Learn how prestressed concrete uses high-strength steel tendons to introduce deliberate compression into a concrete member. Compare pre-tensioning and post-tensioning, understand tendon profiles, anchorages, bonded and unbonded systems, prestress losses, construction sequencing, common applications and the safety issues that make prestressing specialist work.
Concrete is strong in compression but comparatively weak in tension. Prestressing introduces compression before the full service load arrives, so later tensile stresses first have to overcome some of that pre-compression before cracking or large tensile stress develops.
Steel strand, wire or bar is used as the prestressing element because it can carry a large tensile force.
The tendon is tensioned either before the concrete is cast or after it has hardened sufficiently.
The tendon force is transferred into the concrete by bond, anchorages or a combination of system actions.
The pre-compressed member can control tensile stress, deflection and cracking more efficiently in suitable designs.
Both systems put high-strength steel into tension and concrete into compression, but the timing of stressing and the way the force reaches the concrete are different.
The steel is tensioned against external abutments before concrete is placed. After the concrete reaches the required transfer condition, the tendons are released and the force transfers into the member mainly through bond.
The concrete is cast first. Once it has achieved the specified strength or condition, tendons are tensioned with hydraulic stressing equipment and locked off at anchorages that transfer the prestressing force into the concrete.
This simplified figure shows the concept only. A tendon located below the member centroid over a simple span can create an upward balancing effect and compress the concrete, helping counter service-load bending effects.
The concrete member is only one part of the system. Tendons, anchorages, ducts, grout, reinforcement and specialist stressing procedures all contribute to performance.
High-strength strand, wire or bar carries the tensile prestressing force.
Post-tensioned systems use anchorage hardware to transfer large concentrated forces into the concrete.
Ducts guide bonded tendons; sheathing protects unbonded monostrand systems and permits relative movement during stressing.
Bonded post-tensioning grout fills the tendon duct after stressing to bond and protect the prestressing steel.
Anchorage zones can need substantial reinforcement to resist bursting, spalling and local force effects.
Calibrated jacks, pumps, gauges and records are part of controlled force application and verification.
Prestressed concrete is structural concrete in which a deliberate compressive stress is introduced before the member experiences its full design service loading. The prestress is most commonly created by tensioning high-strength steel tendons and transferring that tendon force into the concrete.
The idea is useful because ordinary concrete performs very well in compression but cracks readily when its tensile strength is exceeded. Reinforced concrete accepts that cracking may occur and uses reinforcing steel to carry tensile force across cracks. Prestressed concrete goes a step further: it changes the initial stress condition so some or all of the tensile stress created by service loads is offset by pre-compression.
Prestressing can improve serviceability and structural efficiency. In the right application it can reduce cracking, reduce deflection, allow longer spans, make shallower members possible, reduce the quantity of conventional reinforcing steel and help precast units remain stable during handling and service.
Those advantages do not make prestressed concrete automatically “better” than reinforced concrete. Prestressing introduces specialist materials, stressing equipment, construction controls, detailed anchorage regions and additional durability considerations. The system is selected when the structural or construction benefits justify that complexity.
Pre-tensioning is normally associated with precast production. High-strength strands are stretched between strong abutments on a casting bed before concrete is placed. The concrete is cast around the tensioned steel and cured until it reaches the required transfer strength.
When the strands are released from the abutments, they try to shorten. Because they are bonded to the surrounding hardened concrete, their force is transferred into the concrete over a transfer length near each end. This shortening compresses the member.
Pre-tensioning suits repetitive factory production because the large stressing bed and abutments can be reused. Common examples include bridge girders, railway sleepers, piles, floor planks, hollow-core flooring, poles and other precast members.
The repeated manufacturing environment also makes concrete strength, tendon position and transfer procedures easier to control than they would be on a temporary construction site.
In post-tensioning, the concrete member is cast before the main tendon force is applied. Ducts or sheathed tendons are placed in the formwork at the required profile. After the concrete reaches the specified stressing strength, a hydraulic jack pulls the tendon. Wedges or other anchorage components lock the tendon force into the member when the jack is released.
The Concrete Institute of Australia's practice note on anchorage zones describes the post-tensioning anchorage as a system that transfers prestressing force into the concrete through a bearing plate or embedded casting with a lock-off device. The concrete immediately behind that anchorage is a highly stressed three-dimensional region and needs appropriate design and reinforcement.
Bonded post-tensioning typically uses one or more strands inside a duct. The tendon can move relative to the duct while it is being stressed. Once stressing and anchoring are complete, a cementitious grout is injected into the duct.
