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4.4 USING PRE-CAST BRIQUETTES, SHOTCRETE, OR GROUTING
An alternative to casting the units into the concrete mix directly is to precast the
gauges into briquettes of the same mix as the mass concrete and then place the
briquettes in the structure prior to concrete placement. The briquettes should be
constructed no more than three days prior to installation, but no less than one
day prior, and should be continuously cured with water while awaiting
placement in the mass concrete.
strain gauges may also be used in shotcrete, as well as in holes drilled
GEOKON
in rock or concrete that are subsequently grouted. When used in shotcrete,
special care should be taken to protect the lead wires, such as encasing them in
conduit or heavy tubing. The gauges may be placed by packing the immediate
area around the gauge by hand and then proceeding with the shotcrete
operation.
4.5 CABLE SPLICING AND TERMINATION
The cable from the strain gauges can be protected by using flexible conduit,
which can be supplied by GEOKON.
Terminal boxes with sealed cable entries are available from
for all types
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of applications. These allow many gauges to be terminated at one location with
complete protection of the lead wires. The interior panel of the terminal box can
have built-in jacks or a single connection with a rotary position selector switch.
Contact
for specific application information.
GEOKON
Because the vibrating wire output signal is a frequency rather than a current or
voltage, variations in cable resistance have little effect on gauge readings;
therefore, splicing of cables has no ill effects, and in some cases may be
beneficial. The cable used for making splices should be a high-quality twisted
pair type, with 100% shielding and an integrated shield drain wire. When
splicing, it is very important that the shield drain wires be spliced
together. Always maintain polarity by connecting color to color.
Splice kits recommended by
incorporate casts, which are placed
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around the splice and are then filled with epoxy to waterproof the connections.
When properly made, this type of splice is equal or superior to the cable itself in
strength and electrical properties. Contact
for splicing materials and
GEOKON
additional cable splicing instructions.
Cables may be terminated by stripping and tinning the individual conductors and
then connecting them to the patch cord of a readout box. Alternatively, a
connector may be used which will plug directly into the readout box or to a
receptacle on a special patch cord.
4.6 LIGHTNING PROTECTION
Unlike numerous other types of instrumentation available from
,
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embedment strain gauges do not have any integrated lightning protection
components, such as transorbs or plasma surge arrestors. Usually this is not a
problem, as these types of gauges are installed within concrete or grout and are
somewhat isolated from potentially damaging electrical transients. However,
there may be occasions where some sort of lightning protection is desirable.
One such example is where the gauge is in contact with rebar that may be
exposed to direct or indirect lightning strikes. In addition, if the instrument cable
is exposed, it may be appropriate to install lightning protection components, as
the transient could travel down the cable to the gauge and possibly destroy it.
MODEL 4200 SERIES STRAIN GAUGES | INSTALLING THE GAUGES IN CONCRETE | 9
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