Precast Inc. - May/June 2008 - (Page 9) Synthetic fibers specifically designed for precast concrete at dosages typically above 4.5 pounds per cubic yard may require a mix design adjustment for the same reasons as for steel fibers. New synthetic fiber geometries developed specifically for precast concrete ensure an adequate distribution of fibers throughout a mix without any unusual batching or placement requirements. When synthetic fibers are added to a mix after initial batching, a full dispersion will normally occur within two to three minutes. Most synthetic fibers are inert and do not affect the water-cement ratio of a mix. However, they do affect the cohesiveness of plastic concrete, and a reduction in slump may occur. In these cases, a superplacticizer may be considered helpful. One of the advantages of synthetic fibers is that plastic concrete will tend to bleed less and more uniformly. Consequently, it is important that finishing not be started too early. Finishing should be performed with only steel or magnesium trowels and floats, since wood floats are more abrasive and may reveal excess fibers on the surface. Cellulose fibers provide a desirable balance between mechanical, physical and durability characteristics when placed in a cement matrix. Cellulose fibers processed for the reinforcement of concrete offer somewhat high levels of elastic modulus and bond strength. Cellulose fibers have small effective diameters ranging from 1 micron to 100 microns (a human hair is about 50 microns) that are comparable to the cement particle size, and therefore promote close packing and development of a dense, bulk interface microstructure in the matrix. The relatively high surface area and the close spacing of cellulose fibers, when combined with their desirable mechanical characteristics, make them quite effective in the containment and stabilization of microcracks in the concrete matrix. Cellulose fibers’ physical properties seem to vary widely depending on the Figure 1. Figure 2. authority quoted and the pulping techniques used. The use of cellulose fibers is considered an inexpensive alternative to synthetic fibers. A leading manufacturer of precast concrete products with hundreds of employees. We are looking for great people with outstanding people skills in all job areas. Experience in Precast or concrete construction preferred in our manufacturing jobs. We are recruiting in California, Nevada, and Arizona. Please visit our website at: www.jensenprecast.com or phone our HR Department at (775) 352-2700. We are an equal opportunity employer. Conclusion Fiber reinforcement can be a valuable enhancement to precast concrete products by improving the toughness and impermeability of the finished product. Fibers can also be employed to permit earlier handling of precast products to reduce production cycle times. Enhancements through use of fibers can prove to be valuable for the continued promotion of concrete as a versatile and inherently durable building material. www.jensenprecast.com An Equal Opportunity Employer MAY/JUNE 2008 | WWW.PRECAST.ORG 9 http://www.jensenprecast.com http://www.jensenprecast.com http://www.precast.org
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