Wind, traffic, movement of people, changes in temperature or changes to a building itself can cause almost imperceptible stretches in concrete. Strain gauges can pick up changes at the microstrain level – in the order of 0.0001% change in length. This can be used in monitoring building loads over seconds or years. If a building has cracks, special crack monitors can be used to track whether they are growing, and how they fluctuate each day. Existing stresses can also be estimated within certain structures. This is particularly useful for old pre-stressed elements where the load on the strands is unknown.