Add to basket Add to wishlist Description Concrete remains the most common material for bridge construction around the world, and prestressed concrete is frequently the material of choice. Extensively illustrated throughout, this invaluable book brings together all aspects of designing prestressed concrete bridge decks into one comprehensive volume. Commonly used for bridges with spans between 25m and m, prestressed concrete bridges provide economic, durable and aesthetic solutions in most situations. This book clearly explains the principles behind the design and construction of prestressed concrete bridges and illustrates the interaction between the two. All the different types of deck arrangement and the construction techniques used are covered.
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Forces on post-tensioned concrete with profiled curved tendon Post-tensioned tendon anchorage; four-piece "lock-off" wedges are visible holding each strand Post-tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast. At each end of a tendon is an anchorage assembly firmly fixed to the surrounding concrete.
Once the concrete has been cast and set, the tendons are tensioned "stressed" by pulling the tendon ends through the anchorages while pressing against the concrete. The large forces required to tension the tendons result in a significant permanent compression being applied to the concrete once the tendon is "locked-off" at the anchorage. Each added segment is supported by post-tensioned tendons Tendon encapsulation systems are constructed from plastic or galvanised steel materials, and are classified into two main types: those where the tendon element is subsequently bonded to the surrounding concrete by internal grouting of the duct after stressing bonded post-tensioning ; and those where the tendon element is permanently debonded from the surrounding concrete, usually by means of a greased sheath over the tendon strands unbonded post-tensioning.
When the tendons are tensioned, this profiling results in reaction forces being imparted onto the hardened concrete, and these can be beneficially used to counter any loadings subsequently applied to the structure. This grouting is undertaken for three main purposes: to protect the tendons against corrosion ; to permanently "lock-in" the tendon pre-tension, thereby removing the long-term reliance upon the end-anchorage systems; and to improve certain structural behaviors of the final concrete structure.
This bundling makes for more efficient tendon installation and grouting processes, since each complete tendon requires only one set of end-anchorages and one grouting operation. Ducting is fabricated from a durable and corrosion-resistant material such as plastic e. Fabrication of bonded tendons is generally undertaken on-site, commencing with the fitting of end-anchorages to formwork , placing the tendon ducting to the required curvature profiles, and reeving or threading the strands or wires through the ducting.
Following concreting and tensioning, the ducts are pressure-grouted and the tendon stressing-ends sealed against corrosion. Above Installed strands and edge-anchors are visible, along with prefabricated coiled strands for the next pour.
Below End-view of slab after stripping forms, showing individual strands and stressing-anchor recesses. Unbonded post-tensioning differs from bonded post-tensioning by allowing the tendons permanent freedom of longitudinal movement relative to the concrete. This is most commonly achieved by encasing each individual tendon element within a plastic sheathing filled with a corrosion -inhibiting grease , usually lithium based.
Anchorages at each end of the tendon transfer the tensioning force to the concrete, and are required to reliably perform this role for the life of the structure. Permanent corrosion protection of the strands is provided by the combined layers of grease, plastic sheathing, and surrounding concrete. Where strands are bundled to form a single unbonded tendon, an enveloping duct of plastic or galvanised steel is used and its interior free-spaces grouted after stressing.
In this way, additional corrosion protection is provided via the grease, plastic sheathing, grout, external sheathing, and surrounding concrete layers. The bare steel strand is fed into a greasing chamber and then passed to an extrusion unit where molten plastic forms a continuous outer coating. Finished strands can be cut-to-length and fitted with "dead-end" anchor assemblies as required for the project.
Comparison between bonded and unbonded post-tensioning[ edit ] Both bonded and unbonded post-tensioning technologies are widely used around the world, and the choice of system is often dictated by regional preferences, contractor experience, or the availability of alternative systems.
Either one is capable of delivering code-compliant, durable structures meeting the structural strength and serviceability requirements of the designer. Once cured, this grout can transfer the full tendon tension force to the concrete within a very short distance approximately 1 metre.
As a result, any inadvertent severing of the tendon or failure of an end anchorage has only a very localised impact on tendon performance, and almost never results in tendon ejection from the anchorage. This results in significantly higher tensile strains in the tendons than if they were unbonded, allowing their full yield strength to be realised, and producing a higher ultimate load capacity. With the tendons fixed to the concrete at each side of the crack, greater resistance to crack expansion is offered than with unbonded tendons, allowing many design codes to specify reduced reinforcement requirements for bonded post-tensioning.
As a result, bonded structures may display a higher capacity to resist fire conditions than unbonded ones. Additional lead time may need to be allowed for this fabrication process. Improved site productivity The elimination of the post-stressing grouting process required in bonded structures improves the site-labour productivity of unbonded post-tensioning.
In extremes, unbonded tendons can resort to a catenary -type action instead of pure flexure, allowing significantly greater deformation before structural failure. Research on the durability performance of in-service prestressed structures has been undertaken since the s,  and anti-corrosion technologies for tendon protection have been continually improved since the earliest systems were developed.
Also critical is the protection afforded to the end-anchorage assemblies of unbonded tendons or cable-stay systems, as the anchorages of both of these are required to retain the prestressing forces. Failure of any of these components can result in the release of prestressing forces, or the physical rupture of stressing tendons. Modern prestressing systems deliver long-term durability by addressing the following areas: Tendon grouting bonded tendons Bonded tendons consist of bundled strands placed inside ducts located within the surrounding concrete.
