Mechanical and Cement Stabilization of Reclaimed Asphalt Pavement-Gravely soil Blends for Sustainable Road Subbase Applications in Kenya

The increasing depletion of natural gravel resources and the growing accumulation of reclaimed asphalt pavement (RAP) materials have intensified the need for sustainable alternatives for road construction. This study evaluated the engineering performance of gravely soil mechanically stabilized with RAP and chemically stabilized with Ordinary Portland Cement (OPC) for use as road subbase material. Gravely soil and RAP obtained from Kiambu County, Kenya, were blended at RAP contents of 20, 40, 60, and 80% by dry weight, followed by stabilization with 2–5% OPC. Laboratory testing included particle size distribution, Atterberg limits, moisture content, specific gravity, Modified Proctor compaction, swell, soaked California Bearing Ratio (CBR), Unconfined Compressive Strength (UCS), and direct shear tests in accordance with relevant British Standards and the Kenya Road Design Manual. RAP incorporation improved grading characteristics, increased maximum dry density, reduced optimum moisture content, and significantly lowered plasticity and swell while enhancing bearing capacity. Cement stabilization further improved the engineering performance through the formation of cementitious bonds, resulting in higher strength and greater dimensional stability. The optimum mixture, comprising 60% RAP, 40% gravely soil, and 4% OPC, achieved a Plasticity Index of 13.4%, swell of 0.4%, soaked CBR of 120%, and UCS of 3.1 MPa, satisfying the requirements for cement-stabilized subbase materials. The findings demonstrate that RAP-Gravely–cement stabilization is an effective and sustainable approach for upgrading marginal gravely soils into durable pavement materials, reducing dependence on natural aggregates, minimizing construction waste, and supporting sustainable road infrastructure development.