Effect of Alkali Treatment on Mechanical Properties of Concrete Reinforced with Phragmites Australis Fibres

Abstract This paper investigates the influence of both untreated and alkali-treated Phragmites australis fibres (PAF) on the fresh and hardened properties of fibre-reinforced concrete. Various concrete mixes were prepared with fibre contents ranging from 0–2%, and the effects on workability, density, compressive strength, splitting tensile strength, and flexural strength were systematically evaluated over 7, 14, and 28 days of curing. Results showed that increasing PAF content led to reduced workability and density, with alkali-treated fibres exhibiting slightly better performance due to improved dispersion and reduced water absorption. Mechanical properties improved at low fibre dosages (0.5–1%), particularly with NaOH-treated fibres, which demonstrated superior bonding with the cement matrix. However, higher fibre volumes negatively impacted strength due to poor dispersion and increased porosity. Overall, the findings suggest that low to moderate contents of treated PA fibres can enhance concrete performance while promoting the use of sustainable, plant-based materials in construction applications.