Instruments for measuring loss of prestress in prestressed concrete:
The pre-stressing force applied to a concrete element initially reduces due to various factors such as elastic shortening of the concrete, anchorage losses, and frictional losses. Over time, the pre-stressing force further decreases due to creep, shrinkage, temperature effects, and relaxation of steel, which may result in a reduction in the load carrying capacity of the structure. It is important to note that there is no direct method available to measure the loss of pre-stress. However, it can be estimated by monitoring changes in stress levels in the pre-stressing cables through periodic measurements. The force in the pre-stressing steel can be measured using Vibrating Wire (VW) Load Cells or Vibrating Wire (VW) Strain Gauges.
Vibrating Wire (VW) Load Cell:
The system consists of a ring-shaped housing made of alloy steel, inside which a set of equally spaced vibrating wire gauges are mounted parallel to each other. The studs of the vibrating wire gauges are attached to a diaphragm that serves as the load cell. The load cells are designed with a center hole to accommodate tendons or anchor cables. These load cells are available in a range of capacities, from 100 to 5000 kilonewtons (KN).
The tension of a vibrating wire can be changed by the pressure applied to the diaphragm. The natural frequency of the wire is directly proportional to its tension, which means that by measuring the natural frequency of the wire, we can determine the pressure on the diaphragm. The natural frequency of the wire can be expressed using the equation: f = (T/(2l * m))^0.5, where f is the natural frequency of the wire, l is the length of the wire, T is the tension of the wire, and m is the mass per unit length. The VW Load cell operates by electrically plucking the vibrating wire at its resonance frequency, which varies with the tension in the wire. This induces an alternating current in the coil, which is then detected by the readout unit.
Fig: Vibrating Wire (VW) Load Cell Arrangement
The VW load cell is positioned between two plates at the pre-stressing end of the girder. Its cable is connected to a common readout unit, which typically measures the period or frequency of the wire to obtain the force in the cable. The readout unit can be linked to a PC, and data can be analyzed using custom-made software. An initial reading is taken shortly after the load cell is installed, and subsequent readings are taken at each stage of pre-stressing. However, it may not be possible to measure the force after the cable is cement-grouted, as the duct may be filled with grease or gel. In such cases, the pre-stressing force can be periodically measured during service.
Summary:
The V.W. load cell system is ideal for new construction projects but may not be suitable for existing structures. This is because the V.W. load cell measures the force in pre-stressing wires, and once the cable duct is cement grouted, the force transfer to the load cells may not occur. However, if the cable duct is filled with grease, oil, or other substances, the load cells can still provide accurate force measurements for the pre-stressing steel. Therefore, in systems where the cable duct is cement grouted, the load cell system may not be helpful. On the other hand, the load cell system can be utilized during the pre-stressing process to monitor the force in the pre-stressing steel and prevent overstressing.
Vibrating Wire Strain Gauge:
The Vibrating Wire Strain Gauge is a useful tool for measuring changes in strain over time. By periodically measuring the strain using this gauge, one can obtain information about how the strain is changing. The stress can then be calculated by multiplying the measured strain with the modulus of elasticity, which relates the strain to the stress. Additionally, the pre-stress force, or the force applied to pre-stress cables, can be obtained by multiplying the calculated stress with the cross-sectional area of the pre-stress cables.
Fig: Vibrating Wire Strain Gauge
The V. W. Strain Gauge operates based on the principle of using a tensioned wire that vibrates at a frequency proportional to the tension in the wire when it is plucked. The gauge is designed with a wire held in tension between two end flanges. Changes in force in the concrete result in a change in the distance between the two flanges, which in turn changes the tension of the wire. By applying calibration factors to the frequency measurement, the strain can be calculated.
Vibrating Wire Strain Gauges are of two types i.e.
There are two types of strain gauges: weldable type and embedment type. Weldable type strain gauges are attached to the surface of a structure and do not work when embedded. On the other hand, embedment type strain gauges are designed to be embedded within the structure, making them more suitable for our specific purpose.
VW embedded strain gauge:
VW embedded strain gauges are designed to be embedded in concrete, either before or immediately after placement, to assess changes in strain and subsequently changes in pre-stressing force. These gauges are available in 6 and 10-inch lengths and are known for their superior performance.
For new construction, the strain gauge is embedded in the concrete around the pre-stressing cable during the placement of the concrete. The leads of the gauge are then taken out and connected to a read-out or data logger unit. The datum reading of the gauge is recorded immediately after installation, and subsequent readings are taken at each stage of pre-stressing during construction and at pre-decided intervals during service.
For existing structures, a bore is drilled in the concrete at desired locations, and the strain gauge is installed. After installation, the bore is filled up again, and the leads of the gauges are taken out and connected to a data logger unit. Datum and subsequent readings of strain are taken at pre-decided intervals.