Figure 10.1 is a simple but classic example of droplet erosion. The droplets fall from the roof of a tall building (church) directly onto a concrete sidewalk. In this case, the velocity is small, maybe 10 m/ s, but the duration is large-the droplets have been falling on this sidewalk since 1964 (48 years), and the concrete matrix has been eroded one sand particle at a time, leaving the concrete aggregate standing proud. This is not a life-threatening
situation. The concrete sidewalk just gets rougher and nobody pays much attention, but this illustration shows how droplet erosion works.