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Q. How do fibers affect the workability of concrete? Do fibers require additional water for placement?
A. The addition of fibers to a concrete mix alters its rheology, often resulting in an apparent slump loss. Despite this, the energy required to consolidate and place fresh fiber-reinforced concrete (FRC) is not greater than for plain concrete. FRC mixtures typically appear more cohesive than plain concrete.
For fibers to be uniformly distributed throughout the concrete, they must be well-coated with paste. Uniform distribution is essential to achieve the desired mechanical properties of the fiber-reinforced concrete. At low to moderate fiber dosages, the use of water-reducing admixtures—such as mid-range or high-range water reducers—is generally sufficient to maintain the desired slump for placement without altering the water-cementitious materials ratio (w/cm). At moderate to high fiber dosages, an increase in paste content may be required to ensure proper coating and uniform distribution of fibers, which is critical for performance.
It is important to note that while fibers can influence the workability of concrete, they do not inherently require additional water for placement. Adjustments to the mix, such as adding water-reducing admixtures or increasing paste content, are designed to maintain workability without changing the w/cm ratio. The specific adjustments depend on the type, geometry, and dosage of fibers, as well as the requirements of the concrete mix design.
References: ACI 544.1R, ACI 544.3R
Topics in Concrete: Concrete Fundamentals, Fiber Reinforced Concrete