Answer: There are really two responses here. The first defines permeability, and the second determines how fibers affect permeability.
Permeability is defined as the property that governs the rate of fluid flow through a porous solid (Ref. 1). The porosity of cement paste determines the permeability of concrete (Ref. 2). The water-cement ratio is one of the largest contributors to porosity development, but, on the whole, porosity is determined through chemical interaction. Since the fibers themselves are chemically inert and have no interaction with cementitious reactions, it is not possible for the fibers to decrease the permeability of concrete.
However, this is not to say that fluid will not flow through the concrete at a different rate when fibers are present. In a simple sense, fibers are small, flexible aggregates. As such, they create a small matrix of aggregates within the larger concrete matrix. If moisture flows through concrete via the cement paste, then the fibers create a more tortuous path. The more tortuous the path, the longer it takes the water takes to traverse it.
The simple conclusion might be to assume that if you added fibers the water would take longer to pass through the concrete. However, if the cement porosity is high to begin with, the addition of fibers won't matter.
The best way to make concrete more impermeable is to decrease the cement paste porosity via a combination of chemical or mineral admixtures and maintain a low water-cement ratio.
- ASTM C 94, "Standard Specification for Ready-Mixed Concrete," ASTM, West Conshohocken, PA, Section 188.8.131.52, 2000.
- S. Kosmatka and W. Panarese, Design and Control of Concrete Mixtures, 13th ed., Portland Cement Association, 5240 Old Orchard Rd, Skokie, IL 6007-1083, page 29.