Browsing by Subject "Creep compliance"

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An evaluation of the tensile strength of asphalt binder and mortar with recycled asphalt additives at low temperatures
(2017-08) Rodriguez, Rogelio Gabriel; Bhasin, Amit
A standard grading system is used to evaluate the properties and expected performance of an asphalt binder. The most common grading system used in the US today is the performance grading (PG) system. This system prescribes an operational temperature range within which the binder is expected to perform satisfactorily without resulting in rutting, fatigue cracking or thermal cracking as a distress. On the low-temperature and thermal cracking end, this system uses both a metric related to stiffness and rate of relaxation (m-value) to establish a low temperature grade for a given asphalt binder. Although these parameters provide some information on the rate of relaxation and overall rigidity of the asphalt binder they don’t include an evaluation of the tensile strength of the asphalt binder. The tensile strength of the binder can furnish critical information about its resistance to cracking including thermal cracking. The asphalt industry has become particularly interested in the impact of recycled asphalt pavement (RAP) and additives such as rejuvenators on the behavior and performance of the material. The goal of this study was to evaluate the tensile strength characteristics of asphalt binders and composites in addition to their stiffness characteristics. The study was conducted in three parts: (1) the evaluation of tensile strength of asphalt binders with similar performance grades, (2) the evaluation of tensile strength of asphalt binders as a function of the amount of recycled asphalt binder added to the virgin binder, and (3) the influence of recycled asphalt and rejuvenators on the stiffness and tensile strength of asphalt mortars. In addition to these three objectives, this study also developed simplified methods to prepare and evaluate the tensile strength of asphalt binders and mortars as a material characterization tool to screen or evaluate the efficacy of RAP, rejuvenators, and other additives. Results show that similarly graded binders have a significant variability both in terms of their stiffness and strength and that rejuvenators can be used with RAP to achieve a balance of stiffness and strength.
Response of asphalt matrix under multi-axial stress state
(2014-05) Sakib, Nazmus; Bhasin, Amit
The pavement system is subjected to complex stress states under vehicular loading. A combination of axial and shear stress has been identified as a potential cause of top down cracking (or more precisely near surface cracking) in asphalt surface. Therefore, in terms of modeling the material response a pertinent question is whether the typical one-dimensional viscoelastic properties of the material are affected by a multi-axial stress state. Such changes are referred to as interaction non-linearity. The objective of this study was to evaluate whether or not asphalt composites are susceptible to such interaction effects. The study was conducted using fine aggregate matrix (FAM), which comprises graded sand and asphalt binder. To provide multi-modal loading, the rectangular prismatic FAM specimens were used with the Arcan apparatus. This apparatus ensures low bending stress and offers adjustments in the setup to provide different proportions of axial and shear stress. Finite element modeling was done to evaluate the stress state for different orientations of the sample in the Arcan apparatus. For measurement of strain, the study used digital image correlation (DIC), which is an optical, non-contact measurement technology. The strain thus measured was used to compute shear compliance. Fitting parameters of the shear compliances were estimated for power-law and Prony series for different loading orientations. When compared, the measured shear compliances do not show perceivable variation with respect to different proportion of axial stress applied in conjunction. However, further testing with different temperatures and other magnitudes of shear stress is necessary. This study is the first step to allow modeling of stress and crack propagation behavior near the pavement surface where complex stress state is present.