Geokon 4911A User manual

Installation Manual
Models 4911A/4911
VW Rebar Strain Meters
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon, Inc.
The information contained herein is believed to be accurate and reliable. However, Geokon, Inc. assumes no responsibility for
errors, omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 1989, 1996, 2004, 2007,2008,2009,2010, 2012, 2013 Geokon, Inc.
(Doc Rev O 6/13)

Warranty Statement
Geokon, Inc. warrants its products to be free of defects in materials and workmanship, under normal
use and service for a period of 13 months from date of purchase. If the unit should malfunction, it must
be returned to the factory for evaluation, freight prepaid. Upon examination by Geokon, if the unit is
found to be defective, it will be repaired or replaced at no charge. However, the WARRANTY is VOID
if the unit shows evidence of having been tampered with or shows evidence of being damaged as a
result of excessive corrosion or current, heat, moisture or vibration, improper specification,
misapplication, misuse or other operating conditions outside of Geokon's control. Components which
wear or which are damaged by misuse are not warranted. This includes fuses and batteries.
Geokon manufactures scientific instruments whose misuse is potentially dangerous. The instruments are
intended to be installed and used only by qualified personnel. There are no warranties except as stated
herein. There are no other warranties, expressed or implied, including but not limited to the implied
warranties of merchantability and of fitness for a particular purpose. Geokon, Inc. is not responsible for
any damages or losses caused to other equipment, whether direct, indirect, incidental, special or
consequential which the purchaser may experience as a result of the installation or use of the product.
The buyer's sole remedy for any breach of this agreement by Geokon, Inc. or any breach of any
warranty by Geokon, Inc. shall not exceed the purchase price paid by the purchaser to Geokon, Inc. for
the unit or units, or equipment directly affected by such breach. Under no circumstances will Geokon
reimburse the claimant for loss incurred in removing and/or reinstalling equipment.
Every precaution for accuracy has been taken in the preparation of manuals and/or software, however,
Geokon, Inc. neither assumes responsibility for any omissions or errors that may appear nor assumes
liability for any damages or losses that result from the use of the products in accordance with the
information contained in the manual or software.

TABLE of CONTENTS
Page
1. INTRODUCTION..................................................................................................................................................1
2. INSTALLATION ....................................................................................................................................................2
2.1. PRELIMINARY TESTS...........................................................................................................................................2
2.2. REBAR STRAIN METER INSTALLATION ...............................................................................................................2
2.2.1. Model 4911A...........................................................................................................................................2
2.2.2. Model 4911 "Sister Bar"........................................................................................................................3
2.3. CABLE INSTALLATION ........................................................................................................................................5
3. TAKING READINGS............................................................................................................................................5
3.1. OPERATION OF THE GK-403 READOUT BOX......................................................................................................5
3.2 OPERATION OF THE GK404 READOUT BOX ........................................................................................................5
3.3 OPERATION OF THE GK405 READOUT BOX ........................................................................................................6
FOR FURTHER DETAILS CONSULT THE GK405 INSTRUCTION MANUAL......................................................................6
3.4. MEASURING TEMPERATURES.............................................................................................................................7
4. DATA REDUCTION ..............................................................................................................................................7
4.1. STRAIN CALCULATION .......................................................................................................................................7
4.2. TEMPERATURE CORRECTION..............................................................................................................................8
THE STRAIN CORRECTED FOR TEMPERATURE (I.E.DUE TO LOAD CHANGES ONLY).....................................................8
NOTE THAT THE ACTUAL STRAIN,THE ONE THAT COULD BE MEASURED WITH A TAPE MEASURE,I.E THE STRAIN
ACTUALLY UNDERGONE BY THE CONCRETE IS GIVEN BY THE EXPRESSION................................................................8
4.3. ENVIRONMENTAL FACTORS................................................................................................................................9
4.4. SHRINKAGE EFFECTS..........................................................................................................................................9
4.5. CONVERTING STRAINS TO LOADS.......................................................................................................................9
5. TROUBLESHOOTING.......................................................................................................................................11
APPENDIX A -SPECIFICATIONS ......................................................................................................................12
A.1. REBAR STRAIN METERS...................................................................................................................................12
A.2 THERMISTOR (SEE APPENDIX BALSO)..............................................................................................................12
APPENDIX B - THERMISTOR TEMPERATURE DERIVATION ...................................................................13
APPENDIX C – DERIVING THE CALIBRATION FACTOR, C, FROM THE TEST DATA........................14

