Abstract:
Based on a comprehensive review of current United States transportation agency specifications, it was found that there is a large variation in current limits set on the shape of aggregate particles that are considered as suitable to be used in an unbound base layer. Previous studies found in the literature indicated that the shape of the aggregate may affect the overall performance of the base course, but the extent of that effect has not been quantitatively documented. Therefore, a well-defined limit on particle shape for unbound aggregate base does not exist. This research is providing a detailed quantitative comparison of the effect of aggregate shape on unbound aggregate base as it relates to performance within an unbound base layer.
The experimental program for this study included particle shape analyses, compaction, matrix interlocking characterization (Bailey method), resilient modulus, and permanent deformation testing. Four different types of aggregates including diabase, hornfels, dolostone, and slate were included in the study and aggregates from each rock type were separated and sieved to create varying particle shapes within the same mineralogy and open- and dense-graded aggregate gradations used in road construction. Samples with consistent mineralogy were compared with each other to understand the effect of particle shape of aggregates with a specific mineralogy and samples with different mineralogy (but with same gradation) were compared with each other to understand the variation of the findings with differences in mineralogy.
The findings of the study showed that the durability of the aggregate was dominated by the mineralogy, but marked variations in durability were observed because of particle shape. Additionally, matrix interlocking characterization by the Bailey method showed that the effect of particle shape was more pronounced in the open-graded materials since the coarse particle made up the load carrying skeleton of the structure. The overall effect of particle shape on the performance, as it was defined in this study, of typically designed flexible pavement structures appear to be minor. However, particle shape was noted to significantly affect non-typical flexible pavement structures, specifically an open-graded base layer supporting a thin asphalt surface.
