For slab-on-ground, shotcrete and precast applications, fiber-reinforced concrete is specified as either micro-synthetic (typically 0.6–1.2 kg/m³, polypropylene 6–20 denier) for plastic-stage crack control, macro-synthetic (3–8 kg/m³, 30–50 mm length) or steel fiber (20–40 kg/m³, hooked-end 30–60 mm, aspect ratio 50–80) for post-crack residual strength [S3][S4].
Leading suppliers organize the concrete fiber product tree the same way: micro fibers, macro fibers, steel fibers, and blended fibers are the four families every spec must select between before the batch sheet is written, and each family carries its own dosage band and mixing discipline [S3][S4].
Fiber Families and What Each One Actually Solves
SikaFiber's product guide splits the offering into four families — micro fibers, macro fibers, steel fibers, and blended fibers — each mapped to a specific failure mode [S3]. Micro fibers (polypropylene or nylon, generally 6–54 mm) suppress plastic shrinkage cracking during the first 24 hours; macro synthetic and steel fibers carry post-crack load across a crack opening; blended products stack both mechanisms in a single mix [S3][S4]. TenaBrix's portfolio follows the same taxonomy on the polypropylene side, with separate micro and macro ranges plus steel options, confirming this four-bucket model is industry-standard rather than a single-vendor convention [S4].
Choose micro fiber when the design issue is plastic shrinkage and impact resistance at the surface; choose macro or steel when ACI 360 or equivalent slab design calls for a defined residual strength factor R₃ in the range 0.30–0.50; choose blended when the spec lists both plastic-crack control and residual-load requirements in the same mix [S3][S4]. For background on how these fibers behave during pour and consolidation, the concrete vibrator selection guide is the natural companion — wrong head size or frequency balls up fibers and defeats the dosage.
Dosage Bands, Aspect Ratio and Slump Targets
Typical field bands: micro-synthetic 0.6–1.2 kg/m³, macro-synthetic 3–8 kg/m³, hooked-end steel 20–40 kg/m³, with the lower half of each band used in slabs and the upper half in shotcrete linings and pile caps [S3][S4]. Aspect ratio (length ÷ diameter) is the second control lever: steel fibers commonly land at 50–80 for slab-on-ground and 60–80 for shotcrete, while macro-synthetic fibers run 60–100 with length 30–50 mm [S3].
Slump at the point of fiber addition should sit 25–50 mm above the target slump for the placement method, because fibers of 30 mm or longer consume 15–25 mm of slump as they disperse; do not chase the lost slump by adding water at the truck — use a water-reducing admixture or rework the aggregate grading instead. A standard 100 mm slump slab with 30 kg/m³ of 50 mm hooked-end steel should land at the chute between 75 and 100 mm after fiber addition, not the other way around [S3].
Mixing Sequence, Addition Point and Dispersion Checks
Fibers must be added after the sand and coarse aggregate have wetted out, not on top of dry cement or into the back of a dry truck — the cement coats the fibers and forms balls that survive mixing. The accepted sequence is: charge aggregate, add ~70% mix water, add cementitious materials, add fibers while the drum turns at mixing speed, add the remaining water plus admixtures, then mix a further 70–120 revolutions at mixing speed (typically 6–9 minutes for a 9 m³ truck) before discharge [S3].
SikaFiber's batching instructions set a hard rule: never add fibers to a dry or under-watered load, never add to the back of the truck, and never extend mixing beyond the manufacturer's recommended window because fiber breakage above that point can cut effective length by 10–30% and silently degrades the R₃ value [S3]. On-site dispersion checks are visual: open two random 0.05 m³ samples at mid and end of pour, sieve through a 10 mm mesh, and confirm no fiber balls larger than 25 mm and uniform distribution across the sample [S3][S4].
