Methodology for Pre-stressing of HTS Strands

This item of work deals with the activity of Pre-stressing of H.T.S strands of PSC girders with approved materials as per the specifications of the Contract Agreement.

General

This item of work shall be carried out in compliance with Clause 1800 of MoRTH Specifications. The materials shall conform to Section 1000 and the grout mix shall conform to Section 1808 of MoRTH Specifications.

Procedure

1. Cable Fabrication & Laying

• The length of cable required shall be computed from the drawing including allowances for profiling and gripping length of jack.
• The strand shall be cut evenly using an abrasive cutter to the required length.
• Both ends of the cable shall be numbered to maintain correct configuration in the bearing plate and prevent twisting in the duct.
• The duct shall be profiled as per approved ordinates and cable shall be threaded manually into the duct.
• Grout vents shall be provided wherever required.
• Tube ducts of the anchorage shall be fixed with end blocks as shown in drawings.
• The ducts shall be connected to tube units.
• The cable consisting of the specified number of strands shall be tied with binding wire at regular intervals and threaded into sheathing.
• Both ends of the duct shall be closed after strand installation and protruding strands shall be wrapped properly to avoid dirt entry during concreting.

2. Concreting

• During concreting, cables shall be moved manually at suitable intervals to avoid concrete blockage inside ducts.
• The strands beyond the bearing plate shall be cleaned using petrol after concreting to remove dust and rust.

3. Stressing Operation

• Stressing of cables may be carried out from both ends.
• Bearing plates shall be cleaned and fixed in proper configuration.
• Wedges shall be fixed to bearing plates and tightened.
• Jacks shall be mounted properly and aligned with the cable.
• Master grips of the jack shall be inserted over strands and fixed into pulling plates.
• Stressing pumps shall be connected to jacks through hose pipes.
• Correction factor shall be calculated from pressure and elongation readings.
• Initial pressure of 50 kg/cm² shall be applied to remove cable slackness.
• Pressure shall then be increased gradually and elongation readings recorded.
• Tensioning force shall be verified by pressure gauge readings and elongation comparison.

Acceptance Criteria During Stressing

• If calculated elongation is achieved before specified pressure, stressing shall continue till specified pressure is reached provided elongation does not exceed 1.05 times calculated elongation.
• If specified pressure is reached before calculated elongation, stressing may continue in intervals of 5 kg/cm² provided pressure does not exceed 1.05 times specified pressure.
• If elongation remains less than 0.95 times calculated elongation at 1.05 times specified pressure:
  • Check functioning of jack, pump and hose pipes.
  • Detension the cable and check for duct blockage.
• On achieving full load, wedges shall be locked hydraulically.
• Jack shall be released and retracted.
• The same procedure shall be repeated for all cables.

4. Grouting

• All protruding ends of strands shall be cut after prestressing.
• Nozzles and valves shall be fitted to grout holes.
• Ducts shall be flushed with lime water before grouting.
• Grout shall be prepared using Ordinary Portland Cement and non-shrink grout admixture.
• Grouting shall commence with low pressure until grout flows out from the other end.
• The outlet shall be closed once grout consistency becomes uniform.
• After achieving desired pressure, inlet valve shall be closed.

Equipment / Machinery

• Hydraulic Jack
• Pressure Gauge
• Compressor
• Grouting Equipment
• Grout Cube Moulds

Tolerances