EXPERIMENTAL RESULTS AND DISCUSSION Figure 2 shows the load-deflection curves for concrete with different types of fibers. A significant difference in performance between steel and polypropylene fibers is found in the static flexural test. The flexural toughness is defined as the potential to absorb the energy with cracking. The area enclosed by load-deflection curve shows the flexural toughness. Japan Society of Civil Engineers recommends to use the flexural toughness factor σb obtained by Eqn. 1. σ δ b b tb T l bh = 2 (1) where, δtb: 1/150 of span length, Tb: the area enclosed by load-deflection curves within the deflection equal to δtb, l: length of span, b: width of beam, h: height of beam. 0.0 20.0 40.0 60.0 80.0 Load - kN 0.0 1.0 2.0 3.0 4.0 5.0 6.0 Deflection - mm Steel fiber 150 mm 150 150 150 Polypropylene fiber T b ○○ ○ ○ ○ ○ ○ ○ □ □ □ □ □□ Figure 2: Load-Deflection curves for plain concrete reinforced with different type of fibers The flexural toughness factors σb of concrete with polypropylene and steel fibers are 1.95 N/mm2 and 4.36 N/mm2, respectively. When the flexural toughness factor is used, the flexural toughness of concrete with polypropylene fiber is estimated as half as that with steel fiber. The load deflection curves of the two types of beam are shown in Figures 3 and 4. The results shown in Figure 3 and 4 were obtained from the beam with 2 stirrups and 4 stirrups, respectively. Irrespective of the number of stirrups, the largest increase in toughness was obtained when the concrete beams were reinforced with steel fibers followed by those reinforced with polypropylene fibers. It is clear that the flexural toughness factors σb can follow the toughness of concrete beams at failure. Figures 5 and 6 show the number of cracks with load cycles. In Figure 5, the number of cracks of concrete with steel fibers is compared with that of control. It is noticed that, steel fibers have excellent performance in resisting crack initiation and propagation; thus; the ability of resistance to cyclic loading is increased greatly. In Figure 6, the number of cracks of concrete with polypropylene fibers is compared with that of control. The number of cracks of control specimen before the applying of cyclic load is more than that of specimen with fibers. The number of cracks of control specimen was increased rapidly after the applying of cyclic load and 0.0 20.0 40.0 60.0 80.0 Load - kN 0 25 50 75 100 Deflection - mm Shear failure Control Shear failure Polypropylene fiber Steel fiber 1,500 mm Figure 3: Load-Deflection curves for RC beam reinforced with different type of fibers 0.0 20.0 40.0 60.0 80.0 Load - kN 0 25 50 75 100 Deflection - mm Control Polypropylene fiber Steel fiber 1,500 mm Figure 4: Load-Deflection curves for RC beam reinforced with different type of fibers
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