Seven-wire and nineteen-wire steel strands make up the bulk of construction, transmission, and stay-cable shipments, with 1×7 helically-laid products dominating the prestressed-concrete market and meeting the dimensional and tensile envelope of ASTM A416 [S2][S3].
The same product family splits cleanly into three specifier decisions: bare versus galvanized sheathing, bonded versus unbonded prestressing, and structural versus conductor-core duty. Each axis changes material, cost, and the standard the coil is certified against — picking a strand without locking those three is a common sourcing error.
What counts as a steel strand
A steel strand is two or more steel wires helically laid together, in a fixed geometry, to behave as a single tension member. The most common geometry is 1×7 — six outer wires helically laid around one central king wire — followed by 1×19 (one core wire, six intermediate, twelve outer) for larger structural and stay-cable sections, and 1×3 for low-tonnage tie and stay work [S3]. Production of a single mill typically runs 200–2,000 tonnes per month of finished strand, with Cold-Drawing and stranding/twisting as the two principal forming steps [S2].
The end product is classified by surface treatment, by prestressing duty, and by the reference standard. Surface treatments in the market are bare (bright), galvanized, and grease-filled-with-PE-sheathed; prestressing duty divides into bonded (grouted) and unbonded (PE-sheathed, monostrand anchor); and standards include ASTM A416 for the seven-wire prestressed strand family and GB/T 5224 for the equivalent Chinese specification [S2][S3]. A typical Chinese mill operating ten strand lines, one unbonded line, twenty-two wire-drawing machines, and ten twisters, with 100,000 m² of plant on a registered capital of 100 million yuan, reports a combined bonded/unbonded annual output in the 400,000–430,000 t range [S1].
The three classification axes specifiers actually use
Axis one is the standard-and-tensile envelope. ASTM A416 covers two grades: Grade 250 (regular) at a minimum ultimate strength of 1,725 MPa (250 ksi) and Grade 270 (high-tensile) at 1,860 MPa (270 ksi), each in 1×7 geometry, common diameters 9.53 mm (3/8 in), 12.7 mm (1/2 in), and 15.24 mm (0.6 in) [S2]. GB/T 5224 mirrors this for the Chinese market and is the spec carried by the bulk of the bonded/unbonded export tonnage [S1][S3].
Axis two is corrosion protection. Bare (bright) strand is the cheapest and is used in grouted post-tensioning, in guy/messenger wire, and in steel core for ACSR overhead conductors [S2][S3]. Galvanized strand — zinc-coated wires before stranding, class A or class B zinc weights — goes into coastal bridges, transmission-guying, and ACSR cores where service life in aggressive atmospheres is the driver [S3]. Unbonded strand (also called monostrand or PE-sheathed) takes the corrosion protection further: a grease-filled HDPE sheath is extruded over the 1×7 strand, locking it out of direct contact with the surrounding concrete and allowing individual tendons to be stressed and de-stressed through dedicated anchor heads [S1][S3].
Axis three is construction — 1×3, 1×7, 1×19, and the rarer 1×37. Increasing wire count trades the larger single-wire cross-section of a 1×7 for finer outer wires, which raises flexibility and fatigue behaviour for stay cables but reduces the cross-sectional efficiency of the strand under pure tension [S3]. The selection of construction interacts with the anchor system: 1×7 monostrand is the standard fit for unbonded post-tensioned slabs and bridges, while 1×19 and 1×37 are the norm for stay cables and large-span prestressed members.
Comparison: bonded vs unbonded vs galvanized structural strand
Comparing the three mainstream options against four decision criteria clarifies most specification disputes. On raw cost per kN of guaranteed ultimate load, bare bonded strand is the cheapest and unbonded PE-sheathed strand carries a meaningful premium for the extrusion and grease-fill step [S1]. On corrosion performance, the order is galvanized > unbonded > bare bonded, with unbonded protected by an HDPE barrier rather than by sacrificial zinc [S3]. On typical applications, bare bonded anchors in grouted post-tensioned bridge beams and slabs; unbonded monostrands anchor in flat-slab post-tensioning and in segmental construction; galvanized strand sits in transmission-line guys, ACSR cores, and coastal cable-stay systems [S2][S3]. On replaceability, only the unbonded monostrand can be individually de-tensioned and replaced without demolishing the surrounding concrete — bonded strands, once grouted, are permanent [S1][S3].
Where each strand class earns its place
ASTM A416 1×7 galvanised and bare 7-wire strand is the workhorse of the overhead transmission market, used as the steel core of ACSR conductors and as the messenger/guy wire on distribution lines, with current offers published in the US$690/t band at 5-tonne minimum order and 2,000 t/month mill capacity [S2]. For the related wire-rod and bright-wire supply chain — which feeds the stranding operation — the wider carbon-steel-plate selection grade, thickness and certification map tracks the upstream specification decisions that flow down to the king wire.
Bonded prestressed 1×7 strand is the dominant reinforcement inside precast bridge girders, hollow-core slabs, railway sleepers, and LNG tank walls; in China, GB/T 5224 governs these deliveries and 7-wire strand at 15.20 mm and 17.80 mm is the bulk of mill output [S1][S3]. Unbonded monostrand is the standard tendon in cast-in-place post-tensioned residential slabs, beam-slab systems, and ground anchors, where the PE sheath and grease allow long-term corrosion protection without grouting; a 1×7 12.7 mm unbonded coil plus matching anchor head is the typical bill-of-material [S1].
Limits, failure modes, and what to watch at receiving
The most common in-service failure of prestressed strand is stress-corrosion cracking and hydrogen-embrittlement at the anchor head, especially when bare strand is used in chloride-exposed environments or when the grease layer in unbonded strand is incomplete [S3]. Specifiers should reject any unbonded coil where the PE sheath is damaged or the grease fill is patchy, since the corrosion pathway is then no different from bare strand. For galvanized structural strand, the second-most common in-service issue is coating loss at the lay-points of the outer wires, where the helical contact stresses against the king wire and the adjacent wires wear the zinc; class B (heavier) zinc weights are the conservative answer for coastal service [S3].
Related upstream reading that pairs with this strand classification map is the Steel Fiber in Concrete spec trade-off discussion, where fibre-reinforced concrete competes with bonded prestressed strand for crack control in slabs — and the Embedded Part Installation field-procedure walkthrough, which covers the anchor plate and bearing hardware the strand terminates into.
For the relevant spec sheets and selection criteria, see steel strand, prestressing strand, and alloy steel.