PCD

• Prestressing wire is a single unit made of steel.
• A bundle of several strands forms a tendon
• Strands- Two, three or seven wires are wound to form a prestressing strand
• Tendons- A group of strands or wire are wound to form a prestressing tendon.
• When you call a tendon a bonded tendon? • When there is adequate bond between the prestressing tendon and concrete
• How is grouted post tension tendons done? • If there is concrete all around the tendon
• When will you consider a tendon is unbonded • -When there is no bond between the prestressing tendon and concrete & when grout is not applied after post-tensioning, the tendon is an unbonded tendon
• What are the stages of loading - Initial (a) During tensioning of steel (b) At transfer of prestress to concrete - Intermediate - Final (a) At service (b) At ultimate
• During tensioning of steel - the stress is transferred to the stee
• ) At transfer of prestress to concrete - from the steel the stress is transferred to the concrete
• Differentiate the sub two stages in the final stage(at service & at ultimate) - At service - during operation - At ultimate - during extreme event where the member can reach the final strength
• Advantages of prestressing of concrete over traditional reinforced concrete

  • Remains uncracked under service loads
  • Reduction of steel corrosion
  • Increase in durability
  • Full section is utilized
  • Increase in shear capacity
  • Suitable to be used in pressure vessels, liquid retaining structures
  • Improved performance (resilience) under dynamic and fatigue loading
  • High span-to-depth ratios
  • Larger spans possible with prestressing (bridges, buildings with large column-free spaces)
  • For the same span, less depth compared to RC member
  • Reduction in self-weight
  • More aesthetic appeal due to slender sections
  • More economical sections
  • Suitable for precast construction
  • Rapid construction
  • Better quality control
  • Reduced maintenance
  • Suitable for repetitive construction
  • Multiple use of formwork (reduction of formwork)
  • Availability of standard shapes
• What are the limitations of prestressing - Skilled technology (hence, not as common as reinforced concrete) - Use of high strength materials is costly - There is additional cost in auxilliary equipments - Need for quality control and inspection
• Mechanical - the devices includes weights with or without lever transmission, geared tranmission in conjunction with pulley blocks screw jacks with or without gear drives and wire-winding machines. This type of prestressing is adopted for mass scale production
• Hydraulic - simplest type of prestressing, producing large prestressing force. The hydraulic jack used for the tensioning of tendons, comprises of calibrated pressure gauges which direclty indicate the magnitued of force developed during the tensioning.
• ) Electrical - the steel wires are electrically heated and anchored before placing concrete in the moulds. This type of prestressing is also known as thermo-electric prestressing
• Chemical - expansive cements are used and the degree of expansion is controlled by varying the curing conditions. The expansive action of cement is restrained while setting. This generates tensile forces in the tendons and compressive stresses in concrete.
• four sources of prestressing force 1. mechanical 2. hydraulic 3. Electrical 4. chemical
• Enumerate the types of prestressing 1) Source of prestressing force 2) External or Internal Prestressing 3) Pre-tensioning or post-tensioning 4) Linear or circular prestressing 5) Full, limited or partial prestressing 6) Uniaxial, biaxial or multi-axial prestressing
• Pre-tensioning - the tension is applied to the tendons before casting of the concrete. The pre-compression is transmitted from steel to concrete through bond over the transmission length near the ends
• Post tensioning - the tension is applied to the tendons (located in a duct) after hardening of the concrete. The pre-compression is transmitted from steel to concrete by the anchorage device (at the end blocks).
• Linear prestressing - when prestressed members are straight or flat in direction of prestressing (based on the direction)
• Circular prestressing - when prestressed members are curved in the direction of
• Full prestressing - when the level of prestressing is such that no tensile stress is allowance in concrete under service loads.
• Limited prestressing - when the level of prestressing is such that the tensile stress under service loads is within the cracking stress of concrete.
• Partial prestressing - when the level of prestressing is such that under tensile stresses due to service loads, the crack width is within the allowable limit.