Geokon 4370 User manual

Instruction Manual
Model 4370
Concrete Stressmeter
No part of this instruction manual may be reproduced, by any means, without the written consent of Geokon®.
The information contained herein is believed to be accurate and reliable. However, Geokon®assumes no responsibility for errors,
omissions or misinterpretation. The information herein is subject to change without notification.
Copyright © 2003-2019 by Geokon®
(Doc Rev G, 05/1/19)

Warranty Statement
Geokon 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 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 or any breach of any warranty by Geokon shall not exceed the purchase
price paid by the purchaser to Geokon 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 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
1. INTRODUCTION..................................................................................................................... 1
2. OPERATING PRINCIPLE ..................................................................................................... 1
3. INSTALLATION...................................................................................................................... 2
4. LIGHTNING PROTECTION ................................................................................................. 4
3. TAKING READINGS .............................................................................................................. 5
3.1 GK-404 READOUT BOX.......................................................................................................... 5
3.1.1 Operating the GK-404..................................................................................................... 5
3.2 GK-405 READOUT BOX.......................................................................................................... 6
3.2.1 Connecting Sensors......................................................................................................... 6
3.2.2 Operating the GK-405..................................................................................................... 6
3.3 GK-403 READOUT BOX (OBSOLETE MODEL)......................................................................... 7
3.3.1 Connecting Sensors......................................................................................................... 7
3.3.2 Operating the GK-403..................................................................................................... 7
3.4 MEASURING TEMPERATURES.................................................................................................. 7
6. DATA REDUCTION................................................................................................................ 8
6.1 STRESS CALCULATION............................................................................................................ 8
6.2. TEMPERATURE CORRECTION ................................................................................................. 9
7. TROUBLESHOOTING ......................................................................................................... 10
APPENDIX A. SPECIFICATIONS.......................................................................................... 11
A.1 4370 CONCRETE STRESSMETER ........................................................................................... 11
A.2 THERMISTOR........................................................................................................................ 11
APPENDIX B. THERMISTOR TEMPERATURE DERIVATION...................................... 12
APPENDIX C. TYPICAL CALIBRATION REPORT........................................................... 13

FIGURES
FIGURE 1-THE MODEL 4370 CONCRETE STRESSMETER. ................................................................ 1
FIGURE 2-STRESSMETER READY FOR CONCRETE POUR ................................................................... 2
FIGURE 3-CONCRETE BEING POURED AROUND THE STRESSMETER.................................................. 2
FIGURE 4-ANOTHER TYPICAL STRESS METER INSTALLATION.......................................................... 3
FIGURE 5-LIGHTNING PROTECTION SCHEME .................................................................................. 4
FIGURE 6-LEMO CONNECTOR TO GK-404...................................................................................... 5
FIGURE 7-LIVE READINGS –RAW READINGS ................................................................................. 6
FIGURE 8-TYPICAL CALIBRATION REPORT................................................................................... 13
TABLES
TABLE 1-4370 SPECIFICATIONS.................................................................................................... 11
TABLE 2-THERMISTOR RESISTANCE VERSUS TEMPERATURE........................................................ 12
EQUATIONS
EQUATION 1-DIGITS TO CONCRETE STRESS (LINEAR EQUATION) .................................................. 8
EQUATION 2-DIGITS TO CONCRETE STRESS (POLYNOMIAL EQUATION)......................................... 9
EQUATION 3-POLYNOMIAL GAUGE FACTOR C ............................................................................... 9
EQUATION 4-RESISTANCE TO TEMPERATURE ............................................................................... 12

