Simple Approach for Determination of Optimum Gradation Considering the Combined Index and Angularity Number of Stone Aggregates

A satisfactory bituminous mix should have aggregate gradation, which will ensure maximum density and minimum voids in the mix to fulfill requirements of bituminous mix design specifications. In India organizations such as Indian Roads Congress and Ministry of Surface Transport (MOST) [1] have laid down specifications regarding the range of percentage passing of different size of aggregate in the mix. The conventional procedure is to aim at mid values of these specified ranges for the design of bituminous mix. However, this mid point gradation does not produce a satisfactory mix as it often fails to fulfill the specified requirements of the bituminous mix. This is because the density and voids in a mix depend on the shape factors namely Angularity Number (AN), Flakiness Index (FI) and Elongation Index (EI) of different aggregates in the mix. An experimental study has been made to develop a relationship between the aggregate shape factors in terms of Combined Index (CI) and Angularity Number and the gradation index ‘n’ in the Fuller – Thompson ideal gradation equation (2) to arrive at the optimum gradation. The results obtained using the conventional mid point of gradation range and those using the proposed optimum gradation concept have been compared in this paper. It is generally observed that the mixes prepared at the optimum gradation considering the CI and AN have higher density and lower voids compared to those at mid point of gradation ranges. The empirical equation developed in the study could be advantageously made use of for mix design. Index Terms Aggregates, Combined Index, Angularity Number, Dry density, Voids, Optimum Gradation