Prestressing Strand

What is Prestressing Strand

Prestressing strand is a tendon made by twisting several high-carbon, cold-drawn steel wires together into a single helical rope, then stress-relieving and stabilizing the assembly under tension. The standard product is the 1x7 seven-wire strand: one slightly oversized central wire (the king wire) surrounded by six outer wires laid in a regular helix. The strand is tensioned and then held against hardened concrete so that the concrete is permanently compressed, which is the essence of prestressing. Because concrete is strong in compression but weak in tension, locking in compression lets a member resist far larger service loads, span further, and crack much less than conventional reinforced concrete.

The principle is best understood by contrast with ordinary reinforcing bar. Rebar is passive: it only carries tension once the surrounding concrete has already cracked and transferred load into the steel. A prestressing tendon is active: it carries a large force the moment it is tensioned, before any service load arrives, and that force pre-compresses the concrete. To make this efficient the steel must be very strong, so prestressing strand uses high-carbon steel drawn to a tensile strength of roughly 1670 to 2160 MPa, against 400 to 600 MPa for typical rebar. The trade is that this high-strength steel is far less ductile, which is why anchorage, stressing and corrosion protection are engineered so carefully.

The industrial history runs from the 1928 patents of French engineer Eugene Freyssinet, who first recognized that creep and shrinkage made low-strength prestressing impractical and that only high-strength steel held at high stress could keep an effective force after losses. Seven-wire strand emerged in the United States in the 1950s and was standardized as ASTM A416 in 1957. The low-relaxation grade, produced by a thermo-mechanical stabilizing treatment under tension, displaced the older stress-relieved (normal-relaxation) product through the 1970s and 1980s and is now the global default. Unbonded mono-strand systems, with each strand greased and sheathed in plastic, opened up post-tensioned building slabs from the 1960s onward.

In application scale, strand spans from small precast elements to the longest bridges built. A single 15.2 mm (0.6 inch) Grade 270 strand carries a minimum breaking force of about 260 kN (58.6 kips); a large bridge tendon may bundle 19, 27 or even 55 such strands inside one duct to deliver several thousand kilonewtons. Annual world consumption of PC strand runs into the millions of tonnes, concentrated in bridges, building slabs, railway sleepers, prestressed pipe and tank wrapping, ground and rock anchors, and stay-cable systems. There is no single universal strand: the seven-wire geometry is near-universal, but diameter, grade, relaxation class and coating are matched to each application.

Four engineering properties decide whether a given strand is fit for purpose: breaking strength (and the related 0.1 percent proof force), relaxation class, ductility expressed as elongation at maximum force, and corrosion protection. The first three are intrinsic to the steel and its processing; the fourth depends on coating and on the surrounding grout, duct or concrete. A strand that meets the strength number but fails on relaxation or corrosion class will underperform in service even though it looks identical on the reel.

Strand Types and Geometry

Prestressing tendons come in several geometries, and choosing the wrong one is a common early error that forces a redesign of ducts, anchorages or jacking equipment. The primary families are single wire, two-wire and three-wire strand, the dominant seven-wire strand, and the large nineteen-wire strand used for stay cables. The table below summarizes the structures and where each is used.

Seven-wire strand dominates by tonnage because it balances high breaking force with the flexibility to follow curved tendon profiles. The six outer wires share load through the helical twist while the central king wire, made about two percent larger in diameter, holds the lay together and prevents the outer wires from nesting. The lay length, typically 12 to 16 times the strand diameter, is a controlled parameter: too short a lay wastes axial strength to the helix angle, too long a lay loosens the rope. Strand can be supplied plain (smooth) or indented, where shallow dimples are rolled into the outer wires to increase bond in pretensioned members, governed by ASTM A886 for indented seven-wire strand.

Compacted strand is drawn through a die after stranding so the wires deform from round into a denser cross-section, raising the steel area packed into a given outside diameter. A compacted 15.2 mm strand can carry a higher breaking force than a standard 15.2 mm strand, which lets engineers fit more capacity into an existing duct or anchorage footprint. The penalty is higher unit cost and slightly different anchorage wedge requirements, so compacted strand is reserved for force-limited or space-limited tendons rather than routine work.