1
1. INTRODUCTION
The Geokon Model 4370 Concrete Stressmeter is designed to measure stresses in concrete.
Conventional ways of doing this suffer from some drawbacks: for instance, strain gages can
measure strains but the conversion of strains to stress is made difficult because of changing
modulus with time, shrinking and swelling due to varying moisture content, and creep under
sustained loads. Most of these problems can be overcome using hydraulic Flat Jack type stress
cells; however, these cells are subject to a strong temperature dependence, which can also cause
de-coupling of the cell from the surrounding concrete requiring a means of re-inflating the cells
after the initial concrete curing period.
The Model 4370 Concrete Stressmeter is designed to overcome these problems by, in effect,
making a stressmeter out of concrete so that it will have the same properties of
shrinkage/swelling, modulus variation, temperature dependence, and creep potential, as the
surrounding concrete.
2. OPERATING PRINCIPLE
The Model 4370 Concrete Stressmeter is shown in Figure 1.
Figure 1 - The Model 4370 Concrete Stressmeter.
In essence, the stressmeter comprises a short vibrating wire load cell, in series with an 18-inch-
long cylinder of concrete. This concrete cylinder has the same properties as the surrounding
concrete but is de-bonded from it by means of a smooth-walled, porous plastic tube and is
coupled at its ends to the surrounding concrete by means of a flange at one end and a piece of all-
thread rod at the other. The vibrating wire load cell measures the load imposed on the inner
concrete cylinder by stresses in the surrounding concrete. This load, when divided by the cross-
sectional area of the inner cylinder, gives the stress in the surrounding concrete. Variations of
moisture content in the surrounding concrete are felt also by the inner concrete so that shrinkage
and swelling are the same both inside the cell and out, leading to no net change in the load cell
readout. (This is not strictly true due to the short length of the metal load cell portion, which
behaves differently, but the effect is kept small by the large difference in the relative lengths of
the concrete cylinder versus the length of the load cell).
A thermistor is included inside the cell for the measurement of temperatures.

3. INSTALLATION
The Stressmeter is first wrapped in A Tyvec type material for additional de-bonding power. The
stressmeter is then positioned in line with the direction in which the stress is to be measured and
is tied off to the rebar cage or a small size supplemental rebar cage, using iron wire, or nylon tie-
wraps. A typical installation is shown in Figure 2. Be sure to leave enough slack in the cable so
that the stressmeter can be un-tied and positioned vertically for filling.
Figure 2 - Stressmeter ready for concrete pour
As Figure 2 shows, the far end of the stress meter is left open so that when the concrete is poured
around the stressmeter the cell can be untied from the rebar and held vertical so that the same
concrete can be packed inside the cell, and then the end flange is pushed inside the end of the
tube and securely taped in place. It is of the utmost importance that there be no spaces left
inside the cell, so the concrete must be very carefully packed, vibrated and rodded to get
rid of all traces of air. Cables from the stressmeter should be routed carefully so that they are
protected from the concrete placement and from traffic.
Additional installation photos are shown in Figure 3 and Figure 4.
Figure 3 - Concrete being poured around the stressmeter

Figure 4 - Another typical stress meter installation

4. LIGHTNING PROTECTION
The Model 4370 Stressmeter, unlike numerous other types of instrumentation available from
Geokon, do not have any integral lightning protection components, i.e. transzorbs or plasma
surge arrestors. Usually this is not a problem as the stressmeters are installed within concrete or
grout and somewhat isolated from potentially damaging electrical transients. However, there
may be occasions where some sort of lightning protection is desirable, for example where the
stressmeter is in contact with rebar that may be exposed to direct or indirect lightning strikes.
Also, if the instrument cable is exposed, it may be appropriate to install lightning protection
components, as the transient could travel down the cable to the stressmeter and possibly destroy
it.
Note the following suggestions:
•If the stressmeter is connected to a terminal box or multiplexer components such as plasma
surge arrestors (spark gaps) may be installed in the terminal box/multiplexer to provide a
measure of transient protection. Terminal boxes and multiplexers available from Geokon
provide locations for installation of these components.
•Lighting arrestor boards and enclosures are available from Geokon that install at the exit
point of the instrument cable from the structure being monitored. The enclosure has a
removable top so, in the event the protection board (LAB-3) is damaged, the user may
service the components (or replace the board). A connection is made between this enclosure
and earth ground to facilitate the passing of transients away from the gauge.
•Plasma surge arrestors can be epoxy potted into the gauge cable close to the sensor. A ground
strap would connect the surge arrestor to earth ground, either a grounding stake or the rebar
itself.
Consult the factory for additional information on these or alternate lightning protection schemes.
Figure 5 - Lightning Protection Scheme

