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This title provides an assessment of pre-stressed concrete, unbonded tendons, properties of materials, losses in pre-stress, suitable types of structures, design of floor and roof slabs of buildings using unbonded tendons, design of other structures using unbonded tendons, work on site and post-tensioning systems.
The durability of post-tensioning tendons depends undoubtedly on the durability of the materials used, but there are design concept specifics which are also of major importance: the post-tensioning layout and layers of protection such as concrete cover and selected materials in view of the aggressivity of the environment for instance. It is well known that sustainability principles guide the Engineer from the very beginning, at the project conception, during construction and the service life of a structure. Decisions made during conceptual and design stage have the largest influence on the durability and sustainability of post-tensioning tendons. fibBulletin 33 addresses the specifics for prestressed concrete structures: the durability of post-tensioning tendons. It should be noted that it does not repeat topics that have been addressed in other fib bulletins and which is common for both reinforced concrete and prestressed concrete structures. Pre-tensioning, which is used extensively in the precast industry, is not considered here, although conclusions and recommendations herein may, in many cases, also be applicable. This recommendation was prepared by Working Party 5.4.2, Durability specifics for prestressed concrete structures, in cooperation with fib Commission 9,Reinforcing and prestressing materials and systems. A preliminary version of this recommendation served as the basic document for the second workshop on "Durability of post-tensioning tendons", held on 11-12 October 2004 in Zurich. This workshop was a follow-up to the first workshop held in Ghent in 2001. Bulletin 33 includes revisions corresponding to the agreed results of the Zurich workshop.
Post-tensioning and grouting operations can be dangerous if the required care is not taken in planning, in site preparations and in execution. For prestressed concrete a good working environment is also a prerequisite for high quality. Many accidents in this type of work may be attributed to a lack of training, poor supervision, poor planning or over-familiarity with the process. This guide to good practice highlights important safety measures which are particularly applicable to prestressed concrete, dealing with precautions necessary for post-tensioning and grouting operations on site.
The book combines history with academic notes for use at the university level, presenting design examples from actual jobs with applications and detailing for the practicing engineer. Chapter 1 tells the history of post-tensioned concrete as only Ken Bondy can tell it. Chapters 2-8 are the notes Dirk Bondy uses to teach Design of Prestressed Concrete Structures at UCLA and Cal Poly-San Luis Obispo. Chapters 9-13 are design examples that address many of the decisions faced by practicing engineers on typical projects. Chapters 13-14 cover the art of detailing and observing the construction of post-tensioned concrete. This knowledge was obtained over many years of working on our own projects and listening and learning from the the pioneers of post-tensioned concrete. Chapter 15 covers the slab on grade industry, which represents more sales of post-tensioning tendons than all other post-tensioning applications combined. Chapter 16 discusses the challenging application of post-tensioning-external post-tensioning.
Abstract: Post-tensioning concrete technology increases the resistance of flexural concrete members. This technology allows for the production of slenderer sections, and sequentially less usage of material preserving the sustainability concept in construction engineering. Post-tensioning process can be done using bonded or unbonded steel tendons. The unbonded tendons are thought to have better resistance to corrosion for structures exposed to severe environmental conditions. The unbonded tendon's steel strands are painted with grease and covered with plastic sheathing to prevent the moisture from reaching the steel strands thus they can provide high corrosion resistance. According to the ACI 318-19 and other codes of practice, the stress in the unbonded tendon at the ultimate limit state is limited to less than or equal to the tendon's yield stress. On the other hand, the bonded tendon's stress at this state is determined to be more than or equal to the tendon's yield stress. This limitation for the unbonded tendons restrained the widespread usage of the unbonded system. Through this research, six-simply supported one-way slabs; two with bonded tendons, two with unbonded tendons and two with unbonded tendons and non-prestressing steel reinforcement are tested in flexure to failure. The post-tension slabs are of 4.0-meters in span and the flexural tests are carried in the AUC structural engineering laboratory in a four-point loading scheme. The ultimate stress of the unbonded tendons are measured at the failure stage. The results of both systems are compared against each other and against the provisions of the ACI 318-19. The unbonded post-tension slabs with non-prestressing steel reinforcement showed higher failure loads than the bonded and unbonded slabs without reinforcement. The ACI 318-19 provisions were critically reviewed versus the results of the experimental investigation. The review reveals that the limitation of the unbonded tendon's ultimate stress is not accurate and can be reviewed.