Single PC wire and small two- or three-wire strand remain relevant for pretensioned mass-production items such as railway sleepers, electric poles and hollow-core floor planks, where many small tendons spaced closely give better bond and crack distribution than a few large ones. PC wire is covered by EN 10138-2 and ASTM A421, separate from the strand standards. Nineteen-wire strand and bundled multi-strand cables serve cable-stayed bridges and very large ground anchors, where a single tendon must reach thousands of kilonewtons and the assembly is engineered as a complete cable system with its own corrosion-protected sheathing.

A separate geometric distinction is bonded versus unbonded strand. Bare strand is used where it will later be grouted into a duct or cast directly into concrete, bonding to the surrounding material along its full length. Unbonded strand is factory-coated with corrosion-inhibiting grease and extruded inside a high-density polyethylene (HDPE) sheath, so it can slide freely and transfer force only at the anchorages. This is a property of how the strand is supplied and installed rather than of the steel itself, and it is examined further in Chapter 4.

Strength Grades and Relaxation Classes

Two intrinsic properties define a strand grade: tensile strength and relaxation class. Tensile strength sets the breaking force; relaxation class sets how much of the locked-in force survives over the life of the structure. The table below lists the mainstream grades across the three major standards, all for the seven-wire structure.

Grade naming differs by standard but the steel is comparable. ASTM A416 names grades in ksi, with Grade 250 (1725 MPa) and Grade 270 (1860 MPa). EN 10138-3 and GB/T 5224 name the same property in MPa, so the European Y1860S7 and the Chinese 1x7-1860 are the close equivalents of ASTM Grade 270. Grade 270 / 1860 MPa is the global workhorse: it gives high force per strand and is widely stocked. Higher classes (Y1960S7 at 1960 MPa, and the EN-listed Y2060S7 and Y2160S7) trade a little ductility for more force and are used where tendon count or anchorage size must be minimized. The minimum 0.1 percent proof force is set as a fixed fraction of the breaking force, commonly about 88 to 89 percent depending on grade, which is what the spec sheet calls Fp0.1 or the load at 1 percent extension.

Relaxation is the silent loss of prestress. When a tendon is stretched and then held at constant length, the steel slowly relaxes and sheds tension over time. It is measured in a standard test as the percentage of force lost after 1000 hours at 20 degrees Celsius, starting from an initial force equal to 70 percent of the actual maximum force. Two classes exist. Normal-relaxation (also called stress-relieved, EN relaxation Class 1) loses on the order of 8 percent. Low-relaxation strand (EN relaxation Class 2) loses about 2.5 percent or less, achieved by drawing the strand through a stabilizing thermo-mechanical treatment under tension during manufacture.

The practical consequence is large. Lower relaxation means a smaller long-term prestress loss, so the designer can rely on more effective force remaining in the tendon decades after stressing, which directly affects deflection, crack control and ultimate capacity. Because the gain is so cheap to manufacture, low-relaxation strand has displaced normal-relaxation strand almost completely in new construction; most current ASTM, EN and GB strand is supplied low-relaxation by default, and many specifications no longer permit the normal-relaxation product at all. When reading a legacy drawing, confirm which class was assumed, because substituting one for the other changes the calculated losses.

Relaxation is not an isolated number. It interacts with the other time-dependent losses (concrete creep and shrinkage) and with temperature: relaxation accelerates sharply at elevated temperature, which matters for steam-cured precast and for any tendon that may see fire or sustained heat. For high-temperature or unusually high-stress service, request the manufacturer relaxation curve at the relevant initial stress and temperature rather than relying on the single 70 percent, 20 degrees Celsius datum.

Coatings, Standards and Anchorage

Strand spends decades at high stress, where corrosion can cause stress-corrosion cracking or hydrogen embrittlement and a sudden brittle failure. Protection is therefore selected to match the exposure, and it is layered: the strand coating, the duct or sheath, and the surrounding grout or concrete each form a barrier. The table below summarizes the main coating types and their governing references.

