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WELCOME
CONSTRUCTION SUPERVISION CONSULTANT, PMBPBANGLADESH ARMY
PRESTRESSING OF BRIDGE GIRDER
INTRODUCTION
ADVANTAGE OF PRESTRESSING
The prestressing of concrete has several advantages as compared to traditionalreinforced concrete (RC) without prestressing. A fully prestressed concrete member isusually subjected to compression during service life. The advantages of prestressconcrete are as follows:
Section remains uncracked under service loads.
Reduction of steel corrosion and thereby increase in durability.
Full section is utilised.
● Higher moment of inertia.
● Less deformations (improved serviceability).
Increase in shear, bending and torsional capacities.
ADVANTAGE OF PRESTRESSING
Improved performance (resilience) under dynamic and fatigue loading.
Sections can behave elastically.
Larger spans possible with prestressing (bridges, buildings).
For the same span, less depth compared to RC member.
● Reduction in self weight.
● More aesthetic appeal due to slender sections.
● More economical sections.
ADVANTAGE OF PRESTRESSING
Suitabality for precast construction
● Rapid construction
● Better quality control
● Reduced maintenance
● Suitable for repetitive construction
● Multiple use of formwork
● Availability of standard shapes
LIMITATIONS OF PRESTRESSING
● Prestressing needs skilled manpower
● The use of high strength materials is costly
● There is additional cost in auxiliary equipments
● There is need for quality control and inspection
TERMINOLOGY
Post-TensioningThe application of a compressive force to the concrete by stressing tendons or barsafter the concrete has been cast and cured. The force in the stressed tendons orbars in transferred to the concrete by means of anchorages.
WiresPrestressing wire is a single unit made of steel.
StrandsAn assembly of several high strength steel wires wound together. Stands usuallyhave six outer wires helically wound around a single straight wire of a similardiameter.
TendonA group of strands or wires are wound to form a prestressing tendon.
TERMINOLOGY
AnchorageAn assembly of various hardware components which secure a tendon at its endsafter it has been stressed and imparts the tendon force into the concrete.
Anchor Plate (Tube Unit)The part of the anchorage which bears directly on the concrete and through whichthe tendon force is transmitted.
WedgesA small conically shaped steel component placed around a strand to grip andsecure it by wedge action in a tapered hole through a wedge plate.
Wedge PlateA circular steel component of the anchorage containing a number of tapered holesthrough which the strands pass and are secured by conical wedges.
GIRDER SECTION INCLUDING REBAR PLACEMENT
Reinforcement placing (Side view) Reinforcement placing (Top view)
GIRDER SECTION INCLUDING REBAR PLACEMENT
End Section Mid Section
SHEATHING DUCT
Raw Material for Sheathing Duct
SHEATHING DUCT
Preparation of Sheathing Duct
SHEATHING DUCT
Prepared Sheathing Duct
PRESTRESSING STRAND
Coil Strand (7 wire)Wire
STRESSING CABLE & THEIR PLACEMENT WITHIN RABAR
Cable Profile checking of cable ductCable within the rebar inside sheating duct
ANCHORAGE MATERIALS
Tube Unit (Anchor Plate) Wedge Plate Wedges
CONCRETE MIX DESIGN
Concrete class : C50
28 days cube strength : 50 mpa
Materials
Cement : Bashundhara (Ordinary Portland Cement) OPC
Fine Aggregate : Sylhet Sand, Bangladesh
