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Effective Inspection and Monitoring of Post-tensioned Structures and a Review
of Preventative Maintenance Issues
Dick FeastAmey Infrastructure Services
Principal ConsultantProfessional Services:
Whole Life Asset Management
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The Advantages of Using Post-tensioned Structures
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The Advantages of Post-tensioned Structures
Post tensioning techniques have allowed designers and contractors to build bridges and viaducts in what seemed impossible places:-– At great heights, over rivers and valleys– Elevated roadways over restricted sites– Dramatically bridging navigable seaways
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The Advantages of Post-tensioned Structures
A technique that allows long beams lengths to be achievedReduces deflections and allows high/span depth ratiosImproves durability by the reduction or elimination of crackingMore efficient utilization of steel and concrete material
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Post-tensioned bridges in Hong Kong
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Post-tensioned Bridges Hong Kong
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Post-tensioned Bridges Hong Kong
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Post-tensioned Bridges in Singapore
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Post-tensioned Bridges in USA
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The Hong Kong Bridge StockThe Importance of post-tensioned bridges to Hong Kong:
– The most common structures in Hong Kong are multi-span concrete viaducts
– More than 30% of these structures are post-tensioned– Most of the large span “landmark” concrete
structures are post-tensioned – Post-tensioned bridges are vitally important elements
of the infrastructure of Hong Kong
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Disadvantages of Post-tensioned Bridges are:-
The concentrations of tendons into several large cables make bridges vulnerable to localised corrosionThe risk of corrosion of tendons depends on the quality and integrity of the cement grout to protect them from the ingress of oxygen, water, chlorides and other deleterious elements
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Disadvantages:-
The consequences of the failure of the principal tensile load-carrying component are usually catastrophicA possible lack of redundancy within the structure triggering a catastrophic collapse
This must therefore be prevented at all costs
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United Kingdom
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United States
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Palau
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Belgium
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Belgium
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Particular Concerns Arise When:-
The grouting of ducts are incompleteThe air is moistWater is availableSalts can enter the system
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In-complete Grouting
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Air is Moist
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Water is Available
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Salts Can Enter the System
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The Hong Kong Bridge StockThe potential problems of Hong Kong’s bridge stock are:-– Most bridges are influenced by a marine
environment– They experience very heavy summer rainfalls– Very few bridge decks have waterproof
membranes – The climatic conditions can be very humid that
can assist the carbonation of concrete– Many viaducts have simply supported spans
with leaking bridge joints
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The Hong Kong Bridge Stock
– Many viaducts have half joints details that are very difficult to inspect and maintain
– The structures are heavily trafficked – Post-tensioned segmental constructed structures
can be vulnerable at their construction joints– During periods of high construction boom it is,
unfortunately, a fact that workmanship and supervision qualities fall (50% of HK’s Bridge stock has been constructed over the last 10 years)
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Historic Reasons for the Problems
Early post-tensioned structures suffered from the following:-– Poor workmanship, particularly in grouting
activities especially using spacers within the ducts and with untrained staff
– Poor detailing, poor quality of joints on segmental construction, fragile vent pipe details
– Poor supervision, especially when high volumes of work were required
– Poor structural waterproofing
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Historic Reasons for the Problems
– Poor quality of grout specifications with high water/cement contents and few admixtures
– Grouts suffered from high shrinkage and water lenses often occurred
– With time the water evaporated leaving voids within the ducts
– When fresh cement was used some grouts suffered excessive heat generation, hardened prematurely and stopped the continuous flow of grout through the duct
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Poor Grouting – 1960’s
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Poor Grouting – 1970’s
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Poor Grouting – 1980’s
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Poor Grouting – 2001
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The Vulnerable Locations Are:-
Joints in segmental constructionConstruction jointsHigh points in tendon profiles Low points in tendon profilesAnchorages
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The Need for Effective Inspection and Monitoring
To give bridge owners confidence that their post-tensioned bridge stock is in good and a safe condition it is essential to implement effective Inspection and Monitoring regimes of Post-tensioned BridgesA comprehensive and logical approach is required
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The Need for Effective Inspection and Monitoring
From the UK’s experience the following Advice Notes were developed for managing a Special Post-tensioned Bridge inspection programmeThe main documents being:-– Prioritisation for inspection (BA54/93)
