How the two differ in kind
Both of these are answers to the same question, and on expansive clay the question is not how to stop the ground moving. Nothing on either drawing stops a clay soil taking up water at its edges through a wet winter and giving it back to a thirsty tree in August. Both foundations are plates designed to ride that movement without passing enough differential deflection into the house to crack the walls above, and both are designed from the same document — the geotechnical report's plasticity, its edge moisture variation distance, its differential swell. What separates them is how each plate is made stiff enough to do that, and what the method costs you for the rest of the building's life.
Ordinary reinforcement is passive. A bar sitting in concrete does nothing whatsoever until the concrete around it has stretched, which in practice means until it has cracked; the design accepts that and puts steel where it will be needed afterwards. Post-tensioning is active. Strand is tensioned against the already-hardened slab and locked off, so the concrete is left permanently squeezed and the force is in the section whether or not anything ever moves. Tension has to spend that compression before it can begin to open anything. The consequences cascade: a precompressed slab is stiffer for its depth, so ribs get shallower and further apart and the passive mat largely disappears; it needs no grid of control joints, because the shrinkage those joints existed to manage is being held closed; and it can be poured as one large jointless area under an open plan where a conventional slab would carry a sawn grid beneath the floor covering.
Then the rest of the bill arrives, in a currency no quote shows. A conventional slab is inert steel in concrete and it is worked on the way concrete has always been worked on — cut it, core it, pier under it, dowel an addition onto its edge, and the worst case is that you sever a passive bar and lap a new one across the patch. A post-tensioned slab has live force stored in it and keeps that force for as long as it stands. Every later penetration starts with locating tendons; a tendon that must go is a de-tension and splice by a specialist rather than a saw cut; and the drawing showing where the strands run stops being a record and becomes a document the building cannot be maintained safely without. That is the trade in one line. One slab performs better. The other one can be touched by anybody.
The factors that actually differ
Show
| Post-tensioned slab | Conventionally reinforced slab | |
|---|---|---|
| Where the stiffness comes from | Force locked into the concrete. Strand stressed against the hardened slab leaves the section permanently compressed and behaves as one stiff plate, which buys the same deflection limit out of a thinner slab, shallower ribs at wider spacing, and far less steel by weight. | Section geometry. Nothing is compressed, so stiffness is bought with depth: deeper ribs at closer centres, more excavation, more concrete, and a mat of passive bar that contributes nothing until the concrete around it has already strained. |
| Control joints | Largely absent. Precompression is holding shrinkage closed, so a house slab is poured as one large area — no sawing window to hit, and no joint grid to telegraph through polished concrete or large-format tile. | Part of the design. The grid has to be laid out, sawn inside its window, and then lived with under whatever goes on top. Miss the window and the slab picks its own crack instead of taking the one you offered. |
| Movement while it is being stressed | The slab physically shortens as the strand is tensioned, so it wants a slip surface beneath it and nothing rigid gripping it. A keyed perimeter, a cast-in stack, an abutting wall — anything resisting that shortening absorbs prestress the concrete was supposed to get. | Nothing comparable happens. Concrete still shrinks as it dries — that is the movement the joint grid exists to steer — but there is no stressing-day shortening to make room for, so the slab can be keyed, stepped and tied into whatever sits beside it without any design force being absorbed by the restraint. |
| Cutting, coring and drilling afterwards | Every penetration begins by locating tendons, and a live strand is never sawn — it is de-tensioned by a post-tensioning contractor, cut, then re-anchored or spliced. Small openings placed between tendons in the field of the slab are often approved; anything near an anchorage is a different order of problem. | A saw, a core barrel and a patch. Severed bar is passive and the loss is local, so the repair is dowelling into sound concrete and lapping fresh steel across the opening — routine work for any concrete crew. |
| Foundation repair and underpinning | Constrained on three fronts: pier positions must miss the tendons, a prestressed plate does not respond to jacking the way a conventional one does, and the set of firms willing to design and warrant remedial work on it is much smaller. | The repair industry's home ground. Cored access, piers, jacking and grouting are standard products, priced competitively by firms who see the same slab every week and will stand behind the result. |
