Evaluation of Indirect Tensile Strength as Design Criteria for Superpave Mixtures

Abstract

Many factors contribute to the degradation of asphalt pavements. When high quality materials are used, distresses are typically due to traffic loading, resulting in rutting or fatigue cracking. The presence of water (or moisture) often results in premature failure of asphalt pavements in the form of isolated distress caused by debonding of the asphalt film from the aggregate surface or early rutting/fatigue cracking due to reduced mix strength. Moisture sensitivity has long been recognized as an important mix design consideration. The tensile strength is primarily a function of the binder properties. The amount of asphalt binder in a mixture and its stiffness influence the tensile strength. Tensile strength also depends on the absorption capacity of the aggregates used. At a given asphalt content, the film thickness of asphalt on the surface of aggregates and particle-to-particle contact influences the adhesion or tensile strength of a mixture. Various studies have repeatedly proved that the tensile strength increases with decreasing air voids. The tensile strength of a mixture is also strongly influenced by the consistency of the asphalt cement, which can influence rutting. Thus, tensile strength plays an important role as a design and evaluation tool for Superpave mixtures

Moisture damage of asphalt pavements is a serious problem. The presence of moisture tends to reduce the stiffness of the asphalt mix as well as create the opportunity for stripping of the asphalt from the aggregate. This, in combination with repeated wheel loadings, can accelerate pavement deterioration. Strength loss is now evaluated by comparing indirect tensile strengths of an unconditioned control group to those of the conditioned samples. If the average retained strength of the conditioned group is less than eighty-five percent of the control group strength, the mix is determined to be moisture susceptible. This research study shows that reliance on the Tensile Strength Ratio (TSR) values only may be misleading in many cases. The individual values of tensile strength of conditioned and unconditioned specimens in conjunction with TSR values should be employed in assessing the effect of water damage on the performance of pavements. This study found that a minimum tensile strength should be established for a given ESAL range. The fatigue life of the mixtures decreases exponentially with decreasing tensile strength. This trend is explained by the loss in stiffness, the initiation of cracks, and stripping. There exists a minimum tensile strength for a given ESAL level that can be used as a surrogate criterion for fatigue life estimation. This research study also shows that the mixtures with lower tensile strength have higher rut depths. Rut depths of mixtures were shown to increase with decreasing tensile strength, which can be attributed to the fact that the aggregate structure is affected due to moisture damage and subsequent loss in tensile strengths of the mixtures. This study suggests that tensile strength can be used as a design parameter in the Superpave mix design stage and a modified mix design procedure is proposed based on individual tensile strength.