MAN-221 Concrete Surface Mount Vibrating Wire Strain
Concrete Surface Mount Vibrating Wire Strain Gauge
Section 1 : Foreword
The Soil Instruments Concrete Surface Mount Vibrating Wire Strain Gauge, as with all our equipment, has been designed to operate consistently in a construction site environment and is, therefore, relatively robust.
However, it is essential that the equipment covered by this manual is both operated and maintained by competent and suitably qualified personnel. They must READ AND UNDERSTAND the procedures outlined in this manual before attempting installation or operation of the equipment on site.
Soil Instruments will not accept for repair under guarantee, instruments that have been neglected or mishandled in any way.
Section 2 : Introduction
The Soil Instruments Concrete Surface Mount Vibrating Wire Strain Gauge is intended primarily for long-term strain measurements of strain changes on concrete or rock surfaces
The means of attachment is by using anchors grouted into drill holes.
The nature of the instrument dictates that it is used to measure changes in strain (i.e. change in length per unit length).
Strains are measured using the vibrating wire principle: a length of steel wire is tensioned between two mounting blocks that are welded to the steel surface being studied. Deformations (i.e. Strain changes) of the surface will cause the two mounting blocks to move relative to each other, thus altering the tension in the steel wire. The tension is measured by plucking the wire and measuring its resonant frequency of vibration.
The wire is plucked, and its resonant frequency measured, by means of an electromagnetic coil positioned next to the wire.
Portable readouts or fixed data acquisition systems are available from Soil Instruments will provide the necessary excitation to pluck the wire and will convert the measured frequency to display the reading directly in microstrain.
Note: actual cable length dependent on order and coil is secured with a cable tie, after positioning gauge
Section 3 : Equipment Supplied
For each installation you should have been supplied with:
Two groutable anchors/mounting blocks
Sensor unit (gauge)
Each gauge is supplied with a pickup coil fitted with a user specified length of cable.
The cable can be spliced to other cables for routing to a terminal location. Cables can be routed over distances in excess of 1000 metres without degradation of signal.
Together with these a setting bar and drill template should be used to accurately locate the groutable anchors/mounting blocks, such bars and templates are available from Soil Instruments.
Although gauges are checked prior to leaving Soil Instruments, damage can occur during transit. It is suggested that the gauges are visually checked immediately upon receipt. Additionally it is prudent to check the operation using a Vibrating Wire readout device to ensure steady readings; if an audio signal is available on the readout device this can give a good indication of the quality of the signal.
Before installation, a note should be made of the batch factors for the Strain Gauges for future data interpretation.
Section 4 : Preparation of the Gauges
Where a number of gauges are to be installed on a structural element it is essential that each gauge and its associated cables are accurately and effectively identified. A permanent marking system should be adopted to ensure gauges can be identified throughout their working lifetime and this information safely stored for future reference.
Check the resistance between the two lead wires. It should be around 150 ohms. If the gauge contains a thermistor, check its resistance. Check the reading against that which should be obtained at the existing ambient temperature.
Any faulty units should be returned to the factory. Gauges should not be opened in the field.
The choice of gauge installation location is a critical factor in the design of an instrumentation scheme.
Prior to installation of the gauges it is essential to plan the cable routing from the gauge location to the terminal position. The cables should be marked and where possible, extended before the gauge is installed.
Two approaches to cable routing are commonly adopted, each with their own merits.
(a) Each gauge sensor coil is factory fitted with a length of cable to reach from the gauge location to the terminal. This removes the need for jointing cables on site in, often, unsatisfactory conditions. It is, however, the slightly more expensive option and involves the handling and tying of, often, long lengths of cable.
(b) The gauge sensor coils are factory fitted with a short (say 3m) length of cable and the cable from each gauge in an array is routed to a high quality cable joint where it is connected to a multicore cable.
The multicore cable is then routed to the terminal location.
With the cable routing planned and the gauges marked with their appropriate location description, mounting the gauges can begin.
Section 5 : Gauge Mounting
The Soil Instruments Concrete Surface Mount Vibrating Wire Strain Gauge is attached to mounting blocks, which must first be grouted into the surface to be studied.
A setting bar is used to correctly space apart the two blocks. Firstly the two mounting blocks are fitted over the ends of the setting bar and the jig is used to position them correctly. Then the setscrews in the mounting blocks are tightened down onto the setting bar. Avoid excessive tightening as this only damages the setting bar unduly.
Use the drill template to mark the position of the two mounting holes.
Drill 2 holes of 20mm diameter to a depth to suit the rebar stems on each mounting block. Ensure all dust and debris has been removed from the holes prior to grouting the instrument in place.
Once grout has set (refer to manufacturers’ guide) remove setting jig taking care not to lose the three set screws.