The grout performs two major functions: it creates bond between the tendon and surrounding structure along the duct length, and it contributes to corrosion protection by filling the internal void around the steel. Austroads ATS 5327 covers post-tensioning of concrete bridge elements and specifically includes system components, installation, stressing, grouting and corrosion protection for internal and external post-tensioning systems.
Unbonded systems commonly use an individual high-strength strand coated with corrosion-protective grease and enclosed in plastic sheathing. The strand is free to move relative to the concrete along most of its length and is connected structurally at its anchorages.
Unbonded monostrand is widely associated with building slabs in some construction markets. Because the tendon force is not continuously transferred through cement grout, anchorage condition and tendon protection are particularly important to long-term performance.
| Feature | Bonded Post-Tensioning | Unbonded Post-Tensioning |
|---|---|---|
| Tendon enclosure | Strand(s) inside a duct | Individual sheathed strand is common |
| After stressing | Duct is normally grouted | No cement grout bond along tendon length |
| Force transfer in service | Anchorage plus bond along grouted tendon | Primarily through end anchorages |
| Corrosion protection | Grout plus system protection details | Grease/sheathing plus anchorage protection |
| Typical use | Bridges, beams, large structural elements | Building slab systems in suitable designs |
| Modification implications | Still requires engineered tendon-location/cutting procedure | Cutting can release force over a substantial tendon length |
Prestressing steel is much higher strength than ordinary reinforcing bar. Seven-wire strand is a common prestressing product, although wires and bars are also used. The high steel strength is important because a significant part of the initial stress is lost over time through several mechanisms; enough effective prestress must remain after those losses.
Prestressing steel also requires careful handling. Kinks, severe bending, mechanical damage, heat, corrosion or contamination can compromise performance. CCAA's Guide to Concrete Construction includes dedicated sections on reinforcing and prestressing steel properties and on prestressing-steel handling and fixing.
The tendon profile is the vertical and horizontal path followed by the prestressing steel through the member. In a continuous slab or beam, the profile is often draped so it is low in positive-moment regions and high over supports where negative bending occurs.
This profile is not cosmetic. Tendon eccentricity changes how the prestressing force influences bending. A curved tendon can also create transverse forces that help balance part of the applied loading.
A draped tendon under tension tends to straighten. The interaction between the curved tendon and concrete creates forces on the member that can oppose part of the gravity loading. Engineers often use this behaviour as one way to understand or design post-tensioned floor systems.
The tendon force immediately after jacking is not the same as the long-term effective prestress. Some force is lost during stressing and anchoring, and additional loss develops with time.
| Prestress Loss | What Causes It | Most Relevant To |
|---|---|---|
| Friction | Contact and curvature between tendon and duct/sheathing | Post-tensioned tendons |
| Anchorage seating | Small movement as wedges or anchorage components lock off | Post-tensioning |
| Elastic shortening | Concrete shortens when prestress is introduced | Both systems |
| Concrete creep | Time-dependent strain under sustained compression | Both systems |
| Concrete shrinkage | Time-dependent concrete volume change | Both systems |
| Steel relaxation | Prestressing steel stress reduces gradually under sustained strain | Both systems |
Design calculations account for these effects so the member has the required effective prestress through its design life. Stressing records and elongation checks are also important because they provide evidence that the installed tendon behaviour matches the design assumptions within acceptable tolerances.
At a post-tensioning anchorage, a very large tendon force enters a relatively small concrete area. The stress is not immediately uniform across the member depth or width. Local bursting, splitting and spalling forces develop as that concentrated load spreads into the section.
The Concrete Institute of Australia's CPN 29 specifically highlights this complex three-dimensional stress interaction behind post-tensioning anchorages. Anchorage-zone reinforcement is therefore an engineered detail rather than optional “extra mesh”.
Prestressing cannot be transferred safely into concrete that is too weak. Pre-tensioned members require adequate concrete strength at strand release. Post-tensioned members require adequate strength before the jacks apply and anchor the design tendon force.
The specified strength at transfer or stressing is a project design requirement and should be verified by the prescribed testing method. It should not be replaced by a generic “number of days after the pour” rule because strength development changes with mix, temperature and curing.
Good grouting should fill the duct and encapsulate the prestressing steel without leaving significant voids. Poorly grouted zones can permit water or aggressive agents to reach the tendon and create durability risks that are difficult to detect from outside the structure.
The Concrete Institute of Australia lists fib technical guidance on both durability of post-tensioning tendons and grouting of tendons in prestressed concrete, while Austroads ATS 5327 includes grouting and anchorage protection as explicit parts of the bridge post-tensioning specification.