To ensure full protection to the bundled strands, the ducts must be pressure-filled with a corrosion-inhibiting grout, without leaving any voids, following strand-tensioning. Tendon coating unbonded tendons Unbonded tendons comprise individual strands coated in an anti-corrosion grease or wax, and fitted with a durable plastic-based full-length sleeve or sheath. The sleeving is required to be undamaged over the tendon length, and it must extend fully into the anchorage fittings at each end of the tendon.
Such tendons are composed of individual strands, grease-coated and sleeved, collected into a strand-bundle and placed inside encapsulating polyethylene outer ducting. The remaining void space within the duct is pressure-grouted, providing a multi-layer polythene-grout-plastic-grease protection barrier system for each strand.
Anchorage protection In all post-tensioned installations, protection of the end-anchorages against corrosion is essential, and critically so for unbonded systems. Several durability-related events are listed below: Ynys-y-Gwas bridge, West Glamorgan, Wales, A single-span, precast-segmental structure constructed in with longitudinal and transverse post-tensioning.
Corrosion attacked the under-protected tendons where they crossed the in-situ joints between the segments, leading to sudden collapse. Inadequate concrete cover in the side abutments resulted in tie-down cable corrosion , leading to a progressive failure of the main bridge span and the death of one person.
The moratorium was lifted in An unauthorised chemical was added to the tendon grout to speed construction, leading to corrosion of the prestressing strands and the sudden collapse of one span, injuring many spectators. Corrosion from road de-icing salts was detected in some of the prestressing tendons, necessitating initial closure of the road while additional investigations were done.
Subsequent repairs and strengthening using external post-tensioning was carried out and completed in Genoa bridge collapse , The Ponte Morandi was a cable-stayed bridge characterised by a prestressed concrete structure for the piers, pylons and deck, very few stays, as few as two per span, and a hybrid system for the stays constructed from steel cables with prestressed concrete shells poured on. The concrete was only prestressed to 10 MPa, resulting in it being prone to cracks and water intrusion, which caused corrosion of the embedded steel.
Churchill Way flyovers, Liverpool , England The flyovers were closed in September after inspections revealed poor quality concrete, tendon corrosion and signs of structural distress. Demolition is planned for The prestressing of concrete allows "load-balancing" forces to be introduced into the structure to counter in-service loadings. This provides many benefits to building structures: Longer spans for the same structural depth Load balancing results in lower in-service deflections, which allows spans to be increased and the number of supports reduced without adding to structural depth.
Reduced structural thickness For a given span, lower in-service deflections allows thinner structural sections to be used, in turn resulting in lower floor-to-floor heights, or more room for building services.
Faster stripping time Typically, prestressed concrete building elements are fully stressed and self-supporting within five days. At this point they can have their formwork stripped and re-deployed to the next section of the building, accelerating construction "cycle-times". Reduced material costs The combination of reduced structural thickness, reduced conventional reinforcement quantities, and fast construction often results in prestressed concrete showing significant cost benefits in building structures compared to alternative structural materials.
Prestressed Concrete Bridges : Design and Construction
Home Prestressed Concrete Bridges Prestressed Concrete Bridges, Second edition, is the comprehensive reference for practising bridge engineers on the design and construction of prestressed concrete bridges. Offering complete coverage of the design and construction of prestressed concrete bridges in a single resource, this book is an essential aid for maximising your efficiency on projects and expanding your existing knowledge. The book covers all types of deck arrangements and construction techniques - including in-situ slabs, precast beams, segmental construction, launched bridges and cable-stayed structures - and illustrates the interaction between design and construction. Outlines the fundamentals of the design of prestressed concrete bridges, presenting the latest analysis methods and design techniques. Fully updated for bridge design to Eurocodes.
Table of Contents
Extensively illustrated throughout, this invaluable book brings together all aspects of designing prestressed concrete bridge decks into one comprehensive volume. Commonly used for bridges with spans between 25m and m, prestressed concrete bridges provide economic, durable and aesthetic solutions in most situations. This book clearly explains the principles behind the design and construction of prestressed concrete bridges and illustrates the interaction between the two. All the different types of deck arrangement and the construction techniques used are covered.
Prestressed Concrete Bridges: Design and Construction
Hewson Prestressed concrete decks are commonly used for bridges with spans between 25m and m and provide economic, durable and aesthetic solutions in most situations where bridges are needed. Concrete remains the most common material for bridge construction around the world, and prestressed concrete is frequently the material of choice. Extensively illustrated throughout, this invaluable book brings together all aspects of designing prestressed concrete bridge decks into one comprehensive volume. The book clearly explains the principles behind both the design and construction of prestressed concrete bridges, illustrating the interaction between the two. It covers all the different types of deck arrangement and the construction techniques used, ranging from in-situ slabs and precast beams; segmental construction and launched bridges; and cable-stayed structures. Included throughout the book are many examples of the different types of prestressed concrete decks used, with the design aspects of each discussed along with the general analysis and design process.
Prestressed Concrete Bridges - Nigel Hewson