Placement, Finishing and Curing Adjustments
Fiber-reinforced slabs need a different finishing window than plain concrete. Bull floating must happen before bleed water disappears; closing the surface too early traps bleed water under the densified layer and creates blisters. Power troweling is permitted on macro and steel-fiber mixes at 30 kg/m³ or below, but pan-style trowels beat blades at the first pass because they push fibers flat rather than snagging them; above 30 kg/m³ of steel, expect hand-trowel-only finishing in tight areas [S3].
Joint spacing is the second adjustment: ACI 360 panel aspect ratios hold, but the maximum panel length for a 150 mm slab typically drops from 6.0 m (plain) to 4.5 m (fiber-only) when fibers replace rebar, and saw-cut depth goes to one-third of slab thickness rather than one-quarter because fibers bridge the kerf and limit uncontrolled cracking if the saw is late. Curing compounds go down as soon as the surface will not mar, normally within 30 minutes of final troweling; wet burlap or curing blankets follow the concrete admixture supplier's water-retention guidance [S3].
Standards, Documentation and Test Methods That Travel With the Mix
Specifying fibers without naming the test method is the most common specification defect. Plastic-crack reduction for micro fibers is usually quoted per ASTM C1579 (restrained shrinkage) or EN 14889-2; residual flexural strength for macro and steel fibers is reported per ASTM C1609 (third-point loading, residual strength R₃₀,150) or EN 14651 for steel, with the lower of the two values used when project specs cross-reference both systems [S3].
Documentation that should travel with each delivery: fiber type, length, denier or equivalent diameter, aspect ratio, dosage in kg/m³, batching instructions, MSDS, and a per-batch dispersion QA sheet — and the SikaFiber SikaFiber batching guide template is the cleanest public reference for that list, including uniform visual check-off boxes the batch operator signs [S3]. The batching plant encyclopedia page covers how fiber dosing integrates with the weigh-hopper and aggregate moisture compensation steps.
Common Failure Modes and Field Fixes
Three failure modes account for the majority of rejected fiber pours: fiber balling (caused by dry-batch addition or wrong sequence), surface flaking (caused by fibers protruding through the finished surface when length exceeds 50 mm in a troweled slab), and loss of residual strength (caused by over-mixing breaking the fibers or under-dosing against a too-high aspect-ratio spec) [S3][S4].
The fix for balling is to discard the affected load, never push it, and to revise the sequence per the batching guide; for surface flaking, switch to a 30–40 mm fiber in troweled slabs or move to a hand-float finish; for residual strength losses, the dosage band is widened only after the mix designer revisits aspect ratio, not as a field adjustment. When fibers are used as a partial replacement for rebar in slab-on-ground, document the post-crack residual strength per ASTM C1609 and keep the certificate with the QA package — this is what stands between the slab and a future crack-width claim [S3].
Sourcing, Lead Time and What to Ask the Supplier
Fiber lead time is shorter than rebar but longer than admixtures: most polypropylene fibers ship ex-warehouse in 7–14 days for EU and North America, while hooked-end steel in 25–40 mm lengths runs 21–35 days because of melt scheduling at the steel mill [S3][S4]. Both FibeRego and SikaFiber publish country-level dealer networks; TenaBrix positions itself on the polypropylene synthetic side with both micro and macro ranges and ships to multiple regional markets [S1][S3][S4].
Five questions that go on every RFQ: (1) fiber type and exact polymer or steel grade, (2) length, diameter, aspect ratio, (3) dosage band per cubic metre for the application, (4) test certificates per ASTM C1609 or EN 14651 with R₃₀,150 values, and (5) batching instructions document [S3]. Reject any submittal that comes back with a dosage stated in "bags per yard" without the kg/m³ conversion spelled out, and reject any tech sheet that does not list the standard it tests against [S3]. For the production-line economics on the batching side, the dry-mix mortar plant selection write-up covers the dosing hardware these fibers share with dry-blend operations.
Trackable signal number two: wider adoption of ASTM C1609 R₃ values above 0.40 in slab-on-ground design, which would push the steel dosage band from 20–40 kg/m³ toward the upper half of that range on most commercial slabs [S3].