Fabrication Plant Operations and Prestressing Procedures
Domain 4 covers what happens in the plant: bed layout, tensioning, concrete placement, detensioning, post-tensioning, and grouting. It rewards knowledge of hold points and documented measurements.
Pretensioning operations
Strands are placed to the shop drawing pattern and tensioned with a calibrated jack. Force is verified by gauge pressure and confirmed by elongation, typically within ±5% of the specified force. Concrete is placed and cured; detensioning occurs only after f'ci is verified by field-cured cylinders. Release is slow and symmetrical, in the sequence shown on the drawings. Flame cutting is common but must not overheat or nick the strand.
Post-tensioning operations
Ducts, anchorages, and confinement steel are set before placement; check profile, tendon count, and cover. Stressing occurs after the specified concrete strength is reached. Each tendon gets a pressure and elongation record. Grout promptly after acceptance, using vents and the specified grout.
Worked example
Calculated elongation is 6.3 in. The typical ±7% tolerance gives 5.86–6.74 in. A measured 6.6 in. is accepted and recorded. A measured 5.6 in. is not accepted; the inspector stops stressing, checks the gauge calibration, looks for a blocked or kinked duct, and requires re-evaluation before grouting.
Traps
- Stressing before f'ci is verified.
- Detensioning in a random sequence, causing cracking or erratic camber.
- Using a jack or gauge past its calibration date.
- Grouting without functioning vents, leaving voids at high points.