Durability of Flax Fibre Reinforced Concrete

Abstract

Flax fibre-reinforced concrete has been found to perform as well as concrete reinforced with fibres made from glass and plastic in terms of its resistance to plastic shrinkage cracking. However, flax fibres alone have been found to lose their strength and stiffness when subjected to simulated concrete pore solutions. This thesis describes an experimental program to measure the degradation of the mechanical properties of flax fibre-reinforced concrete subjected to wet-dry and freeze-thaw cycling. Three types of flax fibre (untreated, Duralin treated, and silane treated) were used at lengths and volume fractions of 38 mm and 0.3%, respectively, to determine whether different fibre types would generate different mechanical properties.

The compressive strength, flexural strength and flexural toughness were found for all types of concrete, both before and after cycling, to determine if weathering cycles produced degradation of these mechanical properties. Flax fibre-reinforced concrete was found to experience no significant degradation when compared to the unreinforced concrete. Duralin and silane fibre-reinforced concrete were also found to encounter no significant degradation or improvement when compared to untreated flax fibre or unreinforced concrete. This was found after subjecting all four types of concrete to both wet-dry and freeze-thaw cycling.

Non-destructive testing found that the dynamic modulus of elasticity of beams subjected to wet-dry cycling gradually changed as cycling went on. This was attributed to the changing of the microstructure in the cement paste, as moisture was cycled in and out. Degradation of the cement paste itself was not found to be the cause of this gradual change in dynamic modulus of elasticity.

Analysis performed on scanning electron microscope images of the failed fibres, on the crack surface of the beams, showed that there were no correlations between the final conditions of each fibre type and the type of weathering it was subjected to. Energy-dispersive x-ray spectroscopy analysis also found that elements from the cement paste adhered the surface of the fibres, once mixed in the concrete.