Post-Tensioning

Post-tensioning is one half of prestressed concrete. The Concrete Society's published description of the principle is short: concrete is strong in compression and weak in tension, so tensile stresses caused by bending are counteracted by putting the section into compression before the load arrives. In post-tensioning that compression is applied after the concrete has been cast and has hardened. Ducts are built into the element, tendons are threaded through them, hydraulic jacks pull the tendon against one end while the other end is anchored, and when the jack releases, the stored energy in the steel transfers into the concrete through the end anchorages as a compressive force. The ducts are then normally filled with a cement grout to protect the steel from corrosion and to help transfer the prestressing force into the concrete. Post-tensioning can also be applied with tendons or bars sitting outside the concrete section and connected back to it, an arrangement described in the same guidance as external post-tensioning, whose distinguishing characteristic is that the steel remains accessible for inspection and monitoring.

The sector covered by this page is the specialist trade that does that work: the design of the post-tensioning to a structural engineer's performance specification, the supply of the system, the fixing of ducts and anchorages before the pour, the stressing operation, the grouting, the records, and - increasingly - the investigation, remedial grouting, strengthening, de-tensioning and demolition of structures that were post-tensioned decades ago. It is a licence-to-operate trade in a way that most concrete work is not, because the certification scheme described later in this page is written into the specifications that let a contractor bid at all.

The document framework is layered and unusually clear about which document does what. Design of concrete structures in the UK sits under Eurocode 2. The first-generation BS EN 1992 series remains the applicable set for UK works until BSI withdraws it on 30 March 2028, with the second generation - including BS EN 1992-1-1:2023, published 30 November 2023 and covering general rules and rules for buildings, bridges and civil engineering structures - published progressively through a period of coexistence that BSI's own product record describes as running from 2023 to 2028. Eurocode 2 does not, on its own, tell a designer how to lay out and check a post-tensioned floor. That gap is filled by The Concrete Society's Technical Report 43, Post-tensioned concrete floors, whose third edition was issued in 2025 as a guide to design and construction covering structural form and behaviour, materials, the design process, details, alterations and demolition; the second edition of 2005, with its 2007 addendum, is now superseded. For durability of grouted post-tensioning in bridges and buildings, the reference is Technical Report 72, Durable post-tensioned concrete structures, published in 2010 as a revision of Technical Report 47 on durable bonded post-tensioned concrete bridges, extended to cover buildings as well as bridges and aimed at design, detailing, specification, materials, construction methods and testing for grouted post-tensioning with internal or external tendons.

Execution sits under BS EN 13670:2009, Execution of concrete structures, which covers the placing and stressing of prestressing steel among the rest of the concrete works. The materials have their own standards: strand and wire to BS 5896:2012, prestressing bar to BS 4486:1980, steel strip sheaths for ducts to BS EN 523:2003, and grout to the BS EN 445, 446 and 447 set published together on 30 November 2007. Anchorages and couplings are tested to BS EN 13391:2004 and assessed for European Technical Assessment against EAD 160004-00-0301, the European Assessment Document for post-tensioning kits that carried forward the requirements of the earlier guideline ETAG 013.

For highway structures the management regime is separate and explicit. DMRB CS 465, Management of post-tensioned concrete bridges, published in March 2020 in place of the earlier BD 54/15, sets out a process of risk review, risk assessment and risk management for the post-tensioned bridge stock. Its published summary is blunt about why it exists: tendons can be vulnerable to corrosion and severe deterioration where internal grouting of ducts is incomplete and moist air, water and contaminants can enter the duct system, and safety-critical defects in post-tensioned concrete are typically hidden, very difficult to detect and may produce a brittle mode of failure. Railway structures are managed under Network Rail's own standard for post-tensioned concrete bridges, which a CROSS expert panel identifies as NR/2/CIV/032/MOD03, issued in 2021.

The general construction regime applies as it does everywhere else. Construction work sits under CDM 2015, and the CROSS report on a stressing failure discussed later on this page turns on the contractor's risk assessment and method statement under those regulations. Demolition sits under BS 6187:2011, the code of practice for full and partial demolition, and the National Federation of Demolition Contractors' guidance DRG102:2022 on deconstruction of structures names post-tensioned elements specifically among the special forms of construction that pre-demolition information is expected to identify.

One current point of fact needs stating precisely because it changed recently and a great deal of published commentary is now out of date. The Building Safety Regulator has been a standalone body since 27 January 2026, moving out of the Health and Safety Executive to become an executive non-departmental public body sponsored by the Ministry of Housing, Communities and Local Government, under the Building Safety Regulator (Establishment of New Body and Transfer of Functions etc.) Regulations 2026, SI 2026/20. GOV.UK's own announcement of that date describes it as a step towards a single construction regulator. The BSR is not part of HSE, and descriptions that place it there describe the position before January 2026.

Finally, what this page is not. It is not a design guide and it does not answer the question of which system suits a given structure, because the published guidance does not answer that either: bonded and unbonded systems have different characteristics, different failure behaviour and different consequences for later alteration, and matching them to a structure is the designer's judgement on the specific facts. Where this page names a document, the document governs. Where it names a certification body or a regulator, their current published position governs, not this summary of it.

Start with the tendon. In UK building work the tensile element is almost always seven-wire strand to BS 5896:2012, the British Standard for uncoated high tensile steel wire and strand for the prestressing of concrete, published on 31 May 2012. High tensile alloy steel bars are covered separately by BS 4486, and bar systems have their own certification appendix because they behave and are handled differently. Strand arrives in coils, is delivered clean and free of permanent bends, kinks, pitting and scale, and is kept away from welding operations - the CARES model specification is explicit that corrosion which cannot be removed by hand and has permanently marked the surface makes the strand unusable. That fussiness has a reason: the strand is working at a large fraction of its breaking load for the life of the structure, and a pit is a stress raiser in a component that has no redundancy.

The tendon runs inside something. In bonded slab systems that is a flat duct, formed from spirally wound or seam-folded galvanised metal strip, and the flexibility of the duct sets the limit on the curvature the tendon can follow. BS EN 523:2003 covers uncoated corrugated steel strip sheaths up to 130mm nominal internal diameter and their connectors, and is explicit that it does not cover plastic sheaths; plastic duct systems are used where a higher level of protection is wanted, and the CARES model specification points designers at fib bulletin 75 for those. In unbonded systems the strand is greased and individually sheathed in extruded high density polyethylene or polypropylene, and the sheath is the permanent protection rather than a former for grout.

The geometry is the design. A post-tensioned tendon is not a straight line: it is draped, high over the supports and low in the span, so that when it is stressed the curvature pushes upward against the load in the span and downward over the columns. That upward push is why a post-tensioned floor deflects less than a reinforced concrete floor of the same depth, and why the tendon profile matters more than almost anything else on site. The Post-Tensioning Association's procurement guidance puts tendon drape at the head of its list of details that decide whether the finished slab has the strength the design assumed - a tendon fixed out of position, or floated up during the pour, produces a slab that looks identical and behaves differently.

