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4.2. 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 of stress in the concrete if the concrete is restrained in any way, and these
stresses are superimposed on any other load related stresses.
Temperature can also affect the strain gage itself since increasing temperatures will cause
the vibrating wire to elongate and thus to 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 in which the gage
is embedded or to which the gage is attached. If the concrete expanded by exactly the same
amount as the wire then the wire tension would remain constant and no correction would be
necessary.
The effect of temperature on the 4200ER strain gage is complex - it varies depending on
the strain level.A Typical temperature correction factor to be applied to the 4200ER-10,000
is as follows:
Temperature Correction Factor = +(0.000401*R1 - 1.067)(T1-T0)
Where
R1 is the current gage reading,.
T1 is the current temperature in degrees C,
T0 is the initial temperature in degrees C.
This correction factor was developed by testing four gages 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 C).
When using the polynomial expression to calculate the strain, this correction factor must be
applied to the current reading R1. The modified value of R1 is then inserted into the
polynomial.
Thus the modified value of R1 to be inserted into the polynomial is
R1+ (0.000401*R1 – 1.067) x (T1 - T0)