5.01 Setting the Strain Gauge
When the mounting blocks are grouted in place, the strain gauge is now slid into the holes of the mounting blocks, being careful to see that the V grooved end of the gauge lies inside the mounting block without the slot. The cone point setscrew should be tightened hard down into the V groove in this end of the gauge.
Clip the plucking coil over the gauge, secure with the cable ties and connect it to the readout box. Check that readings can be recorded. Reading frequency can be made to increase by gripping the coil assembly and pushing in the direction of the free end. Readings can be made to decrease by pushing directly on the free end.
When the desired reading is obtained, (midrange – 2500 , compressive readings approximately 3500 and tensile strains approximately 1500 .) the free end is secured inside the mounting block by tightening down hard on the oval point setscrews. The reading may alter slightly during this operation, which is normal.
The final position of the gauge should be accurately recorded since this detail will be required for interpretation of its readings.
Section 6 : Protection of the Installation
All efforts must be taken to protect the gauge. Where the gauge is installed in a vulnerable position good practice is to spray the area with marker paint as a warning and/or cover the gauge with a protective cover.
These can be ordered separately from Soil Instruments.
6.01 Cable Routing and Protection
Having established and marked the preferred route of the cables and decided upon the type of cabling arrangement to be adopted, begin running the cables from the gauges furthest from the readout location.
Where cables are not to be ducted, they should be routed, in a position where they are least likely to be damaged, using strong tape or cable ties. Cable should be supported every 400-500mm and care should be taken to avoid over stretching the cables, especially where movement/loading could take place.
Where significant movement could take place, the cable ties should be left a little slack and sufficient cable left free and positioned so that it cannot be damaged.
Once the cables have been fixed a full set of readings should be recorded for each instrument.
Since the gauges are used to record changes in data and not absolute data, the stage when the "Base" or
"Zero" reading is recorded is flexible.
Where loading tests are to be performed, the Base reading for the test data should be recorded just prior to the beginning of the test.
Data from Vibrating Wire instruments can be recorded in 3 formats; Period, Linear or Engineering Units. The required format should be established prior to any test beginning and remain consistent for the duration of the monitoring program.
A monitoring schedule should be established by the engineers responsible for the structure, so that the monitoring personnel are aware of the data gathering requirements.
Section 7 : Data Interpretation
Data from strain gauges is generally presented in micro strain length per unit length:- where strain is the ratio of the change in
Practical K factor = 36150 (Gauge calibration constant)
Conversion of Period and Linear Units to micro-strain is carried out using either of the formulae detailed below;
Where; = Change in strain in micro-strain
= Gauge Calibration Constant
= Base reading in Period units x 10
= Current reading in Period units x 10
= Batch Factor supplied with each gauge
Please note: when is positive the resultant strain is tensile.
Where; = Change in micro-strain
= Gauge Calibration Constant
= Base reading in
= Current reading in
= Batch Factor supplied with each gauge
Please note: when is positive the resultant strain is tensile.
The calculation of Load in a member using data from strain gauges is often complex. The fundamental problem is determining the composite Young Modulus (E) of the member, since it is often difficult to accurately determine the properties of the in-situ materials.
Once a Young Modulus is calculated, the following equations can be used to calculate the loading on the structural member at the location of the Strain Gauge.
Force (F) = Stress (S) x Area (A)
Where A = Cross sectional area in m²
Where F units = Newton’s
Where S units = N/m²
Stress (S) = Young Modulus of Elasticity (E) x Strain ( )
Where E units = N/m²
Steel pipe outside diameter = 1.016m
Steel pipe inside diameter = 0.984m
Calculated change from the strain gauges = 54.688
Young Modulus of Elasticity of the steel pipe = 200,000,000,000 N/m²
Stress = E x
= 200,000,000,000 x 0.000054688
Area = r²
= x (outside diameter /2)² - x (inside diameter /2)²
= x (1.016 /2)² - x (0.984 /2)²
= x (0.508)² - x (0.492)²
= x 0.258064 - x 0.242064
= 0.810732 m² – 0.760466 m²
= 0.050266 m²
Force = S x A
= 10937600 x 0.050266
Section 8 : Temperature Effects
It is best practice to record temperature when you record strain readings. You can then use the temperature data as well as strain data to analyse the behaviour of the structure.
Temperature induced expansions and contractions can cause real changes in stress in the concrete if the concrete is restrained, these are superimposed on other load related stresses.
Differences between the coefficient of expansions of the concrete and that of the steel in the strain gauge itself give rise to an apparent change in strain in the concrete. This apparent change can be corrected for using the equation below.
is the change in strain,
is the thermal coefficient of concrete in
/°C is the thermal coefficient of the gauge:12.2
/°C is the current temperature is the initial temperature
Section 9 : Troubleshooting Guide
If a failure of any vibrating wire instrument or its electrical cable is suspected, the following steps can be followed. The instrument themselves are sealed and cannot be opened for inspection. The “Troubleshooting
Flowchart” should also be followed if any instrument failures are suspected.