– Planning and execution of the inspection (BA50/93)
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Special Post tensioned Bridge Inspections
The initial inspection programme of a particular bridge stock is based on a priority ranking system
In the UK the priority ranking system is based on the following attributes:-
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Special Post tensioned Bridge Inspection
Bridge Factors - Age of the structureStructural form Presence of vulnerable details
Site Factors - Traffic on the bridgeTraffic under the bridgeStrategic importance of the route
5 No Priority Ratings - 1 (Highest) to 5 (Lowest)(Note:- The highest priority to be inspected first)
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Special Post tensioned Bridge Inspections
Phase 1: Desk Top StudyTo determine the fundamental design and construction detailsTo review previous inspection and maintenance records for the bridge
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Special Post tensioned Bridge Inspections
Phase 2 : Preliminary Site InspectionTo verify design and construction detailsIdentify any signs of distressPlan appropriate investigation programme
Produce a “Technical Plan”
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Special Post tensioned Bridge Inspections
Phase 3 : Detailed Site InvestigationCorrosion detectionSamplingMaterial testingVoid detection Void volume measurementsInternal examination of voidsStress determinations
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PHASE 3 PHASE 3 -- SITE INVESTIGATIONSITE INVESTIGATION
Project Manager's Report
No
Yes
Further Tests ?Yes
TendonsTestsMaterial Voids
ConditionsStress
TestingLoad
Corrosion Detection Sampling
Site Investigation
Void Detection Internal Examination
Is Further Testing Required No
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Cover Meter with Probe for Duct Locating
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Radar Survey Locating PT Ducts
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Video Inspection of PT Ducts
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Void Determination
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Half cell Potential Survey
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Resistivity MeasurementsCONCRETE RESISTIVITY MEASUREMENTS
INDICATION OF PROBABILITY OF CORROSION
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Chloride DeterminationCONCRETE DUST SAMPLES FOR
CHEMICAL ANALYSIS
LEVEL OF CHLORIDE CONTAMINATION INDICATES LIKELIHOOD OF PITTING CORROSION
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Insitu Stress Determination - Concrete
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Instu Stress Determination - Tendons
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Managing Problems Identified
Re-grouting existing voids to ductsAdding external post-tensioned tendonsAdding carbon fibre plates or fibre reinforced polymersInstalling suitable propping systems to prevent catastrophic failures occurring – Note, such propping system will not be utilized
until a structural failure occurs
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Additional External Post-tensioning
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Carbon Fibre Plate Strengthening
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Improving Existing Bridge Durability
Installing bridge deck waterproof membranesRe-place leaking bridge jointsRe-move bridge joints and make the structure semi-intregalIf having semi-intregal abutments construct inspection galleries Apply suitable waterproof coatings, i.e. silaneInstall permanent corrosion monitoring systems
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The Importance of Bridge Deck Waterproof Membranes
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Taf Fawr Bridge Durability Defects
Severe local damage had occurred to the precast concrete unitsSevere corrosion was occurring to the reinforcement and the tendons contained within the precast unitsThere was a 30% loss of prestress in the bottom flange
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Taf Fawr Bridge Demolished in 1986
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Clifton BridgeClifton Bridge
Major refurbishment carried out in 1994Corrosion damage to post-tensioned tendons located in the top of the deckAn ineffective waterproof membrane caused the problem
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Previous Waterproofing Problems to Hong Kong BridgesThe reasons for the failures:-
Low adhesion between waterproof membrane and bituminous surfacingReduced pavement thicknessUse of open grade friction coursePremature aggregate strippingMix design of wearing coursesInadequate compaction
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Previous Waterproofing Problems to Hong Kong Bridges
Weak friable aggregateExcessive dust coating on aggregatesHigh ambient temperature
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Solutions to these ProblemsThe minimum structural thickness to the pavements to be 100mm i.e. binder and wearing course to be 100mm
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Solutions to these ProblemsThe waterproof membrane to be used is to have a stiff modulus of elasticityThe waterproof membrane must not be thicker than 2.5mmThe bridge deck needs to have sub-surface drainage facilitiesThe tack coat to the waterproof membrane needs to be of a bitumen “thick” type and be polymer-modified
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Solutions to these ProblemsThe Binder needs to be a polymer-modified Hot-Rolled Asphalt (typically 35% Aggregate Content)The Binder must be laid at a temperature high enough to activate the tack coatThe Binder must be well compacted and have a voids content no greater than 4 to 5% at the Membrane/binder interface
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Solutions to these Problems
The wheels of the paving machine needs to be clean at all times and not disturb the laid tack coatThe Binder / regulating course must have a thick polymer-modified bond coat at their interfaceThe primer coat to the surfacing wearing course needs to be thick and polymer-modified
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New Materials
Gussasphalt
Waterproofing system and combined pavement system
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eschlimannAsphalt hat viele Gesichter.
