Using Fibers for Plastic Shrinkage Control

Question:
What are the benefits of adding fibers to concrete countertops?

Answer:
Fibers are used in concrete for a variety of reasons, but not all fibers do the same thing or have the same effect on concrete. Factors that can influence their effectiveness include the fiber size, shape and material and the amount of fibers used. Using the wrong fiber type or not enough fibers can result in disappointment or failure.

Fibers are generally added to concrete as secondary reinforcement for shrinkage control (versus primary reinforcement, which is structural in nature). Shrinkage occurs as the concrete sets and transforms from a workable paste into a hard, solid mass. This is especially true in concrete slabs exposed to heat or wind. The matrix of fibers helps to stabilize the wet concrete and distribute the shrinkage stresses so that large cracks are minimized or eliminated.

Fibers are often advertised as capable of replacing welded wire mesh. This is true, but only when the wire mesh is used for plastic shrinkage control. The confusion stems from the fact that welded wire mesh can also be used as structural (primary) reinforcement, while synthetic fibers cannot.

The most commonly used fibers are synthetic (polypropylene or nylon), but some are made of natural materials, such as cellulose fibers. None of these fiber materials is stiff or strong enough to provide significant tensile reinforcement to uncracked concrete. And if the concrete cracks, they simply stretch too much to do any good. After the concrete hardens, these fibers don’t contribute anything to resisting external structural tensile stresses. There is a class of fibers, however, that provides some resistance to external structural tensile stresses. See the article “Structural Fibers Can Minimize Crack Appearance” for more information about these fibers.