| Who has to be on the job, and when | A licensed post-tensioning designer, a tendon package made for that specific lot, and a certified stressing crew who come back AFTER the pour, once the concrete has reached the strength specified for stressing. That return visit is a hold point nobody else on site can clear. | A local engineer's detail or a prescriptive schedule, plus any competent concrete crew. The reinforcement is finished before the truck arrives and nobody has to come back to finish the foundation. |
| What the design number actually is | Not the reading on the jack. Seating at the wedges, friction along the tendon profile, elastic shortening, creep, shrinkage and relaxation all subtract, and the slab has to work on what survives — which is why stressing is signed off with measured elongations rather than declared done. | The bar's area and its yield strength, on the day it was tied and for the life of the slab. Nothing decays but the steel itself, and what the site inspection measures is geometry — bar size, spacing, cover, lap length — all of it before the pour, because there is no force in the section to go back and confirm afterwards. |
| Corrosion, and what it costs when it happens | Concentrated at the ends. Unbonded strand is greased inside a sheath, so the exposure is at the anchorage pockets in the slab edge — and wire stressed near its usable limit fails suddenly and lets go of the whole tendon. Encapsulated anchorage systems and honest edge drainage exist for exactly this reason. | Distributed and slow. A corroding bar loses section gradually and announces itself as rust staining and spalling well before it is structural, and what is affected is the piece of slab around it rather than a line across it. |
| How the cost scales | Mostly fixed per slab. An engineered design for your lot, a made-to-order tendon package, and a crew mobilising and returning cost close to the same on a small slab as a large one, while the material saving grows with area — so it amortises well across a big slab or a repeated plan and badly across one modest one. | Close to proportional. Steel by weight, tying labour by intersection, concrete by volume in the deeper ribs. It costs roughly what it covers, quoted by crews who all price it the same way. |
Which one, and when
Choose post-tensioned slab when…
- The geotechnical report calls for a stiffened raft AND post-tensioning is the ordinary answer where you are building — designers, tendon suppliers and stressing crews all working weekly. Where it is routine it is competitive; where it is exotic you pay for the exoticism twice, once to build it and again to maintain it.
- The slab is the finish, or close to it. Polished concrete, large-format tile and long open-plan spans all suffer from a sawn joint grid and from cracks free to open, and a jointless precompressed slab is the only one of the two that avoids both.
- The slab is large, or the plan is being built repeatedly. Fixed design and mobilisation costs spread across area and across repetition, which is precisely why production builders on expansive clay default to it.
- The layout is genuinely final. Plumbing set, no future bathroom, no workshop, no lift, no addition — you are accepting a slab that must not be cut, and you will keep the tendon drawing somewhere the next owner will find it.
Choose conventionally reinforced slab when…
- Something is going through this slab later. A drain relocated, a bathroom added, a machine or a hoist anchored down, an addition doweled to the edge — each of those is ordinary concrete work on a conventional slab and a specialist engagement on a post-tensioned one.
- Post-tensioning is not routine in your market. One stressing crew within reach, no engineer who details it regularly, and repair firms who will not warrant work on it: that combination prices it as a specialty and leaves you owning an orphan.
- It is a one-off on a difficult lot. The whole post-tensioning apparatus — design, package, mobilisation, return visit — lands on a single slab with nothing to amortise against.
- The pour cannot be one free-floating plate. Tying into an existing structure, a heavily stepped or restrained slab, or staged construction all fight the shortening that stressing depends on.
- The building will outlive its paperwork — a rental, an outbuilding, anything that changes hands more often than its drawings do. A conventional slab needs no document to be worked on safely. A post-tensioned one does.
Now run your own numbers
This page holds no prices on purpose — a national average is wrong for almost every real project. Quantify both options with your dimensions and your local quotes.
Frequently asked questions
- Is post-tensioning actually better on expansive clay?
