REPORT: PRESTRESSED CONCRETEMETHOD: POST-TENSIONING

POST-TENSIONINGIn Post-tension, the tendons are tensioned after the concrete has hardened. Commonly, metal or plastic ducts are placed inside the concrete before casting. After the concrete hardened and had enough strength, the tendon was placed inside the duct, stressed, and anchored against concrete. Grout may be injected into the duct later. This can be done either as precast or cast-in-place.

Post-tensioning is a method of reinforcing (strengthening) concrete or other materials with high-strength steel strands called tendons. Post-tensioning allows construction that would otherwise be impossible due to either site constraints or architectural requirements. Requires specialized knowledge and expertise to fabricate assemble and install. After adequate curing of concrete, reinforcing tendons (placed inside the voids of the structure) are tensioned/stretched by jacks on the sides & grouts filled with appropriate mix.Applications Structural members beams, bridge-deck panels, Roof Slabs, Concrete Silos Etc

PROCESS, WHICH MAY BE PERFORMED IN A CASTING YARD, IS BASICALLY A SIX-STEP PROCESS: Concrete forms are assembled with flexible tubes placed in the forms and held at specified locations. Concrete is then placed in the forms and allowed to cure to a prescribed strength. Tendons are placed in the tubes. In some systems, a complete tendon assembly is placed in the forms prior to the placing of concrete. The tendons are tensioned by jacking against an anchorage device or end plate that, in some cases, has been previously embedded in the end of the member. The anchorage device will incorporate some method for gripping the tendon and holding the load. If the tendons are to be bonded, the space in the tubes around the tendons may be grouted using a pumped grout. Some members use unbounded tendons. The end anchorages may be covered with a protective coating.

BENEFITS OF POST-TENSIONING STRUCTURES Greater flexibility of design Post Tensioning allows highly creative architectural approaches and very demanding geometry requirements. Increased deflection control and improved crack control virtually crack-free concrete delivers long term durability Faster construction program early strength stressing promotes a faster cycle time, allowing the structure to progress more rapidly. Lower construction material costs Post Tensioning can decrease the size of a structural member and reduce the quantity of concrete and steel required for the superstructure. Reduced environmental impact less material creates fewer carbon emissions in terms of production and transportation. Reduced construction costs a faster cycle time and decreased amount of material minimize costs.

TYPES OF POST-TENTIONINGThere are two main types of post-tensioning. Bonded (grouted) post-tensioning Unbonded post-tensioningBONDED POST-TENSIONED CONCRETE: Bonded post-tensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing process (in situ). The concrete is cast around plastic, steel or aluminium curved duct, to follow the area where otherwise tension would occur in the concrete element. A set of tendons are fished through the duct and the concrete is poured. When the tendons have stretched sufficiently, according to the design specifications (see Hooke's law), they are wedged in position and maintain tension after the jacks are removed, transferring pressure to the concrete. The duct is then grouted to protect the tendons from corrosion. This method is commonly used to create monolithic slabs for house construction in locations where expansive soils (such as adobe clay) create problems for the typical perimeter foundation All stress from seasonal expansion and contraction of the underlying soil are taken in to the entire tensioned slab, which supports the building without significant flexure. Post-tensioning is also used in the construction of various bridges; both after concrete is cured after support by false work and by the assembly of prefabricated sections, as in the segmental bridge.

THE ADVANTAGES OF THIS SYSTEM OVER UNBONDED POST-TENSIONING ARE: Large reduction in traditional reinforcement requirements as tendons cannot destress in accidents. Tendons can be easily 'weaved' allowing a more efficient design approach. Higher ultimate strength due to bond generated between the strand and concrete. No long term issues with maintaining the integrity of the anchor/dead end.

UNBONDED POST-TENSIONED CONCRETE: Unbounded post-tensioned concrete differs from bonded post- tensioning by providing each individual cable permanent freedom of movement relative to the concrete. To achieve this each individual tendon is coated with grease (generally lithium based) and covered by a plastic sheeting formed in an extrusion process. The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors embedded in the perimeter of the slab. The main disadvantage over bonded post-tensioning is the fact that a cable can destress itself and burst out of the slab if damaged (such as during repair on the slab). THE ADVANTAGES OF THIS SYSTEM OVER BONDED POST-TENSIONING ARE:

The ability to individually adjust cables based on poor field condition (for eg: shifting a group of 4 cables around on opening by placing 2 to either side). The procedure of post-stress grouting is eliminated. The ability to destress the tendons before attempting repair work.REFERENCES:INTERNET:- http://www.astm.org http://www.concrete.org http://www.post-tensioning.org

PRESENTED PAPER:-

REINFORCED CONCRETE BY : DR.Ibrahim Assakkaf