A STUDY ON DEVELOPMENT IN ENGINEERING PROPERTIES OF DENSE GRADE BITUMINOUS MIXES WITH COAL ASH BY USING NATURAL FIBER

Abstract

Flexible pavements under heavy traffic loading often experience permanent deformation, rutting, and fatigue cracking, demanding bituminous mixes with enhanced shear strength and structural stability. This laboratory study evaluates the volumetric and mechanical properties of Dense Bituminous Macadam (DBM) modified using naturally available Sisal fiber as a stabilizing additive and industrial coal ash (passing 75microns sieve) as a sustainable alternative mineral filler replacing conventional Ordinary Portland Cement (OPC). Aggregates conforming to MoRTH (5th Revision) gradation limits and 60/70 penetration grade bitumen were utilized. Marshall test specimens were prepared with 75 compaction blows on each face across varying binder contents (5.0% to 6.0%) and fiber concentrations (0% to 0.5% by total mix weight, cut to 15 to 20mm lengths). Comparative filler analysis demonstrated that coal ash delivers comparable structural performance to OPC, achieving a peak unmodified stability of 14.38 kN at 5.5% bitumen content. Experimental results established the Optimum Binder Content (OBC) as 5.5% and the Optimum Fiber Content (OFC) as 0.3%. At the optimum mix design (5.5% OBC with 0.3% OFC), the modified DBM mix exhibited a maximum Marshall Stability of 14.55 kN and a reduced flow value of 3.1 mm. Volumetric analysis confirmed full compliance with MoRTH specifications. The insertion of Sisal fiber forms a reinforced three-dimensional matrix within the bituminous mastic, mitigating binder drain-down and increasing shear resistance under dynamic loads. The study demonstrates that combining natural Sisal fiber with byproduct coal ash yields an economically viable, eco-friendly, and high-performance flexible pavement material. Keywords: Dense Bituminous Macadam (DBM), Sisal Fiber, Coal Ash Filler, Marshall Mix Design, Optimum Binder Content (OBC), Optimum fiber content (OFC), Volumetric Parameters, MoRTH Specifications.