Tendon Elongation Outside Tolerance on a Post-Tensioned Flat Plate
On a post-tensioned flat plate, a 24 m long unbonded tendon made from 15.2 mm diameter, 1860 MPa low-relaxation strand is stressed from one end. The approved stressing schedule gives a jacking force of 208 kN and a computed elongation of 165 mm after allowance for wobble and anchor set. The jack gauge reads the correct pressure, but the measured elongation is 142 mm. The jack calibration certificate is 11 weeks old, and the gauge was zeroed before stressing. The tendon profile is a shallow parabola with two high points. The strand was installed three weeks ago and is dry. The contractor wants to accept the tendon because the gauge pressure is correct and proposes to cut the tails. Another tendon on the same pour measured 168 mm. The inspector must decide whether this tendon can be accepted and whether the discrepancy points to a strand problem or a jacking problem.
1. Evaluate the measured elongation against the usual acceptance tolerance. Can the tendon be accepted on the gauge pressure alone?
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The usual field tolerance for tendon elongation is about ±7% of the computed elongation, unless the project specifies otherwise. Seven percent of 165 mm is 11.6 mm, giving an acceptable range of roughly 153.4–176.6 mm. The measured 142 mm is about 23 mm short, or roughly 14% low, so it is outside the tolerance. The 168 mm reading on the adjacent tendon is within tolerance.
The gauge pressure alone is not sufficient evidence that the correct force is in the tendon. Elongation is the primary field check on the actual force because jack friction, gauge error, pump relief, or a miscalibrated jack can make the gauge read correctly while the force is low. Since the tendon is outside tolerance, do not accept it and do not cut the tails. Cutting the tails removes the ability to re-stress or investigate. The tendon should remain unaccepted pending a calibrated load check and a review of the strand and tendon installation.
2. List the likely causes of the low elongation and give a logical diagnostic sequence.
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Likely causes include: jack or gauge calibration error; a load cell or pressure gauge that is not reading true force; jack friction or a pump relief valve issue; strand with the wrong area or modulus; a strand that is hung up, kinked, or restrained in the anchorage; a tendon profile or free length different from the assumed one; higher-than-assumed friction or wobble; anchor seating or wedge slip; and measurement error in the elongation.
Diagnostic sequence: first verify the jack and gauge calibration records and date, then re-stress using a calibrated load cell or a verified master gauge. Perform a lift-off check to determine the actual force in the strand. Check the strand certification for area, grade, and modulus, and verify the free length and tendon profile. Compare with adjacent tendons; a single outlier points to a local strand or anchorage problem, while a systematic pattern points to jack or gauge calibration.
If the measured force at the correct gauge pressure is low, investigate the jacking equipment. If the force is correct but elongation remains low, investigate strand seating, binding, free length, or strand properties. Re-stress only within the code and PTI stress limits, and replace the strand if it cannot be shown to have the required force.
3. What records and acceptance evidence should be required for this tendon and the remaining tendons before the tails are cut?
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Require a complete stressing record for each tendon: tendon identification, jack and gauge identification, calibration date, jacking force, gauge pressure, computed and measured elongation, anchor set, date and time, ambient and concrete temperature, and any unusual observations. Confirm the installer is qualified and that the special inspector witnessed the stressing.
For the low tendon, require the load-cell or lift-off results, the equipment calibration check, the strand certification, and the EOR's written disposition. If the tendon is re-stressed, record the new pressure, force, and elongation. Do not cut tails until the tendon has been accepted by the EOR or the inspector acting within delegated authority.
For the remaining tendons, review the elongation distribution. If other tendons are outside tolerance, or if the same jack or gauge was used, expand the investigation and recalibrate before further stressing. If all others are within tolerance and the low tendon is isolated, document that the issue was localized. Retain all records with the PT installation file.