3
2.2. Placing the Gage in Concrete
The Model 4200ER strain gages are normally set into the concrete structure in one of two
ways: either by casting the unit(s) into the concrete mix directly or by pre-casting the unit(s)
into briquettes that are subsequently cast into the structure.
When casting the gage directly into the structure care must be taken to avoid applying any
large forces to the end blocks during installation. The model 4200ERcan be wired into
position by wiring directly to the tube (see Figure 3). The wires should not be tied too tightly
since rebar and/or tension cables tend to move during concrete placement and vibration.
Care should be taken not to damage the cable with the vibrator. The gage can also be
placed directly into the mix if it can be assured that the orientation will be correct after the
gage placement.
Attached to rebar with blocking
Suspended between rebars
Coil Assembly
Instrument Cable
Nylon Ty-rap
Wire Tie
Rebar or Tensioned Cable
Nylon Ty-rap
Wire Tie
Wire Tie
Instrument Cable
Wire Tie
Coil Assembly
(VCE-4200)
(VCE-4200)
Figure 3 - Attaching VCE-4200X Strain Gages to Rebar
Note the following instructions to suspend the model VCE-4200X strain gage between rebar:
1.
Wrap a layer of self-vulcanizing rubber tape around the gage in the two places
shown in Figure 3 (around the tie points). The rubber layer serves as a shock
absorber, dampening any vibrations of the suspension system. Sometimes,
without the rubber layers, as the tie wires are tightened the resonant frequency
of the tie wires interferes with the resonant frequency of the gage. This results
in unstable readings or no readings at all. This effect disappears once the
concrete has been placed.
2. Select a length of soft iron tie wire, the kind normally used for tying rebar cages
together. Twist it 2 times around the body of the strain gage, over the rubber strips,
about 3 cm from the gage ends.
3. Twist two loops in the wire, one on either side of the gage, at a distance of about
3cm
from the gage body. Repeat this process at the other end of the gage.
4. Position the gage between the rebar and twist the wire ends twice around the rebar,
then around itself.
5. Tighten the wire and orient the gage by twisting on the loops.
6. Slip on the plucking coil and affix using a hose clamp. Tie the instrument cable off to
one of the rebar using nylon Ty-Raps™.
Wood or
Styrofoam
Block
Rubber Tape
Rubber Tape