At each end sits an anchorage. Stressing anchorages are the live ends, where the jack works: an anchor head with tapered wedges that grip the strand, a bearing plate that spreads the force into the concrete, and a trumpet or sleeve that guides the strand into the duct. Fixed anchorages are the dead ends, and the European Assessment Document for post-tensioning kits describes them as either mechanical devices or anchorages formed by bonding the tensile element into the concrete. The concrete immediately behind an anchorage carries an intense, locally concentrated force, so the anchorage zone is reinforced with bursting steel designed for the purpose - a detail whose absence is exactly what a CROSS commenter looked for in the photograph of a burst slab end discussed later on this page.

The stressing sequence follows the concrete. Because the force is applied against hardened concrete, the concrete has to be strong enough first. Partial stressing at an early age is commonly used to control early-age cracking, and the CARES model specification requires stressing not to start before the concrete has reached the transfer strength set by the post-tensioning designer, with that strength established from match-cured cubes taken at the point of placement - cured at the same temperature as the element they represent - and, because strength varies between the first and last loads of a pour, based on cubes taken from the last load. The jack then extends the strand, the wedges seat, and the force locks off with a small loss as the wedges draw in. That draw-in is one of the terms the designer allows for when calculating what the tendon should have stretched.

Which brings in the check that makes post-tensioning auditable in a way ordinary reinforcement is not. The force in a stressed strand can be inferred from how far it moved. The designer calculates a theoretical extension from the tendon profile, the strand area and modulus, assumed friction and wobble coefficients and the allowance for wedge draw-in; the operatives measure the actual extension before and after stressing; and the two are compared. The CARES model specification requires those measurements to an accuracy of 2% or 2mm, whichever is greater, requires actual extensions to fall within the limits given in BS EN 13670, and requires the post-tensioning designer to have checked and signed off the actual extensions before any tendon is cut and sealed. The Post-Tensioning Association's guidance describes the same check as one of the advantages of the system and records that discrepancies are expected to be taken seriously and corrected. In plain site terms: if a tendon has not stretched far enough, something is gripping it that should not be, and if it has stretched too far, something has moved that should not have.

After lock-off, the ends are cropped and sealed. Strand is cropped mechanically; the CARES model specification records that flame cutting is not acceptable because heat changes the properties of the strand. Anchorage components are then sealed against water and aggressive agents, with grouted systems needing a seal that can resist the grout pressure to come, and unbonded anchorages commonly greased and capped with a watertight cap under a stated minimum concrete cover. In a bonded system the last operation is grouting the duct, which is significant enough to have its own section on this page - and, as the CARES model specification notes, bonded tendons cannot reach their full ultimate design capacity until the grouting is complete, so a designer may have to consider the reduced capacity of the slab in that temporary condition.

The single most consequential fact about any post-tensioned element is whether its tendons are bonded to the concrete along their length or not. Everything downstream turns on it: how the element behaves at the ultimate limit state, what happens if a tendon is damaged, whether a hole can be cut through the slab, how the structure is investigated in thirty years, and how it is safely demolished in eighty.

The Post-Tensioning Association's published description is compact. In a bonded system the tendons run through small continuous flattened ducts which are grouted after stressing, creating bond between the concrete and the tendons; the ducts are formed from spirally wound or seam-folded galvanised metal strip, and duct flexibility limits the curvature the tendon profile can achieve. In an unbonded system the tendon is not grouted and remains free to move independently of the concrete between its end anchorages. The association's guidance records that this makes no difference to serviceability design or to performance under normal working conditions, but that it changes both the design theory and the structural performance at the ultimate limit state, and that the greater resistance to accidental damage of bonded construction is often an important consideration when a system is chosen.

The mechanics behind that sentence are worth spelling out. A bonded tendon is tied to the concrete continuously by the grout, so strain in the concrete at any section produces strain in the tendon at that section; the tendon stress at failure can rise locally and the element behaves in a way that is broadly familiar from reinforced concrete. An unbonded tendon has no such local tie: it is a single element stretched between two anchorages, and its strain is an average over its whole length, so its contribution at the ultimate limit state is calculated differently and the element relies more on ordinary bonded reinforcement for local ductility. The corrosion protection differs too. In a bonded tendon, the grout is the protection along the length and the anchorage seal protects the ends. In an unbonded monostrand, the grease and the extruded sheath are the protection along the length, and the anchorage is a sealed, greased and capped assembly - which is why the CARES model specification sets a minimum sheath thickness of 1.0mm unless the project says otherwise, caps the friction between coating and strand at 60N per metre, and requires that friction to be established by the test method in the ETAG 013 annex.

Accidental damage is where the difference becomes a site risk rather than a design nuance. Cut a bonded tendon and the prestress is lost only locally: the force re-anchors into the concrete over a development length either side of the cut, in the same way a pre-tensioned strand transfers its force at the end of a precast beam. Cut an unbonded tendon and the entire length de-tensions, because nothing between the anchorages was holding it. The CROSS expert panel commentary on modifications to post-tensioned slabs describes the bonded case directly: once tendons are cut, the force previously balanced by the removed portion must transfer into the new concrete end over the development length of the strands, that zone has to be checked for its revised capacity, and beyond the development length the tendons continue to carry their original forces - provided the tendons were originally fully bonded with adequate grouting. The same commentary notes that there have been cases of inadequate grouting revealed when existing structures were modified, which is why a bonded plug is commonly formed before strands are cut, to cover the possibility of partial bond at the interface.

That plug has produced its own safety issue. A CROSS report published in November 2025 describes a refurbishment where slab openings required post-tensioning tendons to be cut back and re-anchored with a high-tensile epoxy resin grout, and where the epoxy manufacturer's data sheet gave a maximum working temperature of 100 degrees C. A one-dimensional heat transfer calculation showed the epoxy could exceed that temperature well before the building's prescribed 120-minute period of structural adequacy under the standard fire curve, and the eventual solution was 75mm of vermiculite-based insulation to the underside and cut edges of the slab. The expert panel's comment is that the preferred plug material is a high-strength, low-shrink cementitious one with fire properties similar to ordinary Portland cement, and that if an epoxy plug is used, creep under normal loading needs consideration alongside fire resistance. The report also makes the point that cutting tendons to form openings is not rare - communication stairs, atria, lecture theatres and viewing mezzanines are all named.

Bonded and unbonded systems are also not mutually exclusive on one project. The association's guidance records that post-tensioned floors may be bonded, unbonded, or a combination of both. And the choice is not confined to buildings: the Hammersmith Flyover strengthening in 2012 installed horizontal 19-strand tendons that were unbonded inside pre-grouted HDPE ducts, an arrangement chosen because the new post-tensioning had to be durable, replaceable and re-stressable in a structure whose original 1960s post-tensioning had corroded.

National habits differ, which matters when reading guidance from elsewhere. The CROSS expert panel commenting on the epoxy plug report noted that its comments applied to bonded tendon systems only, because bonded systems have been the preferred type in Australia and New Zealand, while unbonded systems have been adopted in some other countries. UK practice runs both, and the CARES certification scheme reflects that by covering bonded and unbonded floors in the same model specification while reserving its higher-durability appendix for the grouted systems used in major structures.