3. TAKING READINGS
3.1 GK-404 Readout Box
The Model GK-404 Vibrating Wire Readout is a portable, low-power, handheld unit that can run
continuously for more than 20 hours on two AA batteries. It is designed for the readout of all
Geokon vibrating wire gauges and transducers; and is capable of displaying the reading in either
digits, frequency (Hz), period (µs), or microstrain (µε). The GK-404 also displays the
temperature of the stressmeter (embedded thermistor) with a resolution of 0.1 °C.
3.1.1 Operating the GK-404
Before use, attach the flying leads to the GK-404 by aligning the red circle on the silver
“Lemo” connector of the flying leads with the red line on the top of the GK-404 (Figure
6). Insert the Lemo connector into the GK-404 until it locks into place.
Figure 6 - Lemo Connector to GK-404
Connect each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
To turn the GK-404 on, press the “ON/OFF” button on the front panel of the unit. The
initial startup screen will be displayed. After approximately one second, the GK-404 will
start taking readings and display them based on the settings of the POS and MODE
buttons.
The unit display (from left to right) is as follows:
•The current Position: Set by the POS button. Displayed as a letter A through F.
•The current Reading: Set by the MODE button. Displayed as a numeric value
followed by the unit of measure.
•Temperature reading of the attached gauge in degrees Celsius.
Use the POS button to select position B and the MODE button to select Dg (digits).
(Other functions can be selected as described in the GK-404 Manual.)
The GK-404 will continue to take measurements and display readings until the unit is
turned off, either manually, or if enabled, by the Auto-Off timer. If the no reading
displays or the reading is unstable, see Section 7 for troubleshooting suggestions.
For further information, please refer to the GK-404 manual.

3.2 GK-405 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; and the GK-405 Remote Module, which is housed in a weatherproof enclosure and
connects to the vibrating wire gauge to be measured. The two components communicate
wirelessly. The Readout Unit can operate from the cradle of the Remote Module, or, if more
convenient, can be removed and operated up to 20 meters from the Remote Module.
3.2.1 Connecting Sensors
Connecting sensors with 10-pin connectors:
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
Connecting sensors with bare leads:
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.2.2 Operating the GK-405
Press the button labeled “POWER ON”. A blue light will begin blinking, signifying that
the Remote Module is waiting to connect to the handheld unit. Launch the GK-405
VWRA program by tapping on “Start” from the handheld PC’s main window, then
“Programs” then the GK-405 VWRA icon. After a few seconds, the blue light on the
Remote Module should stop flashing and remain lit. The Live Readings Window will be
displayed on the handheld PC. Choose display mode “B”. Figure 7 shows a typical
vibrating wire output in digits and thermistor output in degrees Celsius. If no reading
displays or the reading is unstable, see Section 7 for troubleshooting suggestions. For
further information, consult the GK-405 Instruction Manual.
Figure 7 - Live Readings – Raw Readings

3.3 GK-403 Readout Box (Obsolete Model)
The GK-403 can store gauge readings and apply calibration factors to convert readings to
engineering units. The following instructions explain taking gauge measurements using Mode
“B”. Consult the GK-403 Instruction Manual for additional information.
3.3.1 Connecting Sensors
Connecting sensors with 10-pin connectors:
Align the grooves on the sensor connector (male), with the appropriate connector on the
readout (female connector labeled senor or load cell). Push the connector into place, and
then twist the outer ring of the male connector until it locks into place.
Connecting Sensors with Bare Leads:
Attach the GK-403-2 flying leads to the bare leads of a Geokon vibrating wire sensor by
connecting each of the clips on the leads to the matching colors of the sensor conductors,
with blue representing the shield (bare).
3.3.2 Operating the GK-403
1) Turn the display selector to position “B”.
2) Turn the unit on.
3) The readout will display the vibrating wire output in digits. The last digit may change
one or two digits while reading.
4) The thermistor reading will be displayed above the gauge reading in degrees
centigrade.
5) Press the “Store” button to record the value displayed.
If the no reading displays or the reading is unstable, see Section 7 for troubleshooting
suggestions. The unit will turn off automatically after approximately two minutes to
conserve power.
3.4 Measuring Temperatures
Each Vibrating Wire Stressmeter is equipped with a thermistor for reading temperature. The
thermistor gives a varying resistance output as the temperature changes. Geokon readout boxes
will read the thermistor and display temperature in °C automatically. To read the thermistor
using an ohmmeter, complete the following:
1) Connect the ohmmeter to the two thermistor leads coming from the stressmeter. (Usually
white and green.) Since the resistance changes with temperature are large, the effect of cable
resistance is usually insignificant.
2) Look up the temperature for the measured resistance in Table 2 in Appendix B.

