Prestressed Concrete Design Overview
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Questions and Answers

Which of the following is NOT an advantage of prestressed concrete over reinforced concrete?

  • Increased spans
  • Increased durability (correct)
  • Smaller deflections
  • Smaller section sizes

    • What is the main benefit of prestressed concrete having a larger second moment of area?

  • Increased durability
  • Reduced tensile stress
  • Smaller deflections (correct)
  • Increased compressive strength

    • In what scenario is the compressive stress in prestressed concrete maximized?

  • When tendons are stressed before casting (correct)
  • When tendons are stressed simultaneously with casting
  • When tendons are stressed after casting
  • When tendons are stressed before and after casting

    • Why can a given section span further with prestressed concrete compared to reinforced concrete?

    <p>The smaller section size reduces self-weight in prestressed concrete. (D)</p> Signup and view all the answers

    What is the key difference between pre-tensioned and post-tensioned concrete?

    <p>The time at which the prestressing tendons are stressed (D)</p> Signup and view all the answers

    Which of these statements accurately describes pre-tentioned concrete?

    <p>Tendons are typically placed in a steel bed form and stressed before concrete placement. (A)</p> Signup and view all the answers

    Who first proposed the idea of prestressing to counteract stresses due to loads?

    <p>Austrian Engineer Mandl (A)</p> Signup and view all the answers

    What significant development occurred in 1907 regarding prestressed concrete?

    <p>The reporting on losses of prestress (C)</p> Signup and view all the answers

    In what year did Dischinger demonstrate the use of unbonded tendons?

    <p>1928 (D)</p> Signup and view all the answers

    What was a major problem encountered with early prestressed concrete slabs in the mid-1950s?

    <p>Weight and large deflections (C)</p> Signup and view all the answers

    Which company was associated with the manufacture of slabs using embedded tensioned wires in 1888?

    <p>Dohring of Germany (A)</p> Signup and view all the answers

    What was a key result of Jackson's patent application in 1886?

    <p>Construction of prestressed concrete pavements (B)</p> Signup and view all the answers

    What does the US building post-tensioning industry credit for its existence?

    <p>The first lift slab buildings (B)</p> Signup and view all the answers

    What issue arose due to flexural creep in early prestressed concrete slabs?

    <p>Large deflections (B)</p> Signup and view all the answers

    Which development was associated with the introduction of tensioning in the construction of pavements?

    <p>The creation of artificial stone pavements (B)</p> Signup and view all the answers

    What is the primary reason for introducing prestress to a beam?

    <p>To counteract stresses from external loads (D)</p> Signup and view all the answers

    Which of the following statements about reinforced concrete is correct?

    <p>Reinforced concrete combines the strengths of concrete and steel. (D)</p> Signup and view all the answers

    What happens to a row of books lacking prestressing when weight is applied?

    <p>It has zero tensile capacity and cannot support itself. (B)</p> Signup and view all the answers

    How is prestress typically introduced into reinforced concrete?

    <p>By tensioning the steel reinforcement within the concrete (A)</p> Signup and view all the answers

    What is the basic function of the steel reinforcement in reinforced concrete?

    <p>To handle tensile stresses that concrete cannot (C)</p> Signup and view all the answers

    According to the principles of prestressed concrete, what is the initial state of internal stresses?

    <p>They should be introduced adequately and suitably. (C)</p> Signup and view all the answers

    Which type of beam will experience tension when loaded?

    <p>A loaded simply supported beam (A)</p> Signup and view all the answers

    What is the tensile capacity of concrete used in prestressed concrete?

    <p>Zero or negligible (C)</p> Signup and view all the answers

    What does prestressing do to the behavior of concrete structures?

    <p>It reduces the likelihood of cracking under load. (A)</p> Signup and view all the answers

    What is the key to effective prestressed concrete?

    <p>Ensuring internal stresses are effective and appropriately introduced (D)</p> Signup and view all the answers

    Flashcards

    Prestressed Concrete

    Concrete that has been pre-tensioned or post-tensioned to reduce tension stresses.

    Advantages of Prestressed Concrete

    Includes smaller section sizes, reduced deflections, increased spans, and improved durability.

    Smaller Section Sizes

    Prestressed concrete allows for stiffer sections, enabling smaller sizes without losing strength.

    Post-tensioning

    A method of prestressing applied after casting the concrete.

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    Pre-tensioning

    A prestressing method that applies tension to steel strands before the concrete is cast.

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    Second Moment of Area

    A measure of a section's stiffness; larger for prestressed concrete leading to less deflection.

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    Prestressing

    The process of applying initial compressive stress to concrete to improve its performance.

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    Compressive Stress

    A type of stress that compresses materials, enhancing stability under loads.

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    Tensile Strength of Concrete

    Concrete's strength in resisting pulling forces is virtually zero.

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    Reinforced Concrete

    Concrete combined with steel bars to improve tensile strength.

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    Deflected Beam

    A beam that bends under load, causing tension and compression.

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    Internal Stresses

    Forces developed within a material to counteract external loads.

