Effect of alkali treatment on mechanical properties of concrete reinforced with phragmites australis fibres

Abstract

This study evaluates the influence of untreated and alkali-treated Phragmites australis fibres (PAF) on the fresh and hardened properties of concrete. Concrete mixes with fibre contents ranging from 0% to 2% were tested for workability, density, compressive strength, splitting tensile strength, flexural strength, and ultrasonic pulse velocity (UPV) at 7, 14, and 28 days of curing. Results showed that the addition of PAF reduced workability and density, though treated fibres maintained slightly higher slump (about 3% at 1% fibre) and density (up to 2% higher at 28 days) compared to untreated fibres. Mechanical performance improved significantly at low fibre dosages (0.5-1%), particularly with treated fibres. At 28 days, compressive strength increased by 6% (34 MPa vs. 32 MPa) at 1% fibre, splitting tensile strength improved by 12% (3.14 MPa vs. 2.81 MPa for the control), and flexural strength improved by 32% (10.35 MPa vs. 7.85 MPa for the control). Beyond 1% fibre content, mechanical properties declined due to fibre clustering and increased porosity. UPV results confirmed that alkali treatment enhanced internal homogeneity, with treated mixes achieving up to 3% higher velocities compared to untreated counterparts at the same fibre volume. Overall, the findings demonstrate that alkali-treated Phragmites australis fibres, when used at controlled dosages (0.5-1%), can improve concrete’s mechanical performance while providing an eco-friendly alternative to synthetic fibres.