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Prestress Concrete

C"oncrete in which there has been introduced internal stresses of such magnitude and distribution that the stresses resulting from given external loadings are counteracted to a desired degree. In reinforced concrete members the prestress is commonly introduced by tensioning the steel reinforcement.

This internal stress is induced into the member by either of the following prestressing methods:
Pre-Tensioning   :  Apply prestress to steel strands before casting concrete;
Post-Tensioning   :  Apply prestress to steel tendons after casting concrete

Pre Tensioning

Pre-tensioned concrete is almost always done in a precast plant. A pre- tensioned Prestressed concrete member is cast in a preformed casting bed. The BONDED wires (tendons) are tensioned prior to the concrete hardening. After the concrete hardens to approximately 75% of the specified compressive strength f’c, the tendons are released and axial compressive load is then transmitted to the cross-section of the member.

Front Desk Architects

The tensile force in the stressing steel is resisted by one of three methods:

  • Abutment method - an anchor block cast in the ground.
  • Front Desk Architects
  • Strut method - the bed is designed to act as a strut without deformation when tensioning forces are applied.
  • Front Desk Architects
  • Mould method - tensioning forces are resisted by strong steel moulds.
  • Front Desk Architects

    It is usual in pretensioning factories to locate the abutments of the stressing bed a considerable distance apart so that a number of similar units can be stressed at the same time, end to end using the same tendon. This arrangement is called the "Long Line Process".

    Post Tensioning

    Post-tensioning is the application of a compressive force to the concrete at some point in time after casting. When the concrete has gained strength a state of prestress is induced by tensioning steel tendons passed through ducts cast into the concrete, and locking the stressed tendons with mechanical anchors. The tendons are then normally grouted in place.

    Grouted Post-Tensioning System (Bonded PT System)

  • Quite an old method.
  • Predominantly meant for bridge construction.
  • Multi-strands (many cables) are anchored with a common anchorage.
  • Grouting is necessary for Protection of cables (Primary Reason)restricting the movement of cable within duct to surrounding concrete.
  • Front Desk Architects Front Desk Architects Front Desk Architects
    Unbonded Post-Tensioning System (Mono-Strand PT System)

  • Evolved from Grouted Post-Tensioning System before 1950 for Building Construction Application
  • No need of grouting (Grouting activity is eliminated)
  • Saving of Cost and Time and Material ?Installation made easier and flexible
  • Mono-strand System
  • Each cable can be managed individually
  • Induces less stress concentration to green concrete
  • It can work efficiently in lesser depths where ducts can not work efficiently
  • Better elastic behavior as far as earthquake event is concerned. Post-quake residual deformation is lesser.
  • Post Tensioning Patented Systems

  • Freyssinet system
  • Giffod-udall-CCL
  • Lee-McCall
  • Magnel-Blaton
  • Procedure of Post Tensioning

    Front Desk Architects Front Desk Architects
    Placing tendons

  • Tendons are housed in tendon ducts and the ducts are fixed in their predetermined alignment, level and profile. (Sometimes the tendons are threaded through the ducts after concreting.)
  • The tendon ducts shall be securely tied to the reinforcement to prevent dislodgment during concreting.
  • Concreting
  • Insitu concrete is cast. It is then cured as normal reinforced concrete.
  • Stressing (Tensioning)

  • Stressing can be carried out when the concrete has achieved sufficient strength. Stressing is performed by using a hydraulic jack.
  • During stressing, the readings of load and extension are recorded. When about one-half of the designed stressing load has been reached, a graph of load against extension is plotted. An extension correction is obtained from the plot by extrapolation method.
  • Stressing is continued until the designed load has been reached. Check if the corrected extension matches with the theoretical value. If it does, the tendon can be wedged to the anchorage. Otherwise, remedial action shall be taken.
  • Grouting
  • The ducts are grouted with cement grout through the grout holes/tubes to protect the tendons from corrosion.

  • Prestress Concrete Uses

  • Railway Sleepers;
  • Communications poles;
  • Pre-tensioned precast “hollowcore” slabs;
  • Pre-tensioned Precast Double T units - for very long spans (e.g., 16 m span for car parks);
  • Pre-tensioned precast inverted T beam for short-span bridges;
  • Pre-tensioned precast PSC piles;
  • Pre-tensioned precast portal frame units;
  • Post-tensioned ribbed slab;
  • In-situ balanced cantilever construction - post-tensioned PSC;
  • This is “glued segmental” construction;
  • Precast segments are joined by post-tensioning;
  • PSC tank - precast segments post-tensioned together on site. Tendons around circumference of tank;
  • Barges;

  • Prestress Concrete Advantage

  • Prestressing minimises the effect of cracks in concrete elements by holding the concrete in compression.
  • Prestressing allows reduced beam depths to be achieved for equivalent design strengths.
  • Prestressed concrete is resilient and will recover from the effects of a greater degree of overload than any other structural material.
  • If the member is subject to overload, cracks, which may develop, will close up on removal of the overload.
  • Prestressing enables both entire structural elements and structures to be formed from a number of precast units, e.g. Segmented and Modular Construction.
  • Lighter elements permit the use of longer spanning members with a high strength to weight characteristic.
  • The ability to control deflections in prestressed beams and slabs permits longer spans to be achieved.
  • Prestressing permits a more efficient usage of steel and enables the economic use of high tensile steels and high strength concrete.
  • More efficient members (i.e., smaller members to carry same loads)
  • Much less cracking since member is almost entirely in compression
  • Precast members have very good quality control
  • Precast members offer rapid field erection

  • Prestress Concrete DisAdvantage

  • More expensive in materials, fabrication, delivery
  • Heavy precast members require large cranes
  • Somewhat limited design flexibility
  • Small margin for error
  • More complicated design

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    References

  • Construction of Prestressed Concretes 2nd Edt., Ben C. Gerwick, Jr. (1993), Wiley Inter.
  • Science.
  • Modern Prestressed Concrete Design Principles and Construction Methods 4th Edt., James R.
  • Libby (1990), Van Nostrand Reinhold.
  • An Introduction to Prestressed Concrete, A.H. Allen ((1992), British Cement Association.
  • VSL Construction Systems,    www.vsl.com
  • Construction Technology Vol. 3 2nd Edt., R. Chudley (1991), Longman.
  • Civil Engineering Construction IV Vol. 4, S.A.R Jufri & R.J. Wellman (1992), Hong Kong Polytechnic.