Vibrating Wire Concrete Stressmeter

The Geokon Model 4370 Vibrating Wire Concrete Stressmeter is designed to measure stresses in concrete.

Conventional methods of measuring stresses in concrete suffer from certain drawbacks: strain gages can measure strains but the conversion of strains to stress is made difficult because of changing modulus over time, shrinking and swelling due to varying moisture contents, and creep under sustained loads. Most of these problems can be overcome by the use of hydraulic flatjack type stress cells; however, these cells are subject to a strong temperature dependence, which can also cause de-coupling of the cell from the surrounding concrete, in turn requiring a means of re-inflating the cells after the initial concrete curing period. By making a stressmeter, in effect, out of concrete, these issues are circumvented in a creative manner.

The Model 4370 comprises a small vibrating wire load cell in series with a cylinder of concrete. The concrete cylinder has the same properties as the surrounding concrete but is de-bonded from it by means of a smooth-walled, porous plastic tube and is coupled at its ends to the surrounding concrete by means of two flanges. The vibrating wire load cell measures the load imposed on the inner concrete cylinder by stresses in the surrounding concrete. This load, when divided by the cross sectional area of the inner cylinder, gives the stress in the surrounding concrete. Variations of moisture content in the surrounding concrete are felt also by the inner concrete so that shrinkage and swelling are the same both inside and out, leading to no net change in the load cell readout. (This is not strictly true due to the short length of the metal load cell portion, which behaves differently, but the effect is kept small by the large difference in the relative lengths of the concrete cylinder versus the length of the load cell). A thermistor is included inside the cell for the measurement of temperatures.

Data is collected by the usual means, portable manual readout or automated datalogger.

For more information, please contact Geokon »