Pre stressed concrete- modular construction technology

1. PRE STRESSED
CONCRETE:
MODULAR
CONSTRUCTION
TECHNOLOGY
PREPARED BY:
VIVEK PAUL
SEM VI
KMEA COLLEGE OF ARCHITECTURE

2. 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)

3.  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

4. 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.

5.  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.

6.  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.

7. 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.

8. 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.

9. 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.

10. 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.

11. 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

12. 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.

13. 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.

14. 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.

15. 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.

16. Application:
Bridges
Slabs in buildings
Water Tank
Concrete Pile
Thin Shell Structures
Offshore Platform
Nuclear Power Plant
Repair and Rehabilitations

17. 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.

18. 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.

19. 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.

20. 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

21. 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.

24. 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.

25. 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.

26. 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: