Post-tension slab stressing field guide

Post-tensioning, and what compression buys the slab

Post-tensioning is a method that pulls high-strength steel tendons through concrete after it has cured, then anchors them so the slab is squeezed into compression for the life of the structure. The concrete is poured around the tendons but they are not stressed yet. Once the concrete reaches strength, a hydraulic jack pulls each tendon tight, the anchor locks the force in, and the whole slab is now precompressed. That precompression is the whole point.

Concrete is strong in compression and weak in tension, and a flat slab spanning between supports goes into tension on the bottom at midspan and the top over the supports. A conventionally reinforced slab lets that tension crack the concrete and counts on the rebar to hold the crack tight. A post-tensioned slab does something different. It loads the concrete with so much built-in compression that the service loads have to overcome that compression before the section ever sees net tension, so the slab spans farther on less depth, deflects less, and the cracks that do form stay closed.

That is why you see post-tensioning on long-span parking decks, transfer slabs, and big flat plates where a conventional slab would be too thick or too cracked. It is also why the work is unforgiving. The force in those tendons is enormous, it is locked in permanently, and almost every part of the job, the strength gate, the elongation check, the cutting and coring, the demolition, traces back to that one fact. The slab is a loaded spring, and it stays loaded.

Bonded or unbonded: which post-tensioning is this?

The first thing to know on any PT job is which system you have, because bonded and unbonded behave differently in service and on the day you have to cut into the slab. Unbonded is the common building slab. Bonded shows up on bridges, transfer girders, and heavy structural work.

Unbonded post-tensioning uses a monostrand: a single seven-wire strand coated in corrosion-inhibiting grease and run inside a continuous plastic sheath, usually high-density polyethylene. The grease and the sheath let the strand slide freely through the concrete during stressing, and they are the strand's permanent corrosion protection. The strand is never bonded to the concrete along its length. It transfers all of its force to the slab only at the two anchors. The consequence matters: cut or break an unbonded tendon anywhere and you lose the prestress along the entire length of that tendon, often violently.

Bonded post-tensioning runs the strands inside a corrugated metal or plastic duct cast into the concrete. After the tendon is stressed and locked off, the duct is injected full of cementitious grout, which bonds the strand to the surrounding concrete along its whole length. Now the force transfers through the bond, not just at the anchors. Damage one spot and the force redistributes locally through the grout instead of dumping the whole tendon, so a bonded system is more forgiving of a localized hit. The grout is also the corrosion protection, which is why grouting voids are the long-term failure people chase on bonded work. There is a hybrid in between, a greased and sheathed strand grouted inside a duct, used more in Europe than in North America. Confirm the system from the drawings and the PT supplier before you do anything, because the repair, the cutting rule, and the corrosion story all change with it.

The system: strand, anchor, sheathing, and pocket

The strand is the muscle. Building post-tensioning uses seven-wire prestressing strand to ASTM A416, six wires laid helically around a slightly larger center wire, in 0.5 in or 0.6 in diameter, almost always Grade 270 and low-relaxation. Grade 270 means a specified tensile strength of 270 ksi, and the cross-sectional areas to carry in your head are about 0.153 sq in for the 0.5 in strand and 0.217 sq in for the 0.6 in. That is the steel doing tens of thousands of pounds of work in a strand you can hold in one hand.

The anchor is what holds it. At each end a steel anchor casting bears against the concrete, and the strand is gripped by a set of tapered wedges that seat into a matching taper in the casting. When the jack releases, the strand tries to pull back, the wedges bite, and that biting action is what locks the force in. The wedges are a consumable, they leave teeth marks in the strand, and a wedge that does not seat clean is a tendon that does not hold. One end is the stressing end, where the jack works against a pocket former that leaves a recess for the jack and the tail. The other is commonly the dead end, an anchor pre-set on the strand at the shop or buried in the slab, that takes no jack.

Around the strand is the sheathing, the grease-filled plastic on an unbonded tendon or the duct on a bonded one. Where the sheath meets the anchor, an encapsulated system wraps the connection in a sealed plastic transition and a cap so there is no bare steel exposed from one end to the other. Learn the parts by name, because the inspection and the failure modes all land on one of them: the strand, the wedges, the casting, the sheath, or the seal at the pocket.

What is the tendon profile and why does the drape matter?

