Post-tensioning isn't a rival to prestressing — it's one of the two ways prestress force actually gets put into concrete. The other is pretensioning, done before the pour instead of after.
In the field, "prestressed concrete" often gets used loosely to mean the precast beams and planks that arrive on a truck already under compression, while "post-tensioned concrete" refers to the cast-in-place slabs and garages stressed on site — as if the two were separate systems. They're not. Both are prestressed concrete: concrete that has a deliberate internal compressive force built into it so that, once service loads are applied, the concrete stays mostly in compression instead of cracking under tension it can barely resist. The only real difference is when that force is applied relative to the concrete curing, and what that timing forces the mechanism to be.
Concrete is strong in compression and weak in tension, so both methods work the same underlying trick: apply an eccentric compressive force through the tendon, positioned below the section's centroid, so that force creates its own upward bending moment that counteracts the downward moment from gravity loads. Under service load, the net stress at the bottom fiber stays compressive, or only mildly tensile, instead of cracking the way an ordinary reinforced concrete member would. Pretensioning can only do this by casting concrete around an already-taut strand and later transferring that stress through bond, because there's no hardened concrete yet to anchor against. Post-tensioning does the opposite: it waits for the concrete to cure to sufficient strength, then jacks the tendon against the concrete's own end faces through steel anchorage hardware. Same structural principle, same core equation for combined stress (f = P/A ± Pey/I), just two different ways of getting the force P into the member depending on whether hardened concrete already exists to react against.
No — post-tensioning is a prestressing method. "Prestressed concrete" is the umbrella term (ACI 318's own chapter on the subject is titled Prestressed Concrete, and it covers both) for any concrete member that has an internal compressive force deliberately induced before service loads are applied. Pretensioning and post-tensioning are simply its two methods, distinguished only by when the tendon is stressed relative to the concrete curing and, as a direct consequence, how the force gets transferred — bond versus mechanical anchorage. The reason the field jargon splits them apart in casual conversation is that they show up in different settings — pretensioning almost exclusively at precast plants with reusable fixed abutments, post-tensioning almost exclusively in cast-in-place field work where no such abutment exists — not because one of them is somehow a lesser or different category of prestressed concrete.
Explains why pretensioned and post-tensioned concrete are both prestressed concrete, not two competing systems — the only real difference is when the tendon is stressed relative to the concrete curing, and the mechanism that follows from that timing. Pretensioning stresses strand against fixed abutments before the pour and transfers force by bond once concrete cures and strand is released; post-tensioning stresses a tendon through a duct after the concrete has already cured, transferring force through mechanical end anchorages. Covers the shared eccentric-compression principle, why each method's setting is dictated by whether hardened concrete exists yet to react against, and bonded vs. unbonded post-tensioning.
Both methods induce an eccentric compressive force below the section's centroid so that force's own upward moment offsets the downward bending moment from gravity loads, keeping the concrete predominantly in compression under service load rather than cracking under tension it can't resist well. Pretensioning achieves this by stressing strand against fixed abutments before any concrete exists, casting concrete around the taut strand, and later cutting the strand so bond transfers the force into the surrounding concrete over a transfer length near each end. Post-tensioning achieves the same end state in reverse order — casting concrete with an empty duct first, letting it cure to adequate strength, then threading and jacking a tendon through that duct against the member's own hardened concrete ends, anchored mechanically rather than by bond.
Pretensioning fundamentally requires something rigid to stress the strand against before the concrete exists — fixed, reusable abutments capable of reacting the full jacking force — which is why it's essentially confined to precast plants producing repeatable products like beams, double tees, and hollow-core planks. Cast-in-place work has no such abutment available before the pour; the concrete itself doesn't exist yet to react against, and building a temporary abutment strong enough for every pour isn't practical. Post-tensioning solves this by flipping the order: the concrete is cast first (with an embedded duct), and once it has cured to adequate strength, it becomes its own reaction structure — the tendon is jacked against steel anchorage plates bearing directly on the member's own hardened concrete ends, which is why post-tensioning is the practical default for cast-in-place slabs, parking structures, transfer girders, and segmental bridge construction.
Post-tensioning itself splits further into two variants after stressing. In bonded post-tensioning, the duct is filled with cementitious grout after stressing, which bonds the tendon to the surrounding concrete along its length — behaving structurally similar to a pretensioned member from that point forward, and providing a secondary, distributed load path if a tendon were ever severed locally. In unbonded post-tensioning, common in many flat-plate floor slabs, the strand is individually coated in corrosion-inhibiting grease and wrapped in a plastic sheathing that permanently prevents bond — the tendon relies on its end anchorages for its entire service life, and can move slightly relative to the surrounding concrete under load, which changes some serviceability and ultimate-strength behavior compared to a bonded tendon.
Yes. Prestressed concrete is the umbrella category for any concrete member with a deliberately induced internal compressive force applied before service loads act on it. Pretensioning and post-tensioning are its two methods, distinguished by when the tendon is stressed relative to the concrete curing — not by whether the result counts as prestressed.
Pretensioning requires stressing the strand against fixed abutments before the concrete is poured, so the abutments — not the concrete — react the jacking force initially. A cast-in-place slab has no such abutment available before the pour; the concrete itself is what would need to exist to react against, and it doesn't yet. Post-tensioning solves this by casting the concrete first and stressing against the member's own cured concrete afterward.
Transfer length is the distance, measured from the end of a pretensioned member, over which the strand's prestress force fully transfers into the surrounding concrete via bond after the strand is cut and released. It's a critical zone for shear and anchorage checks near member ends, since the strand isn't yet developing its full effective prestress force within that length.
Because the eccentric prestress force sits below the beam's centroid, it creates an upward bending moment on its own, before any gravity load acts. Once the strand is released and that force transfers into the concrete, the beam bows upward slightly (camber) as a direct result — a visible sign that the intended eccentric compression is actually present, and something precasters intentionally account for in bearing and connection details.
Bonded post-tensioning grouts the duct after stressing, bonding the tendon to the concrete along its length and providing a distributed, redundant load path similar to pretensioned strand. Unbonded post-tensioning leaves the tendon permanently coated and sheathed so it never bonds, relying entirely on its end anchorages for the life of the structure — common in flat-plate floor slabs for its lower friction losses and simpler field installation, but more dependent on anchorage integrity if a tendon is ever damaged.
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