The two systems also leave different traces for a surveyor. A bonded system generates a grouting record for every duct and can be interrogated later by drilling into ducts and looking for voids. An unbonded system generates no grout record at all, and its vulnerable points are the anchorages: the CARES model specification's requirements for greasing anchorage components, fitting a watertight cap and providing a minimum concrete end cover to that cap exist because a monostrand anchorage at a slab edge is a small assembly of steel components sitting close to the weather. The same specification notes that ordinary steel ducts with a folded seam are not leak-tight, that plastic duct systems are considered where higher protection is wanted such as in car parks, and that PVC is not used because chloride ions can be released from it in certain conditions - three sentences that between them explain most of what an investigation finds thirty years later.

The most common UK application by area is the building floor. The Concrete Centre describes post-tensioned slabs as typically flat slabs, band beam and slab arrangements or ribbed slabs, and as the thinnest slab type available, because the concrete is worked to its strength and mostly kept in compression, with prestress also used to counteract deflection. The published benefits list is consistent across the trade bodies: minimum structural thickness, longer spans for the same depth, fewer columns, flat soffits that make services distribution simple, rapid construction, reduced material use, and the ordinary concrete benefits of fire resistance, acoustic performance and thermal mass. The Post-Tensioning Association's procurement note carries a chart of UK post-tensioned slab area constructed per year from 1990 to 2010, rising to roughly 1.5 million square metres a year by the end of that period - a useful reminder that a very large stock of UK buildings now contains tendons that somebody will one day want to drill through.

Transfer structures are the second building application, and they are where post-tensioning earns its keep most obviously. A transfer slab or transfer beam carries columns or walls from the floors above and redistributes them onto a different grid below, typically because a residential tower sits over a car park or a retail floor. The forces are large, the depths are constrained, and prestress both reduces the depth needed and controls the deflection and cracking that a heavily loaded transfer element would otherwise suffer. The Concrete Centre maintains transfer slabs as a distinct building element in its structural design guidance for exactly that reason.

Bridges are the historic heartland and the source of most of the sector's published caution. Post-tensioning appears in bridges in several distinct families: in-situ box girders stressed in stages as construction proceeds; precast segmental decks, where short segments are match-cast and then squeezed together by longitudinal tendons crossing the joints; balanced cantilever construction, where segments are added alternately either side of a pier and each new segment is stressed back into the completed cantilever; and externally post-tensioned decks, where tendons run inside the box void rather than inside the concrete and are accessible for inspection. Each has its own risk profile, and the segmental family carries the heaviest history - the joints between segments are the discontinuity where water, chlorides and oxygen have historically reached the steel.

Containment structures form a third group. Water retaining tanks, reservoirs, digesters and silos are commonly post-tensioned circumferentially so that the ring compression keeps the wall in compression against the outward pressure of the contents, which is how a concrete tank is kept from cracking and leaking rather than simply reinforced against it. The design sits under BS EN 1992-3:2006, the part of Eurocode 2 covering liquid retaining and containing structures, which gives additional rules for plain, reinforced and prestressed concrete containing liquids or granular solids; its published scope is equally clear about what it does not cover, including very low or very high temperature storage, hazardous materials whose leakage would be a major health or safety risk, pressurised vessels, large dams and gas tightness. Nuclear containment structures sit in their own category with their own assurance: the CARES post-tensioning scheme manual carries a dedicated appendix, PT(N), for the supply, installation and monitoring of post-tensioning systems in concrete structures for nuclear projects.

Ground anchors are post-tensioning turned into the earth. A grouted anchor is a tendon bonded into a drilled hole in rock or soil, stressed against a bearing plate at the surface, and used to hold back a retaining wall, tie down a slab against uplift, or stabilise a slope or a dam. The UK code of practice is BS 8081:2015+A2:2018, which gives recommendations for the design, construction, stressing, testing, monitoring and maintenance of grouted anchors, covering ground assessment, materials, the effect of water on durability, analysis, structural design and execution. Execution on site sits under BS EN 1537:2013, the European standard for ground anchors, which superseded the 2000 edition and covers anchors grouted into the ground that are stressed and tested, for permanent or temporary use, including drilling methods chosen to minimise ground disturbance, borehole collapse, groundwater change and softening of the borehole wall. Anchors carry the same two obsessions as any other post-tensioning: proving the load actually went in, and protecting the steel for the design life.

Two smaller applications are worth naming because they turn up in refurbishment work. Post-tensioned ground-bearing slabs are used in industrial floors to reduce joints and cracking; and post-tensioning is used as a strengthening technique on existing structures, adding new tendons to a structure that was never post-tensioned or replacing capacity lost from one that was. The Hammersmith Flyover work is the best-documented UK example of the second: an elevated section of the A4 completed in 1961, found by routine and special inspections to have significant corrosion in post-tensioning installed in the early 1960s, closed as an emergency just before Christmas 2011, reopened in January 2012 with reduced load limits and lane closures, and then strengthened by installing additional post-tensioning above and below the top slab within a widened central reservation - about 200 linear metres of the existing central reservation removed by hydrodemolition and wall sawing, vertical prestressing bars cored in and stressed, new blisters cast, and horizontal tendons installed and stressed, in a £4 million package that ran from late January to May 2012 to reopen the road before the Olympics.

Post-tensioning installation happens inside somebody else's programme. The concrete frame contractor is fixing reinforcement and formwork, the services contractor wants penetrations, and the post-tensioning gang has to get ducts, tendons, anchorages, bursting steel and vents into the same slab and keep them where the design put them. The CARES model specification treats the whole operation as a quality plan exercise: the contractor gives the contract administrator a plan covering proposed materials, equipment and method statements for site activities, with work instructions, quality procedures, records, inspection and test arrangements and work acceptance procedures.

The duct comes first. The model specification requires duct, vent and connection material to be strong enough to resist damage during construction, giving as examples smooth galvanised steel of at least 0.35mm wall thickness, corrugated galvanised steel of at least 0.30mm, or high density polyethylene or polypropylene of at least 2.0mm. The internal cross-sectional area of the duct is required to be at least twice the net area of the prestressing steel it carries - the working space that lets grout get all the way round the strand. Ducting has to stop cement paste from the concrete getting in, must not cause harmful electrolytic action or degrade the tendon, and has to transmit force from the grout to the surrounding concrete. Joints and connections are sealed with a water-resistant fabric-based tape suitable for wet galvanised or plastic duct and inert to the concrete, grout and steel. The specification also carries a note that ordinary steel ducts with a folded seam are not leak-tight, that plastic duct systems are considered where a higher level of protection is wanted such as in car parks, and that PVC is not used because chloride ions can be released from it in certain conditions.

