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Cycles of wetting and drying can change the microstructure of vegetable fibers through a mechanism known as hornification, which modifies the polymeric structure of the fiber-cells resulting in a higher dimensional stability. In the present work the influence of hornification on the sisal fiber-matrix bond adhesion as well as in the sisal fiber dimensional stability and mechanical behaviour under direct tension was evaluated. Furthermore, cementitious composites reinforced with randomly dispersed hornified sisal fibers were developed and characterized under bending loads. The results show that the tensile strength and strain at failure of the hornified sisal fibers were increased by about 5% and 39%, respectively, whereas the modulus of elasticity was reduced by 9%. The fibers also presented higher dimensional stability with the hornification process. The fiber-matrix bonding was improved and the pull-out resistance of the fibers submitted to ten cycles of wetting and drying was increased by about 40% to 50%. The higher fiber-matrix bond strength contributed to an increase in the ductility and post-cracking behaviour of the composite. The fracture process was characterized by the formation of multiple cracks with the hornified sisal fibers presenting a higher ability to bridge and arrest the cracks.

To design building elements using sisal fibre reinforced mortar composites, the stress-strain curves of the composites both under tensile and compression load is needed. In this study short sisal fibre-cement based composites were developed and their stress-strain behaviour under compression characterized experimentally. The composites consisted of two mortar matrices, one self-compacting and one of normal consistency, reinforced with randomly distributed short sisal fibre (25 and 50 mm long) in volume fractions ranging from 2% to 6%. Based on the experimental results a compressive constitutive law for the composites was proposed based on the damage theory developed by Mazars (1986). This theory was used to model the ascending branch of the stress-strain curve and a damage parameter associated to the fibre-reinforcing index is proposed to allow the modelling of the post-peak behaviour of the composites. The modified model was then validated using results available in the literature. The experimental results obtained in the study indicated that the addition of short sisal fibres to cement matrices tends to reduce its elastic modulus, peak stress and strain and to increase its toughness. However, the use of a self-compacting matrix allowed better sisal fiber dispersion and composites with superior performance were obtained. The modified analytical model was able to predict with good accuracy the ascending and descending branch of the stress-strain curves of the sisal fiber-mortar composites and allowed evaluating the effect of fibre reinforcing index on material damage. In the ascending branch, an increase in the damage from 40% to 70% is recorded for fiber volume ranging from 2 to 6%. In the descending branch, on the other hand, the variation of fiber volume allowed a reduction of the damage from 65% to 60%.

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