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31
If, for whatever reason, the actual strain of the concrete member is required, (e.g., the change of
unit length that would be measured by a dial gage attached to the surface,) this is given by the
equation:
με
=(R
-R
)B + (T
-T
)C
actual
1
0
1
0
1
Equation 13 - Actual Strain
Where C
represents the coefficient of expansion of steel = +12.2 microstrain/°C.
1
Equation 13 may seem less than intuitive and therefore requires some explanation. As an
example, assume first that the strain gage is inside a concrete slab that is perfectly restrained at
its ends. If the temperature rises by one degree C, then the vibrating wire undergoes an expansion
of +12.2 microstrains and (R
-R
)B would be -12.2 microstrains, therefore the result of Equation
1
0
5 in Section 6.5 would be zero actual strain in the concrete slab.
On the other hand, if the concrete slab is free of all restraint, and experiences a temperature
change of + one degree C, then the concrete would expand 10 microstrains, while the vibrating
wire would expand 12.2 microstrains. The value of (R
-R
)B would then be -2.2 microstrains
1
0
(the vibrating wire would slacken slightly), and Equation 5 would yield a value of +10
microstrains.
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