Long-Term Deformation Monitoring

The resisting capacity of a structure is entirely defined by the comparison of the stresses that actually develop in the materials that constitute it with the limiting constraints of these materials. The most appropriate means of evaluating the variations of constraints is the measurement of strains, which are directly related to constraints.

Measurement of the strains is carried out by extensometers, for which two families can be distinguished: short-base measurements for sensors with dimensions of the order of one centimeter, like strain gauges, and measurements in long base of the order of a few tens of centimeters to several meters, of Optical Strand type.

The short base allows a very punctual measurement of the strain. It is therefore limited to the case of homogeneous materials such as steel. In the case of heterogeneous materials such as concrete, wood or masonry, measurements in a short base are tainted by the error due to the importance of the local disparities: the measurement will be very different on a micro-crack or between two cracks, on a masonry joint or on a block.

The long base measurement makes it possible to remedy this error by averaging the strain over a length that is relevant to the current dimensions of the structures. For example, a cracked critical zone will be followed for itself, instead of individual cracks.

The Optical Strand, as a long base extensometer, therefore responds to the need to follow the mechanical behavior of structures by measuring the most significant quantities for a resistance analysis of materials, i.e. variations of the strains related to the stresses.

Optical Strand technology also allows a measurement without dead time, at high frequency (100 Hz) and with an immediate and inexpensive electronic treatment. This particularity and its great robustness make it suitable for long-term continuous monitoring, allowing access to a long-term behavior as well as to the dynamic behavior of a structure.

In the long term, measurements performed at high frequency and averaged at an appropriate time scale (usually between 1 minute and 1 hour) make it possible to record the complete kinetics of the progressive evolutions of the structure: differential settlements, creep or prestress losses. The large number of measures taken also makes it possible to use statistical methods to eliminate seasonal phenomena due to temperature and to study a long-term mechanical phenomenon as such, free from the uncertainty linked to the variation of conditions that affect traditional topographic methods.

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