Two pavement distress mechanisms have become more prevalent in recent years: near-surface rutting and surface-initiated wheelpath cracking. Some possible factors causing these failure mechanisms include higher traffic volumes, use of lower-quality materials, and changes in tire type and structure. Although all of these factors may play a role, this study concentrated on the effects of changes in tire type and structure on surface distress. Within the last decade, trucking companies have shifted from operation on bias ply tires to an exclusive use of radial tires and the gradual introduction of wide-base (super-single) radial tires. This prevalence of radial tires causes a major change in pavement surface loading characteristics and is shown to help explain the development of surface rutting and cracking. Tire contact stresses were measured for bias ply, radial, and wide-base radial truck tires at various loads and inflation pressures to investigate the effects of tire structure, loading, and inflation pressure on surface loads applied to pavements. It was determined that contact stresses vary significantly for the different types of tires investigated. The observed variations were explained by the differences in tire construction. In fact, tire structure appeared to have a greater influence on contact stresses than variations in either load or inflation pressure for a given tire type. It was shown that the specific characteristics of the complex contact stresses under truck tires have a strong influence on asphalt pavement cracking and rutting and must be considered for proper design and evaluation of asphalt pavements.

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An alternative laboratory asphalt aging process that could be used to simulate the aging effects of hot mixing on modified asphalts was developed and evaluated. The rotavapor apparatus, which has been used for recovery of asphalt from solution, was modified to work as an aging device for asphalts and modified asphalts. The rotavapor apparatus was modified such that the vacuum connection was replaced by an air pump with a controlled air flow. To reduce the variation of aging condition due to variation of the temperatures in the oil bath of the rotavapor apparatus, an insulated covering for the oil bath was constructed and used in this aging process. Evaluation results indicate that the aging severity of the modified rotavapor aging process is affected by variables such as process temperature, duration, and sample weight. All these factors could be adjusted to achieve the desirable level of asphalt aging. Because of the flexibility in controlling these variables in the modified rotavapor process, it appears that this method could be used to simulate the aging effects of hot mix plant process effectively.

The Strategic Highway Research Program (SHRP) has proposed a short-term oven aging (STOA) process to simulate the aging effects of hot mixing and construction process on asphalt mixtures, and a long-term oven aging (LTOA) process to simulate the effects of additional aging of asphalt mixtures in service for 5 to 10 years. In the SHRP Superpave Binder Specifications, the conventional Rolling Thin Film Oven (RTFO) Test is used to simulate the effects of the hot-mix process on a binder, and the Pressure Aging Vessel (PAV) is used to simulate additional aging of the binder in service. This study evaluated the effects of the SHRP STOA and LTOA by comparing their effects on the asphalt binders with those produced by the RTFOT and the PAV processes.

The flow properties of asphalts are usually characterized as Newtonian, pseudoplastic, dilatant, Bingham plastic, and thixotropic. Asphalts always exhibit some degree of elasticity. Therefore, even asphalts with Newtonian flow properties cannot be truly classified as Newtonian materials. In general, we can consider these flow categories to be a function of the shear susceptibility (complex flow) of the material. Most paving grade asphalts will exhibit Newtonian-like (C = 1.0) flow properties at or near conventional mix temperatures. At high shear rates the material may develop pronounced dilatant behavior (C > 1.0). However, at low temperature (<25°C) we often observe a greater degree of pseudoplastic (C < 1.0) behavior. The use of a fixed or specified shear rate for computation of asphalt viscosities at lower temperatures is often misleading and may result in erroneous interpretation of test results. The advantages of using constant power viscosity (ηi) is discussed in conjunction with viscosity-temperature regression analyses for evaluation of temperature susceptibility. This paper presents a thorough and concise overview of rheological types with emphasis on the need for rheological measurements throughout a range in temperature. The presentation discusses measurement methods and relies heavily on procedures purported by H. E. Schweyer as being excellent for the characterization of paving and roofing asphalts. The theoretical basis of the Schweyer constant stress rheometer is fully presented. The operation of the rheometer and procedure for analysis of data are described and illustrated by an example. The application of low-temperature viscosity test results using the Schweyer rheometer is also shown by an example.