Structure and support · Floor Structures
Post-Tensioned Concrete Slab System
A reference account of what changes when a concrete slab is actively compressed rather than passively reinforced: a load path switched on after the pour, an edge condition whose job is to release the slab rather than hold it, and openings that cannot be formed once the concrete is down.
Educational reference entry. How this system behaves depends on climate, jurisdiction, loading, substrate, the adjacent systems it meets, how the building is used, the manufacturer's instructions and qualified professional design.
Overview
What post-tensioned slab is
In a post-tensioned slab the concrete is cast first and the force is applied afterwards. Tendons run within the depth of the plate, sheathed or ducted so they are not bonded to the concrete around them while the work is being done, and once the concrete has gained enough strength they are tensioned against it and locked off at anchorages at the slab's edges. From that moment the slab is carrying a load that has nothing to do with the building standing on it.
That is the whole distinction from a conventionally reinforced plate, and it is a distinction of mechanism rather than of material. Ordinary reinforcement is passive: it does nothing at all until the concrete around it has already strained, and its contribution is a reaction to what the structure is being asked to do. A tendon is active: it is working in an empty building, at night, in a slab nobody is standing on, and it goes on working for as long as the anchorages hold it.
The consequence that gives this system its identity is that the slab has to be able to move. Tensioning pulls the anchorages towards one another and the concrete between them shortens. Anything that holds the plate still while that is happening — a stiff core cast into it, a column it was cast hard against, a perimeter detail that grips — resists the shortening and takes part of the force that was intended for the concrete. The prestress is not lost, it is delivered somewhere else.
Against the other ways a concrete floor plate is built, the boundary is again the mechanism. A composite metal deck floor works by making a profiled sheet and a concrete topping act together; a precast plank floor works by tying separately made units into one plate; a cast frame works by continuity of passive reinforcement through every junction. None of those creates an anchorage zone, a stressing sequence, or a requirement that the element be free to shorten, and those are exactly the roles this entry exists to describe.
Terminology
Common names and aliases
These names describe the same assembly. The encyclopedia keeps one entry per system so that a trade term or a regional name never becomes a second, thinner page.
- post-tensioned slab
- PT slab
- post-tensioned floor plate
- stressed concrete floor
- prestressed in-situ slab
- post-tensioned concrete floor
Purpose
What this system is intended to do
The job the assembly exists to perform, conceptually. An intention is not a guarantee that any particular build achieves it.
- Place the concrete in compression before the building loads arrive, so the material is working in the state it deals with most reliably.
- Carry the floor load through a tendon path arranged by the designer within the depth of the plate, rather than through the concrete and passive reinforcement alone.
- Hand the whole of that internal force into the structure at a limited number of anchorage positions, each of which is a local condition rather than a fitting.
- Leave the plate free to shorten towards those anchorages while the force is applied, and to go on moving as the concrete continues to change afterwards.
- Fix the position of every opening, penetration and cast-in item before the pour, because afterwards the plate has no areas that are known to be clear.
Composition
The roles this system is made of
What each part does in the assembly and what it depends on — never a product, a thickness, a fixing or a determination that anything is adequate for the role.
Order is not meaningful in this system. These roles interact as parts of one whole rather than stacking up in a sequence, so the list below is not a build-up and nothing should be read into the order it appears in.
- Primary supportTendons within the slab depth
The tensioned elements running through the plate that apply the force. They follow a path through the depth that the designer sets, and that path is the load arrangement itself rather than a detail of it. Once the slab is cast the path cannot be altered, and nothing about its position is visible from either face.
- AttachmentAnchorages at the slab edge
The positions where a tendon's force is handed into the concrete. An anchorage behaves as a zone rather than as a component: the force arrives concentrated at a point and has to spread into the plate, which is why the concrete immediately behind it is treated differently from the concrete anywhere else.
- Primary supportLocal reinforcement behind each anchorage
Reinforcement placed around and behind the anchorage to deal with the effects of a concentrated force entering the slab there. It exists because of the anchorage rather than because of anything the floor is carrying, so it appears only where the anchorages are and nowhere else in the plate.
