Code of Practice for Prestressed Concrete
1980 Edition

IS 1343 specifies the following for cement and admixtures in prestressed concrete:

• Cement Types:

  • Ordinary Portland Cement (OPC) conforming to IS 269 (33 grade) or IS 8112 (43 grade).
  • Portland Pozzolana Cement (PPC) as per IS 1489 (Parts 1 & 2).
  • Portland Slag Cement (PSC) following IS 455.
  • High Alumina Cement, if specified, complying with IS 6452.

• Admixtures:

  • Chemical and mineral admixtures adhering to IS 9103 standards to enhance workability and durability.
  • Common admixtures include water reducers, retarders, and superplasticizers.
  • Admixtures should not impair the bond between prestressing steel and concrete or affect durability negatively.

• Durability and Mix Design:

  • Cement content typically limited to 450 kg/m³ to minimize shrinkage and cracking risks.
  • Durability clauses of IS 456 must be followed according to exposure conditions.

This ensures the production of prestressed concrete with high strength, durability, and optimal bonding characteristics.

IS 1343 provides comprehensive guidelines on corrosion protection of post-tensioned prestressing steel:

• General Requirements (Clause 12.1.5): Anchorages, steel sheathing ducts, and prestressing steel must be adequately protected from corrosion during construction and service.

• For Unbonded Steel (Clause 12.1.5.1): Protection approaches include hot-dip coatings with zinc or zinc-aluminum alloys, use of non-corrosive sheaths such as HDPE or FRP, epoxy coatings, and extruded sheaths filled with corrosion inhibitors like grease or wax. Replaceable corrosion inhibitors with monitoring provisions may also be employed. External anchorages and cables should be enclosed within protective casings.

• For Bonded Steel (Clause 12.1.5.3): Primary protection is provided by dense, alkaline concrete cover and high-quality cement grout. Additional measures include coated steel (zinc, zinc-aluminum, epoxy) and impervious sheathing. Effective bonding between sheathing, grout, and concrete is crucial. External components require protective casing.

• During Construction (Clause 12.1.5.4): Steel and sheathing must be stored and handled with corrosion-preventive treatments such as oil films or passivating agents. If exposure exceeds two weeks, periodic flushing with oil-water mixtures every three months is recommended. Permanent corrosion protection, such as pressure grouting, should be applied within one week after prestressing.

These layered corrosion prevention methods ensure long-term durability of post-tensioned prestressed concrete elements.

IS 1343 outlines the following quality assurance requirements for prestressed concrete construction:

1. General Principles (Clauses 10.1.1 & 10.1.2): Ensure the completed structure's properties align with design assumptions by using suitable materials, skilled workmanship, and maintaining quality during the service life. Quality assurance encompasses both inputs (materials, workmanship, equipment) and outputs (placed concrete). Each concreting stage must be inspected before progressing.

2. Quality Assurance Plan (Clause 10.1.3): Each stakeholder, including contractors, suppliers, and subcontractors, must develop a QA plan integrated within the overall project QA framework. This plan should define responsibilities, control and inspection procedures, and document all activities.

3. Documentation: This includes material test reports and manufacturer certifications, concrete mix design data, pour cards for concrete placement authorization, site inspection records and field tests, reports on non-conformance and change orders, quality control charts, and statistical quality analyses.

4. Special Emphasis: Use of quality control charts is recommended for continuous monitoring of concrete production.

This comprehensive system ensures traceability, responsibility, and consistent quality management throughout prestressed concrete construction.

IS 1343 addresses loss of prestress from slip and relaxation as follows:

• Slip Loss (Clause 19.5.2.5): Occurs due to movement of wires at the anchorage or strain within anchorage zones. This loss is explicitly accounted for in design and is generally a small initial decrease occurring immediately after tensioning. Slip values are usually obtained from tests or manufacturer data.

• Relaxation Loss (Clause 19.5.2.3): Defined as the decrease in stress in prestressing steel under sustained strain over time, influenced by steel grade, initial stress level, and duration. IS 1343 recommends adopting standard relaxation values derived from experimental data or literature.

• The total prestress loss considered in design also includes creep and shrinkage of concrete.

Design Procedure:

1. Calculate initial prestressing force.
2. Subtract losses due to anchorage slip, relaxation, creep, and shrinkage.
3. Use the effective prestressing force for serviceability and strength evaluations.

This method ensures accurate accounting of reductions in prestress during the service life of the structure.

IS 1343 prescribes detailed testing methods for prestressing steel and grouting materials:

1. Prestressing Steel Tensile Test (Clause 1.2):

   • Apply tensile stress in incremental steps at approximately 20%, 40%, 60%, and 80% of the characteristic tensile strength (f_p).
   • Increase the load at around 100 MPa per minute.
   • Maintain the load at 80% of f_p for one hour.
   • Then increase the load gradually until failure.
   • For prestressing systems where locking occurs after stressing, adapt the procedure to simulate actual tendon geometry and anchorage conditions.

2. Fatigue Test (Clause 2.2):

   • Utilize a tensile testing machine equipped with a pulsator operating at up to 500 cycles per minute.
   • The upper load level is constant at 65% of f_p.
   • Maintain a constant force range (difference between maximum and minimum forces).
   • Ensure uniform force distribution and avoid secondary oscillations.
   • Use specialized devices for grouted tendons to avoid fretting damage.

3. Acceptance Tests (Clause 2.5):

   • Determine modulus of elasticity, relaxation loss after 1000 hours at 20°C, ultimate tensile strength, and stress-strain curves.
   • Ensure modulus of elasticity variation does not exceed 2.5% between maximum and minimum values.

4. Specimen Requirements (Clause 3.0):

   • Minimum free length of specimens must be 3 meters.
   • If multiple grades are used with the same anchorage, test the highest strength grade.

These procedures guarantee that prestressing steel and grout materials meet the necessary performance standards for safe and durable prestressed concrete constructions.