360R-06 Design of Slabs-on-Ground
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| 10.3.2.1 Random crack control—Steel fibers are
commonly used for random crack control. As in the case with conventional reinforcement, the fibers do not prevent cracking, but serve to hold cracks tight such that the slab performs as intended during its service life. The degree of random crack control by the fibers is directly related to the fiber type and quantity. 10.3.2.2 Crack width opening—As with conventional reinforcement, steel fibers at volumes of 0.25 to 0.5% (33 to 66 lb/yd 3 [20 to 39 kg/m 3 ]) can increase the number of cracks, and thus reduce the average crack widths. Steel FRC, when used in combination with conventional deformed or smooth continuous reinforcement, will have synergistic effects, and can be designed to share the applied tensile forces with the continuous reinforcement, thereby adding to the crack width opening control. The degree of crack width control is directly related to the fiber type and quantity. 10.3.2.3 Flexural toughness (ductility)—Flexural toughness of steel FRC is determined by testing beams in a laboratory using JSCE SF4 or ASTM C 1399. It is generally accepted that the presence of steel fibers in quantities < 0.5% by volume, as would be expected in most slabs-on-ground, will not affect the first crack strength (modulus of rupture) of concrete. Steel fibers, however, greatly affect the deformation characteristics of a beam after first crack. Toughness is a measure of the post-cracking energy-absorbing capacity of steel FRC, and is defined as the area under the test beam load-deflection curve. Residual strength factors R e,3 and average residual strength (ARS), determined according to JSCE SF4 and ASTM C 1399, respectively, are used in slab- on-ground design. These factors represent an average value of load-carrying capacity of the test beam over a deflection interval. ARS is reported in psi (MPa) and represents a portion of the modulus of rupture. R e,3 is reported as a percentage of the modulus of rupture. Further discussions of these test methods can be found in ACI 544.2R, ASTM STP 169C, and ACI SP-155 (Stevens et al. 1995). The residual strength factor R e,3 will be used in this document to represent the postcrack characteristics of steel FRC. The degree of flexural toughness is directly related to the mixture proportion and all mixture constituents, including fiber type and quantity. tải về 2.35 Mb. Chia sẻ với bạn bè của bạn:
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