Post-Tensioning Operations: The Concrete Construction Method That Stresses Steel After Placement

Post-tensioning (PT) stresses high-strength steel tendons after concrete cures, compressing concrete to enhance structural performance. Tendons run through ducts (bonded systems) or sheathing (unbonded systems) embedded in concrete. After concrete reaches required strength, tendons stressed by hydraulic jacks. Stress transferred to concrete creates compression resisting service loads. PT enables longer spans, thinner slabs, reduced cracking, and material efficiency. Parking structures, elevated slabs, transfer beams, and bridges commonly use PT. PTI (Post-Tensioning Institute) provides standards.

Understanding PT helps GCs coordinate this specialty scope. This post covers post-tensioning operations.

Bonded vs Unbonded

Two main PT systems:

Two PT system types. Unbonded — individual 0.5 or 0.6 inch strands in plastic sheathing with grease coating allowing strand to slip during tensioning. Bonded — multi-strand tendons (2-31 strands) in metal duct, grouted after tensioning bonding strands to concrete. Unbonded simpler installation — common in slabs. Bonded provides higher structural performance — common in beams, transfer girders, parking structures.

Unbonded PT Operations

Unbonded PT process:

Unbonded PT process. Tendons installed during rebar placement following design profile (typically draped). Anchorages at slab edges. Concrete placed and cured to specified strength (usually 75-80% of design). Tendons stressed by hydraulic jack at one end (or both ends for long spans). Excess tendon cut after stressing. Anchor pockets patched. Documentation of stressing forces and elongations.

Bonded PT Operations

Bonded PT additional steps:

Bonded PT adds grouting after stressing. Ducts installed in concrete. Tendons inserted (after concrete cures sometimes). Stressing. Grouting fills duct around tendons — cement grout under pressure. Once cured, grout bonds tendons to concrete — fully bonded behavior. More steps and more potential issues than unbonded. Higher performance for specific applications.

Stressing Operations

Stressing is critical operation:

Stressing operations critical. Hydraulic jacks apply specified force — typically 70-80% of ultimate tendon strength. Elongation measurement (how much tendon stretches under force) verifies actual force. Comparison to calculated elongation within tolerance. PT contractor specialty performs. Documentation each tendon. Anchor sets (slip during anchoring) sometimes considered. Stressing sequence per design.

Concrete Strength

Strength critical for stressing:

Concrete must reach specified strength before stressing. Typically 3,000 psi minimum, some designs higher. Cylinder testing verifies. Premature stressing damages concrete — anchor zones particularly. Curing affects strength gain. Schedule planning critical — stressing too early causes problems; too late delays project. Field-cured cylinders best represent slab.

PT Coordination

Coordination across trades:

PT coordination across trades. PT contractor specialty installer. Coordination with concrete contractor for placement. Reinforcing contractor for cage detailing around tendons. MEP coordination critical — penetrations through PT slabs prohibited without engineering review (drilling can hit tendon causing failure). Future drilling/cutting prohibited without locator. Inspection by PT contractor.

Common Applications

PT applications widespread:

PT applications widespread. Parking structures — long spans, durability through service. Elevated slabs in office buildings reduce slab thickness. Hotel and condominium slabs. Transfer beams supporting columns over large openings below. Mat slabs for high loads. Bridge decks and beams. Specialty (water tanks, silos) using PT for circumferential compression.

Bottom Line

Post-tensioning (PT) stresses high-strength steel tendons after concrete cures, compressing concrete to enhance structural performance. Unbonded systems use individual sheathed tendons; bonded systems use multi-strand tendons in grouted ducts. PT enables longer spans, thinner slabs, and material efficiency. Operations include installation, concrete placement, curing, stressing, and grouting (bonded). Coordination across PT contractor, concrete, reinforcing, and MEP trades. Future drilling prohibited without locating tendons. Applications include parking structures, elevated slabs, transfer beams, mat slabs, bridges. PTI provides standards. For GCs coordinating PT scope, understanding operations supports successful structural delivery. Specialty contractors execute; GC coordinates.