Fiber-Reinforced Concrete (Synthetic vs Steel) for Slab-on-Grade
Why this matters
Fiber-reinforced concrete (FRC) replaces or supplements traditional welded-wire fabric and rebar in slab-on-grade construction with discrete fibers mixed into the concrete itself. The choice between synthetic and steel fibers, and within each family between micro and macro grades, drives both performance and cost. The wrong fiber choice gives the homeowner a slab that cracks anyway, or a commercial floor that fails fatigue testing under fork-lift loads. Specifying FRC correctly requires understanding what each fiber class controls: plastic shrinkage, hardened-state cracking, post-crack residual strength, or all three.
What ASTM C1116 actually defines
ASTM C1116 is the umbrella spec for fiber-reinforced concrete. It defines four fiber types: Type I (steel), Type II (glass), Type III (synthetic), and Type IV (natural). Within each type the spec separates micro-fiber (typically under 0.012 inch / 0.3 mm diameter, used for plastic shrinkage control) from macro-fiber (over 0.012 inch, used for hardened-state structural performance). The spec calls for fibers to be batched at a producer-controlled dose, mixed to disperse, and tested for residual flexural strength per ASTM C1609 when structural performance is the design intent.
Synthetic micro-fiber
Polypropylene or polyester micro-fibers (Sika Fiber PSF, Forta-Fiber Stealth, BASF MasterFiber M, Euclid PSI-Fiberstrand) are typically 1/2 to 3/4 inch long, 0.001 to 0.005 inch diameter, dosed at 0.1 to 0.15 percent by volume (roughly 1.5 pounds per cubic yard). They control plastic shrinkage cracking only; that is the cracking that happens at the surface in the first few hours of cure as water evaporates and the still-plastic concrete cannot bridge the tensile stress.
Use synthetic micro-fiber when: you want plastic shrinkage protection on top of conventional rebar or WWF reinforcement; the slab is interior and protected from heavy loading; the project budget cannot support macro-fiber or steel; the architect wants a clean finished surface without visible fiber.
Do not use synthetic micro-fiber as the only reinforcement on a structural slab. It does not contribute meaningful hardened-state strength. ACI 360 (slabs-on-grade) and PCA guidance are explicit on this distinction.
Synthetic macro-fiber
Engineered polypropylene or polyolefin macro-fibers (Forta-Fiber Ferro, Sika Fiber MX-150, BASF MasterFiber MAC, Euclid Tuf-Strand) are typically 1.5 to 2.25 inches long, 0.012 to 0.030 inch diameter, dosed at 0.3 to 0.6 percent by volume (roughly 4 to 8 pounds per cubic yard). These fibers carry post-crack tensile load; once the slab develops a hairline crack, the fibers crossing the crack hold the two faces together and prevent crack opening.
Use synthetic macro-fiber when: the slab is residential or light-commercial slab-on-grade where plastic shrinkage protection plus modest post-crack capacity is sufficient; the design wants to eliminate WWF entirely; the slab will not see fork-lift or high-impact loading; the floor will be polished or covered (visible fiber ends can telegraph through some thin coatings).
Documented residual flexural strength per ASTM C1609 at the specified dose is the bid acceptance test; manufacturer published values give the design engineer the data they need.
Steel fiber
Hooked-end carbon steel fibers (Bekaert Dramix, Sika SikaFiber Steel, ArcelorMittal HE) are typically 1 to 2.5 inches long, 0.025 to 0.040 inch diameter, dosed at 30 to 80 pounds per cubic yard. Steel fibers deliver the highest post-crack residual strength of any fiber class; they also raise the slab's fatigue resistance and impact resistance significantly. Steel fiber is the standard for industrial floors, distribution-center slabs, and structural slab-on-grade designed to ACI 544 or post-tensioning supplement.
Use steel fiber when: the slab will see fork-lift, pallet-jack, or heavy-vehicle loading; the architect designed for jointless or super-flat construction (FF/FL targets above 60/40); the engineer specified post-crack performance per ACI 544 and ASTM C1609.
Do not use steel fiber when: the slab is exposed exterior (rust streaks at the fiber ends become permanent stain marks; corrosion can spall the cover); the floor is polished and steel fiber appearance is a visual problem; pumping is required (some pumps will jam on steel-fiber-rich mixes; coordinate with the pump operator and producer).
What about welded-wire fabric
WWF (W2.9 x W2.9 6x6 sheets, the residential default) carries hardened-state tensile load if positioned properly in the slab and tied to chairs that keep it at the design depth. The problem with WWF is positioning: contractors who lay the sheets on grade and intend to "pull it up" with hooks during the pour usually end up with the wire flat on the substrate, where it contributes almost nothing. Macro-fiber distributed throughout the matrix removes the positioning problem.
A defensible spec for residential slab-on-grade replaces 6x6 W2.9xW2.9 WWF with 4 pounds per cubic yard of polyolefin macro-fiber and proven equivalent performance per ASTM C1609. For larger or commercial work, the engineer of record makes the substitution call.
Dosing and mixing
Dose fibers at the batch plant, not at the chute. Synthetic fibers can be added at the chute in some cases but distribution is less reliable. Specify dose by weight per cubic yard and require the producer to log it on the truck ticket. Mix at least 80 revolutions of the truck drum at high speed after adding fibers to disperse them; balling indicates inadequate mixing time or excessive dose for the mix design.
Slump loss from fibers is real: 0.5 to 1 inch slump loss per 4 pounds per cubic yard of macro-fiber, dependent on fiber length and aspect ratio. Adjust the mix water reducer (ASTM C494 Type A or F) rather than adding mix water. Verify slump per ASTM C143 at the placement.
Joint and control-joint planning
Fiber reinforcement does not eliminate the need for control joints. ACI 360 and PCA guidance both call for control joints in slab-on-grade at intervals of 24 to 36 times the slab thickness (a 4 inch slab gets 8 to 12 foot joint spacing). FRC reduces but does not remove drying-shrinkage cracking; the joints give the cracks a designated location. Joints can be early-entry saw cut (Soff-Cut and equivalent) or conventional wet-cut.
References
- ASTM C1116, "Standard Specification for Fiber-Reinforced Concrete," and ASTM C1609, "Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete."
- ACI 360R, "Guide to Design of Slabs-on-Ground," and ACI 544.4R, "Guide to Design with Fiber-Reinforced Concrete."
- Portland Cement Association (PCA), "Design and Control of Concrete Mixtures," fiber sections.
- Manufacturer technical data sheets for Bekaert Dramix, Forta-Ferro, Sika MX-150, BASF MasterFiber MAC, and Euclid Tuf-Strand.