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30 | TEMPERATURE CORRECTION | GEOKON
APPENDIX D. TEMPERATURE CORRECTION WHEN
USED ON CONCRETE
In a free field, where no loads are acting, the thermal concrete strains are given
by the following equation:
µε
= (T
– T
) x CF
thermal
1
0
2
EQUATION 9: Thermal Concrete Strains
CF
represents the coefficient of expansion of concrete. Unless this figure is
2
known, assume a nominal value of 10.4 microstrains/C.
If the actual strain of the concrete member is required, (i.e., the change of unit
length that would be measured by a dial gauge attached to the surface,) you can
arrive at this using this equation:
µε
= (R
– R
)B + (T
– T
actual
1
0
1
EQUATION 10: Actual Strain
Where CF
represents the coefficient of expansion of steel = 12.2 microstrains/
1
C, and (R
– R
)B is the apparent strain recorded by the readout box.
1
0
To calculate the strain in the concrete due to load changes only:
µε
= µε
– µε
= (R
load
actual
thermal
1
EQUATION 11: Strain Due to Load Changes Only
Note the following example, where B = 0.91
R
= 3000 microstrain, T
= 20 °C
0
0
R
= 2900 microstrain, T
= 30 °C
1
1
µε
= (2900 – 3000) x 0.91 = –91
apparent
µε
= (2900 – 3000) x 0.91 = + (30 – 20) x 12.2 = 31
actual
µε
= (30 – 20) x 10.4 = 104
thermal
µε
= (2900 – 3000) x 0.91 + (30 – 20) x (12.2 – 10.4) = –73
load
Note: Because assumptions have been made regarding the thermal coefficients
for the concrete, these equations should only be used as a general guide.
Explanation: The apparent compressive strain, indicated by the readout box
after application of the batch factor, B, is (R
strain in the concrete had not changed, the steel vibrating wire would have
expanded and gone slack by the equivalent of (30 – 20) x 12.2 = –12.2 microstrain,
therefore the concrete must have actually expanded by +31 microstrain to
account for the observed apparent strain. The concrete should have expanded
by (30 – 20) x 10.4 = +104 microstrain on account of the temperature increase, the
fact that it didn't reach this value must mean that there has been a
superimposed buildup of compressive strain equal to 104 – 31 = –73 microstrains.
This, multiplied by Young's Modulus, will give the actual stress in the concrete
caused by the imposed load change.
) x CF
0
1
– R
)B + (T
– T
) x (CF
– CF
0
1
0
1
(compressive)
(tensile)
(tensile)
(compressive)
– R
) x B = –91 microstrain. If the
1
0
)
2
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