PRECAST CONSTRUCTION AND PRE-STRESSING

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• Q1: Define Pre-cast members.
  Ans: The members which are constructed in the factory and are assembled at the site are known as pre-cast members.
• Q2: Define Pre-stressing.
  Ans: Pre-loading of a structural member in such a way that stresses introduced due to this preloading are opposite to those that will be induced later on when external loads are applied on that member.
• Q3: Name the methods of Pre-stressing.
  Ans: The two methods of pre-stressing are:

  • ➔ Pre-Tensioning
  • ➔ Post-Tensioning
• Q4: What type of steel is used in pre-stressed concrete?
  Ans: High-strength steel, such as high-tensile steel wires, bars, or strands are used in pre-stressed concrete.
• Q5: Why is high strength concrete needed for pre-stressed concrete?
  Ans: High-strength concrete offers greater resistance to tension, shear, bond, and bearing, and is less susceptible to shrinkage and creep.
• Q6: Define Pre-Tensioning.
  Ans: In Pre-Tensioning, the prestressing force is applied to the steel tendons before the concrete is placed.
• Q7: Define Post-Tensioning.
  Ans: In Post-Tensioning, the prestressing force is applied to the steel tendons after the concrete is placed and cured.
• Q8: What is the ratio of concrete used in foundation?
  Ans: In Foundations, the ratio of concrete used is 1:1.5:3.
• Q9: What are the advantages of precasting?
  Ans: Some of the advantages are:

  • ➔ Improved quality
  • ➔ Faster construction
  • ➔ Reduced on-site labor cost
  • ➔ Overall cost reduction
• Q10: What are some limitations of precasting?
  Ans: Some limitations include:

  • ➔ Transportation cost
  • ➔ Cost of custom design
• Q11: What are the advantages of prestressing?
  Ans: Advantages of prestressing include:

  • ➔ Good quality
  • ➔ High strength
  • ➔ Larger span
  • ➔ Reduced cracking
• Q12: Name some pre-cast members.
  Ans: Common precast members include:

  • ➔ Columns
  • ➔ Beams
  • ➔ Stairs
  • ➔ Floor slabs
• Q13: Name some forms of prestressing steel.
  Ans: Forms of pre-stressing steel include:

  • ➔ Wires
  • ➔ Cables
  • ➔ Strands
  • ➔ Tendons
• Q14: Define Wires.
  Ans: Pre-stressing wire is a single unit made of steel. The diameters of the wires are 2.5, 3.0, 4.0, 5.0, 7.0 and 8.0 mm.
• Q15: Define Strands.
  Ans: Two, three or seven wires are wound together to form a pre-stressing strand.
• Q16: Define Tendons.
  Ans: A group of strands or wires are wound to form a pre-stressing tendon. A tendon is a tensioned element that introduces the prestressing force into the concrete.
• Q17: Define Cable.
  Ans: A group of tendons form a pre-stressing cable. The cables are primarily used in bridges.
• Q18: Define Bars.
  Ans: A tendon can be made up of a single steel bar. The diameter of a bar is much larger than that of a wire. Bars are available in sizes: 10, 12, 16, 20, 22, 25, 28, and 32 mm.
• Q19: Name some industries in Pakistan that do precasting.
  Ans: Some of the industries are:

  • ➔ Izhar Concrete Pvt. Ltd.
  • ➔ Precast Building Systems (Agro Group)
  • ➔ Gammon Pakistan Precast
  • ➔ TEG (Taha Engineering Group)
  • ➔ Shaheen Precast Associates