Rethinking concrete reinforcement: smarter designs with FORTA fibers

How fiber geometry determines structural performance

Fiber reinforcement works through bond and interlock between the fiber and the surrounding concrete matrix. The geometry of the fiber determines both when it engages and how much force it can carry across a crack.

Twisted steel fibers, such as Helix micro-rebar, are one inch long with a diameter of 0.02 inches. Dylan Field explained that the twisted profile creates mechanical interlock at the fiber-matrix interface, allowing the fiber to engage at smaller crack widths than hooked-end steel fibers, which require the crack to widen enough to pull the hook straight before generating resistance. The average tensile strength of the Helix micro-rebar is 246 ksi, and the fiber is zinc-coated to provide corrosion resistance in aggressive environments.

The IAPMO evaluation report governs code compliance for structural applications and uses a direct tension testing methodology that validates fiber performance across a broader range of applications than flat-work flexural tests alone. An IAPMO-certified product datasheet gives the engineer a documented basis for specifying fibers in applications beyond standard slabs-on-grade.

Three fiber categories and when to use each

The session organized synthetic and steel fiber products into three practical categories defined by their structural function.

Micro fibers, typically polypropylene at around one pound per cubic yard, address plastic shrinkage cracking in residential and light commercial flat work. They are distributed throughout the mix at high dosages and bridge micro-cracks that form in the first hours after placement. Their contribution to hardened concrete strength is limited; the role is preventing surface crack initiation before the concrete sets.

Fibrillated network fibers at approximately 1.5 pounds per cubic yard form a three-dimensional reinforcement web in the mix. This category can replace welded wire mesh for temperature and shrinkage applications in slabs, offering more consistent distribution than mesh, which is prone to displacement during placement.

Macro structural fibers, including Helix micro-rebar at three to 7.5 pounds per cubic yard, provide post-crack residual tensile strength in hardened concrete. When a slab cracks under load, macro fibers bridge the crack and carry tensile stress across it. This is the category that permits the engineer to credit fiber reinforcement in structural calculations under ACI 318 and ACI 544.4R provisions, subject to ASTM C1609 performance verification.

Code compliance pathways: ICC and IAPMO evaluation reports

Two evaluation report frameworks support code-compliant fiber specification in the United States.

The ICC evaluation report focuses on modulus of rupture increases for flat work. It provides a pathway for substituting fiber reinforcement for minimum temperature and shrinkage steel in slabs under ACI 318-19 Section 26.4.1.5, when testing confirms the required residual strength. This is the most commonly used compliance route for slab-on-grade applications.

The IAPMO evaluation report uses direct tension testing methodology, which covers a broader range of applications including structural reinforcement and members loaded primarily in tension. Paige Beichner noted that engineers working outside standard horizontal flat work will generally need the IAPMO report to support their design basis, because the direct tension test captures performance in loading conditions that the modulus of rupture test does not reflect.

Both evaluation reports reference specific fiber products. Specifying a performance value, such as a minimum residual strength at a prescribed deflection from ASTM C1609, rather than specifying by product name alone, gives contractors flexibility to bid alternative products while ensuring the structural requirement is verifiable.

Contractor benefits and project examples

The most consistent labor benefit cited in the session was approximately one day saved per 10,000 square feet of industrial slab by eliminating welded wire mesh handling and placement. Dylan Field noted that for large warehouse floors, this schedule saving can be significant relative to total concrete placement time.

Project applications covered in the session included elevator decks in a Detroit high-rise building, warehouse slabs in North Carolina, and agricultural irrigation channels. The elevator deck application illustrated how steel micro-rebar can be used in confined concrete placements where mesh placement and consolidation are difficult. The agricultural channel application demonstrated durability performance in an environment with continuous moisture exposure.

Fibers are preferably introduced at the batch plant during the mixing cycle rather than added on-site at the truck. Batch plant addition ensures complete mixing and reduces the risk of fiber balling or uneven distribution. Surface finishing timing is more critical with fiber mixes: troweling too early can bring surface fibers to the top, so finishing should wait until the concrete has stiffened adequately.