Fiber-Reinforced Concrete Slab

By Dan Martel, P.E. | February 4, 2021

Recently we have been asked a lot about fiber reinforcing, and if it should be used in concrete slabs-on-grade. Fiber-reinforced concrete, or FRC, is a common building structural element, and it consists of cement, sand and aggregate, and very small fibers. The fibers can be made from steel, glass, synthetic/polymer materials, or natural materials like jute. Typically, FRC is specified to control tiny cracking that naturally occurs when concrete cures and experiences freeze/thaw cycles. In some cases, it can be used in lieu of steel reinforcing welded wire fabric or bars.

Without much experience specifying fiber reinforcing, we did some research and discovered:

  • Fiber reinforcing can improve long-term serviceability of the concrete.
  • Fiber reinforcing can increase residual strength of concrete, but does not significantly improve compressive strength, the most important property in concrete design.
  • Fiber reinforcing can improve resistance to impact, toughness (durability), and fire resistance.
  • Steel fibers have the greatest effect on crack control. But apparently concrete with steel fibers is much more difficult to work during placement. Typically, additives like high-range water reducers are required to increase the slump and make the wet concrete more workable.
  • When specifying, good experience with the materials is needed. The concrete supplier should be consulted, and mix designs approved by the engineer, before FRC is committed to for a project.

Slabs on grade typically require only minimal reinforcing in the form of small reinforcing bars or welded wire reinforcement to control cracking. For residences, the IRC does not require any reinforcing at all in slabs on grade. We conclude that when reinforcing is provided for crack control purposes and is not required for the strength of the concrete, fiber reinforcing can be used in lieu of steel reinforcing for slabs on grade. We have updated our standard details to reflect this, and it has been used recently on buildings we have designed.

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