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ACI 544.4R
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ACI 544.4R 88th Edition, January 1, 1988 Design Considerations for Steel Fiber Reinforced Concrete
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Description / Abstract:
INTRODUCTION
Steel fiber reinforced concrete (SFRC) and mortar made with hydraulic cements and containing fine or fine and coarse aggregates along with discontinuous discrete steel fibers are considered in this report. These materials are routinely used in only a few types of applications at present (1988), but ACI Committee 544 believes that many other applications will be developed once engineers become aware of the beneficial properties of the material and have access to appropriate design procedures. The contents of this report reflect the experience of the committee with design procedures now in use.
The concrete used in the mixture is of a usual type, although the proportions should be varied to obtain good workability and take full advantage of the fibers. This may require limiting the aggregate size, optimizing the gradation, increasing the cement content, and perhaps adding fly ash or other admixtures to improve workability. The fibers may take many shapes. Their cross sections include circular, rectangular, half-round, and irregular or varying cross sections. They may be straight or bent, and come in various lengths. A convenient numerical parameter called the aspect ratio is used to describe the geometry. This ratio is the fiber length divided by the diameter. If the cross section is not round, then the diameter of a circular section with the same area is used.
The designer may best view fiber reinforced concrete as a concrete with increased strain capacity, impact resistance, energy absorption, and tensile strength. However, the increase in these properties will vary from substantial to nil depending on the quantity and type of fibers used; in addition, the properties will not increase at the same rate as fibers are added.
Several approaches to designing members with steel fiber reinforced concrete (SFRC) are available that are based on conventional design methods supplemented by special procedures for the fiber contribution. These methods generally modify the internal forces in the member to account for the additional tension from the fibers. When supported by full-scale test data, these approaches can provide satisfactory designs. The major differences in the proposed methods are in the determination of the magnitude of the tensile stress increase due to the fibers and in the manner in which the total force is calculated. Other approaches that have been used are often empirical, and they may apply only in certain cases where limited supporting test data have been obtained. They should be used with caution in new applications, only after adequate investigation.
Generally, for structural applications, steel fibers should be used in a role supplementary to reinforcing bars. Steel fibers can reliably inhibit cracking and improve resistance to material deterioration as a result of fatigue, impact, and shrinkage, or thermal stresses. A conservative but justifiable approach in structural members where flexural or tensile loads occur, such as in beams, columns, or elevated slabs (i.e., roofs, floors, or slabs not on grade), is that reinforcing bars must be used to support the total tensile load. This is because the variability of fiber distribution may be such that low fiber content in critical areas could lead to unacceptable reduction in strength.
In applications where the presence of continuous reinforcement is not essential to the safety and integrity of the structure, e.g., floors on grade, pavements, overlays, and shotcrete linings, the improvements in flexural strength, impact resistance, and fatigue performance associated with the fibers can be used to reduce section thickness, improve performance, or both.
ACI 318 does not provide for use of the additional tensile strength of the concrete in building design and, therefore, the design of reinforcement must follow the usual procedure. Other applications provide more freedom to take full advantage of the improved properties of SFRC.
There are some applications where steel fibers have been used without bars to carry flexural loads. These have been short-span elevated slabs, e.g., a parking garage at Heathrow Airport with slabs 3 ft-6 in. (1.07 m) square by 2l/2 in. (10 cm) thick, supported on four sides (Anonymous 1971). In such cases, the reliability of the members should be demonstrated by full-scale load tests, and the fabrication should employ rigid quality control.
Some full-scale tests have shown that steel fibers are effective in supplementing or replacing the stirrups in beams (Williamson 1978; Craig 1983; Sharma 1986). Although it is not an accepted practice at present, other full-scale tests have shown that steel fibers in combination with reinforcing bars can increase the moment capacity of reinforced concrete beams (Henager and Doherty 1976; Henager 1977a).
Steel fibers can also provide an adequate internal restraining mechanism when shrinkage-compensating cements are used, so that the concrete system will perform its crack control function even when restraint from conventional reinforcement is not provided. Fibers and shrinkage-compensating cements are not only compatible, but complement each other when used in combination (Paul et al. 1981). Guidance concerning shrinkage-compensating cement is available in ACI 223.1R.
ASTM A 820 covers steel fibers for use in fiber reinforced concrete. The design procedures discussed in this report are based on fibers meeting that specification.
Additional sources of information on design are available in a selected bibliography prepared by Hoff (1976-l982), in ACI publications SP-44 (1974) and SP- 81 (1984), in proceedings of the 1985 U.S.-Sweden joint seminar edited by Shah and Skarendahl (1986), and the recent ACI publication SP-105 edited by Shah and Batson (1987).
