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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