A DEMEC (DEmountable MEChanical) strain gauge is a device for monitoring strain in concrete or masonry structures. They generally have gauge lengths like100mm, 200mm, 500mm, etc. There are small metal discs (or other shapes) with small indents that are epoxied or otherwise attached to the structure. There is a bar which is used to set the spacing of the DEMEC points and another bar that’s used to calibrate the gauge. The gauge has two sharp points which fit into the indents on the DEMEC points and a mechanical or digital indicator to read strain. For quasi-static load testing or monitoring cracks in a structure, you can place as many DEMEC points as you want and take measurements at different load levels or times.