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

5
2.3. Cable Installation
As noted in the installation sections, route the instrument cables along the structural rebar and
tie off using nylon cable ties every 2-3 feet (1 meter) to secure. Outside of the instrumented
structure, the cable should be protected from accidental damage caused by moving equipment
or other construction activity.
Cables may be spliced to lengthen them, without affecting gage readings. Always waterproof
the splice completely, especially when embedding within the concrete, preferably using an
epoxy based splice kit such the 3M Scotchcast, model 82-A1. These kits are available from
the factory.
3. TAKING READINGS
3.1. Operation of the GK-403 Readout Box
The GK-403 can store gage readings and also apply calibration factors to convert readings to
engineering units. Consult the GK-403 Instruction Manual for additional information on Mode
"G" of the Readout. The following instructions will explain taking gage measurements using
Mode "B".
Connect the Readout using the flying leads or in the case of a terminal station, with a connector.
The red and black clips are for the vibrating wire gage, the white and green clips are for the
thermistor and the blue for the shield drain wire.
1. Turn the display selector to position "B". Readout is in digits (Equation 1).
2. Turn the unit on and a reading will appear in the front display window. The last digit may
change one or two digits while reading. Press the "Store" button to record the value
displayed. If the no reading displays or the reading is unstable see section 5 for
troubleshooting suggestions. The thermistor will be read and output directly in degrees
centigrade.
3. The unit will automatically turn itself off after approximately 2 minutes to conserve power.
3.2Operation of the GK404 Readout Box
The GK404 is a palm sized readout box which displays the Vibrating wire value and the
temperature in degrees centigrade.
The GK-404 Vibrating Wire Readout arrives with a patch cord for connecting to the vibrating
wire gages. One end will consist of a 5-pin plug for connecting to the respective socket on the
bottom of the GK-404 enclosure. The other end will consist of 5 leads terminated with alligator
clips. Note the colors of the alligator clips are red, black, green, white and blue. The colors
represent the positive vibrating wire gage lead (red), negative vibrating wire gage lead (black),
positive thermistor lead (green), negative thermistor lead (white) and transducer cable drain wire
(blue). The clips should be connected to their respectively colored leads from the vibrating wire
gage cable.

6
Use the POS (Position) button to select position Band the MODE button to select Dg (digits).
Other functions can be selected as described in the GK404 Manual.
The GK-404 will continue to take measurements and display the readings until the OFF button
is pushed, or if enabled, when the automatic Power-Off timer shuts the GK-404 off.
The GK-404 continuously monitors the status of the (2) 1.5V AA cells, and when their combined
voltage drops to 2V, the message Batteries Low is displayed on the screen. A fresh set of 1.5V
AA batteries should be installed at this point
3.3 Operation of the GK405 Readout Box
The GK-405 Vibrating Wire Readout is made up of two components:
•the Readout Unit, consisting of a Windows Mobile handheld PC running the GK-405
Vibrating Wire Readout Application
•the GK-405 Remote Module which is housed in a weather-proof enclosure and connects to
the vibrating wire sensor by means of:
1) Flying leads with alligator type clips when the sensor cable terminates in bare wires or,
2) by means of a 10 pin connector..
The two components communicate wirelessly using Bluetooth®, a reliable digital
communications protocol. The Readout Unit can operate from the cradle of the Remote Module
(see Figure 6) or, if more convenient, can be removed and operated up to 20 meters from the
Remote Module
Figure 6 GK405 Readout Unit
For further details consult the GK405 Instruction Manual

7
3.4. Measuring Temperatures
Each Vibrating Wire Rebar Strain Meter is equipped with a thermistor for reading temperature.
The thermistor gives a varying resistance output as the temperature changes. Usually the white
and green leads are connected to the internal thermistor.
1. If an ohmmeter is used connect the ohmmeter to the two thermistor leads coming from the
strain meter. (Since the resistance changes with temperature are so large, the effect of
cable resistance is usually insignificant.)
2. Look up the temperature for the measured resistance in Table B-1. Alternately the
temperature could be calculated using Equation B-1.
Note: The GK-403, GK-404 and GK-405 readout boxes will read the thermistor and display
temperature in °C automatically.
4. DATA REDUCTION
4.1. Strain Calculation
The basic units utilized by Geokon for measurement and reduction of data from Vibrating Wire
Rebar Strain Meters are "digits". Calculation of digits is based on the following equation;
Digits T =



