Damage Evolution of Fiber/Mortar Interface During Fiber Pullout
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about 10 to 20 gm in thickness. The remaining area scattered among the dense CH crystals is a porous zone that contains some C-S-H gel and possibly some ettringite particles (AFt: calcium aluminum trisulfate). Around the thick CH layer is a distinct porous layer of C-S-H gel, and only beyond that porous layer is the bulk cement paste microstructure observed. The region surrounding the fiber thus contains a very porous and weak layer parallel to the fiber at a distance of at least 10 gm away from the fiber surface. Although the microstructure of the fiber-mortar interface has been reasonably understood, the microstructural change of interface during the fiber pullout process, such as interfacial debonding and frictional wear are known in considerably less detail. To the authors' knowledge, only Pinchin and Tabor [6] attempted to explain the decrease in interfacial friction of steel fiber-mortar during the pullout process based on the surface compaction of hydrated cement paste observed by Soroka and Sereda [7]. Pinchin and Tabor attribute the significant friction decrease during a small amount of steel fiber pullout to densification or compaction, but not to wear, on the mortar surface. They argued that the compaction in their pullout test occurs on a very fine scale in the order of 0.1-0.3 gtm near the embedded steel fiber and is difficult to detect. Since their conclusions are based on the final stage of the pullout test, i.e., at total fiber pullout, it does not reflect the whole pullout process. Polymeric Fiber-Concrete Interface Polymeric fibers have high yield strength, but unlike steel fiber, they have lower elastic moduli and transverse strengths than cementitious materials. Therefore, the interfacial damage mechanism of polymeric fiber-mortar is different from that of steel fiber-mortar. Polymeric fibers also have less corrosion in harsh environments, but high creep effect. Based on these properties, polymeric fibers have been used to reinforce FRC in the early stage when the matrix is weak, and of low modulus, or to resist impact load and corrosion. Polypropylene fiber was the first polymeric fiber applied to concrete in forms of monofilament or fibrillated film. Baggott and Gandhi [8] studied continuous monofilament polypropylene fiber (340 gim) reinforced cement beam under tensile load. They observed defects of up to 10 gim on the polypropylene fiber interface. One typical damage observed was the chiseling out of a long shaving of fiber by matrix particles (Fig. 2). The application and study of nylon FRC so far are not as extensive as polypropylene FRC although nylon fiber exhibits good toughness and durability. Wang, et al. [3] investigated the nylon-cement interface and observed peeling and fibrillation at the fiber surface (Fig. 3). They concluded that the increased interfacial friction during fiber pullout is due to the increase in surface abrasion. In this study, the interfacial microstructures and the damage evolution of steel, nylon and polypropylene FRC's are investigated microscopically with sc
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