Carefully observe the surface of the playground, walking slowly as you look for patterns of cracks. Be sure to look for cracks where there are objects in the pavement such as poles, utility covers, and buildings, as well as in open areas. Try to find several examples of different patterns of cracks.
Force applied to an area is called stress. Stresses in paved surfaces cause cracking. When cracks appear in an open area of the paved surface (when the crack isn’t touching an object), you may notice three main cracks with 120° angles between each crack. This pattern forms when the stress is equal in all directions. Uniform stress can be caused by expansion or contraction due to changes in temperature.
Cracks that form around objects develop because the embedded objects cause unequal stress. The cracks generally are perpendicular to the stress, creating two 90° angles.
Cracks that develop at a corner, such as the corner of the building, form two 135° angles.
The 120° angle pattern that you may have found in an open area of the playground can also be found in the natural world. When an area of lava dries, for example, this same pattern forms. That’s because the lava shrinks as it dries (just as the pavement contracts when it’s cold), creating stress throughout the lava, and it cracks to relieve that stress. Cracks that meet at 120° angles are the shortest possible cracks that can get the job done—and nature always finds ways to take the shortest path and to expend the least amount of energy.
Looking for patterns isn’t just something you can do on a playground. In the days and weeks ahead, look for patterns of cracks on sidewalks, driveways, and other surfaces.
Any opinions, findings, conclusions, or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.
© 2010 Exploratorium | The museum of science, art and human perception