Presentation on prestressing prepared by Rais Uddin

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