Evaluation of a mixture's moisture sensitivity is currently the final step in the Superpave volumetric mix design process. 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. The damage due to moisture is controlled by the specific limits of the tensile strength ratios (TSR) or the percent loss in tensile strength of the mix. Loss in the pavement strength due to moisture damage indicates that the individual tensile strength of the mixtures after conditioning will govern the rutting and fatigue life of the mixtures. A total dependency and reliance on the TSR values only may be misleading in many cases. There has been no concerted effort at national or state level towards establishing the quantitative causal effects of failing to meet the minimum prescribed value of TSR or loss in tensile strength. Tensile strength plays an important role in the performance of a mixture under fatigue, rutting and moisture susceptibility. A recent pilot research study conducted at NCSU has shown that there exists a relationship between the indirect tensile strength of a mixture and its estimated fatigue and rutting life. A minimum tensile strength value can be calculated at a given traffic level for fatigue life and an allowable rut depth for rutting and can be used as an additional criterion for evaluating the moisture susceptibility of a mix. Thus, the individual values of tensile strength of conditioned and unconditioned specimens along with TSR values should be employed in assessing the effect of water damage on the performance of pavements.
The primary objectives of this research study will be to:Evaluate the tensile strengths of conditioned and unconditioned specimens and their tensile strength ratios (TSRs) for mixtures with different aggregates and gradations.Conduct a comparative study on the effects of hydrated lime and a liquid anti-stripping agent on tensile strength and TSR values of the mixtures.Develop the relationship between the tensile strength for mixtures with different aggregates and gradations and their fatigue performance as estimated using the Frequency Sweep Test at Constant Height, Diametral Tensile Fatigue Test and Indirect Tensile Test.Conduct a detailed study to investigate the rutting performance of mixtures with different aggregates and gradations using the Repeated Shear Test and Constant Height and develop its relationship with the tensile strengths of the mixtures.Develop a minimum tensile strength criterion along with TSRs for mixtures with different aggregates and gradations.
The proposed study will provide a simple, reasonable and dependable method for mix design and performance evaluation of Superpave mixtures. The minimum tensile strength criteria developed from different correlations could be used along with the TSR values as a part of the Superpave mix design criteria."
