tendon

In prestressed concrete work, a tendon is the steel element, usually high-strength wire or a bundle of wires, that carries the initial compressive force through the concrete member. Before the concrete cures or after it has gained strength, the tendon is pulled tight and anchored at the ends, squeezing the entire slab or beam. This pre-compression counteracts the tensile stresses that would otherwise crack the concrete when live loads are applied.

Tendons come in two main forms: single strands of stress-relieved wire (typically 0.5 to 0.6 inches diameter) or multiple strands bundled together and often sheathed in plastic or grease. Strand tendons are more common in bridge decks and long-span beams because they can be tensioned to 175 to 210 ksi depending on the grade. Wire tendons are smaller and used in shorter elements. Some tendons are grouted after tensioning to bond them to the concrete; others remain unbonded and rely purely on anchorage at the ends.

The placement strategy distinguishes between pretensioning and post-tensioning. In pretensioning, the tendon is anchored before the concrete is poured; the concrete bonds to the stretched steel as it cures. In post-tensioning, ducts are cast into the concrete first, the concrete cures fully, then the tendon is threaded through the ducts and pulled. Post-tensioning gives greater flexibility for curved or draped tendons and is often used for site-cast bridge elements and parking structures.

Common failure modes include stress corrosion cracking of the steel if tendon specifications are exceeded or water reaches ungrouted strands, loss of tension from anchorage slip or creep in the concrete, and rupture if the tendon is over-stressed during jacking. Proper cover, grouting, and inspection of anchorages are critical to durability.

The term "tendon" borrows from anatomy, reflecting how the steel element acts like a sinew: it pulls on and supports the entire structure. The vocabulary underscores that prestressing is not simple reinforcement but active structural tension management. Tendon design and specification must account for concrete strength at release, jacking equipment limitations, and long-term losses from shrinkage and relaxation.