Then the tendon, and the profile. Fixing tolerances in the model specification are asymmetric for a reason: vertical accuracy is plus or minus 5mm or tighter at the post-tensioning designer's discretion, while horizontal accuracy is 50mm in beams and 150mm in floor slabs. Vertical position controls the drape and therefore the balancing force, so it is policed to millimetres; lateral position mostly affects where the load is picked up, so it is policed to centimetres. Tendons are fixed and supported at centres not exceeding 1m and securely enough to prevent movement and flotation during construction - concrete is a dense fluid and an unsecured duct will float. A practical note in the same document records that flat ducts are quite flexible vertically and that it is common to install the strand inside the duct first and then profile the tendon, because the strand stiffens the assembly and helps produce a smooth profile rather than a series of kinks.

Deviations around obstructions are allowed but controlled. Unbonded tendons may be deviated around openings and columns with the contract administrator's agreement, with the change of direction occurring away from the opening and trimmer bars provided against corner cracking. Curved profiles in plan are achieved by faceting straight lengths of flat duct, with the model specification giving a minimum radius of 50m, a maximum facet angle of 7 degrees and a minimum facet length of 6m, and pointing to circular ducts for tighter radii. Alterations to the post-tensioning designer's drawings are authorised by that designer before concrete is poured, which is the paperwork equivalent of not moving somebody else's tendon because it is in your way.

Vents are the part that only matters later, and are therefore the part most often compromised. Vents are fixed at injection and exit points, and where tendon drape exceeds 500mm, intermediate vents are fixed at tendon high points; they extend roughly 500mm above the slab surface, and all inlets and outlets are marked to identify the tendon and the location along it. The model specification adds a note on considering intermediate vents on tendons over 20m, with vent spacing not exceeding 30m. Every one of those requirements exists because grout is a fluid being pushed uphill and downhill through a long thin tube, and air trapped at a high point becomes a void at the highest point of the tendon - which is where the bending moment is largest over the support.

Anchorages come with their own assurance trail. The model specification requires anchorages to be CE marked and to meet the minimum performance requirements of BS EN 13391 as defined in the CARES appendix for anchorage production, or for the post-tensioning kit to hold a European Technical Assessment against ETAG 013 or EAD 160004, with documentary evidence of conformity available on site if requested. BS EN 13391:2004 itself specifies the test procedures for anchorages and couplings of post-tensioning systems, and its published scope carries a note that unbonded tendons need additional tests and requirements that the standard does not cover. Around the anchorage sits the bursting reinforcement, installed as drawn - the Post-Tensioning Association's procurement note lists anchorages, installed as shown on the drawings and with the correct reinforcement around them, as one of the few critical components the system relies on.

Two record-keeping habits complete the installation. First, tendon installation is recorded: date, strand source, coil number, heat or cast number, anchorage batch, wedge batch, duct batch, supervisor, operatives, the location of the products within the structure, and the drawing number and revision status. That trail is what allows a future investigation to know what is buried where. Second, and unglamorous but consequential, the actual position of tendons is marked on the slab soffit to show their plan location, by a system agreed with the contract administrator, and the model specification notes that where services are to be fixed to a slab it is important the slab is clearly identified as post-tensioned construction on all drawings and relevant documents, that all details relating to tendon positioning for potential alterations or demolition are held in the operation and maintenance manual supplied by the post-tensioning designer, and that where tendon position is not accurately and authoritatively documented, reinforcement detection equipment is used to locate tendons before any cutting or drilling. Those three sentences are the difference between a routine fixing job and a call to the structural engineer twenty years later.

Stressing is the day the slab becomes a machine. A hydraulic jack is clamped to a strand, pressure is raised, and a force measured in hundreds of kilonewtons is pulled into a steel element the thickness of a finger, restrained by a wedge assembly seated in concrete that was fluid a week ago. Everything about the operation - the sequence, the timing, the exclusion behind the jack, the paperwork - exists because that energy has exactly two acceptable destinations: into the concrete through the anchorage, or nowhere.

The first control is the concrete. Stressing does not begin before the concrete has achieved the transfer strength specified by the post-tensioning designer, and that strength is established from match-cured cubes taken at the point of concrete placement rather than from cubes cured in a tank in the site cabin. The CARES model specification requires match curing - the cubes held at the same temperature as the element they represent - and requires transfer strength to be based on cubes from the last load poured, on the reasoning that strength can differ significantly between the first and last concrete into a slab. Partial stressing at an early age is common practice to control early cracking, and the Post-Tensioning Association's procurement note describes the timing question in those terms: early enough that shrinkage has not gone too far, late enough that the concrete can resist the prestress force.

The second control is the equipment. Stressing jacks and their load measuring systems carry a current calibration certificate traceable to national standards, no more than six months old at the time of stressing, with the calibration done by a UKAS accredited company, and the equipment capable of establishing tendon load to a tolerance of plus or minus 2%. Gauge and jack serial numbers are recorded against the tendons they stressed. The reason for that pedantry is that the gauge reading is the only direct evidence of force, and a jack reading 5% high produces a structure that is 5% under-stressed and looks perfect.

The third control is the sequence. The designer specifies any restrictions on stressing sequence and increments, and the jacking force is not to exceed the values in the applicable design standard. Sequence matters because a slab stressed in the wrong order can be pulled out of shape, can crack at restraints such as stiff cores and columns, and can carry unbalanced forces into elements that were designed for the completed condition rather than the intermediate one.

The fourth control is the measurement already described: extensions taken before stressing and after lock-off to 2% or 2mm, whichever is greater, compared against the designer's theoretical extension, checked and signed off by the post-tensioning designer before any tendon is cut and sealed, with the completed stressing records made available to the contract administrator within a week. The stressing record itself is a defined list: date, strength and age of the cubes, minimum age of concrete at transfer, stressing equipment, calibration date, supervisor and operators, gauge and jack serial numbers, tendon identification, theoretical and actual extensions with corresponding loads and initial and final jacking loads, and the drawing number and revision status. If anything fails during stressing, whether a system component or the concrete, the model specification requires the cause to be investigated thoroughly, a formal report to go to the contract administrator, and no further stressing on that element until the proposed corrective action is approved.

That clause is not theoretical, and the trade has a published example of why it is there. CROSS report 882, published in April 2020, describes the end of a slab bursting during a stressing operation: the live end of the tendon being tensioned moved as the fixing in the concrete failed, destroying the slab over an area of roughly 1.5m by 1.5m, and an operative struck by debris sustained relatively minor injuries in an incident whose consequences, as the report notes, could have been much worse. The investigation considered over-stressing, concrete strength and structural design, and the contractor, the post-tensioning installer and the post-tensioning designer concluded that localised under-strength concrete had been used - the result of a site method of grouting out the mobile concrete pump line and discharging it into the permanent works. The recorded outcomes are instructive because they are all procedural rather than technical: grout from pump lines is not to be discharged into the slab area or form part of the permanent works; quality assurance checks to make sure neither pump-line grout nor heavy rain is incorporated into the permanent concrete, especially at the start of a pour; the post-tensioning contractor's segregation area was implemented in full after the incident, with additional blast mats of plywood or tarpaulin; risk assessments and method statements reviewed; and the post-tensioning contractor encouraged to put concerns about programme, structure or safety in writing at the time rather than mentioning them verbally. The report also records that a similar incident had happened on an earlier project when heavy rain on the day of a pour caused local weakness in the concrete and failure at the end of a cable. A commenter on the same report observed that the photograph of the damaged anchorage showed no evident anti-bursting reinforcement at the anchorage location.

