PPT-Introduction to Prestressed Concrete PDF

CEPC0510 PRESTRESSED CONCRETE DESIGN PRE-REQUISITE: Reinforced Concrete Design (CEPC0506)  History of Prestressed Concrete History of Prestressing Jackson of San Francisco applied for The idea of prestressing to counteract...

CEPC0510 PRESTRESSED CONCRETE DESIGN PRE-REQUISITE: Reinforced Concrete Design (CEPC0506)  History of Prestressed Concrete History of Prestressing Jackson of San Francisco applied for The idea of prestressing to counteract a patent for “construction of artificial the stresses due to loads was first put 1886 stone and concrete pavements”, in 1896 forward by the Austrian Engineer which prestress was introduced by Mandl tensioning the reinforcing rods set in sleeves. Dohring of Germany manufactures M Koenan of Germany further slabs and small beams in 1888 using developed the subject in 1907 by 1888 embedded tensioned wires in 1907 reporting on the losses of prestress concrete to avoid cracks. due to elastic shortening of concrete. History of Prestressing Problems were encountered during Dischinger demonstrated the used of unbonded tendons, in the Mid lifting in these early slabs because of 1950s their weight, and large deflections 1928 construction of a major bridge of the developed after construction due to deep girder type, in which flexural creep. prestressing wires we placed inside the girder without any bond. The US building post-tensioning industry owes its existence to lift slab Mid construction. The first lift slab 1950s buildings were built in this country in the mid 1950s using non-prestressed slabs. Basic Principle of Prestressing Basic Principle of Prestressing Application of Compressive Stress: Compressive Compressive Stress Stress The row of books has ZERO tensile capacity. Therefore, the “beam” of books cannot even carry its self-weight. That’s why, we provide an external initial stress (prestress) which compresses the books together. Basic Principle of Prestressing Application of Compressive Stress: Compressive Compressive Stress Stress The row of books has ZERO tensile capacity. Therefore, the “beam” of books cannot even carry its self-weight. That’s why, we provide an external initial stress (prestress) which compresses the books together.  What is Reinforced Concrete? What is Reinforced Concrete? Concrete is strong to compression but weak in tension. Steel is strong in tension. Reinforced concrete uses concrete to resist compression and hold to bars in position and uses steel to resist tension. Tensile strength of concrete is neglected (i.e. zero) Unstressed Beam Deflected Beam Review of Principles of Reinforced Concrete Principles of Reinforced Concrete 1 Unloaded Simply Supported Beam Compression 2 Loaded Simply Supported Beam As Tension  Introduction of Prestressed Concrete Introduction of Prestressed Concrete Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from external loads are counteracted to a desired degree. In reinforced concrete members, the prestress is commonly introduced by tensioning the steel reinforcement. Compressive Compressive Stress Stress 1 Tendons Stressed 2 Tendons apply stress Compressive Compressive Stress Stress Compressive Compressive Stress Stress 3 Prestressed Deflection (upward) 4 Total Deflection (flat)  Advantages of Prestressed Concrete Advantages of Prestressed Concrete Smaller Section Sizes Since Prestressed Concrete uses the whole concrete section, the second moment of area is bigger and so the section is stiffer. Compression Compression As Tension As Reinforced Concrete Prestressed Concrete Smaller Deflections The larger the second moment of area greatly reduces defections for a given size. Increased Spans The smaller section size reduces self-weight. Hence a given section can span further with prestressed concrete than it can with ordinary reinforced concrete. Durability Since the entire section remains in compression, no cracking of the concrete can occur and hence there is little penetration of the cover Methods of Prestressing Methods of Prestressing There are two methods of prestressing: Pre-tensioning: Apply prestress to steel strands before casting. Post-tensioning: Apply prestress to steel strands after casting. Pre-tensioned Post-tensioned Pre-tensioned vs. Post-tensioned Prestressed Concrete Prestressed concrete is the umbrella term that describes both pre-tensioned and post- tensioned concrete. The “pre” and “post” prefixes describe when the prestressing tendons are stressed (pulled or jacked) relative to the concrete being placed and cured. Pre-tensioned Prestressed Concrete In pre-tensioned concrete the tendons are typically placed in steel bed forms and stressed prior to the concrete being placed. The prestressing steel is usually straight, or “harped” in straight segments. These tendons are bare and bond directly to the concrete. Once the concrete is cured the tendons are released from the bed and the tension force is transferred through bond into the concrete, compressing the concrete. Pretensioned concrete is almost always associated with precast concrete, or concrete cast in beds within a pre-casting plant. The most typical pre tensioned structural members are single tees, double tees, beams, girders and planks. Post-tensioned Prestressed Concrete In post tensioned concrete the tendons are typically draped in a parabolic profile in the concrete forms, unstressed. These tendons are coated in grease within plastic sheathing so that they won’t bond to the concrete and can slide within the sheathing when they are stressed after the concrete had been placed and hardened. Post-tensioned concrete is almost always associated with cast in place concrete, or concrete cast at the actual site in its final location. The most typical post-tensioned structural members are one-way slabs, two-way slabs, beams and girders