Internal tendons lie within the concrete section. External tendons run outside the main concrete section but inside the structural envelope or box-girder space, typically deviating through designed saddles or diaphragms.
External prestressing can make inspection, replacement or future strengthening more accessible in some bridge systems, but it introduces its own deviation, anchorage, vibration and protection requirements.
| Application | Common Prestressing Approach | Why Prestressing Helps |
|---|---|---|
| Precast bridge girders | Often pre-tensioned | Long span capacity, crack control and efficient precast production |
| Hollow-core floor units | Pre-tensioned | Lightweight repetitive long-span flooring |
| Railway sleepers | Pre-tensioned | Repeated factory production and resistance to service cracking |
| Building flat slabs | Post-tensioned | Longer spans, deflection control and reduced slab depth in suitable layouts |
| Transfer beams / band beams | Post-tensioned | High loads and serviceability control |
| Segmental / box-girder bridges | Post-tensioned | Continuity, segment compression and long-span construction |
| Strengthening existing structures | External post-tensioning in suitable cases | Adds controlled structural force without rebuilding the full member |
| Feature | Reinforced Concrete | Prestressed Concrete |
|---|---|---|
| Tension strategy | Reinforcement carries tensile force after loading/cracking | Pre-compression reduces service tensile stress before full loading |
| Steel | Conventional reinforcing bars/mesh | High-strength prestressing strand, wire or bar plus conventional reinforcement |
| Construction equipment | Standard reinforcement/concreting equipment | Specialist stressing systems and procedures required |
| Service cracking | Controlled cracking is common and designed for | Can be substantially reduced or controlled depending on design class |
| Span efficiency | Very effective over many conventional spans | Often attractive for longer spans or depth-sensitive structures |
| Future modification | Rebar still must be located and structural capacity checked | Tendon location and stored-energy risk add another level of control |
Yes. Prestressing reduces or controls tensile stresses but does not make concrete immune to cracking. Cracks can occur from overload, restrained shrinkage, thermal effects, detailing, construction defects, corrosion, impact, settlement, fire or other actions.
Structural design may permit some tension or cracking depending on the member, exposure and serviceability criteria. The phrase “prestressed” should never be interpreted as “crack-proof”.
Only with an appropriate process. Post-tensioned tendons should be located before drilling or coring. A new penetration can also reduce slab capacity even if it misses every tendon, because it removes concrete and may cut conventional reinforcement.
The Post-Tensioning Institute of Australasia identifies post-cut holes through PT slabs as a specialist issue and publishes guidance intended to help engineers, contractors and owners locate and create penetrations safely. Cutting a tendon is not an acceptable accidental outcome.
Existing drawings, construction records and physical locating methods may all be used. The exact method depends on the tendon system, slab construction, access and penetration size. Ground-penetrating radar, cover meters, scanning and specialist PT locating methods are commonly considered.
Where the consequences of striking a tendon are high, the locating process and penetration approval should be formalised rather than left to an individual drill operator.
Building services change over the life of a structure. New hydraulic, electrical, mechanical or fire-service penetrations can be required long after the original concrete has been cast. A tendon layout that was obvious during construction may be invisible years later.
Good asset records, tendon marking strategies, scanning procedures and a permit-to-core or engineered penetration process help protect both the structure and the worker.
Stressing is specialist high-energy work. Tendon force is applied using hydraulic equipment and retained by anchorage components. Work zones need to consider the possibility of strand, wedge, anchorage or equipment failure and the path in which components could travel.
PTIA states that stressing should be carried out by experienced personnel assessed as competent to operate stressing equipment and understand the relevant safety requirements. Project-specific exclusion zones, barriers, inspections, equipment calibration and stressing procedures are part of professional post-tensioning work.
Prestressing steel is highly stressed and must remain protected from corrosion. The protection strategy differs by system: concrete cover, duct quality, grout, sheathing, grease, anchorage caps and drainage details can all form part of the durability system.
Durability problems can be difficult to identify because much of the tendon system is concealed. Water entry through anchorages, leaking joints or defective grout can therefore be particularly serious in exposed structures.
Inspection strategy depends on structure type and tendon system. Visible anchorages, leakage, rust staining, cracking, spalling and unusual deflection can be warning signs, but absence of visible damage does not prove concealed tendons are sound.
For bridge assets and other critical structures, tendon condition assessment may involve specialist non-destructive investigation, opening selected anchorage/duct locations, grout assessment and structural analysis.
Australian building concrete design commonly references AS 3600, Concrete Structures. Standards Australia also provides commentary material intended to help users interpret the standard. Bridge structures are addressed through the AS 5100 Bridge Design series and road agencies can apply additional specifications such as Austroads post-tensioning requirements.