For guidance regarding proportioning, mixing, placing, finishing, and testing for workability of steel fiber reinforced concrete, the designer should refer to ACI 544.3R.
Steel fiber reinforced concrete (SFRC) and mortar made with hydraulic cements and containing fine or fine and coarse aggregates along with discontinuous discrete steel fibers are considered in this report. These materials are routinely used in only a few types of applications at present (1988), but ACI Committee 544 believes that many other applications will be developed once engineers become aware of the beneficial properties of the material and have access to appropriate design procedures. The contents of this report reflect the experience of the committee with design procedures now in use.
The concrete used in the mixture is of a usual type, although the proportions should be varied to obtain good workability and take full advantage of the fibers. This may require limiting the aggregate size, optimizing the gradation, increasing the cement content, and perhaps adding fly ash or other admixtures to improve workability. The fibers may take many shapes. Their cross sections include circular, rectangular, half-round, and irregular or varying cross sections. They may be straight or bent, and come in various lengths. A convenient numerical parameter called the aspect ratio is used to describe the geometry. This ratio is the fiber length divided by the diameter. If the cross section is not round, then the diameter of a circular section with the same area is used.
The designer may best view fiber reinforced concrete as a concrete with increased strain capacity, impact resistance, energy absorption, and tensile strength. However, the increase in these properties will vary from substantial to nil depending on the quantity and type of fibers used; in addition, the properties will not increase at the same rate as fibers are added.
Several approaches to designing members with steel fiber reinforced concrete (SFRC) are available that are based on conventional design methods supplemented by special procedures for the fiber contribution. These methods generally modify the internal forces in the member to account for the additional tension from the fibers. When supported by full-scale test data, these approaches can provide satisfactory designs. The major differences in the proposed methods are in the determination of the magnitude of the tensile stress increase due to the fibers and in the manner in which the total force is calculated. Other approaches that have been used are often empirical, and they may apply only in certain cases where limited supporting test data have been obtained. They should be used with caution in new applications, only after adequate investigation.
Generally, for structural applications, steel fibers should be used in a role supplementary to reinforcing bars. Steel fibers can reliably inhibit cracking and improve resistance to material deterioration as a result of fatigue, impact, and shrinkage, or thermal stresses. A conservative but justifiable approach in structural members where flexural or tensile loads occur, such as in beams, columns, or elevated slabs (i.e., roofs, floors, or slabs not on grade), is that reinforcing bars must be used to support the total tensile load. This is because the variability of fiber distribution may be such that low fiber content in critical areas could lead to unacceptable reduction in strength.
In applications where the presence of continuous reinforcement is not essential to the safety and integrity of the structure, e.g., floors on grade, pavements, overlays, and shotcrete linings, the improvements in flexural strength, impact resistance, and fatigue performance associated with the fibers can be used to reduce section thickness, improve performance, or both.
ACI 318 does not provide for use of the additional tensile strength of the concrete in building design and, therefore, the design of reinforcement must follow the usual procedure. Other applications provide more freedom to take full advantage of the improved properties of SFRC.
There are some applications where steel fibers have been used without bars to carry flexural loads. These have been short-span elevated slabs, e.g., a parking garage at Heathrow Airport with slabs 3 ft-6 in. (1.07 m) square by 2l/2 in. (10 cm) thick, supported on four sides (Anonymous 1971). In such cases, the reliability of the members should be demonstrated by full-scale load tests, and the fabrication should employ rigid quality control.
Some full-scale tests have shown that steel fibers are effective in supplementing or replacing the stirrups in beams (Williamson 1978; Craig 1983; Sharma 1986). Although it is not an accepted practice at present, other full-scale tests have shown that steel fibers in combination with reinforcing bars can increase the moment capacity of reinforced concrete beams (Henager and Doherty 1976; Henager 1977a).
Steel fibers can also provide an adequate internal restraining mechanism when shrinkage-compensating cements are used, so that the concrete system will perform its crack control function even when restraint from conventional reinforcement is not provided. Fibers and shrinkage-compensating cements are not only compatible, but complement each other when used in combination (Paul et al. 1981). Guidance concerning shrinkage-compensating cement is available in ACI 223.1R.
ASTM A 820 covers steel fibers for use in fiber reinforced concrete. The design procedures discussed in this report are based on fibers meeting that specification.
Additional sources of information on design are available in a selected bibliography prepared by Hoff (1976-l982), in ACI publications SP-44 (1974) and SP- 81 (1984), in proceedings of the 1985 U.S.-Sweden joint seminar edited by Shah and Skarendahl (1986), and the recent ACI publication SP-105 edited by Shah and Batson (1987).
For guidance regarding proportioning, mixing, placing, finishing, and testing for workability of steel fiber reinforced concrete, the designer should refer to ACI 544.3R.