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Module 4D8Prestressed Concrete

Lent Term 2010

Lecture 2 – Current Practice

Post-tensioned concreteDuct cast into concrete –tendon threaded

Tendon is tensioned by jack

Tendon anchored by wedges

End blocksHigh point load on the concrete face

causes lateral tension forcesSteel spirals placed duct to resist local forces

Other steel required for splitting if multiple tendons are used

Heavy jacks in confined spaces

Tendons curved - in elevation - and in plan

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Tendon spacing adjusted in slabs

- and in nuclear reactor

containment vessels

Kylesku Bridge, Scotland

GrangetownGrangetownViaductViaduct Grangetown Viaduct, Cardiff

• Precast segments• Counter-cast – new segment cast

against its neighbour, so fit is guaranteed

• Balanced cantilever construction• Erected by gantry or crane• New segments prestressed to existing

segments

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Joint with shear keys

Prestressing anchorage in web

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Bideford Bridge, Devon

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Coquet Bridge, NorthumberlandCoquet Bridge, Northumberland

Coquet Bridge• All tendons in top or bottom flanges• Flanges cast in-situ• Webs thin and precast – no need to

provide space for anchorages• No heavy lifting on site• Claimed to be cheaper to build, but

not repeated

Pre-tensioned concreteTendons tensioned between external anchor blocks

Concrete cast in mould around tendons

Tendons released and contract –concrete placed in compression

Long-line prestressingSeveral beams can be cast on same tendons

Tendons can be deflected, but corrosion problems

Collapse caused by corrosion at deflector points

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Strands debonded at ends only

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Inverted-T bridgesSolid or voided-slab bridges

Beam & slab bridges

Permanent formwork• Precast beam supports weight of wet concrete • Composite beam supports live loads

Variations Precast flooring systems

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External prestressing

• It is the force that is needed, not the tendon

• Not important that the tendons are inside the concrete

• Can be convenient to place the tendons outside the concrete

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Wentbridge Viaduct, Yorkshire

Exe Viaduct, M5, DevonExe Viaduct, M5, Devon

Prestressing tendons

Road drain

Joint in drain

Repair to plastic sleeve

Leak from drain

22ndnd Severn Crossing ApproachSevern Crossing Approach

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Bridge at Hope, Derbyshire• The first (?) stressed-ribbon bridge in

the UK

• A thin concrete slab is hung from cables to form a catenary

• It is then axially prestressed to resist the tension forces without cracking

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But its not all good news - this

Can turn into this No. of bridges built each year

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year of Construction

Num

ber

Reinforced Prestressed

Grade 4: Advanced section loss or spalling (or worse)

0

2

4

6

8

10

12

14

16

18

20

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Year of Construction

% o

f Wea

k B

ridge

s

Reinforced Prestressed

Rust!

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Ynys-y-Gwas

Ynys-y-Gwas• Bridge collapsed without warning at

dawn in February, with no imposed load• Built in 1950s, segmental both

longitudinally and transversely• No top slab• Poor mortar joints• Bad grouting of tendonsLed to prestressing being banned in UK

from 1992 - 1996

Ban lifted• Improved specification of grouts• Improved training of operatives• Ducts to be sealed and pressure

tested• Ducts to be plastic and

electrically insulated• Improved durability provisions,

especially at anchors

Do you want to be rich?

Invent a method of inspecting reinforcing bars and prestressing tendons inside concrete that allows general corrosion and pitting corrosion to be investigated