The physical exclusion arrangements follow from the same energy. Nobody stands behind a jack: if a wedge fails to grip or a strand breaks, the jack and any debris travel along the tendon axis. The segregation area in front of and behind the stressing position, blast protection over the anchorage where blow-out is a credible risk, and control of who may enter the zone while pressure is on are all normal features of a post-tensioning method statement, and CROSS's learning outcomes for the incident above put it plainly: large stressing forces and stored energy are generated during the process, the operations are carried out by people with relevant training and competence, and where concrete blow-out is a concern it is raised on site so the risk assessment and method statement required under CDM 2015 can be updated. Regular toolbox talks with the work crews are named in the same list.

After lock-off, strands are cropped mechanically, never by flame, and the anchorage is sealed. For grouted tendons the seal has to resist the grout pressure that follows, commonly using proprietary non-shrink mortars and bonding agents, with the model specification noting that anchors are usually cast into pockets in edge beams and the pockets sealed with mortar or render, and that this detail deserves attention because mortar and render can be permeable and shrink. Exposed anchorages attract higher levels of protection such as end caps. For unbonded tendons the anchorage components are greased with material of similar specification to the tendon grease and covered with a watertight cap under a stated minimum concrete cover. Then, for bonded systems, the ducts get grouted - and that is where the long-term story of the structure is decided.

Grouting looks like the tidy-up at the end of the post-tensioning job. It is not. For a bonded tendon the grout does two structural jobs and one durability job, and the durability job is the one that decides whether the structure lasts a century. BS EN 446:2007, the European standard covering grouting procedures for prestressing tendons, sets out the functions of grouting in terms the whole industry uses: protecting the prestressing steel against corrosion, providing bond between the prestressing steel and the duct where the design requires it, allowing compressive stresses to transfer transversely to internal tendons, and filling voids where water might otherwise accumulate and cause frost damage. Take the grout away and a bonded tendon is a bare high-strength steel element sitting in a damp tube inside a structure nobody can see into.

The grout specification lives in BS EN 447:2007, which covers the materials that may be used to make cement grouts and the required properties and composition, applicable to tendons in all types of structures including bridges and buildings. Testing sits in BS EN 445:2007, which specifies the apparatus, techniques and procedures for the sieve test, the fluidity test, the inclined tube test, the wick-induced bleeding test, the compressive strength test and the density test. BS EN 446 places those tests in a three-level regime: initial type and audit testing against EN 447, suitability testing to confirm the selected grout for a specific project, and inspection testing during production of grout on that project. All three standards were published on the same day, 30 November 2007, and are the reference set that UK post-tensioning specifications point at.

The UK site practice built on top of that set is stricter than the standard in several places, and the CARES model specification marks its departures explicitly. Grout is pre-bagged material requiring only the addition of a measured quantity of water, certified under the CARES appendix for pre-bagged grout, stored to the manufacturer's instructions and used within its marked shelf life. The water/cement ratio is capped at 0.35, tighter than the standard's baseline, because bleed water - water in the grout that is not chemically bound - is the mechanism that produces low density grout, cracking, shrinkage and porosity. Volume change is bracketed between minus 0.5% and plus 2.5% on the wick-induced test. Fluidity is tested per grout mix at mixing and again after 30 minutes; bleeding and volume change are tested by wick-induced test once per day; three cubes are taken per grouting session at the mixer, with density recorded from those cubes. Suitability tests are carried out in advance with the materials, plant and personnel proposed for the site, far enough ahead that adjustments can be made.

The acceptance criterion for the finished job is the sentence that matters most. Grouting is normally shown to leave no void with a radial dimension greater than 5% of the maximum duct sectional dimension, or any void that poses a risk to the integrity of the tendon, with particular attention to avoiding bleed collection or void formation at high points in the ducts and at anchorages. Grout trials are typically considered for tendons longer than 50m or with particularly severe profiles, and for structures in the more aggressive chloride exposure classes.

The procedure itself is a sequence of small disciplines that each close off a known failure. Ducts are kept free from contamination from storage through installation and are thoroughly clean before grouting. They are blown through with oil-free compressed air, not water, and the blowing-through is recorded tendon by tendon. Grouting is not carried out if the temperature of the structure adjacent to the tendons is expected to fall below 5 degrees C in the following 24 hours, and the filled duct is protected from shock and vibration and kept above that temperature for 24 hours after injection. Injection is continuous and slow enough to avoid segregation. Vents are closed one after another in the direction of flow, and the injection tubes are then sealed under a positive pressure not exceeding 0.1MPa. There is a written procedure for corrective action if the pump blocks or breaks down, including backup equipment or flushing the duct out. Grouting takes place as soon as practicable after stressing, extension approval and cropping, and no longer than 28 days from stressing without the contract administrator's approval. The contract administrator or the contractor's representative is invited to witness a representative sample of the operation and to countersign the grouting record.

That record is itself a defined list: materials used with batch numbers; date, time and conditions; ambient and structure temperatures and weather; grout properties including temperature, bleed, volume change and fluidity; details of any interruptions and problems such as blockages, loss of grout or loss of pressure; the supervisor and operatives; and the manufacturer's technical and safety data sheets. Calibration records are kept for the pressure gauges used in grouting, the gauges and jacks used in stressing, and temperature probes.

None of this discipline was invented in a committee room. It came out of a decade of finding voids in structures that were supposed to be solid. fib bulletin 20, the 2002 guide to good practice on grouting of tendons in prestressed concrete, records the history plainly: in the 1990s cause for concern emerged, first in the UK and then in many other countries, that the grout protecting internally ducted tendons was in some cases inadequate; major investigations followed, including intrusive physical examination of ducts mainly in bridges, and procedures, processes and specifications were rewritten. FIP launched a task group in 1998 to review its 1990 grouting guidelines; the UK's Concrete Society published Technical Report 47 on durable post-tensioned concrete bridges in 1996 as the culmination of four years of investigative research, containing new specifications and procedures aimed at improving grouting quality; the Post-Tensioning Institute in the United States published a grouting specification in 2001; and an international workshop on the durability of post-tensioning tendons was held in Ghent in November 2001. The bulletin describes the consensus that emerged as a multi-layer protection strategy - waterproofing, dense impermeable concrete, sealed ducts and good quality grout - with design detailing and rainwater management treated as significant. Its most quoted conclusion is about the material itself: the properties of common grout made simply from cement and water can be very variable and sometimes unpredictable, and such grout is not recommended. The new understanding it lists - the effects of an unstable grout, bleeding and how to avoid it, the importance of training and proper procedures, mix design, trials and new test procedures - is recognisably the specification UK sites work to today.

