posttensioning

Posttensioning is a method of prestressing concrete where tendons are placed within channels (ducts) inside the concrete and later tensioned to introduce compression, enhancing flexural capacity and crack control. It allows longer spans, thinner slabs, or lighter structures compared with conventional reinforced concrete.

It was developed in the 1920s by French engineer Eugène Freyssinet, forming modern prestressed concrete. In posttensioning, prestress is applied by stressing tendons after concrete cures, unlike pretensioning. Two main systems exist: bonded posttensioning, in which ducts are grouted to bond the tendons, and unbonded posttensioning, where tendons are protected from the concrete with sheaths or grease and remain free to move.

Components include high-strength steel tendons (wires, strands, or bars), ducts or sleeves, end anchors to transfer tension to the concrete, and grout for bonded systems or protective coatings/grease for unbonded systems. Duct placement, corrosion protection, and anchor detailing are key design elements.

Process: ducts are installed in the formwork, concrete is poured and cured, tendons are threaded or placed, after curing jacks apply tension, and anchors secure the tension. In bonded systems grout fills ducts to bond tendons to concrete; in unbonded systems tendons remain in a protective sheath and are anchored at ends.

Applications include slabs for parking structures and floors, beams, transfer girders, bridges, and tanks. Benefits include increased stiffness and load capacity, improved crack control, reduced thickness and deflections, and potential cost savings in long spans, though higher material and installation costs and the need for skilled workmanship apply.

Limitations include sensitivity to installation quality, grout integrity in bonded systems, corrosion risk if grout or coatings fail, creep and friction losses, and inspection needs. Proper design, detailing, and maintenance are essential.