Chap7 - HOME PAGE CHAPTER 7 Fibers Fibers have been used in...

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Fibers have been used in construction materials for manycenturies. The last three decades have seen a growinginterest in the use of fibers in ready-mixed concrete, pre-cast concrete, and shotcrete. Fibers made from steel,plastic, glass, and natural materials (such as wood cellu-lose) are available in a variety of shapes, sizes, and thick-nesses; they may be round, flat, crimped, and deformedwith typical lengths of 6 mm to 150 mm (0.25 in. to 6 in.)and thicknesses ranging from 0.005 mm to 0.75 mm(0.0002 in. to 0.03 in.) (Fig. 7-1). They are added to concreteduring mixing. The main factors that control the perform-ance of the composite material are:1.Physical properties of fibers and matrix2.Strength of bond between fibers and matrixAlthough the basic governing principles are the same,there are several characteristic differences between con-ventional reinforcement and fiber systems:1. Fibers are generally distributed throughout a givencross section whereas reinforcing bars or wires areplaced only where required2.Most fibers are relatively short and closely spaced ascompared with continuous reinforcing bars or wires3.It is generally not possible to achieve the same area ofreinforcement to area of concrete using fibers as com-pared to using a network of reinforcing bars or wiresFibers are typically added to concrete in low volumedosages (often less than 1%), and have been shown to beeffective in reducing plastic shrinkage cracking.Fibers typically do not significantly alter freeshrinkage of concrete, however at high enough dosagesthey can increase the resistance to cracking and decreasecrack width (Shah, Weiss, and Yang 1998).ADVANTAGES AND DISADVANTAGES OF USING FIBERSFibers are generally distributed throughout the concretecross section. Therefore, many fibers are inefficientlylocated for resisting tensile stresses resulting from appliedloads. Depending on fabrication method, random orienta-tion of fibers may be either two-dimensional (2-D) orthree-dimensional (3-D). Typically, the spray-up fabrica-tion method has a 2-D random fiber orientation where asthe premix (or batch) fabrication method typically has a3-D random fiber orientation. Also, many fibers areobserved to extend across cracks at angles other than 90ºor may have less than the required embedment length fordevelopment of adequate bond. Therefore, only a smallpercentage of the fiber content may be efficient in resistingtensile or flexural stresses. “Efficiency factors” can be aslow as 0.4 for 2-D random orientation and 0.25 for 3-Drandom orientation. The efficiency factor depends on fiberlength and critical embedment length. From a conceptualpoint of view, reinforcing with fibers is not a highly effi-cient method of obtaining composite strength.

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