The same lesson is documented on the other side of the Atlantic. The FHWA's Post-Tensioning Tendon Installation and Grouting Manual, publication FHWA-NHI-13-026 issued in May 2013, covers materials, systems, duct and tendon installation, jacks and stressing methods, and grouting equipment, testing and inspection, and includes a section on grouting problems and solutions such as interrupted grout flow and excessive grouting pressure. Its corrosion protection chapter deals with sealing inlets and outlets, protection at anchorages and temporary protection during construction, and the accompanying FHWA article records a technical advisory on assessing and managing bridges whose tendons were installed with grout containing elevated levels of chloride - a reminder that the grout itself can be the contaminant.

Post-tensioning is one of the few construction trades where the entry ticket is a certificate and the certificate is written into the specification. The body that issues it in the UK is CARES, the UK Certification Authority for Reinforcing Steels, which operates a certification scheme for the supply and installation of post-tensioning systems in concrete structures alongside its longer-established reinforcing steel schemes. CARES states the scheme's origin directly: its development, and its acceptance by the post-tensioning industry, played a key part in the Highways Agency lifting its moratorium on grouted post-tensioned bridges in September 1996.

The scheme is a set of appendices, each covering a different part of the supply chain, and knowing which is which shortens any conversation about scope. Appendix PT1 covers the supply and installation of strand post-tensioning systems in concrete structures and is aimed at major structures with a high durability requirement - CARES describes it as being for Concrete Society TR72-type contracts, for internal and external systems, with a certified specialist contractor supplying, installing, stressing and grouting the system. Appendix PT2 covers the supply and or installation of post-tensioning systems in concrete structures excluding highway structures, and is the appendix for general structures such as concrete frames, floor slabs and containment structures; it allows one certified specialist to supply the system and another to install, stress and grout it. Appendix PT4 covers bar post-tensioning systems. Appendix PT5 covers void grouting: a certified specialist investigating ducts with voids, locating them, establishing their characteristics and disposition, and regrouting them - a scheme that exists because the remedial market exists. Appendix PT3 covers the production and supply of prestressing anchorages, requiring compliance with the performance requirements of EAD 160004. Appendices PT6 to PT12 and PT14 cover the material supply chain: feedstock rod, prestressing bar to BS 4486, wire and strand to BS 5896, pre-bagged grout to the BS EN 445, 446 and 447 set, the stocking and distribution of BS 5896 products, bar wire and strand to the Australian and New Zealand standard, and special filling products such as grease or wax. A separate appendix, PT(N), covers supply, installation and monitoring for nuclear projects.

People are certified as well as companies. Appendix PT9 is the registration scheme for post-tensioning operatives: CARES requires post-tensioning companies to provide training for supervisors and operatives and to assess its effectiveness, and once an individual reaches a satisfactory level, evidence of achievement goes to CARES and an identity card is issued. Those cards are worn on site. The CARES model specification reinforces the same point in contract language: all post-tensioning operations are carried out by operatives with appropriate knowledge, training and proven experience in similar operations, with supervisors and operators trained and certified to the PT9 requirements; trainees are adequately supervised, with a note that the ratio of trainees to experienced trained staff is balanced to circumstances and normally does not exceed 50%; and all site duct fixing, tendon installation, stressing and grouting is undertaken by suitably trained and experienced staff under the direct supervision of the post-tensioning contractor, with site operatives employed by other contractors on the site not used for post-tensioning work.

Certification carries audit with it. CARES states that certification is issued only after a thorough audit of the operations and quality management on the project being assessed, and the Post-Tensioning Association's procurement note describes the ongoing regime: the contractor's offices audited once a year and a minimum of two sites audited every year, including a check that the workforce hold CARES cards. The supplier's quality system complies with CARES requirements and ISO 9001 and covers system production and the key on-site and off-site activities. The model specification adds that access to the works is granted to CARES for quality auditing by arrangement, and that the contract administrator may request an additional surveillance audit at their own cost.

The scheme also solves a chicken-and-egg problem for new entrants. CARES operates a two-stage certification as a concession to companies that do not yet have a suitable post-tensioning contract to be assessed on, which avoids firms having to wait until they have won a contract before applying and reduces the potential for disparate and anti-competitive bidding. Where full certification cannot be obtained for want of a suitable contract, CARES's suggested specification wording requires a letter of pre-certification instead.

That suggested wording is the mechanism by which the scheme reaches the site. CARES publishes model text limiting bidding for, and undertaking of, contracts to supply and install post-tensioning systems to companies with full certification under its scheme, with the pre-certification route as the stated exception. The Post-Tensioning Association's procurement guidance carries the same wording and records a set of association recommendations around it: that CARES accredited post-tensioning companies are used as a minimum and ideally that the contractor is also a PTA member; that the specialist contractor carries out both supply and installation so that components and equipment are compatible and installers are trained in that system; that the specification requires a CARES certified contractor; and that the client's designer ensures a single engineer takes overall responsibility for the design and that the post-tensioning design is compliant. The same note describes the three procurement routes seen in practice - a performance specification with the post-tensioning contractor designing, supplying and installing; the structural engineer designing the post-tensioning and the specialist supplying and installing; and the frame contractor buying products and installing them itself - and records the association's view that the third is not recommended because of the difficulty of ensuring adequately trained personnel install, stress and grout the system.

Verifying a certificate is a small job with a defined method. The association's guidance describes requesting a copy of the CARES certificate, checking that the company named on it is the company that will do the work - noting that there are companies in the industry with similar names - checking that the scope on the certificate matches the works on site, bonded or unbonded, bridges or buildings, and checking current status on the CARES database of certified companies rather than relying on a paper copy of unknown age.

The awkward property of post-tensioned concrete is that its condition is not visible. DMRB CS 465's published summary states the problem for bridges in one sentence: the primary problem with post-tensioned bridges that sets them apart from other types is the difficulty of establishing the internal condition of the tendons, because external signs of distress are not generally expected to occur. It adds that for most forms of concrete bridge construction defects tend to be relatively easy to interpret and produce ductile failure modes, whereas safety-critical defects in post-tensioned concrete are typically hidden, very difficult to detect, and may produce a brittle mode of failure. Visual inspection alone may not give warning of imminent collapse. At the same time, intrusive investigation is expensive and potentially damaging to the structure, and the standard's position is that it is carried out where there is a clear need.

The management model CS 465 sets out is therefore risk-based: a process of risk review, risk assessment and risk management across the stock, to provide assurance about the safety of post-tensioned bridges, with the most vulnerable structures monitored and reinspected after an appropriate interval because uncertainty about the condition of the post-tensioning increases with time. The standard, published in March 2020, replaced BD 54/15, and an ICE webinar record from April 2020 describes the lineage: a consultancy structures investigation group trialled the draft standard's processes on post-tensioned bridges during 2013 and 2014, from desk studies to site investigations, completed risk review and risk assessment reports for over 340 post-tensioned bridges from 2015, and then re-drafted the standard as CS 465 for the DMRB update. Non-destructive testing has its own DMRB document, CS 464, whose published summary names detecting voids in post-tensioned concrete as one of the typical uses of NDT on highway structures - gaining increased assurance about the integrity of earlier construction where similar types have been shown to be defective.