The steps below and the Troubleshooting Flowchart are applicable generally to any vibrating wire instrument.
Before any of the following steps are followed, the readout unit should be used to verify the stability of the reading and the audio signal from the portable logger should be heard. The period reading from the instrument should not vary by more than 2 units and the audio signal should be crisp and of a consistent tone and duration. An unstable (wildly fluctuating) reading from an instrument or an unsteady audio signal are both indications of possible problems with instruments or their related electrical cables.
If a portable data logger is giving faulty readings or audio signals from all instruments, a faulty readout unit must be suspected. Another readout unit should be used to check the readings from the transducers and Soil Instruments should be
consulted about the faulty readout unit.
Before proceeding to Steps 2 and 3, if possible the continuity should be checked between conductors and earthing screen of the electrical cable. If a continuity exists, a damaged cable is confirmed.
The resistance across the two conductors of the electrical cable should be checked. This can be done using a multimeter device across the two exposed conductors if the cable has not been connected to a terminal cabinet, or can be done just as easily across the two conductors if the instrument has been connected to such a terminal (or Dataloggers).
The resistance across the two conductors should be approximately of the order of 80 to 180 . The majority of these resistances arise from the instrument and the remainder from the electrical cable connected to the instrument.
If the resistance across the two conductors is much higher than the values quoted in “Step 2” (or is infinite), a damaged cable must be suspected.
If the resistance across the two conductors is much lower than the values quoted in “Step 1” (say 80 or less) it is likely that cable damage has occurred causing a short in the circuit.
If the resistance is within the values quoted in “Step 1” (i.e. 80 to 180 ), AND no continuity exists between conductor and earth screen and on checking the reading from the instrument, it proves to be still unstable or wildly fluctuating, it must be assumed that the integrity of the circuit is good. A faulty instrument must be suspected and Soil Instruments should be consulted.
If the location on site of cable damage is found, the cable can be spliced in accordance with recommended procedure with suitably qualified personnel.
Appendix A: Vibrating Wire Data
Frequency Units (f): The tension of a wire can be measured by registering the frequency (note) at which it naturally vibrates. If the wire is "plucked" electronically the frequency at which it vibrates can be measured. The most common units used to express frequency are Hertz (Hz) or Kilohertz (KHz).
The disadvantage of these units is that there is no "linear" conversion from Hertz to "change in wire tension".
Linear Units (L): In order to overcome the problem of a linear conversion described above, the frequency value can be squared, thereby rendering it linear, but quite large. To reduce its size it is often divided by 1000 (or multiplied by 10
3). The expression f
/1000 (or f commonly adopted as a "linear" digital output.
3) is the most
Period Units (P): Electronic devices and digital technology often utilise the "counter" function available in some common circuits.
Period Units represent the time taken for the wire to vibrate over one full oscillation, expressed in seconds. Due to the very small size of the number generated most equipment manufacturers display the unit multiplied by 10000000 (or 10
The relationship between Period Units and frequency units is expressed as P 1 frequency
Period units are, therefore, convenient to measure but do not have a linear relationship to "change in wire tension".
Calibration Constants: Each instrument is supplied with a Calibration Constant value, to convert the raw data into engineering units.
The value of the calibration constant will vary depending upon the engineering units into which the data is to be converted and the readout units. For example, the data from piezometers may convert into Kg/cm
, mH20, Bar, Psi, etc., and, therefore, the Calibration Constant for each will be different.
Some instruments have "Generic" Calibration Constants and others are calibrated to generate the
Constant. The constant is generated by using the following calculation.
Constant (K) = Range
Reading @ Full Range - Reading @ Range Zero x 10
Appendix B: Trouble Shooting Flowchart
Is reading from portable logger stable, sensible and audio signal steady?
There is no reason to suspect a faulty instrument
Does a continuity exist between earthing screen and conductor?
A damaged cable or damaged cable joint are suspected
A severed cable is suspected causing very high or infinite resistance.
R is very high
Check magnitude of resistance (R) between conductors.
R is between 80 & 180 Ohm
A faulty readout is suspected. Check reading of instrument with another unit.
R < 80 Ohm
A damaged cable is suspected causing a short.
(See step 4)
It must be suspected that the portable logger used first is faulty. Contact
Soil Instruments Ltd.
Is reading OK with alternative logger?
A faulty instrument is possible. Contact
Soil Instruments Ltd.
Bell Lane, Uckfield, East Sussex
+44 (0) 1825 765044
TN22 1QL United Kingdom f: +44 (0) 1825 744398 e: w:
Soil Instruments Ltd. Registered in England. Number: 07960087. Registered Office: 5th Floor, 24 Old Bond Street, London, W1S 4AW