Geokon 4911, Руководство по установке, Страница: 19 / 27

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4.3 Environmental Factors
Since the purpose of the strain meter installation is to monitor site conditions, factors which may
affect these conditions should always be observed and recorded. Seemingly minor effects may
have a real influence on the behavior of the structure being monitored, and may give an early
indication of potential problems. Some of these factors include, but are not limited to: blasting,
rainfall, tidal levels, traffic, temperature and barometric changes, weather conditions, changes in
personnel, nearby construction activities, excavation and fill level sequences, seasonal changes,
etc.
4.4 Shrinkage Effects
A well know property of concrete is its propensity to shrink as the water content diminishes and
to swell as it absorbs water. This shrinkage and swelling can give rise to large strain changes that
are not related to load or stress. The magnitude of these strains can be several hundred
microstrains.
It is difficult to compensate for these unwanted strains. An attempt may be made to keep the
concrete under a constant condition of water content, but this is frequently impossible on
concrete structures exposed to varying weather conditions. The shrinkage and/or swelling effect
may be measured by casting a strain gauge inside a concrete block that remains unloaded, yet
still exposed to the same moisture conditions as the active gauges. Strains measured on this
gauge may be used as a correction factor.
4.5 Converting Strains to Loads
The load in any structural element to which the strain meter is attached is given by the formula:
L = E µ A
Equation 5 - Strain to Load Formula
Where;
L is the load.
E is the elastic modulus of the structural element in the appropriate units.
µ
is the strain in microstrains.
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.