Design Studies of Indiana Bituminous Concrete Surface Mixtures

For several years, the bituminous laboratory of the Joint High­ way Research Project has been concerned with the testing of bitu­ minous-aggregate mixtures and with the development of knowledge that would lead to an understanding of the properties of bituminous mixtures as satisfactory paving materials. At the Thirty-Eighth Annual Road School, a paper entitled, “Role of the Laboratory in the Design of Bituminous Mixes,” was presented which included test data from the laboratory for the evaluation of a number of mixture variables including aggregate gradation, asphalt content, grade of asphalt cement, aggregate type, and testing speed (1, 2). In this testing program two types of tests were used, the triaxial test and the Marshall test. The triaxial test is a rational one from which fundamental strength properties of a material may be obtained. This test has been found to be very useful as a research tool, but it is not well-adapted for use in the field either as a means of design­ ing or controlling a bituminous mixture. For such use, the Marshall test has advantages of simplicity and convenience. The apparatus used in the Marshall test is shown in Figure 1. The specimen is a cylinder 4-inches in diameter and 2j^-inches high. A unique feature of the test is the method of loading the specimen. Unlike most compression tests, the Marshall specimen is loaded on the cylindrical surface rather than on the plane faces. The data normally obtained from a test of this type are the total load at failure (stability) and the total deformation of the specimen at failure expressed in hundredths of an inch (flow).