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Temperature can also affect the strain gage. 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 coefficient of the expansion of steel is different than the coefficient of the
expansion of concrete. Because of this, a correction for temperature is required equal to:
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 gages is 17.3 microstrains/°C.)
1
C
the coefficient of expansion of concrete: ~10 microstrains/°C. (Users should use their own
is
2
values for C
if known.)
2
Load related strain in concrete, (a composite of both external load and temperature effects,)
corrected for temperature, is given by:
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. (Note: when (R
1
positive, the strain is tensile.)
B is the batch gage factor suppled with each gage.
T
T
C
and C
are the same values as shown in Equation 4
0,
1,
1
2
A theoretical example of the above is shown on the next page.
+ (T
1
Equation 4 – Correction for Temperature Effects on the Gage
µ
= (R
– R
load
1
– T
) (C
– C
)
0
1
2
) B + (T
– T
) (C
0
1
0
.
– C
)
1
2
17
− R
) is
1
0
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