Hooked-end steel fibers are deformed at both tips so the bent anchorage engages the cementitious matrix as a crack opens, while straight steel fibers are smooth cylindrical wire with no end deformation and depend entirely on chemical bond plus frictional slip [S2][S6].
This single geometric difference cascades into every mechanical property that matters for steel fiber reinforced concrete (SFRC): pull-out resistance, post-crack residual strength, flexural toughness, and fresh-mix workability all move in opposite directions between the two shapes [S3][S4].
How the Two Shapes Generate Resistance Inside Concrete
Straight steel fibers transfer load to the matrix through interfacial bond and sliding friction; once debonding occurs, the pull-out resistance drops sharply because the fiber can slide out without obstruction [S3]. A single straight fiber pulled from cementitious composite loses load capacity rapidly after debonding, limiting reinforcing effect at short embedded lengths [S3].
Hooked-end fibers add a second stage: after bond failure, the bent hook must straighten, plastically deform, and pull through the matrix, which sustains resistance over a longer slip distance [S3]. MDPI work on hook shape optimization (2023) shows that a larger hook angle reduces peak pull-out stress but increases the ratio of residual pull-out resistance, producing a slower load decay after first crack [S3]. Cold-drawn hooked-end wire typically reaches tensile strength above 1100 MPa, and the deformed ends are described in manufacturer literature as "the best anchoring design" because they slowly deform during extraction rather than slipping cleanly [S5].
Compressive, Tensile, and Flexural Numbers From Side-by-Side Tests
A direct comparative study (University of Babylon, E3S Web of Conferences 427, 2023) tested straight and hooked fibers at matched dosages of 0.5, 1.0, and 1.5% by volume: straight fibers used 13 mm length x 0.2 mm width, hooked fibers used 35 mm length x 0.55 mm width, so the aspect ratios were not identical and results must be read with that caveat [S4].
Compressive strength gains at 0.5, 1.0, and 1.5% fiber content were 7.27%, 20%, and 21.4% for straight versus 1.41%, 11.52%, and 20.81% for hooked, meaning straight fibers were more effective on cube compressive strength at the same dosage in this dataset [S4]. Hooked-end fibers consistently delivered lower percentage gains in these tests, yet the gap narrows on splitting and direct tensile strength at higher dosage, and independent flexural work (Sulthan 2022, Geomate Journal) reports that hooked shapes achieve greater flexural strength and higher fracture energy than straight shapes when the comparison is normalized to equivalent fiber volume and anchorage capacity [S8].
Standard Classifications and Common Dimensional Envelopes
Both shapes fall under the same material classifications: ASTM A820/A820M lists five types (Type I cold-drawn wire, Type II cut sheet, Type III melt-extracted, Type IV mill-cut, Type V modified cold-drawn wire) and explicitly states the fibers may be straight or deformed [S2]. EN 14889-1 mirrors this with five groups (Group I cold-drawn wire, Group II cut sheet, Group III melt-extracted, Group IV shaved cold-drawn wire, Group V milled from blocks) and similarly allows straight or deformed geometry [S2].
Common catalog envelopes for cold-drawn hooked-end loose fiber run from 0.50 mm to 1.00 mm diameter and 30 mm to 60 mm length, with L/D ratios between 50 and 80 and tensile strength rated above 1150 MPa in published spec sheets [S5]. Bekaert's Type I designation corresponds to the higher tensile strength, higher anchorage, hooked-end family, while Type II fibers are typically shorter and lower-strength, often straight or lightly deformed [S7]. Engineers comparing fiber selections should treat the standard classification as a material-source tag and the shape call-out as the performance tag, because buyers and contractors specify on shape even when the underlying material is the same [S2].
Where Each Shape Wins, and Where It Loses
Hooked-end fibers dominate when post-crack behavior matters: industrial floors, tunnel linings, shotcrete, precast elements, bridge decks, and any slab expected to carry load across a propagating crack [S1][S9]. The mechanical anchorage acts as a "crack bridge" that keeps pulling as the crack widens, sustaining residual load capacity beyond first crack, which straight fibers cannot do at equivalent dosage [S1][S3].
Straight fibers win on workability, surface finish, and pumping because the smooth profile reduces balling, lowers mix friction, and distributes more uniformly at high dosage; they are also cheaper per kilogram and easier to dose in automated plants [S4][S6]. On applications dominated by compressive load with limited flexural demand (certain precast blocks, mass concrete toppings, some dry-mix shotcrete), straight fibers are the more economical and effective choice, and the Babylon study's compressive data is the clearest evidence of that [S4]. Trade-off summary: choose hooked-end when toughness and crack control drive the design; choose straight when pumpability, finish quality, and compressive efficiency at low dosage drive it [S1][S4][S6].
Mix Design and Dosage Signals Worth Tracking
Dosage in SFRC is normally specified as percent by volume, and the comparative dataset above shows that for both shapes the property gains keep climbing from 0.5% to 1.5% by volume, with no plateau reached in that range [S4]. Practical industry guidance treats 20–40 kg/m³ as a typical working window for hooked-end fiber in industrial slabs, with higher doses (up to 60 kg/m³) used where heavy dynamic or impact loading is expected [S1][S9].
Two trackable signals for the next 6 months: (a) published revisions or addenda to ASTM A820/A820M and EN 14889-1 that may tighten the deformed-vs-straight performance criteria, since both standards currently allow either geometry under the same type; (b) fresh comparative pull-out datasets at matched L/D ratio, because the Babylon results are confounded by the 13 mm x 0.2 mm straight versus 35 mm x 0.55 mm hooked geometry, and matched-aspect-ratio test data would sharpen the picture considerably [S2][S4]. For broader context on how reinforcement choices propagate into adjacent plant equipment decisions, the screw conveyor trough loading logic used for fiber dosing lines follows similar volumetric thinking, and a parallel reinforcement question in fencing is dissected in welded mesh versus steel safety fence panels.
Spec-level background on the components involved: steel fiber, carbon fiber, and concrete fiber.