pre stressed concrete
TRANSCRIPT
Ramireddy subbarami reddyEngineering college
KadanuthalaDEPARTMENT OF CIVIL
ENGINEERINGPRESENTED BYKALYAN KUMAR Y133R1A0118
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
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.
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.
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.
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
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.
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.
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.
TYPES OF PRESTRESSED CONCRETE
• There are two types of prestressed concrete.
• They are Pre- tensioned concrete Post tensioned concrete
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.
PRE-TENSIONED CONCRETE
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
POST-TENSIONED 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 aluminum curved duct, to follow the area where otherwise tension would occur in the concrete element.
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.
TEST ON PRESTRESSED CONCRETE
POST- TENSIONED SPLICECAST-IN-PLACE SPLICEFLEXURAL TEST
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.
POST- TENSIONED SPLICE
POST- TENSIONED SPLICE
POST- TENSIONED SPLICE
POST- TENSIONED SPLICE
RESULTMEMBER 6”*12” cylinder compressive strengths
1 day 7 days 28 days
Beam slab
3790 psi4180 psi
4990 psi5060 psi
5380 psi5440 psi
CAST-IN-PLACE SPLICEThe 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)
CAST-IN-PLACE SPLICE
CAST-IN-PLACE SPLICE
RESULT
MEMBER 6" x 12" Cylinder compressive strength
1 day 7 days 28 days
BeamSpliceSlab
4630 psi5060 psi4250 psi
6160 psi6190 psi5100 psi
7030 psi6960 psi5690 psi
FLEXURAL TEST
POST TENSION CABLES
ADVANTAGESTake full advantages of high strength concrete and high
strength steelNeed less materialsSmaller and lighter structureNo cracksUse the entire section to resist the loadBetter corrosion resistanceVery effective for deflection controlBetter shear resistance
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.
APPLICATION• Bridges• Slabs in buildings• Water tank• Concrete pile• Thin shell structures• Offshore platform• Nuclear power plant• Repair and rehabilitations
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.
HARRODS CREEK ARCH BRIDGE HARRODS CREEK ARCH BRIDGE WIDENINGWIDENING
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.