×
−
23
110
or
Digits Hz =
2
1000
Equation 1 - Digits Calculation
Where: T is the period in seconds. Hz is the frequency in cycles per second.
To convert digits to strain the following equation applies;
εapparent = (R1- R0) ×C
Equation 2 – Apparent Strain
Where: R0is the initial reading in digits, usually obtained at installation or at the
commencement of a test.
R1is the current reading in digits.
C is the calibration factor from the supplied calibration sheet (see Figure 7).
For example, assume an initial reading, R0, of 8000 digits, a current reading, R1, of 7700, and a
calibration factor, C, of 0.343 microstrain per digit.
εapparent = (7700 - 8000) ×0.343 = - 102.9 µε (compression)

8
4.2. Temperature Correction
Rebar strain meters are usually embedded in concrete and strained by the concrete, the
assumption being that the strain in the meter is equal to the strain in the concrete. When
the temperature changes, the concrete expands and contracts at a rate slightly less than the
rate of the steel of the vibrating wire. The coefficients of expansion are:
Steel (Ksteel):
12.2 ppm/°C
6.7 ppm/°F
Concrete (Kconcrete):
≈10 ppm/°C
≈5.5 ppm/°F
Difference (K):
2.2 ppm/°C
1.2 ppm/°F
Table 1 - Thermal Coefficients
Hence a correction is required to the apparent strains equal to the difference of these two
coefficients. See Equation 3.
εload related = ((R1 - R0) ×C) + ((T1- T0) ×K)
Equation 3 – Load Related Strain
Where: T0is the initial temperature recorded at the time of installation.
T1is the current temperature.
K is the thermal coefficient from Table 1.
The strains thus calculated are due to load changes only.
Using the same example : R0= 8000 digits on channel B and R1= 7700 digits on channel B
T0= 20°C and T1= 60°C (during the concrete curing).
The strain corrected for temperature (i.e. due to load changes only)
= (7700 – 8000) 0.343 + (60 – 20) (12.2 – 10) = – 14.9 µstrain (compression)
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
εactual = ((R1 - R0) ×C) + ((T1- T0) ×Ksteel)
Equation 4 –Actual strain
The apparent strain =
(7700 – 8000) x 0.343 = – 103 µstrain (compression)
The load related strain =
(7700 – 8000) x 0.343 + (60 – 20) x (12.2 – 10) = – 15 µstrain (compression)
The actual strain =
(7700 – 8000) x 0.343 + (60-20) x (12.2) = +385 µstrain (tension)
From this example it can be seen that while the concrete was actually expanding by 385
microstrains due to the temperature increase the apparent strain was 103 microstrains in
compression and the actual change of strain due to increased stress in the concrete was only
15 microstrains compression.

9
4.3. Environmental Factors
Since the purpose of the strain meter installation is to monitor site conditions, factors which may
affect these conditions should be observed and recorded. Seemingly minor effects may have a
real influence on the behavior of the structure being monitored and may give an early indication
of potential problems. Some of these factors include, but are not limited to: blasting, rainfall,
tidal or reservoir levels, excavation and fill levels and sequences, traffic, temperature and
barometric changes, changes in personnel, nearby construction activities, seasonal changes,
etc.
4.4. Shrinkage Effects
A well know property of concrete is its propensity to shrink as the water content diminishes,or
for the concrete to swell as it absorbs water. This shrinkage and swelling can give rise to large
apparent strain changes that are not related to load or stress changes. The magnitude of the
strains can be several hundred microstrain. It is difficult to compensate for these unwanted
strains. An attempt may be made, or it may occur naturally, to keep the concrete under a
constant condition of water content. But this is frequently impossible on concrete structures
exposed to varying weather conditions. Sometimes an attempt is made to measure the
shrinkage and/or swelling effect by casting a strain gage inside a concrete block that remains
unloaded but exposed to the same moisture conditions as the active gages. Strains measured
on this gage may be used as a correction.
4.5. Converting Strains to Loads
The load L in any structural element to which the rebar strain gage or Sister-Bar Strain Gage is
attached is given by the formula
L = E νA
Where E is the elastic modulus of the structural element, in the appropriate units
νis the strain in microstrain and
A is the cross-sectional area in the appropriate units
Where strain gages are installed in concrete piles it is standard practice to install them in pairs
on either side of the neutral axis, at each depth horizon. This is done so that any strains
imposed by bending can be cancelled out by taking the average strain of the two strain gages. It
is also standard practice to install a pair of strain gages close to the top of the pile where the
measure strain is used to calculate E, the modulus of the concrete.