- Primary supportOrdinary passive reinforcement
Bar placed for the duties the tendons do not perform: over supports, around openings, at edges, and wherever the plate has to hold itself together before and while the force is being applied. The two kinds of steel are not substitutes for one another, and a slab of this kind usually carries both.
- SubstrateThe plane the slab bears on, and how much it grips
Whatever the plate sits on where it sits on something continuous — a prepared base, a separating layer, or a supporting structure. Because the slab has to move towards its anchorages, how much friction or bond that plane offers is part of the design rather than a by-product of what happened to be underneath.
- Edge and terminationA perimeter arranged to release the plate
The condition at every edge of the slab and at every vertical element it runs into. This is the place where the designer either allows the shortening or prevents it, which makes an edge detail that grips the concrete a structural decision even when it is drawn by somebody treating it as a finish.
- Jointing and sealingConstruction joints, pour breaks and joints left to move
The planes where one pour stops and the next is cast against it, and the separate joints left open so movement can be released. They are different things doing different work, and the sequence in which the pours are made is read together with the sequence in which the tendons are stressed.
- Service zoneOpenings, penetrations and cast-in items
Every hole, sleeve, drain, socket and fixing that will pass through the plate or be buried in it. All of them belong to the pre-pour coordination, because after the pour the slab contains no area that can be assumed to be free of a tendon.
- Jointing and sealingAnchorage recesses and their making good
The pockets left at the stressing positions so the tendons can be reached, filled once the force is in. Filling them is what makes the anchorage invisible, so from that moment the only reliable record of where the anchorages are is the one that was drawn.
Interaction
How the parts work together
The reason this is a system rather than a list of parts: what depends on what, and what stops working when one part is changed.
Compression only arrives if the plate is allowed to move. Tensioning pulls the anchorages towards each other and the concrete between them shortens; anything holding the slab still resists that shortening and takes part of the force instead. A stiff core cast into the plate, a column the slab was cast hard against, or an edge detail that grips will each do it, and each of them is then receiving an action the element concerned may never have been designed to receive.
A tendon is one continuous element from one anchorage to the other, so nothing about it is local. Move an anchorage and the path through the depth changes along the whole length of that tendon rather than near the end that was moved. That is why an apparently modest change at one edge of a plate is a redesign of the plate, and why the position of the edges is settled before the tendon arrangement rather than after it.
Passive reinforcement and tendons occupy the same concrete and act at different moments. The bar waits until the concrete has strained and then reacts; the tendon has been acting since it was locked off. Because they are not interchangeable, an argument that a slab is heavily reinforced says nothing about whether a tendon can be disturbed, and an argument that it is post-tensioned says nothing about whether the ordinary reinforcement can be cut.
A penetration formed afterwards is not a hole in a slab; it is a cut tendon. The force that tendon was carrying is released along its whole length, not within the size of the opening and not within the bay the opening was made in. This is the single respect in which a post-tensioned plate behaves least like the reinforced slab it resembles, and it is the reason this entry exists as a separate system rather than as a note on another one.
The separating plane, the joints and the perimeter are three answers to one question, which is where the shortening goes. Settle any one of them and the others are already partly decided. Leave the question unresolved and it is still answered on site, by whichever part of the surrounding construction turns out to be stiffest.
Materials
Material families commonly met in each role
Commonly encountered, not recommended. Whether a material suits a given project depends on the whole assembly, the exposure, the manufacturer's documentation and qualified professional review.
Primary support
Cast concrete families, and the admixtures used to modify how a mix behaves while it is being placed and while it gains strength, are commonly encountered in plates of this kind. What a particular mix is required to achieve, and by when, is a structural determination for the designer rather than a property of the family.
Primary support
Reinforcing steel families are commonly encountered as the passive reinforcement and as the local reinforcement behind anchorages. Their presence in a post-tensioned plate is additional to the tendons rather than an alternative to them.
Attachment
The plates, castings and edge components at anchorage positions are commonly formed in these metal families. Which components belong together, and how they are protected once the recess is filled, comes from the manufacturer's documentation and the designer.
