Effect of fiber modulus of elasticity on the long term properties of micro-fiber reinforced cementitious composites

Abstract

Fibers for reinforcing cementitious composites are typically short and randomly dispersed in the matrix. Consequently, most of the fibers are inclined to the cracks that develop in the cement matrix and suffer from bending stress as these cracks open. For brittle fibers, such as carbon fibers, the bending stress may lead to flexural fiber rupture before the fiber attains its full capacity in direct tension. As a result, the efficiency of these fibers may be reduced. This phenomenon is not expected to occur in ductile fibers, which can yield locally rather than rupture. Predictions of a theoretical model show that the bending stress increases as the matrix becomes denser and suffer (an event which occurs as the matrix ages or due to the addition of silica-fume) and decreases for fibers of lower modulus of elasticity. Therefore, a reduction in strength with time in composites with dense matrices is expected for very brittle fibers of high modulus, moderate or no reduction for low modulus brittle fibers, and no reduction in strength is expected for ductile fibers. The long term properties of cementitious composites reinforced with various microfibers was studied to validate the model; PAN and Pitch type carbon fibers represented brittle fibers of high and low modulus, respectively; polypropylene and polyacrylonitrile fibers represented ductile fibers. The results showed good agreement with the theoretical model.