2. Classification of prestressed concreteClassification of prestressed concrete
membersmembers..
• Externally and Internally Prestressed Members
• A member can be prestressed either by external
reaction offered by rigid abutments or by tensioned
tendon. The former method is called external
prestressing and the later is called internal
prestressing.
• In the external prestressing instead of providing a
tendon from which the prestress can be applied,
necessary prestressing force can be applied by
compressing the members by jacking against
abutments.
3. Linear or circularLinear or circular
prestressingprestressing
• The term circular prestressing is applied tb
prestressing circular ike cylindrical tanks, silos and
pipes. In this case the tendons are provided in the
form of rings. tressing is a term applied to
prestressing straight members like beams and slabs.
4. Pre-tensioning and post-Pre-tensioning and post-
tensioningtensioning
• In pre-tensioned members, the tendons are
tensioned even before casting the concrete.
• One end of reinforcement (i e tendon) is secured to
an abutment while the other end of the
reinforcement is pulled by using jack and this end is
then fixed to another abutment. The concrete is
now poured. After the concrete has cured and
hardened the ends of the reinforcement are
released from the abutments. The reinforcement
which tends to resume its original length wi11
compress the surrounding it by bond action.
5. A post tensioned memberA post tensioned member
• A post-tensioned member is one in which the
reinforcement is tensioned after he concrete has
fully hardened The beam is first cast leaving ducts
for placing the tendons. The ducts are made in a
number of ways by leaving corrugated steel tube in
the concrete by providing steel spirals, sheet metal
tubing, rubber hose or any other duct forming unit
in the form work.
• .
6. A post tensioned memberA post tensioned member
• When the concrete has hardened and developed
its strength, the tendon is passed through the duct.
One end an anchor and is fixed to one end of the
member.
• Now the other end of the tendon is pulled by a jack
which is butting against the end of the member. The
jack simultaneously pulls the tendon and
compresses the concrete. After the tendon is
subjected to the desired stress, this end of the
tendon is also properly anchored to the concrete.
To avoid crushing of concrete due to excessive
bearing stress, a distribution plate is provided at
each end
7. Losses of PrestressLosses of Prestress
• Loss of prestress during the tensioning process
• Loss of prestress at the anchoring stage
• Losses occurring subsequently.
