posttension

Posttensioning is a method of prestressing concrete in which high-strength steel tendons, usually strands or bars, are threaded through ducts cast into the concrete. After the concrete reaches sufficient strength, typically 3 to 7 days, hydraulic jacks pull these tendons to a predetermined load, then lock them in place with anchorages. This creates a permanent compressive force within the concrete that resists tensile stresses from loads and temperature change.

The process differs fundamentally from pretensioning, in which steel is tensioned before concrete is poured. Posttensioning works well for long-span beams, slabs, and structures where flexibility in layout matters. Common tendon arrangements include straight runs through beams, draped profiles in slabs that follow the bending moment diagram, and harped layouts with multiple anchor points. Seven-wire strands are the most common tendons, typically 0.5 inch or 0.6 inch diameter, stressed to 70 to 80 percent of ultimate tensile strength.

Ducts are usually corrugated plastic tubing or steel pipe embedded in the formwork before the pour. After posttensioning, the ducts are grouted with cementitious material to prevent corrosion of the steel and bond the strands more fully to the concrete. Grouting also transfers shear forces along the tendon length, which matters for anchorage zone design and ultimate strength calculations.

Common problems and design details

Incomplete grouting is a persistent issue and can leave voids where corrosion starts. Tendon friction losses occur as strands bend within ducts; designers account for these using wobble and curvature loss formulas. Anchorages must be sized to develop full tendon stress without spalling the concrete. Unbonded tendons, which skip grouting, are faster to install but limit structural capacity and are restricted in some seismic zones. PT also creates concentrated bearing stresses at anchor points that require bearing plates and often local reinforcement.

The name reflects the order of operations: concrete first, then tension applied. Posttensioning is common in bridge decks, parking structures, and building slabs where span-to-depth ratios and load requirements favor the method. It costs more than reinforced concrete due to equipment and labor, but reduces concrete volume, deflection, and cracking, which often justify the expense in long-span or heavily loaded applications.