What a post-tensioned special inspection actually involves is a graded sequence. It starts with the paperwork: original drawings, tendon layouts, stressing and grouting records, the operation and maintenance manual, and any record of alterations. It moves to the surface: mapping the deck for signs that matter in this specific failure mode - calcite staining and efflorescence at joints and duct positions, cracking along tendon lines, evidence of water tracking, and failed waterproofing above. Non-destructive methods then locate ducts and, within limits, indicate whether they are full. Where the evidence justifies it, the investigation becomes intrusive: holes are drilled into ducts at high points and low points, ducts are inspected with borescopes, water and voids are recorded, and in some cases strand is exposed and sampled so that section loss and condition can be measured directly. The findings feed a structural assessment that has to state what it assumed about the grouting - which is exactly where one recent UK case went wrong.

CROSS report 1450, published in August 2025, sets out that case in detail. A reporter raised concerns about a post-tensioned segmental concrete viaduct built in 1970, formed as a series of precast box segments held together by post-tensioned high tensile steel tendons. Post-tensioned special inspections in 1998 and 2014 had revealed ungrouted ducts and voiding; a more detailed survey in 2021 exposed significantly worse conditions, including snapped wires and voids at the tendons, and the reporter's view is that the earlier inspections had captured only a small proportion of the problem. Regrouting begun in 2024 uncovered further areas of voids and water ingress. The structural assessment carried out in 1997 had assumed fully grouted ducts and had not been revised to reflect the 1998 findings; a 2023 reassessment found the segmental joints were not always in compression under serviceability loading, and assuming ungrouted tendons reduced the live load capacity; a 2025 reassessment, taking the 2024 regrouting into account, concluded the bridge could safely carry a single traffic lane rather than two, with reduced surfacing thickness. Some ducts filled with water within weeks of inspection. The report frames the whole thing around three lines of defence against corrosion - effective waterproofing, compression across the joints, and full grout coverage - all three of which were compromised in this structure. The CROSS expert panel's comments add that the 16-year gap between special inspections, and the early warning signs noted in intermediate condition inspections but not acted on, are concerning, and that post-tensioned concrete bridges are treated as priority risk structures.

The same report is the clearest published statement of where UK bridge policy now sits. It records the reporter's account that a general moratorium on the construction of new post-tensioned concrete box girder bridges has been in place in the UK since 1992, following collapses attributed to tendon corrosion, and the expert panel's statement that the moratorium for precast segmental construction using internal grouted tendon systems is generally still in place for National Highways - although, after rigorous improvements in grouting materials, procedures and testing, departures from standards have been accepted, with the Water Orton Viaduct on the HS2 project named as an example. The panel points at Concrete Society TR72 as the documentation of those improvements, and at Network Rail's own standard for the management of post-tensioned concrete bridges for railway structures. It also names the historical anchor for the whole regime: the collapse of the Ynys-y-Gwas bridge in Wales in 1985, cited as a reminder of the consequences of corrosion in ungrouted ducts, in a passage that observes the institutional memory of such failures appears to be fading.

Buildings get a lighter but parallel treatment. There is no building equivalent of CS 465, and the practical investigation trigger is usually an alteration rather than a safety review: somebody wants a new riser, a stair, an atrium or a plant opening through a post-tensioned slab. The starting point is the record set the installation was required to produce - tendon positions marked on the soffit, and tendon positioning details held in the operation and maintenance manual for potential alterations or demolition. Where those records are missing or cannot be trusted, the CARES model specification's position is that reinforcement detection equipment is used to locate tendon positions before any cutting or drilling work on the slab. The Post-Tensioning Association publishes a dedicated guidance note on post-formed holes through post-tensioned slabs, covering both bonded and unbonded systems and applicable to flat slabs, beams and band beam slabs, aimed at engineers, contractors and building owners who need to locate and cut penetrations safely after construction. And the CROSS report on epoxy plugs is a reminder that the design of the modification does not end when the tendon is safely cut - the replacement anchorage has to survive everything the slab will meet, including a fire.

Every post-tensioned structure ends its life holding the energy that was put into it on stressing day. Demolition is the process of getting that energy out in a controlled order, and it is the reason post-tensioned demolition is a designed operation rather than a machine and a driver.

The demolition code of practice, BS 6187:2011, gives recommendations for full and partial demolition of buildings and structures, covering the management of the process, maintaining structural stability including temporary support, identifying and establishing responsibilities, managing environmental and health and safety hazards, carrying out risk assessments and managing safe exclusion zones. The trade guidance that sits under it names post-tensioning explicitly. The National Federation of Demolition Contractors' guidance DRG102:2022 on deconstruction of structures lists, among the pre-demolition information gathered, original construction drawings identifying special forms of construction including pre-stressed and post-tensioned elements, and information on the condition of those special elements. Its section on the demolition design programme is the clearest short statement of the hazard: the programme takes account of the presence of post-tensioned concrete and the possible structural effects of cutting the stressing tendons, giving as examples the ejection of tension bolts or anchor blocks, tendon acceleration causing injury or damage, and structural breach.

Specialist demolition engineers describe the same hazards as a checklist. Andun's published summary of the challenges of post-tensioned demolition names accidental damage to tendons, damaging anchorages, staged construction, stress reversal, non-grouted tendons, the condition of the structure and loss of shear capacity. Two of those need unpacking because they are unfamiliar outside the trade. Staged construction and stress reversal mean that a post-tensioned structure was assembled in a sequence, with each stage stressed into the previous one; taking it apart in a different order can put an element into a state it was never designed for, in particular reversing the sign of the moment so that a section with tendons in its top is asked to hog when it was designed to sag, or the reverse. Loss of shear capacity means that prestress contributes to a section's resistance to shear, so removing the compression by cutting tendons reduces the shear capacity of the section that is left, and often does so in a zone where the temporary works are trying to hold the structure up.

The Churchill Way flyovers in Liverpool are the best documented UK example of the whole sequence. The two flyovers opened in 1970 and closed at the end of September 2018; an engineering report received by Liverpool City Council in February 2019 revealed multiple irreversible defects; replacement was estimated at between £50m and £60m, and demolition was delivered at £6.75m between September and December 2019. Each flyover was a continuous post-tensioned structure over 240m long, with individual spans varying from 14m to 33m, one fixed central bearing and longitudinal bearings elsewhere.

The engineering the demolition needed is instructive. The first question asked was whether the tendons were bonded or unbonded - in the demolition engineer's plain phrasing, whether the tendons would re-anchor when cut or release their tensile force. Checks of the grouting around the tendons at numerous points, taken at the high and low points of the tendon profile to confirm bond throughout, established that the tendons could be treated as bonded. Even with that answer, the risk assessment identified two failure mechanisms to be mitigated when the first spans were cut. The first was shear failure: the re-anchorage length required for a tendon can be significant, the reduction in compression reduces the shear capacity of the section, and the distance between the proposed cut line and the point of high shear on the adjacent span was around 1.5m, below the maximum theoretical re-anchorage length calculated - so a prop was designed to hold the weight of the bridge in that worst case. The second was bending failure: if the anchorages failed, the bending capacity of the span would be significantly reduced, so the propping had to be able to carry the span in that scenario too. Proprietary props could not take the loads involved and bespoke modular props were designed for the scheme. Columns were propped as well, for stability once the first spans were out and given the potential for movement on the longitudinal bearings.

