Internal deformations (strain) of a concrete mass are measured using a vibrating strain wire element, sealed within a gauge tube and tensioned between two end flanges. With the gauge embedded within concrete, deformation of the concrete mass causes the two gauge flanges to move relative to one another, thus altering the tension in the steel wire. As the wire tension changes, its natural frequency of vibration changes.

An externally mounted, housed electromagnetic excitation coil, within a sealed PVC housing at the mid-point of the gauge tube, is used to excite the strain wire. Connected by tough 4-core, screened cable, the coil is activated by an electrical pulse from a vibrating wire readout, which then measures the resulting frequency of vibration. Using a gauge factor, the measured frequency may be converted directly into units of strain.

Of stainless steel construction, the150mm active length gauges may be pre-attached to rebar or tensioning cables either by tie wire wrapped around the heat shrink sleeved gauge tube or by attachment to a 2,3 or 4 directional rosette, thereby measuring strain in several directions.

Alternatively, the gauge may be pre-cast into a concrete briquette, for subsequent casting into the structure, or grouting into holes drilled into existing structures.

Frequency readings maybe taken manually with a portable Soil Instruments Model 1030 vibrating wire logger or automatically and remotely, with a Campbell Scientific data logger. Additionally, recorded data maybe imported automatically into I-Site software for tabular and graphical data presentation.

Long cable lengths may be used without significant deterioration of signal quality. Attached cable lengths are user specified.

The gauge combines accuracy, sensitivity and long-term stability with a rugged and waterproof construction. Pressure testing is to 15 bar. Over-voltage protection is provided at the gauge coil by a gas tube arrestor.

The gauge may be supplied set for mid-range, providing either extension or compression measurements or set for compression only.

Simple in principle.
Accurate and sensitive.
Waterproof.
Suitable for long cable lengths.
Long-term stability.
Degree of care required to avoid mechanical damage during embedment.
Integral Thermistor.

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