An experimental investigation on the mechanical and thermal characteristics of eco-friendly concrete with coconut shell aggregate and coir fiber as reinforcement

Abstract

Coconut shell (CS) and coir fiber (CF) are components of coconut waste abundantly available in subtropical and tropical regions. This study aims to develop eco-friendly concrete by partially replacing coarse aggregate with CS and incorporating CF as reinforcement, while ensuring its mechanical properties are comparable to those of ordinary concrete—with no need for additional cement. To evaluate the effects of CS and CF on the mechanical and thermal properties of concrete, two groups of specimens were prepared: CS0, without CS, and CS10, with CS replacing 10 % of natural gravel (NG) by volume. Each group contained various CF contents ranging from 0 % to 0.32 % by weight of concrete for mechanical and thermal property testing. The results showed that, for both groups, compressive strength, flexural strength, and splitting tensile strength initially increased and then decreased with increasing CF content. However, replacing 10 % NG with CS exerted a negative effect on the mechanical properties of the concrete. The mechanism by which CS and CF influence mechanical properties was revealed through failure zone observations and microscopic analysis. CS failed by separating from the matrix at the concave-side interface, whereas NG failed primarily through fracture of the gravel itself. Additionally, the variations in thermal conductivity with temperature and relative humidity were examined, showing a monotonic increase. For both groups, thermal conductivity decreased with increasing CF content, with an additional 20 % reduction observed in group CS10. The concrete with CS replacing 10 % NG and incorporating 0.24 % CF exhibited mechanical properties comparable to those of ordinary concrete, with a thermal conductivity of 0.4 W/m·K at 20 ℃ in dry state. These findings demonstrate the combined incorporation of CS and CF at appropriate contents can serve as an effective approach for consumption reduction and energy conservation in the construction industry.