The Kentucky Department of Highways in cooperation with the Bureau of Public Roads is conducting a continuing investigation of the fundamental mechanical properties of flexible pavement materials. The ultimate objective of this investigation is to gain sufficient knowledge of the fundamental mechanical behavior of these materials to support the establishment of a responsive flexible pavement design procedure. A preliminary report, issued in 1964, contained the results of the preparatory phase of this investigation. This report summarized from a theoretical point of view efforts that had been made to quantify the mechanical response of viscoelastic materials to known conditions of loading. In addition, it described the development of a rotating coaxial cylinder viscometer which was designed to measure the creep response in shear of solid or semi-solid bituminous materials. The usefulness of this viscometer was verified by testing a rubberized asphalt cement at several temperature and torque levels. It was found that rubber, when added to the asphalt cement in significant quantities, tended to increase, at the higher temperatures, the steady-state viscosity, the stiffness, the retardation time of the viscoelastic response, and the complexity of flow. The current report summarizes results of a second phase of the continuing investigation in which the preparatory efforts have been expanded to encompass the creep testing of 13 asphalt cements in a second rotating coaxial cylinder viscometer. The 13 asphalt cements were selected to represent a variety of crude sources, penetration grades, and manufacturing processes. Design and construction of the viscometer, which were accomplished as a portion of this phase, reflect the basic features of the earlier viscometer modified on the basis of the recommendations contained in the first report. In analyzing the data reported herein, efforts were made to apply existing theories for simple ideal materials such as the Newtonian liquid and the Bingham plastic whenever these theories produced results in reasonable accord with the actual data. In many instances, however, it was necessary to combine these simple theories with somewhat more complex concepts of linear visco-elasticity in order to adequately characterize the deformation properties of the materials.