10
Figure 7 - Sample Model 4911 Calibration Sheet

11
5. TROUBLESHOOTING
Maintenance and trouble shooting of Vibrating Wire Rebar Strain Meters are confined to
periodic checks of cable connections. Once installed, the meters are usually inaccessible and
remedial action is limited.
Consult the following list of problems and possible solutions should difficulties arise. Consult
the factory for additional troubleshooting help.
Symptom: Strain Meter Readings are Unstable
Is the readout box position set correctly? If using a datalogger to record readings
automatically are the swept frequency excitation settings correct? Channel A of the GK-401
and GK-403 can be used to read the strain meter. To convert the Channel A period display
to digits use Equation 1.
Is there a source of electrical noise nearby? Most probable sources of electrical noise are
motors, generators and antennas. Make sure the shield drain wire is connected to ground
whether using a portable readout or datalogger. If using the GK-401 Readout connect the
clip with the green boot to the bare shield drain wire of the strain meter cable. If using the
GK-403 connect the clip with the blue boot to the shield drain wire.
Does the readout work with another strain meter? If not, the readout may have a low battery
or be malfunctioning.
Symptom: Strain Meter Fails to Read
Is the cable cut or crushed? This can be checked with an ohmmeter. For the 4911A,
nominal resistance between the two gage leads (usually red and black leads) is 50Ω, ±5Ω,
the same for the 4911,50Ω, ±5Ω. 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). If the resistance reads infinite, or very high (megohms), a cut wire must be
suspected. If the resistance reads very low (<20Ω) a short in the cable is likely.
Does the readout or datalogger work with another strain meter? If not, the readout or
datalogger may be malfunctioning.
Symptom: Thermistor resistance is too high.
Is there an open circuit? Check all connections, terminals and plugs. If a cut is located in
the cable, splice according to instructions in Section 2.3.
Symptom: Thermistor resistance is too low.
Is there a short? Check all connections, terminals and plugs. If a short is located in the
cable, splice according to instructions in Section 2.3.
Water may have penetrated the interior of the strain meter. There is no remedial action.

12
APPENDIX A -SPECIFICATIONS
A.1. Rebar Strain Meters
Model:
4911A
4911 "Sister Bar"
Range:
2500 µε
Rebar Sizes Available:¹
#6, #7, #8, #9, #10, #11
#4, #5
Sensitivity:
0.025% FSR
Accuracy:
0.25% FSR
Linearity:
0.25% FSR
Operating Temperature:
-40 to +90° C
-40 to 200° F
Operating Frequency:
1200-2800 Hz
Coil Resistance:
50+/- 5 Ω
50+/-5 Ω
Length:
43.5", 1105 mm
36", 914 mm
Materials:
Grade 60 Rebar and High Strength Steel
Electrical Cable:
2 twisted pair (4 conductor) 22 AWG
Foil shield, PVC jacket, nominal OD=6.3 mm (0.250")
Table A-1 Model 4911A/4911 Strain Meter Specifications
Notes:
¹ Consult the factory for other sizes available.
A.2 Thermistor (see Appendix B also)
Range: -80 to +150° C
Accuracy: ±0.5° C

