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Nylon Cable Tie
Rebar or
Instrument Cable
Tensioned Cable
4200
Wire Tie
Coil Assembly
Wood or
Wire Tie
Polystyrene
Block
FIGURE 10: Attaching Gauges to Rebar
4.
INSTALLING THE GAUGES IN CONCRETE
strain gauges are typically set into concrete in one of two ways:
GEOKON
1.
Cast the units directly into the concrete mix (see Sections 4.1 through 4.3)
2.
Cast them into briquettes that are subsequently cast into the concrete
structure (see Section 4.4).
When casting the gauge directly into the concrete mix, take care to avoid
applying any large forces to the end blocks during installation. This is most
imperative when installing Model 4202 gauges.
Model 4202 Note: Do not wrap an iron tie wire around the body of the gauge;
doing so could cause damage due to its delicate construction. Instead, use the
holes in the end blocks to affix the gauge to the rebar, being sure that the gauge
is not tensioned or compressed in the longitudinal direction.
Model 4200L Note: This gauge is specifically designed to allow strains to be
measured in curing concrete. However, do not bury the gauge more than one
meter deep; doing so could damage the gauge.
4.1 DIRECT ATTACHMENT TO REBAR
1.
Place two pieces of wood or polystyrene foam between the gauge and the
rebar as shown in Figure 10 on this page.
2.
Use soft iron tie wire, similar to the kind that is normally used for tying rebar
cages together. Run the wire around the body of the strain gauge and
around the rebar. Twist the wire around itself to hold the gauge in place.
3.
Tie the instrument cable off to the rebar using nylon cable ties.
Note: Don't tie the wires too tightly, since rebar and tension cables tend to
move during concrete placement and vibration. Take care not to damage the
cable with the vibrator. The gauge can also be placed directly into the mix if it
can be assured that the orientation will be correct after the gauge placement.
4.2 SUSPENSION METHOD
1.
Wrap a layer of self-vulcanizing rubber tape 3 cm from each gauge end, as
shown in Figure 11 on page 8.
Model 4210 Note: This step is not necessary
The layers of rubber serve as a shock absorber, dampening any vibration of
the suspension system. Without the rubber layers, the resonant frequency
of the tie wires might interfere with the resonant frequency of the gauge
when the tie wires are tightened. This can result in unstable readings, or in
no readings at all. However, this problem disappears once the concrete has
been placed.
For a method that avoids this potential problem, see Section 4.3.
2.
Use soft iron tie wire, the kind normally used for tying rebar cages together.
Wrap the wire around the rubber strips twice.
3.
Twist two loops in the wire, one on either side of the gauge, at a distance of
3 cm from the gauge body. Repeat this process at the other end.
4.
Position the gauge between the rebar and twist the wire ends twice around
the rebar, then around itself.
5.
Tighten the wire and orient the gauge by twisting the loops between the
gauge and the rebar.
MODEL 4200 SERIES STRAIN GAUGES | INSTALLING THE GAUGES IN CONCRETE | 7
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