Prestressed Concrete

Introduction

Prestressed concrete uses pre-applied compression in a member to counteract tensile stresses from service loads. By strategically compressing the section before it sees loads, engineers can achieve longer spans, slender members, crack control, and improved deflection performance. From bridge girders and segmental spans to floor slabs, parking structures, tanks, and stadium roofs, prestressing delivers performance that conventional reinforced concrete struggles to match.

Design the system: tendon profile + force + losses + anchors + detailing must align with the global load path, realistic analysis, and inspection plan.

What Is Prestressed Concrete & Why Use It?

In prestressed members, high-strength steel tendons are tensioned to introduce compressive stress into the concrete. Under service loads, these pre-compressions reduce or eliminate tensile stresses, limiting cracking and deflection and boosting fatigue life. The approach is economical when span/depth ratios are aggressive, cracking must be minimized (parking decks, water tanks), or construction speed matters (precast plants).

Span & Depth: Longer spans with shallower members; increased headroom and architectural flexibility.
Serviceability: Reduced deflection and crack width → better durability and watertightness.
Speed & Repetition: Pre-tensioned precast members enable factory quality and fast erection.
Lateral Systems: Prestressed diaphragms and coupling beams can improve stiffness within seismic design strategies.

Common Applications

Pre-tensioned double tees and I-girders, post-tensioned floor/post-tensioned beam-and-slab systems, segmental bridges, tanks, containment structures, transfer girders, and long-span canopies.

Pre-Tensioning vs. Post-Tensioning

Two delivery methods dominate practice, each with distinct fabrication and detailing requirements.

Pre-Tensioning: Tendons are tensioned against fixed abutments before casting. After concrete reaches release strength, strands are cut and stress transfers via bond. Ideal for precast plants (tees, bulbs, planks) with repetitive forms and tight QA.
Post-Tensioning: Tendons are installed in ducts or sheaths cast within the member and stressed after concrete reaches stressing strength. Forces are anchored with end hardware; in bonded systems, grout is injected to bond tendon to concrete; in unbonded systems, tendons remain greased and sheathed with force transmitted at anchors and via friction.

Did you know?

Bonded tendons provide load distribution after local damage and better crack control; unbonded tendons simplify replacement and allow tendon elongation monitoring but rely on anchorage zones for force transfer.

Materials, Tendons & Anchorage Zones

Prestressing steel is typically 7-wire strands (e.g., 0.5–0.6 in) or bars with high tensile strength. Concrete is proportioned for high early strength (release/stressing), low creep/shrinkage, and durability. Anchorages, bursting reinforcement, and duct details are critical to performance and inspectability.

Strands: Low-relaxation, high-strength steel; corrosion protection via grout (bonded) or grease sheaths (unbonded).
Ducts/Sheaths: Corrugated steel or HDPE for bonded post-tensioning; plastic sheathing for unbonded mono-strand systems.
Anchorages: Wedges and anchor heads concentrate forces; provide confinement and bursting reinforcement in end zones.
Concrete: Strength at release and at transfer must be met; mix design should control heat and shrinkage for long members.

Prestress Fundamentals (Concept)

\( \sigma(x) \approx \frac{P}{A} \pm \frac{P\,e(x)\,y}{I} \;-\; \frac{M_s(x)\,y}{I} \)

\(P\)Effective prestress

\(e(x)\)Tendon eccentricity

\(M_s\)Service moment

Prestress Losses: Getting to Effective Force

The force you stress is not the force you keep. Estimate short- and long-term losses and verify with measurements to assure service behavior.

Immediate Losses: Anchorage set, seating, elastic shortening, friction (curvature & wobble) for post-tensioning.
Time-Dependent: Steel relaxation, concrete creep, and shrinkage; temperature effects for long tendons.
Verification: Elongation checks during stressing, lift-off tests, and long-term monitoring where critical.

Effective Prestress (Concept)

\( P_\text{eff} = P_\text{jack} \,(1 – \Delta_\text{immed} – \Delta_\text{time}) \)

\(\Delta_\text{immed}\)Immediate loss fraction

\(\Delta_\text{time}\)Time-dependent loss fraction

Design Tip

Profile tendons to fight the moment diagram: high eccentricity at midspan for gravity, reverse near supports for negative moments. Reducing wobble lowers friction losses and improves stressing uniformity.

Design & Analysis: Strength, Service, and Stability

Prestressed members must satisfy stress limits at transfer and service, ultimate strength, shear and torsion, and overall stability. Coordinate tendon layout with the member’s role in the global system and with wind design and seismic design demands.

Stress Limits: Check concrete stresses at transfer and service at top/bottom fibers; limit tensile stress to reduce cracking where watertightness matters.
Flexural Strength: Compute nominal capacity with prestressing force and reinforcement; confirm ductility and neutral axis depth criteria.
Shear & Torsion: Prestress improves diagonal cracking resistance; design shear reinforcement and consider draped tendon vertical components.
Deflection & Camber: Estimate short- and long-term deflections including creep and shrinkage; coordinate camber with architectural interfaces.
Composite Action: For precast beams with cast-in-place decks, verify shear connectors, construction stages, and time-dependent effects.

System Workflow

Set service stress goals → choose tendon layout/profile → estimate losses and \(P_\text{eff}\) → check service stresses/deflection → size for ultimate flexure and shear → verify anchors/end blocks → coordinate camber and construction stages with the project schedule.

Serviceability, Cracking & Vibration

Prestress is primarily a serviceability tool—use it to manage crack widths and deflection under frequent loads. For floors and pedestrian bridges, confirm vibration and comfort using realistic damping assumptions; lighter members may need supplemental stiffness or damping.

