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The role of the fiber reinforcement in concrete is not to increase strength, although modest strength increases may occur. Rather, the role of discrete, discontinuous, randomly oriented fibers is to bridge the cracks that develop in concrete either as it is subjected to environmental changes (such as drying) or as it is loaded. If the fibers are strong enough, stiff enough, and present in sufficient quantity and develop sufficient bonds with the cementitious matrix, they will serve to keep the crack widths small and will permit the FRC to withstand significant stresses over a relatively large strain capacity in the post-cracking (or strain-softening) stage. 

In other words, by bridging the cracks that develop, the fibers can provide a considerable amount of post-cracking ductility. This is often referred to as toughness. The toughness may be defined in terms of the area under the load vs. the deflection curve. The stress is transferred from the matrix to the fibers in two ways:

  • At early stages of loading, the stress is transmitted from the matrix to the fibers through an elastic shear-transfer mechanism, which, however, has only a small effect on the limit of proportionality or first-crack stress of the FRC.

  • After the initial cracking, and at more advanced stages of loading, debonding along the fiber–matrix interface occurs, and frictional slip becomes the process controlling the stress transfer.

The degree of frictional slip affects both the ultimate strength and strain of the FRC. At this stage, the fibers may increase the strength of the FRC by transferring loads and stresses across the cracked matrix. In addition, and more importantly, they may increase the toughness of the FRC by providing energy absorption mechanisms related to the pullout processes of the fibers.