Prestressed Concrete Prestressed Concrete I. *Unfavorable effects induced from the presence of...

PrestressedPrestressed Concrete ConcreteI.I. *Unfavorable effects induced from the presence of cracking in *Unfavorable effects induced from the presence of cracking in

reinforced concrete:reinforced concrete:

1.1. To expose the reinforcement to corrosion; To decrease bending To expose the reinforcement to corrosion; To decrease bending stiffness; To reduce shear strength; To increase deflection; To rstiffness; To reduce shear strength; To increase deflection; To reduce watertightness; To detract from the appearance of the steduce watertightness; To detract from the appearance of the structureructure

II. *Prestressing is a technique of introducing stresses of predeterII. *Prestressing is a technique of introducing stresses of predetermined magnitude into a structural member to improve its behamined magnitude into a structural member to improve its behavior.vior.

III. *Purpose of prestressingIII. *Purpose of prestressing

1.1. To eliminate cracking in reinforced concreteTo eliminate cracking in reinforced concrete

2.2. To reduce deflectionsTo reduce deflections

3.3. To increase shear strengthTo increase shear strength

11 Tuesday, April 18, 2023Tuesday, April 18, 2023

Prestressed ConcretePrestressed ConcreteIV. *Methods of prestressingIV. *Methods of prestressing

1.1. by tensioned wires fixed at their ends (post-tensioning)by tensioned wires fixed at their ends (post-tensioning)

2.2. by tensioned wires gripped by the concrete (pretensioning)by tensioned wires gripped by the concrete (pretensioning)

3.3. by applying an external load (by applying an external load (external prestressingexternal prestressing))

V. *Pre-tensioningV. *Pre-tensioning

1.1. The method in which the wires are stretched before concreting is The method in which the wires are stretched before concreting is referred to as pretensioning.referred to as pretensioning.

2.2. The grip of concrete on wires increases slightly when the wires are The grip of concrete on wires increases slightly when the wires are released from the stretching device due to a slight shortening and released from the stretching device due to a slight shortening and swelling of the wires. swelling of the wires.

VI. *Post-tensioning VI. *Post-tensioning

1.1. ProcessProcess

1)1) Cast the concrete in which a sheath or other wrapping is preembeCast the concrete in which a sheath or other wrapping is preembeded which allows the wires to move freely during stressingded which allows the wires to move freely during stressing


Prestressed ConcretePrestressed ConcreteV. Post-tensioning V. Post-tensioning

1)1) Anchor the wires or tendons at the ends of the beam by fixing tAnchor the wires or tendons at the ends of the beam by fixing to an anchor plate, or other devices such as wedging with concreo an anchor plate, or other devices such as wedging with concrete coneste cones

2)2) Stress the wires or tendonsStress the wires or tendons3)3) Grout the sheath Grout the sheath 4)4) Disadvantage: the wires must be protected from corrosion by forcing Disadvantage: the wires must be protected from corrosion by forcing

cement grout into the sheath. cement grout into the sheath. 2.2. Use of grout in post-tensioning: Use of grout in post-tensioning: 1)1) Protect wires from corrosionProtect wires from corrosion2)2) Provide a bond between the wires and the sheath and thus with the cProvide a bond between the wires and the sheath and thus with the c

oncreteoncrete3)3) Supplement the resistance of the wires to slip, without which the secuSupplement the resistance of the wires to slip, without which the secu

rity of the wires would depend entirely on the permanence of the end rity of the wires would depend entirely on the permanence of the end anchoragesanchorages


Prestressed ConcretePrestressed ConcreteVII. Applying external load: This method entails prestressing an arcVII. Applying external load: This method entails prestressing an arc

h slab or arch rib spanning between two unyielding abutments. h slab or arch rib spanning between two unyielding abutments. A loaded column can be regarded as being prestressed against sA loaded column can be regarded as being prestressed against stresses due to bending moments, as also can an arch if it is uniftresses due to bending moments, as also can an arch if it is uniformly compressed by a large dead load.ormly compressed by a large dead load.

VIII. Usage and Advantages of PCVIII. Usage and Advantages of PC

1.1. Examples of PC members: sleepers, hollow beams, concrete pilExamples of PC members: sleepers, hollow beams, concrete piles of precast blocks, cylindrical water tankses of precast blocks, cylindrical water tanks

2.2. The steels are mostly in the shape of wires or strands but sometThe steels are mostly in the shape of wires or strands but sometimes as bars.imes as bars.

3.3. The prestressed concrete increases the shear strength and reduThe prestressed concrete increases the shear strength and reduces radically both the deflections and the tensile cracks at services radically both the deflections and the tensile cracks at service loads in such structures and thereby enables these high-strence loads in such structures and thereby enables these high-strength materials to be used effectively. gth materials to be used effectively.


Prestressed ConcretePrestressed Concrete4.4. Prestressed concrete is particularly suited to prefabrication on Prestressed concrete is particularly suited to prefabrication on

a mass-production basisa mass-production basis

5.5. Prestressing increases the ultimate moment of resistance of the Prestressing increases the ultimate moment of resistance of the concrete and the resistance to shear forces and in some cases reconcrete and the resistance to shear forces and in some cases reduces the amount of steel required to resist shear.duces the amount of steel required to resist shear.

6.6. The concrete and the steel are strictly tested during the prestreThe concrete and the steel are strictly tested during the prestressing operation, and a lower factor of safety is justified.ssing operation, and a lower factor of safety is justified.

IX.IX. * *Disadvantages of PCDisadvantages of PC

1.1. Because of the large forces carried by the tendons, the concrete Because of the large forces carried by the tendons, the concrete is vulnerable to crushing at points where tendons / wires are anis vulnerable to crushing at points where tendons / wires are anchored or change direction.chored or change direction.

2.2. The high stresses in PC increase the axial and bending deformaThe high stresses in PC increase the axial and bending deformation caused by creep.tion caused by creep.


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