Resilient modulus is a property for bound and unbound pavement materials characterizing the elastic behavior of materials under dynamic repeated loading. Resilient modulus is an important design parameter for pavement structures because it represents the structural strength of pavement layers through which the thickness design is based on. In Turkey, the layer thickness design is performed using resilient modulus determined empirically from various published sources. Determining a layer modulus using empirical methods causes inaccurate design solutions, which directly affects the structural performance and the overall cost of pavement construction. In this study, the resilient moduli of bituminous mixtures are measured in the laboratory by the indirect tensile test procedure for eight asphalt concrete samples according to NCHRP and ASTM procedures. The measured moduli of samples based on the two procedures are compared with the predicted values calculated from various empirical methods using aggregate and binder properties. An evaluation of each estimation method is presented on the basis of its accuracy level. The results show that the Witczak predictive equation produces the closest estimation to the modulus of samples for both laboratory measurement methods.

The major Hot-Mix Asphalt (HMA) input for mechanistic-empirical (M-E) flexible pavement design is the dynamic complex modulus obtained from either the uniaxial or triaxial compressive dynamic modulus test. Furthermore, as part of the performance-based mix design process, the triaxial dynamic modulus has been selected to predict rutting and fatigue cracking, and the Indirect Tension (IDT) creep compliance test to predict low-temperature thermal cracking. The creep compliance and dynamic modulus are measured responses (viscoelastic functions) of viscoelastic materials under transient and cyclic loading, respectively. Under the assumptions of linearity, linear viscoelastic functions are equivalent. Moreover, these properties should be the same whether they are obtained from a uniaxial compressive or IDT test. For this dissertation, we tested the applicability of linear viscoelastic (LVE) theory to HMA mixes and determined whether HMA need to be modeled as a bimodular material to analyze IDT creep compliance test results. The need to model HMA as a bimodular material is a result of a number of studies that suggest that HMA tensile and compressive properties are different. A testing program was developed to experimentally measure the uniaxial compression, and IDT creep compliance, and the uniaxial compression dynamic modulus for different HMA mixes. The uniaxial compressive creep compliance and dynamic modulus master curves are constructed and the shift factors obtained from each test are compared. Interconversion between the creep compliance and dynamic modulus experimental results confirm the applicability of LVE theory for the HMA mixes investigated. Based on the applicability of LVE theory, a methodology to determine HMA LVE properties from the combined creep compliance and dynamic modulus test results was developed. As a practical application that is relevant to the M-E flexible pavement design procedure, LVE theory was used and compared to proposed approximate methods to perform the conversion of testing frequency to loading time. Specifically, dynamic modulus results were converted to relaxation modulus, creep compliance, and resilient modulus. Finally, the HMA IDT creep compliance test results at low and intermediate temperature (<20oC) were successfully analyzed using a HMA bimodular material model based on the Ambartsumyan model. The difference between the compressive modulus and the modulus calculated from the IDT test using Hondros' stress distribution is calculated. In addition, a method to determine the compressive-to-tensile modulus ratio using uniaxial compressive and IDT test results is illustrated for one of the tested HMA mixes. / Ph. D.

The recent changes in the Texas Department of Transportation (TxDOT) hot mix asphalt (HMA) mix design procedures to ensure that the mixture types routinely used on Texas highways are not prone to rutting raised concerns that these mixture types are now more susceptible to fatigue cracking. The primary goal of this study was to evaluate fatigue cracking test methods and recommend that which is both simple and robust, especially in qualifying commonly used Texas mixture types. One way to minimize fatigue cracking is through material screening and selection of appropriate mix designs that are representative of fatigue-resistant HMA mixes. However, there are not many standardized laboratory fracture resistance tests that have been universally adopted for routine mix design and/or screening purposes for HMA fatigue resistance. In this study, four different fracture test methods: the Overlay Tester (OT), Direct Tension (DT), Indirect Tension (IDT), and Semicircular Bending (SCB) tests were comparatively evaluated for their potential application as surrogate tests for routine fracture resistance evaluation and screening of HMA mixes in the laboratory. The evaluation criteria included: rationality of the test concept and correlation to field performance, repeatability and variability, simplicity and practicality of the sample fabrication process, and simplicity of data analysis. Results and key findings based on the laboratory fatigue resistance characterization of various commonly used Texas coarse- and fine-graded HMA mixes (Type B, C, and D) are presented in this paper. Overall, preliminary findings indicated that no monotonically-loaded test would be appropriate as a surrogate fatigue resistance test; however, the SCB test showed potential as a repeated-loading test. Suggested SCB test improvements include developing the repeated SCB test protocol, determining the appropriate failure criterion, and correlating laboratory performance to field performance.

Over the past decades, several new technologies/materials (such as WMA, RAP, rubber, polymers, bio-binders…etc.) were incorporated into asphalt mixtures. However, current mix-design specifications evaluate all mixtures containing these different additives/technologies based on volumetric. Further, RAP incorporation in asphalt mixtures is still limited, and the influence of lowered production temperatures on RAP contribution in RAP-WMA mixtures is understudied. To tackle these issues, this study presents a comprehensive evaluation of the effect of production factors ( RAP content and source, binder grade, and production temperatures) on the effective binder in WMA-RAP mixtures, and the role of the effective binder in controlling mixture performance.The experimental program included evaluation of the compaction, cracking, and rutting performance of WMA-RAP mixtures produced with a different combination of the production factors. The Semi-Circular Bend (SCB) test at intermediate temperatures was used for cracking evaluation, while the Indirect Tension Test at High Temperatures (IDT-HT) was used for rutting evaluation. Further, the study included rheological characterization of extracted binder from the mixtures to investigate the role of the effective binder on cracking performance. The results showed that the effective binder properties are changed significantly with changes in the production factors, as measured by the extracted binder rheological properties. Also, the properties of the effective binder showed a direct control of the mixture performance as measured by the IDT-HT strength and the flexibility index obtained from the SCB test. Binder selection limits were developed for lab-produced WMA-RAP mixtures based on the Glover-Rowe parameter. Finally, a validation study was conducted using data from four different projects, including a field project in Texas, FHWA’s accelerated loading facility, a laboratory mixture study in Wisconsin, and a New Hampshire DOT study to confirm the refine the findings of this study. / Civil Engineering

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