The tendon profile is the up-and-down path the tendon follows through the slab, and it is the design, not a detail you eyeball. Tendons are draped: high over the supports and low at midspan, following the shape of the tension in the slab. High over the column where the slab wants to crack on top, low at midspan where it wants to crack on the bottom. That curved path is what lets the tendon push back against the load along the whole span, and the force the engineer designed assumes the strand sits exactly on that profile.

The profile is held by support bars and chairs set to the height the placing drawings call out at each point along the tendon, the same way a rebar mat is held to its cover. Get the chair heights wrong and the tendon is in the wrong place, so the force is in the wrong place, and the slab does not behave the way it was designed even if the stressing numbers all check out. The PT placing drawings give the height of the tendon above the form at marked points, usually as a profile schedule, and the field check is a tape from the form to the strand at those points before the pour. Confirm the high points, the low points, and the spacing of the supports so the strand does not sag between chairs and flatten the drape.

There is one more thing to capture before the concrete buries any of it. Mark where the tendons run. The day someone needs to core a hole or set an anchor in that slab, the only thing standing between them and a cut tendon is a record or a scan of where the steel is. You never drill a post-tensioned slab without locating the tendons first, and the cheapest time to know where they are is now, while you can still see them. The same locating discipline runs through the concrete scanning work covered in the scanning guide.

How strong does the concrete have to be before stressing?

You do not stress until the concrete has reached the strength the drawings require, confirmed by cylinder breaks, and that strength gate is the first hold point on a PT pour. A common minimum is around 3000 psi at the anchorage before any stressing, but the number lives on the structural drawings and the PT supplier's specification, not in a rule of thumb, and it is often higher. The strength has to be there because the entire tendon force concentrates into a small block of concrete behind each anchor.

Stress too early and you crush the anchorage zone. The casting drives into concrete that has not gained enough strength to take the bearing, and you get spalling, cracking, or blowout right behind the anchor, which is a repair nobody wants and a delay that costs more than waiting did. The way crews get burned is reading a calendar instead of a cylinder. Concrete gains strength on temperature, not on age, so a cold pour that is three days old can be well short of a warm pour at the same age.

The confirmation comes from field-cured cylinders, not just the standard lab-cured set. The lab cylinders tell you the mix is capable. The field-cured cylinders, kept beside the slab in the same weather, tell you the concrete in the actual slab has the strength right now, which is the question stressing actually asks. Many specs call for partial stressing at one strength and full stressing at a higher one, so the slab can carry its own forms early and take full force later. Break the cylinders, read the number against the drawing, and only then bring out the jack.

The stressing operation

Stressing is done with a hydraulic jack that grips the strand tail, pulls it to a target force, and holds while the wedges seat. The jack reads in hydraulic pressure on a gauge, and the force in the strand comes off a calibration chart that pairs that specific jack and gauge to a force. That is why the jack and gauge are a calibrated matched set with paperwork. A common jacking force is about 0.80 of the strand's specified tensile strength, but the design force is on the stressing schedule and that is the number you stress to, read through the calibration chart, not a generic pressure off the pump.

The sequence follows the supplier's drawing, and you follow it in order. Tendons are stressed in a pattern that brings the slab into compression evenly, not one corner at a time, because uneven stressing can crack a slab that is only partly precompressed. After the jack reaches force, it is released and the wedges seat, and the strand draws back slightly as the wedges bite. That draw-in, the anchor set or seating loss, is commonly around 1/4 in to 3/8 in, and it is a real loss of elongation that the calculation has to account for.

Length decides whether you stress from one end or both. Short and medium tendons are usually stressed from one end, with a dead anchor at the far end, and friction along the tendon means the far end carries somewhat less force than the jack end. Long tendons are stressed from both ends, one end then the other, so friction does not starve the middle, and on a both-end tendon the lowest force ends up near the center rather than at a dead anchor. The drawings call out which tendons are one-end and which are two-end. Lock-off is the moment the wedges hold and the jack comes off, and from that point the force is in the slab to stay.

Why elongation is the real proof of force

The gauge tells you the pressure. The elongation tells you whether the force actually got into the strand, and that is why elongation is the acceptance measurement, not the gauge alone. A strand stretched to a known force elongates a predictable amount, set by the force, the length, the steel area, and the strand's modulus of elasticity. Measure how far the strand actually moved and compare it to what it should have moved, and you have an independent check on the force that a stuck gauge or a fouled jack cannot fake.

