Overview
ISO 21914:2019 specifies a laboratory test method to obtain bending moment–curvature curves for fibre‑reinforced cementitious composites (FRCCs) using a four‑point bending test on prism specimens. The standard applies to FRCCs that exhibit separated multiple cracks (deflection‑hardening behaviour) under pure bending prior to peak load. Results are used to quantify bending performance, estimate tensile properties and support quality control, material development and design validation.
Key topics and requirements
- Specimen geometry and fabrication
- Prism cross‑sections: 150 × 150 mm or 100 × 100 mm; side length must be ≥ three times the fibre length.
- Overall length ≥ 3.5 × D; cross‑section tolerance ±0.5%.
- Minimum number of specimens: 6. If more than one‑third fail to show multiple cracks, the standard does not apply.
- Maximum aggregate size ≤ 1/4 side length; continuous placement recommended (no internal vibration).
- Test setup
- Four‑point bending loading apparatus with two upper and two lower rollers; supports must allow horizontal movement.
- Specimen span: 3 × D (±2%).
- Specimen rotated 90° from casting orientation during test.
- Instrumentation and measurement
- Two LVDTs for average curvature measurement set at 0.15 D and 0.85 D from lower surface; contact length equals pure bending span (S/3); distance between LVDTs = 0.7 D.
- LVDT accuracy: 1/500 mm or better. Digital recording interval ≤ 5 s (1 s recommended).
- Loading and procedure
- Controlled continuous loading; recommended machine displacement rate 0.3 ± 0.2 mm/min (rate may be increased during softening if reported).
- Test until significant opening of one crack; discard tests where damage concentrates outside constant‑moment span.
- Outputs and calculations
- Produce the bending moment–curvature curve; the standard provides formulae to compute bending moment and average curvature and prescribes reporting items (mix, fibre type/volume, curing, maximum bending moment, curvature at first crack and at peak, number of cracks, etc.).
- Annex A gives a method to estimate tensile strength and average ultimate tensile strain from the bending moment–curvature data using assumed stress/strain distributions.
Applications and users
ISO 21914:2019 is practical for:
- Materials laboratories and R&D teams evaluating FRCC mixes and fibre types.
- Quality control in prefabrication and production of fibre‑reinforced elements.
- Structural engineers and designers requiring bending performance data for modelling and design validation of FRCC components.
- Standards bodies and researchers comparing deflection‑hardening behaviour across materials.
Common uses include characterizing multiple‑cracking performance, deriving tensile‑property estimates for design checks, and benchmarking mixes for durability or serviceability.
Related standards
- ISO 19044 (alternative methods for FRCCs that do not show multiple cracks)
- ISO 1920‑3 (making and curing test specimens)
- ISO 1920‑4 (testing strength of hardened concrete)
Keywords: ISO 21914:2019, FRCC, fibre‑reinforced cementitious composites, four‑point bending test, bending moment‑curvature, LVDT, deflection hardening, tensile strength estimation.