LIST of FIGURES, TABLES and EQUATIONS
Page
FIGURE 1-MODEL 4911A REBAR STRAIN METER..................................................................................................... 1
FIGURE 2-MODEL 4911 REBAR STRAIN METER........................................................................................................ 1
FIGURE 3-MODEL 4911A INSTALLATION ................................................................................................................... 3
FIGURE 4-MODEL 4911 "SISTER BAR"INSTALLATION .............................................................................................. 4
FIGURE 5-MODEL 4911 "SISTER BAR"INSTALLATION DETAIL.................................................................................. 4
FIGURE 6GK405 READOUT UNIT................................................................................................................................ 6
EQUATION 1-DIGITS CALCULATION............................................................................................................................ 7
EQUATION 2–APPARENT STRAIN .............................................................................................................................. 7
TABLE 1-THERMAL COEFFICIENTS............................................................................................................................. 8
EQUATION 3–LOAD RELATED STRAIN....................................................................................................................... 8
FIGURE 7-SAMPLE MODEL 4911 CALIBRATION SHEET............................................................................................10
TABLE A-1 MODEL 4911A/4911 STRAIN METER SPECIFICATIONS...........................................................................12
EQUATION B-1 CONVERT THERMISTOR RESISTANCE TO TEMPERATURE ................................................................13
TABLE B-1 THERMISTOR RESISTANCE VERSUS TEMPERATURE...............................................................................13
FIGURE C-1 REBAR STRAIN METER SCHEMATIC.......................................................................................................14
EQUATION C-1 TOTAL STRAIN CALCULATION ............................................................................................................14
EQUATION C-2 ZONE 2CALCULATION EQUATION C-3 ZONE 3CALCULATION......................................................14
TABLE C-1 UNBONDED SECTION DIMENSIONS...........................................................................................................15
TABLE C-2 MICROSTRAIN CONVERSION FACTORS....................................................................................................15

1
1. INTRODUCTION
Geokon Vibrating Wire Rebar Strain Meters are designed primarily for monitoring the stresses in
reinforcing steel in concrete structures, such as bridges, concrete piles and diaphragm walls.
The strain meter is comprised of a length of high strength steel, bored along its central axis to
accommodate a miniature vibrating wire strain gage. Readout of load or stress is achieved
remotely using a portable readout or datalogging system available from Geokon.
The Model 4911A Vibrating Wire Rebar Strain Meter consists of a short length of high strength
steel welded between two 18" (457 mm) long sections of reinforcing bar. It is designed to be
welded between sections of structural concrete reinforcing bar. The cable exits from the strain
meter via a compression fitting. See Figure 1.
Strain Meter Body
Rebar
Instrument Cable
Compression Fitting
Strain Gage
Electromagnetic Coil
Heat Shrink
Rebar
Thermistor
Weld
Weld
914 mm
36”
Figure 1 - Model 4911A Rebar Strain Meter
The Model 4911 Vibrating Wire Rebar Strain Meter or "Sister Bar" consists of a short length of
high strength steel welded between two 14" (356 mm) long sections of reinforcing bar. It is
designed to be wire tied in parallel with the structural rebar. The small diameter of the bar
minimizes its affect on the sectional modulus of the concrete. The cable exits from the strain
meter through a small block of protective epoxy. See Figure 2.
Figure 2 - Model 4911 Rebar Strain Meter
Both models of strain meters are robust, reliable and easy to install and read, and are
unaffected by moisture, cable length or contact resistance. The long term stability of these
instruments has proven to be excellent.
Strain Meter Body
Rebar
Instrument Cable
Protective Epoxy
Strain Gage
Electromagnetic Coil
Heat Shrink
914 mm
36"
Rebar
Thermistor
(encapsulated)
Weld

