Optimising Marshall Stability of Coir Fiber Reinforced Dense Bituminous Macadam (DBM) Grade-II with VG-40 Bitumen

Abstract

This study evaluates the effect of coir fiber incorporation on the Marshall properties and volumetric characteristics of Dense Bituminous Macadam (DBM) Grade II mixes at a fixed Optimum Bitumen Content (OBC) of 4.67 %. Coir fibers were chopped into five lengths (5 mm, 10 mm, 15 mm, 20 mm, and 25 mm) and added at contents ranging from 0.1 % to 0.7 % by weight of bitumen. Standard Marshall specimens were prepared and tested for stability, flow, bulk specific gravity, voids in mineral aggregate (VMA), voids filled with bitumen (VFB), and air voids (Vv). Results indicate that Marshall stability increased significantly with fiber addition, peaking at 17.51 kN for 0.4 % content and 15 mm fiber—approximately 31 % higher than the control (13.38 kN). VFB reached a maximum of 76.19 % at 0.5 % content and 15 mm length. Flow values remained near control up to 0.3 % but increased sharply beyond 0.4 %, indicating reduced compatibility. Optimal performance is achieved at 0.4 %–0.5 % fiber content with 15 mm length, balancing strength and volumetric efficiency.