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EXAMPLE:
If:
R
= 3000 in position D
0
R
= 2900 in position D
1
T
= 20 °C
0
T
= 30 °C
1
B = 0.975 (batch calibration factor)
Then:
The apparent strain = (2900 – 3000) 0.975 = –97.5 strain (compression).
The load-related strain, corrected for temperature effects on the gauge =
(2900 – 3000) 0.975 + (30 – 20) (12.2 – 10) = –75.5 strain (compression).
Note: The actual strain undergone by the concrete, (i.e., that which would be
measured by a measuring scale) is given by the formula:
= (R
– R
) B + (T
– T
actual
1
0
1
EQUATION 6: Actual Strain
Which in the current example = (2900 – 3000) 0.975 + (30 – 20) (12.2) = 24.5
strain (expansion).
See Appendix G for further information.
6.5.1 MODEL 4200-6 / 4200-7 CORRECTIONS
The effect of temperature on the 4200-6 and 4200-7 strain gauges is complex; it
varies depending on the strain level. A typical temperature correction factor to
be applied to the 10,000 4200-7 model is as follows:
Temperature Correction Factor = (0.000401*R
Where:
R
is the current gauge reading.
1
T
is the current temperature in degrees Celsius.
1
T
is the initial temperature in degrees Celsius.
0
This correction factor was developed by testing four gauges at three different
parts of their range (i.e., at microstrain levels of 4000, 8000, and 12000), at five
different temperature levels, i.e., -40, -20, 0, 20, 40, and 60 degrees Celsius).
When using the polynomial expression to calculate the strain, this correction
factor must be applied to the current reading R
inserted into the polynomial.
Thus, the modified value of R
R
+ (0.000401*R
– 1.067) x (T
1
1
6.6 SHRINKAGE EFFECTS
A well-known property of concrete is its propensity to shrink as the water
content diminishes, and to swell as it absorbs water. This shrinkage and swelling
can give rise to large strain changes that are not related to load or stress. The
magnitude of these strains can be several hundred microstrain.
It is difficult to compensate for these unwanted strains. An attempt may be
made to keep the concrete under a constant condition of water content, but this
is frequently impossible on concrete structures exposed to varying weather
conditions. The shrinkage and/or swelling effect may be measured by casting a
strain gauge inside a concrete block that remains unloaded, yet still exposed to
the same moisture conditions as the active gauges. Strains measured on this
gauge may be used as a correction factor.
6.7 CREEP EFFECTS
It is also well-known that concrete will creep under a sustained load. What may
seem to be a gradually increasing load, as evidenced by a gradually-increasing
MODEL 4200 SERIES STRAIN GAUGES | DATA REDUCTION | 17
) (C
)
0
1
- 1.067)(T
1
. The modified value of R
1
to be inserted into the polynomial is:
1
- T
)
1
0
-T
)
1
0
is then
1
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