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    Tensioning Steel Reinforcement

    Applying tension to steel reinforcement within concrete to induce prestress.

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    Simply Supported Beam

    A beam supported at both ends, allowing free rotation and no moment.

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    Unstressed Beam

    A beam that has not been preloaded or prestressed before loading.

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    Counteracting Stresses

    The process of balancing internal stresses against external loads to maintain integrity.

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    Patent Application by Jackson

    Jackson filed for a patent in 1886 for prestressing concrete pavements.

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    Mandl's Idea

    Austrian Engineer Mandl introduced prestressing to counteract loading stresses in 1896.

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    Dohring's Contribution

    Dohring manufactured slabs with embedded tensioned wires in 1888 to prevent cracks.

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    M Koenan's Research

    Koenan reported on prestress losses in concrete due to elastic shortening in 1907.

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    Unbonded Tendons

    Dischinger demonstrated the use of unbonded tendons in prestressed concrete in 1928.

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    Challenges of Early Slabs

    Early slabs faced lifting issues and large deflections after construction.

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    Post-Tensioning Industry

    The US post-tensioning industry originated from mid-1950s lift slab constructions.

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    Lift Slab Construction

    First lift slab buildings in the US were constructed in the mid-1950s using non-prestressed slabs.

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    Study Notes

    Prestressed Concrete Design

    • Course code: CEPC0510
    • Prerequisite: Reinforced Concrete Design (CEPC0506)

    History of Prestressed Concrete

    • 1886: Jackson of San Francisco patented "construction of artificial stone and concrete pavements," introducing prestress by tensioning reinforcing rods in sleeves.
    • 1888: Dohring (Germany) developed slabs and beams using embedded tensioned wires in concrete to prevent cracking.
    • 1896: Mandl (Austrian engineer) first proposed the idea of prestressing to counteract stresses from loads.
    • 1907: Koenan (Germany) further explored prestressing, noting losses due to concrete's elastic shortening.
    • 1928: Dischinger used unbonded tendons in a major bridge construction, placing prestressing wires inside the girder without bonding.
    • Mid-1950s: The US building post-tensioning industry emerged, driven by lift slab construction, with mid-1950s buildings utilizing non-prestressed slabs.

    Basic Principle of Prestressing

    • Concrete has zero tensile capacity, meaning it cannot carry its own weight without external support.
    • Prestressing applies an initial compressive stress to counteract tensile stresses from external loads.
    • This initial stress increases the concrete's overall strength and durability.

    Reinforced Concrete

    • Concrete's strength lies in its compressive properties.
    • Concrete is weak in tension.
    • Reinforced concrete uses steel to resist tension.
    • Steel's tensile strength is utilized, while concrete resists compression.

    Review of Principles of Reinforced Concrete

    • Unloaded Simply Supported Beam: A basic beam structure without applied load.
    • Loaded Simply Supported Beam: A beam with loads applied, showcasing compression and tension forces.

    Introduction of Prestressed Concrete

    • Prestressed concrete introduces initial internal stresses to counteract external loads.
    • These internal stresses are introduced by tensioning steel reinforcement.
    • The initial stress is introduced before the concrete sets, creating compression within the concrete.

    Advantages of Prestressed Concrete

    • Smaller Section Sizes: The whole concrete section is used, leading to a stiffer structure with a larger moment of area, resulting in less deflection.
    • Increased Spans: The smaller cross-section reduces the self-weight, allowing for longer spans compared to ordinary reinforced concrete.
    • Durability: No cracking on the concrete occurs, minimizing penetration of the cover.

    Methods of Prestressing

    • Pre-tensioning: Steel strands are stressed before the concrete is cast.
    • Post-tensioning: Steel strands are stressed after the concrete is cast.

    Pre-tensioned vs. Post-tensioned Prestressed Concrete

    • Pre-tensioned: Tendons are typically placed in steel beds, usually straight, or "harped" in segments. They bond directly to the concrete, and the prestress is transferred as the concrete cures. Precast components are common.
    • Post-tensioned: Tendons are coated and draped in a parabolic profile within the concrete forms. These are typically unstressed. Sheathing within plastic ensures these tendons are contained, and they are post-stressed. Cast in place members are typical.

    Pre-tensioned Prestressed Concrete

    • Tendons are typically placed in steel beds, then stressed prior to concrete placement.
    • Steel is usually straight or "harped" (formed into segments) within the tendons.
    • Tendons bond directly with concrete.

    Post-tensioned Prestressed Concrete

    • Tendons are draped parabolically in the forms—and thus are unstressed.
    • Tendons are coated in grease and housed in plastic sheathing, thus not bonding with the concrete during initial placement.
    • Prestress is applied after concrete placement and hardening.

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    Related Documents

    PPT-Introduction to Prestressed Concrete PDF

    Description

    This quiz covers the foundational aspects of prestressed concrete design, including its historical development and key principles. Explore the evolution of prestressing techniques and their applications in modern construction. Perfect for students in the CEPC0510 course.

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