Creep compliance of Hot Mix Asphalt (HMA) is an important property to characterize the material's viscoelatic behavior. It is used to predict HMA thermal cracking at low temperature and permanent deformation at high temperatures. There are several experimental methods to measure the creep compliance. Two of these methods were used in this thesis; uniaxial compressive and indirect tension (IDT) creep compliance. The tests were conducted at five temperatures (-15, 5, 20, 30, and 40°C) with a static loading for 1000-sec to characterize two typical HMA mixes used in Virginia, a base and a surface mix. Creep compliance master curves (CCMC) were developed by shifting the curves to a reference temperature using time-temperature superposition. Three mathematical functions, Prony series, power and sigmoidal, were fitted to the experimental data using regression analysis. Uniaxial CCMC were also predicted based on dynamic modulus measurements using method for interconversion of vicoelastic properties recommended in the literature. Finally, the susceptibility of the mixes to thermal cracking was evaluated based on the creep compliance measurements at low temperature. The regression analysis showed that the three mathematical models considered are appropriate to model the CCMC over a wide ranger of reduced times. The sigmoidal model provided the best fit over the entire range of reduced times investigated. This model also produced the best results when used in the interconversion procedures. However, there were noticeable differences between the CCMC predicted using interconversion and the experimental measurements, probably due to nonlinearity in the material behavior. The m-values for the base mix were higher using the creep results measured with both configurations. / Master of Science

The objective of this study was to develop a model to predict the viscoelastic material functions of a vinyl ester (VE) polymer (Derakane 441-400, Ashland Co.,) with variations in its material properties. Short-term tensile creep/creep recovery experiments were conducted at two stress levels and at four temperatures below the glass transition temperature of the VE polymer, with 10 replicates for each test configuration. Experimental strains in both the longitudinal and transverse directions were measured using a digital image correlation technique. The measured creep strain versus time responses were subsequently used to determine the creep compliances using the generalized viscoelastic constitutive equation with a Prony series representation. The variation in the creep compliances of Derakane 441-400 was described by formulating the probability density functions (PDFs) and the corresponding cumulative distribution functions (CDFs) of the creep compliances using the two-parameter Weibull and log-normal distributions. The maximum likelihood estimation technique was used to obtain the Weibull shape and its scale parameters and the log-normal location and its scale parameters. The goodness-ofit of the distributions was determined by performing Kolmogorov-Smirnov (K-S) hypothesis tests. Based on the K-S test results, the Weibull distribution is a better representation of the creep compliances of Derakane 441-400 when compared to the log-normal distribution. Additionally, the Weibull scale and shape parameters of the creep compliance distributions were shown to be time and temperature dependent. Therefore, two-dimensional quadratic Lagrange interpolation functions were used to characterize the Weibull parameters to obtain the PDFs and subsequently the CDFs of the creep compliances for the complete design temperature range during steady state creep. At each test temperature, creep compliance curves were obtained for CDF values of 0.05, 0.50 and 0.95 and compared with the experimentally obtained lowest, mean and highest creep compliances, respectively. The predicted creep compliances of Derakane 441-400 in the design space are in good agreement with the experimental data.

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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 effects of vapor-grown carbon nanofiber (VGCNF) weight fraction, applied stress, and temperature on the viscoelastic responses (creep strain, creep rate, and creep compliance) of VGCNF/vinyl ester (VE) nanocomposites were studied using a central composite design (CCD). The nanocomposite test articles were fabricated by high shear mixing, casting, curing, and post-curing in an open face mold under a nitrogen environment. Short-term creep/creep recovery experiments were conducted at prescribed combinations of temperatures (23.8 – 69.2 C), applied stresses (30.2 – 49.8 MPa), and VGCNF weight fractions (0.00 – 1.00 parts of VGCNF per hundred parts of resin, phr) determined from the CCD. The response surface models (RSMs) for predicting these viscoelastic responses were developed using the least squares method and an analysis of variance procedure. The response surface estimates indicate that increasing the VGCNF weight fraction decreases the creep resistance of the VGCNF/VE nanocomposites at high temperatures (46.5 – 69.2 C).

Cracking is one of the most severe distress modes of asphaltpavements. Thus characterising the fracture resistance properties of asphaltmixtures is the key issue for improving the performance relatedmixture design. The present master thesis project addresses the implementationof the theoretical framework, which is used to characterise thefracture resistance of mixtures based on the SuperPave indirect tensiletest (IDT). An open source Matlab-based software for analysing resilientmodulus, Poisson’s ratio, creep parameters and fracture resistance parametershas been developed. The software analyses the the IDT results, toestimate mixture’s fracture resistance based on hot mix asphalt FractureMechanics. Predictions form the field specimens concerning the fractureresistance obtained from IDT are compared with observed field performance.

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. / text

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