The cutting itself was sequenced. Saw cutting and tendon release were planned in order, and on the curved sections of deck the outside cut was made first, then the inside cut, then the mid-section. Self-propelled modular transporters underneath were jacked to carry 80% of the dead load before the cuts were made, so the deck could not sag as it was released. Spans were lifted, driven out, lowered onto designed steel grillages and broken down at ground level; the last four spans of each flyover were removed by traditional methods working back to the abutment, with propping used because of the span lengths. The design output for a three-month programme ran to more than 200 drawings, 30 sets of calculations, two approvals in principle, and category 3 checks by independent checkers for both the structural assessment and the temporary works, with the lead designer role taken under BS 5975.

Three general points fall out of that case for anyone meeting a post-tensioned structure at end of life. First, the records are the starting point and their absence is a cost: at Churchill Way the limited availability of as-built information was one of the reasons strengthening was ruled out. Second, bonded or unbonded is not a paperwork question but the question that sets the method, and where records do not answer it, intrusive checks at the high and low points of the profile are how it gets answered. Third, partial demolition inside an occupied or retained building is the same problem in miniature - cutting an opening in a post-tensioned slab releases force into a structure that is still in service, which is why the alteration guidance, the detection sweep before drilling, the bonded plug and the fire performance of the re-anchorage all belong to the same conversation.

Prestressed concrete comes in two families, and confusing them on site produces bad decisions. Both exist for the same reason. The FHWA's summary of the principle puts a number on it: the tensile strength of concrete is only about 10% of its compressive strength, so plain concrete members used in bridge construction are likely to crack when loaded, and prestressing places the structure under compression in the areas where loading causes tension so that the prestressing steel accepts the tensile stresses instead. The difference between the two families is when the steel is tensioned relative to the concrete.

The Concrete Society's description separates them on exactly that point. In pre-tensioning, the strands are tensioned first, against fixed abutments or bulkheads in a casting bed, and the concrete is cast around them; once the concrete has gained sufficient strength the strands are released and their force transfers into the concrete. In post-tensioning, the concrete is cast around ducts, hardens and reaches a minimum compressive strength, and the tendons - installed in ducts embedded in the concrete or running outside it - are stressed afterwards and anchored against the hardened concrete.

That single difference produces a chain of practical consequences. Pre-tensioning is a factory process, because it needs a long stressing bed with abutments capable of holding the full force until the concrete is ready. Post-tensioning is a site process, because the structure itself provides the reaction. Pre-tensioned strands are straight or deflected in a few positions, because a casting bed can only do so much; post-tensioned tendons follow a continuous drape shaped to the bending moment diagram of the finished structure. Pre-tensioned members arrive complete, and the prestress is already in them when they are lifted; post-tensioned members are inert until stressing day and only then become the structure they were designed to be.

The force transfer differs in a way that matters for anyone cutting into either. A pre-tensioned strand has no end anchorage at all: the force transfers by bond over a transmission length near the member end, which is why the ends of pre-tensioned beams and hollowcore units are congested with the effects of that transfer and why nothing is cut near them. A bonded post-tensioned tendon has both - a mechanical anchorage at the ends and bond along its length after grouting - so a cut tendon re-anchors by bond over a development length, in the same manner as pre-tensioning. An unbonded post-tensioned tendon has only its end anchorages, so there is no re-anchorage mechanism at all and cutting it de-tensions the full length.

Corrosion protection differs in the same pattern. In pre-tensioned units the concrete cover is the protection, and the strand sits directly in dense factory-produced concrete cured under controlled conditions. In bonded post-tensioning, the protection is a chain of components produced or installed on site - the duct, the grout, the anchorage seal, and the waterproofing above - which is why the whole grouting apparatus described earlier exists, and why the failures the industry learned from were failures of that chain rather than of the strand itself. In unbonded post-tensioning, the protection is the factory-applied grease and sheath, with the anchorages as the vulnerable detail.

Inspection follows from that too. A pre-tensioned member shows distress in familiar ways, with cracking that an experienced inspector can read. A bonded post-tensioned member may show nothing at all, which is the point DMRB CS 465 makes about hidden safety-critical defects and brittle failure modes. External post-tensioning - tendons running inside a box void rather than inside the concrete - was developed partly to bring the steel back within reach of inspection and monitoring, as the Concrete Society's description notes.

Quality control sits in different places, and so does the paperwork. Pre-tensioned units are made in a works under factory production control and arrive as finished components whose prestress was applied and verified before they left; the site sees a delivered element. Post-tensioning is completed inside the permanent structure, which is why the record trail described earlier on this page - tendon installation records with coil and heat numbers, stressing records with jack calibration dates and measured extensions, grouting records with batch numbers, temperatures and any interruption - is generated on site by the specialist contractor and countersigned there. It is also why the certification scheme attaches to the installer and the operatives as well as to the products.

The two families also fail differently in a fire and under later modification. A pre-tensioned unit's strands sit in cover concrete and behave predictably as the section heats. A modified post-tensioned slab may contain a re-anchorage plug formed in the last twenty years, whose material properties are not the concrete's - the CROSS report on epoxy plugs turns entirely on that point, and its expert panel describes the state of a cut bonded tendon as akin to a pre-tensioned strand system, transferring its force over a development length into the concrete rather than into a mechanical anchorage. In other words, cutting a bonded post-tensioned tendon converts that end of it into a pre-tensioned detail, with all the end-zone behaviour that implies.

In practice, most structures mix the two. A precast segmental bridge is built from precast segments, which may themselves be reinforced or pre-tensioned, and then post-tensioned longitudinally so that the tendons crossing the joints hold the segments in compression - the arrangement described in the CROSS viaduct report. A building might have pre-tensioned hollowcore floors sitting on post-tensioned band beams. A demolition or alteration survey therefore does not ask which family the structure belongs to, but which family each element belongs to, and where the anchorages, transmission zones and joints are in each case; the NFDC's pre-demolition information list names pre-stressed and pre- or post-tensioned elements together for that reason. Standards split along the same line: the material standards for strand and bar are shared, execution sits under BS EN 13670 for both, duct and grout standards apply only to post-tensioning, and the design and durability guidance splits into the post-tensioning-specific documents named throughout this page.

BuildPedia would rather show where the evidence runs out than round it off. Post-tensioning is documented in paid standards, paid technical reports and access-controlled scheme manuals, so a larger share of this page than usual rests on published product records, freely available model specifications and trade guidance rather than the primary texts. These were the open points at the time of writing, and each is a place to check the source rather than this page.