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per-tensioned concrete and post-tensioned concrete

presentation construction

amier saifuddien

on 27 November 2012

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Transcript of per-tensioned concrete and post-tensioned concrete

NURUL IZZAH BINTI RASIDI AA11037 DR. KHAIRUNISA BINTI MUTHUSAMY LECTURER: CONSTRUCTION ENG. PRE-TENSIONING POST-TENSIONING DEFINITION Pre-tensioning is accomplished by stressing wires or strands, called tendons, to predetermined amount by stretching them between two anchorages prior to placing concrete as shown in fig.1. The concrete is then placed and tendons become bounded to concrete throughout their length. After concrete has hardened, the tendons are released by cutting them at the anchorages. The tendons tend to regain their original length by shortening and in this process transfer through bond a compressive stress to the concrete. APPLICATION HOW IT'S BEEN PRODUCED Used in building materials-can
reduce tensile stress in heavy loads. Replacing tensile stress in a structure with compression stress, using cable or rod tendons. Examples are balcony elements, lintels, floor slabs, beams or foundation piles. An innovative bridge construction
method using pre-stressing is the
stressed ribbon bridge design. Pre-tensioned concrete Building Materials 1) 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. 2) Once the concrete attains the desired strength for prestressing, the tendons are cut
loose from the abutments. 3) 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). 4) During the cutting of the tendons, the prestress is transferred to the concrete with elastic shortening and camber of the member. Advantages of
Pre-tensioning Pre-tensioning is suitable for precast members produced in bulk. In pre-tensioning large anchorage device is not present. Pretension is the easiest controlled of the bonded stressings with the least chance of error in the bonding process. Tension caused by the steel is spread throughout the length of the concrete since it is bonded within the concrete along the length of the member Disadvantages of
Pre-tensioning 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. DEFINITION Post tensioning is a technique for reinforcing concrete. Post-tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete. Post-tensioned concrete means that the concrete is poured and then the tension is applied—but it is still stressed before the loads are applied so it is still prestressed. HOW IT'S BEEN PRODUCED In post-tensioning systems, the ducts for the tendons (or strands) are placed along with the reinforcement before the casting of concrete. The tendons are placed in the ducts after the casting of concrete.

The duct prevents contact between concrete and the tendons during the tensioning operation. Unlike pre-tensioning, the tendons are pulled with the reaction acting against the hardened concrete.

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. APPLICATION Post-tensioning applications in almost all facets of construction Allows extremely long span bridges to be constructed without the use of temporary intermediate supports. System of choice for parking structures since it allows a high degree of flexibility in the column layout, span lengths and ramp configurations.

Allows bridges to be built to very demanding geometry requirements, including complex curves, variable superelevation and significant grade changes. Advantages of
Post-tensioning Post-tensioning is suitable for heavy
cast-in-place members.
The waiting period in the casting bed is less.
The transfer of prestress is independent of transmission length
It reduces or eliminates shrinkage cracking—therefore no joints, or fewer joints, are needed
Cracks that do form are held tightly together
Disadvantages of
Post-Tensioning the requirement of anchorage device and grouting equipment more expensive for the equipments compared than pre-tensioning
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