- Better at the specific job of staying stiff and staying crack-tight, yes — and that is a real advantage on soil that lifts at the edges one season and drops away under the centre the next. But it is not the thing that makes a foundation right. Both methods are designed from the same geotechnical report, and a conventional stiffened raft designed to accurate soil parameters will outperform a post-tensioned slab designed to assumed ones. Neither stops the clay moving, either. Perimeter drainage, gutters discharging well away from the slab, and trees kept at a sensible distance change the moisture regime the foundation has to survive more than the choice of reinforcement does. Spend on the site investigation first; this argument is downstream of it.
- Can I cut into a post-tensioned slab?
- Yes, carefully, with help — and never by putting a saw into it and hoping. The sequence is: find the tendon drawing, scan the area to confirm the drawing tells the truth, and take the proposal to an engineer before anything is cut. A small opening placed in the field of the slab between tendons is frequently approved. A tendon that genuinely has to go is de-tensioned deliberately by a post-tensioning contractor, cut, then re-anchored or spliced. The strand is holding real stored energy, so cutting one live is a way to be injured rather than merely a way to damage a foundation, and the zone around an anchorage is the part to leave alone entirely. This is what the warning placards on these slabs are for; if yours has one, believe it, and make sure the next tradesman with a core drill sees it before you do.
- Which one costs more?
- The two costs are shaped differently enough that there is no single answer, and this site will not manufacture one. Post-tensioning is material-light and service-heavy: less concrete and much less steel by weight, set against an engineered design for your lot, a made-to-order tendon package, and a stressing crew who mobilise and return. Most of that is fixed per slab, so it amortises over area and over repetition. Conventional reinforcement is the mirror image — heavier materials and more tying labour, but nearly all of it proportional to the slab and quoted by any local crew. Two things move the answer more than the arithmetic does: how routine post-tensioning is in your market, and the cost of changing the slab later, which arrives years after the quote and never appears on it. Run the rebar takeoff, get the post-tensioned design priced, and compare the two on your actual slab rather than on the general case.
- Do post-tensioned slabs crack?
- They do. Precompression makes cracking much harder to start and holds what does form tight rather than open, which is the entire basis for pouring them without a joint grid — but it is a resistance, not an immunity. The usual causes are restraint while the slab is shortening under stressing, re-entrant corners where the plan steps in, and ground movement beyond what the design assumed. The practical difference is what a crack MEANS. On a conventional slab it is expected, it is what the joints were placed to steer, and the steel's job starts there. On a post-tensioned slab a crack that is open rather than hairline is a signal worth an engineer's opinion, because something has overcome a compression that was meant to be permanent.
- I am buying a house on a post-tensioned slab. What should I ask for?
- The tendon layout drawing, first and last. It is not a record, it is an operating requirement, and a slab without one starts every future job with a scan and an engineer. Then the foundation design, the geotechnical report it was based on, the stressing records with their measured elongations, and a straight answer to whether any tendon has ever been cut, damaged or repaired. Walk the slab edge where the anchorage pockets are: patched pockets, rust staining or water standing against that edge are all worth an inspector's eye, because the edge is where an unbonded system is vulnerable. None of that should scare you off a well-documented slab — a post-tensioned foundation with its paperwork intact is a good thing to own. It is the undocumented one that quietly transfers a problem to you.
- Why does the post-tensioned side only have a loss calculator?
- Because that is an honest picture of the difference between them. On the conventional side you can do your own takeoff: a grid, its spacing, its laps, a stick count you can check against a delivery ticket — the rebar calculator does that arithmetic and shows every step of it, so you can audit the answer instead of trusting it. It covers the flat mat, not the cages in a stiffened raft's ribs, and on a foundation it defers to the engineer's schedule the way every slab tool here does. On the post-tensioned side there is nothing to take off, because the slab is a design rather than a quantity and it comes from a licensed post-tensioning engineer with the soil report open. The calculator offered here handles one piece of that design: the prestress given up when the wedges settle their fixed few millimetres into the anchorage, which bites hardest on short tendons because the same seating distance is spread over less length to absorb it. It is one loss among several, and it does not pretend to be the design. That asymmetry is worth sitting with before you choose, because it is also the asymmetry you will live in — one of these foundations you can understand on your own, and one of them you will always be phoning somebody about.