The basic element of a post-tensioning system is called a tendon.
A post-tensioning tendon is made up of one or more pieces of prestressing steel,
coated with a protective coating, and housed inside of a duct or sheathing. A
tendon will have anchors on each end to transmit the forces into the structure.
Long tendons may also have intermediate anchors along their length. The
prestressing steel can be a high strength steel strand (typical in horizontal
applications) or a high strength steel bar (typical in vertical applications).
Prestressing steel is manufactured to applicable ASTM requirements. Typical
strand sizes are 0.50 in. and 0.60 in. diameters, and bar sizes can typically
range from 1 in. to 2.5 in. To get an idea of the high strength of this type of
steel, a typical steel strand used for post-tensioning will yield at about
243,000 psi. In comparison, a typical piece of rebar will yield at about 60,000
Tension is applied to prestressing steel by using a hydraulic stressing
jack. The jack bears against one of the anchors that is embedded in the
concrete and pulls the steel to a predetermined force. As the tensioning is
occurring, the steel is being elongated, and the concrete or masonry element is
being compressed. When the proper tensioning force is reached, the prestressing
steel is anchored in place. The anchors are designed to provide a permanent
mechanical connection, forever keeping the steel in tension, and the concrete in
duct or sheathing that houses the prestressing steel provides one layer of
corrosion protection. A tendon with a duct that contains multiple pieces of
prestressing steel strand is commonly called a multistrand
tendon, and a tendon in which a single prestressing steel strand is covered
in a plastic sheathing is commonly referred to as a monostrand
tendon. Inside the duct or sheathing, the prestressing steel is covered in a
protective coating that provides another level of corrosion protection. This
coating can be a specially formulated type of grease, or it can be a specially
designed type of grout. When grease is used, the prestressing steel is
permanently free to move relative to the sheathing and the tendon is referred to
as an unbonded tendon. When
grout is used, the steel is permanently bonded to the sheathing and is referred
to as a bonded tendon.