1 Lecture 4: Post-Tensioning Systems and Devices: ECE 411 Design of Concrete Structures II Dr. Moonis Zaheer
1 Lecture 4: Post-Tensioning Systems and Devices: ECE 411 Design of Concrete Structures II Dr. Moonis Zaheer
1 Lecture 4: Post-Tensioning Systems and Devices: ECE 411 Design of Concrete Structures II Dr. Moonis Zaheer
1.1 Introduction
Prestressing systems have developed over the years and various companies have patented their products.
Detailed information of the systems is given in the product catalogues and brochures published by
companies. There are general guidelines of prestressing in Section 12 of IS 1343: 1980. The information
given in this section is introductory in nature, with emphasis on the basic concepts of the systems.
The prestressing systems and devices are described for the two types of prestressing, pre-tensioning and
post-tensioning, separately. This section covers post-tensioning. In posttensioning, the tension is applied
to the tendons after hardening of the concrete. The stages of post-tensioning are described next.
Unlike pre-tensioning, the tendons are pulled with the reaction acting against the hardened concrete. If
the ducts are filled with grout, then it is known as bonded post-tensioning. The grout is a neat cement
paste or a sand-cement mortar containing suitable admixture. The grouting operation is discussed later
in the section.
In unbonded post-tensioning, as the name suggests, the ducts are never grouted and the tendon is held
in tension solely by the end anchorages. The following sketch shows a schematic representation of a
grouted post-tensioned member. The profile of the duct depends on the support conditions. For a simply
supported member, the duct has a sagging profile between the ends. For a continuous member, the duct
sags in the span and hogs over the support.
Figure 1: Post-tensioning
Among the following figures, the first photograph shows the placement of ducts in a box girder of a simply
supported bridge. The second photograph shows the end of the box girder after the post-tensioning of
some tendons.
1) Casting of concrete.
The stages are shown schematically in the following figures. After anchoring a tendon at one end, the
tension is applied at the other end by a jack. The tensioning of tendons and pre-compression of concrete
occur simultaneously. A system of self-equilibrating forces develops after the stretching of the tendons.
1.4 Devices
The essential devices for post-tensioning are as follows.
1) Casting bed
2) Mould/Shuttering
3) Ducts
4) Anchoring devices
5) Jacks
6) Couplers (optional)
In post-tensioned members the anchoring devices transfer the prestress to the concrete. The devices are
based on the following principles of anchoring the tendons.
1) Wedge action
2) Direct bearing
1.4.3 Jacks
The working of a jack and measuring the load were discussed in Section 1.3, “Pretensioning Systems and
Devices”. The following figure shows an extruded sketch of the anchoring devices.
1.4.4 Couplers
The couplers are used to connect strands or bars. They are located at the junction of the members, for
example at or near columns in post-tensioned slabs, on piers in posttensioned bridge decks.
The couplers are tested to transmit the full capacity of the strands or bars. A few types of couplers are
shown.
1.4.5 Grouting
Grouting can be defined as the filling of duct, with a material that provides an anticorrosive alkaline
environment to the prestressing steel and also a strong bond between the tendon and the surrounding
grout. The major part of grout comprises of water and cement, with a water-to-cement ratio of about 0.5,
together with some water-reducing admixtures, expansion agent and pozzolans.
The following figure shows a grouting equipment, where the ingredients are mixed and the grout is
pumped.
The following photos show the construction of post-tensioned box girders for a bridge. The first photo
shows the fabricated steel reinforcement with the ducts for the tendons placed inside. The top flange will
be constructed later. The second photo shows the formwork in the pre-casting yard. The formwork for
the inner sides of the webs and the flanges is yet to be placed. In the third photo a girder is being post-
tensioned after adequate curing. The next photo shows a crane on a barge that transports a girder to the
bridge site. The completed bridge can be seen in the last photo.
(a) Reinforcement cage for box girder (b) Formwork for box girder