Sisal fibre pull-out behaviour as a guide to matrix selection for the production of sisal fibre reinforced cement matrix composites

Abstract

Natural fibre reinforced cement composites are promising potential materials for use in panelised construction. The structural properties of these composite materials are yet to be fully understood. As the role of the natural fibre is to reinforce the composite in tension and improve the toughness of the system, the mechanisms of failure and the effect of sisal reinforcement in potential matrices were investigated. This study sought to understand fibre pullout as a failure mechanism in sisal fibre reinforced cement composites and to select optimum cement composite matrices from a range of potential cement blends. Single fibre pull-out tests were carried out to determine the optimum matrices for fibre/matrix bonding for three generic types of matrices of varying basicity ratio [(CaO+MgO)/ SiO2] in the Al2O3-SiO2-CaO ternary composition system. The embedded length of the sisal fibres was varied to establish the critical lengths for a specific failure mechanism. Curing time was also taken into account to establish the optimum curing conditions for maximum energy absorption or release on fibre pullout. 5x10x20 mm cement composite beams were cast with a single sisal fibre strand per sample along the neutral axis of the beam length. The beams were subsequently cured in water and tested in a tensile machine, where the fibres were pulled out from the cement composites. The mechanism of failure was determined from; SEM images of the fibres pulled from the matrices, pullout shear stress-strain data, the measured stiffness (dynamic Young modulus) and the porosity of the cement composite matrix. From this data, the matrix with the highest pullout stresses and highest energy absorption was selected. The maximum fibre length to ensure fibre pull-out rather than fibre fracture (critical length) was also determined. The strength of the sisal fibre/matrix bond and anchorage relative to the Young modulus (stiffness) influenced the fibre pullout mechanisms. Matrix shear was the dominant failure mechanism in low stiffness (high porosity) matrices whereas matrix/fibre de-bonding was predominant in high stiffness (low porosity) matrices.