Triaxial Testing of Open-Type Bituminous Mixtures : Technical Paper

Abstract

From field observation, bituminous mixtures with both one-size and open-graded aggregate gradations appear to perform satisfactorily for many service conditions. The results of conventional triaxial testing, on the other hand, show these one-size mixtures to be unstable under present design criteria, while the open-graded mixtures produce acceptable stability values. This illustrates that the present methods and theories of rational triaxial testing as applied to bituminous mixtures with a one-size aggregate fail to evaluate the true in-service stability properties of these paving mixtures. This research study consisted primarily of carefully testing laboratory-compacted specimens by the open-system triaxial compression test to determine their stability under the influence of different variables. These were aggregate gradation, confining pressure, specimen height, and degree of compaction. The two aggregate gradations, open-graded and one-size, were incorporated with an asphalt cement into bituminous mixtures. The variable of specimen height necessitated the molding of specimens whose height to diameter ratios were less than two Therefore, this phase of the study was referred to as the irrational triaxial compression tests. The laboratory results were presented in the form of graphical stress diagrams with the confining pressure plotted against the normal stress. The open-graded mixture produced a direct correlation with the linear Coulomb equation, represented by the two constants, (c) cohesion and (o) angle of internal friction. However, different values of cohesion and angle of internal friction were obtained by the rational and the irrational triaxial tests. The triaxial testing of the one-size mixture showed that low shearing strength was developed at low values of confining pressure and at small values of specimen deformation. With an increase in the confining pressure and/or the amount of specimen deformation, high values of shearing strength were obtained for this paving mixture. For the range of confining pressures investigated, 0 to 150 psi, approximately 90 percent of the potential shearing resistance was mobilized when the test specimen was deformed to a strain of ten percent. The graphs of confining pressure versus normal stress at various strain values indicated a curvilinear relationship between these two parameters. Thus, the shearing strength of this one-size mixture cannot be ascertained wholly by such constants as the cohesion and the angle of internal friction, variables for this paving mixture, The complex interaction of lateral support, rearrangement of aggregate particles, and change in specimen volume prevented the exact evaluation of the effects of specimen height on the triaxial stability of the one-size mixture.