Prestressed concrete structural members typically by tensioning tendons rebound squeeze to make some concrete sections subject to internal pressure magnitude and distribution in order to partially or completely offset the stress load. The presence of pre-tensile stress being tensioned tendons in. Thus, the prestressing is to improve the fracture and deformation capacity of the structural member. Before using previously applied permanent internal stress, and the tendons of the tensile stress and compressive stress of concrete to form a self-balancing system.

According to the order prestressed tendons and pouring concrete, the project is divided into first-tensioned and post-tensioned in two categories. Press force and working conditions of the tendons, is divided into a bonded or unbonded.
Prestressed concrete strand with a plurality of cold-drawn steel wire stranding machine on helically twisted, and by stress relief tempered steel. The entire root breaking force strand of large, flexible, and easy construction.
Prestressing strand different twist structure can be divided into: 1 * 2 strand / strand 1 * 3/1 * 7 strand like. 1 * 7 strand is composed of six outer wires around a central wire (diameter increased 2.5%) Jiaocheng wide range of uses. Strand 1 * 2/1 * 3 strand is used only for pre-tensioned prestressed concrete members.
Unbonded prestressed concrete structure unbonded prestressed steel hinge line, through the rust, corrosion grease and other plastic sleeve coat wrapped constituted new tendons. Not between it and prestressed concrete adhesion, sliding may be permanently, all delivered by prestressed anchor both ends. Such tendons coating materials require high chemical stability of the surrounding material does not react chemically, such as concrete, steel and wrapping material; good corrosion resistance, good lubrication performance, small friction resistance. Of the cladding material is required to have sufficient toughness, abrasion resistance strong, no erosion of the surrounding material.
"Technical specification of unbonded prestressed concrete structures" JGJ92 definition:
2.1.1 unbonded tendons unbonded tendon using special anti-corrosion grease and plastic coated prestressing strand single package, which can maintain the relative sliding between the concrete and pre-stressed.
2.1.2 unbonded prestressed concrete structure unboded Prestressed concretestructure configure the main force unbonded tendons prestressed concrete structure in one or both directions.
2.1.3 In vitro exemal tendon bundle is arranged outside the member section of concrete structures post-tensioned tendons, only connected at the anchorage area and steering block and at member. Unbonded bundle in vitro by single unbonded tendons made.
2.1.4 In vitro prestressed by the prestressing external prestressing after arranged outside the concrete section of the member-tensioned tendons produced.
Unbonded prestressed concrete structure construction relatively simple and can be put unbonded tendons together with non-prestressed reinforcement according to the design curve in the template laid until after the strength of the concrete pouring and then tensioned and unbonded anchor , with both ends of the anchor, to produce structural prestressing effect. Since all delivered by prestressed anchor, anchor it such a structure should be able to play at least 95% of the actual ultimate strength of prestressing steel and does not exceed the expected deformation. Concrete or mortar must be duly protected anchorage after construction to ensure that its anti-corrosion and fire safety requirements. Unbonded prestressed structures help to reduce storey buildings and reduce the structural weight; and improve functional structure, small floor deflection, almost no fracture; large-span floor to increase the use of space, but also easier to use to change the floor; construction convenient, fast; save steel and concrete; can be used in place of flat ribbed slab floors, etc. is reduced, a better economic and social benefits for office buildings, shopping malls, hotels, garages, warehouses, high-rise buildings, structures, bridges.
Currently unbonded prestressed concrete structures are widely used in floor (board) primary and secondary beams, box girder, single plate, double plate, flat slab, a large span flat beam / Tic-Tac-beam / multi-ribbed beams, pool, long structure, externally prestressed bridges.

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