post-tension

Post-tensioning is a method of reinforcing concrete by threading high-strength steel tendons (usually seven-wire strands or solid bars) through ducts cast into the concrete, then pulling those tendons tight after the concrete has cured to design strength, typically 28 days or verified by strength testing. The tendons are anchored at each end with mechanical devices called anchorages, which lock the steel in place under tension. This process compresses the concrete, which counteracts the tensile stresses that develop when the structure carries loads.

The steel used is grade 270 or 300 (in North American notation), with breaking strengths typically between 18,900 and 22,500 pounds per square inch. Tendons may be bonded, where grout is pumped into the duct after tensioning to lock them permanently in place, or unbonded, where the steel remains free to move along its length. The force applied to each tendon ranges from a few tens of thousands of pounds in small members to over a million pounds in long-span bridges or heavy industrial floors.

Post-tensioning differs from pre-tensioning, where tendons are pulled before concrete is poured. Post-tensioning allows longer spans because it can be applied only where needed and adjusted on site. It is common in bridge decks, parking structures, long-span floor systems, and tanks. The method requires skilled crews familiar with the specific anchor hardware, jacking equipment, and sequencing rules, since uneven or out-of-order tensioning can crack the concrete or damage the structure.

Common problems and limits

Corrosion of unbonded tendons is a long-term concern, particularly in coastal environments or where deicing salts are used; bonding with grout provides better protection but cannot guarantee it if the grout is incomplete. Improper grouting, broken tendons during jacking, and incorrect anchor seating pressure are field failures that require careful inspection and remediation. The cost of post-tensioning steel, anchorages, ducts, and labor is higher than mild reinforcing bar, but the savings in concrete volume and long-span capability often justify the expense in floors, bridges, and major structures.

The term reflects the sequence: tension is applied after casting and curing, not before. This simple distinction separates the method from pre-tensioning (used in precast plants for beams and planks) and from passive mild reinforcement that carries loads only through bond and friction rather than active compression.