PRE STRESSED CONCRETE:MODULAR CONSTRUCTION

TECHNOLOGYPREPARED BY:

VIVEK PAULSEM VI

KMEA COLLEGE OF ARCHITECTURE

History…

1938 Hoyer, E., (Germany) Developed ‘long line’ pre-tensioning method.

1940 Magnel, G., (Belgium) Developed an anchoring system for post-tensioning, using flat wedges.

Used high tensile steel wires, with ultimate strength as high as 1725 MPa and yield stress over 1240 MPa. In 1939, he developed conical wedges for end anchorages for post-tensioning and developed double-acting jacks. He is often referred to as the Father of Pre-stressed concrete.

Eugene Freyssinet (France)

In India, the applications of pre-stressed concrete diversified over the years. The first pre-stressed concrete bridge was built in 1948 under the Assam Rail Link Project. Among bridges, the Pamban Road Bridge at Rameshwaram , Tamilnadu , remains a classic example of the use of pre-stressed concrete girders.

Pamban Road Bridge at Rameshwaram, Tamilnadu

Reinforced concrete:

Concrete is strong in compression weak in tension.

Steel in strong in tension

Reinforced concrete uses concrete to resist compression and to hold bars in position and uses steel to resist

tension.

Tensile strength of concrete is neglected (i.e. zero )

R.C beams allows crack under service load.

Pre-stressed concrete was started to be used in building frames, parking structures, stadiums, railway sleepers, transmission line poles and other types of structures and elements.

Materials for pre-stress concrete member

1. Cement, 2. Concrete, 3. Steel.

Cement: Ordinary portland cement, Portland slag cement, Rapid hardening portland cement, High strength ordinary portland cement.

Concrete: Pre-stress concrete requires high strength concrete,

which has high compressive strength comparatively higher tensile strength than ordinary concrete.

The concrete is a material should be compose of gravels or crushed stones, sand, cement.

In pre-stress concrete minimum grade of concrete M20.

Steel: High tensile steel, tendons, strands. In pre-stress concrete high tensile steel with tensile

strength around 2000MPa.

According to IS: 1343-1980 prestress concrete is design.

Pre-stressing is the application of an initial load on the structure so as to enable the structure to counteract the stresses arising during its service period.

Pre-stressed concrete is a form of reinforced concrete that builds in compressive stresses during construction to oppose those found when in use.”

In other words it is a combination of steel and concrete that takes advantages of the strengths of each material.

Concept of pre-stressing:

The metal bands were tighten under tensile stress which creates compression between the staves allowing them to resist internal liquid pressure.

The concept of pre stressing was invented years ago when metal brands were wound around wooden pieces to form barrels.

Principle of pre-stressing: Pre-stressing is a method in which compression force is

applied to the reinforced concrete section.

The effect of pre stressing is to reduce the tensile stress in the section to the point till the tensile stress is below the cracking stress. Thus the concrete does not crack.

It is then possible to treat concrete as a elastic material.

The concrete can be visualized to have two compressive force

i . Internal pre-stressing force. ii . External forces

These two forces must counteract each other.

Forms of Pre-stressing Steel:wire

tendons

strands

bars

Pre-stressing wire is a single unit made of steel.

Two, three or seven wires are wound to form a pre-stressing

strand.

A group of strands or wires are wound to form a pre-

stressing tendon.

A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire.

Types of pre-stressing:I . Pre-tensioning  In Pre-tension, the tendons are tensioned against

some abutments before the concrete is place. After the concrete hardened, the tension force is released. The tendon tries to shrink back to the initial length but the concrete resists it through the bond between them, thus, compression force is induced in concrete. Pretension is usually done with precast members

II . Post tensioning In 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.

Walnut Lane Memorial Bridge:

The first pre-stressed concrete bridge in North America was the Walnut Lane Memorial Bridge in Philadelphia, Pennsylvania. It was completed and opened to traffic in 1951. 

 

The original Walnut Lane Memorial Bridge in Philadelphia’s Fairmount Park (1950). The main span is 160 ft (49 m) and the superstructure is about 50 ft (15 m) above Lincoln Drive.

Advantages: Factory products are possible. Long span structure are possible so that saving of wt

is significant & thus it become economical. Pre-stressed member are tested before use. Dead load are get counter balanced by eccentric pre-

stressing It has high ability to resist the impact. It has high fatigue resistance. It has high live load carrying capacity. It free from cracks from service loads and enable

entire section to take part in resisting moments. Member are free from the tensile stresses.

Disadvantages compared to RC:

Required skilled builders & experienced engineers. Initial equipment cost is very high. Availability of experienced engineers is less. Required complicated formwork. It requires high strength concrete & steel. Pre-stressed concrete is less fire resistant.

Application:

BridgesSlabs in buildingsWater TankConcrete PileThin Shell StructuresOffshore PlatformNuclear Power PlantRepair and Rehabilitations

Tensioning Devices

The various types devices used for tensioning steel are grouped under four principal categories, viz.

1. Mechanical devices:

The mechanical devices generally used include weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear devices and wire-winding machines. These devices are employed mainly for pre-stressing structural concrete components produced on a mass scale in factory.

2. Hydraulic devices:

These are simplest means for producing large prestressing force, extensively used as tensioning devices.

3. Electrical devices: The wires are electrically heated and anchored before placing concrete in the mould. This method is often referred to as thermo-prestressing and used for tensioning of steel wires and deformed bars.

4. Chemical devices:

Expanding cements are used and the degree of expansion is controlled by varying the curing condition. Since the expansive action of cement 90

while setting is restrained, it induces tensile forces in tendons and compressive stresses in concrete.

1. Pretensioning system:

In the pre-tensioning systems, the tendons are first tensioned between rigid anchor-blocks cast on the ground or in a column or unit –mould types pre-tensioning bed, prior to the casting of concrete in the mould. The tendons comprising individual wires or strands are stretched with constant eccentricity or a variable eccentricity with tendon anchorage at one end and jacks at the other. With the forms in place, the concrete is cast around the stressed tendon. The system is shown in Fig. 1 below.

a) prior to pre-stressing

b) effect of pre-stressing, ignoring self-weight

c) Pre-stress plus self-weight

d) Pre-stress plus self-weight and live load

2. Post-tensioned system:

In post-tensioning the concrete unit are first cast by incorporating ducts or grooves to house the tendons. When the concrete attains sufficient strength, the high-tensile wires are tensioned by means of jack bearing on the end of the face of the member and anchored by wedge or nuts. The forces are transmitted to the concrete by means of end anchorage and, when the cable is curved, through the radial pressure between the cable and the duct. The space between the tendons and the duct is generally grouted after the tensioning operation.

Most of the commercially patented prestressing systems are based on the following principle of anchoring the tendons:

1. Wedge action producing a frictional grip on the wire. 2. Direct bearing from the rivet or bolt heads formed at the end of the wire. 3. Looping the wire around the concrete.

Bonded 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 a 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. Once the concrete has hardened, the tendons are tensioned by hydraulic jacks.

When the tendons have stretched sufficiently, according to the design specifications 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 create problems for the typical perimeter foundation.

All stresses from seasonal expansion and contraction of the underlying soil are taken into the entire tensioned slab, which supports the building without significant flexure. Post-stressing is also used in the construction of various bridges.

The advantages of this system over unbonded post-tensioning are:

Large reduction in traditional reinforcement requirements as tendons cannot de-stress 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

Unbonded 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 a grease (generally lithium based) and covered by a plastic sheathing formed in an extrusion process.

The transfer of tension to the concrete is achieved by the steel cable acting against steel anchors 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: