Precast Inc. - November/December 2008 - (Page 24) TECHNICAL Steel Fibers in SCC for Structural Applications A perspective of self-consolidating (SCC), steel fiber-reinforced concrete (SFRC) for precast structural applications. BY LIBERATO FERRARA AND SURENDRA P. SHAH Editor’s Note: This is a condensed version of the article by Ferrara and Shah. Please visit the Precast Inc. magazine archive at www.precast.org for the complete version. The online version includes additional information on optimum fiber grading and dispersion, electrical testing for fibers in SCSFRC, a more comprehensive list of achieving more cost-effective precast solutions and more. o date, steel fibers have been used predominantly to control shrinkage cracks and enhance durability in nonstructural concrete applications in pavement, floors, containers, tunnel linings, septic tanks, curtain walls and repair overlays. But what is the potential for steel fibers in structural precast concrete applications where fibers contribute to loadcarrying capacity? How does self-consolidating concrete (SCC) help distribute steel fibers throughout the concrete mix? In this article, researchers from Northwestern University provide answers to these questions and present case studies related to steel-fiber reinforced concrete (SFRC) using SCC. The precast concrete industry has been looking with increasing interest to the possible uses of SFRC technology, where the fibers may act as a partial or even total replacement of secondary reinforcement. Steel fibers, for example, could replace welded-wire meshes for shear and transverse flexure in prestressed roof elements. Recent studies demonstrate the effectiveness of this solution, and also led to reduced labor costs for reinforcement detailing and placing. T redundancy, may negate the benefits of the fibers. The advantage of adding steel fibers to SCC lies in the self placeability of SCC, which eliminates the need for compaction by vibration. SCC’s rheological stability in the fresh state produces a more uniform dispersion of fibers within structural elements with no downward settlement. Advantages of SCC SCC also provides precast concrete durability benefits. The compactness of the SCC matrix, due to the greater amount of fines, may be beneficial to structural durability. Penetration of water under pressure, chloride and CO2 ingress, and water absorption have been reported to be significantly reduced in comparison to conventional fiber-reinforced concrete. The synergy between SCC and fiber-reinforced technologies leads to an overall economic efficiency in the construction process: increased speed of construction; reduction of labor in casting, vibrating and finishing; decreased energy consumption; and reduced workplace noise. It also has been shown recently that, through a suitable balance of the fresh state properties of concrete (mainly the viscosity of the fluid mixture), fibers can be effectively oriented TABLE 1: SUMMARY OF TYPICAL SCFRC MIX PROPORTIONING Absolute volume of coarse aggregate Paste Fraction (calculated on volume) Mortar Fraction (calculated on volume) Typical w/cm Typical fiber content (volume fraction Vf ) 28% to 32% (total mix volume) 32% to 40% (total mix volume) 68% to 72% (total mix volume) 0.32 to 0.45 30 to 125 kg/m3 (0.3% to 1.6% by volume) Role of SCC in fiber dispersion In order to promote widespread use of SFRC for full loadbearing structures, it is critical to be able to effectively control the dispersion of fibers during the manufacturing and casting process. Uniform dispersion of fibers helps to ensure structural performance. Areas with little or no fibers may occur within a structural element. Spotty fiber dispersion is usually due to improper casting techniques. It is well known that fibers have negative effects on concrete workability, and compaction and vibration cause the fibers to settle downward. Poorly dispersed fibers may jeopardize the structural performance. From a building code perspective, a non-uniform dispersion of fibers may also result in non-uniform material properties and consequently compromise safety factors. These factors, even if partially compensated for by structure 24 NOVEMBER/DECEMBER 2008 | PRECAST INC. http://www.precast.org
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