Amorphous alloy fibers (AAF) raise flexural strength roughly 9% after six months in geothermal water while carbon steel fiber concrete (SFRC) loses 10-15% of its bond strength under the same wet/dry cycling, per a 2025-11-14 study published in Materials and Structures [S4] and a 2021 IOP Materials Express study [S3].
Both fiber classes reinforce cement concrete by bridging microcracks, but the underlying carbon steel wire is a crystalline metal with grains and grain boundaries, while amorphous metal lacks long-range atomic order and therefore corrodes far more slowly, a structural difference first exploited by French researchers in the early 2000s [S3].
Material structure and why it changes corrosion behaviour
Amorphous alloys have a long-range disordered atomic structure with no grains or grain boundaries, which is why the metal material class exhibits far higher corrosion resistivity than crystalline alloys and is now being paired with carbon-steel and synthetic fibers in hybrid HPFRC mixes [S3].
Steel fibers are crystalline carbon steel wires, typically produced as Type I (cold-drawn wire, hooked ends, higher tensile strength) or Type II (cut sheet, lower tensile strength) per ASTM A820-style classifications, with the hooked-end geometry specifically designed to anchor into the concrete matrix [S2].
AAF is produced by rapid solidification of a melt into ribbon or wire, with no plastic deformation steps and no grain structure, which is why it can reach a high effective aspect ratio in concrete without a separate drawing stage [S3]. The smooth, hydrophobic surface of the amorphous fiber does, however, reduce the chemical bond with cement paste, which feeds the strength-vs-ductility trade-off discussed below.
Mechanical performance numbers from the 2025 HPFRC study
In deep-beam four-point bending tests on 100×100×500 mm specimens cured 90 days, the reference steel-fiber mix reached a peak bending stress of 16.78 MPa, the amorphous-fiber mix 19.04 MPa, and the CEM III cement plus steel-fiber mix 20.19 MPa, all at 1.5% fiber by volume [S4].
Tensile strength across the three mixes fell in a tight 8-9 MPa window, but strain-at-localization split sharply: steel-fiber mixes localized at 2.9-3.5% strain (above the 2.1-2.5% yield range of conventional rebar), while the amorphous-fiber mix localized at just 1.4%, indicating a more brittle post-cracking response [S4].
The earlier IOP study reported that amorphous alloy fiber delivers a higher pull-out load and bond strength than crimped steel fiber, but lower interfacial toughness, with the bond advantage linked to a higher effective aspect ratio rather than chemical adhesion [S3].
Durability under wet/dry cycling and chemical exposure
Steel-fiber concrete lost about 10% flexural strength and 15% bond strength after 30 days of wet/dry cycling, while amorphous-alloy-fiber concrete (AAFRC) showed almost no change in the same window, a result the authors attribute to the absence of active corrosion paths in the amorphous structure [S3].
In the 2025 Materials and Structures immersion test, the amorphous-fiber mix gained 9% in flexural strength after submersion in geothermal water, compared with degradation in the steel reference, supporting AAF for aggressive chemical exposure such as precast geothermal tanks and marine structures [S4].
Steel fibers are also known to increase shotcrete rebound and add structural weight because of their high specific gravity, two practical drawbacks that disappear with lighter amorphous fiber dosing, per the IOP work [S3]. For more on marine-grade steel fiber selection, see this Milled Steel Fiber Specs for Marine Concrete: 2026 Selection Map.
Where steel fiber still wins, and where AAF does not
Steel fibers begin resisting crack opening almost immediately after first load thanks to a high elastic modulus, and they develop denser crack patterns with smaller spacing under cyclic loading while retaining stiffness, per the 2025 study [S4][S5].
Amorphous-fiber mixes show wider crack spacing and a noticeable stiffness drop with increased deflection, a behaviour tied to stiffer bond-slip, hydrophobic fiber surface chemistry, and a higher likelihood of fiber rupture rather than pull-out at the crack plane [S4].
Where ductility, impact absorption, and crack-width control under cyclic load drive the design (tunnels, industrial slabs, shotcrete linings, seismic elements), Type I hooked-end steel fibers remain the default; AAF fits where the service environment is the binding constraint, chemical attack, chloride or sulphate exposure, geothermal fluids, and the structural code can tolerate lower post-crack strain.
Hybrid and mix-design considerations
Hybrid steel + amorphous alloy fiber mixes show superior flexural performance to either single-fiber system, because the two fiber types delay micro-crack and macro-crack formation at different scales, according to the IOP 2021 study [S3].
Flowability of fresh concrete drops once any fiber is added, but the higher aspect ratio of amorphous fiber amplifies that workability loss, so water-reducer dosage and maximum aggregate size typically need adjustment at constant fiber volume fraction [S3].
Both fiber types fall under ACI's standard fiber-reinforced concrete categories (steel, glass, synthetic, natural) and are governed by ASTM C1116 for the as-delivered, uniformly mixed product; the standard does not, by itself, define finished structural design, placement, or curing [S5]. For shotcrete dosing and vibration choices on fiber-reinforced pours, this Poker vibrator vibration velocity guide covers head-diameter and frequency ranges that interact with fiber aspect ratio.
Selection checklist for specifying engineers
Specify Type I cold-drawn hooked-end steel fibers (per ASTM A820-style Type I definition) when residual strength, post-crack ductility, and 2.9-3.5% strain capacity are mandatory, and accept periodic inspection for surface corrosion in chloride exposure [S2][S4].
Specify amorphous alloy fiber when the service environment involves chloride, sulphate, geothermal brine, or other aggressive ions, when the structural code allows the lower 1.4% localization strain, and when the higher per-kg fiber cost is offset by eliminating corrosion-related maintenance and shotcrete rebound losses [S3][S4].
For high-risk assets where neither fiber alone is acceptable (deep tunnels, marine piers, chemical tanks), design a hybrid section that combines a steel-fiber structural layer with an amorphous-fiber cover or surface zone, and verify both flexural capacity and durability by 4PBT plus wet/dry cycling before finalizing the mix [S3][S4].
Track two signals going forward: (1) wider commercial availability of AAF at aspect ratios matching Type I steel (currently 30-80 mm cut length), which will determine whether hybrid mixes become routine on commodity infrastructure rather than research pours; (2) any update to ASTM C1116 or EN 14889-1 coverage of amorphous fibers, since current standards are written around steel, glass, synthetic, and natural categories [S3][S5].