MixDesignCalc 2026 — Steel, Synthetic, Basalt & Glass Fiber: Fiber Type Selection, Volume Fraction, Aspect Ratio (L/d), Workability Adjustment, Post-Crack Residual Strength, SFRS Shotcrete, Structural FRC per fib MC 2020 & Batch Quantities
Fiber Reinforced Concrete (FRC) is concrete incorporating discrete, randomly distributed fibers to improve post-cracking performance. While normal concrete is brittle — failing suddenly once a crack forms — FRC maintains load-carrying capacity after cracking through the bridging action of fibers across the crack faces. This post-crack residual tensile strength is the defining property of FRC and the basis of structural FRC design per fib Model Code 2020.
Fibers do not significantly increase the first-crack strength (pre-crack behavior is essentially identical to plain concrete). Their value is entirely in the post-crack behavior — controlling crack widths, providing ductility, and allowing load redistribution. This makes FRC particularly valuable in applications involving flexural loading, impact, fatigue, and crack-width-sensitive structures.
Hooked-end (crimped) steel fibers are the most widely used structural fiber worldwide. The hooked ends provide mechanical anchorage — the fiber pulls out progressively as the crack widens, providing sustained post-crack load capacity over crack mouth opening displacements (CMOD) of 0.5–4.0mm. Typical sizes: 35/0.55mm (Aspect ratio 64), 50/0.75mm (AR 67), 60/0.90mm (AR 67). Available in glued bundle form for easier mixing. Rust staining on exposed surfaces — use stainless steel (SS) for architectural applications.
Select fiber type, enter mix parameters and fiber dosage, then click Calculate FRC Mix. Results include base mix proportions adjusted for FRC, fiber volume fraction, aspect ratio, estimated workability reduction, post-crack residual strength class, EN 14651 strength category, batch quantities and application-specific notes.
| Fiber Type | L (mm) | d (mm) | Aspect Ratio | Sg | Tensile Str (MPa) | Typical Dose | Volume % |
|---|---|---|---|---|---|---|---|
| Hooked-End Steel | 30–60 | 0.50–0.90 | 50–80 | 7.85 | 1000–1500 | 20–80 kg/m³ | 0.25–1.0% |
| Straight Steel | 13–30 | 0.15–0.40 | 60–120 | 7.85 | 700–1200 | 15–40 kg/m³ | 0.2–0.5% |
| Macro PP (structural) | 30–54 | 0.5–0.9 | 50–80 | 0.91 | 450–600 | 3–8 kg/m³ | 0.3–0.9% |
| Micro PP (shrinkage) | 6–18 | 0.018–0.040 | 300–700 | 0.91 | 300–500 | 0.6–1.8 kg/m³ | 0.06–0.2% |
| Basalt Fiber | 12–48 | 0.013–0.020 | 800–3000 | 2.65 | 3000–4800 | 2–6 kg/m³ | 0.08–0.22% |
| AR Glass (GRC) | 12–50 | 0.013 | 900–3800 | 2.65 | 1700–2500 | 2–5 kg/m³ | 0.08–0.19% |
| Stainless Steel (corrosion-resistant) | 30–60 | 0.55–0.80 | 50–80 | 7.90 | 1100–1300 | 20–60 kg/m³ | 0.25–0.8% |
| Residual Strength Class | fR,1k (MPa) | fR,3k (MPa) | Typical Steel Fiber Dose | Applications |
|---|---|---|---|---|
| a (1.0) | 1.0–1.5 | — | 15–20 kg/m³ | Non-structural crack control |
| b (1.5) | 1.5–2.0 | — | 20–30 kg/m³ | Industrial floors (light duty) |
| c (2.0) | 2.0–3.0 | >1.5 | 30–40 kg/m³ | Industrial floors (medium/heavy) |
| d (2.5–3.0) | 2.5–4.0 | >2.0 | 40–50 kg/m³ | Tunnel lining, ground slabs |
| e (3.5–4.0+) | >4.0 | >3.0 | 50–80 kg/m³ | Structural beams, precast, UHPFRC |
Steel fiber ≤ 60 kg/m³ (0.8% vol): Standard drum/pan mixer can accommodate; add fibers last or via conveyor to avoid balling.
Steel fiber 60–80 kg/m³ (0.8–1.0% vol): Extended mixing time; high-energy mixer (pan or twin-shaft) recommended; SP dose may need to increase to 1.5–2.0%.
Steel fiber > 80 kg/m³: Very difficult to mix uniformly without fiber balling (tangling into clumps). Pan or Eirich mixer required; aspect ratio should be reduced to ≤ 60.
PP macro 3–8 kg/m³: Equivalent to 0.3–0.9% volume — manageable in any mixer. Lower residual strength than steel at same volume fraction.
Aspect ratio × volume fraction ≤ 60: Practical limit for uniform mixing in drum mixers. AR × Vf_steel > 60 → fiber balling risk increases significantly.