Classification of prestressed concreteClassification of prestressed concretemembersmembers..• Externally and Internally Prestressed Members• A member can be prestressed either by externalreaction offered by rigid abutments or by tensionedtendon. The former method is called externalprestressing and the later is called internalprestressing.• In the external prestressing instead of providing atendon from which the prestress can be applied,necessary prestressing force can be applied bycompressing the members by jacking againstabutments.
Linear or circularLinear or circularprestressingprestressing• The term circular prestressing is applied tbprestressing circular ike cylindrical tanks, silos andpipes. In this case the tendons are provided in theform of rings. tressing is a term applied toprestressing straight members like beams and slabs.
Pre-tensioning and post-Pre-tensioning and post-tensioningtensioning• In pre-tensioned members, the tendons aretensioned even before casting the concrete.• One end of reinforcement (i e tendon) is secured toan abutment while the other end of thereinforcement is pulled by using jack and this end isthen fixed to another abutment. The concrete isnow poured. After the concrete has cured andhardened the ends of the reinforcement arereleased from the abutments. The reinforcementwhich tends to resume its original length wi11compress the surrounding it by bond action.
A post tensioned memberA post tensioned member• A post-tensioned member is one in which thereinforcement is tensioned after he concrete hasfully hardened The beam is first cast leaving ductsfor placing the tendons. The ducts are made in anumber of ways by leaving corrugated steel tube inthe concrete by providing steel spirals, sheet metaltubing, rubber hose or any other duct forming unitin the form work.• .
A post tensioned memberA post tensioned member• When the concrete has hardened and developedits strength, the tendon is passed through the duct.One end an anchor and is fixed to one end of themember.• Now the other end of the tendon is pulled by a jackwhich is butting against the end of the member. Thejack simultaneously pulls the tendon andcompresses the concrete. After the tendon issubjected to the desired stress, this end of thetendon is also properly anchored to the concrete.To avoid crushing of concrete due to excessivebearing stress, a distribution plate is provided ateach end
Losses of PrestressLosses of Prestress• Loss of prestress during the tensioning process• Loss of prestress at the anchoring stage• Losses occurring subsequently.