PRESTRESSED CONCRETE

Prestressed concrete is basically concrete in which the stresses of suitable magnitude and distribution are introduced to counteract the stresses due to dead and live loads to the desired degree.

Thus, the concrete in the entire section will be compressive and no tensile stresses are permitted

Prestressing in vogue centuries back

i. The earliest example of wooden barrel construction by force fitting of metal band

ii. Shrink fitting of metal tyres of wooden wheels

iii. The forks of cycle

Cement concrete is a homogeneous material and strong in compression and weak in tension

A concrete of M20 will take an ultimate compressive stress of 20 N/mm2 but its tensile strength is hardly 3.1 N/mm2 (0.7 fck)

Steel is used to reinforce the concrete in tension zone. Reinforced concrete is a composite section

In a composite section, strains of different materials are same. The sum of loads taken by individual materials equal to total load

Reinforced concrete has the following disadvantages

i. Concrete takes tension along with steel though steel is designed to take entire tension. In this process, minute cracks develop in the concrete due to concrete unable to take strains along with steel

ii. In long beams, to limit the diagonal tension within limits, deeper sections are required

iii. Reinforced concrete member develops cracks even due to shrinkage

These disadvantages are overcome in prestressed concrete.Advantages of prestressed concrete

i. Cross section more efficiently utilisedii. Members posses improved resistance to shearing

forcesiii. Flexural members are stiffer under working loadsiv. More economicalv. Free from cracks during working loads and have

more durabilityvi. Absorbs energy efficiently due to impact loadsvii. Beams from 10 m to 30 m span are more

economical

Fundamentals of Prestressing

Materials used in prestressing

• Concrete – High strength concrete more than M35, with low shrinkage

• Steel – 5 mm to 7 mm diameter. High tensile steel with proof stress 1000 – 2000 N/mm2 (against 540 N/mm2 for tor steel)

• Steel Tendons – Steel made as single wire or group of wires. Group wires is called tendons.

Loss of prestressingPrestressing force is lost upto 10%

i. Loss due to friction : Loss due to friction between materials

ii. Loss due to curvature : Tendons are not straight but in a curve

iii. Loss due to slip of Anchorages : Wires are stretched and locked – in that process

iv. Loss after prestressing : a. Due to shrinkage of concreteb. Creep of concretec. Elastic shorteningd. Creep in steel

Prestressed concrete is used in

i. Beams

ii. Tanks

iii. Sleepers

TYPES OF PRESTRESSING

i. Linearii. Circular

In Linear prestressing the prestressing wires are in lines. This is used in beams and slabs

In Circular prestressing cables take circular path. This is used in tanks, pipes.

Methods of Prestressingi. Pre tensioningii. Post tensioningIn pre-tensioning, the tendons are tensioned even before casting the concreteOne end of tendon is secured to abutment. The other end is pulled with jacks.

In post tensioning, the beam is cast first leaving ducts for placing the tendonsDepending upon forces, there may be number of ducts

In post tensioning, not a solid beam but a series of blocksCables are inserted and will be prestressed

Post Tensioning in Blocks

End BlockWhatever may be the shape of beam, the end block is a rectangular section. The entire prestressing will be transferred by the end block

Anchorages will be embedded in end blocks

Systems of prestressing

It is the process of tensioning of tendons. Secures firmly to concrete till the lift of member. Many systems are in practice.

i. Freyssinet system

ii. Magnel Blaton system

iii.Gifford Udall system

i. The Freyssinet system : a. High tensile wires 12 No.b. Arranged to form a group into cables with a

spiral spring inside to give clearance between the wires

c. They will be inserted in a metal sheeting cablesd. The cable will be free to move initially and after

prestressing it will be grouted with cement mortar

e. The anchorages consists of a cylinder with central conical hole

Placement of ducts along with reinforcement in a box girder bridge

Freyssinet System

Fluted male cone

Female anchorage with steel spirals

Duct former

Steel wedge

Sandwich Plate

Distribution Plate

Magnel Blaton System

Magnel Blaton System – where 8 wires can be prestressed individually

Stages of prestressing

i. Ducts for tendons (strands) are placed along with reinforcements before casting of concrete

ii. Then it will be concreted

iii. Care should be taken that concrete (cement slurry) should not go into cable

iv. After concreting the PSC wires are moved to and fro so that the concrete if it goes inside cable should not set

v. After concrete set, according to design tendons are stressed in 7 days, 28 days and before live load is allowed

vi. After prestressing and locking in each tendon is grouted with cement slurry so that the concrete and tendon are well bonded.

