In the past several years, 1974–1977, NASA has conducted several research studies to develop an extensive collection of ground truth soil moisture data. As a result of these experiments, moisture data were available from 58 ‘large‐field sites,’ each being 400 × 400 m (40 acre). The field locations were 1 field in Phoenix, Arizona (sampled four times), 28 fields in Jefferson County, Kansas, 23 fields in Finney County, Kansas, and 5 fields in Hand County, South Dakota. At the first three locations, samples were taken in specific vertical increments or horizons (i.e., 0–1; 1–2, 2–5, 5–9, and 9–15 cm). In the South Dakota study, moisture samples were taken in increments from the surface ( i.e., 0–2.5, 0–5, and 0–10 cm). A detailed statistical analysis was made to define the general relationship and ranges of values of the field moisture relative to both the variance (standard deviation) and coefficient of variation (CV) for a given test site and depth increment. On the basis of the results of the variability study it was concluded that (1) moisture variations within any given‘large‐field’ area are inherent and are normally distributed about the mean, (2) neither a single (constant) value of the standard deviation nor coefficient of variation uniquely define the variability over the complete range of mean field moisture contents examined, and (3) using an upper bound standard deviation parameter clearly defines the maximum range of anticipated moisture variability. It was found that 87% of all large‐ moisture content standard deviations were less than 3%, while about 96% of all the computed values had an upper bound of s = 4% for these intensively sampled fields. Using these upper bound magnitudes as an estimate of the population standard deviation and a preselected confidence level, limit of error curves of mean soil moisture measurements for large‐field sites relative to the required number of samples were determined.

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The objective of this research study was to evaluate several mathematical models to be used in calculating the onset of tertiary flow [referred to as the flow number (FN) parameter] for asphalt mixtures. The FN indicates the onset of shear deformation in asphalt mixtures, which is a significant parameter in evaluating rutting in the field. The FN is obtained from the repeated load permanent deformation (RLPD) laboratory test. Current modeling techniques in determining the FN use a polynomial model fitting approach, which works well for most conventional asphalt mixtures. However, analysis and observations on the use of this polynomial model for rubber-modified asphalt mixtures showed problems in identifying the true FN values. The scope of the work included the collection and analysis of more than 300 RLPD test data files, which comprised more than 40 mixtures, a wide range of test temperatures, and several stress levels. A new comprehensive mathematical model was recommended to accurately determine the FN. The results and analysis were evaluated through manual calculations and found to be accurate, rational, and applicable to all mixture types, whether a tertiary stage was reached or not.

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A comprehensive constitutive model for asphalt concrete was calibrated that included viscoelasticity, viscoplasticity, and irreversible microstructural damage in unconfined compression. Three different types of laboratory tests were designed and performed to calibrate each of these response components. Small-strain dynamic modulus tests were used to calibrate the undamaged linear viscoelastic properties. Cyclic creep and recovery tests to failure were performed to calibrate the viscoplastic properties. Constant-rate-of-strain tests to failure were used to calibrate the damage behavior. These tests were performed at a wide range of temperatures, loading rates, and stress levels. Upon calibration of each individual response, the model was validated by predicting the results of other constant-rate-of-strain tests at temperatures and strain rates different from those used in the calibrations. The predictions for these different conditions indicate that the comprehensive model can realistically simulate a wide range of asphalt concrete behavior.

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The main purpose of this paper is to present the development of a set of predictive models for the viscosity and complex shear modulus of asphalt binders. The model development was aimed at overcoming the limitations of current models used in the Mechanistic-Empirical Pavement Design Guide (MEPDG), which has been developed under NCHRP Project 1-37A and refined under NCHRP Project 1-40D. A comprehensive study was completed at Arizona State University to conduct a number of asphalt binder tests and to finalize a large binder characterization database containing 8,940 data points from 41 different virgin and modified binders. This database was used to develop the new models. The first of the models developed in this study is a fully revised version of the widely known ASTM Ai-VTSi viscosity model. This new, revised model takes into account the loading frequency applied on the binder while accurately predicting the viscosity at a specific temperature and loading frequency from the Ai and VTSi values of a binder. The other two new models developed are capable of accurately predicting the dynamic shear modulus (|G*b|) and associated phase angle (δb) of the binder at a specific temperature and loading frequency from the Ai and VTSi values of the binder. The models have been found to be rational, unbiased, accurate, and statistically sound. It is hypothesized that because of mathematical structures similar to those used in the MEPDG, the models developed in this study can be incorporated easily into a future revision of the guide.