Why do sidewalks have cracks?

At any given place, various parts of concreted sidewalks have cracks in them.  Most of these cracks are actually the growing distance or separation between two slabs of concrete.  The uneven soil and constant pressure from being stepped on may cause these natural cracks to widen.  There is also the case of plants and weeds that may have grown bigger in between these natural cracks or separations. The bigger the plant, the more likely it is for the slabs of concrete to move apart from each other and widen the so-called crack.

Thermal expansion may also be the culprit in the formation of cracks in many sidewalks.  This is especially true for sidewalks that are exposed to the sun more often.  The concrete slabs can only take so much heat from the sun and when there is too much of it, the concrete may undergo expansion and this will cause cracking on the surface.  The more heat builds up on the concrete slabs on the sidewalk, the deeper will the crack build-up be.  This natural occurrence explains why sidewalks that are exposed to heat are more often cracked when compared to those that get some shading and protection from nearby trees, buildings, or other structures.

Uneven soil underneath the concrete slabs on many sidewalks will also contribute to cracking.  For these concrete slabs to last, the support underneath them must be even.  Any elevation or depression on some parts will cause them to bend and crack once the pressure of stepping on them becomes too much for the slab to handle.  There are also cases wherein the concrete slabs may not be thick enough to handle heavy foot traffic.  With too many people walking through sidewalks, the thinner slabs will also have a greater tendency to get cracked over time.  Nearby trees may also cause cracking on sidewalk concrete slabs.  Some roots may grow bigger and put some upward pressure on the slabs and thereby causing them to break.