Durability of alkali-sensitive sisal and coconut fibres in cement mortar composites

This paper presents the results of an experimental program designed to assess the durability of sisal and coconut fibres exposed to alkaline solutions of calcium and sodium hydroxide. In addition, the durability and microstructure of the cement mortar composites reinforced with these fibres aged under tap water, exposed to controlled cycles of wetting and drying, as well as to the open air weathering have been studied. The possibility of biological attack of fibres was investigated by conditioning them in tap water for 420 days. It was found that sisal and coconut fibres kept in a calcium hydroxide solution of pH 12 completely lost their flexibility and strength after 300 days. The composites manufactured with short sisal or short coconut fibres and ordinary Portland cement `OPC' matrix presented a significant reduction in toughness after six months of exposure to the open air weathering or after being submitted to cycles of wetting and drying. The embrittlement of the composites can be mainly associated with the mineralization of the fibres due to the migration of hydration products, especially calcium hydroxide, to the fibre lumen, walls and voids

Influência das fibras de aço e das adições minerais na fissuração de tirantes de concreto armado

Neste trabalho é investigada a influência das fibras de aço e das adições minerais (sílica ativa e cinza volante) na fissuração de tirantes de concreto armado. Fibras de aço com gancho nas extremidades e relação de aspecto igual a 65 foram usadas nas frações volumétricas de 0,75%, 1,00% e 1,50%. Dezesseis tirantes de 800 mm de comprimento e seção transversal quadrada de 150 mm de lado reforçados com uma barra de aço CA-50 de 20 mm de diâmetro foram submetidos à tração axial para determinação das aberturas das fissuras. Os resultados mostraram que as fibras reduziram a abertura média de fissuras em até 75% e que as adições minerais melhoraram o comportamento do concreto na fissuração. As aberturas de fissuras foram comparadas aos valores sugeridos pelas recomendações das normas brasileira, americana e européia, que se mostraram inadequadas para estimar a abertura de fissuras em concretos reforçados com fibras

Effect of water absorption on the mechanical properties of cotton fabric-reinforced geopolymer composites

Cotton fabric (CF) reinforced geopolymer composites are fabricated with fibre loadings of 4.5, 6.2 and 8.3 wt%. Results show that flexural strength, flexural modulus, impact strength, hardness and fracture toughness are increased as the fibre content increased. The ultimate mechanical properties were achieved with a fibre content of 8.3 wt%. The effect of water absorption on mechanical and physical properties of CF reinforced geopolymer composites is also investigated. The magnitude of maximum water uptake and diffusion coefficient is increased with an increase in fibre content. Flexural strength, modulus, impact strength, hardness and fracture toughness values are decreased as a result of water absorption. Scanning electron microscopy (SEM) is used to characterise the microstructure and failure mechanisms of dry and wet cotton fibre reinforced geopolymer composites