2. CONTENTS
INTRODUCTION
DEFINITION OF PRESTRESS
PRINCIPLE OF PRE-STRESSING
METHOD OF PRESTRESSING
GIRDER DESIGN
WORKING OF PRESTRESSED CONCRETE
DIFFERENCE B/W ORDINARY AND PRESTRESSED CONCRETE
TYPES OF PRESTRESSED CONCRETE
TEST ON PRESTRESSED CONCRETE
ADVANTAGES AND DISADVANTAGES OF PRESTRESSED CONCRETE
APPLICATION
CASE STUDY
CONCLUSION
3. INTRODUCTION
Prestressed concrete is a method for overcoming concrete's natural
weakness in tension.
In 1904, Freyssinet attempted to introduce permanent acting forces .
To resist elastic forces under loads and was named “Pre Stressing”.
It can be used to produce beams , floors , bridges with a longer span
than is practical with ordinary reinforced concrete.
4. DEFINITION OF PRESTRESS
• Prestress is defined as a method of applying pre-
compression to control the stresses resulting due to
external loads below the neutral axis of the beam
tension developed due to external load which is more
than the permissible limits of the plain concrete.
5. PRINCIPLE OF PRE-STRESSING
• Pre-stressing is a method in which compression force is
applied to the reinforced concrete section.
• Pre-stressing tendons (generally of high tensile steel cable or
rods) are used to provide a clamping load which produces a
compressive stress that balances the tensile stress that the
concrete compression member would otherwise experience
due to a bending load.
6. METHODs OF PRESTRESSING
This classification is based on the method by which the prestressing
force is generated.
Hydraulic Prestressing
Mechanical Prestressing
Electrical Prestressing
Chemical Prestressing
7. GIRDER DESIGN
• For transportation and handling purposes of the pier segments of both tx70 and
Texas u54 girder bridges.
• Diameter are provided in the bottom flange of the pier segments.
• Various corrosion protection systems are available for these 106 thread bars, none
of which bond with the structure.
• Once the pier segment is erected on site, it behaves as a cantilever.
• The preliminary designs for tx70 and Texas u54 girder bridges assumed shored
construction.
• From the preliminary designs, it was noted that the span lengths of 280 ft and 240 ft
for the continuous prestressed concrete bridges using the standard tx70 and texas
u54 girders, respectively are achieved using shoring towers (shored construction)
and by making the girder sections work up to their limits.
8. WORKING OF PRESTRESSED
CONCRETE Pre-stressed concrete refers to a procedure whereby
tensile rods are put in place first and tightened,
followed by concrete pouring.
Compression can be applied after pouring concrete
using bonds. They are tightened once the concrete is
dry
Main disadvantage is that a cable can burst out of the
slab, if the anchoring system fails.
9. DIFFERENCE B/W ORDINARY AND
PRESTRESSED CONCRETE
• Even without a load, the
ordinary concrete beam
must carry its own weight.
• An upward force is created
which in effect relieves the
beam of having to carry its
own weight.
10. TYPES OF PRESTRESSED
CONCRETE
• There are two types of prestressed concrete.
• They are
Pre- tensioned concrete
Post tensioned concrete
11. PRE-TENSIONED CONCRETE Pre-tensioned concrete is cast around
already tensioned tendons.
This method produces a good bond
between the tendon and concrete, which
both protects the tendon from corrosion and
allows for direct transfer of tension.
The cured concrete adheres and bonds to
the bars and when the tension is released it
is transferred to the concrete as
compression by static friction.
13. POST TENSIONED CONCRETE
Post tensioning is a technique for reinforcing concrete.
Post-tensioning tendons, which are prestressing steel cables inside plastic ducts
or sleeves, are positioned in the forms before the concrete is placed.
Afterwards, once the concrete has gained strength but before the service loads
are applied, the cables are pulled tight, or tensioned, and anchored against the
outer edges of the concrete.
They are classified into two types,
Bonded post tensioned concrete
Unbonded post tensioned concrete
15. 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 aluminum
curved duct, to follow the area where otherwise tension would
occur in the concrete element.
16. UNBONDED POST-TENSIONED
CONCRETE
• Unbounded 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.
18. POST- TENSIONED SPLICE
The specimen was loaded through a hydraulic jack at the
centerline of the span. The jack reacted against a steel
test frame bolted to the foundation. Elastomeric bearing
pads were used at the end bearings and between the jack
and the slab. The jack had a capacity of 400 kips, while
the test frame was rated at about 200 kips. The jack had
been pre-calibrated for correspondence between gage
pressure and applied load.
23. RESULT
MEMBER 6”*12” cylinder compressive strengths
1 day 7 days 28 days
Beam
slab
3790 psi
4180 psi
4990 psi
5060 psi
5380 psi
5440 psi
24. CAST-IN-PLACE SPLICE
The splice contemplated in the present application
consists of cast-in place concrete section of perhaps 4
feet in length, reinforced with mild steel bars
projecting from each precast element. Supplemental
stirrups are also used in the splice. (Possible
variations would include post-tensioning the cast-in-
place portion, but this will not be considered herein)
31. ADVANTAGES
Take full advantages of high strength concrete and high
strength steel
Need less materials
Smaller and lighter structure
No cracks
Use the entire section to resist the load
Better corrosion resistance
Very effective for deflection control
Better shear resistance
32. DISADVANTAGES OF
PRESTRESSED CONCRETE
• The availability of experienced builders is scanty.
• Initial equipment cost is very high.
• Availability of experienced engineers is scanty.
• Prestressed sections are brittle
• Prestressed concrete sections are less fire
resistant.
33. APPLICATION
• Bridges
• Slabs in buildings
• Water tank
• Concrete pile
• Thin shell structures
• Offshore platform
• Nuclear power plant
• Repair and rehabilitations
34. CASE STUDY
HARRODS CREEK ARCH BRIDGE WIDENING
• Completion Date: August, 2010
• Cost: $34,00,000
• Designer: Stantec Consulting
• Client/Owner: Louisville Metro Public Works
• Contracter MAC Construction & Excavating, New Albany, Indiana.
• Located on River Road over Harrods Creek near Prospect, Kentucky, 10 miles
northeast of Louisville
• River Road is a Kentucky Scenic Byway and the bridge is located within the
Harrods Creek Historic District.
36. CONCLUSION
• Thus, pre-stressed concrete increases the quality, strength,
span of the structure.
• Since it is cost effective, it is used widely on recent days.
