Ducts are used in post tensioning systems as a conduit for strands. In this method the strands are debonded from concrete and are protected from corrosion too. Ducts are manufactures from galvanized straps as well as HDPE materials. Round ducts have a range of internal diameter from 40-160 mm while round HDPE ducts have diameters in range of 50-140mm. Rectangular galvanized ducts have a width range of 57097mm and HDPE types range from 41 to 90 mm.
HDPE ducts are strong against grout pressure and can tolerate more deformations so they act better against cracking.
Some of the applications of this instrument are:
- Measurement of deformation around tunnels.
- Monitoring rock displacement in metro stations.
- Stability assessment in shafts and mine tunnels.
- Measuring mine roof movement.
Operation and Installation
Several pins are installed around periphery of tunnel by drilling and grouting. The convergence meter is attached from its both ends to the pins and a spring loaded handle pulls the tape to remove the slack. Once the force on tape reaches a preset value, the system locks. Meters and centimeters are read from the tape and millimeters and 1/100 mm is read from the gauge. The same thing repeats in coming days or weeks and rock displacement versus time is plotted. The amount of displacement as well the rate of displacement is used as a guide to judge about the stability of tunnel
- X: Dial Gauge (B) or Digital Gauge (C
- AA: Tape length (15 or 30m