Bare strand is the lowest-cost product and relies entirely on its surroundings: in pretensioning it bonds directly to the concrete, and in bonded post-tensioning it is cement-grouted inside a metal or plastic duct, which both bonds and shields it. It suits benign internal environments. Galvanized strand adds a hot-dip zinc layer that corrodes sacrificially before the steel, well suited to atmospheric and mildly aggressive exposure such as stay cables and permanent rock and soil anchors. Epoxy-coated strand (ASTM A882) carries a fusion-bonded epoxy film over the whole strand, sometimes filled into the interstices between wires, for the aggressive chloride exposure of marine structures and bridge decks treated with de-icing salt. Greased-and-sheathed strand combines corrosion-inhibiting grease with an extruded HDPE sheath and is the standard product for unbonded mono-strand tendons.

Standards govern more than coating. The three dominant strand standards are ASTM A416 in North America, EN 10138-3 in Europe and GB/T 5224 in China, with BS 5896, JIS G3536, AS/NZS 4672 and IS 14268 covering the United Kingdom, Japan, Australia and India. They agree on the seven-wire geometry, the 1000-hour 70 percent relaxation test and a nominal modulus near 195 to 197 GPa, but they differ on grade naming, the preferred diameter set, the exact proof-force ratios and the tolerance bands. A purchase specification must always name the standard plus diameter, grade and relaxation class together, because a bare diameter such as 15.2 mm is ambiguous across standards.

Bonded versus unbonded post-tensioning is the central system choice. In a bonded system, bare strands are threaded through a grouted duct and become mechanically tied to the concrete along their length, which raises ultimate capacity, improves crack control, and adds a grout corrosion barrier; it dominates bridges and heavily loaded members. In an unbonded system, each greased-and-sheathed strand transfers force only at its end anchorages and is free to slide, which is faster to install and standard in building slabs, but offers lower ultimate capacity and depends entirely on the anchorage integrity. Pretensioning is a third route, used in factory precast: strands are tensioned against abutments, concrete is cast around them, and the force transfers by bond when the strand is released.

Anchorage and the anchorage zone are inseparable from strand selection. In post-tensioning, the stressed strand is gripped by a hardened steel barrel-and-wedge anchor: two or three tapered wedges bite into the strand and lock it against a bearing plate that spreads the concentrated force into the concrete. The small seating slip as the wedges grip, on the order of 6 to 8 mm, is itself a prestress loss that the designer must account for. Strand, wedge and anchor are a matched system from one supplier and must not be mixed across brands. In pretensioning, no permanent anchor is needed because force transfers by bond over the transfer length, the short distance near each member end over which the strand stress builds from zero to its full value.

Key Specification Parameters

Reading a strand spec sheet is a core procurement skill. The same product may carry a dozen listed values, but selection turns on a handful: nominal diameter, nominal steel area, minimum breaking force, 0.1 percent proof force, modulus of elasticity, elongation at maximum force, and relaxation class. The two comparison tables below give real Grade 270 ASTM A416 values and the EN 10138-3 reference strand, so the numbers can be checked directly.

The table above is the Grade 270 (1860 MPa) low-relaxation seven-wire strand to ASTM A416, with values rounded as in manufacturer data. For comparison, the EN 10138-3 reference strand Y1860S7 at 15.7 mm has a nominal area of 150 mm², a nominal mass of 1172 g/m, a characteristic maximum force Fm of 279 kN, a maximum value of maximum force of 321 kN, a characteristic 0.1 percent proof force Fp0.1 of 246 kN, a modulus of elasticity taken as 195 GPa, and a minimum total elongation of 3.5 percent. The two standards land on the same 279 kN breaking force at this diameter, which is a useful cross-check that they describe the same product through different naming.

The table below decodes the eight parameters that actually drive a selection decision, with the typical span of values seen in current strand.

