In post-tensioned structures ducts are formed in the concrete through which the strands are passed. During the development of post-tensioning systems, a number of different methods have been used to form ducts. For straight tendons this has sometimes meant using formers that are subsequently removed leaving unlined ducts. Lined ducts are usually formed with welded or spirally wound steel tube; the latter can be bent on site to match the required tendon profile.

In addition to steel, non-metallic dust liners are available, made of high density polyethylene (HDPE), now known as PE80, or polypropylene. They have a number of advantages including being corrosion-resistant, provide better sealing against ingress of contaminants and can be pressure-tested during construction to demonstrate integrity.