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18
Example:
If;
R
= 3000 on channel D
0
R
= 2900 on channel 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 µstrain (compression)
The load related strain, corrected for temperature effects on the gage =
(2900 – 3000) 0.975 + (30 – 20) (12.2 – 10) = –75.5 µstrain (compression)
Please note that the actual strain undergone by the concrete, (e.g., that which would be measured
by a measuring scale,) is given by the formula:
Which in the current example = (2900-3000)0.975 + (30-20)(12.2) = + 24.5 µstrain (expansion).
See Appendix G for further information.
6.6 Shrinkage Effects
A well know 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
microstrains.
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 gage inside a concrete block that remains unloaded, yet still
exposed to the same moisture conditions as the active gages. Strains measured on this gage may
be used as a correction factor.
µ
= (R
– R
) B + (T
actual
1
0
Equation 6 - Actual Strain
– T
) (C
)
1
0
1
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