Introduction; Figure 1 - O Nsite Installation Photo - Geokon 4200 Series Instruction Manual

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1. INTRODUCTION

Geokon Vibrating Wire Embedment Strain Gages are designed for direct embedment in
concrete. This can be accomplished by attaching the gage to rebar or tensioning cables and then
casting the gage into a concrete briquette, which is subsequently cast into the structure, or
grouting the gage into boreholes in the concrete.
Strains are measured using the vibrating wire principle. A length of steel wire is tensioned
between two end blocks that are firmly in contact with the mass concrete. Deformations in the
concrete will cause the two end blocks to move in relation to each other, altering the tension in
the steel wire. This change in tension is measured as a change in the resonant frequency of
vibration of the wire.
Two coils, one with a magnet insert, the other with a pole piece insert, are located close to the
vibrating wire. In use, a pulse of varying frequency (swept frequency) is applied to the coils
causing the wire to vibrate primarily at its resonant frequency.
Portable readouts and dataloggers are available from Geokon. These models, when used in
conjunction with vibrating wire strain gages, will provide the necessary voltage pulses to pluck
the wire. During vibration, a sinusoidal signal is induced in the coils and transmitted to the
readout box where it is conditioned and displayed.
Figure 1 - On Site Installation Photo