The crew marks the strand at a reference before stressing, stresses to the target gauge pressure, and measures the movement at lock-off, accounting for the wedge seating draw-in. That measured elongation gets compared against the calculated, or theoretical, elongation on the stressing schedule. The calculation comes from force times length divided by area times modulus, with the strand modulus, commonly about 28,500 ksi, taken from the mill certificate for that strand. One caution worth keeping straight: force and elongation are correlated, not perfectly proportional, so a single off reading is a flag to investigate, not an automatic reject.

The reason the trade leans on elongation is that it catches the failures the pressure gauge hides. A jack out of calibration, a wedge that slipped, a tendon snagged on a misplaced chair, a strand fouled in its sheath, all of these show up in the elongation when the gauge looks fine. The elongation is how you prove the right force went in, and it is the number the engineer of record looks at first on the stressing report.

P (avg)

The average force in the tendon over its length, after friction and seating losses

L

The tendon length being stressed, in consistent units with the elongation

A (ps)

The cross-sectional area of the prestressing strand, about 0.153 sq in for 0.5 in strand

E

The strand modulus of elasticity, commonly about 28,500 ksi, from the mill certificate

What elongation tolerance is allowed?

Measured elongation is accepted when it falls within plus or minus 7 percent of the calculated elongation, and that figure is the one to carry. ACI 318 recognizes that real friction, real seating, and real strand properties produce a spread between calculated and measured, and it sets a tolerance commonly applied as plus or minus 7 percent, with the Post-Tensioning Institute giving the same band for unbonded building tendons. The structural drawings and the PT specification can tighten it, so confirm the number for the project before you accept on it.

The 7 percent is most defensible as a group or average tolerance rather than a single-tendon pass-fail, because short tendons and odd profiles scatter more. The Post-Tensioning Institute applies the tolerance against the average tendon elongation, capping any tendon near 107 percent of the average. For the short tendons common in residential slab-on-ground foundations, the PTI slab-on-ground guidance widens the allowance, commonly to about plus or minus 10 percent, because a few inches of stretch over a short run is hard to read to 7 percent. Use the band the project and the adopted standard call for, not the one from the last job.

Treat a reading near the edge of the band as a reading to verify, not a reading to wave through. Check the length used, the calibration chart, the seating loss assumed, and the strand the calculation was run for. The tolerance exists to absorb honest variation. It does not exist to absorb a wrong number, and the difference between the two is the investigation in the next section.

When elongation comes back out of tolerance

An out-of-tolerance elongation is a finding to investigate, not a number to accept or to quietly re-stress away. The two directions point at different problems, and reading which one you have is the first move.

High elongation means the strand stretched more than calculated. The usual suspects are lower friction than assumed, a wedge that slipped or seated late so the strand kept moving, or a tendon that is actually understressed, where the force is low and the easy travel reads as extra stretch. Slip at the anchor is the one to rule out fast, because a slipped wedge can read as generous elongation while the locked-in force is short. Pull the lift-off if there is doubt and confirm the strand is holding the force, not just that it moved.

Low elongation means the strand stretched less than calculated. That points to higher friction than assumed, a blockage or a snag in the sheath or duct, a tendon hung up on a misplaced support, or genuine overstressing where the force is high but the strand cannot travel. Low elongation is the more worrying side, because a tendon that will not stretch can be carrying more force than intended right up to the moment it lets go. Find the cause: re-check the calculated length and modulus, look for a kink or a tight radius, and on a bonded tendon suspect the duct. The right close is to identify the mechanism and get the engineer's disposition, in writing, before that tendon is accepted.

Stay out from behind a live tendon

This is the one that kills people, so it gets blunt treatment. Never stand behind a tendon being stressed, and never put any part of yourself in line with the jack. A strand or a wedge that lets go during stressing comes back through the anchor and the jack with enough energy to go through a person. It is not a pinch hazard. It is a projectile hazard, and the projectile is a steel cable under tens of thousands of pounds of tension.

Set up the exclusion zone before the jack pumps. Barricade the area behind every live anchor and behind the jack, post signage that says stressing is in progress and to keep clear, and keep everyone who is not running the jack out of the line of fire. Backing boards, the stressing barricades set behind the live end, are common practice and are placed close behind the anchor to catch a release. The operator works to the side of the jack, not behind it, and a second person verifies the wedges seat without ever stepping into the line.

