The Effect of Vertical Inhomogeneity on Compressive Properties of Asphalt Mixtures

摘要

Vertical inhomogeneity, which is defined as the separation of the original mix design gradation into finer and coarser gradations along the depth of the specimen, is often observed in laboratory compacted asphalt mixture specimens. This phenomenon is believed to be the result of the heavier, coarse aggregates gravitating to the bottom of the mold thus preventing the fine aggregates from sinking. Also, the kneading effort of compaction forces the larger particles to the bottom of the mold. While inhomogeneities exist, the effect of the vertical inhomogeneity on mechanical properties of asphalt mixtures is not known. To study this effect, homogeneous and vertically inhomogeneous asphalt mixtures specimens were fabricated. In order to characterize the degree of inhomogeneity, non-destructive x-ray computed tomography (CT) was used to scan cross-sectional images of the specimens. An index of inhomogeneity was developed to evaluate the level of inhomogeneity of the scanned images. The results clearly showed that the index could distinguish between homogeneity and inhomogeneity. The compressive modulus of the mixture was evaluated at test temperatures of 21°C and 45°C by means of frequency sweep simple performance dynamic modulus test. The resistance of the material to axial permanent deformation was measured using the simple performance flow number test at 45°C. The effects of vertical inhomogeneity on the compressive properties of the specimens were investigated at the two test temperatures. Neither dynamic modulus at both test temperatures nor the permanent axial deformation changed significantly with vertical inhomogeneity. Although, the correlation between the compressive properties and the inhomogeneity index is generally very poor, the correlation is higher at the test temperature of 21°C than at the test temperature of 45°C. The results of this study indicate that simple performance tests are not sensitive to even an extreme level of vertical inhomogeneity. Regardless of the separation of the original mixture to the coarser and the finer gradations, an average property value that is comparable to the properties of homogeneous specimens is measured. This shows that predicting the performance of the material in the field based on the compressive properties of laboratory-made specimens as they are measured using Simple Performance Tests could be reliable.