Coarse Aggregate : Meghaloy, India
Admixture : Master Polyheed 8632
Water Cement Ratio : 0.3
Slump : 150mm ~ 200mm
CONCRETE MIX DESIGN
Batch Weight per cum of Concrete on SSD Condition
Cement : 470 kg
Water : 141 kg
Fine Aggregate : 658.6 kg
Coarse Aggregate : 1152 kg
Admixture : 5.9 kg
Mix Proportion : Cement : Sand : Stone
1 : 1.40 : 2.45
CONCRETING WORK OF PC GIRDER
Girder ready for concreting Concrete Transit Mixer Car Slump Test
CONCRETING WORK OF PC GIRDER
Concreting is going on Form Vibrator
CURING
PRESTRESSING JACK & ITS FUNCTION
Jack (Front Side) Jack (Back Side)
PRESTRESSING JACK & ITS FUNCTION
Jack Setting Prestressing Pump Machine
GIRDER END ARRANGEMENT FOR PRESTRESSING
Tube Unit Wedge Plate
GIRDER END ARRANGEMENT FOR PRESTRESSING
Collar Setting Safety Protection
GIRDER END ARRANGEMENT FOR PRESTRESSING
Master Grip
Inserting Master Grip
Inserting Master Grip
Jack Setting
PRESTRESSING PULL (ELONGATION) FROM BOTH END
1st Reading : 62 mm Final Reading : 163 mm Reading after lock off : 153 mmSlip : 163-153= 10 mm
PRESTRESSING PULL (ELONGATION) FROM BOTH END
Work supervised by CSC Engr. Reading just after stressing : 200 mm Reading after 24 hours : 200 mm After 24 hours slip: 200-200=0
STRUCTURE LOAD ON EACH GIRDER AND HOW IT IS MAINTAINED BY PRESTRESSING
Total load (LL+DL) on each 30m girder = 1643.19 kN
Jacking force on each cable = 2475 kN (As per Contract Drawings)
Jacking force on each girder (3 no cable) = 2475 x 3 = 7425 kN
Hence 7425 kN>1643 kN
Factor of Safety, FS = 7425÷1643 = 4.5 [Minimum FS is required 1.5]
So Girder is safe.
PRESTRESSING PULL IS CONVERTED INTO STRESS
Jacking force, F = 2475 kN [As per Contract Drawings]= 2475÷0.96 [Jack efficiency 96%]= 2578.125 kN= 2578.125 x 1000 N [1 kN = 1000 N]= 2578.125 x 1000÷9.81 kg [1 Kg = 9.81 N]= 262805.81 kg
Jacking Ram Area, A = 563.72 cm2
Stress = Force/Area = F/A = 262805.81/563.72 kg/cm2
= 466.19 kg/cm2
HOGGING & IT’S MEASUREMENT
After Stressing Hogging at centre : 33 mm Measurement taking by level machine
BUCKLING & IT’S MEASUREMENT
After Stressing Reading at end section : 100 mm Reading at max buckling point90 mm
GROUT MIX DESIGN
Water cement ratio : 0.45
Mix proportion : Water : Cement : Admixture (Cebex-100)
22.5 Kg : 50 Kg : 1 Pkt
GROUTING PROCESS
Setting Grout Inlet Pipe Washing Duct Pipe by Air Compressor
Washing Duct Pipe by Air Compressor
GROUTING PROCESS
Pouring Water in Agitator Pouring Cement in Agitator Mixing Grout Materials in Agitator
GROUTING PROCESS
Grouting Pump Machine Grout Come out from Agitator Grouting in Progress
GROUTING PROCESS
Grout Come out from Outlet Pipe Lock off Outlet Pipe Provide Pressure 5.25 Kg/cm2
WEIGHT OF 30M GIRDER
CRANE CAPACITY
PIER CAP SEAT ARRANGEMENT
BEARING PAD (ELASTOMERIC PAD)
Size of Bearing Pad : 500 x 250 x 60mmPlan of Bearing and Pedestal
BEARING PAD (ELASTOMERIC PAD)
BEARING PAD (ELASTOMERIC PAD)
PLACING OF RUBBER BEARING PAD(ELASTOMERIC PAD)
Bearing Pedestal Placing Bearing Pad on Pedestal
PLACING OF GIRDER ON BEARING PAD
TIEING WITH CRANE AT GIRDER END
GIRDER LIFTING AND PLACING
GIRDER LIFTING AND PLACING
LEAVE THE GIRDER AFTER PLACING ON PIER CAP
Cross Girder Rebar is welded for Tieing Girders
GAP BETWEEN FLANGE TO FLANGE OF GIRDER
Design Gap : 40mm Maximum Gap (Actual) : 85mm
GAP BETWEEN FLANGE TO FLANGE OF GIRDER
Design Gap : 40mm Minimum Gap (Actual) : 12mm
GAP BETWEEN TWO GIRDER
Design Gap : 100mm Gap (Actual) : 115mm
CONCLUSION