The dead end is in the line of fire too, not just the jack end, because a failure there fires the strand the other way. Calibrated and maintained equipment is part of the safety case, since a jack that fails under load is its own hazard, but the discipline that actually keeps the crew alive is simple and absolute: while a tendon is live, the space behind it is empty. No shortcuts, no exceptions, no one ducking through to grab a tool.

Cutting the tails and sealing the pocket

After lock-off and after the elongation is accepted, the strand tails sticking out past the anchors get trimmed, and how you cut them matters. Leave a stub past the wedges, commonly a short tail rather than a flush cut, so the wedges are not disturbed. The point is to cut the tail without ruining the grip or the corrosion seal you are about to build over it.

Do not torch the strand right at the anchor in a way that heats the wedges or the strand at the grip. Heat changes the steel, an over-cooked cut near the wedges can anneal and weaken the very spot that has to hold the force, and a careless flame cut is how a held tendon turns loose later. Many crews use an abrasive saw or a shear to keep the heat down at the anchor. Cut the tail, check the wedges are still seated, and keep the flame away from the grip.

Then seal the pocket, because the recess at the anchor is the most exposed point on the whole tendon. On an encapsulated system the anchor gets a sealing cap, often grease-filled, over the wedges and the cut strand, and the pocket is filled with a non-shrink grout flush to the slab face. That seal is the corrosion protection at the one place water and chlorides reach the steel most easily. An unsealed or poorly sealed pocket is one of the most common ways a PT anchor corrodes and fails years later, so the pocket fill is a real step with a real inspection, not a cosmetic patch.

Corrosion protection and the long game

Corrosion is how post-tensioning fails over the long run, so the protection is not an extra, it is the system. A tendon is highly stressed steel, and stressed steel under chloride attack can fail in ways that ordinary rebar does not, including sudden brittle wire breaks. The protection differs by system, and knowing which one you have tells you what to inspect.

On an unbonded monostrand the corrosion protection is the grease inside the sheath along the length, plus the encapsulated anchor and the sealed pocket at each end. The grease has to be continuous and the sheath unbroken, because a gap in the sheath or a dry spot in the grease is where corrosion starts. On a bonded tendon the protection is the duct grout, and it has to fill the duct completely. Voids in the grout, often at high points where air gets trapped, leave bare strand exposed inside the duct, and grout voids are the failure inspectors hunt on bonded structures.

The exposure drives how hard you push on all of this. Parking decks, marine structures, and anything seeing deicing salt are the aggressive cases where the encapsulated and fully grouted detail earns its keep, and where a sloppy pocket seal or a grouting void shows up as a corroded tendon in a decade rather than never. Protect the steel from end to end: continuous sheath and grease or full grout along the length, encapsulated anchors, and sealed pockets. The day the corrosion protection fails is the day the clock starts on the tendon.

Field example: stressing a 100 ft monostrand

Take a single 0.5 in, seven-wire, Grade 270 low-relaxation monostrand, 100 ft long, stressed from one end with a dead anchor at the far end. The strand area is 0.153 sq in, the design jacking force off the stressing schedule is about 33 kip at roughly 0.80 of the strand's tensile strength, and the gauge pressure that produces it comes off the calibration chart for that jack and gauge.

The schedule gives a calculated elongation of about 8.5 in for this tendon, run from the average force, the 100 ft length, the 0.153 sq in area, and a strand modulus near 28,500 ksi, with the seating loss accounted for. The crew marks the strand, stresses to the gauge pressure on the chart, seats the wedges, and measures the strand movement at lock-off. It reads 8.1 in. The difference is minus 4.7 percent, inside the plus or minus 7 percent band, so this tendon is accepted on elongation.

Now change the read and watch what the inspector does. Come back at 9.4 in, plus 10.6 percent, and you do not accept it, you investigate: suspect a slipped wedge or low friction, pull a lift-off, and confirm the locked force before signing anything. Come back at 7.5 in, minus 11.8 percent, and you investigate the other way: suspect high friction or a snag, check the length and modulus used, and look for where the strand is hung up. The number on the tape decides whether you sign, and the record below is what proves you read it right.

