CIVL3811 Chap.3 Post-Tensioned Concrete Systems
Post-Tensioned Concrete Systems
Post-tensioning introduces compression and uses tendon curvature to balance part of the gravity load. This chapter follows the sequence from ducts and anchorages through concrete placement, strength confirmation, stressing and grouting. It separates bonded and unbonded behaviour and groups immediate losses, such as friction and seating, from creep, shrinkage and relaxation over time.
The worked force audit makes the percentage basis explicit so a calculated effective force can be checked rather than accepted as a rule of thumb. A dedicated sequence page connects jack calibration, theoretical extension, measured extension, exclusion zones and acceptance records to the structural result.
The design clinic links thinner floors and longer spans to anchor-zone forces, punching, tendon coordination, inspection and strict control of later penetrations. Students practise choosing floor forms while recognising that specialist construction, future openings and stored tendon force can reverse an apparently efficient selection.
What this chapter covers
- 01
Prestress and precompression
- 02
Draped tendon action
- 03
Bonded and unbonded systems
- 04
Prestress losses
- 05
Floor-system selection
- 06
Anchor zones and penetrations
Worked example · free
Convert jacking force to effective prestress
- +1Calculate immediate losses against the stated original-force basis.
- +1Calculate time-dependent loss on the same basis.
- +1Subtract the total loss from jacking force.
- +1State the retained percentage.
- +1List field and model checks.
Key terms
- Jacking force
- The tendon force applied by the stressing equipment.
- Effective prestress
- The remaining prestressing force after relevant losses.
- Load balancing
- Equivalent upward action created by force in a curved tendon.
- Anchorage zone
- The concrete region where concentrated tendon force enters the member.
- Tendon profile
- The vertical and horizontal path followed by a prestressing tendon.
- Anchorage seating
- A tendon-force reduction caused by wedge or anchorage movement as the applied stressing force is locked off.
- Friction loss
- Prestress reduction along a tendon caused by intended curvature and unintended deviation from the designed profile.
- Measured extension
- The field-observed tendon elongation compared with theoretical expectation as an independent stressing acceptance check.
- Bonded tendon
- A tendon grouted within a duct so force and strain can transfer along its length after completion.
- Unbonded tendon
- A greased and sheathed strand that remains free to move relative to surrounding concrete between anchorages.
Post-Tensioned Concrete Systems FAQ
Why is a tendon draped?
Tendon curvature creates an equivalent upward action that can oppose part of the gravity loading while the prestressing force also precompresses the concrete. The profile is therefore matched to the desired bending response and construction constraints.
Why does effective prestress reduce?
Friction, anchorage seating and elastic shortening create immediate reductions, while creep, shrinkage and steel relaxation act over time. Their magnitude depends on tendon profile, stressing sequence, material behaviour and the reference force used.
Can a stressed slab be drilled freely?
A stressed slab cannot be treated like unreinforced material. Verified tendon and anchorage-zone locations, structural assessment, scanning, an engineered detail and a controlled permit are needed before any cutting or drilling begins.
Why compare theoretical and measured extension?
Jack pressure indicates applied force, while extension responds to tendon length, elastic properties, friction and seating. Comparing both can reveal misidentification, obstruction or profile problems before the stressing result is accepted.
What happens after bonded tendons are stressed?
Accepted tendons are secured and their ducts are grouted to provide bond and corrosion protection. Vents, grout continuity, anchorage pockets and completion records all form part of the permanent structural result.
Why are anchor zones heavily detailed?
Concentrated tendon force enters the concrete at anchorages and creates local bursting and spalling effects. Adequate concrete strength, bearing, reinforcement, access and exclusion control are needed before and during stressing.
How should post-tensioned floor forms be compared?
Compare span, depth, punching, deflection, tendon congestion, services, repetition, stressing access and future alteration. A thinner solution is not automatically better when anchor zones or penetrations dominate the project.
Why must the prestress-loss basis be stated?
Percentages may be defined against original jacking force or applied sequentially to a reducing value. Writing the basis prevents an apparently correct percentage total from producing an inconsistent effective force.
What records should remain for future alterations?
Retain tendon layout, anchor zones, stressing pressures, measured extensions, accepted discrepancies, grouting and as-built changes. These records guide scanning, structural assessment and controlled permits when later services work proposes a penetration.
Exam move
Start with a slab section and draw a draped tendon, anchorage forces, precompression and the upward equivalent action from curvature. Explain the two effects separately before joining them. Build a loss table with mechanism, timing, location and evidence: friction, seating, elastic shortening, creep, shrinkage and relaxation.
Recalculate the effective-force example under both an additive original-force basis and a hypothetical sequential basis so the importance of definition is obvious. Rehearse the site sequence from tendon and anchor inspection through concrete-strength confirmation, calibrated stressing, extension comparison, acceptance, tail treatment and grouting.
For system selection, compare flat plate, banded and one-way arrangements using span, slab depth, punching, services, formwork repetition and future penetrations. End by writing a safe response to a proposed core hole, naming the verified records, scanning, structural review, permit, exclusion zone and as-built update required before drilling.
