fractionation, differential scanning calorimetry, and atomic force microscopy to evaluate the impact of a chemically modified vegetable oil-based recycling agent in terms of bitumen colloidal stability and phase compatibility. The Colloidal Instability Index (CII) is shown to be a useful parameter for monitoring the effect of ageing on rejuvenated bitumen. The rejuvenator tested in this study showed good ageing stability as indicated by the relatively limited increase in CII due to ageing. Differential scanning calorimeter thermal analysis of the bitumen showed an improvement in the glass transition of aged bitumen through the use of rejuvenators. Extended Bending Beam Rheometer testing at isothermal conditions around the glass transition temperature showed that the rejuvenator effect is not affected by physical hardening. The results are used to support a proposed categorisation of the impact mechanisms of recycling agents.

Critical Considerations toward Better Implementation of the Multiple Stress Creep and Recovery Test

AbstractIn recent years, many studies have dealt with the inadequacy of the current Superpave performance grading (PG) specification for accurately capturing the performance properties of modified binders. An alternative binder testing mode that has gained much attention in recent years is the repeated creep and recovery (RCR) test, and more specifically, the multiple stress creep and recovery (MSCR) test. As with any new test procedure, a number of concerns and questions remain with regards to interpretation of results in terms of performance and sufficiency of the number of cycles and stress levels used, as well as representative temperature and stress levels to be used. This paper investigates the aforementioned issues using a set of modified and unmodified binders. The results show that the standard 10 cycles per stress level are not sufficient to reach a stable steady-state creep behavior for modified binders, thus the addition of 20 conditioning cycles are recommended, for a total of 30 cycles for e...

Field Validation of a Thermal Cracking Resistance Specification Framework for Modified Asphalt

Current Superpave® performance grade (PG) binder specifications, such as those used in Wisconsin and many other states, were developed primarily for unmodified binders and are valid only within the linear viscoelastic range of properties. Further, performance benefits from modification are not limited to continuum rheological properties because modified binders often demonstrate superior damage resistance not captured in the current PG system. This paper discusses the results of a recently completed research project sponsored by the Wisconsin Department of Transportation in Madison; the objective was to identify promising procedures through a comparison of test results with field performance and the development of appropriate modified binder specification criteria for thermal cracking resistance. A number of procedures have been introduced in recent years to address the need for damage characterization test procedures for proper assessment and selection of modified binders. In this study, the single-edge notched bending (SENB) procedure was found to correlate well with the observed field thermal cracking when tests were conducted at the project location low-temperature PG specification temperature. The device used was a modified bending beam rheometer (BBR), which could perform displacement-controlled loading with higher load cell capacity, and accommodated notched BBR beams. On the basis of the results of this study, a procedure was introduced for thermal cracking resistance potential of modified binders with the BBR-SENB test at an average annual minimum pavement temperature of +10°C. A preliminary failure limit and acceptance criterion were defined to qualify binder results in terms of the failure properties measured.