The period between 1971 and now has been a time of learning about steel fiber reinforced concrete. One of the primary problems has been failure to recognize that the mixing practices must be even more carefully controlled than is necessary in producing conventional concrete. Varying water content is probably the biggest offender. The presence of the steel fibers stiffens the mix substantially, and there is a consequent tendency, regardless of specifications, to employ high water-to-cement rations to achieve the easily worked mixes preferred by finishers. The resulting concrete may develop 300 to 800 psi less ultimate flexural strength than the material is actually capable of achieving. Although the thesis that the fibers within the matrix must be spaced less than .5 inch apart on the average has been corroborated in laboratories and in the field, work at many different points has proven that the bond area of the fibers is also important. Increasing the effective bond area of the fibers is also important. Increasing the effective bond area of the fibers, keeping the length to diameter ratio constant, gives better strength, whereas merely changing the cross-sectional shape from round to square or to rectangular does not accomplish this objective. Deformation of the fibers also increases mechanical bonding, making it more difficult to pull the fibers from the concrete matrix. The major mixing problem is "balling" of the steel fiber, but this is becoming less importance as mixing techniques are improved. Special feeders have been designed to feed the fibers into the mixing chamber of mixing trucks.
A Progress Report on Concrete Reinforced with Steel Fibers
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