Pre-tensioning Systems and Devices

This section covers the following topics.
• Introduction
•  Stages of Pre-tensioning
•  Advantages of Pre-tensioning
•  Disadvantages of Pre-tensioning
• Devices

 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 pre-tensioning. 
Section 1.4, “Post-tensioning Systems and Devices”, covers post-tensioning.  In pre-tensioning, the tension is  applied to the tendons before casting of the concrete. The
stages of pre-tensioning are described next.

Stages of Pre-tensioning

In pre-tensioning system, the high-strength steel tendons are pulled between two end
abutments (also called bulkheads) prior to the casting of concrete. The abutments are
fixed at the ends of a prestressing bed.
 
Once the concrete attains the desired strength for prestressing,  the tendons are cut
loose from the abutments.

The prestress is transferred to the concrete from the tendons, due to the bond between
them. During the transfer of prestress, the member undergoes elastic shortening. If the
tendons are located eccentrically, the member is likely to bend and deflect (camber).
The various stages of the pre-tensioning operation are summarised as follows.
1)  Anchoring of tendons against the end abutments
2)  Placing of jacks
3)  Applying tension to the tendons
4)  Casting of concrete
5)  Cutting of the tendons.

During the cutting of the tendons, the prestress is transferred to the concrete with elastic
shortening and camber of the member.

The stages are shown schematically in the following figures.



 Advantages of Pre-tensioning

The relative advantages of pre-tensioning as compared to post-tensioning are as
follows. 
•  Pre-tensioning is suitable for precast members produced in bulk. 
•  In pre-tensioning large anchorage device is not present.

 Disadvantages of Pre-tensioning 

The relative disadvantages are as follows. 
•  A prestressing bed is required for the pre-tensioning operation.
•  There is a waiting period in the prestressing bed, before the concrete attains
sufficient strength. 
•  There should be good bond between concrete and steel over the transmission
length.

Devices

The essential devices for pre-tensioning are as follows. 
•  Prestressing bed 
•  End abutments 
•  Shuttering / mould 
•  Jack 
•  Anchoring device   



An extension of the previous system is the  Hoyer system. This system is generally
used for mass production. The end abutments are kept sufficient distance apart, and
several members are cast in  a single line. The shuttering is provided at the sides and
between the members. This system is also called the  Long Line Method.  The
following figure is a schematic representation of the Hoyer system  

 

The end abutments have to be sufficiently  stiff and have good foundations. This is
usually an expensive proposition, particularly when large prestressing forces are
required.  The necessity of stiff and strong foundation can be bypassed by a simpler
solution which can also be a cheaper option.   It is possible to avoid transmitting the
heavy loads to foundations, by adopting self-equilibrating  systems. This is a common
solution in load-testing. Typically, this is done by means of a ‘tension frame’.  The
following figure shows the basic components  of a tension frame.  The jack and the
specimen tend to push the end members.  But the end members are kept in place by
members under tension such as high strength steel rods.     

 
The frame that is generally adopted in a pre-tensioning system is called a stress bench.
The concrete mould is placed within the frame and the tendons are stretched and
anchored on the booms of the frame.  The following figures show the components of a
stress bench. 



The following figure shows the free body diagram by replacing the jacks with the applied
forces.


The following figure shows the stress bench after casting of the concrete.


Jacks 
The jacks are used to apply tension to the tendons. Hydraulic jacks are commonly used.
These jacks work on oil pressure generated by a pump. The principle behind the design
of jacks is Pascal’s law.  The load applied  by a jack is measured by the pressure
reading from a gauge attached to the oil inflow or by a separate load cell.  The following
figure shows a double acting hydraulic jack with a load cell. 



Anchoring Devices
Anchoring devices are often made on the wedge and friction principle. In pre-tensioned
members, the tendons are to be held in tension during the casting and hardening of
concrete. Here simple and cheap quick-release grips are generally adopted.

Harping Devices
The tendons are frequently bent, except in cases of slabs-on-grade, poles, piles etc.
The tendons are bent (harped)  in between the supports with a shallow sag as shown
below.