13
APPENDIX B - THERMISTOR TEMPERATURE DERIVATION
Thermistor Type: YSI 44005, Dale #1C3001-B3, Alpha #13A3001-B3
Resistance to Temperature Equation:
TA B LnR CLnR =+ + −
12732
3
( ) ( ) .
Equation B-1 Convert Thermistor Resistance to Temperature
Where; T =Temperature in °C.
LnR =Natural Log of Thermistor Resistance
A =1.4051 ×10-3 (coefficients calculated over the −50 to +150°C. span)
B =2.369 ×10-4
C =1.019 ×10-7
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
Ohms
Temp
201.1K
-50
16.60K
-10
2417
+30
525.4
+70
153.2
+110
187.3K
-49
15.72K
-9
2317
31
507.8
71
149.0
111
174.5K
-48
14.90K
-8
2221
32
490.9
72
145.0
112
162.7K
-47
14.12K
-7
2130
33
474.7
73
141.1
113
151.7K
-46
13.39K
-6
2042
34
459.0
74
137.2
114
141.6K
-45
12.70K
-5
1959
35
444.0
75
133.6
115
132.2K
-44
12.05K
-4
1880
36
429.5
76
130.0
116
123.5K
-43
11.44K
-3
1805
37
415.6
77
126.5
117
115.4K
-42
10.86K
-2
1733
38
402.2
78
123.2
118
107.9K
-41
10.31K
-1
1664
39
389.3
79
119.9
119
101.0K
-40
9796
0
1598
40
376.9
80
116.8
120
94.48K
-39
9310
+1
1535
41
364.9
81
113.8
121
88.46K
-38
8851
2
1475
42
353.4
82
110.8
122
82.87K
-37
8417
3
1418
43
342.2
83
107.9
123
77.66K
-36
8006
4
1363
44
331.5
84
105.2
124
72.81K
-35
7618
5
1310
45
321.2
85
102.5
125
68.30K
-34
7252
6
1260
46
311.3
86
99.9
126
64.09K
-33
6905
7
1212
47
301.7
87
97.3
127
60.17K
-32
6576
8
1167
48
292.4
88
94.9
128
56.51K
-31
6265
9
1123
49
283.5
89
92.5
129
53.10K
-30
5971
10
1081
50
274.9
90
90.2
130
49.91K
-29
5692
11
1040
51
266.6
91
87.9
131
46.94K
-28
5427
12
1002
52
258.6
92
85.7
132
44.16K
-27
5177
13
965.0
53
250.9
93
83.6
133
41.56K
-26
4939
14
929.6
54
243.4
94
81.6
134
39.13K
-25
4714
15
895.8
55
236.2
95
79.6
135
36.86K
-24
4500
16
863.3
56
229.3
96
77.6
136
34.73K
-23
4297
17
832.2
57
222.6
97
75.8
137
32.74K
-22
4105
18
802.3
58
216.1
98
73.9
138
30.87K
-21
3922
19
773.7
59
209.8
99
72.2
139
29.13K
-20
3748
20
746.3
60
203.8
100
70.4
140
27.49K
-19
3583
21
719.9
61
197.9
101
68.8
141
25.95K
-18
3426
22
694.7
62
192.2
102
67.1
142
24.51K
-17
3277
23
670.4
63
186.8
103
65.5
143
23.16K
-16
3135
24
647.1
64
181.5
104
64.0
144
21.89K
-15
3000
25
624.7
65
176.4
105
62.5
145
20.70K
-14
2872
26
603.3
66
171.4
106
61.1
146
19.58K
-13
2750
27
582.6
67
166.7
107
59.6
147
18.52K
-12
2633
28
562.8
68
162.0
108
58.3
148
17.53K
-11
2523
29
543.7
69
157.6
109
56.8
149
55.6
150
Table B-1 Thermistor Resistance versus Temperature

14
APPENDIX C – DERIVING THE CALIBRATION FACTOR, C, FROM THE TEST DATA
The Model 4911A/4911 Strain Meters are calibrated by loading them in a testing machine hence
the gage factor, C, must be determined after converting loads to strains. This is done as follows:
The central section of the rebar strain meter, the unbonded length, (7.5" or 19.05 cm long)
contains a vibrating wire sensor located axially at the mid-section. See Figure C-1.
ε2a ε2b ε3ε1
L1 L3L2b L2a
Figure C-1 Rebar Strain Meter Schematic
To convert the observed change in readout digits, ∆R, into a strain, εt, for the entire length (7.5
in., 19.05 cm) of the unbonded section, requires solution of the following equation:
( ) ( ) ( )
εε ε ε
t
L L L
L L L
=× + × + ×
+ +
1 1 2 2 3 3
1 2 3
Equation C-1 Total Strain Calculation
Where: εtis the total strain of the unbonded section.
ε1is the strain in zone 1, determined empirically from the equation for the vibrating
wire sensor itself, i.e., ε1= ∆R ×0.359 ×10-6 where ∆R is the change in
readout digits.
ε2, ε3are the strains in zones 2 and 3, respectively, dependent on the load and
cross-sectional area, see Equations C-2 and C-3.
L1is 2.000" (5.08 cm).
L2is 5.000" (12.7 cm).
L3is 0.500" (1.27 cm).
Ε
×
=
2
2aP
ε
Ε×
=
3
3
aP
ε
Equation C-2 Zone 2 Calculation Equation C-3 Zone 3 Calculation
Where: P is the load in pounds or kilograms.
a2and a3are the cross-sectional areas in inches2or cm2. See Table C-1.
Εis the Young's Modulus, 30×106psi or 2.1×106kg/cm2(or MPa ×10.197)
P is also given by the following equation:
FRP ×∆=
Where: P is the applied load in pounds or kilograms.
∆R is the corresponding change in readout digits.
F is the calibration factor expressed as lbs. or kg. per readout digit.