Introduction to VW Strain Gauges
Overview
Vibrating Wire Strain gauges measure strain within a wide range of media including steel and concrete the data from which can be used to calculate a wide range of parameters including load, stress and pressure.
The weldable version can be fixed to steel load-bearing beams, tunnel segments, struts, sheet pile walling and tendons whereas the embedment type is suitable for direct burial and can be used in pile tests, load bearing concrete beams and columns, tunnel segments and concrete foundations. They can be read with a portable readout or connected to a data logger for automatic data acquisition.
Mountings
Standard mountings for VW strain gauges are:
- Surface mount – arc weldable blocks, groutable or bondable anchors.
- Embedment – only available with end flanges
- Spot weldable – tabs which can be spot welded or bonded onto surfaces.
- Re-bar strain meter/Sister bar – varying length of rebar which can be welded or tied into the reinforcing cage.
Mountings are available in different materials to suit the application.
Selection
What should I use to measure concrete strain?
There are two main products for measuring concrete strain, the embedment gauge or the sister bar. The embedment gauge comes in three sizes 50mm, 150mm and 250mm, the standard gauge is 150mm and accounts for 90% of the embedment gauges we sell it can be used in pile cages, diaphragm walls. The 250mm is suitable for direct installation into mass concrete where the 150mm gauge would never survive installation. The 50mm gauges are two small for most applications and have some installation and accuracy issues due to this but can be used where the other gauges cannot fit.
The sister bars are individually calibrated and are much more robust than embedment gauges but are of course more expensive. The only disadvantage is size but special versions are made for the bottom of piles with one anchor side being replaced by a flange.
Sister Bar or Rebar Strain gauge?
There seems to be some confusion through the industry on the correct terminology for a sister bar or a rebar strain meter, so this should only be taken as the authors understanding of the subject. Rebar strain meters come in many different sizes to match the different size reinforcement bars used in piles, diaphragm walls etc. The rebar strain meters are intended to be welded into the reinforcement cage to take the place of a piece of reinforcement of equal size. The rebar strain meter will now measure the strain in the reinforcement cage by becoming a load carrying part of the structure.
A sister bar is typically either a 12mm or 16mm version of the same thing but installed in the cage such that it is not fixed in such a way as it will measure the strain in the reinforcement bar but in such a way that it will measure the strain in the concrete. Sister bars are usually fixed in with some tie wire or cable ties in effect floating in the cage structure until such time as it is surrounded by concrete. Once embedded in the concrete the gauges strain matches (Sisters) that of the concrete. The reason that either 12mm or 16mm are used is that they do not have a sufficient cross section in the measuring zone to impact the accuracy of the strain readings measured.
Installation
There are a number of good working practices which should be adopted when installing VW strain gauges as follows:-
General considerations
- Strain gauges should be handled with care avoiding bending or dropping them
- Strain gauges should be orientated parallel to the axis that is loaded
- To measure axial load an array of gauges should be installed around the axis of bendin
- All equipment should be stored in an environment that is protected from direct sunlight
- Identify the locations where strain gauges will best reflect changes in the parameters to be measured
- Label gauges and cables clearly. Colour coded tapes are commonly used for instrument identification and to mark installation location
Installation of embedment strain gauges
- Gauges are typically mounted on short pieces of re-bar (mounting bars) that are fixed onto reinforcement bars at 90º to the gauge orientation
- Attach the mounting bars to the reinforcement bars using cable ties or tie wire
- Wrap the mounting bars with masking tape at the position of the gauge mounting
- Following installation of the gauge, route the cable to a convenient location
Installation of surface mounted strain gauges
- Select appropriate mounting blocks for the surface to be instrumented e.g. arc weldable, bonded, grouted
- Always use the specific installation tools when attaching the mounting blocks
- Always insert the end of the gauge without the groove into the full mounting block
- Always tension the gauge in accordance with the procedure detailed in the installation manual
- Suitable protective covers against damage and/or thermal effects should be fitted
Installation of spot welded strain gauges
- Surfaces onto which the gauges are to be mounted should always be prepared by grinding and cleaning
- Always use a suitable spot welder to fix one end of the gauge in place
- Always use a purpose built tensioning jig to avoid damage to the gauge
- Always tension the gauge in accordance with the procedure detailed in the installation manual
Protection
Where strain gauges are exposed it is recommended to protect them from physical and thermal effects wherever possible.
Mid-range setting
Unless the specification requires a certain setting the strain gauge should be set to its mid-range.
Operation
Engineering units
Direct frequency and temperature readings of VW strain gauges can be read with the VWR-1 or MP12.
The NexusGEO hand held readout allows the batch calibration to be downloaded directly via the internet from the Geosense GeoHUB or entered manually to convert readings into micro strain.
Troubleshooting
What is a gauge factor?
A gauge factor is the equivalent of the strain gauges calibration factor and this term can be applied across many different types of strain gauges from Foil gauges to VW gauges. For all Geosense gauges and those of other manufacturers a gauge factor is required whether it is individual or otherwise, for without this factor the readings of the strain gauge are no use. The gauge factor is used to convert the output of the gauge into the micro strain readings you are interested in. And possibly then from micro strain into stress or load in/on the medium the gauge is installed.
Geosense VW gauges get their batch gauge factor from the batch of gauges they come from. A selection of gauges from each individual batches are taken and a calibration is then performed on them, taking readings at set intervals of micro strain across the gauges full range. Once we have this set of values the average reading at each intervals are plotted. A best fit straight line is then applied to the data, it is from this analysis the gauge factor is derived. This factor is then given as the factor for each gauge from this batch
Can Geosense gauges be individually calibrated?
Yes our VW gauges can be individually calibrated although we believe batch calibration is generally sufficient and offers the most cost effective solution and is industry standard.
For the Embedment Strain Gauge we believe that a welded gauge is much more reliable and robust than a none welded gauge for embedment directly into concrete and that it is more important to have working gauges than anything else. The modulus of concrete is also variable over batch and time as it strengthens so any increased accuracy of the gauge is unlikely to be helpful and is probably a waste of money for you the user, specifying body or client.
What do my readings mean?
The readings from any VW Strain gauge can be given in two different units.
- Frequency – Hz
- Frequency2 x 10-3
The manufacturer’s readout manual should tell you which units it is using.
Temperature effects
Temperature change can cause real deformation or stress changes as most materials expand and contract with temperature the amount of which is dependent on their coefficient of expansion. In extreme cases large strains especially in steel structures can occur and should be taken into consideration when analysing data from sensors.
Where wide variations in temperature are expected it is recommended to provide thermal insulation to the gauge.
Geosense VW strain gauges are all fitted with thermistors so that the temperature of the sensor can be monitored and the data used to make compensation corrections if required.
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