The stressing record

The stressing record is the document an engineer signs off on, and it is the only proof that the force the design needs actually got into the slab. The inspector witnesses the stressing, and the record is what survives the day. Build it tendon by tendon, at the jack, as the work happens, not reconstructed afterward from memory, because the elongation reading is gone the moment the next tendon is stressed.

Capture, for each tendon, the tendon mark, the jack and gauge used with their calibration reference, the gauge pressure and the force it represents, the calculated elongation, the measured elongation, the percent difference against the tolerance, the concrete strength at stressing and the date, and the accept or investigate call with who made it. When a tendon went out of tolerance, log the investigation and the engineer's disposition, because an out-of-tolerance reading closed without a documented cause is a hole in the record the engineer will find. The table below is the spine of a stressing record an engineer will accept.

PT slab joints: pour strips and the closure strip

Post-tensioned slabs are jointed on a different strategy than conventional slabs, so do not bring the 24-to-36-times-thickness control-joint habit from the joint-layout guide to a PT floor. The precompression holds shrinkage cracks closed and lets a PT slab run far between joints, but the slab also shortens as it is stressed and as it dries, and if it is locked to a stiff wall or core while it shortens, it cracks from the restraint. The PT joint strategy is built around letting the slab shorten before it is tied down.

The common tool is the pour strip, also called a closure or delay strip. A gap, often a few feet up to several feet wide, is left open across the slab between two regions that are poured and stressed separately. The two sides get stressed with the gap open, so each region is free to shorten on its own without dragging on the other, and the gap is left open for a stretch of time, commonly on the order of three to four weeks, while most of the shortening happens. Then the strip is filled and reinforced to tie the regions together. Closing it too early defeats the purpose, because the slab is still shortening into the joint you just filled.

The stressing sequence and the joint and pour-strip layout come from the PT design, and they go together. The slabs on each side of a pour strip have to be fully separate when they are stressed for the strip to do its job. Where the design uses pour strips, the closure timing, the reinforcement across the strip, and any tendons that get stressed or coupled there are all on the PT drawings, and the field follows that sequence rather than improvising joints.

Can you drill or core into a post-tensioned slab?

Not until you have located the tendons and confirmed your hole misses them. You can drill, core, and saw a post-tensioned slab, and it happens all the time, but never blind, because cutting a live tendon is catastrophic. The rule is absolute and it has no convenient exceptions: scan and locate before any core, drill, anchor, or saw cut, every hole, every time.

Here is the physics behind the rule. Each tendon is holding on the order of 24,000 to 33,000 pounds of force, and on an unbonded system that force runs the full length of the strand. Nick it with a core bit or a saw and the tendon can burst out of the slab with enough energy to maim or kill, and at the same time you have just dumped the prestress the slab was counting on, which is a structural problem on top of the safety one. A small drill is not safe cover. Even a shallow anchor into a tendon is a strike.

The way you do it right is to scan with ground-penetrating radar, mark the tendon runs and the rebar on the slab, and lay out the penetration in the clear space between them, the same locating discipline covered in the scanning guide. On a managed job the cutting does not start until the scan is done and the scan report is issued, and that hold point is there for a reason. The as-built tendon layout from the original pour helps, but you still scan, because tendons drape up and down and a plan view does not tell you the depth at your hole. Scan it, mark it, then cut. Never the other way around.

Detensioning: demolition and renovation of a PT slab

Cutting up a post-tensioned slab is specialist work, because every tendon in it is a loaded spring that has to be released under control, not surprised. This is detensioning, and on an unbonded slab it is the demolition hazard that gets people hurt when a crew treats a PT slab like an ordinary one. You do not just start sawing.

The work is to release the tendon force deliberately before the concrete around it comes apart. On an unbonded system that can mean re-stressing a tendon slightly to free the wedges and then letting it down in a controlled way, or cutting in a planned sequence with shielding and exclusion zones so a released strand has nowhere dangerous to go. The energy is the same energy you respect during stressing, only now you are giving it back, and an uncontrolled release during demolition fires the strand exactly like a failure during stressing does.

Renovation cuts that leave the rest of the slab in service are their own problem, because you are severing some tendons while others stay live, and the slab has to be shored and the load path checked so the remaining structure carries the load without the tendons you removed. This is engineered work with a specialty contractor and the engineer of record, not a field call with a demo saw. If the job involves opening up, cutting through, or taking down a post-tensioned slab, the detensioning plan comes first.