Strassenbau- und Asphaltunternehmung
0255075
100
0 5 10 15 20 25 30 35 40 45 50 55
Cos
t[%
]
60
Time [years]
Gussasphalt ΣLCC = 1.00
Replace waterproofing layer and surfacingevery
30 years
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eschlimannAsphalt hat viele Gesichter.
Strassenbau- und Asphaltunternehmung
0255075
100
0 5 10 15 20 25 30 35 40 45 50 55
Cos
t[%
]
60
Time [years]
Hot Rolled Asphalt ΣLCC = 1.26
20 yearsReplace waterproofing layer and surfacing every
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eschlimannAsphalt hat viele Gesichter.
Strassenbau- und Asphaltunternehmung
0255075
100
0 5 10 15 20 25 30 35 40 45 50 55
Cos
ts[%
]
60
Time [years]
Asphalt Concrete ΣLCC = 1.45
15 yearsReplace waterproofing layer and surfacingevery
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eschlimannAsphalt hat viele Gesichter.
Strassenbau- und Asphaltunternehmung
0255075
100
0 5 10 15 20 25 30 35 40 45 50 55 60
Time [years]
Cos
ts[%
]
Gussasphalt ΣLCC = 1.00
30 yearsReplace waterproofing layer and surfacingevery
0255075
100
0 5 10 15 20 25 30 35 40 45 50 55 60
Time [years]
Cos
ts[%
]
Hot Rolled Asphalt ΣLCC = 1.26
20 yearsReplace waterproofing layer and surfacingevery
0255075
100
0 5 10 15 20 25 30 35 40 45 50 55 60
Time [years]
Cos
ts[%
]
Asphalt Concrete ΣLCC = 1.45
15 yearsReplace waterproofing layer and surfacingevery
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eschlimannAsphalt hat viele Gesichter.
Strassenbau- und Asphaltunternehmung
Stone chippings
Wearing Gussasphalt AE PS 8/11 30 mm
Base Gussasphalt AE – S 8/11 30 mm
Steel – Deck 10 mm
Shot blastingConcrete or Steel Primer
Tack Coat
Eliminator Membrane (1st
coat)
STEEL - DECKSTEEL - DECK
Eliminator Membrane (2nd coat)3 mm
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eschlimannAsphalt hat viele Gesichter.
Strassenbau- und Asphaltunternehmung
CONCRETE BRIDGE - DECKCONCRETE BRIDGE - DECK
Stone chippings
Wearing Gussasphalt AE PS 11 35 mm
Base Gussasphalt AE – S 11/16 30 mm
ConcreteShot blastingConcrete or Steel Primer
Tack Coat
Eliminator Membrane (1st coatEliminator Membrane (2nd coat)3 mm
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Sub-surface Drainage Details
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Sub-surface Drainage Details
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Re-placing Bridge Joints
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Damage Caused by Leaking Bridge Joints
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Removal of Bridge JointsSemi-intregal Solutions
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Provision of Inspection Galleries