Investigation Of Structural M25 Grade Concrete Incorporating Manufactured Sand As Fine Aggregate And Reinforced With Alkali Treated Coconut Coir Fibers

Abstract

The depletion of natural river sand and the brittle tensile behaviour of conventional concrete motivate the use of engineered fine aggregates and discrete natural-fibre reinforcement. This experimental investigation evaluates M25 grade structural concrete prepared with 100% Manufactured Sand (M-Sand) as fine aggregate and reinforced with alkali-treated coconut coir fibres. Fifty-millimetre coir fibres were treated in a 5% sodium hydroxide solution for 24 hours and incorporated at 0.0%, 0.5%, 1.0%, and 1.5% by weight of cement. The baseline mix used a water-cement ratio of 0.45 and 0.8% polycarboxylate ether superplasticizer. Fresh-state behaviour was assessed through slump-cone and compaction-factor testing, while compressive strength was measured at 7 and 28 days and splitting tensile and flexural strengths at 28 days. Workability decreased progressively with increasing fibre content, with the reported slump falling from 75 mm for the control condition to 0 mm at the highest fibre level. The detailed mechanical results identified CC1.0 as the optimum mix: 28-day compressive strength reached 28.15 N/mm², splitting tensile strength reached 2.47 N/mm², and flexural strength reached 3.80 N/mm². Relative to CC0.0, the corresponding improvements were 4.84%, 16.51%, and 18.75%. Fibre-reinforced specimens also exhibited improved crack bridging and a transition from abrupt brittle fracture toward pseudo-ductile post-cracking behaviour.