Short Strand Elongation, a Broken Wire, and an Expired Jack Gauge
A bed is stressing twelve 1/2-in, 270-ksi low-relaxation strands to 0.75 fpu (31.0 kips each) for a 100-ft precast girder. The theoretical elongation is 8.5 in. The measured elongation on strand 7 is 8.2 in, about 3.5% short; the jack gauge was calibrated 14 months ago, and the plant procedure requires annual calibration. Ambient strand temperature is 40°F versus the 70°F basis. During stressing, one outer wire on strand 4 breaks at the chuck; the chuck still grips the remaining wires and the load holds. The bed supervisor wants to accept both strands because the gauge was close and only one wire broke. The QC inspector must decide on elongation acceptance, the broken wire, and what records are needed before releasing the bed.
1. Is the 3.5% short elongation acceptable? What checks are needed before deciding?
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A 3.5% short elongation is not automatically acceptable, and the expired gauge makes the force uncertain. The first check is temperature correction. Steel expands about 6.5 x 10^-6 per °F, so a 100-ft strand (1,200 in) at 40°F versus 70°F is about 0.23 in shorter, or roughly 2.7% of the 8.5-in theoretical elongation. Correcting 8.2 in upward by 0.23 in gives about 8.43 in, leaving about 0.8% short, which is much less alarming. The second check is seating slip and chuck take-up, which can account for additional shortfall. The third check is gauge calibration: with a 14-month-old calibration, the inspector cannot defend the jacking force from the gauge alone. Verify the force with a calibrated load cell or recalibrate the gauge before accepting the bed. If the force is verified and the corrected elongation is within the project tolerance, strand 7 may be accepted with a documented explanation. If not, re-stress or replace the strand.
2. What should be done about the broken outer wire and the expired gauge?
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A strand with a broken outer wire should not be accepted as a prestressing tendon. Even though the chuck holds the remaining wires, the strand's area and capacity are reduced, and the break indicates either damaged chuck jaws, improper seating, or a strand defect. The strand should be de-tensioned if practicable, removed, and replaced. The chuck and jaws should be inspected and replaced if worn, and the other strands should be checked for wire breaks or slippage. The expired gauge is a separate nonconformance. Stressing should stop until the gauge is calibrated or a calibrated load cell is used. If other strands were stressed with the same gauge, verify the force on a representative sample or re-stress as required. Do not let the supervisor's 'gauge was close' argument override the need for a calibrated force measurement. Both issues require a nonconformance report and engineering review before bed release.
3. What records and release checks should be completed before the bed is released?
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The release record should include the calculated elongation for each strand, the measured elongation, the temperature at stressing, and any correction applied. It should include the jacking force, gauge identification, calibration date, and load-cell verification if used. Seating slip should be recorded. The broken-wire event on strand 4 should be documented with the replacement strand identity, chuck inspection, and the engineer's acceptance. A nonconformance report should reference the expired gauge and the corrective action. The inspector should verify that all strands have been stressed to the approved force, that no wire breaks remain, and that the bed is safe to release. The record should be signed by the QC inspector and reviewed before the bed is released. If any required check is missing, the bed should remain on hold.