Substrate
Sheet and board families are commonly encountered beneath and against a slab that has to be free to move, where a separating plane or a compressible edge is being formed. Whether a given layer offers the release the design assumes is a question for the designer, not a property of the material.
Jointing and sealing
Joint profiles, embedded joint components and sealant families are commonly encountered where a plate's joints are formed and closed. A joint intended to move and a joint intended to act as though the concrete were continuous are different design decisions, and the products differ with them.
Finish surface
Where the slab is also the walked surface, these treatment and coating families are commonly encountered on it. A surface treatment applied across a joint that was left to move is working against the joint, which is a coordination question rather than a product one.
Boundary
Where this system ends and meets another
The edge of this system. Everything above is inside it; each junction below is a condition at its boundary, where an adjacent system or an adjacent building condition takes over. Junctions are where most assemblies actually fail, so they are set out explicitly rather than left inside the prose. What resolves one is a detail designed for the specific building — not a rule of thumb.
Columns and walls cast with the plate
A vertical element cast monolithically with the slab is a restraint at that point in the plan, and a stiff one is a restraint that will not yield. Which vertical elements the plate is allowed to be tied to, and which are deliberately separated from it, is settled in the frame design rather than discovered when the slab is stressed.
Read about Concrete Frame →A core or wall holding the plate at one position
The elements resisting horizontal action are chosen for stiffness, which is exactly the property that prevents a slab shortening past them. Where the stability elements sit in the plan therefore affects how a post-tensioned plate can be arranged, and the two are decided together rather than one after the other.
Read about Lateral Stability →A floor plate propping perimeter walls
A floor that is propping retaining walls is being held at its edges by the thing it is holding apart. Whether a plate in that position can also be a post-tensioned element, and what the propping action does to its freedom to shorten, is a question for the engineer responsible for the box as a whole.
Read about Basement Structure →A stressed plate cast on prepared ground
At ground level the plate is bearing on a surface along its whole area rather than at supports, so friction against that surface is the restraint that matters. What separates the concrete from what is beneath it, and how much grip that separation leaves, moves from being a construction detail to being part of the structural arrangement.
Read about Ground-Bearing Slab →Anything passing through the plate
A penetration through a post-tensioned slab is agreed before the pour and formed with the plate, not cut into it. That reverses the usual order of work, in which a route is sealed after it has been made, and it is the point at which a services drawing becomes a structural drawing.
Read about Penetration Sealing →Service distribution above and below the slab
Because the plate resists being drilled, services are routed through zones beside it rather than through it wherever that can be arranged. Where a fixing into the soffit is unavoidable, what may be fixed into and where is a matter for the structural designer and for the record of the tendon positions.
Read about Service Void →A build-up laid over the structural plate
An isolated build-up above the slab conceals it, and in doing so conceals the joints beneath it. Where a joint in the structure has to be carried up through the build-up, and where the build-up may run over, is decided from the structural layout rather than from the screed layout.
Read about Floating Screed Build-Up →
Considerations
Topics worth discussing
Which topics genuinely apply to this system and what to raise about them. Measured values, classes and ratings come from a qualified professional's design for the specific building, not from a reference page.
- Movement
- Shortening is not an unwanted side effect of this system, it is how the system works, and it continues after the stressing is finished as the concrete goes on changing. How much movement is expected, over what period, and what is allowed to restrain it are all part of the structural design and belong with the engineer responsible for the plate.
- Buildability
- The order of work is unusually consequential here. Pours, the point in the concrete's life at which stressing is carried out, and the sequence in which tendons are stressed are decided in advance and are not site variables. Access to the stressing positions also has to survive long enough for the work to be done.
- Interfaces
- The perimeter of the plate is where this system is usually decided. Every element the slab meets at its edge is either allowing it to move or preventing it, and a detail drawn by somebody thinking about weathering, finishes or fixings can change a structural assumption without anybody noticing that it did.