Crack Control: Limit tensile stress at service and use bonded tendons or mild steel to control crack spacing and widths.
Deflection: Include creep/shrinkage and tendon relaxation; confirm total plus incremental deflection limits for cladding and partitions.
Dynamics: Verify natural frequencies and accelerations—see structural dynamics.

Crack Control (Concept)

\( \sigma_t^\text{service} \lesssim 0 \;\; \text{via prestress} \Rightarrow w_\text{crack} \downarrow \)

\(\sigma_t^\text{service}\)Service tensile stress

Detailing, Construction & Inspection

Successful projects come from constructible detailing and clear sequencing. Provide access for jacks, accommodate tendon curvature limits, and design robust end zones. Plan special inspections for stressing and grouting operations.

End Zones: Congestion is common. Provide bursting, spalling, and confinement reinforcement; detail jack clearance and stressing pockets.
Duct Layout: Respect minimum radii; coordinate slab openings and penetrations early to avoid cutting tendons.
Stressing Operations: Sequence carefully; monitor elongations; record jack calibration; lock-off per procedures.
Grouting (Bonded PT): Use qualified grout, correct water ratios, and proper venting; document volume and pressure; verify full grout fill to minimize corrosion risk.
Unbonded Tendons: Protect sheaths, maintain cover, and photograph anchor seals; replace damaged sheaths before concrete placement.

Important

Never core or saw-cut near tendons without as-built verification and de-tensioning procedures. Accidental tendon cuts can be catastrophic.

Durability, Fire & Environmental Exposure

Prestressed systems are durable when corrosion protection is robust and cracking is minimized. Exposure class drives cover, grout, and detailing. For fire, ensure required cover, detailing at anchors, and rated assemblies where needed; loss of prestress at high temperature must be considered in performance objectives.

Corrosion Protection: Bonded PT relies on complete, durable grout; unbonded relies on continuous sheathing and grease—inspect at anchors and couplers.
Environmental Classes: Marine and deicing exposures call for low-permeability concrete, enhanced cover, and corrosion-resistant hardware.
Fire: Provide cover and consider passive protection around anchors; check deflection/strength under fire load combinations.

Testing, QA/QC & Submittals

Quality control links design assumptions to field reality. Require comprehensive submittals and lay down clear hold points.

Submittals: Tendon data (grade, relaxation), anchorage hardware, duct type, grout specs, jack calibration, stressing sequence, predicted elongations, and shop drawings tied to analysis.
Pre-Pour: Verify tendon locations/cover, profile heights, blockouts, and rebar congestion relief in end zones.
Stressing: Record jack gauge readings and measured elongations; reconcile with calculated values; flag deviations.
Grouting: Pre-qualified grout, temperature/log sheets, flow/bleed tests, venting sequence, and post-grout verification.
Post-Construction: As-built tendon maps, anchor photos, grout certificates, and inspection schedule for life-cycle maintenance.

Field Acceptance Snapshot

Elongations within tolerance, grout void checks passed, anchors sealed, and camber/deflection measurements within predicted bands → green light for cladding and fit-out.

Codes, Standards & Trusted References

Start with authoritative organizations whose homepages are stable and widely cited:

American Concrete Institute (ACI): Design, materials, and construction guidance for prestressed and post-tensioned concrete. Visit concrete.org.
Precast/Prestressed Concrete Institute (PCI): Bridge and building manuals, details, and best practices. Visit pci.org.
ASTM International: Materials and testing standards for strand, grout, and concrete. Visit astm.org.
FHWA: Prestressed bridge resources and grouting guidance. Visit fhwa.dot.gov.
NIST: Research on concrete materials, durability, and modeling. Visit nist.gov.

For related topics, see our guides on concrete design, confirm structural analysis assumptions, align with wind design and seismic design, and verify anchorage into foundation design with appropriate special inspections.

Frequently Asked Questions

When should I choose bonded vs. unbonded post-tensioning?

Use bonded where redistribution and crack control are key (two-way slabs, transfer girders, segmental bridges). Use unbonded in slabs-on-beams and parking decks where replaceability and simplified detailing help, but protect anchors and sheaths rigorously.

How accurate must tendon elongations be?

Field elongations should generally fall within a reasonable tolerance of calculated values; larger deviations trigger investigation of friction, seating, or duct misalignment. Keep jack calibrations current.

Can I cut openings after stressing?

Only with engineering review, tendon scan/as-builts, and staged de-tensioning if required. Unplanned cuts can release energy suddenly and compromise capacity and serviceability.

Does prestress eliminate the need for mild reinforcement?

No. Mild steel controls local cracking, provides temperature/shrinkage restraint, and supports ultimate strength and ductility—especially in disturbed regions and anchorage zones.

How do I manage camber in precast members?

Share predicted camber ranges, specify compatible bearing details, and plan erection/leveling. Consider match-casting and construction-stage analyses for composite systems.

Key Takeaways & Next Steps

Prestressed concrete delivers long spans, crack control, and speed—when tendon forces, profiles, losses, anchors, and construction methods are tightly aligned. Start with service targets, design tendon geometry to counteract moments, quantify losses, and size for ultimate limit states. Detail end zones and ducts for constructability, and enforce disciplined stressing and grouting with documented QA/QC.

Continue with our pages on concrete design, verify analysis models, confirm load path continuity into foundation design, and plan special inspections. For specifications and research, begin at ACI, PCI, ASTM, NIST, and FHWA. Thoughtful tendon design + robust detailing + rigorous field controls = prestressed structures that perform for decades.