Mechanical behavior of cementitious composites reinforced with high volume content of fibers

Available techniques for manufacturing cement based composites were evaluated. Cementitious composites with high volume concentration of fibers were manufactured using high energy mixing and extrusion techniques. Mechanical properties were evaluated by means of flexural and tensile fracture tests. It is shown that short alumina and carbon fibers are able to stabilize the microcracking process and increase the strength of the composites. Polypropylene fibers were shown to increase the toughness of composites at low volume fractions and significantly increase the strength and toughness at high volume fractions. By combining short alumina and carbon fibers with polypropylene fibers, hybrid composites with high strength and toughness were developed. Fracture surfaces of the composites evaluated using scanning electron microscopy (SEM) reveal that the major mode of composite failure is due to fiber pullout. The fibers across the fracture surface are observed to follow a random 3D distribution.;The compliance calibration technique was used to derive the experimental R-Curves for the composites tested in flexure. The methodology proposed develops the interrelationship between the increased toughness of the composites and the growth of the matrix crack. Both the strengthening and toughening behavior of various single fiber type and hybrid composites can be characterized by this technique.;A fracture mechanics model for the fiber pullout problem was presented. The model was based on the R-Curve approach to formulate the stable debonding process. It was shown that the maximum load was achieved under partial debonding conditions. Results were compared to various experimental data and finite element predictions based on a Coulomb friction approach. This pullout model was used as the bridging force distributed across the faces of a propagating matrix crack. The stress intensity reduction due to the closing pressure distributions and the crack opening profiles were obtained by means of numerical solution of the resulting integral equations. Effects of various fiber and interface parameters such as stiffness, length, diameter, volume fraction, adhesional bond strength, and interfacial toughness on the strength of the composites are presented. Results were also compared to the finite element formulation based on the cohesive crack models.