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6.7 Creep Effects
It is also well known that concrete will creep under a sustained load. What may seem to be a
gradually increasing load, as evidenced by a gradually increasing strain, may actually be strain
due to the concrete creeping under a constant, sustained load.
On some projects, gages 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 expansion is
rather like creep, but in the opposite direction, and is difficult to account for.
6.9 Converting Strain to Load
The load in any structural element to which the strain gage is attached is given by the formula:
Where;
L is the load.
E is the elastic modulus of the structural element in the appropriate units.
µ is the strain in microstrain.
A is the cross-sectional area in the appropriate units.
When installing strain gages 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 gages. It is also standard practice to install a pair of strain
gages close to the top of the pile. The measured strain of these two gages 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 gage. The Effective modulus of the various embedment gages
is shown in the table below.
L = E µ A
Equation 7 - Strain to Load formula
Gage
4200
4200L
4200HT
4200HT-T
4202
4204
4210,4212,4214
Table 4 - Effective Modulus
E, approximate
596,000 psi
56,500 psi
596,000 psi
11,950,000 psi
610,000 psi
596,000 psi
2,350,000 psi
19
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