6. Once the transfer strength has achieved, stressing begins and this is the stage you can assess whether the beam is healthy. The force required is huge, hundreds of tonnes, safety measures have to be observed. As the load is applied, elongation is measured. This elongation is compared with the theoretical elongation done before hand from stress-strain relationship, you have to know the cable and concrete properties.
If the measurement exceeds the theoretical value, it means some wires have broken, perhaps during installation and handling. This can be solved by applying higher load to other wires to compensate the defective wires or Re-check of overall performance by neglecting these wires. That is why your maximum design stress shall not go beyond the allowable elastic stress to allow such deviation, where the steel will not enter the plastic stage, even you over-stress a bit.
If the measurement is less, cement grout may have leaked into the duct cementing the cable. De-stress and re-stress to see whether the cement grout can be broken. However, if large leak occurs, for example, only half the value being measured and cannot break through de-stress and re-stress, then it is a serious problem and the beam may have to be abandoned. The leak position can be roughly estimated from the measured elongation. Perhaps, nowadays you can use the ultra-sound equipment to detectthe leak. But how to repair is still a big question. To abandon such hug beam is also another problem, likely have to break into pieces.
If the beam bows sideways, then the cable is either not placed centrally or dis-located during concreting. Lifting and transportation then becomes dangerous besides creating secondary stress to the bridge deck.
If the beam does not bow upwards, settlement has occurred at the mid-section during concreting.
If the edges crack, the steel base induces too much stress to the corner edges as the beam shortens during stressing. Moveable edge bases should be used.
No comments:
Post a Comment