FIRE RESISTANCE

• Concrete is non combustible

• Failure of concrete members usually due to progressive loss of strength of reinforcing steel or tendons

• Concrete has greater fire resistance than steel

• Reduction in strength of high tensile steel is less at high temperatures compared to ordinary steel

• Greater cover to tendons, PSC will be more fire resistant

• Application of prestressing

i. Floor slabs, columns, beams

ii. Bridges, water tanks

iii. Piles, wall panels, frames, window mullions, fence posts

iv. Railway sleepers

P.S.C. BRIDGES

PSC girders with part of deck as flange are castPrestressing in stages – first stage to take dead loads and erection loads

Girders are launched on piers and aligned

Launching will be by cranes or with the help of launching girderThe gap of alignment will be filled with cement concrete to get continuous road way and wearing coat laidThen second stage prestressing to take live loads

Instead of a single girder, the entire cross section of bridge will be sliced of one meter width and cast so that its weight is hardly one ton or soIf the bridge is of length 30 m, there will be 30 piecesAll these pieces will be lifted and put at proper place on the shuttering providedPSC tendons will be inserted and prestressedThe wearing coat will be laid

Bending moment – zero at supports and max at centre

where

w l2

Max B.M. =8

w l2

Max B.M. =8

MBending stress (σ) = ±

Z

MBending stress (σ) = ±

Z

1Z = bd2

6

1Z = bd2

6

It gives a direct load P and a moment P.e at top and at bottom

Total stress at top = Max. permissible stress in concrete

At bottom = Near to 2000

P P.e P P.eStress = ± i.e. –

A Z A Z

P P.e P P.eStress = ± i.e. –

A Z A Z

P P.e P P.eStress = ± i.e. –

A Z A Z

P P.e+

A Z

P P.e+

A Z

P P.e+

A Z

P P.e+

A Z

P P.e+

A Z

P P.e+

A Z

P P.e+

A Z

P P.e+

A Z

P P.e– σ + + = 0

A Z

P P.e– σ + + = 0

A Z

P P.e– σ + + = 0

A Z

P P.e– σ + + = 0

A Z

P P.e+

A Z

P P.e+

A Z

M P P.e= – + +

Z A Z

M P P.e= – + +

Z A Z

M P P.e= + + –

Z A Z

M P P.e= + + –

Z A Z

METRO RAIL SYSTEM

Mass Rapid Transit System (MRTS)

High capacity and frequency

Grade separation from other traffic

Located in underground tunnels (or) elevated viaducts (or) few grade separated tracks.

Growing cities, growing population and growing traffic – require shift from private mode to public

MRTS is successful with 70% of total transport is public one

London underground railway system – 1863 is first metro system

Technology quickly spread to Europe and USA

Recently the largest growth is in Asia with driverless system

New York city subway in 1904 largest track of 1335 kms of RTS

Shanghai metro – largest length of passenger lines

Tokyo subway, Seoul metropolitan subway and Moscow metro – busiest metro

London Underground Railway systemNew York City Subway

Tokyo Subway

By 1940 – 19 metro rail system 1984 – Swelled to 66 2013 – About 170 metro systems

India like other developing countries lagging behind

First metro rail – Kolkata – commenced in 1984 – Route length 97.5 Kms.

Available land for road transport is insufficient hence underground route

Delhi first phase – 65.11 Km in 2002 Second phase – 125 Km recently completed Mostly elevated i.e. viaduct

First one recognised by UN – Saves power – Regenerative waste by using green house gas

Chennai MRTS – Elevated one – completed in 2007

System normal electrical multiple units and not modern (without automatic doors)

Poor maintenance, lack of security, so not popular

Hyderabad metro – elevated one – 71 Kms in the first phase

First two track elevated transit system

Commenced on 2011 – expected to be completed by 2015

Bengaluru – MRT – Elevated and underground with double line corridors

Total length 33 Kms – First phase expected to be completed by 2013

MRTS – consumes less energy

Echo friendly – Less sound

Averts number of accidents

Efficient in terms of space, occupancy, provides comfort – ultra modern coaches

Modern system – Automatic ticketing

Advanced signalling system – Automatic train protection system

Integrated security

Maximum speed 80 kms/hr. Average 34 km/hr

Peak work hour capacity – more than 3 lakhs passengers