- Maintenance and access
- Almost nothing about the mechanism can be inspected once the recesses are filled. There is no visible difference between a plate whose tendons are where the drawings say and one whose tendons are not, which is why survey before any intrusive work is a normal part of altering a floor of this kind.
- Durability
- The anchorage zones and the tendon path are the parts of the plate whose long-term condition matters most and which are hardest to reach. What protection was provided, and what would be looked at if the plate's condition were ever questioned, are worth establishing while the information still exists.
- Documentation
- This is the system where the record is part of the structure. Tendon layouts, anchorage positions, the stressing record and the as-built location of openings are the only means by which a later owner or designer can know what may be cut, drilled or altered, and their absence is itself a constraint on the building.
Design
Questions the design has to answer
The decisions this assembly turns on. They are questions rather than answers, because the answer depends on the building.
- Which vertical elements is this plate tied to, and which has the design deliberately separated it from?
- Where are the anchorages, and what does the edge of the slab meet at each of them?
- What is the slab bearing on, and does that surface allow it to move as the design assumes?
- Which joints are intended to act as though the concrete were continuous, and which are intended to move?
- Has every opening, sleeve and cast-in item been located before the pour, including the ones the services design has not finished yet?
- How will anyone fixing into this floor after handover know where they may drill and where they may not?
- Is a post-tensioned plate being chosen for a reason that survives the coordination it imposes on everything else?
Boundaries
Commonly misunderstood points
Distinctions that are easy to blur, and the places where a familiar term means something narrower than it sounds.
- A post-tensioned slab is often read as a heavily reinforced one, when the difference is that part of its steel is doing work in an unloaded building and cannot be interrupted.
- The freedom to shorten is treated as a tolerance to be accommodated, when it is the mechanism by which the slab is compressed at all.
- An unplanned restraint is assumed to be a local problem at the restraint, when what it actually does is remove force from the plate it was meant to compress.
- Coring or drilling is thought of as a question of finding a clear spot, when the consequence of a cut tendon is released along the whole of that tendon.
- Filling the anchorage recesses is treated as making good, when it is the moment the assembly stops being inspectable.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- What restraints has the design of this plate assumed, and what has it assumed will not restrain it?
- Where do the tendons run, and what record of that will the building owner be given?
- What is the intended sequence of pouring and stressing, and what depends on it being followed?
- Which joints in this floor are movement joints, and how are they to be carried through the finishes above?
- What is the procedure if an opening is needed in this slab after it has been stressed?
- How is the perimeter detailed so that the slab can shorten, and who is checking that the details drawn by others respect it?
- What survey would be required before any intrusive work in this floor, and who would be qualified to carry it out?
What this page does not do
- This entry describes how the assembly is organised. It states no tendon arrangement, force, sequence or timing, and is not a specification for any particular floor.
- Whether a post-tensioned plate is appropriate for a given building is a structural determination made by a qualified engineer against the loading, the frame, the ground and local requirements.
- Cutting, coring, drilling or chasing a post-tensioned slab is not comparable to the same operation in a conventionally reinforced one, and should never be undertaken on the basis of a general reference.
- Stressing operations and the temporary conditions around them are specialist work carried out by qualified people, and nothing here describes how they are performed.
- If the tendon layout for an existing floor cannot be established, that is itself a finding to take to a structural engineer rather than a gap to be worked around.
Related systems
How this system relates to others
Every link states what the relationship actually is, rather than leaving a bare list of related pages to be read as a suggestion.
Alternatives
Different systems answering the same need. Listing them together is not a comparison and does not suggest one is better — which, if either, suits a project is a design decision.
Applications
Building contexts this system is used in
The functional parts of a building this assembly is commonly met in. This is membership, not a ranking and not a recommendation: several systems answer any one context, and which of them suits a project is a design decision. A context appearing here does not mean the system is suitable, permitted or adequate for it.
- Intermediate Floor · Structure
- Superstructure · Structure
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this assembly involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this system comes up.
Floor Structure and Build-Up Systems
Horizontal decks read together with the stack placed on them, told apart by what spans, what is infill and what is only a topping over both.
Browse all floor structures entries →