15364 Mechanical Properties 153641 Tension

Usually, there are two categories of FRCs according to their tensile response, strain softening, and strain hardening. The strain softening type of fiber-reinforced cement-based composites is usually reinforced with a low volume of short fibers. This kind of composites containing about 1% fiber is typically used for bulk field applications involving a massive volume of concrete. Figure 15.9 illustrates the tensile response of such low volume composites. It shows stress deformation curves for a plain concrete specimen, steel, and a polypropylene fiber reinforced concrete specimen. The length, diameter, and volume fraction of steel fiber are 48 mm, 0.5 mm and 0.5%, respectively, whereas the length and volume fraction of fibrillated polypropylene fiber are 50 mm and 0.5%, respectively. It can be seen from the figure that although all the specimens display a softening behavior after peak load, the area under the load deformation curve is different and it is larger for fiber-reinforced specimens than that for plain concrete specimens. This implies that the fiber-reinforced specimens require more energy to fracture than the plain concrete specimens.

For a strain softening type of failure, the fracture process of the material can be divided into three stages, namely, randomly distributed damage during the loading stage to about 80% of the prepeak load, microcrack localization during the loading period between 80% of prepeak load and 80% of postpeak load, and the major crack propagation during the period after 80% of postpeak load.

A failure mode of multiple matrix cracking can be obtained when the quasibrittle matrix is reinforced with either a high volume fraction ofshort fibers or with aligned continuous fibers. Such high volume fiber composites have been used in thin sheets (e.g., curtain walls). A typical strain-hardening curve for steel-fiber-reinforced cementitious composites is shown in Figure 15.10. There is a special point (marked B in the curve) called the "bend over point'' (BOP) at which the matrix's contribution to the tension capacity reaches a maximum. The stress-strain curve shown in the figure can be roughly divided into four stages:

• Stage 1 is characterized by the elastic behavior of the composite before the initiation of the first matrix cracks. In this stage, a few microcracks are initiated and are randomly dispersed.

• Stage 2 is defined as the period from the initiation to the complete propagation of the first major crack across the specimen's cross-section. The BOP corresponds to the end of stage 2.

• Stage 3 can be called multiple cracking stage. In this stage, the incremental loading of the fibers (at the position of the crack) is transferred back to the matrix through the bond and this process builds up the tensile stresses in the matrix. This can lead to the next matrix crack when the stress

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