Aroon Shenoy

Zero shear viscosity of asphalt binders

Recently there has been considerable interest, especially in Europe, in the use of zero shear viscosity (ZSV) as a specification criterion for asphalt binders. This interest is precipitated by the apparent inability of the current Superpave® criterion, G*/sin(δ), to capture the contribution to rutting resistance afforded by polymer modification. ZSV can be determined directly from long-term creep tests, but such tests are timeconsuming and are often very difficult to perform. Several alternative methods for determining the ZSV have been proposed in the literature, including extrapolating the dynamic viscosity to zero frequency; applying the Cross model to dynamic data; and superimposing multiple short-term, non-steady-state creep tests. A number of methods for determining the ZSV from both creep and dynamic data were evaluated. Laboratory test data for 10 unmodified and modified binders were obtained through a series of creep and dynamic experiments. ZSV values obtained from two of the more promising methods were compared, along with a comparison of the ZSV ranking with the Superpave grading temperature. Two of the methods provided very similar values for the ZSV when applied over a considerable range in test temperature, and the results from the two methods could be used interchangeably for the materials that were tested. The binders ranked quite differently when ranked according to their Superpave grading temperature or their ZSV.

Standardized Procedure for Analysis of Dynamic Modulus |E*| Data to Predict Asphalt Pavement Distresses

A standardized procedure is presented by which various asphalt concrete mixtures can be compared and their expected performance can be assessed in a uniform manner with the simple performance test suggested by NCHRP 9-19: Superpave Support and Performance Models Management. The frequency sweep data generated from the test are available in terms of dynamic modulus |E*| versus frequency at the measurement temperature under different levels of confining stress [0, 20, 30 psi (0, 0.14, 0.21 MPa)]. The moduli versus frequency data at different temperatures are unified to form a single curve for each mixture through a normalizing parameter. The temperature at which the normalizing parameter becomes equal to 1 is designated the specification parameter T s (°C) for assessing mixture performance. Each unified curve is fitted with a constitutive equation from which model parameters are evaluated. Slope B1 in the low-frequency region of the unified curve, when normalized with the term (T/T s ), results in a parameter that is related to asphalt pavement distress at high temperature T. It is shown that B1/T s is related to rut depths measured at different WesTrack (a full-scale test track) sections and the correlation improves with increasing confining stress. There is a good possibility that slope B2 in the high-frequency region of the unified curve may relate to distresses in the intermediate temperature range, such as fatigue cracking. A preliminary check shows this may be true, but data are too limited to draw firm conclusions.

A Simple Quantitative Method for Identification of Failure due to Fatigue Damage

A simple quantitative method is presented which is applicable to any type of fatigue testing that uses sinusoidal strain/stress input and which will work for experimentally identifying points of failure due to fatigue damage of any kind of material being tested. The present work utilizes strain-controlled bending beam fatigue test on asphalt mixtures to demonstrate the efficacy of this method. Distortions in the hysteresis loop or waveform are tracked to pinpoint the appearance of initial microcracks and final point of complete failure due to fatigue damage. Relationship between output signals for consecutive cycles with reference to initial stable cycle is used for computing ‘R 2’. The ‘R2’ drops sharply from initial stable value of 1 to less than 0.5 and eventually to almost 0 with increasing loading cycles. The number of cycles determined from the fitted equation at ‘R2’ = 1 marks the point of first fatigue failure N fff and ‘R2’ = 0 marks the point of complete fatigue failure Ncff.

Do Asphalt Mixtures Correlate Better with Mastics or Binders in Evaluating Permanent Deformation

Researchers have often looked for relationships between mechanical properties of asphalt mixtures and rheological properties of binders when assessing the resistance of mixtures and binders to permanent deformation. When mixtures are subjected to deformation on application of a stress, aggregates act as load-bearing entities, and binders deform in response to applied stress. Intuitively, a correlation must exist between the properties of mixtures and binders. However, a good correlation usually is not observed. There could be a number of reasons for the observed poor correlations, including (a) variability in the data or a possible change in the microstructure of two-phase, polymer-modified asphalt in the presence of aggregates, or (b) strong interactions between the aggregate and the binder that, of course, are not reflected in the binder properties. In such cases, there are reasons to believe asphalt mastics might provide a better correlation because they would account for at least the physicochemical aspects of the aggregate–binder interaction. The present work compared asphalt mixture data initially with mastic data and then separately with binder data. The mastic and binder rheological data were generated with the same equipment under identical conditions of measurement to identify which one correlates better with the mixture data. A good correlation was obtained in only one case when Superpave® shear tester data for mixtures were compared with the dynamic shear rheometer data for binders. In all cases analyzed in this work, no correlation was found between the permanent deformation for mixtures and the rheological properties of the mastics.