tesi A. Caggiano.pdf - EleA@UniSA - Università degli Studi di Salerno

Chapter 1. Introduction1.1.1 Fiber types: material and geometryFiber Reinforced Cementitious Composite (FRCC) is a concrete made of hydraulic cement,aggregates and discrete reinforcing fibers. There are several fiber types employedfor engineering applications and classified according to their material. Particularly thefollowing categories can be listed:• steel fibers of different shapes and dimensions;• glass fibers;• fibers of synthetic materials such as polypropylene, polyethylene and polyolefin,polyvinyl alcohol (PVA);• recycled ones (PET, from waste tires);• natural fibers.Referring to its geometry, fiber is defined as “short” and “discrete”. One of the proprietieswhich characterizes its geometry is represented by the “aspect ratio”: it represents theratio between the fiber length and the equivalent diameter. The latter is calculated asthe diameter of a circle having an area equal to the cross-section of the consideredfiber.The most important short reinforcements, for structural concrete, are surely representedby the steel fibers. The length and diameter of such fibers, used for FRCC, dotypically not exceed the 75mm and 1mm, respectively. As example the Figure 1.1outlines various steel fiber geometries characterized by different shapes for improvingthe fiber-matrix bond and aimed at increasing the efficiency of the bridging effect offibers.Recent researches on Fiber-Reinforced Concrete (FRC) also address to the possibleuse of mixed fibers of different material and/or geometry which can, in principle,play a synergistic role in enhancing flexural and post-cracking response of FRC members.This kind of fiber-reinforced cement-based composites are known as HybridFRC (HyFRC). Experimental tests aimed at investigating the HyFRC failure behaviorin direct tension have been performed, among others, by Sorelli et al. [2005] and Parket al. [2012]. The mechanical behavior measured by means of indirect tensile tests havebeen proposed on Hy-Polypropylene FRC [Hsie et al., 2008], Hy-Steel FRC [Banthia andSappakittipakorn, 2007, Kim et al., 2011] or combining several material fibers: i.e., Carbon/Steel/PolypropyleneFRC [Yao et al., 2003] or Steel/Palm/Synthetic FRC [Dawood2