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The concept of post-tensioning has been recognized for over a century. Interestingly, early developments started with external tendons, but failed to be recognized as a major construction technique for two main reasons:
The concept of post-tensioning has been recognized for over a century. Interestingly, early developments started with external tendons, but failed to be recognized as a major construction technique for two main reasons:
Low tensile performance of early steels in combination with a poor knowledge of concrete creep and shrinkage properties,
Lack of a durable corrosion protection.
With the technological progress, external tendons became increasingly popular in the 1980’s, as a post-tensioning method enabling inspection and, if necessary, replacement of tendons without demolition of structural members. Towards the end of the last century, more than 50 bridges have been built with external tendons, first in France and soon gaining traction in other countries.
FIP published a state-of the-art report in May 1996 to provide a review of the application of external tendons, describing specific material problems and methods for dealing with them. 25 years have passed and, while the engineering principles covered by the FIP report remain unchanged, the context has evolved:
External tendons and construction methods have kept evolving with better materials, ever longer spans, and tighter schedules.
Normalization frame in Europe changed,
Severe durability issues have occurred in some countries from which the industry can extract good knowledge of the causes and how to avoid similar problems in the future.
This new fib bulletin has been prepared with the aim to reflect the current state of the art and encompass the knowledge amassed in the last quarter of century with chapters covering from the design and approval of systems and materials to installation, quality control and monitoring. The last chapter is a compilation of structures worldwide covering all sorts of materials, typologies and construction methods, which might be a source of inspiration for owners and designers alike.
Contents
Preface
1. Introduction
1.1 Definition
1.2 Historical Development
1.3 State of the Art of External Tendons
1.3.1 External tendon types
1.4 Advantages and Disadvantages of External Tendons
1.4.1 Advantages
1.4.2 Disadvantages
1.5 Terms and Definitions
2. Primary components of External Tendons
2.1 Tensile elements
2.1.1 Strands
2.1.2 Wires
2.1.3 Bars
2.2 Ducts
2.2.1 Polymeric ducts
2.2.2 Steel ducts
2.3 Anchorages
2.4 Couplers
2.5 Anchorage Caps
2.6 Deviators
2.6.1 Inserts in straight recess pipes or bore holes
2.6.2 Deviators directly cast in the concrete for bundled deviation
2.6.3 Deviators directly cast in the concrete for single strand deviation
2.7 Fixations
3. Durability of External Tendons
3.1 Corrosion protection for External Tendons
3.1.1 Exposure
3.1.2 Protection measures of prestressing steel
3.1.3 Corrosion protection for anchorages
3.1.4 Protection strategies
3.1.5 Lessons learned
3.2 Resistance of prestressing steel when deviated
3.3 Wear of polymer duct at deviators
3.3.1 Cable factor (transverse pressure at individual tensile elements)
3.3.2 Friction and wear when stressing
3.4 Fire resistance
4. Design and detailing aspects
4.1 General approach to design
4.2 Action on the structure by External Tendons
4.2.1 Initial prestressing force
4.2.2 Losses of prestressing force
4.2.3 Force acting at deviation points
4.2.4 Local forces at deviation points
4.2.5 Stress increase in ULS
4.2.6 Calculation of acting forces
4.2.7 Anchorage and deviation blocks
4.3 Minimum deviation radius
4.4 Provision for future tendons
5. Fabrication, Installation and Replacement
5.1 Tendon fabrication
5.1.1 On-site assembly
5.1.2 Partial prefabrication
5.1.3 Full prefabrication
5.2 Temporary corrosion protection measures
5.3 Jobsite activities before tendon installation
5.4 Tendon installation
5.4.1 On-site assembled tendons
5.4.2 Partially prefabricated tendons
5.4.3 Fully prefabricated tendons
5.5 Stressing
5.6 Duct filling method
5.6.1 Full or partial prefabrication
5.6.2 On-site fabrication
5.7 Final works
5.8 Documentation
5.9 Replacement of external tendons
5.9.1 Replacement of grouted External Tendons
5.9.2 Replacement of external tendons with soft filler
6. Approval Testing
6.1 Resistance to static load test
6.1.1 Verification of anchorages
6.1.2 Verification of deviation zones with small radii of curvature
6.2 Resistance to fatigue load test
6.3 Load transfer to the structure test
6.4 Deviated tendon test verifying the corrosion protection barriers
6.5 Assembly / installation / stressing test
6.6 Duct filling test
7. Quality Assurance, Inspection and Monitoring
7.1 Quality assurance
7.1.1 Quality plan
7.1.2 Quality control testing on components performed by the manufacture
7.1.3 External surveillance for components
7.1.4 Traceability
7.1.5 Quality control of the execution
7.2 Regular and special inspections of external tendons
7.2.1 Regular inspection
7.2.2 Special inspection – Load measurement
7.2.3 Special inspection – Durability assessment
7.3 Long-term monitoring
7.3.1 Monitoring of prestressing force
7.3.2 Monitoring of electrical isolation
7.3.3 Acoustic Emission Measurements
8. Application in bridge construction and repair
8.1 General
8.2 Concrete bridges
8.2.1 Use of combined internal and external tendons in cast-in-place bridges
8.2.2 Use of internal and external tendons in precast bridges
8.2.3 Use of only external tendons in cast-in-place bridges
8.2.4 Use of only external tendons for precast bridges
8.3 Structures made of Ultra High Performance Concrete
8.4 Structural steel and composite structures
8.5 Strengthening of existing structures
8.5.1 Strengthening with longitudinal tendons
8.5.2 Strengthening with transverse tendons
9. References