Standards and agency specifications are revised over time. In 2026 Standards Australia was also circulating a draft revision identified as DR AS 3600:2026. For an actual project, always verify the currently adopted standard, amendments, jurisdictional requirements and project specification rather than relying on the edition mentioned in an online guide.
| Stage | Key Control | Why It Matters |
|---|---|---|
| Design | Tendon force, profile, losses, serviceability and ultimate capacity | Defines structural behaviour |
| System selection | Approved strand, duct, anchor and grout/sheathing system | Components function as one engineered system |
| Installation | Correct tendon profile, support and anchorage location | Position affects prestressing action |
| Concrete | Correct mix, compaction, curing and test results | Concrete must safely receive prestressing force |
| Pre-stress check | Specified transfer/stressing strength achieved | Prevents overstressing immature concrete |
| Stressing | Calibrated equipment, sequence, force and elongation records | Verifies installed prestress |
| Grouting | Complete duct filling and quality control where bonded | Bond and corrosion protection |
| Anchorage protection | Seal/protect system as specified | Durability at a critical tendon location |
| Records | As-built tendon information retained | Essential for future drilling, repair and demolition |
For concrete fundamentals, see What Is Concrete Made Of?, High Strength Concrete Guide, Concrete Water to Cement Ratio Guide, What Is Concrete Admixture? and How to Cure Concrete. For reinforcement planning topics, visit the Reinforcing Steel Calculator and Concrete Strength PSI to MPa converter.
Prestressed concrete is best understood as a complete engineered system rather than concrete plus a different type of steel.
Prestressing changes the initial stress state before full service loading.
Pre-tensioning stresses steel before casting; post-tensioning stresses it after hardening.
Long-term effective prestress is lower than the initial jacking force.
Stressing, grouting, cutting, repair and demolition require trained people and engineered procedures.
Quick answers about prestressing, post-tensioning, tendon force, grouting, cracking, drilling and structural applications.
Prestressed concrete is concrete that receives deliberate internal compression, usually from tensioned high-strength steel tendons, so it can better manage tensile stresses created by service loading.
Reinforced concrete uses passive steel that develops force as the member deforms and cracks. Prestressing actively applies force to the member before the full service load occurs.
The tendon is stressed before concrete is cast. After the concrete reaches the required strength, the tendon is released and transfers prestress into the member primarily by bond.
The concrete is cast first. Tendons are stressed after the concrete has gained sufficient strength and are locked off at anchorages that transfer the tendon force into the member.
A bonded tendon is typically stressed inside a duct and then grouted so the tendon becomes bonded to the surrounding structure and receives grout-based corrosion protection.
An unbonded system commonly uses individually sheathed and greased strand that remains free to move relative to the concrete along most of its length and transfers force through end anchorages.
Tendon profile is selected to suit bending demand. A draped tendon can also create forces that help balance part of the applied gravity loading.
They are reductions from the initial tendon force caused by effects such as friction, anchorage seating, elastic shortening, concrete creep and shrinkage, and steel relaxation.
Yes. Prestressing improves stress and crack control but cannot prevent every crack caused by overload, shrinkage, temperature, durability problems, construction defects or other actions.
Only after tendons and reinforcement have been located and the penetration has been assessed under an appropriate engineered process. Never assume a clear-looking surface is tendon-free.
In bonded post-tensioning, grout fills the duct, bonds the tendon to the structure and contributes to corrosion protection.
It is a well-established construction system when designed and executed correctly, but stressing stores significant energy. Installation, stressing, de-tensioning, repair and demolition require specialist procedures and competent personnel.
For design or construction, use the current project specification and the applicable current standard or agency specification. These sources provide useful Australian context and specialist guidance.
Includes principles of reinforced and prestressed concrete, prestressing steel properties, handling/fixing, concrete site practices, curing, properties and testing.
Open CCAA GuidePractice-note resource covering the highly stressed anchorage zone behind post-tensioning anchorages and the different force-transfer mechanism in pre-tensioned systems.
View Current Practice NotesTechnical specification for bridge post-tensioning covering components, tendon installation, stressing, grouting, corrosion protection and quality controls.
Read ATS 5327Australasian specialist industry body providing accreditation, training and technical guidance for post-tensioning systems.
Visit PTIAStandards Australia describes AS 3600 as the Concrete Structures Standard and provides commentary material supporting design and construction interpretation.
Read Standards Australia UpdateHighlights the stored-energy and structural-stability risks involved when removing tensioned tendons during demolition of prestressed concrete.
Read Demolition Guidance