Instruments for measuring loss of prestress in prestressed concrete:
The initial pre-stressing force applied to the concrete element reduces due to elastic shortening of the concrete, anchorage losses, frictional losses etc. The pre-stressing force further reduces with the passage of time due to creep, shrinkage, temperature effect and relaxation of steel. The loss of pre-stressing force may reduce load carrying capacity of the structure.
It will be worthwhile to note that no direct method is available to measure loss of pre-stress. However, it can be ascertain by knowing the changes in stress levels in pre-stressing cables. The change in stress level can be known by periodical measurement of the same. The force in pre-stressing steel can be measured by using Vibrating Wire (VW)Load Cells or Vibrating Wire (VW) Strain Gauges.
Vibrating Wire (VW) Load Cell:
It consist of a set of vibrating wire gauges mounted parallel to each other equally spaced in a ring in an alloy steel cylindrical housing. The studs of vibrating wire are attached with diaphragm of load cell. The load cells are manufactured with a centre hole to accommodate tendons or anchor cables. These load cells are available in various ranges from 100 to 5000 KN.
The change in pressure on the diaphragm will change the tension of vibrating wire. The natural frequency of wire is proportional to the tension of the wire. Thus, by measuring the natural frequency of wire pressure on diaphragm can be known. Natural frequency of the wire can be co-related with the diaphragm pressure. The natural frequency of wire can be expressed as below:
where f = Natural frequency of the wire
l = Length of the wire
T = Tension of wire
m = Mass/unit length
The operation of VW Load cell is based on electrical plucking of the vibrating wire. The wire, vibrating at its resonance frequency, which varies with the tension in the wire, induces an alternative current in the coil which is detected by the read out unit.
Fig: Vibrating Wire (VW) Load Cell Arrangement
The VW load cell, sandwiched between two plates, is installed at the pre-stressing end of the girder. The cable of load cell is connected to common read out unit. Read out units generally measures the period or frequency of the wire from which force in cable can be obtained.
The read out unit can be connected with P.C. and data can be analysed using custom-made software for this purpose. Initial reading is taken soon after the load cell is installed. Subsequent readings are taken at each stage of pre-stressing. However, after cement grouting of cable it may not be possible to measure the force. If the cable duct is filled with grease or gel, the pre-stressing force can be measured periodically during service.
Summary: From the above, it can be noted that V.W. load cell system is suitable for new construction and not for existing structure. V. W. load cell measures the force in pre-stressing wire. Once the cable duct is cement grouted the force transfer to load cells does not take place.
However, if the cable duct is filled with grease, oil etc. load cell continue to give the force in pre-stressing steel. Hence, in the present system where cable duct is cement grouted load cell system will not help. Load cell system can also be used to monitor the force in pre stressing steel, at the time of pre-stressing, to avoid overstressing of the same.
Vibrating Wire Strain Gauge:
With the help of Vibrating Wire Strain Gauge, the change in strain can be obtained with periodical measurement of strain. The stress can be calculated by multiplying strain with modulus of elasticity. The pre-stress force can be obtained by multiplying stress with cross sectional area of pre-stress cables.
Fig: Vibrating Wire Strain Gauge
V. W. Strain Gauge operates on the principle that a tensioned wire, when plucked, vibrates at a frequency that is proportional to the tension in the wire. The gauge is constructed in such a way that a wire is held in tension between two end flanges. Force in concrete changes the distance between its two flanges and results in a change in the tension of the wire. Strain is then calculated by applying calibration factors to the frequency measurement.
Vibrating Wire Strain Gauges are of two types i.e.
i) weldable type and
ii) embedment type.
Weldable type strain gauges are fixed on the surface of the structure. They do not function in embedded state. Therefore, embedment type strain gauges are more suited for our purpose.
VW embedded strain gauge:
VW embedded strain gauge is embedded in concrete nearer to pre-stressing cable to assess change in strain and subsequently change in pre-stressing force. The gauge may be installed either before or immediately after placement of the concrete. Their unique design provides superior performance. These are available in 6 and 10-inch lengths.
For new construction the strain gauge is embedded in concrete around pre-stressing cable during placement of concrete. The leads of the gauge are taken out and connected to read out or data logger unit. The datum reading of the gauge is recorded immediately after installation of gauge. Subsequent readings are taken at each stage of pre-stressing during construction and at pre-decided interval during service.
For existing structure, bore is drilled in concrete at desired locations and strain gauge is installed. After Installation of strain gauge, the bore is again filled up. The leads of the gauges are taken out and connected to data logger unit. Datum and subsequent reading of strain are taken at pre-decided intervals.
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