Short-gage and long-gage sensors: Applicability and interpretation of measurement

Depending on the geometric basis of measurement (or gage-length), discrete deformation sensors can be classified as a short-gage sensors (typically 10 to 100 mm) or long-gage sensors (typically 0.25 to 10 m). Strain field in homogenous materials is continuous and although the strain can vary a lot in terms of sign and magnitude in distant points of a structure, strain variations are low, quasi constant, over very short segments. Thus, the measurement of a short-gage sensor approximates very well the real strain value at location of the measurement, while the long-gage measurement may give slightly less accurate result, depending on strain variation along its length. Nevertheless, both types of sensors can be used for global structural monitoring of structures built of homogenous construction materials. However, frequently used construction materials, and notably concrete, are inhomogeneous and can be affected by local defects, such as cracks, air pockets and inclusions. All these defects introduce discontinuities in the mechanical material properties. The gage length of a deformation sensor can cross several discontinuities that influence the measurement and its interpretation. Short-gage sensor will provide accurate information concerning the local strain value, but analysis at global structural level may be very inaccurate or even impossible. Contrary, the long-gage sensor will still provide with very useful information on global structural behavior. Basic notions concerning the gage length of a deformation sensor is presented in this paper. Gage length influence on measurement accuracy is analyzed depending of strain variation magnitude for both homogeneous and inhomogeneous materials. Guidelines for determination of applicability and selection of sensor gage lengths depending on project specifications are developed.