Short, small-diameter steel fibers can improve the engineering properties of concrete notably flexural load-carrying ability, fatigue resistance and impact strength. Yet there are several disadvantages with this type of fiber. The first is that they have a relatively low aspect ratio, which is a measure of the bond area of a given volume of fiber. Because of their relatively low aspect ratio (typically less than 60) and because of their smooth, straight form, this type of fiber is relatively inefficient. Under the conditions just described, balling or clumping together of the fibers can occur.
Also available are packets of collated, larger-diameter fiber; the low aspect ratio provides for good handling and mixing characteristics and eliminate the balling problem. The high aspect ratio, coupled with mechanical deformations (hooked ends), means that concrete properties can be improved to desired levels even though fiber additions are less than 100 pounds per cubic yard. Experience with these fibers has shown that fiber stiffness and section properties contribute substantially to the engineering performance of steel-fiber-reinforced concrete.
More recently, a large-diameter, stiff steel fiber with lengths up to 3.0 inches has become available. This deformed fiber has a crescent-shaped cross section with an effective diameter in the range of 0.030 to 0.035 inch. They are easily mixed in concrete with no need to initially separate or screen them and with no balling problems. The corrugated shape improves bond between fibers and the cement matrix. The coupling of these two features provides high reinforcement efficiency. This type of fiber has been used in applications which include cast-in-place concrete, shotcrete and precast concrete. The concretes have been used in shotcrete domes, modular housing, floor slabs, underground vaults, dam repairs, driveways and castable refractories.
