Material :Prestressing

Tendon

INTRODUCTION : TENDONS

Tendons are a single or group of prestressing elements and their anchorage assemblies, which impart a compressive force to a structural member. Also included are ducts, grouting attachments and grout. The main prestressing element is usually a high strength steel member made up of a number ofstrands,wires, or bars. 

Prestressing tendons (generally of hightensile steel rods ) are used to provide a clamping load which produces a compressive  that balances the tensile stress that the concrete compression member would otherwise experience due to a bending load.

ADVANTAGES OF PRESTRESS CONCRETE

●Very rapid speed of erection

●Good quality control

●Entire building can be precast-walls, floors,beams,etc.

●Rapid construction on site

●High quality because of the controlled conditions in the factory

●Prestressing is easily done which can reduce the size and number of the structural members.

DISADVANTAGES OF PRESTRESS CONCRETE

● Very heavy members

● Camber in beams and slabs

● Very small margin for error

● Connections may be difficult

● Somewhat limited building design flexibility

● Economics of scale demand regularly shaped buildings.

● Need for repetition of forms will affect building design.

● Joints between panels are often expensive and complicated.

● Skilled workmanship is required in the application of the panel on site.

● Cranes are required to lift panels.

TYPES OF TENDON

● Cable● Strand● Wires● Bars ( high tensile and round bars )

Form Of Pre-Stressing Steel

Introduction : CableConstruction of post-tensioned slabs on

grade is very similar to using reinforcing steel, except for the tensioning step.

Cables are arranged as indicated by the engineer and chaired to run through the center of the slab.

For residential construction, tendons at 48 inches on center are common. Commercial foundations will have much more steel. Tendons can be easily routed around obstructions.

Characteristic and Used for Tendon cable A residential post-tensioned concrete slab will typically

be 8 inches thick and use 3000 psi concrete. Once the concrete has gained strength to 2000 psi, typically within the 3 to 10 days recommended by PTI, the tendons are stressed.

Tendons today are seven high-strength steel wires wound together and placed inside a plastic duct. At each end a PT anchor is located and these are located in pockets embedded into the slab edge. When the strands are stressed, the wires will stretch—about 4 inches for a 50 foot strand—to apply 33,000 pounds of load. Stressing should only be done by qualified workers. After stressing, the tendon is cut off and the pocket in which the anchors are located is filled with grout to protect them from corrosion.

Continuous… Larger structural concrete members may also be

post-tensioned, especially in bridges and floors and beams in parking structures.

The process is very similar to that used for slabs, except on a bigger scale. One interesting difference is that the tendons will often be "draped" so that they are low at the midpoint of a beam and high at the supports—this places the steel at the point of highest tension where it can keep the concrete held together tightly. With structural members the duct is often grouted full following stressing to bond the strand to the concrete along its entire length—these are called bonded tendons. Unbonded tendons, used in residential slabs, remain free to move within the duct and are protected from corrosion by grease.

METHOD INSTALLATIO

N CABLES

Pre-stressing wire is a single unit made of steel, mainly used in post-tensioning systems for pre-

stressing concrete, is cold drawn and stress relieved with a yield stress of about 1300 MPa. Wire diameters most commonly used in New

Zealand are 5mm, 7mm, and 8mm. 

TENDONS WIRES

• Low relaxation ensures that no noticeable loss of tension will occur in time, therefore a long lasting compressive force on concrete.

• Deviated tension is current in post-tensioned structures, stay-cables and even some prefabrication methods.

• Stress corrosion and hydrogen embrittlement are known to be a threat for high-tensile structural steels, especially in environments containing chlorides. For several decades.

SPECIFICATIONS AFTER STRESS RELIEVING

Seperate wire

Spun together helically

Form strands

Introduction : StrandsStrand, which is used in both pre and post-tensioning is made by winding seven cold drawn wires together on a stranding machine. Six wires are wound in a helix around a centre wire which remains straight. Strands of 19 or 37 wires are formed by adding subsequent layers of wire. With wire tendons and strands, it may be desirable to form a cable to cope with the stressing requirements of large post-tensioning applications. Cables are formed by arranging wires or strands in bundles with the wires or strands parallel to each other.

Different types of strands

• TWO WIRES ARE SPUN TOGETHER TO FORM THE STRAND

Two wire strands

• Three wires are spun together to form the strand

Three wire strands

• Six wires are spun around a central wire.

Seven wire strands

Technical data

Characteristic

of strands

which is used in both pre and post-tensioning is made by

winding seven cold drawn wires together on a stranding machine.

Six wires are wound in a helix around a centre wire

which remains straight.

The prestressing steel can be a high strength steel strand (typical

in horizontal applications) or a high strength steel bar (typical in

vertical applications). 

When used

strands type

tendon??

To adequately protect against losses of prestress and to use the materials economically requires that the initial stresses at prestressing be at the allowable upper limits of the material. This imposes high stresses, which the member is unlikely to experience again during its working life.

Because the construction system is designed to utilise the optimum stress capability of both the concrete and steel, it is necessary to ensure that these materials meet the design requirements

•

This requires control and responsibility from everyone involved in prestressed concrete work - from the designer right through to the workmen on the site.

A tendon can be made up of a single steel bar. Post-tensioning bars are high strength steel bars. The diameter of a bar is much larger than that of a wires. These bar type tendons are used in certain types of post-tensioning systems.

When employed with threaded anchorages has the advantages of eliminating the possibility of pull-in at the anchorages and of reducing anchorage costs.

Introduction : Bar

The  normally available sizes are from 10mm to 44mm (5/8 to 1-3/4in) diameter with 10, 12, 16, 20, 22, 25, 28 and 32 mm dias being more common. The bars are usually threaded with very coarse thread.

The alloy steel from which these bars are made has a yield stress in the order of 620 MPa.

This gives bar tendons a lower strength to weight ratio than either wires or strands.

Bar : Characteristic

50Mpa Deformed Bar

Type of Tendons

Bar

Specifications● For  members, the high-tensile steel, used

generally, consists of wires, bars or strands. The high tensile strength of steel is generally achieved by marginally increasing the carbon content in steel in comparison to mild steel. High-tensile steel usually contains 0.6 to 0.85% carbon, 0.7 to 1 % manganese, 0.05% of sulphur and phosphorus.

● The high carbon steel ingots are hot rolled into rods and cold drawn through a process of dies to reduce the diameter and increase the tensile strength. The durability of wires gets improved due to the cold-drawing operation. The cold-drawn wires are then tempered to improve their properties.

● Tempering or ageing or stress relieving by heat treatment of wires at 150-420°C improves the tensile strength. These cold-drawn wires are generally available in nominal sizes of 2.5, 3, 4, 5, 7 and 8 mm diameter.

Codes of Properties : Tendons

� Plain hard-drawn steel wire conforming to IS: 785 (Part 1)-1966 and IS: 1785(Part 2)-1967,

� Cold-drawn Steel wire, BS : 5896

� High tensile steel bar conforming to IS: 2090- 1962

� Uncoated stress relieved strands conforming to IS: 6006-1970

The 3D Model Of Pre-Stressed Concrete

Pre-Stressed Production using wire in India

Multi-Strand System