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| DATA REDUCTION | GEOKON
strain, may actually be strain due to the concrete creeping under a constant,
sustained load.
On some projects, gauges have been cast into concrete blocks in the laboratory
and kept loaded by means of springs inside a load frame. In this manner, the
creep phenomenon can be quantified.
6.8
EFFECT OF AUTOGENOUS GROWTH
Some older concretes that have a particular combination of aggregates and
alkaline cements may expand with time as they undergo a chemical change and
recrystallization. This is called autogenous growth and is like creep, but in the
opposite direction, and is difficult to quantify.
6.9
CONVERTING STRAIN TO LOAD
The load in any structural element to which the strain gauge is attached is given
by the formula:
EQUATION 7:
Strain to Load Formula
Where:
L
is the load.
E
is the elastic modulus of the structural element in the appropriate units.
is the strain in microstrain.
A
is the cross-sectional area in the appropriate units.
When installing strain gauges in concrete piles it is standard practice to install
them in pairs on either side of the neutral axis. This allows any strains imposed
by bending to be corrected by taking the average strain of the two gauges. It is
also standard practice to install a pair of strain gauges close to the top of the
pile. The measured strain of these two gauges is used to calculate the modulus
of the concrete.
6.10 EFFECTIVE MODULUS
For some concrete strain measurements during the early stages of curing it is
important to know the effective modulus of the strain gauge. The effective
modulus of the various embedment gauges is shown in the following table:
TABLE 6:
Effective Modulus
Model
E, approximate
4200
596,000 psi
4200L
56,500 psi
4200-6 / 4200-7
N/A
4200HT
596,000 psi
4200HT-T
11,950,000 psi
4202
610,000 psi
4204
596,000 psi
4210 / 4212 / 4214
2,350,000 psi
L
E
A
=
