1. 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
2. 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
3. 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
4. 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
5. These disadvantages are overcome in prestressed
concrete.
Advantages of prestressed concrete
i. Cross section more efficiently utilised
ii. Members posses improved resistance to shearing
forces
iii. Flexural members are stiffer under working loads
iv. More economical
v. Free from cracks during working loads and have
more durability
vi. Absorbs energy efficiently due to impact loads
vii. Beams from 10 m to 30 m span are more
economical
7. 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.
8. Loss of prestressing
Prestressing 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 concrete
b. Creep of concrete
c. Elastic shortening
d. Creep in steel
10. TYPES OF PRESTRESSING
i. Linear
ii. 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.
11. Methods of Prestressing
i. Pre tensioning
ii. Post tensioning
In pre-tensioning, the tendons are tensioned even
before casting the concrete
One end of tendon is secured to abutment. The
other end is pulled with jacks.
12.
13. In post tensioning, the beam is cast first leaving
ducts for placing the tendons
Depending upon forces, there may be number of
ducts
14. In post tensioning, not a solid beam but a series of
blocks
Cables are inserted and will be prestressed
Post Tensioning in Blocks
15. End Block
Whatever 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
16. 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
17. 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 cables
d. 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
23. 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.
24. 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
25. • Application of prestressing
i. Floor slabs, columns, beams
ii. Bridges, water tanks
iii. Piles, wall panels, frames, window mullions, fence
posts
iv. Railway sleepers
26. P.S.C. BRIDGES
PSC girders with part of deck as flange are cast
Prestressing in stages – first stage to take dead
loads and erection loads
27. Girders are launched on piers and aligned
Launching will be by cranes or with the help of
launching girder
The gap of alignment will be filled with cement
concrete to get continuous road way and wearing coat
laid
Then second stage prestressing to take live loads
28. 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 so
If the bridge is of length 30 m, there will be 30
pieces
All these pieces will be lifted and put at proper
place on the shuttering provided
PSC tendons will be inserted and prestressed
The wearing coat will be laid
29. Bending moment – zero at supports and max at centre
where
w l2
Max B.M. =
8
w l2
Max B.M. =
8
M
Bending stress (σ) = ±
Z
M
Bending stress (σ) = ±
Z
1
Z = bd2
6
1
Z = bd2
6
30. 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.e
Stress = ± i.e. –
A Z A Z
P P.e P P.e
Stress = ± i.e. –
A Z A Z
P P.e P P.e
Stress = ± 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
31. 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.
32. 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
33. 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
35. By 1940 – 19 metro rail system
1984 – Swelled to 66
2013 – About 170 metro systems
India like other developing countries lagging
behind
36. First metro rail – Kolkata – commenced in 1984
– Route length 97.5 Kms.
Available land for road transport is insufficient
hence underground route
37. 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
38. 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
39. 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
40. MRTS – consumes less energy
Echo friendly – Less sound
Averts number of accidents
Efficient in terms of space, occupancy, provides
comfort – ultra modern coaches
41. 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
