Pre-tensioning involves applying tension to steel tendons before casting concrete. The tendons are anchored and tensioned using jacks, then concrete is cast around them. After the concrete reaches strength, the tendons are cut loose, transferring their prestress to the concrete. Key stages of pre-tensioning include anchoring tendons, applying tension with jacks, casting and curing concrete, and cutting tendons once concrete has gained strength. Special equipment like prestressing beds, molds, jacks, anchoring devices, and harping devices are used in the pre-tensioning process.
2. WHAT IS PRE-TENSIONING ?
IN PRE-TENSIONING, THE TENSION IS APPLIED TO THE
TENDONS BEFORE CASTING OF CONCRETE.
FOR PRE-TENSIONED PRESTRESSED CONCRETE, THE GRADE
OF CONCRETE SHALL NOT BE LESS THAN M40. [AS PER IS
1343:2012]
3. PRE-TENSIONING STAGE'S SUMMARY
ANCHORING OF TENDONS AGAINST THE ABUTMENTS
PLACING OF JACKS
APPLYING TENSION TO THE TENDON
CASTING OF CONCRETE
CURING OF CONCRETE
CUTTING OF TENDONS
4. PRE-TENSIONING STAGE'S DETAILS
IN PRE-TENSIONING SYSTEM, THE HIGH STRENGTH STEEL
TENDONS ARE PULLED BETWEEN TWO END ABUTMENTS,
ALSO CALLED BULKHEADS, PRIOR TO CASTING OF
CONCRETE. THE ABUTMENTS ARE FIXED TO THE ENDS OF A
PRESTRESSING BED.
ONCE THE CASTED CONCRETE ATTAINS THE DESIRED
STRENGTH FOR THE TRANSFER OF PRESTRESS, THE TENDONS
ARE CUT LOOSE FROM THE ABUTMENTS.
5. PRE-TENSIONING STAGE'S DETAILS
THE PRESTRESS IS TRANSFERRED TO THE CONCRETE FROM
THE TENDONS, DUE TO THE BOND AT THEIR INTERFACE.
DURING THE TRANSFER OF PRESTRESS, THE MEMBER
UNDERGOES ELASTIC SHORTENING.
IF THE TENDONS ARE LOCATED ECCENTRICALLY,
THE MEMBER IS LIKELY TO BEND AND DEFLECT UPWARDS
[CAMBER].
7. ADVANTAGES OF PRE-TENSIONING
SUITABLE FOR PRECAST MEMBERS THAT ARE TO BE
PRODUCED IN BULK.
ABSENCE OF LARGE ANCHORAGE DEVICE.
8. DISADVANTAGES OF PRE-TENSIONING
REQUIREMENT OF PRE-STRESSING BED.
WAITING PERIOD IN PRE-STRESSING BED, BEFORE CONCRETE
ATTAINS SUFFICIENT STRENGTH.
REQUIRES GOOD BOND BETWEEN CONCRETE AND STEEL
TENDONS OVER THE TRANSMISSION LENGTH.
9. WHAT IS TRANMISSION LENGTH ?
The prestress is transferred over a certain length from
each end of a member which is called the transmission
length or transfer length (Lt). The stress in the tendon is
zero at the ends of the members. It increases over the
transmission length to the effective prestress (fpe) under
service loads and remains practically constant beyond it
14. JACK
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.
15. 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. The
following figure provides some
examples of anchoring devices.
16. 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.
![WHAT IS PRE-TENSIONING ?
IN PRE-TENSIONING, THE TENSION IS APPLIED TO THE
TENDONS BEFORE CASTING OF CONCRETE.
FOR PRE-TENSIONED PRESTRESSED CONCRETE, THE GRADE
OF CONCRETE SHALL NOT BE LESS THAN M40. [AS PER IS
1343:2012]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)