Establishing the Location and Magnitude of Critical Tensile Strains and Octahedral Shear Stresses for Design of Asphalt Concrete Pavement Systems
Touma, Bassam Elias
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https://hdl.handle.net/2142/72190
Description
Title
Establishing the Location and Magnitude of Critical Tensile Strains and Octahedral Shear Stresses for Design of Asphalt Concrete Pavement Systems
Author(s)
Touma, Bassam Elias
Issue Date
1992
Doctoral Committee Chair(s)
Carpenter, Samuel H.
Department of Study
Civil Engineering
Discipline
Civil Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Civil
Abstract
In recent years, our pavements experienced premature surface cracking and rutting. Traditional mechanistic design procedures account for two modes of failure in asphalt pavements: Fatigue and Rutting.
Fatigue in the surface layer is accounted for by limiting the tensile strain at the bottom of that layer, the value required by the fatigue equations used to predict the service life of pavements. Only the bottom of the layer is investigated with the assumption that the maximum tensile strains always occur at that location. Theoretical and field studies suggest that the tensile strain is not always maximum at the bottom of the layer, and that increasing the thickness of the surface layer will not necessarily reduce the maximum tensile strain in the layer rather, the maximum will shift towards the surface. Under such conditions, the stress state at the that location is different than "flexure" and our current procedures are likely to be error.
Rutting in the surface layer is usually taken care of by utilizing good quality material and mix design, while rutting in the subgrade is usually accounted for by limiting the vertical strain on top of that layer. Due to the premature rutting in the surface layer experienced today, it is necessary to have a means to address this issue from the point of view of the existing stress state at the time of loading. The Octahedral Shear Stress is a parameter that describes the complete stress state in the material and accurately relates to the onset of volumetric deformation.
Procedures were developed to investigate the magnitude and location of the maximum tensile strain and maximum octahedral shear stress in the asphalt layer for two and three layer systems. The computer program ELSYM5 was utilized to generate the data which involved over 200,000 computer runs. Both, single and dual wheel loading were considered along with a wide spectrum of pavement cross sections. The results indicated that the location and magnitude of the maximum stresses and strains varied with the number of layers, thicknesses, modular ratios and, loading, and that they occurred near the surface frequently, which might explain the premature failures experienced by our roads today.
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