Engineering Properties of Functionally Layered Concrete Containing Rubber Aggregate

Abstract

Concrete remains the most widely used construction material globally due to its high compressive strength, versatility, and cost-effectiveness. This study investigates the engineering properties of Functionally Layered Crumb Rubber Concrete (FLCRC) to develop a sustainable structural material that incorporates recycled tyre rubber without significantly compromising mechanical or durability performance. The research investigates the material and durability properties of FLCRC and examines how functional gradation, when strategically positioned to place fine and coarse crumb rubber concrete below the neutral axis, can balance strength, ductility, and environmental benefits. Motivated by the need to reduce natural aggregate consumption and mitigate waste tyre accumulation, an extensive experimental programme was undertaken to assess the fresh, mechanical, durability, and microstructural properties of FLCRC using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDX). Furthermore, full-scale one-way reinforced slabs were tested under four-point bending to evaluate flexural behaviour. The findings indicated that the inclusion of 20% fine crumb rubber reduced compressive strength by approximately 10%, while functional gradation mitigated this reduction to about 5%. Similarly, a 15% replacement of coarse aggregate with crumb rubber resulted in a 36% reduction in compressive strength; however, the loss was reduced to approximately 21% when functional layering was adopted. Although durability analyses indicated increased porosity and carbonation in the rubberised layers (up to ~7% and ~17% for fine and coarse rubber, respectively, and ~35% and ~63% for carbonation depth), the structural behaviour remained comparable to that of conventional concrete slabs. From a sustainability perspective, modern structural design increasingly evaluates environmental performance through embodied carbon, which quantifies the total greenhouse gas emissions associated with the production, processing, and transportation of construction materials. The partial substitution of natural aggregates with recycled rubber can contribute to reducing the embodied carbon of concrete by diverting waste tyres from landfill and decreasing the demand for natural aggregates. Consequently, despite observed changes in durability indicators, FLCRC slabs demonstrated comparable load-carrying capacity and failure mechanisms to conventional slabs, while offering potential environmental benefits, including reduced embodied carbon and improved resource efficiency. Overall, FLCRC offers a sustainable, structurally viable approach to incorporating recycled rubber into structural concrete applications.