Q.: Over the years I have observed a trend of lower compressive strength results when fibers are used in concrete mixes. I have suspected that additional water demand for workability is the main reason for this. What have you experienced in your strength results when using fibers in comparable concrete mixes?
A.: Don Smith, R&D manager for SI Concrete Systems, says "You are exactly on the right track with your thinking. Actually fiber-reinforced concrete does not have worse workability (at normal doses of fiber) than plain concrete, but it does appear stiffer to the eye or in a slump cone. Therefore the normal finisher will add water to compensate. This is not a suggested practice and will lead to lower compressive strength. If finishers would go ahead and start placing the concrete, they would see that it is not actually stiffer and harder to work but simply more cohesive. This is one way that fibers improve concrete. The fiber alone does not have any effect on compressive strength unless the water is adjusted."
Others, however, say that fibers do reduce workability. "One way to overcome this problem is reduce the amount of fibers added instead of 11/2 pounds/cubic yard (which the suppliers would like you to use) reduce it to 1 pound/cubic yard. You will find there is a trade-off of the benefit of the extra 1/2 pound of fiber and the amount of water added. Your finishers will be a lot happier, and your strengths will rise."
Some fiber companies have already addressed the workability problem by developing fibers that are more user-friendly by being thinner, changing the physical properties, reducing the dosage, or a combination of these. The new fibers are more "transparent" to the finisher.Some have noticed an increased chance of blisters and delamination when fibers are in the mix, even on bull floated or Fresno floated slabs. This may be because the fibers slow the bleeding of the concrete. Finishers should be cautioned about sealing the surface before all of the bleed water has risen.
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Aggregate Research Industries Web site (www.aggregateresearch.com) includes topic-specific forums where forum members can pose questions and respond, creating an interactive discussion group. The question and answer in this Problem Clinic entitled "Lower strength with fibers?" was based on ARI concrete construction forum postings.