Low Strand Elongation in a Pretensioning Bed
At a long-line pretensioning plant, a 15.2 mm seven-wire low-relaxation strand (nominal area 140 mm², certified Ep = 195 GPa, fpu = 1,860 MPa) is tensioned in a 91.4 m bed to 0.75fpu, or 195.3 kN. The jack gauge was last calibrated 13 months ago; the plant's annual calibration is due. Bed temperature is 6°C, while the gauge certificate is referenced to 21°C. Strand No. 14 records 617 mm elongation; adjacent strands record 650 mm and 655 mm. The strand certificate lists Ep = 195 GPa, and the wedges are new. The operator wants to accept the strand because the gauge pressure 'looks right,' but the inspector asks for a check before transfer. No strand slip marks are visible, and the dead-end chuck appears seated.
1. Calculate the theoretical elongation and determine whether the measured value is outside a normal acceptance band after temperature correction.
Read the worked answer
Using δ = PL/(AE): P = 195,300 N, L = 91,400 mm, A = 140 mm², E = 195,000 MPa. δ = (195,300 × 91,400)/(140 × 195,000) = 653.9 mm. The measured 617 mm was taken at 6°C, so correct to the 21°C basis: Δδ = αLΔT = 11.7×10⁻⁶ × 91,400 × 15 = 16.0 mm. Corrected measured elongation = 617 + 16 = 633.0 mm. Difference = 653.9 − 633.0 = 20.9 mm, or 3.2% low. That is inside the commonly specified ±5% elongation band, but the band is only meaningful if the jacking force is verified by a calibrated gauge and there is no anchor slip. The reading alone does not justify rejection, but it does require the calibration and slip checks before transfer.
2. What checks distinguish a gauge/calibration problem from strand slip or seating at the anchorage?
Read the worked answer
First verify the force measurement chain: current calibration of the jack gauge, master gauge or load cell, pump relief, and gauge certificate basis. A gauge that is 13 months old is outside the plant's annual interval, so its pressure may not represent 195.3 kN. Second, mark the strand at the dead end and live end and re-stress; observe whether the mark moves relative to the chuck. Third, compare pull-in or wedge seating on the low strand with adjacent strands; abnormal seating reduces measured elongation while the gauge still shows full pressure. Fourth, verify strand certificate area and modulus and measure actual diameter; a smaller effective area or lower modulus would increase, not decrease, calculated elongation. Fifth, check free length and bed abutment movement. If the calibrated force is correct and elongation is within ±5% after correction, accept; if the mark moves or force is low, re-tension or replace the wedge/strand.
3. State the acceptance decision and the hold points before bed transfer.
Read the worked answer
Do not transfer the bed yet. The expired gauge is a hold point: recalibrate the gauge or use a calibrated master gauge/load cell to confirm the force in strand No. 14. Re-measure elongation and check anchor marks. If force is confirmed at 195.3 kN and corrected elongation remains within 5%, accept the strand and document the temperature correction, calibration verification, and adjacent-strand comparison in an NCR or inspection report. If force is low or slip is evident, re-tension the strand, replace damaged wedges, and re-measure. Transfer only after the engineer/inspector releases the hold. The 3.2% corrected difference is not, by itself, a rejectable elongation, but the calibration nonconformance must be closed first.