Breaking force and proof force are the load-side anchors of any calculation. Fm (the minimum breaking strength, called MUTS in North American practice) is the guaranteed failure load; Fp0.1 is the force at the 0.1 percent proof point, where permanent set begins, set as a fixed fraction of Fm. Designers do not stress to these limits: code practice for low-relaxation strand caps the jacking force near 80 percent of MUTS and the force just after lock-off near 70 percent of MUTS, leaving margin for seating, friction and time-dependent losses. Modulus of elasticity matters because it converts jacking force into the strand extension the crew reads off the ram during stressing; EN allows 195 GPa and PCI practice uses 28,500 ksi (about 196.5 GPa), both slightly below solid wire because the helix lets the outer wires straighten under load.

Elongation at maximum force (Agt) is the ductility check: EN 10138-3 sets a minimum total elongation of 3.5 percent, which guards against the brittle failure that high-strength steel can otherwise exhibit. Relaxation class, covered in Chapter 3, governs long-term loss. Mass per length drives both dead-load and commercial pricing, since strand trades by weight. Coating is the corrosion class from Chapter 4. A complete strand call-out names all of these together, for example: 15.2 mm, Grade 270 / Y1860S7, low-relaxation (Class 2), bare for grouted bonded tendon, to ASTM A416 or EN 10138-3.

Selection Decision Factors

To turn the preceding chapters into a specific purchase, follow the decision sequence below. Most selection mistakes are not a single wrong number but a decision taken at the wrong level, such as fixing a diameter before the prestressing system is chosen. These eight steps can serve as a fixed RFQ template.

1. Prestressing system first: Decide pretensioning (factory precast, bond transfer) versus post-tensioning, and within post-tensioning bonded (grouted duct) versus unbonded (greased and sheathed). This sets whether you order bare or greased-and-sheathed strand and dictates the duct and anchorage hardware.
2. Diameter and grade: Choose the diameter (commonly 12.7 mm / 0.5 in or 15.2 mm / 0.6 in) and grade (Grade 270 / Y1860S7 is the default) to deliver the required tendon force with a sensible strand count. A 15.2 mm Grade 270 strand gives about 261 kN minimum breaking force; size the tendon so each strand works within code stress limits.
3. Relaxation class: Specify low-relaxation (EN Class 2) for all structural work; confirm the legacy drawing assumption before substituting, since relaxation class changes the calculated long-term losses.
4. Coating and exposure: Match coating to environment per Chapter 4: bare for internal grouted tendons, galvanized for atmospheric anchors and cables, epoxy (ASTM A882) for marine and de-icing-salt decks, greased-and-sheathed for unbonded tendons. For severe exposure, layer barriers.
5. Anchorage system: Select a matched barrel-and-wedge anchorage from a recognized supplier sized to the strand and tendon force. Strand, wedge and anchor are a single qualified system and must not be mixed across brands. Confirm the European Technical Assessment or equivalent approval for the system.
6. Stressing parameters: Define jacking force (commonly about 80 percent of MUTS for low-relaxation strand), lock-off force (about 70 percent of MUTS), expected elongation from the strand modulus, and the wedge seating slip (about 6 to 8 mm) so the site crew can verify force by both gauge pressure and measured extension.
7. Standard and certification: Name the governing standard (ASTM A416, EN 10138-3, GB/T 5224, BS 5896 or JIS G3536) and require mill test certificates reporting breaking force, proof force, elongation and relaxation for the heat supplied. Cross-region projects may need more than one approval.
8. Total cost of ownership: Compare delivered strand price by weight against the cost of duct, grout or sheathing, anchorage hardware, and stressing labour, not strand alone. A cheaper strand that forces a larger tendon count or extra anchorages rarely wins on installed cost.

One last commonly overlooked dimension is system serviceability and provenance: traceable heat numbers, an unbroken coil identity from mill to site, anchorage spare-part availability, and the supplier capability to provide the relaxation curve and stress-corrosion test data for the exact heat. These seem like paperwork at the purchasing stage, but a prestressing tendon cannot be inspected or replaced once it is grouted into a structure, so the quality of the documentation is the quality of the asset for its whole service life. Established strand and system suppliers include ArcelorMittal, Bekaert, Insteel, Sumiden Wire and Tycsa on the strand side, and Freyssinet, VSL, DYWIDAG and BBR on the anchorage and system side.