Reason behind difference between measured field elongation & theoretical elongation for Pre-stressed concrete girder:
- Incorrect number of strands placed in the tendon.
- Excessive wobble of ducts increase friction & decrease elongation.
- Usually smooth duct placement reduces friction & increase elongation.
- Even, layered strand placement reduces friction & increase elongation, practically when strands are pushed into the duct.
- A pressure cell does not detect a change in jack efficiency. This may cause faulty reading.
- Elongation calculations may be wrong due to a) incorrect modulus of elasticity (E) or are of strand (A) b) Incorrect or varying tendon lengths due to skew or sharp radii c) Different co-efficient of friction between girders on sharply curved structures d) different tendon paths in girder
- Incorrect method of measuring elongations.
- Slippage of strand during stressing, especially if the strand area is small (below 0.151 in2 or 97.4 mm2.
- Gage damaged or indicator not zeroed.
The cause of any inconsistent elongations among the tendons of a structure must be determined as soon as possible. Do not cut off excess strand until the problem is resolved. In the event it is necessary to de-tension a tendon, stressing contractor must have suitable equipment available for this purpose. The contractor’s de-tensioning plan must be acceptable to the Engineer. It is recommended that the Engineer discuss the de-tensioning procedure with the pre-stressed concrete committee.
When a frame is stressed from two ends, first end stressing results should be compared to theoretical first end calculations. Including the first and second results will usually make any discrepancies less apparent, because second end results tend to offset some of the difference.
As a general practice, strands should not be cut off until all tendons in the structure are fully stressed.
However, due to a variety of different reasons, field measured elongation can differ by as much as 5% to 10%, even with the updated coefficients. This is accepted as long as the variations are understood and explained; but deviations between elongations of similar tendons of the same structure should not vary more than (+ 4%). Remember each case must be carefully examined to ensure compliance with the working force required.