6. DATA REDUCTION
6.1 Stress Calculation
To convert digits to concrete stress, use the linear or polynomial equation from the calibration
report provided with the stressmeter. See Appendix C for a sample calibration report
The linear equation is as follows:
Concrete Stress (S) = (R1– R0) ×G
Equation 1 - Digits to Concrete Stress (Linear Equation)
Where;
R1is the current reading.
R0is the initial reading, usually obtained at installation just before the concrete is poured.
(NOTE: For greater accuracy use the regression zero found on the calibration report as R0.)
G is the linear gauge factor, in MPa/digit or psi/digit taken from the calibration report.
The cross-sectional area of the Stressmeter is equal to 5.31 sq in.
For example:
If;
Initial field zero reading = 7035 digits
Regression zero = 7071
R1= 5219 digits
G = -0.01748 MPa/digit
Then;
The calculated concrete stress using Equation 1 and the initial field zero =
S = (5219 – 7035) x -0.01748 = 31.74 MPa
The calculated concrete stress using Equation 1 and the regression zero =
S = (5219 −7071) ×-0.01748 = 32.37 MPa

The polynomial equation is as follows:
Concrete Stress (S) = AR12+ BR1+ C
Equation 2 - Digits to Concrete Stress (Polynomial Equation)
Where;
R1is the is the current reading
A and B are the polynomial gauge factors, in MPa/digit or psi/digit taken from the calibration
report.
C is calculated using the equation:
S = AR12+ BR1
Equation 3 - Polynomial gauge Factor C
Where;
S = 0
R1is the initial field zero reading
A and B are the polynomial gauge factors, in MPa/digit or psi/digit taken from the calibration
report.
For example:
If;
A = -6.568E-07 MPa/digit
B = -0.009799 MPa/digit
Initial field zero reading = 7035 digits
Current reading = 5219 digits
Then;
C = -6.568E-07*7035^2 - 0.009799*7035 = 101.44
And the calculated concrete stress using =
S = 6.658E-07*5219^2 - 0.009799*5219+101.44 = 32.41 MPa
Compare these values to the actual applied stress of 32.47MPa
6.2. Temperature Correction
The Model 4370 Concrete Stressmeter has a coefficient of thermal expansion very similar to the
concrete surrounding it so in most cases correction is not necessary.

7. TROUBLESHOOTING
Maintenance and troubleshooting of concrete stressmeters is confined to periodic checks of cable
connections and maintenance of terminals. Once installed, the gages 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 gauge 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?
Is the strain reading outside the specified range (either compressive or tensile) of the
instrument?
Is there a source of electrical noise nearby? Most probable sources of electrical noise are
motors, generators and antennas. Move the equipment away from the installation or install
electronic filtering. Make sure the shield drain wire is connected to ground whether using a
portable readout or datalogger.
Does the readout work with another gauge? If not, the readout may have a low battery or be
malfunctioning.
Symptom: Strain gauge Fails to Read
Is the cable cut or crushed? This can be checked with an ohmmeter. Nominal resistance
between the two gauge leads (usually red and black leads) is 50Ω, ±10Ω. Remember to add
cable resistance when checking (22 AWG stranded copper leads are approximately 14.7Ω
/1000' or 48.5Ω/km, multiply by two for both directions). If the resistance reads infinite, or
very high (megohms), a cut wire must be suspected. If the resistance reads very low (<10Ω) a
short in the cable is likely. Splicing kits and instructions are available from the factory to
repair broken or shorted cables. Consult the factory for additional information.
Does the readout or datalogger work with another strain gauge? If not, the readout or
datalogger may be malfunctioning.

APPENDIX A. SPECIFICATIONS
A.1 4370 Concrete Stressmeter
Standard Range
-3 MPa to +25 Mpa
Resolution
10 kPa
Accuracy1
±0.25% F.S.
Temperature Range2
-20 to +80 °C
Length ⨉Diameter
600 ⨉76 mm (I.D. 66 mm)
Table 1 - 4370 Specifications
Notes:
1 Load cell accuracy
2 Other ranges available on request
A.2 Thermistor
(See Appendix B also)
Range: -80 to +150° C
Accuracy: ±0.5° C

APPENDIX B. THERMISTOR TEMPERATURE DERIVATION
Thermistor Type: YSI 44005, Dale #1C3001-B3, Alpha #13A3001-B3
Resistance to Temperature Equation:
T= 1
A+B(LnR)+C(LnR)3-273.15 °C
Equation 4 - 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
Table 2 - Thermistor Resistance versus Temperature
55.6
150

APPENDIX C. TYPICAL CALIBRATION REPORT
Figure 8 - Typical Calibration Report