Post-tensioned (PT) concrete structures consist of high-strength steel tendons placed in concrete that are tensioned after the concrete has gained sufficient compressive strength (i.e. hardened). Tensioning the tendons provides a precompression force in the concrete, which can result in smaller members, thinner slabs, and longer spans, as compared to conventionally reinforced concrete. The tendons are placed in a duct or wrapped in paper or plastic sheathing, so they remain unbonded from the concrete. This construction method is used for buildings, bridges, and parking structures.
PT structures were introduced in the U.S. in the 1950s, initially for the nuclear industry and for the lift-slab construction method. They became more common by the late 1960s for a variety of building types. Mono-strand tendons became predominant in the late 1960s and largely replaced earlier tendon types (e.g. bar and button-headed wire systems) by the late 1970s.
PT tendons are susceptible to corrosion from water and chloride (salt) ingress in similar ways as conventional reinforcing steel. However, unlike conventionally reinforced concrete, the effects of corrosion can result in the tendons losing tension and the concrete losing pre-compression. In some cases, this can happen suddenly and with sufficient force, a tendon can erupt from the slab (see image).
Eruptions most commonly occur through the underside of a slab, but can also erupt from the top side or out of a stressing end anchor (at the end of the tendon). Vulnerable locations for PT tendons are at end anchorages, at intermediate anchorages (typically at construction joints), and at high and low points in the drape of the tendons. Tendons can also fail when cored through or drilled into—failed slab tendons do not always erupt from slabs when they fail. Another indication of failed tendons is cracking in the slab in the negative moment region or near mid-span, or increased deflection of beams or slabs. The tendon in the photo failed due to corrosion from moisture and chloride ingress at a construction joint.
Methods of protecting the tendons have changed over time. Initially, tendons were wrapped in kraft paper with grease to permit the tendons to remain unbonded from the concrete and permit them to be tensioned after the concrete cured. Plastic sheathings were introduced in the late 1960s and have improved over time to become more water-resistant. Current codes require tendons to be fully encapsulated, so they are watertight from end-to-end. The grease used in the tendons was initially provided to reduce friction during initial tensioning, but it now also serves to protect the tendons from moisture and corrosion.
Failure of PT tendons reduces the capacity of the structure, and erupted tendons or suspected loss of post-tensioning force should be evaluated by a structural engineer. Repairs to failed PT tendons commonly consist of splicing in new sections of the tendon by extracting damaged sections and re-threading a new strand into the original sheathing that remains in the structure, installing splicing couplers, and restressing the tendons. Repairs require special care and an experienced contractor.
Authors
Kenneth Itle, AIA, is an architect and associate principal with Wiss, Janney, Elstner Associates (WJE) in Northbrook, Illinois, specializing in historic preservation. He can be reached at kitle@wje.com.
Andrew Lobbestael is a structural engineer and senior associate with Wiss, Janney, Elstner Associates (WJE) in Detroit, Michigan. He can be reached at alobbestael@wje.com.
The opinions expressed in Failures are based on the authors’ experiences and do not necessarily reflect that of The Construction Specifier or CSI.