2
2. INSTALLATION
2.1. Preliminary Tests
It is always wise, before installation commences, to check the strain meters for proper function.
Each strain meter is supplied with a calibration sheet that shows the relationship between
readout digits and microstrain and also shows the initial no load zero reading. The strain meter
electrical leads (usually the red and black leads) are connected to a readout box (see section 3)
and the zero reading given on the sheet is now compared to a current zero reading. Under
normal circumstances the two readings should not differ by more than about 25 digits (10
microstrain). Shipping shocks may, however, cause larger shifts. If the reading is within 100
digits (40 microstrain) of the factory zero, and is stable, it is safe to proceed with the installation.
By pulling on the strain meter it should be possible to change the readout digits, causing them to
rise as tension increases.
Checks of electrical continuity can also be made using an ohmmeter. For the 4911A, resistance
between the gage leads should be approximately 50 Ω, ±10 Ω, for the 4911, 50 Ω, ±10 Ω.
Remember to add cable resistance when checking (22 AWG stranded copper leads are
approximately 14.7Ω/1000' or 48.5Ω/km, multiply by 2 for both directions). Between the green
and white should be approximately 3000 ohms at 25° (see Table B-1), and between any
conductor and the shield should exceed 2 megohm.
Note: Do not lift the strain meter by the cable.
2.2. Rebar Strain Meter Installation
2.2.1. Model 4911A
The normal procedure is to weld the strain meter in series with the reinforcing steel that is to be
instrumented on the site. For a typical installation see Figure 3. The strain meter is long
enough so that it may be welded in place without damaging the internal strain gage element
(Figure 1). However, care should still be taken to ensure that the central portion of the strain
meter does not become too hot as the plucking coil and protective epoxy could melt. In order to
prevent this it may be necessary to place wet rags between the weld area and the coil housing.
Also, take care not to damage or burn the instrument cable when welding. After welding, route
the instrument cable along the rebar system and tie it off every 3-4 feet (1 meter) using nylon
cable ties. Avoid using iron tie wire to secure the cable as the cable could be cut.
Be sure when installing the strain meters to note the location and serial numbers of all
instruments. This is necessary for applying the proper calibration factors and determining strain
characteristics when reducing data.

3
Material to be excavated
Tieback
Instrument Cables
Guide Wall
Rebar Reinforcement
Concrete
Rebar Strain Meters
(2 places, opposite)
Rebar Strain Meters
(2 places, opposite)
Tieback
Rebar Strain Meters
(2 places, opposite)
Diaphragm Wall
(installed after excavation)
(installed after excavation)
Figure 3 - Model 4911A Installation
2.2.2. Model 4911 "Sister Bar"
The "Sister Bar" is usually installed using standard iron tie wire. Normally ties near the ends
and at the one third points are sufficient if the gage is being wired to a larger section of rebar or
to horizontal bars. Wiring at the one third points alone is sufficient if the gage is being wired in
parallel to the structural rebar. See Figures 4 and 5. Route the instrument cable along the
rebar system and tie it off every 3-4 feet (1 meter) using nylon cable ties. Avoid using the tie
wire on the instrument cable as it could cut the cable.
Be sure when installing the strain meters to note the location and serial numbers of all
instruments. This is necessary for applying the proper calibration factors and determining load
characteristics when reducing data.

4
Instrument Cables
Rebar Reinforcement or
Concrete
Rebar Strain Meters
(2 or 3 places)
Rebar Strain Meters
(2 or 3 places)
Rebar Strain Meters
(2 or 3 places)
Pile
Pre-Stressing Cables
Figure 4 - Model 4911 "Sister Bar" Installation
Reinforcing Rebar
Rebar Strain Meter
Wire Tie
Instrument Cable
or Strand
Wire Tie
Tied to Reinforcing Rebar Tied to Reinforcing Rings
Reinforcing Rebar
or Strand
(2 places)
Rebar Strain Meter
Instrument Cables
(3 places, 120° apart)
Figure 5 - Model 4911 "Sister Bar" Installation Detail