Double-hooked end steel fibers consistently out-perform single-hooked ends on single-fiber pull-out load and on residual flexural capacity, but the gap narrows once concrete dosage, aspect ratio, and matrix strength are held constant, per a 2025 quantitative bond study [S2].
For a process engineer, the right pick is not "always double hook". It is a trade among four numbers: pull-out work, post-crack residual strength, fresh-concrete workability, and unit cost, with ASTM A820/A820M-22 governing the type declaration on the mill cert [S1].
What "hooked end" actually means in a mill cert
A hooked end steel fiber is a cold-drawn wire cut to length, with one or both ends bent into a hook that bears against the surrounding matrix during pull-out [S1]. In the ASTM A820/A820M-22 type system, cold-drawn wire fibers fall in Type I and "modified" cold-drawn wire fibers fall in Type V, and the standard requires checks on finished-fiber dimensions plus physical properties on every lot [S1]. A spec sheet that lists only a product name and a length is therefore not enough; the type code, bend angle tolerance, declared tensile strength, and a bend-test result must also be present.
Hook count is the variable that matters most for anchorage. Single-hooked (1D) fibers have one bent end; double-hooked (2D or "3D" in some catalogs) fibers have two bent ends. The Sulthan and Saloma 2022 study on self-compacting concrete, using EFNARC workability, ASTM C-469 elastic modulus, and ASTM C-1609 flexural tests, ranked hook count as 3D < 4D < 5D on energy absorption, with reported values of 147.23 J, 166.16 J, and 178.03 J respectively for 3D, 4D, and 5D fibers at the same dosage [S5].
How the hook changes pull-out mechanics
Pull-out of a straight fiber is dominated by chemical adhesion, then by friction along the embedded length; once debonding is complete, load drops fast and residual capacity is low [S4]. A hook adds a third phase: the bent end must plastically rotate or straighten before the fiber can leave the crack plane, so the load-versus-slip curve stays elevated over a much longer slip range [S2].
The 2025 study by Zou and co-authors isolated three shape parameters: number of end hooks, fiber diameter, and wire tensile strength. Their quantitative analysis confirmed that, within the range tested, increasing the number of end hooks raises fiber-matrix bond strength more efficiently than increasing diameter alone, and that tensile strength controls the failure mode (pull-out versus rupture) [S2]. The 2023 MDPI optimization work by Wang et al. reached the same conclusion from a different angle: optimized end-hook geometry slowed the post-peak drop in pull-out resistance compared with ordinary hooks of the same diameter and tensile strength [S4].
A practical limit exists. If anchorage is too aggressive for the wire's tensile strength, the fiber ruptures at the hook root before it can mobilize the full slip, which wastes the geometry. The Jamil Alwan "equivalent pulley" model and Wang's inclined-hook extension both treat the hook as a rotational friction element, which is why a well-shaped hook smooths the descent of the post-peak curve rather than just lifting the peak [S2].
Single-hook vs double-hook: side-by-side decision criteria
Across the public data, double-hooked ends win on three measurable criteria and lose on two. Pull-out work per fiber is higher because the second hook adds a second pulley stage [S2]. Residual flexural strength of SFRC beams is higher, with the 3D-to-5D ladder in the Sulthan data showing a 13 percent step from 3D to 4D and a further 7 percent step from 4D to 5D in absorbed energy at the same volume fraction [S5]. Compressive strength of the hardened composite is essentially unaffected by hook count; a single hook can match or slightly exceed double-hook on cube compressive strength at the same dosage, because compression failure is governed by the matrix, not by anchorage [S9].
Where double-hook loses is fresh-concrete behavior and unit cost. Higher hook count increases fiber surface area and entanglement, so slump flow, T-500, V-funnel, and L-box all degrade as hook count rises; the same study recorded slump flow dropping as 3D dosage climbed, with the best workability at 3D type with a fiber volume fraction of 0.5 (slump flow 677 mm, T-500 4.36 s, V-funnel 9.35 s, L-box 0.92) [S5]. On price, the extra bending step and higher steel mass per performance unit push the per-kg cost of double-hooked above single-hooked in most catalogs [S1].
When single-hook is the right answer
Specify single-hooked end steel fiber when the failure mode is bending-driven, not impact-driven, and when the mix has tight workability targets. Industrial floor slabs, ground-supported pavements, and tunnel linings fall in this group: post-crack residual strength is the design driver, not peak impact, and the matrix usually caps hook benefit before the second hook can be mobilized [S1][S3].
Single-hook is also the right pick for shotcrete where pumpability and rebound loss dominate cost, and for any mix where the supplier's dosage recommendation is below about 30 kg/m^3, because at low dosage the second hook is statistically unlikely to sit across a crack and so adds cost without adding anchored fibers [S1]. For dosing accuracy in bag-fed ready-mix trucks, the direct-add paper and PVA bag approach keeps loose single-hook fibers dispersed without the clumping risk that comes with higher-aspect-ratio double-hook products.
When double-hook earns its premium
Specify double-hooked (or higher) end steel fiber when the failure mode is impact, seismic, or blast, and where the design uses residual flexural strength (f_R1, f_R3 in EN 14889-1 terminology) as a code-binding value. Precast tunnel segments, seismic coupling beams, impact-resistant machine foundations, and high-dose precast elements are the canonical cases [S3].
Here the second hook pays for itself because the higher residual capacity allows reduction of traditional rebar or mesh, and the slower post-peak decay (visible in the Wang optimized-hook curves) keeps the composite load-bearing through larger crack openings [S4]. For procurement during tight supply windows, going through an authorized channel rather than a broker protects the mill-cert chain that proves the Type I or Type V declaration [S1].
Limits, failure modes, and sourcing signals to watch
The main failure mode for any hooked fiber is rupture at the hook root, not pull-out, when the wire tensile strength is too low for the chosen hook geometry [S2]. The main practical failure mode on site is balling, where high-aspect-ratio double-hook fibers clump if the mixer sees them before the aggregate is fully wetted; glued bundles break down during mixing only if the water-soluble adhesive is correct for that mix temperature [S1]. Corrosion at the crack face is a separate service-life limit, not a pull-out limit, and ongoing work on hook-end fiber coatings is the main corrosion mitigation in the public literature [S3].
Trackable signals for the next buying cycle: published pull-out curves for the specific hook angle and diameter on the data sheet (not just an average value), EN 14889-1 system performance class (e.g. 1a, 2a, 2b) declared on the DoP, and any change in the ASTM A820/A820M-22 edition that tightens the bend-test acceptance band [S1]. Background on how steel fiber sits in the wider fiber family, including concrete fiber and steel fiber reference pages, is a good cross-check before locking the spec.
Detailed specification references: single girder crane.