Evaluation of the Combined Effect of Flakiness and Elongation in the Strength Parameters of First Coat Stone Chippings for Double Bituminous Surface Treatment

Abstract

This study investigates the influence of the combined flakiness and elongation index (CFEI) on the mechanical strength parameters—Los Angeles abrasion (LAA), aggregate impact value (AIV), and aggregate crushing value (ACV)—of stone chippings used in double bituminous surface treatment (DBST) for low-volume roads in Nepal. Natural stone chippings sourced from Ghyampe Khola, Sindhuli, and crushed aggregates from the Roshi Quarry, Kavrepalanchok, were used as primary materials. Specimens were prepared by blending base aggregates with varying proportions of flaky and elongated particles (0 to 50% CFEI) in five different flaky-to-elongated ratios (0:100 to 100:0). A second series examined the effect of blending natural stone chippings (40% CFEI) with crushed aggregates (10–35% CFEI) in ratios of 85:15, 70:30, and 50:50. Results demonstrate that as CFEI increases, all three strength parameters deteriorate, with LAA rising from 27.08 to 38.83%, AIV from 20.13 to 28.65%, and ACV from 22.05 to 34.98%. Flaky aggregates were found to have a significantly greater negative impact on strength than elongated aggregates. The findings indicate that strength specifications are satisfied at CFEI values up to 42% for LAA and 43% for AIV, suggesting the current 35% CFEI limit in SSRBW-2016 may be revised. The optimal blending strategy—a 50:50 mix of natural and crushed aggregates with 30% CFEI—successfully reduced the combined index to within specification while maintaining all required strength characteristics.