Table of Contents
Advertisement
16 | DATA REDUCTION | GEOKON
R
Note: When (R
) is positive, the strain is tensile.
1
0
B is the batch gauge factor suppled with each gauge.
6.4 STRAIN RESOLUTION
When using the GK-403 Readout, refer to the following table for the strain
resolution in microstrains.
Position
Model
4200 / 4200HT
D
4200-6 / 4200-7
D
4202
E
4210 / 4212 / 4214
B
TABLE 5: Strain Resolution
Note: For some gauges the reading may fluctuate by one digit, so this
resolution may not be useful.
6.5 TEMPERATURE CORRECTIONS
Temperature variations of considerable magnitude are not uncommon,
particularly during concrete curing; therefore, it is always advisable to measure
temperatures along with the measurement of strain.
Temperature-induced expansions and contractions can give rise to real changes
in the stress of the concrete if the concrete is restrained in any way. These
stresses are superimposed on any other load-related stresses.
Temperature can also affect the strain gauge. Increasing temperatures will
cause the vibrating wire to elongate and thus go slack, indicating what would
appear to be a compressive strain in the concrete. This effect is balanced to
some degree by a corresponding stretching of the wire, caused by expansion of
the concrete. If the concrete expanded by exactly the same amount as the wire,
the wire tension would remain constant, and no correction would be necessary.
However, the steel expansion coefficient is different from the concrete
expansion coefficient. Due to this difference, a temperature correction is
required equal to:
T
–
T
C
–
C
1
0
1
2
EQUATION 4: Correction for Temperature Effects on the Gauge
Where:
T
is the initial temperature.
0
T
is the current temperature.
1
C
is the coefficient expansion of steel: 12.2 microstrains/°C.
1
(C
for Model 4200HT gauges is 17.3 microstrains/°C.)
1
C
is the coefficient of expansion of concrete: ~10 microstrains/°C. (Users
2
should use their own values for C
Load-related strain in concrete (a composite of both external load and
temperature effects) corrected for temperature, is given by:
=
R
–
R
B
+
T
–
1
0
1
load
EQUATION 5: True, Load-Related Strain Corrected for Temperature
Where:
R
is the initial reading.
0
R
is the current reading from the readout box, taken in position D or E.
1
- R
Note: When (R
) is positive, the strain is tensile.
1
0
B is the batch gauge factor suppled with each gauge.
T
T
C
and C
are the same values as shown in Equation 4 on page 16.
0,
1,
1,
2
A theoretical example of the above is shown below.
Strain Resolution (microstrain)
±0.1
N/A
±0.1
0.1 x gauge factor
if known.)
2
C
T
–
C
0
1
2
Advertisement
Table of Contents
