Strain gauges are electrical devices used to measure deformation, pressure, load, torque, position, among other things, using the piezoresistive effect, which is the property that certain materials have of changing the nominal value of their electrical resistance when they are subjected to certain stresses.

Gauges have many applications in civil engineering, which will be explained below.

Strain gauges can be used to measure strain in metal structures. A case of their application is illustrated below. A steel truss was built, to which the load was applied using a chain. Said load was applied by a hydraulic jack.

The data obtained by the strain gauges are sent to a data acquisition system, which is an electronic device responsible for recording the data.[1]​.

This procedure was done on a scale truss, but it is also carried out for larger metal structures such as steel bridges, trusses for large capacity warehouses, etc. This way it can be studied and defined what capabilities they may have when supporting different forces and the appropriate design.[2]​.

Applications in reinforced concrete structures

Strain gauges are used in reinforced concrete to study the behavior of this material. For this, two metal plates are placed on the concrete, so that the strain gauges can adhere.[3]​[4]​ The properties of these plates must be studied in advance. since they will be on the concrete, which will be subjected to different loads.

Strain gauges are the most commonly used instruments for measuring stresses, and therefore for determining stresses in concrete structures. In parallel, during the last ten years, fiber optic systems have been developed, which have the capacity to provide extensive information, with the drawback of being much less economical than other methods, which is why they are only used in very special applications.[5][4]​.

A strain gauge takes measurements of the relative displacement between two supports that are fixed to the structure oriented parallel to each other. These measurements are collected using a vibrating wire or a resistive sensor. The strain gauge has to be installed with its axis parallel to the main direction of the strain and the respective stress to be measured. In order to obtain the axial force and bending moment of a structural element, one gauge has to be installed parallel to the structural element and at least two other gauges must be installed: one on the extrados and a second on the intrados. In simple concrete structures, the gauges are embedded in the concrete during casting, while in reinforced concrete elements they are usually welded or glued to the reinforcing steels.