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Geotechnical News March 2011
GEOTECHNICAL INSTRUMENTATION NEWS
strain gauges can be installed inside
the structure during casting in order to
directly record the strain state during
different construction phases. From
these measurements, stresses and
internal forces can then be derived. To
obtain reliable estimates of the forces
and stresses, one must use correct
assumptions about concrete behaviour
aswellasaproperconversionprocedure.
The creep behaviour of concrete,
shrinkage and hardening should all
be considered to avoid macroscopic
errors. This is particularly true with
regard to concrete structures which
undergo loads only a few days after
casting, such as temporary supports,
tunnel linings or pier foundations. In
the following, a conversion procedure
aimed to properly simulate concrete
behaviour is described and its
application to real monitoring cases
is presented. We show the effect of
each strain contribution and the errors
which could result as a consequence of
following too simplified a conversion
procedure.
General Description of Strain
Gauges and Their Installation
Procedure
Strain gauges are the most commonly
used instruments for measuring strains,
and consequently for determination
of stresses in concrete structures.
As a possible alternative, fibre optic
systems have been developing during
the last decade. These are able to
provide extensive information, but
are considerably more expensive than
other methods, and hence are usually
only used for special applications. A
strain gauge measures, by means of
a vibrating wire or resistive sensor,
the relative displacement between
two supports that are fixed to the
structure and orientated parallel to the
instrument. The strain gauge has to be
installed with its main axis parallel
to the direction of the strain (with its
consequent stress) to be measured. In
order to derive axial force and bending
moment of a structural element, strain
gauges have to be installed parallel to
the longitudinal axis of the structural
element and at least two should be
installed: one at the extrados and
a second at the intrados. In plain
concrete structures, strain gauges are
embedded within the concrete during
casting, whereas in reinforced concrete
elements they are usually welded
or glued to reinforcement bars (see
, 1b). The sensor records the
deformation electronically, hence it
is possible to connect the instrument
to a data acquisition system so as to
record data and to undertake real-time
monitoring. Strain gauges are usually
equipped with a thermal sensor in order
to record the surrounding temperature
during the readings and to estimate the
contribution of thermal strain to the
structural element.
During installation, it is important
to take some precautions to obtain ac-
curate and reliable results:
• Protect strain gauges by a proper
shield to avoid possible damages
during concreting due to the con-
crete flow or concrete vibrators;
this can be achieved by placing
a polystyrene casing around the
gauge, if welded gauges are used,
or a steel sheet around the sensor
when placing embedded gauges;
• Protect cables by PVC pipes to
avoid potential damage during the
different construction phases;
• Verify operation of each instrument
by taking a first reading before
casting, to allow for replacement of
malfunctioning strain gauges;
• Perform a data acquisition imme-
diately after wiring so as to verify
operation of the data acquisition
system.
Conversion Procedure
Assumptions
As previously stated, in order to obtain
reliable information about stresses
within the structure, a proper conversion
procedure should be adopted to obtain
stresses from measured strains.
As first step, if the instrument is not
thermally self-compensated, as it is the
case for vibrating wire gauges, a cor-
rection must be applied to the readings.
A procedure will usually be described
by the instrument manufacturer, in
order to compensate readings for the
thermal errors in the gauge itself (as op-
posed to the effect that temperature has
on the strain in the concrete or steel). If
resistive sensors are used instead, they
are usually self-compensated by the
Wheatstone bridge system.
Once the total strain (corrected
for thermal errors in the gauge itself)
is measured, various concrete strain
components have to be considered, in
addition to instantaneous strain due
to stress increments, in order to take
into account the complex behaviour
of concrete. Thermal (concrete and
steel) strain, shrinkage and creep strain
should all be considered. Moreover,
the effect of variations in the Young’s
modulus of concrete during the hard-
ening process has to be assessed with
regard to the relationship between elas-
tic strain and stresses. A proper estima-
tion of such contributions is critical to
understanding the strain behaviour of
concrete structures, particularly if the
Figure 1a. Strain gage welded to steel bar.
Figure 1b. Strain gage embedded in concrete.