What to document

The PT record is the spine of the QA file, because almost nothing about post-tensioning can be verified after the fact. The strength at stressing is a moment in time, the elongation is gone after lock-off, and the tendon layout disappears under the finish. Document it as it happens, tendon by tendon and pour by pour, and tie each record to a location in the structure so two years out somebody can stand in the building and find what was proven where.

Beyond the per-tendon stressing data, capture the system and materials, the strand size and grade and the ASTM A416 certification, the tendon profile and chair-height verification before the pour, the cylinder breaks that opened the strength gate, the pour-strip closure dates, and the pocket sealing. The table is the per-tendon core of it; the rest rides alongside in the QA package.

Common mistakes

• Stressing before the concrete reaches strength, crushing or spalling the anchorage zone.
• Reading a calendar instead of field-cured cylinders to decide the concrete is ready.
• Accepting an elongation that is out of the plus or minus 7 percent band without investigating the cause.
• Trusting the gauge pressure alone and skipping the elongation check that proves the force.
• Standing behind the jack or in line with a live tendon, or letting anyone into the exclusion zone.
• Drilling, coring, or saw cutting a PT slab without scanning and locating the tendons first.
• Flame-cutting the strand tail at the anchor and annealing the wedges or the grip.
• Leaving the anchor pocket unsealed or poorly grouted, so corrosion starts at the most exposed point.
• Setting chairs to the wrong height, so the tendon is off its design profile even when stressing checks out.
• Treating a post-tensioned slab like a conventional one during demolition, with no detensioning plan.

Field checklist

Standards and references

The structural drawings, the PT supplier's stressing schedule, and the engineer of record govern, full stop. They set the force, the profile, the stressing sequence, the strength gate, and the acceptance, and where the contract documents are stricter than any value here, they win. Everything below is the framework those drawings are built on.

ACI 318, the building code for structural concrete, carries the post-tensioning design and construction provisions, including the recognition that measured and calculated elongation differ and the tolerance commonly applied as plus or minus 7 percent. ACI 423 covers prestressed concrete and unbonded tendon work. The Post-Tensioning Institute, the PTI, publishes the field manuals and the certification programs that the trade runs on, including its stressing and elongation guidance and the inspector certifications, and the plus or minus 7 percent tolerance on average elongation, widened for short slab-on-ground tendons, traces to ACI 318 together with PTI field and inspection manual guidance. The exact section, table, and document numbers shift between editions and revisions, so confirm them against the adopted edition before you cite one on a submittal.

On the materials side, ASTM A416 is the specification for the seven-wire prestressing strand, including Grade 270 and the low-relaxation type. The post-tensioning work on most structural projects is a code-required special inspection, performed by a qualified inspector independent of the installer, witnessing the stressing and reviewing the elongation record. Name the standard that controls the point, and let the project specification and the PT supplier's details override any rule of thumb when they are stricter.

Units, terms, and conversions

Post-tensioning carries its own vocabulary, and the same idea reads differently across a drawing set, a supplier submittal, and a metric spec. Strand diameter is in inches on US jobs, 0.5 in and 0.6 in, and in millimeters elsewhere, about 13 mm and 15 mm. Force shows in kip, thousands of pounds, where 33 kip is about 147 kN, and strand strength in ksi, where Grade 270 is about 1860 MPa. Elongation reads in inches and millimeters, where 8 in is about 200 mm.

The terms below are the ones that travel across the whole operation, from the strand on the reel to the signed stressing record.

Post-tension

Stressing tendons after the concrete has cured, putting the slab into compression

Monostrand

A single seven-wire strand, greased and sheathed, the common unbonded building tendon

Bonded / unbonded

Bonded strands are grouted to the concrete in a duct; unbonded slide free, anchored only at the ends

Elongation

The measured stretch of the strand under load, the independent proof the design force went in

Lock-off / anchor set

The moment the wedges seat and hold; the small strand draw-in is the seating loss, about 1/4 in to 3/8 in

Plus or minus 7 percent

The common acceptance band for measured versus calculated elongation, per ACI 318 and PTI

Exclusion zone

The barricaded, cleared area behind every live tendon and the jack during stressing

Jacking force / fpu

The force applied by the jack, commonly near 0.80 of the strand tensile strength, fpu

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